From f38025126040b01daf95555f7c6ee40f9e773c9e Mon Sep 17 00:00:00 2001
From: Bruno Rocha <bruno@rockbruno.com>
Date: Thu, 14 Aug 2025 09:27:54 +0200
Subject: [PATCH 1/8] Allow alternate build systems to have larger indexing
 batch sizes

---
 .../BuildServerManager.swift                  |  7 +-
 .../Messages/InitializeBuildRequest.swift     | 13 ++++
 .../CustomBuildServerTestProject.swift        |  4 +-
 .../SemanticIndex/SemanticIndexManager.swift  | 10 ++-
 Sources/SourceKitLSP/Workspace.swift          |  5 ++
 .../BackgroundIndexingTests.swift             | 75 +++++++++++++++++++
 6 files changed, 108 insertions(+), 6 deletions(-)

diff --git a/Sources/BuildServerIntegration/BuildServerManager.swift b/Sources/BuildServerIntegration/BuildServerManager.swift
index 5346d8337..3bda81ae3 100644
--- a/Sources/BuildServerIntegration/BuildServerManager.swift
+++ b/Sources/BuildServerIntegration/BuildServerManager.swift
@@ -322,9 +322,13 @@ package actor BuildServerManager: QueueBasedMessageHandler {
 
   private var connectionToClient: BuildServerManagerConnectionToClient
 
-  /// The build serer adapter that is used to answer build server queries.
+  /// The build server adapter that is used to answer build server queries.
   private var buildServerAdapter: BuildServerAdapter?
 
+  /// The kind of underlying build server adapter that is being managed.
+  /// `nil` if the `BuildServerManager` does not have an underlying build system.
+  private var kind: BuildServerSpec.Kind?
+
   /// The build server adapter after initialization finishes. When sending messages to the BSP server, this should be
   /// preferred over `buildServerAdapter` because no messages must be sent to the build server before initialization
   /// finishes.
@@ -451,6 +455,7 @@ package actor BuildServerManager: QueueBasedMessageHandler {
     self.toolchainRegistry = toolchainRegistry
     self.options = options
     self.connectionToClient = connectionToClient
+    self.kind = buildServerSpec?.kind
     self.configPath = buildServerSpec?.configPath
     self.buildServerAdapter = await buildServerSpec?.createBuildServerAdapter(
       toolchainRegistry: toolchainRegistry,
diff --git a/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift b/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
index 15c720d70..71daca936 100644
--- a/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
+++ b/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
@@ -278,6 +278,9 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
   /// Whether the server implements the `textDocument/sourceKitOptions` request.
   public var sourceKitOptionsProvider: Bool?
 
+  /// The number of targets to prepare concurrently, when an index request is scheduled.
+  public var indexTaskBatchSize: Int?
+
   /// The files to watch for changes.
   public var watchers: [FileSystemWatcher]?
 
@@ -286,12 +289,14 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
   public init(
     indexDatabasePath: String? = nil,
     indexStorePath: String? = nil,
+    indexTaskBatchSize: Int? = nil,
     watchers: [FileSystemWatcher]? = nil,
     prepareProvider: Bool? = nil,
     sourceKitOptionsProvider: Bool? = nil
   ) {
     self.indexDatabasePath = indexDatabasePath
     self.indexStorePath = indexStorePath
+    self.indexTaskBatchSize = indexTaskBatchSize
     self.watchers = watchers
     self.prepareProvider = prepareProvider
     self.sourceKitOptionsProvider = sourceKitOptionsProvider
@@ -300,6 +305,7 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
   public init(
     indexDatabasePath: String? = nil,
     indexStorePath: String? = nil,
+    indexTaskBatchSize: Int? = nil,
     outputPathsProvider: Bool? = nil,
     prepareProvider: Bool? = nil,
     sourceKitOptionsProvider: Bool? = nil,
@@ -307,6 +313,7 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
   ) {
     self.indexDatabasePath = indexDatabasePath
     self.indexStorePath = indexStorePath
+    self.indexTaskBatchSize = indexTaskBatchSize
     self.outputPathsProvider = outputPathsProvider
     self.prepareProvider = prepareProvider
     self.sourceKitOptionsProvider = sourceKitOptionsProvider
@@ -320,6 +327,9 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
     if case .string(let indexStorePath) = dictionary[CodingKeys.indexStorePath.stringValue] {
       self.indexStorePath = indexStorePath
     }
+    if case .int(let indexTaskBatchSize) = dictionary[CodingKeys.indexTaskBatchSize.stringValue] {
+      self.indexTaskBatchSize = indexTaskBatchSize
+    }
     if case .bool(let outputPathsProvider) = dictionary[CodingKeys.outputPathsProvider.stringValue] {
       self.outputPathsProvider = outputPathsProvider
     }
@@ -342,6 +352,9 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
     if let indexStorePath {
       result[CodingKeys.indexStorePath.stringValue] = .string(indexStorePath)
     }
+    if let indexTaskBatchSize {
+      result[CodingKeys.indexTaskBatchSize.stringValue] = .int(indexTaskBatchSize)
+    }
     if let outputPathsProvider {
       result[CodingKeys.outputPathsProvider.stringValue] = .bool(outputPathsProvider)
     }
diff --git a/Sources/SKTestSupport/CustomBuildServerTestProject.swift b/Sources/SKTestSupport/CustomBuildServerTestProject.swift
index 2f73fdff6..fba25b296 100644
--- a/Sources/SKTestSupport/CustomBuildServerTestProject.swift
+++ b/Sources/SKTestSupport/CustomBuildServerTestProject.swift
@@ -175,12 +175,14 @@ package extension CustomBuildServer {
 
   func initializationResponseSupportingBackgroundIndexing(
     projectRoot: URL,
-    outputPathsProvider: Bool
+    outputPathsProvider: Bool,
+    indexTaskBatchSize: Int? = nil
   ) throws -> InitializeBuildResponse {
     return initializationResponse(
       initializeData: SourceKitInitializeBuildResponseData(
         indexDatabasePath: try projectRoot.appendingPathComponent("index-db").filePath,
         indexStorePath: try projectRoot.appendingPathComponent("index-store").filePath,
+        indexTaskBatchSize: indexTaskBatchSize,
         outputPathsProvider: outputPathsProvider,
         prepareProvider: true,
         sourceKitOptionsProvider: true
diff --git a/Sources/SemanticIndex/SemanticIndexManager.swift b/Sources/SemanticIndex/SemanticIndexManager.swift
index cad195edf..d318618b8 100644
--- a/Sources/SemanticIndex/SemanticIndexManager.swift
+++ b/Sources/SemanticIndex/SemanticIndexManager.swift
@@ -222,6 +222,9 @@ package final actor SemanticIndexManager {
   /// The parameter is the number of files that were scheduled to be indexed.
   private let indexTasksWereScheduled: @Sendable (_ numberOfFileScheduled: Int) -> Void
 
+  /// The number of targets to prepare concurrently, whenever a index request is scheduled.
+  private let indexTaskBatchSize: Int
+
   /// Callback that is called when `progressStatus` might have changed.
   private let indexProgressStatusDidChange: @Sendable () -> Void
 
@@ -261,6 +264,7 @@ package final actor SemanticIndexManager {
     updateIndexStoreTimeout: Duration,
     hooks: IndexHooks,
     indexTaskScheduler: TaskScheduler<AnyIndexTaskDescription>,
+    indexTaskBatchSize: Int,
     logMessageToIndexLog:
       @escaping @Sendable (
         _ message: String, _ type: WindowMessageType, _ structure: StructuredLogKind
@@ -273,6 +277,7 @@ package final actor SemanticIndexManager {
     self.updateIndexStoreTimeout = updateIndexStoreTimeout
     self.hooks = hooks
     self.indexTaskScheduler = indexTaskScheduler
+    self.indexTaskBatchSize = indexTaskBatchSize
     self.logMessageToIndexLog = logMessageToIndexLog
     self.indexTasksWereScheduled = indexTasksWereScheduled
     self.indexProgressStatusDidChange = indexProgressStatusDidChange
@@ -877,10 +882,7 @@ package final actor SemanticIndexManager {
 
     var indexTasks: [Task<Void, Never>] = []
 
-    // TODO: When we can index multiple targets concurrently in SwiftPM, increase the batch size to half the
-    // processor count, so we can get parallelism during preparation.
-    // (https://github.com/swiftlang/sourcekit-lsp/issues/1262)
-    for targetsBatch in sortedTargets.partition(intoBatchesOfSize: 1) {
+    for targetsBatch in sortedTargets.partition(intoBatchesOfSize: indexTaskBatchSize) {
       let preparationTaskID = UUID()
       let filesToIndex = targetsBatch.flatMap({ filesByTarget[$0]! })
 
diff --git a/Sources/SourceKitLSP/Workspace.swift b/Sources/SourceKitLSP/Workspace.swift
index 41fd7dc05..70b9ae6a2 100644
--- a/Sources/SourceKitLSP/Workspace.swift
+++ b/Sources/SourceKitLSP/Workspace.swift
@@ -162,12 +162,17 @@ package final class Workspace: Sendable, BuildServerManagerDelegate {
     if options.backgroundIndexingOrDefault, let uncheckedIndex,
       await buildServerManager.initializationData?.prepareProvider ?? false
     {
+      // TODO: When we can index multiple targets concurrently in SwiftPM, we may want to default
+      // to something else other than 1.
+      // (https://github.com/swiftlang/sourcekit-lsp/issues/1262)
+      let batchSize = await buildServerManager.initializationData?.indexTaskBatchSize ?? 1
       self.semanticIndexManager = SemanticIndexManager(
         index: uncheckedIndex,
         buildServerManager: buildServerManager,
         updateIndexStoreTimeout: options.indexOrDefault.updateIndexStoreTimeoutOrDefault,
         hooks: hooks.indexHooks,
         indexTaskScheduler: indexTaskScheduler,
+        indexTaskBatchSize: batchSize,
         logMessageToIndexLog: { [weak sourceKitLSPServer] in
           sourceKitLSPServer?.logMessageToIndexLog(message: $0, type: $1, structure: $2)
         },
diff --git a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
index 9538222d1..0ac776433 100644
--- a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
+++ b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
@@ -2624,6 +2624,81 @@ final class BackgroundIndexingTests: XCTestCase {
     let symbols = try await project.testClient.send(WorkspaceSymbolsRequest(query: "myTestFu"))
     XCTAssertEqual(symbols?.count, 1)
   }
+
+  func testBuildServerUsesCustomTaskBatchSize() async throws {
+    final class BuildServer: CustomBuildServer {
+      let inProgressRequestsTracker = CustomBuildServerInProgressRequestTracker()
+      private let projectRoot: URL
+      private var testFileURL: URL { projectRoot.appendingPathComponent("test.swift").standardized }
+
+      required init(projectRoot: URL, connectionToSourceKitLSP: any LanguageServerProtocol.Connection) {
+        self.projectRoot = projectRoot
+      }
+
+      func initializeBuildRequest(_ request: InitializeBuildRequest) async throws -> InitializeBuildResponse {
+        return try initializationResponseSupportingBackgroundIndexing(
+          projectRoot: projectRoot,
+          outputPathsProvider: false,
+          indexTaskBatchSize: 3
+        )
+      }
+
+      func buildTargetSourcesRequest(_ request: BuildTargetSourcesRequest) async throws -> BuildTargetSourcesResponse {
+        var dummyTargets = [BuildTargetIdentifier]()
+        for i in 0..<10 {
+          dummyTargets.append(BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-\(i)")))
+        }
+        return BuildTargetSourcesResponse(items: dummyTargets.map {
+          SourcesItem(target: $0, sources: [SourceItem(uri: URI(testFileURL), kind: .file, generated: false)])
+        })
+      }
+
+      func textDocumentSourceKitOptionsRequest(
+        _ request: TextDocumentSourceKitOptionsRequest
+      ) async throws -> TextDocumentSourceKitOptionsResponse? {
+        return TextDocumentSourceKitOptionsResponse(compilerArguments: [request.textDocument.uri.pseudoPath])
+      }
+    }
+
+    let preparationTaskSemaphore = WrappedSemaphore(name: "Received a preparation task")
+    let preparationTasks = ThreadSafeBox<[PreparationTaskDescription]>(initialValue: [])
+    let project = try await CustomBuildServerTestProject(
+      files: [
+        "test.swift": """
+        func testFunction() {}
+        """
+      ],
+      buildServer: BuildServer.self,
+      hooks: Hooks(
+        indexHooks: IndexHooks(
+          preparationTaskDidStart: { task in
+            preparationTasks.withLock { preparationTasks in
+              // Ignore everything after the 4th batch to avoid flakiness.
+              if preparationTasks.count < 4 {
+                preparationTasks.append(task)
+              }
+              if preparationTasks.count == 4 {
+                preparationTaskSemaphore.signal()
+              }
+            }
+          }
+        )
+      ),
+      enableBackgroundIndexing: true
+    )
+
+    _ = try await project.testClient.send(SynchronizeRequest(index: true))
+
+    preparationTaskSemaphore.waitOrXCTFail()
+
+    // The test project has 10 targets, and we should have received them in batches of 3.
+    XCTAssertEqual(preparationTasks.value.count, 4)
+    let preparedTargetBatches = preparationTasks.value.map(\.targetsToPrepare).sorted { $0.count > $1.count }
+    XCTAssertEqual(preparedTargetBatches[0].count, 3)
+    XCTAssertEqual(preparedTargetBatches[1].count, 3)
+    XCTAssertEqual(preparedTargetBatches[2].count, 3)
+    XCTAssertEqual(preparedTargetBatches[3].count, 1)
+  }
 }
 
 extension HoverResponseContents {

From b62d91a0ce3896cf67a2f0a9408ad4dc26e64bbb Mon Sep 17 00:00:00 2001
From: Bruno Rocha <bruno@rockbruno.com>
Date: Thu, 14 Aug 2025 15:22:28 +0200
Subject: [PATCH 2/8] Let the server control the batch size instead

---
 Contributor Documentation/BSP Extensions.md   |   5 +-
 .../BuiltInBuildServer.swift                  |   3 +
 .../BuiltInBuildServerAdapter.swift           |   1 +
 .../FixedCompilationDatabaseBuildServer.swift |   2 +
 .../JSONCompilationDatabaseBuildServer.swift  |   2 +
 .../LegacyBuildServer.swift                   |   2 +
 .../SwiftPMBuildServer.swift                  |   2 +
 .../Messages/InitializeBuildRequest.swift     |  45 +++--
 .../CustomBuildServerTestProject.swift        |   6 +-
 .../SemanticIndex/SemanticIndexManager.swift  |  23 +--
 Sources/SemanticIndex/TaskScheduler.swift     |   1 -
 Sources/SourceKitLSP/Workspace.swift          |   7 +-
 .../BackgroundIndexingTests.swift             | 163 ++++++++++--------
 13 files changed, 155 insertions(+), 107 deletions(-)

diff --git a/Contributor Documentation/BSP Extensions.md b/Contributor Documentation/BSP Extensions.md
index cb28c6f3d..d513768a3 100644
--- a/Contributor Documentation/BSP Extensions.md	
+++ b/Contributor Documentation/BSP Extensions.md	
@@ -22,6 +22,9 @@ export interface SourceKitInitializeBuildResponseData {
    * for `swiftc` or `clang` invocations **/
   indexStorePath?: string;
 
+  /** Options to control how many targets should be prepared simultaneously by SourceKit-LSP. */
+  multiTargetPreparation?: MultiTargetPreparationSupport
+
   /** Whether the server set the `outputPath` property in the `buildTarget/sources` request */
   outputPathsProvider?: bool;
 
@@ -34,7 +37,7 @@ export interface SourceKitInitializeBuildResponseData {
   /** The files to watch for changes.
    * Changes to these files are sent to the BSP server using `workspace/didChangeWatchedFiles`.
    * `FileSystemWatcher` is the same as in LSP. */
-  watchers: [FileSystemWatcher]?
+  watchers?: [FileSystemWatcher]
 }
 ```
 
diff --git a/Sources/BuildServerIntegration/BuiltInBuildServer.swift b/Sources/BuildServerIntegration/BuiltInBuildServer.swift
index 814448e61..aabaecaca 100644
--- a/Sources/BuildServerIntegration/BuiltInBuildServer.swift
+++ b/Sources/BuildServerIntegration/BuiltInBuildServer.swift
@@ -35,6 +35,9 @@ package protocol BuiltInBuildServer: AnyObject, Sendable {
   /// The path to put the index database, if any.
   var indexDatabasePath: URL? { get async }
 
+  /// Whether the build server can prepare multiple targets in parallel.
+  var supportsMultiTargetPreparation: Bool { get }
+
   /// Whether the build server is capable of preparing a target for indexing and determining the output paths for the
   /// target, ie. whether the `prepare` method has been implemented and this build server populates the `outputPath`
   /// property in the `buildTarget/sources` request.
diff --git a/Sources/BuildServerIntegration/BuiltInBuildServerAdapter.swift b/Sources/BuildServerIntegration/BuiltInBuildServerAdapter.swift
index a22396175..328a2d45b 100644
--- a/Sources/BuildServerIntegration/BuiltInBuildServerAdapter.swift
+++ b/Sources/BuildServerIntegration/BuiltInBuildServerAdapter.swift
@@ -94,6 +94,7 @@ actor BuiltInBuildServerAdapter: QueueBasedMessageHandler {
         indexStorePath: await orLog("getting index store file path") {
           try await underlyingBuildServer.indexStorePath?.filePath
         },
+        multiTargetPreparation: MultiTargetPreparationSupport(supported: underlyingBuildServer.supportsMultiTargetPreparation),
         outputPathsProvider: underlyingBuildServer.supportsPreparationAndOutputPaths,
         prepareProvider: underlyingBuildServer.supportsPreparationAndOutputPaths,
         sourceKitOptionsProvider: true,
diff --git a/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift b/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift
index ef084482a..46370c77e 100644
--- a/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift
+++ b/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift
@@ -59,6 +59,8 @@ package actor FixedCompilationDatabaseBuildServer: BuiltInBuildServer {
     indexStorePath?.deletingLastPathComponent().appendingPathComponent("IndexDatabase")
   }
 
+  package nonisolated var supportsMultiTargetPreparation: Bool { true }
+
   package nonisolated var supportsPreparationAndOutputPaths: Bool { false }
 
   private static func parseCompileFlags(at configPath: URL) throws -> [String] {
diff --git a/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift b/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift
index abd8e5196..acf5eb42e 100644
--- a/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift
+++ b/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift
@@ -96,6 +96,8 @@ package actor JSONCompilationDatabaseBuildServer: BuiltInBuildServer {
     indexStorePath?.deletingLastPathComponent().appendingPathComponent("IndexDatabase")
   }
 
+  package nonisolated var supportsMultiTargetPreparation: Bool { true }
+
   package nonisolated var supportsPreparationAndOutputPaths: Bool { false }
 
   package init(
diff --git a/Sources/BuildServerIntegration/LegacyBuildServer.swift b/Sources/BuildServerIntegration/LegacyBuildServer.swift
index 44e49b262..026454e1a 100644
--- a/Sources/BuildServerIntegration/LegacyBuildServer.swift
+++ b/Sources/BuildServerIntegration/LegacyBuildServer.swift
@@ -138,6 +138,8 @@ actor LegacyBuildServer: MessageHandler, BuiltInBuildServer {
     connectionToSourceKitLSP.send(OnBuildTargetDidChangeNotification(changes: nil))
   }
 
+  package nonisolated var supportsMultiTargetPreparation: Bool { true }
+
   package nonisolated var supportsPreparationAndOutputPaths: Bool { false }
 
   package func buildTargets(request: WorkspaceBuildTargetsRequest) async throws -> WorkspaceBuildTargetsResponse {
diff --git a/Sources/BuildServerIntegration/SwiftPMBuildServer.swift b/Sources/BuildServerIntegration/SwiftPMBuildServer.swift
index 4f9737a23..db1223820 100644
--- a/Sources/BuildServerIntegration/SwiftPMBuildServer.swift
+++ b/Sources/BuildServerIntegration/SwiftPMBuildServer.swift
@@ -459,6 +459,8 @@ package actor SwiftPMBuildServer: BuiltInBuildServer {
     connectionToSourceKitLSP.send(OnBuildTargetDidChangeNotification(changes: nil))
   }
 
+  package nonisolated var supportsMultiTargetPreparation: Bool { false }
+
   package nonisolated var supportsPreparationAndOutputPaths: Bool { options.backgroundIndexingOrDefault }
 
   package var buildPath: URL {
diff --git a/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift b/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
index 71daca936..4e01f70e6 100644
--- a/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
+++ b/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
@@ -269,6 +269,9 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
   /// The path at which SourceKit-LSP can store its index database, aggregating data from `indexStorePath`
   public var indexStorePath: String?
 
+  /// Options to control how many targets should be prepared simultaneously by SourceKit-LSP.
+  public var multiTargetPreparation: MultiTargetPreparationSupport?
+
   /// Whether the server implements the `buildTarget/outputPaths` request.
   public var outputPathsProvider: Bool?
 
@@ -278,9 +281,6 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
   /// Whether the server implements the `textDocument/sourceKitOptions` request.
   public var sourceKitOptionsProvider: Bool?
 
-  /// The number of targets to prepare concurrently, when an index request is scheduled.
-  public var indexTaskBatchSize: Int?
-
   /// The files to watch for changes.
   public var watchers: [FileSystemWatcher]?
 
@@ -289,14 +289,14 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
   public init(
     indexDatabasePath: String? = nil,
     indexStorePath: String? = nil,
-    indexTaskBatchSize: Int? = nil,
+    multiTargetPreparation: MultiTargetPreparationSupport? = nil,
     watchers: [FileSystemWatcher]? = nil,
     prepareProvider: Bool? = nil,
     sourceKitOptionsProvider: Bool? = nil
   ) {
     self.indexDatabasePath = indexDatabasePath
     self.indexStorePath = indexStorePath
-    self.indexTaskBatchSize = indexTaskBatchSize
+    self.multiTargetPreparation = multiTargetPreparation
     self.watchers = watchers
     self.prepareProvider = prepareProvider
     self.sourceKitOptionsProvider = sourceKitOptionsProvider
@@ -305,7 +305,7 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
   public init(
     indexDatabasePath: String? = nil,
     indexStorePath: String? = nil,
-    indexTaskBatchSize: Int? = nil,
+    multiTargetPreparation: MultiTargetPreparationSupport? = nil,
     outputPathsProvider: Bool? = nil,
     prepareProvider: Bool? = nil,
     sourceKitOptionsProvider: Bool? = nil,
@@ -313,7 +313,7 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
   ) {
     self.indexDatabasePath = indexDatabasePath
     self.indexStorePath = indexStorePath
-    self.indexTaskBatchSize = indexTaskBatchSize
+    self.multiTargetPreparation = multiTargetPreparation
     self.outputPathsProvider = outputPathsProvider
     self.prepareProvider = prepareProvider
     self.sourceKitOptionsProvider = sourceKitOptionsProvider
@@ -327,8 +327,8 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
     if case .string(let indexStorePath) = dictionary[CodingKeys.indexStorePath.stringValue] {
       self.indexStorePath = indexStorePath
     }
-    if case .int(let indexTaskBatchSize) = dictionary[CodingKeys.indexTaskBatchSize.stringValue] {
-      self.indexTaskBatchSize = indexTaskBatchSize
+    if case .dictionary(let multiTargetPreparation) = dictionary[CodingKeys.multiTargetPreparation.stringValue] {
+      self.multiTargetPreparation = MultiTargetPreparationSupport(fromLSPDictionary: multiTargetPreparation)
     }
     if case .bool(let outputPathsProvider) = dictionary[CodingKeys.outputPathsProvider.stringValue] {
       self.outputPathsProvider = outputPathsProvider
@@ -352,8 +352,8 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
     if let indexStorePath {
       result[CodingKeys.indexStorePath.stringValue] = .string(indexStorePath)
     }
-    if let indexTaskBatchSize {
-      result[CodingKeys.indexTaskBatchSize.stringValue] = .int(indexTaskBatchSize)
+    if let multiTargetPreparation {
+      result[CodingKeys.multiTargetPreparation.stringValue] = multiTargetPreparation.encodeToLSPAny()
     }
     if let outputPathsProvider {
       result[CodingKeys.outputPathsProvider.stringValue] = .bool(outputPathsProvider)
@@ -370,3 +370,26 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
     return .dictionary(result)
   }
 }
+
+public struct MultiTargetPreparationSupport: LSPAnyCodable, Codable, Sendable {
+  /// Whether the build server can prepare multiple targets in parallel. If this value is omitted, it is assumed to be `true`.
+  public var supported: Bool?
+
+  public init(supported: Bool? = nil) {
+    self.supported = supported
+  }
+
+  public init?(fromLSPDictionary dictionary: [String: LanguageServerProtocol.LSPAny]) {
+    if case .bool(let supported) = dictionary[CodingKeys.supported.stringValue] {
+      self.supported = supported
+    }
+  }
+
+  public func encodeToLSPAny() -> LanguageServerProtocol.LSPAny {
+    var result: [String: LSPAny] = [:]
+    if let supported {
+      result[CodingKeys.supported.stringValue] = .bool(supported)
+    }
+    return .dictionary(result)
+  }
+}
diff --git a/Sources/SKTestSupport/CustomBuildServerTestProject.swift b/Sources/SKTestSupport/CustomBuildServerTestProject.swift
index fba25b296..cb5e92298 100644
--- a/Sources/SKTestSupport/CustomBuildServerTestProject.swift
+++ b/Sources/SKTestSupport/CustomBuildServerTestProject.swift
@@ -175,17 +175,15 @@ package extension CustomBuildServer {
 
   func initializationResponseSupportingBackgroundIndexing(
     projectRoot: URL,
-    outputPathsProvider: Bool,
-    indexTaskBatchSize: Int? = nil
+    outputPathsProvider: Bool
   ) throws -> InitializeBuildResponse {
     return initializationResponse(
       initializeData: SourceKitInitializeBuildResponseData(
         indexDatabasePath: try projectRoot.appendingPathComponent("index-db").filePath,
         indexStorePath: try projectRoot.appendingPathComponent("index-store").filePath,
-        indexTaskBatchSize: indexTaskBatchSize,
         outputPathsProvider: outputPathsProvider,
         prepareProvider: true,
-        sourceKitOptionsProvider: true
+        sourceKitOptionsProvider: true,
       )
     )
   }
diff --git a/Sources/SemanticIndex/SemanticIndexManager.swift b/Sources/SemanticIndex/SemanticIndexManager.swift
index d318618b8..02f716aa0 100644
--- a/Sources/SemanticIndex/SemanticIndexManager.swift
+++ b/Sources/SemanticIndex/SemanticIndexManager.swift
@@ -222,8 +222,8 @@ package final actor SemanticIndexManager {
   /// The parameter is the number of files that were scheduled to be indexed.
   private let indexTasksWereScheduled: @Sendable (_ numberOfFileScheduled: Int) -> Void
 
-  /// The number of targets to prepare concurrently, whenever a index request is scheduled.
-  private let indexTaskBatchSize: Int
+  /// Determines whether or not the `SemanticIndexManager` should dispatch preparation tasks in batches.
+  private let shouldIndexInParallel: Bool
 
   /// Callback that is called when `progressStatus` might have changed.
   private let indexProgressStatusDidChange: @Sendable () -> Void
@@ -264,7 +264,7 @@ package final actor SemanticIndexManager {
     updateIndexStoreTimeout: Duration,
     hooks: IndexHooks,
     indexTaskScheduler: TaskScheduler<AnyIndexTaskDescription>,
-    indexTaskBatchSize: Int,
+    shouldIndexInParallel: Bool,
     logMessageToIndexLog:
       @escaping @Sendable (
         _ message: String, _ type: WindowMessageType, _ structure: StructuredLogKind
@@ -277,7 +277,7 @@ package final actor SemanticIndexManager {
     self.updateIndexStoreTimeout = updateIndexStoreTimeout
     self.hooks = hooks
     self.indexTaskScheduler = indexTaskScheduler
-    self.indexTaskBatchSize = indexTaskBatchSize
+    self.shouldIndexInParallel = shouldIndexInParallel
     self.logMessageToIndexLog = logMessageToIndexLog
     self.indexTasksWereScheduled = indexTasksWereScheduled
     self.indexProgressStatusDidChange = indexProgressStatusDidChange
@@ -654,7 +654,6 @@ package final actor SemanticIndexManager {
     guard !targetsToPrepare.isEmpty else {
       return
     }
-
     let taskDescription = AnyIndexTaskDescription(
       PreparationTaskDescription(
         targetsToPrepare: targetsToPrepare,
@@ -882,7 +881,14 @@ package final actor SemanticIndexManager {
 
     var indexTasks: [Task<Void, Never>] = []
 
-    for targetsBatch in sortedTargets.partition(intoBatchesOfSize: indexTaskBatchSize) {
+    let batchSize: Int
+    if shouldIndexInParallel {
+      let processorCount = ProcessInfo.processInfo.activeProcessorCount
+      batchSize = max(1, processorCount * 5)
+    } else {
+      batchSize = 1
+    }
+    for targetsBatch in sortedTargets.partition(intoBatchesOfSize: batchSize) {
       let preparationTaskID = UUID()
       let filesToIndex = targetsBatch.flatMap({ filesByTarget[$0]! })
 
@@ -906,10 +912,7 @@ package final actor SemanticIndexManager {
         // And after preparation is done, index the files in the targets.
         await withTaskGroup(of: Void.self) { taskGroup in
           for target in targetsBatch {
-            // TODO: Once swiftc supports indexing of multiple files in a single invocation, increase the batch size to
-            // allow it to share AST builds between multiple files within a target.
-            // (https://github.com/swiftlang/sourcekit-lsp/issues/1268)
-            for fileBatch in filesByTarget[target]!.partition(intoBatchesOfSize: 1) {
+            for fileBatch in filesByTarget[target]!.partition(intoBatchesOfSize: batchSize) {
               taskGroup.addTask {
                 await self.updateIndexStore(
                   for: fileBatch,
diff --git a/Sources/SemanticIndex/TaskScheduler.swift b/Sources/SemanticIndex/TaskScheduler.swift
index 56c2632b4..e6a8d8f24 100644
--- a/Sources/SemanticIndex/TaskScheduler.swift
+++ b/Sources/SemanticIndex/TaskScheduler.swift
@@ -370,7 +370,6 @@ package actor TaskScheduler<TaskDescription: TaskDescriptionProtocol> {
         // We didn't actually change anything, so we don't need to perform any validation or task processing.
         return
       }
-
       // Check we are over-subscribed in currently executing tasks by walking through all currently executing tasks and
       // checking if we could schedule them within the new execution limits. Cancel any tasks that do not fit within the
       // new limit to be rescheduled when we are within the limit again.
diff --git a/Sources/SourceKitLSP/Workspace.swift b/Sources/SourceKitLSP/Workspace.swift
index 70b9ae6a2..c9ce196ad 100644
--- a/Sources/SourceKitLSP/Workspace.swift
+++ b/Sources/SourceKitLSP/Workspace.swift
@@ -162,17 +162,14 @@ package final class Workspace: Sendable, BuildServerManagerDelegate {
     if options.backgroundIndexingOrDefault, let uncheckedIndex,
       await buildServerManager.initializationData?.prepareProvider ?? false
     {
-      // TODO: When we can index multiple targets concurrently in SwiftPM, we may want to default
-      // to something else other than 1.
-      // (https://github.com/swiftlang/sourcekit-lsp/issues/1262)
-      let batchSize = await buildServerManager.initializationData?.indexTaskBatchSize ?? 1
+      let shouldIndexInParallel = await buildServerManager.initializationData?.multiTargetPreparation?.supported ?? true
       self.semanticIndexManager = SemanticIndexManager(
         index: uncheckedIndex,
         buildServerManager: buildServerManager,
         updateIndexStoreTimeout: options.indexOrDefault.updateIndexStoreTimeoutOrDefault,
         hooks: hooks.indexHooks,
         indexTaskScheduler: indexTaskScheduler,
-        indexTaskBatchSize: batchSize,
+        shouldIndexInParallel: shouldIndexInParallel,
         logMessageToIndexLog: { [weak sourceKitLSPServer] in
           sourceKitLSPServer?.logMessageToIndexLog(message: $0, type: $1, structure: $2)
         },
diff --git a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
index 0ac776433..f942c11df 100644
--- a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
+++ b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
@@ -2625,81 +2625,94 @@ final class BackgroundIndexingTests: XCTestCase {
     XCTAssertEqual(symbols?.count, 1)
   }
 
-  func testBuildServerUsesCustomTaskBatchSize() async throws {
-    final class BuildServer: CustomBuildServer {
-      let inProgressRequestsTracker = CustomBuildServerInProgressRequestTracker()
-      private let projectRoot: URL
-      private var testFileURL: URL { projectRoot.appendingPathComponent("test.swift").standardized }
-
-      required init(projectRoot: URL, connectionToSourceKitLSP: any LanguageServerProtocol.Connection) {
-        self.projectRoot = projectRoot
-      }
-
-      func initializeBuildRequest(_ request: InitializeBuildRequest) async throws -> InitializeBuildResponse {
-        return try initializationResponseSupportingBackgroundIndexing(
-          projectRoot: projectRoot,
-          outputPathsProvider: false,
-          indexTaskBatchSize: 3
-        )
-      }
-
-      func buildTargetSourcesRequest(_ request: BuildTargetSourcesRequest) async throws -> BuildTargetSourcesResponse {
-        var dummyTargets = [BuildTargetIdentifier]()
-        for i in 0..<10 {
-          dummyTargets.append(BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-\(i)")))
-        }
-        return BuildTargetSourcesResponse(items: dummyTargets.map {
-          SourcesItem(target: $0, sources: [SourceItem(uri: URI(testFileURL), kind: .file, generated: false)])
-        })
-      }
-
-      func textDocumentSourceKitOptionsRequest(
-        _ request: TextDocumentSourceKitOptionsRequest
-      ) async throws -> TextDocumentSourceKitOptionsResponse? {
-        return TextDocumentSourceKitOptionsResponse(compilerArguments: [request.textDocument.uri.pseudoPath])
-      }
-    }
-
-    let preparationTaskSemaphore = WrappedSemaphore(name: "Received a preparation task")
-    let preparationTasks = ThreadSafeBox<[PreparationTaskDescription]>(initialValue: [])
-    let project = try await CustomBuildServerTestProject(
-      files: [
-        "test.swift": """
-        func testFunction() {}
-        """
-      ],
-      buildServer: BuildServer.self,
-      hooks: Hooks(
-        indexHooks: IndexHooks(
-          preparationTaskDidStart: { task in
-            preparationTasks.withLock { preparationTasks in
-              // Ignore everything after the 4th batch to avoid flakiness.
-              if preparationTasks.count < 4 {
-                preparationTasks.append(task)
-              }
-              if preparationTasks.count == 4 {
-                preparationTaskSemaphore.signal()
-              }
-            }
-          }
-        )
-      ),
-      enableBackgroundIndexing: true
-    )
-
-    _ = try await project.testClient.send(SynchronizeRequest(index: true))
-
-    preparationTaskSemaphore.waitOrXCTFail()
-
-    // The test project has 10 targets, and we should have received them in batches of 3.
-    XCTAssertEqual(preparationTasks.value.count, 4)
-    let preparedTargetBatches = preparationTasks.value.map(\.targetsToPrepare).sorted { $0.count > $1.count }
-    XCTAssertEqual(preparedTargetBatches[0].count, 3)
-    XCTAssertEqual(preparedTargetBatches[1].count, 3)
-    XCTAssertEqual(preparedTargetBatches[2].count, 3)
-    XCTAssertEqual(preparedTargetBatches[3].count, 1)
-  }
-}
+// WIP, not ready for review
+//   func testBuildServerUsesCustomTaskBatchSize() async throws {
+//     final class BuildServer: CustomBuildServer {
+//       let inProgressRequestsTracker = CustomBuildServerInProgressRequestTracker()
+//       private let projectRoot: URL
+//       private var testFileURL: URL { projectRoot.appendingPathComponent("test.swift").standardized }
+
+//       nonisolated(unsafe) var preparedTargetBatches = [[BuildTargetIdentifier]]()
+
+//       required init(projectRoot: URL, connectionToSourceKitLSP: any LanguageServerProtocol.Connection) {
+//         self.projectRoot = projectRoot
+//       }
+
+//       func initializeBuildRequest(_ request: InitializeBuildRequest) async throws -> InitializeBuildResponse {
+//         return try initializationResponseSupportingBackgroundIndexing(
+//           projectRoot: projectRoot,
+//           outputPathsProvider: false,
+//         )
+//       }
+
+//       func buildTargetSourcesRequest(_ request: BuildTargetSourcesRequest) async throws -> BuildTargetSourcesResponse {
+//         var dummyTargets = [BuildTargetIdentifier]()
+//         for i in 0..<10 {
+//           dummyTargets.append(BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-\(i)")))
+//         }
+//         return BuildTargetSourcesResponse(items: dummyTargets.map {
+//           SourcesItem(target: $0, sources: [SourceItem(uri: URI(testFileURL), kind: .file, generated: false)])
+//         })
+//       }
+
+//       func textDocumentSourceKitOptionsRequest(
+//         _ request: TextDocumentSourceKitOptionsRequest
+//       ) async throws -> TextDocumentSourceKitOptionsResponse? {
+//         return TextDocumentSourceKitOptionsResponse(compilerArguments: [request.textDocument.uri.pseudoPath])
+//       }
+
+//       func prepareTarget(_ request: BuildTargetPrepareRequest) async throws -> VoidResponse {
+//         preparedTargetBatches.append(request.targets.sorted { $0.uri.stringValue < $1.uri.stringValue })
+//         return VoidResponse()
+//       }
+//     }
+
+//     let project = try await CustomBuildServerTestProject(
+//       files: [
+//         "test.swift": """
+//         func testFunction() {}
+//         """
+//       ],
+//       buildServer: BuildServer.self,
+//       enableBackgroundIndexing: true,
+//     )
+
+//     // Wait for indexing to finish without elevating the priority
+//     // Otherwise, task re-scheduling would cause the test to become flaky
+//     let semaphore = WrappedSemaphore(name: "Indexing finished")
+//     let testClient = project.testClient
+//     Task(priority: .low) {
+//       await assertNoThrow {
+//         try await testClient.send(SynchronizeRequest(index: true))
+//       }
+//       semaphore.signal()
+//     }
+//     try semaphore.waitOrThrow()
+
+//     let buildServer = try project.buildServer()
+//     let preparedBatches = buildServer.preparedTargetBatches.sorted { $0[0].uri.stringValue < $1[0].uri.stringValue }
+//     XCTAssertEqual(preparedBatches, [
+//       [
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-0")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-1")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-2")),
+//       ],
+//       [
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-3")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-4")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-5")),
+//       ],
+//       [
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-6")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-7")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-8")),
+//       ],
+//       [
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-9")),
+//       ]
+//     ])
+//   }
+// }
 
 extension HoverResponseContents {
   var markupContent: MarkupContent? {

From ef3a6dd1627835ebedb61fde0e76e3fa351e0a44 Mon Sep 17 00:00:00 2001
From: Bruno Rocha <bruno@rockbruno.com>
Date: Thu, 14 Aug 2025 15:57:59 +0200
Subject: [PATCH 3/8] (Temporary) Remove unit test

---
 .../BackgroundIndexingTests.swift             | 89 -------------------
 1 file changed, 89 deletions(-)

diff --git a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
index f942c11df..262623384 100644
--- a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
+++ b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
@@ -2625,95 +2625,6 @@ final class BackgroundIndexingTests: XCTestCase {
     XCTAssertEqual(symbols?.count, 1)
   }
 
-// WIP, not ready for review
-//   func testBuildServerUsesCustomTaskBatchSize() async throws {
-//     final class BuildServer: CustomBuildServer {
-//       let inProgressRequestsTracker = CustomBuildServerInProgressRequestTracker()
-//       private let projectRoot: URL
-//       private var testFileURL: URL { projectRoot.appendingPathComponent("test.swift").standardized }
-
-//       nonisolated(unsafe) var preparedTargetBatches = [[BuildTargetIdentifier]]()
-
-//       required init(projectRoot: URL, connectionToSourceKitLSP: any LanguageServerProtocol.Connection) {
-//         self.projectRoot = projectRoot
-//       }
-
-//       func initializeBuildRequest(_ request: InitializeBuildRequest) async throws -> InitializeBuildResponse {
-//         return try initializationResponseSupportingBackgroundIndexing(
-//           projectRoot: projectRoot,
-//           outputPathsProvider: false,
-//         )
-//       }
-
-//       func buildTargetSourcesRequest(_ request: BuildTargetSourcesRequest) async throws -> BuildTargetSourcesResponse {
-//         var dummyTargets = [BuildTargetIdentifier]()
-//         for i in 0..<10 {
-//           dummyTargets.append(BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-\(i)")))
-//         }
-//         return BuildTargetSourcesResponse(items: dummyTargets.map {
-//           SourcesItem(target: $0, sources: [SourceItem(uri: URI(testFileURL), kind: .file, generated: false)])
-//         })
-//       }
-
-//       func textDocumentSourceKitOptionsRequest(
-//         _ request: TextDocumentSourceKitOptionsRequest
-//       ) async throws -> TextDocumentSourceKitOptionsResponse? {
-//         return TextDocumentSourceKitOptionsResponse(compilerArguments: [request.textDocument.uri.pseudoPath])
-//       }
-
-//       func prepareTarget(_ request: BuildTargetPrepareRequest) async throws -> VoidResponse {
-//         preparedTargetBatches.append(request.targets.sorted { $0.uri.stringValue < $1.uri.stringValue })
-//         return VoidResponse()
-//       }
-//     }
-
-//     let project = try await CustomBuildServerTestProject(
-//       files: [
-//         "test.swift": """
-//         func testFunction() {}
-//         """
-//       ],
-//       buildServer: BuildServer.self,
-//       enableBackgroundIndexing: true,
-//     )
-
-//     // Wait for indexing to finish without elevating the priority
-//     // Otherwise, task re-scheduling would cause the test to become flaky
-//     let semaphore = WrappedSemaphore(name: "Indexing finished")
-//     let testClient = project.testClient
-//     Task(priority: .low) {
-//       await assertNoThrow {
-//         try await testClient.send(SynchronizeRequest(index: true))
-//       }
-//       semaphore.signal()
-//     }
-//     try semaphore.waitOrThrow()
-
-//     let buildServer = try project.buildServer()
-//     let preparedBatches = buildServer.preparedTargetBatches.sorted { $0[0].uri.stringValue < $1[0].uri.stringValue }
-//     XCTAssertEqual(preparedBatches, [
-//       [
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-0")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-1")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-2")),
-//       ],
-//       [
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-3")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-4")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-5")),
-//       ],
-//       [
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-6")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-7")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-8")),
-//       ],
-//       [
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-9")),
-//       ]
-//     ])
-//   }
-// }
-
 extension HoverResponseContents {
   var markupContent: MarkupContent? {
     switch self {

From 7c96ab66ad916655738bd341a178d8086b6c6f0e Mon Sep 17 00:00:00 2001
From: Bruno Rocha <bruno@rockbruno.com>
Date: Thu, 14 Aug 2025 15:58:47 +0200
Subject: [PATCH 4/8] Remove unused kind param

---
 Contributor Documentation/BSP Extensions.md              | 5 +++++
 Sources/BuildServerIntegration/BuildServerManager.swift  | 5 -----
 Sources/SKTestSupport/CustomBuildServerTestProject.swift | 2 +-
 Sources/SemanticIndex/SemanticIndexManager.swift         | 1 +
 Sources/SemanticIndex/TaskScheduler.swift                | 1 +
 Tests/SourceKitLSPTests/BackgroundIndexingTests.swift    | 1 +
 6 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/Contributor Documentation/BSP Extensions.md b/Contributor Documentation/BSP Extensions.md
index d513768a3..de5947b2b 100644
--- a/Contributor Documentation/BSP Extensions.md	
+++ b/Contributor Documentation/BSP Extensions.md	
@@ -39,6 +39,11 @@ export interface SourceKitInitializeBuildResponseData {
    * `FileSystemWatcher` is the same as in LSP. */
   watchers?: [FileSystemWatcher]
 }
+
+export interface MultiTargetPreparationSupport {
+  /** Whether the build server can prepare multiple targets in parallel. If this value is omitted, it is assumed to be `true`. */
+  supported: bool;
+}
 ```
 
 ## `build/taskStart`
diff --git a/Sources/BuildServerIntegration/BuildServerManager.swift b/Sources/BuildServerIntegration/BuildServerManager.swift
index 3bda81ae3..679ddcbec 100644
--- a/Sources/BuildServerIntegration/BuildServerManager.swift
+++ b/Sources/BuildServerIntegration/BuildServerManager.swift
@@ -325,10 +325,6 @@ package actor BuildServerManager: QueueBasedMessageHandler {
   /// The build server adapter that is used to answer build server queries.
   private var buildServerAdapter: BuildServerAdapter?
 
-  /// The kind of underlying build server adapter that is being managed.
-  /// `nil` if the `BuildServerManager` does not have an underlying build system.
-  private var kind: BuildServerSpec.Kind?
-
   /// The build server adapter after initialization finishes. When sending messages to the BSP server, this should be
   /// preferred over `buildServerAdapter` because no messages must be sent to the build server before initialization
   /// finishes.
@@ -455,7 +451,6 @@ package actor BuildServerManager: QueueBasedMessageHandler {
     self.toolchainRegistry = toolchainRegistry
     self.options = options
     self.connectionToClient = connectionToClient
-    self.kind = buildServerSpec?.kind
     self.configPath = buildServerSpec?.configPath
     self.buildServerAdapter = await buildServerSpec?.createBuildServerAdapter(
       toolchainRegistry: toolchainRegistry,
diff --git a/Sources/SKTestSupport/CustomBuildServerTestProject.swift b/Sources/SKTestSupport/CustomBuildServerTestProject.swift
index cb5e92298..2f73fdff6 100644
--- a/Sources/SKTestSupport/CustomBuildServerTestProject.swift
+++ b/Sources/SKTestSupport/CustomBuildServerTestProject.swift
@@ -183,7 +183,7 @@ package extension CustomBuildServer {
         indexStorePath: try projectRoot.appendingPathComponent("index-store").filePath,
         outputPathsProvider: outputPathsProvider,
         prepareProvider: true,
-        sourceKitOptionsProvider: true,
+        sourceKitOptionsProvider: true
       )
     )
   }
diff --git a/Sources/SemanticIndex/SemanticIndexManager.swift b/Sources/SemanticIndex/SemanticIndexManager.swift
index 02f716aa0..182df9fc1 100644
--- a/Sources/SemanticIndex/SemanticIndexManager.swift
+++ b/Sources/SemanticIndex/SemanticIndexManager.swift
@@ -654,6 +654,7 @@ package final actor SemanticIndexManager {
     guard !targetsToPrepare.isEmpty else {
       return
     }
+
     let taskDescription = AnyIndexTaskDescription(
       PreparationTaskDescription(
         targetsToPrepare: targetsToPrepare,
diff --git a/Sources/SemanticIndex/TaskScheduler.swift b/Sources/SemanticIndex/TaskScheduler.swift
index e6a8d8f24..56c2632b4 100644
--- a/Sources/SemanticIndex/TaskScheduler.swift
+++ b/Sources/SemanticIndex/TaskScheduler.swift
@@ -370,6 +370,7 @@ package actor TaskScheduler<TaskDescription: TaskDescriptionProtocol> {
         // We didn't actually change anything, so we don't need to perform any validation or task processing.
         return
       }
+
       // Check we are over-subscribed in currently executing tasks by walking through all currently executing tasks and
       // checking if we could schedule them within the new execution limits. Cancel any tasks that do not fit within the
       // new limit to be rescheduled when we are within the limit again.
diff --git a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
index 262623384..9538222d1 100644
--- a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
+++ b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
@@ -2624,6 +2624,7 @@ final class BackgroundIndexingTests: XCTestCase {
     let symbols = try await project.testClient.send(WorkspaceSymbolsRequest(query: "myTestFu"))
     XCTAssertEqual(symbols?.count, 1)
   }
+}
 
 extension HoverResponseContents {
   var markupContent: MarkupContent? {

From 25c5a52e967ccc0fe533ea6d055adbdcaf692f55 Mon Sep 17 00:00:00 2001
From: Bruno Rocha <bruno@rockbruno.com>
Date: Fri, 15 Aug 2025 09:50:44 +0200
Subject: [PATCH 5/8] Handle prepare task cancellation based on the purpose

---
 .../PreparationTaskDescription.swift          | 23 +++++++++++----
 .../SemanticIndex/SemanticIndexManager.swift  | 29 +++++--------------
 Sources/SourceKitLSP/Workspace.swift          |  9 +++++-
 3 files changed, 33 insertions(+), 28 deletions(-)

diff --git a/Sources/SemanticIndex/PreparationTaskDescription.swift b/Sources/SemanticIndex/PreparationTaskDescription.swift
index 4d45be438..f19a83352 100644
--- a/Sources/SemanticIndex/PreparationTaskDescription.swift
+++ b/Sources/SemanticIndex/PreparationTaskDescription.swift
@@ -39,6 +39,9 @@ package struct PreparationTaskDescription: IndexTaskDescription {
 
   private let preparationUpToDateTracker: UpToDateTracker<BuildTargetIdentifier, DummySecondaryKey>
 
+  /// The purpose of the preparation task.
+  private let purpose: TargetPreparationPurpose
+
   /// See `SemanticIndexManager.logMessageToIndexLog`.
   private let logMessageToIndexLog:
     @Sendable (_ message: String, _ type: WindowMessageType, _ structure: StructuredLogKind) -> Void
@@ -63,6 +66,7 @@ package struct PreparationTaskDescription: IndexTaskDescription {
     targetsToPrepare: [BuildTargetIdentifier],
     buildServerManager: BuildServerManager,
     preparationUpToDateTracker: UpToDateTracker<BuildTargetIdentifier, DummySecondaryKey>,
+    purpose: TargetPreparationPurpose,
     logMessageToIndexLog:
       @escaping @Sendable (
         _ message: String, _ type: WindowMessageType, _ structure: StructuredLogKind
@@ -72,6 +76,7 @@ package struct PreparationTaskDescription: IndexTaskDescription {
     self.targetsToPrepare = targetsToPrepare
     self.buildServerManager = buildServerManager
     self.preparationUpToDateTracker = preparationUpToDateTracker
+    self.purpose = purpose
     self.logMessageToIndexLog = logMessageToIndexLog
     self.hooks = hooks
   }
@@ -119,14 +124,22 @@ package struct PreparationTaskDescription: IndexTaskDescription {
     to currentlyExecutingTasks: [PreparationTaskDescription]
   ) -> [TaskDependencyAction<PreparationTaskDescription>] {
     return currentlyExecutingTasks.compactMap { (other) -> TaskDependencyAction<PreparationTaskDescription>? in
-      if other.targetsToPrepare.count > self.targetsToPrepare.count {
-        // If there is an prepare operation with more targets already running, suspend it.
-        // The most common use case for this is if we prepare all targets simultaneously during the initial preparation
-        // when a project is opened and need a single target indexed for user interaction. We should suspend the
-        // workspace-wide preparation and just prepare the currently needed target.
+      if other.purpose == .forIndexing && self.purpose == .forEditorFunctionality {
+        // If we're running a background indexing operation but need a target indexed for user interaction,
+        // we should prioritize the latter.
         return .cancelAndRescheduleDependency(other)
       }
       return .waitAndElevatePriorityOfDependency(other)
     }
   }
 }
+
+/// The reason why a target is being prepared. This is used to determine the `IndexProgressStatus`
+/// and to prioritize preparation tasks when several of them are running.
+package enum TargetPreparationPurpose: Comparable {
+  /// We are preparing the target so we can index files in it.
+  case forIndexing
+
+  /// We are preparing the target to provide semantic functionality in one of its files.
+  case forEditorFunctionality
+}
diff --git a/Sources/SemanticIndex/SemanticIndexManager.swift b/Sources/SemanticIndex/SemanticIndexManager.swift
index 182df9fc1..5335a0edb 100644
--- a/Sources/SemanticIndex/SemanticIndexManager.swift
+++ b/Sources/SemanticIndex/SemanticIndexManager.swift
@@ -145,15 +145,6 @@ private struct InProgressPrepareForEditorTask {
   let task: Task<Void, Never>
 }
 
-/// The reason why a target is being prepared. This is used to determine the `IndexProgressStatus`.
-private enum TargetPreparationPurpose: Comparable {
-  /// We are preparing the target so we can index files in it.
-  case forIndexing
-
-  /// We are preparing the target to provide semantic functionality in one of its files.
-  case forEditorFunctionality
-}
-
 /// An entry in `SemanticIndexManager.inProgressPreparationTasks`.
 private struct InProgressPreparationTask {
   let task: OpaqueQueuedIndexTask
@@ -222,8 +213,8 @@ package final actor SemanticIndexManager {
   /// The parameter is the number of files that were scheduled to be indexed.
   private let indexTasksWereScheduled: @Sendable (_ numberOfFileScheduled: Int) -> Void
 
-  /// Determines whether or not the `SemanticIndexManager` should dispatch preparation tasks in batches.
-  private let shouldIndexInParallel: Bool
+  /// The size of the batches in which the `SemanticIndexManager` should dispatch preparation tasks.
+  private let indexTaskBatchSize: Int
 
   /// Callback that is called when `progressStatus` might have changed.
   private let indexProgressStatusDidChange: @Sendable () -> Void
@@ -264,7 +255,7 @@ package final actor SemanticIndexManager {
     updateIndexStoreTimeout: Duration,
     hooks: IndexHooks,
     indexTaskScheduler: TaskScheduler<AnyIndexTaskDescription>,
-    shouldIndexInParallel: Bool,
+    indexTaskBatchSize: Int,
     logMessageToIndexLog:
       @escaping @Sendable (
         _ message: String, _ type: WindowMessageType, _ structure: StructuredLogKind
@@ -277,7 +268,7 @@ package final actor SemanticIndexManager {
     self.updateIndexStoreTimeout = updateIndexStoreTimeout
     self.hooks = hooks
     self.indexTaskScheduler = indexTaskScheduler
-    self.shouldIndexInParallel = shouldIndexInParallel
+    self.indexTaskBatchSize = indexTaskBatchSize
     self.logMessageToIndexLog = logMessageToIndexLog
     self.indexTasksWereScheduled = indexTasksWereScheduled
     self.indexProgressStatusDidChange = indexProgressStatusDidChange
@@ -660,6 +651,7 @@ package final actor SemanticIndexManager {
         targetsToPrepare: targetsToPrepare,
         buildServerManager: self.buildServerManager,
         preparationUpToDateTracker: preparationUpToDateTracker,
+        purpose: purpose,
         logMessageToIndexLog: logMessageToIndexLog,
         hooks: hooks
       )
@@ -882,14 +874,7 @@ package final actor SemanticIndexManager {
 
     var indexTasks: [Task<Void, Never>] = []
 
-    let batchSize: Int
-    if shouldIndexInParallel {
-      let processorCount = ProcessInfo.processInfo.activeProcessorCount
-      batchSize = max(1, processorCount * 5)
-    } else {
-      batchSize = 1
-    }
-    for targetsBatch in sortedTargets.partition(intoBatchesOfSize: batchSize) {
+    for targetsBatch in sortedTargets.partition(intoBatchesOfSize: indexTaskBatchSize) {
       let preparationTaskID = UUID()
       let filesToIndex = targetsBatch.flatMap({ filesByTarget[$0]! })
 
@@ -913,7 +898,7 @@ package final actor SemanticIndexManager {
         // And after preparation is done, index the files in the targets.
         await withTaskGroup(of: Void.self) { taskGroup in
           for target in targetsBatch {
-            for fileBatch in filesByTarget[target]!.partition(intoBatchesOfSize: batchSize) {
+            for fileBatch in filesByTarget[target]!.partition(intoBatchesOfSize: indexTaskBatchSize) {
               taskGroup.addTask {
                 await self.updateIndexStore(
                   for: fileBatch,
diff --git a/Sources/SourceKitLSP/Workspace.swift b/Sources/SourceKitLSP/Workspace.swift
index c9ce196ad..5f5749329 100644
--- a/Sources/SourceKitLSP/Workspace.swift
+++ b/Sources/SourceKitLSP/Workspace.swift
@@ -163,13 +163,20 @@ package final class Workspace: Sendable, BuildServerManagerDelegate {
       await buildServerManager.initializationData?.prepareProvider ?? false
     {
       let shouldIndexInParallel = await buildServerManager.initializationData?.multiTargetPreparation?.supported ?? true
+      let batchSize: Int
+      if shouldIndexInParallel {
+        let processorCount = ProcessInfo.processInfo.activeProcessorCount
+        batchSize = max(1, processorCount * 5)
+      } else {
+        batchSize = 1
+      }
       self.semanticIndexManager = SemanticIndexManager(
         index: uncheckedIndex,
         buildServerManager: buildServerManager,
         updateIndexStoreTimeout: options.indexOrDefault.updateIndexStoreTimeoutOrDefault,
         hooks: hooks.indexHooks,
         indexTaskScheduler: indexTaskScheduler,
-        shouldIndexInParallel: shouldIndexInParallel,
+        indexTaskBatchSize: batchSize,
         logMessageToIndexLog: { [weak sourceKitLSPServer] in
           sourceKitLSPServer?.logMessageToIndexLog(message: $0, type: $1, structure: $2)
         },

From b2526c080410f3c541a2b725a241ecd2425367b2 Mon Sep 17 00:00:00 2001
From: Bruno Rocha <bruno@rockbruno.com>
Date: Fri, 15 Aug 2025 09:50:48 +0200
Subject: [PATCH 6/8] Revert "(Temporary) Remove unit test"

This reverts commit 19491c96058f83a73684466ef8ea4bfeb5225329.
---
 .../BackgroundIndexingTests.swift             | 89 +++++++++++++++++++
 1 file changed, 89 insertions(+)

diff --git a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
index 9538222d1..83675f923 100644
--- a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
+++ b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
@@ -2626,6 +2626,95 @@ final class BackgroundIndexingTests: XCTestCase {
   }
 }
 
+// WIP, not ready for review
+//   func testBuildServerUsesCustomTaskBatchSize() async throws {
+//     final class BuildServer: CustomBuildServer {
+//       let inProgressRequestsTracker = CustomBuildServerInProgressRequestTracker()
+//       private let projectRoot: URL
+//       private var testFileURL: URL { projectRoot.appendingPathComponent("test.swift").standardized }
+
+//       nonisolated(unsafe) var preparedTargetBatches = [[BuildTargetIdentifier]]()
+
+//       required init(projectRoot: URL, connectionToSourceKitLSP: any LanguageServerProtocol.Connection) {
+//         self.projectRoot = projectRoot
+//       }
+
+//       func initializeBuildRequest(_ request: InitializeBuildRequest) async throws -> InitializeBuildResponse {
+//         return try initializationResponseSupportingBackgroundIndexing(
+//           projectRoot: projectRoot,
+//           outputPathsProvider: false,
+//         )
+//       }
+
+//       func buildTargetSourcesRequest(_ request: BuildTargetSourcesRequest) async throws -> BuildTargetSourcesResponse {
+//         var dummyTargets = [BuildTargetIdentifier]()
+//         for i in 0..<10 {
+//           dummyTargets.append(BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-\(i)")))
+//         }
+//         return BuildTargetSourcesResponse(items: dummyTargets.map {
+//           SourcesItem(target: $0, sources: [SourceItem(uri: URI(testFileURL), kind: .file, generated: false)])
+//         })
+//       }
+
+//       func textDocumentSourceKitOptionsRequest(
+//         _ request: TextDocumentSourceKitOptionsRequest
+//       ) async throws -> TextDocumentSourceKitOptionsResponse? {
+//         return TextDocumentSourceKitOptionsResponse(compilerArguments: [request.textDocument.uri.pseudoPath])
+//       }
+
+//       func prepareTarget(_ request: BuildTargetPrepareRequest) async throws -> VoidResponse {
+//         preparedTargetBatches.append(request.targets.sorted { $0.uri.stringValue < $1.uri.stringValue })
+//         return VoidResponse()
+//       }
+//     }
+
+//     let project = try await CustomBuildServerTestProject(
+//       files: [
+//         "test.swift": """
+//         func testFunction() {}
+//         """
+//       ],
+//       buildServer: BuildServer.self,
+//       enableBackgroundIndexing: true,
+//     )
+
+//     // Wait for indexing to finish without elevating the priority
+//     // Otherwise, task re-scheduling would cause the test to become flaky
+//     let semaphore = WrappedSemaphore(name: "Indexing finished")
+//     let testClient = project.testClient
+//     Task(priority: .low) {
+//       await assertNoThrow {
+//         try await testClient.send(SynchronizeRequest(index: true))
+//       }
+//       semaphore.signal()
+//     }
+//     try semaphore.waitOrThrow()
+
+//     let buildServer = try project.buildServer()
+//     let preparedBatches = buildServer.preparedTargetBatches.sorted { $0[0].uri.stringValue < $1[0].uri.stringValue }
+//     XCTAssertEqual(preparedBatches, [
+//       [
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-0")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-1")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-2")),
+//       ],
+//       [
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-3")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-4")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-5")),
+//       ],
+//       [
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-6")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-7")),
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-8")),
+//       ],
+//       [
+//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-9")),
+//       ]
+//     ])
+//   }
+// }
+
 extension HoverResponseContents {
   var markupContent: MarkupContent? {
     switch self {

From 8da0ff7dd92711ba9068bd3c04362bc6de1f6103 Mon Sep 17 00:00:00 2001
From: Bruno Rocha <bruno@rockbruno.com>
Date: Fri, 15 Aug 2025 10:08:08 +0200
Subject: [PATCH 7/8] Allow custom batch size, write test for batch sizes

---
 Contributor Documentation/BSP Extensions.md   |   6 +-
 .../FixedCompilationDatabaseBuildServer.swift |   2 +-
 .../JSONCompilationDatabaseBuildServer.swift  |   2 +-
 .../LegacyBuildServer.swift                   |   2 +-
 .../Messages/InitializeBuildRequest.swift     |  15 +-
 .../CustomBuildServerTestProject.swift        |   4 +-
 Sources/SourceKitLSP/Workspace.swift          |  10 +-
 .../BackgroundIndexingTests.swift             | 167 ++++++++----------
 8 files changed, 109 insertions(+), 99 deletions(-)

diff --git a/Contributor Documentation/BSP Extensions.md b/Contributor Documentation/BSP Extensions.md
index de5947b2b..ba263001c 100644
--- a/Contributor Documentation/BSP Extensions.md	
+++ b/Contributor Documentation/BSP Extensions.md	
@@ -41,8 +41,12 @@ export interface SourceKitInitializeBuildResponseData {
 }
 
 export interface MultiTargetPreparationSupport {
-  /** Whether the build server can prepare multiple targets in parallel. If this value is omitted, it is assumed to be `true`. */
+  /** Whether the build server can prepare multiple targets in parallel. */
   supported: bool;
+
+  /** The number of targets to prepare in parallel.
+   * If not provided, SourceKit-LSP will calculate an appropriate value based on the environment. */
+  batchSize?: int;
 }
 ```
 
diff --git a/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift b/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift
index 46370c77e..a6545669f 100644
--- a/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift
+++ b/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift
@@ -59,7 +59,7 @@ package actor FixedCompilationDatabaseBuildServer: BuiltInBuildServer {
     indexStorePath?.deletingLastPathComponent().appendingPathComponent("IndexDatabase")
   }
 
-  package nonisolated var supportsMultiTargetPreparation: Bool { true }
+  package nonisolated var supportsMultiTargetPreparation: Bool { false }
 
   package nonisolated var supportsPreparationAndOutputPaths: Bool { false }
 
diff --git a/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift b/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift
index acf5eb42e..75b602517 100644
--- a/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift
+++ b/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift
@@ -96,7 +96,7 @@ package actor JSONCompilationDatabaseBuildServer: BuiltInBuildServer {
     indexStorePath?.deletingLastPathComponent().appendingPathComponent("IndexDatabase")
   }
 
-  package nonisolated var supportsMultiTargetPreparation: Bool { true }
+  package nonisolated var supportsMultiTargetPreparation: Bool { false }
 
   package nonisolated var supportsPreparationAndOutputPaths: Bool { false }
 
diff --git a/Sources/BuildServerIntegration/LegacyBuildServer.swift b/Sources/BuildServerIntegration/LegacyBuildServer.swift
index 026454e1a..57569177d 100644
--- a/Sources/BuildServerIntegration/LegacyBuildServer.swift
+++ b/Sources/BuildServerIntegration/LegacyBuildServer.swift
@@ -138,7 +138,7 @@ actor LegacyBuildServer: MessageHandler, BuiltInBuildServer {
     connectionToSourceKitLSP.send(OnBuildTargetDidChangeNotification(changes: nil))
   }
 
-  package nonisolated var supportsMultiTargetPreparation: Bool { true }
+  package nonisolated var supportsMultiTargetPreparation: Bool { false }
 
   package nonisolated var supportsPreparationAndOutputPaths: Bool { false }
 
diff --git a/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift b/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
index 4e01f70e6..e4ecb0928 100644
--- a/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
+++ b/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
@@ -372,17 +372,25 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
 }
 
 public struct MultiTargetPreparationSupport: LSPAnyCodable, Codable, Sendable {
-  /// Whether the build server can prepare multiple targets in parallel. If this value is omitted, it is assumed to be `true`.
+  /// Whether the build server can prepare multiple targets in parallel.
   public var supported: Bool?
 
-  public init(supported: Bool? = nil) {
+  /// The number of targets to prepare in parallel.
+  /// If not provided, SourceKit-LSP will calculate an appropriate value based on the environment.
+  public var batchSize: Int?
+
+  public init(supported: Bool? = nil, batchSize: Int? = nil) {
     self.supported = supported
+    self.batchSize = batchSize
   }
 
   public init?(fromLSPDictionary dictionary: [String: LanguageServerProtocol.LSPAny]) {
     if case .bool(let supported) = dictionary[CodingKeys.supported.stringValue] {
       self.supported = supported
     }
+    if case .int(let batchSize) = dictionary[CodingKeys.batchSize.stringValue] {
+      self.batchSize = batchSize
+    }
   }
 
   public func encodeToLSPAny() -> LanguageServerProtocol.LSPAny {
@@ -390,6 +398,9 @@ public struct MultiTargetPreparationSupport: LSPAnyCodable, Codable, Sendable {
     if let supported {
       result[CodingKeys.supported.stringValue] = .bool(supported)
     }
+    if let batchSize {
+      result[CodingKeys.batchSize.stringValue] = .int(batchSize)
+    }
     return .dictionary(result)
   }
 }
diff --git a/Sources/SKTestSupport/CustomBuildServerTestProject.swift b/Sources/SKTestSupport/CustomBuildServerTestProject.swift
index 2f73fdff6..5c082fb2c 100644
--- a/Sources/SKTestSupport/CustomBuildServerTestProject.swift
+++ b/Sources/SKTestSupport/CustomBuildServerTestProject.swift
@@ -175,12 +175,14 @@ package extension CustomBuildServer {
 
   func initializationResponseSupportingBackgroundIndexing(
     projectRoot: URL,
-    outputPathsProvider: Bool
+    outputPathsProvider: Bool,
+    multiTargetPreparation: MultiTargetPreparationSupport? = nil
   ) throws -> InitializeBuildResponse {
     return initializationResponse(
       initializeData: SourceKitInitializeBuildResponseData(
         indexDatabasePath: try projectRoot.appendingPathComponent("index-db").filePath,
         indexStorePath: try projectRoot.appendingPathComponent("index-store").filePath,
+        multiTargetPreparation: multiTargetPreparation,
         outputPathsProvider: outputPathsProvider,
         prepareProvider: true,
         sourceKitOptionsProvider: true
diff --git a/Sources/SourceKitLSP/Workspace.swift b/Sources/SourceKitLSP/Workspace.swift
index 5f5749329..4f4dcf63a 100644
--- a/Sources/SourceKitLSP/Workspace.swift
+++ b/Sources/SourceKitLSP/Workspace.swift
@@ -162,11 +162,15 @@ package final class Workspace: Sendable, BuildServerManagerDelegate {
     if options.backgroundIndexingOrDefault, let uncheckedIndex,
       await buildServerManager.initializationData?.prepareProvider ?? false
     {
-      let shouldIndexInParallel = await buildServerManager.initializationData?.multiTargetPreparation?.supported ?? true
+      let shouldIndexInParallel = await buildServerManager.initializationData?.multiTargetPreparation?.supported ?? false
       let batchSize: Int
       if shouldIndexInParallel {
-        let processorCount = ProcessInfo.processInfo.activeProcessorCount
-        batchSize = max(1, processorCount * 5)
+        if let customBatchSize = await buildServerManager.initializationData?.multiTargetPreparation?.batchSize {
+          batchSize = customBatchSize
+        } else {
+          let processorCount = ProcessInfo.processInfo.activeProcessorCount
+          batchSize = max(1, processorCount / 2)
+        }
       } else {
         batchSize = 1
       }
diff --git a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
index 83675f923..e1f3b0cb4 100644
--- a/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
+++ b/Tests/SourceKitLSPTests/BackgroundIndexingTests.swift
@@ -2624,96 +2624,85 @@ final class BackgroundIndexingTests: XCTestCase {
     let symbols = try await project.testClient.send(WorkspaceSymbolsRequest(query: "myTestFu"))
     XCTAssertEqual(symbols?.count, 1)
   }
-}
 
-// WIP, not ready for review
-//   func testBuildServerUsesCustomTaskBatchSize() async throws {
-//     final class BuildServer: CustomBuildServer {
-//       let inProgressRequestsTracker = CustomBuildServerInProgressRequestTracker()
-//       private let projectRoot: URL
-//       private var testFileURL: URL { projectRoot.appendingPathComponent("test.swift").standardized }
-
-//       nonisolated(unsafe) var preparedTargetBatches = [[BuildTargetIdentifier]]()
-
-//       required init(projectRoot: URL, connectionToSourceKitLSP: any LanguageServerProtocol.Connection) {
-//         self.projectRoot = projectRoot
-//       }
-
-//       func initializeBuildRequest(_ request: InitializeBuildRequest) async throws -> InitializeBuildResponse {
-//         return try initializationResponseSupportingBackgroundIndexing(
-//           projectRoot: projectRoot,
-//           outputPathsProvider: false,
-//         )
-//       }
-
-//       func buildTargetSourcesRequest(_ request: BuildTargetSourcesRequest) async throws -> BuildTargetSourcesResponse {
-//         var dummyTargets = [BuildTargetIdentifier]()
-//         for i in 0..<10 {
-//           dummyTargets.append(BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-\(i)")))
-//         }
-//         return BuildTargetSourcesResponse(items: dummyTargets.map {
-//           SourcesItem(target: $0, sources: [SourceItem(uri: URI(testFileURL), kind: .file, generated: false)])
-//         })
-//       }
-
-//       func textDocumentSourceKitOptionsRequest(
-//         _ request: TextDocumentSourceKitOptionsRequest
-//       ) async throws -> TextDocumentSourceKitOptionsResponse? {
-//         return TextDocumentSourceKitOptionsResponse(compilerArguments: [request.textDocument.uri.pseudoPath])
-//       }
-
-//       func prepareTarget(_ request: BuildTargetPrepareRequest) async throws -> VoidResponse {
-//         preparedTargetBatches.append(request.targets.sorted { $0.uri.stringValue < $1.uri.stringValue })
-//         return VoidResponse()
-//       }
-//     }
-
-//     let project = try await CustomBuildServerTestProject(
-//       files: [
-//         "test.swift": """
-//         func testFunction() {}
-//         """
-//       ],
-//       buildServer: BuildServer.self,
-//       enableBackgroundIndexing: true,
-//     )
-
-//     // Wait for indexing to finish without elevating the priority
-//     // Otherwise, task re-scheduling would cause the test to become flaky
-//     let semaphore = WrappedSemaphore(name: "Indexing finished")
-//     let testClient = project.testClient
-//     Task(priority: .low) {
-//       await assertNoThrow {
-//         try await testClient.send(SynchronizeRequest(index: true))
-//       }
-//       semaphore.signal()
-//     }
-//     try semaphore.waitOrThrow()
-
-//     let buildServer = try project.buildServer()
-//     let preparedBatches = buildServer.preparedTargetBatches.sorted { $0[0].uri.stringValue < $1[0].uri.stringValue }
-//     XCTAssertEqual(preparedBatches, [
-//       [
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-0")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-1")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-2")),
-//       ],
-//       [
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-3")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-4")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-5")),
-//       ],
-//       [
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-6")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-7")),
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-8")),
-//       ],
-//       [
-//         BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-9")),
-//       ]
-//     ])
-//   }
-// }
+  func testBuildServerUsesCustomTaskBatchSize() async throws {
+    final class BuildServer: CustomBuildServer {
+      let inProgressRequestsTracker = CustomBuildServerInProgressRequestTracker()
+      private let projectRoot: URL
+      private var testFileURL: URL { projectRoot.appendingPathComponent("test.swift").standardized }
+
+      nonisolated(unsafe) var preparedTargetBatches = [[BuildTargetIdentifier]]()
+
+      required init(projectRoot: URL, connectionToSourceKitLSP: any LanguageServerProtocol.Connection) {
+        self.projectRoot = projectRoot
+      }
+
+      func initializeBuildRequest(_ request: InitializeBuildRequest) async throws -> InitializeBuildResponse {
+        return try initializationResponseSupportingBackgroundIndexing(
+          projectRoot: projectRoot,
+          outputPathsProvider: false,
+          multiTargetPreparation: MultiTargetPreparationSupport(supported: true, batchSize: 3)
+        )
+      }
+
+      func buildTargetSourcesRequest(_ request: BuildTargetSourcesRequest) async throws -> BuildTargetSourcesResponse {
+        var dummyTargets = [BuildTargetIdentifier]()
+        for i in 0..<10 {
+          dummyTargets.append(BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-\(i)")))
+        }
+        return BuildTargetSourcesResponse(items: dummyTargets.map {
+          SourcesItem(target: $0, sources: [SourceItem(uri: URI(testFileURL), kind: .file, generated: false)])
+        })
+      }
+
+      func textDocumentSourceKitOptionsRequest(
+        _ request: TextDocumentSourceKitOptionsRequest
+      ) async throws -> TextDocumentSourceKitOptionsResponse? {
+        return TextDocumentSourceKitOptionsResponse(compilerArguments: [request.textDocument.uri.pseudoPath])
+      }
+
+      func prepareTarget(_ request: BuildTargetPrepareRequest) async throws -> VoidResponse {
+        preparedTargetBatches.append(request.targets.sorted { $0.uri.stringValue < $1.uri.stringValue })
+        return VoidResponse()
+      }
+    }
+
+    let project = try await CustomBuildServerTestProject(
+      files: [
+        "test.swift": """
+        func testFunction() {}
+        """
+      ],
+      buildServer: BuildServer.self,
+      enableBackgroundIndexing: true,
+    )
+
+    try await project.testClient.send(SynchronizeRequest(index: true))
+
+    let buildServer = try project.buildServer()
+    let preparedBatches = buildServer.preparedTargetBatches.sorted { $0[0].uri.stringValue < $1[0].uri.stringValue }
+    XCTAssertEqual(preparedBatches, [
+      [
+        BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-0")),
+        BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-1")),
+        BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-2")),
+      ],
+      [
+        BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-3")),
+        BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-4")),
+        BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-5")),
+      ],
+      [
+        BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-6")),
+        BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-7")),
+        BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-8")),
+      ],
+      [
+        BuildTargetIdentifier(uri: try! URI(string: "dummy://dummy-9")),
+      ]
+    ])
+  }
+}
 
 extension HoverResponseContents {
   var markupContent: MarkupContent? {

From a61079cdecb21911eb5f3b8ff7de14e31eb9a45c Mon Sep 17 00:00:00 2001
From: Bruno Rocha <bruno@rockbruno.com>
Date: Mon, 18 Aug 2025 09:58:29 +0200
Subject: [PATCH 8/8] Default multi-target preparation to true

---
 Contributor Documentation/BSP Extensions.md                  | 5 +++--
 .../FixedCompilationDatabaseBuildServer.swift                | 2 +-
 .../JSONCompilationDatabaseBuildServer.swift                 | 2 +-
 Sources/BuildServerIntegration/LegacyBuildServer.swift       | 2 +-
 .../Messages/InitializeBuildRequest.swift                    | 1 +
 Sources/SourceKitLSP/Workspace.swift                         | 2 +-
 6 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/Contributor Documentation/BSP Extensions.md b/Contributor Documentation/BSP Extensions.md
index ba263001c..2701ef4ab 100644
--- a/Contributor Documentation/BSP Extensions.md	
+++ b/Contributor Documentation/BSP Extensions.md	
@@ -41,8 +41,9 @@ export interface SourceKitInitializeBuildResponseData {
 }
 
 export interface MultiTargetPreparationSupport {
-  /** Whether the build server can prepare multiple targets in parallel. */
-  supported: bool;
+  /** Whether the build server can prepare multiple targets in parallel.
+   * If this value is not provided, it will be assumed to be `true`. */
+  supported?: bool;
 
   /** The number of targets to prepare in parallel.
    * If not provided, SourceKit-LSP will calculate an appropriate value based on the environment. */
diff --git a/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift b/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift
index a6545669f..46370c77e 100644
--- a/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift
+++ b/Sources/BuildServerIntegration/FixedCompilationDatabaseBuildServer.swift
@@ -59,7 +59,7 @@ package actor FixedCompilationDatabaseBuildServer: BuiltInBuildServer {
     indexStorePath?.deletingLastPathComponent().appendingPathComponent("IndexDatabase")
   }
 
-  package nonisolated var supportsMultiTargetPreparation: Bool { false }
+  package nonisolated var supportsMultiTargetPreparation: Bool { true }
 
   package nonisolated var supportsPreparationAndOutputPaths: Bool { false }
 
diff --git a/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift b/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift
index 75b602517..acf5eb42e 100644
--- a/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift
+++ b/Sources/BuildServerIntegration/JSONCompilationDatabaseBuildServer.swift
@@ -96,7 +96,7 @@ package actor JSONCompilationDatabaseBuildServer: BuiltInBuildServer {
     indexStorePath?.deletingLastPathComponent().appendingPathComponent("IndexDatabase")
   }
 
-  package nonisolated var supportsMultiTargetPreparation: Bool { false }
+  package nonisolated var supportsMultiTargetPreparation: Bool { true }
 
   package nonisolated var supportsPreparationAndOutputPaths: Bool { false }
 
diff --git a/Sources/BuildServerIntegration/LegacyBuildServer.swift b/Sources/BuildServerIntegration/LegacyBuildServer.swift
index 57569177d..026454e1a 100644
--- a/Sources/BuildServerIntegration/LegacyBuildServer.swift
+++ b/Sources/BuildServerIntegration/LegacyBuildServer.swift
@@ -138,7 +138,7 @@ actor LegacyBuildServer: MessageHandler, BuiltInBuildServer {
     connectionToSourceKitLSP.send(OnBuildTargetDidChangeNotification(changes: nil))
   }
 
-  package nonisolated var supportsMultiTargetPreparation: Bool { false }
+  package nonisolated var supportsMultiTargetPreparation: Bool { true }
 
   package nonisolated var supportsPreparationAndOutputPaths: Bool { false }
 
diff --git a/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift b/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
index e4ecb0928..13579335c 100644
--- a/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
+++ b/Sources/BuildServerProtocol/Messages/InitializeBuildRequest.swift
@@ -373,6 +373,7 @@ public struct SourceKitInitializeBuildResponseData: LSPAnyCodable, Codable, Send
 
 public struct MultiTargetPreparationSupport: LSPAnyCodable, Codable, Sendable {
   /// Whether the build server can prepare multiple targets in parallel.
+  /// If this value is not provided, it will be assumed to be `true`.
   public var supported: Bool?
 
   /// The number of targets to prepare in parallel.
diff --git a/Sources/SourceKitLSP/Workspace.swift b/Sources/SourceKitLSP/Workspace.swift
index 4f4dcf63a..dba9df84c 100644
--- a/Sources/SourceKitLSP/Workspace.swift
+++ b/Sources/SourceKitLSP/Workspace.swift
@@ -162,7 +162,7 @@ package final class Workspace: Sendable, BuildServerManagerDelegate {
     if options.backgroundIndexingOrDefault, let uncheckedIndex,
       await buildServerManager.initializationData?.prepareProvider ?? false
     {
-      let shouldIndexInParallel = await buildServerManager.initializationData?.multiTargetPreparation?.supported ?? false
+      let shouldIndexInParallel = await buildServerManager.initializationData?.multiTargetPreparation?.supported ?? true
       let batchSize: Int
       if shouldIndexInParallel {
         if let customBatchSize = await buildServerManager.initializationData?.multiTargetPreparation?.batchSize {