fix(anthropic): strip all thinking blocks from historical messages

bladin · bladin · commit df763b31101d · 2026-06-01T13:58:00.000+08:00
Strip ALL thinking blocks from historical assistant messages to avoid "Invalid signature in thinking block" errors when Anthropic thinking signatures expire. Anthropic thinking signatures are time-limited and single-use. Replaying them causes API errors that brick sessions until manual reset. The current-turn thinking block is preserved by the stream handler, not by transcript replay. Fixes #88932
diff --git a/src/llm/providers/anthropic.test.ts b/src/llm/providers/anthropic.test.ts
@@ -79,7 +79,7 @@ describe("Anthropic provider", () => {
     expect(config.defaultHeaders?.["cf-aig-authorization"]).toBe("Bearer gateway-token");
   });
 
-  it("preserves provider-signed Anthropic thinking text on replay", async () => {
+  it("strips all thinking blocks from historical messages to avoid expired signature errors", async () => {
     const highSurrogate = String.fromCharCode(0xd83d);
     const signedThinking = `keep${highSurrogate}signed`;
     let capturedPayload: unknown;
@@ -154,18 +154,7 @@ describe("Anthropic provider", () => {
 
     const payload = capturedPayload as { messages: Array<{ role: string; content: unknown[] }> };
     const assistantMessage = payload.messages.find((message) => message.role === "assistant");
-    expect(assistantMessage?.content).toEqual([
-      {
-        type: "thinking",
-        thinking: signedThinking,
-        signature: "sig_1",
-      },
-      {
-        type: "thinking",
-        thinking: "sanitizesynthetic",
-        signature: "reasoning_content",
-      },
-    ]);
+    expect(assistantMessage?.content).toEqual([]);
   });
 
   it("clamps max adaptive effort when the Claude model does not advertise it", async () => {
diff --git a/src/llm/providers/transform-messages.ts b/src/llm/providers/transform-messages.ts
@@ -107,22 +107,12 @@ export function transformMessages<TApi extends Api>(
           if (block.redacted) {
             return isSameModel ? block : [];
           }
-          // For same model: keep thinking blocks with signatures (needed for replay)
-          // even if the thinking text is empty (OpenAI encrypted reasoning)
-          if (isSameModel && block.thinkingSignature) {
-            return block;
-          }
-          // Skip empty thinking blocks, convert others to plain text
-          if (!block.thinking || block.thinking.trim() === "") {
-            return [];
-          }
-          if (isSameModel) {
-            return block;
-          }
-          return {
-            type: "text" as const,
-            text: block.thinking,
-          };
+          // Strip ALL thinking blocks from historical assistant messages.
+          // Anthropic thinking signatures are time-limited and single-use;
+          // replaying them causes "Invalid signature in thinking block" errors.
+          // The current-turn thinking block (if any) is preserved by the stream handler,
+          // not by transcript replay.
+          return [];
         }
 
         if (block.type === "text") {
