fix(config): deep merge agent overrides with reusable deepMerge utility (#27)

Author: junhoyeo

src/agents/utils.ts

@@ -5,6 +5,7 @@ import { librarianAgent } from "./librarian"
 import { exploreAgent } from "./explore"
 import { frontendUiUxEngineerAgent } from "./frontend-ui-ux-engineer"
 import { documentWriterAgent } from "./document-writer"
+import { deepMerge } from "../shared"
 
 const allBuiltinAgents: Record<AgentName, AgentConfig> = {
   oracle: oracleAgent,
@@ -18,16 +19,7 @@ function mergeAgentConfig(
   base: AgentConfig,
   override: AgentOverrideConfig
 ): AgentConfig {
-  return {
-    ...base,
-    ...override,
-    tools: override.tools !== undefined
-      ? { ...(base.tools ?? {}), ...override.tools }
-      : base.tools,
-    permission: override.permission !== undefined
-      ? { ...(base.permission ?? {}), ...override.permission }
-      : base.permission,
-  }
+  return deepMerge(base, override as Partial<AgentConfig>)
 }
 
 export function createBuiltinAgents(

src/index.ts

@@ -42,7 +42,7 @@ import { builtinTools, createCallOmoAgent, createBackgroundTools } from "./tools
 import { BackgroundManager } from "./features/background-agent";
 import { createBuiltinMcps } from "./mcp";
 import { OhMyOpenCodeConfigSchema, type OhMyOpenCodeConfig } from "./config";
-import { log } from "./shared/logger";
+import { log, deepMerge } from "./shared";
 import * as fs from "fs";
 import * as path from "path";
 import * as os from "os";
@@ -89,10 +89,7 @@ function mergeConfigs(
   return {
     ...base,
     ...override,
-    agents:
-      override.agents !== undefined
-        ? { ...(base.agents ?? {}), ...override.agents }
-        : base.agents,
+    agents: deepMerge(base.agents, override.agents),
     disabled_agents: [
       ...new Set([
         ...(base.disabled_agents ?? []),
@@ -105,10 +102,7 @@ function mergeConfigs(
         ...(override.disabled_mcps ?? []),
       ]),
     ],
-    claude_code:
-      override.claude_code !== undefined || base.claude_code !== undefined
-        ? { ...(base.claude_code ?? {}), ...(override.claude_code ?? {}) }
-        : undefined,
+    claude_code: deepMerge(base.claude_code, override.claude_code),
   };
 }
 

src/shared/deep-merge.ts

@@ -0,0 +1,53 @@
+const DANGEROUS_KEYS = new Set(["__proto__", "constructor", "prototype"]);
+const MAX_DEPTH = 50;
+
+function isPlainObject(value: unknown): value is Record<string, unknown> {
+  return (
+    typeof value === "object" &&
+    value !== null &&
+    !Array.isArray(value) &&
+    Object.prototype.toString.call(value) === "[object Object]"
+  );
+}
+
+/**
+ * Deep merges two objects, with override values taking precedence.
+ * - Objects are recursively merged
+ * - Arrays are replaced (not concatenated)
+ * - undefined values in override do not overwrite base values
+ *
+ * @example
+ * deepMerge({ a: 1, b: { c: 2, d: 3 } }, { b: { c: 10 }, e: 5 })
+ * // => { a: 1, b: { c: 10, d: 3 }, e: 5 }
+ */
+export function deepMerge<T extends Record<string, unknown>>(base: T, override: Partial<T>, depth?: number): T;
+export function deepMerge<T extends Record<string, unknown>>(base: T | undefined, override: T | undefined, depth?: number): T | undefined;
+export function deepMerge<T extends Record<string, unknown>>(
+  base: T | undefined,
+  override: T | undefined,
+  depth = 0
+): T | undefined {
+  if (!base && !override) return undefined;
+  if (!base) return override;
+  if (!override) return base;
+  if (depth > MAX_DEPTH) return override ?? base;
+
+  const result = { ...base } as Record<string, unknown>;
+
+  for (const key of Object.keys(override)) {
+    if (DANGEROUS_KEYS.has(key)) continue;
+
+    const baseValue = base[key];
+    const overrideValue = override[key];
+
+    if (overrideValue === undefined) continue;
+
+    if (isPlainObject(baseValue) && isPlainObject(overrideValue)) {
+      result[key] = deepMerge(baseValue, overrideValue, depth + 1);
+    } else {
+      result[key] = overrideValue;
+    }
+  }
+
+  return result as T;
+}

src/shared/index.ts

@@ -7,3 +7,4 @@ export * from "./snake-case"
 export * from "./tool-name"
 export * from "./pattern-matcher"
 export * from "./hook-disabled"
+export * from "./deep-merge"