diff --git a/.changeset/fix-google-thought-signature.md b/.changeset/fix-google-thought-signature.md
new file mode 100644
index 00000000..15be3f53
--- /dev/null
+++ b/.changeset/fix-google-thought-signature.md
@@ -0,0 +1,5 @@
+---
+"perstack": patch
+---
+
+fix: support Google/Vertex thought_signature in multi-turn conversations with Gemini 3 models
diff --git a/packages/core/src/schemas/message-part.ts b/packages/core/src/schemas/message-part.ts
index 2dffd2d9..7c57eed5 100644
--- a/packages/core/src/schemas/message-part.ts
+++ b/packages/core/src/schemas/message-part.ts
@@ -129,6 +129,8 @@ export interface ToolCallPart extends BasePart {
   toolName: string
   /** Arguments to pass to the tool */
   args: Record<string, unknown>
+  /** Provider-specific metadata for this tool call (e.g., Google thought_signature) */
+  providerMetadata?: Record<string, Record<string, unknown>>
 }
 
 export const toolCallPartSchema = basePartSchema.extend({
@@ -136,6 +138,7 @@ export const toolCallPartSchema = basePartSchema.extend({
   toolCallId: z.string(),
   toolName: z.string(),
   args: z.record(z.string(), z.unknown()),
+  providerMetadata: z.record(z.string(), z.record(z.string(), z.unknown())).optional(),
 })
 toolCallPartSchema satisfies z.ZodType<ToolCallPart>
 
diff --git a/packages/runtime/src/messages/message.ts b/packages/runtime/src/messages/message.ts
index 3877e352..64206248 100644
--- a/packages/runtime/src/messages/message.ts
+++ b/packages/runtime/src/messages/message.ts
@@ -237,6 +237,7 @@ function toolCallPartToCoreToolCallPart(part: ToolCallPart): ToolCallModelPart {
     toolCallId: part.toolCallId,
     toolName: part.toolName,
     input: part.args,
+    providerOptions: part.providerMetadata as ToolCallModelPart["providerOptions"],
   }
 }
 function thinkingPartToCoreThinkingPart(part: ThinkingPart): {
diff --git a/packages/runtime/src/state-machine/states/generating-tool-call.ts b/packages/runtime/src/state-machine/states/generating-tool-call.ts
index 42eedaee..6e74c8b3 100644
--- a/packages/runtime/src/state-machine/states/generating-tool-call.ts
+++ b/packages/runtime/src/state-machine/states/generating-tool-call.ts
@@ -34,10 +34,16 @@ type ClassifiedToolCall = {
   toolName: string
   input: Record<string, unknown>
   adapter: SkillAdapter
+  providerMetadata?: Record<string, Record<string, unknown>>
 }
 
 function classifyToolCalls(
-  toolCalls: Array<{ toolCallId: string; toolName: string; input: unknown }>,
+  toolCalls: Array<{
+    toolCallId: string
+    toolName: string
+    input: unknown
+    providerMetadata?: Record<string, Record<string, unknown>>
+  }>,
   skillManager: SkillManager,
 ): ClassifiedToolCall[] {
   return toolCalls.map((tc) => {
@@ -47,6 +53,7 @@ function classifyToolCalls(
       toolName: tc.toolName,
       input: tc.input as Record<string, unknown>,
       adapter,
+      providerMetadata: tc.providerMetadata,
     }
   })
 }
@@ -64,6 +71,7 @@ function buildToolCallParts(toolCalls: ClassifiedToolCall[]): Array<Omit<ToolCal
     toolCallId: tc.toolCallId,
     toolName: tc.toolName,
     args: tc.input,
+    providerMetadata: tc.providerMetadata,
   }))
 }