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 4a5e52ad..2dffd2d9 100644
--- a/packages/core/src/schemas/message-part.ts
+++ b/packages/core/src/schemas/message-part.ts
@@ -144,14 +144,17 @@ export interface ThinkingPart extends BasePart {
   type: "thinkingPart"
   /** The thinking content */
   thinking: string
-  /** Signature for redacted thinking blocks (Anthropic) */
+  /** Signature for thinking blocks (required by Anthropic and Google) */
   signature?: string
+  /** Provider namespace for the signature (defaults to "anthropic" for backward compatibility) */
+  signatureProvider?: "anthropic" | "google" | "vertex"
 }
 
 export const thinkingPartSchema = basePartSchema.extend({
   type: z.literal("thinkingPart"),
   thinking: z.string(),
   signature: z.string().optional(),
+  signatureProvider: z.enum(["anthropic", "google", "vertex"]).optional(),
 })
 thinkingPartSchema satisfies z.ZodType<ThinkingPart>
 
diff --git a/packages/runtime/src/helpers/thinking.test.ts b/packages/runtime/src/helpers/thinking.test.ts
index 54b69a2b..4288e202 100644
--- a/packages/runtime/src/helpers/thinking.test.ts
+++ b/packages/runtime/src/helpers/thinking.test.ts
@@ -20,8 +20,18 @@ describe("@perstack/runtime: thinking", () => {
       const result = extractThinkingParts(reasoning)
 
       expect(result).toEqual([
-        { type: "thinkingPart", thinking: "First thought", signature: undefined },
-        { type: "thinkingPart", thinking: "Second thought", signature: undefined },
+        {
+          type: "thinkingPart",
+          thinking: "First thought",
+          signature: undefined,
+          signatureProvider: undefined,
+        },
+        {
+          type: "thinkingPart",
+          thinking: "Second thought",
+          signature: undefined,
+          signatureProvider: undefined,
+        },
       ])
     })
 
@@ -45,6 +55,57 @@ describe("@perstack/runtime: thinking", () => {
           type: "thinkingPart",
           thinking: "Thinking with signature",
           signature: "test-signature-123",
+          signatureProvider: "anthropic",
+        },
+      ])
+    })
+
+    it("extracts thoughtSignature from Google providerMetadata", () => {
+      const reasoning: ReasoningPart[] = [
+        {
+          type: "reasoning",
+          text: "Google thinking",
+          providerMetadata: {
+            google: {
+              thoughtSignature: "google-sig-456",
+            },
+          },
+        },
+      ]
+
+      const result = extractThinkingParts(reasoning)
+
+      expect(result).toEqual([
+        {
+          type: "thinkingPart",
+          thinking: "Google thinking",
+          signature: "google-sig-456",
+          signatureProvider: "google",
+        },
+      ])
+    })
+
+    it("extracts thoughtSignature from Vertex providerMetadata", () => {
+      const reasoning: ReasoningPart[] = [
+        {
+          type: "reasoning",
+          text: "Vertex thinking",
+          providerMetadata: {
+            vertex: {
+              thoughtSignature: "vertex-sig-789",
+            },
+          },
+        },
+      ]
+
+      const result = extractThinkingParts(reasoning)
+
+      expect(result).toEqual([
+        {
+          type: "thinkingPart",
+          thinking: "Vertex thinking",
+          signature: "vertex-sig-789",
+          signatureProvider: "vertex",
         },
       ])
     })
@@ -65,6 +126,7 @@ describe("@perstack/runtime: thinking", () => {
       expect(result).toHaveLength(3)
       expect(result[0]?.signature).toBeUndefined()
       expect(result[1]?.signature).toBe("sig-1")
+      expect(result[1]?.signatureProvider).toBe("anthropic")
       expect(result[2]?.signature).toBeUndefined()
     })
   })
diff --git a/packages/runtime/src/helpers/thinking.ts b/packages/runtime/src/helpers/thinking.ts
index 013400b8..3f394b66 100644
--- a/packages/runtime/src/helpers/thinking.ts
+++ b/packages/runtime/src/helpers/thinking.ts
@@ -4,8 +4,10 @@ import type { ThinkingPart } from "@perstack/core"
  * Reasoning part from AI SDK generateText result.
  * This matches the AI SDK ReasoningPart type.
  *
- * For Anthropic Extended Thinking, signature is in providerMetadata.anthropic.signature
- * (not providerOptions - that's for input, providerMetadata is for output)
+ * Signatures are provider-specific:
+ * - Anthropic: providerMetadata.anthropic.signature
+ * - Google: providerMetadata.google.thoughtSignature
+ * - Vertex: providerMetadata.vertex.thoughtSignature
  */
 export interface ReasoningPart {
   type: "reasoning"
@@ -14,6 +16,12 @@ export interface ReasoningPart {
     anthropic?: {
       signature?: string
     }
+    google?: {
+      thoughtSignature?: string
+    }
+    vertex?: {
+      thoughtSignature?: string
+    }
   }
 }
 
@@ -22,19 +30,39 @@ export interface ReasoningPart {
  * Used to preserve thinking blocks in conversation history for providers
  * that require them (Anthropic, Google).
  *
- * Note: For Anthropic, signature is required for all thinking blocks
- * when including them in conversation history.
+ * The signature and its provider namespace are preserved so that
+ * the correct providerOptions can be set when converting back to
+ * AI SDK messages for multi-turn conversations.
  */
 export function extractThinkingParts(
   reasoning: ReasoningPart[] | undefined,
 ): Omit<ThinkingPart, "id">[] {
   if (!reasoning) return []
-  return reasoning.map((r) => ({
-    type: "thinkingPart" as const,
-    thinking: r.text,
-    // Signature is in providerMetadata for Anthropic (output from API)
-    signature: r.providerMetadata?.anthropic?.signature,
-  }))
+  return reasoning.map((r) => {
+    const { signature, signatureProvider } = extractSignature(r)
+    return {
+      type: "thinkingPart" as const,
+      thinking: r.text,
+      signature,
+      signatureProvider,
+    }
+  })
+}
+
+function extractSignature(r: ReasoningPart): {
+  signature: string | undefined
+  signatureProvider: ThinkingPart["signatureProvider"]
+} {
+  if (r.providerMetadata?.anthropic?.signature) {
+    return { signature: r.providerMetadata.anthropic.signature, signatureProvider: "anthropic" }
+  }
+  if (r.providerMetadata?.google?.thoughtSignature) {
+    return { signature: r.providerMetadata.google.thoughtSignature, signatureProvider: "google" }
+  }
+  if (r.providerMetadata?.vertex?.thoughtSignature) {
+    return { signature: r.providerMetadata.vertex.thoughtSignature, signatureProvider: "vertex" }
+  }
+  return { signature: undefined, signatureProvider: undefined }
 }
 
 /**
diff --git a/packages/runtime/src/messages/message.ts b/packages/runtime/src/messages/message.ts
index 0622bfc6..3877e352 100644
--- a/packages/runtime/src/messages/message.ts
+++ b/packages/runtime/src/messages/message.ts
@@ -244,10 +244,15 @@ function thinkingPartToCoreThinkingPart(part: ThinkingPart): {
   text: string
   providerOptions?: Record<string, Record<string, string>>
 } {
+  if (!part.signature) {
+    return { type: "reasoning", text: part.thinking }
+  }
+  const provider = part.signatureProvider ?? "anthropic"
+  const signatureKey = provider === "anthropic" ? "signature" : "thoughtSignature"
   return {
     type: "reasoning",
     text: part.thinking,
-    providerOptions: part.signature ? { anthropic: { signature: part.signature } } : undefined,
+    providerOptions: { [provider]: { [signatureKey]: part.signature } },
   }
 }
 function toolResultPartToCoreToolResultPart(part: ToolResultPart): ToolResultModelPart {