diff --git a/src/agents/transcript-policy.test.ts b/src/agents/transcript-policy.test.ts
index 15c03250e..2e6c673f2 100644
--- a/src/agents/transcript-policy.test.ts
+++ b/src/agents/transcript-policy.test.ts
@@ -122,6 +122,15 @@ describe("resolveTranscriptPolicy", () => {
     expect(policy.preserveSignatures).toBe(false);
   });
 
+  it("does not preserve signatures for kimi-coding provider (#39798)", () => {
+    const policy = resolveTranscriptPolicy({
+      provider: "kimi-coding",
+      modelId: "k2p5",
+      modelApi: "anthropic-messages",
+    });
+    expect(policy.preserveSignatures).toBe(false);
+  });
+
   it("enables turn-ordering and assistant-merge for strict OpenAI-compatible providers (#38962)", () => {
     const policy = resolveTranscriptPolicy({
       provider: "vllm",
diff --git a/src/agents/transcript-policy.ts b/src/agents/transcript-policy.ts
index 4296f6239..264eb9410 100644
--- a/src/agents/transcript-policy.ts
+++ b/src/agents/transcript-policy.ts
@@ -123,7 +123,8 @@ export function resolveTranscriptPolicy(params: {
       (!isOpenAi && sanitizeToolCallIds) || requiresOpenAiCompatibleToolIdSanitization,
     toolCallIdMode,
     repairToolUseResultPairing,
-    preserveSignatures: isAnthropic,
+    // kimi-coding uses anthropic-messages API but cannot handle re-sent thinkingSignature blobs (#39798)
+    preserveSignatures: isAnthropic && provider !== "kimi-coding",
     sanitizeThoughtSignatures: isOpenAi ? undefined : sanitizeThoughtSignatures,
     sanitizeThinkingSignatures: false,
     dropThinkingBlocks,