fix(agents): set preserveSignatures to isAnthropic in resolveTranscriptPolicy

When preserveSignatures was unconditionally false, stripThoughtSignatures
removed msg_-prefixed thought_signature fields from Anthropic thinking
blocks, breaking the API contract that requires thinking/redacted_thinking
blocks to be returned byte-identical. Now preserveSignatures is true only
for Anthropic (and Bedrock-Anthropic) providers.

Closes #32526

Made-with: Cursor

src/agents/transcript-policy.test.ts

@@ -76,6 +76,50 @@ describe("resolveTranscriptPolicy", () => {
     expect(policy.sanitizeMode).toBe("full");
   });
 
+  it("preserves thinking signatures for Anthropic provider (#32526)", () => {
+    const policy = resolveTranscriptPolicy({
+      provider: "anthropic",
+      modelId: "claude-opus-4-5",
+      modelApi: "anthropic-messages",
+    });
+    expect(policy.preserveSignatures).toBe(true);
+  });
+
+  it("preserves thinking signatures for Bedrock Anthropic (#32526)", () => {
+    const policy = resolveTranscriptPolicy({
+      provider: "amazon-bedrock",
+      modelId: "us.anthropic.claude-opus-4-6-v1",
+      modelApi: "bedrock-converse-stream",
+    });
+    expect(policy.preserveSignatures).toBe(true);
+  });
+
+  it("does not preserve signatures for Google provider (#32526)", () => {
+    const policy = resolveTranscriptPolicy({
+      provider: "google",
+      modelId: "gemini-2.0-flash",
+      modelApi: "google-generative-ai",
+    });
+    expect(policy.preserveSignatures).toBe(false);
+  });
+
+  it("does not preserve signatures for OpenAI provider (#32526)", () => {
+    const policy = resolveTranscriptPolicy({
+      provider: "openai",
+      modelId: "gpt-4o",
+      modelApi: "openai",
+    });
+    expect(policy.preserveSignatures).toBe(false);
+  });
+
+  it("does not preserve signatures for Mistral provider (#32526)", () => {
+    const policy = resolveTranscriptPolicy({
+      provider: "mistral",
+      modelId: "mistral-large-latest",
+    });
+    expect(policy.preserveSignatures).toBe(false);
+  });
+
   it("keeps OpenRouter on its existing turn-validation path", () => {
     const policy = resolveTranscriptPolicy({
       provider: "openrouter",

src/agents/transcript-policy.ts

@@ -123,7 +123,7 @@ export function resolveTranscriptPolicy(params: {
       (!isOpenAi && sanitizeToolCallIds) || requiresOpenAiCompatibleToolIdSanitization,
     toolCallIdMode,
     repairToolUseResultPairing,
-    preserveSignatures: false,
+    preserveSignatures: isAnthropic,
     sanitizeThoughtSignatures: isOpenAi ? undefined : sanitizeThoughtSignatures,
     sanitizeThinkingSignatures: false,
     dropThinkingBlocks,