From 98e89f59399bfe075ad4b8691396d76425c651b3 Mon Sep 17 00:00:00 2001
From: Shakker <shakkerdroid@gmail.com>
Date: Sun, 12 Apr 2026 04:11:42 +0100
Subject: [PATCH] fix: gate immutable thinking replay repair to anthropic

---
 .../pi-embedded-runner/replay-history.ts      |  1 +
 src/agents/session-transcript-repair.test.ts  | 47 ++++++++++++++++++-
 src/agents/session-transcript-repair.ts       | 10 +++-
 3 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/src/agents/pi-embedded-runner/replay-history.ts b/src/agents/pi-embedded-runner/replay-history.ts
index a3061f57ce7..29fe1692eef 100644
--- a/src/agents/pi-embedded-runner/replay-history.ts
+++ b/src/agents/pi-embedded-runner/replay-history.ts
@@ -418,6 +418,7 @@ export async function sanitizeSessionHistory(params: {
     : sanitizedImages;
   const sanitizedToolCalls = sanitizeToolCallInputs(droppedThinking, {
     allowedToolNames: params.allowedToolNames,
+    preserveImmutableThinkingTurns: policy.validateAnthropicTurns,
   });
   const repairedTools = policy.repairToolUseResultPairing
     ? sanitizeToolUseResultPairing(sanitizedToolCalls, {
diff --git a/src/agents/session-transcript-repair.test.ts b/src/agents/session-transcript-repair.test.ts
index a7e6ac8ad71..dcb36dba379 100644
--- a/src/agents/session-transcript-repair.test.ts
+++ b/src/agents/session-transcript-repair.test.ts
@@ -403,7 +403,10 @@ describe("sanitizeToolCallInputs", () => {
       },
     ]);
 
-    const out = sanitizeToolCallInputs(input, { allowedToolNames: ["read"] });
+    const out = sanitizeToolCallInputs(input, {
+      allowedToolNames: ["read"],
+      preserveImmutableThinkingTurns: true,
+    });
 
     expect(out).toEqual([]);
   });
@@ -432,12 +435,52 @@ describe("sanitizeToolCallInputs", () => {
       },
     ]);
 
-    const out = sanitizeToolCallInputs(input, { allowedToolNames: ["sessions_spawn"] });
+    const out = sanitizeToolCallInputs(input, {
+      allowedToolNames: ["sessions_spawn"],
+      preserveImmutableThinkingTurns: true,
+    });
 
     expect(out).toEqual([]);
     expect(JSON.stringify(out)).not.toContain(secret);
   });
 
+  it("keeps generic thinking turns mutable when immutable preservation is disabled", () => {
+    const input = castAgentMessages([
+      {
+        role: "assistant",
+        content: [
+          {
+            type: "thinking",
+            thinking: "Let me normalize this tool name.",
+            thinkingSignature: "sig_generic",
+          },
+          {
+            type: "toolCall",
+            id: "call_read",
+            name: " read ",
+            arguments: { path: "README.md" },
+          },
+        ],
+      },
+    ]);
+
+    const out = sanitizeToolCallInputs(input, { allowedToolNames: ["read"] });
+    const assistant = out[0] as Extract<AgentMessage, { role: "assistant" }>;
+    expect(assistant.content).toEqual([
+      {
+        type: "thinking",
+        thinking: "Let me normalize this tool name.",
+        thinkingSignature: "sig_generic",
+      },
+      {
+        type: "toolCall",
+        id: "call_read",
+        name: "read",
+        arguments: { path: "README.md" },
+      },
+    ]);
+  });
+
   it.each([
     {
       name: "trims leading whitespace from tool names",
diff --git a/src/agents/session-transcript-repair.ts b/src/agents/session-transcript-repair.ts
index cd845c0219e..b7196c0745d 100644
--- a/src/agents/session-transcript-repair.ts
+++ b/src/agents/session-transcript-repair.ts
@@ -176,7 +176,7 @@ function isReplaySafeThinkingAssistantTurn(
       return false;
     }
   }
-  return sawToolCall || content.some((block) => isThinkingLikeBlock(block));
+  return sawToolCall;
 }
 
 function makeMissingToolResult(params: {
@@ -232,6 +232,7 @@ export type ToolCallInputRepairReport = {
 
 export type ToolCallInputRepairOptions = {
   allowedToolNames?: Iterable<string>;
+  preserveImmutableThinkingTurns?: boolean;
 };
 
 export type ErroredAssistantResultPolicy = "preserve" | "drop";
@@ -269,6 +270,7 @@ export function repairToolCallInputs(
   let changed = false;
   const out: AgentMessage[] = [];
   const allowedToolNames = normalizeAllowedToolNames(options?.allowedToolNames);
+  const preserveImmutableThinkingTurns = options?.preserveImmutableThinkingTurns === true;
 
   for (const msg of messages) {
     if (!msg || typeof msg !== "object") {
@@ -281,7 +283,11 @@ export function repairToolCallInputs(
       continue;
     }
 
-    if (msg.content.some((block) => isThinkingLikeBlock(block))) {
+    if (
+      preserveImmutableThinkingTurns &&
+      msg.content.some((block) => isThinkingLikeBlock(block)) &&
+      countRawToolCallBlocks(msg.content) > 0
+    ) {
       // Signed Anthropic thinking blocks must remain byte-for-byte stable on
       // replay. Preserve the turn only if every sibling tool call is already
       // valid and requires no redaction or normalization. Otherwise drop the