fix(agents): preserve trailing assistant tool-call turns during session trim

amknight · amknight · commit deaa6f656758 · 2026-05-01T21:20:48.000+10:00
Mirror the outbound guard (stripTrailingAssistantPrefillTurns): skip assistant entries containing toolCall/toolUse/functionCall blocks so transcript repair can synthesize missing tool results. Addresses PR review feedback from clawsweeper on #75606.
diff --git a/src/agents/session-file-repair.test.ts b/src/agents/session-file-repair.test.ts
@@ -409,6 +409,73 @@ describe("repairSessionFileIfNeeded", () => {
     expect(result.trimmedTrailingAssistantMessages ?? 0).toBe(0);
   });
 
+  it("preserves trailing assistant messages that contain tool calls", async () => {
+    const { file } = await createTempSessionPath();
+    const { header, message } = buildSessionHeaderAndMessage();
+    const toolCallAssistant = {
+      type: "message",
+      id: "msg-asst-tc",
+      parentId: null,
+      timestamp: new Date().toISOString(),
+      message: {
+        role: "assistant",
+        content: [
+          { type: "text", text: "Let me check that." },
+          { type: "toolCall", id: "call_1", name: "read", input: { path: "/tmp/test" } },
+        ],
+        stopReason: "toolUse",
+      },
+    };
+    const original = `${JSON.stringify(header)}\n${JSON.stringify(message)}\n${JSON.stringify(toolCallAssistant)}\n`;
+    await fs.writeFile(file, original, "utf-8");
+
+    const result = await repairSessionFileIfNeeded({ sessionFile: file });
+
+    expect(result.repaired).toBe(false);
+    expect(result.trimmedTrailingAssistantMessages ?? 0).toBe(0);
+    const after = await fs.readFile(file, "utf-8");
+    expect(after).toBe(original);
+  });
+
+  it("trims non-tool-call assistant but stops at tool-call assistant", async () => {
+    const { file } = await createTempSessionPath();
+    const { header, message } = buildSessionHeaderAndMessage();
+    const toolCallAssistant = {
+      type: "message",
+      id: "msg-asst-tc",
+      parentId: null,
+      timestamp: new Date().toISOString(),
+      message: {
+        role: "assistant",
+        content: [{ type: "toolUse", id: "call_1", name: "read" }],
+        stopReason: "toolUse",
+      },
+    };
+    const plainAssistant = {
+      type: "message",
+      id: "msg-asst-plain",
+      parentId: null,
+      timestamp: new Date().toISOString(),
+      message: {
+        role: "assistant",
+        content: [{ type: "text", text: "stale" }],
+        stopReason: "stop",
+      },
+    };
+    const original = `${JSON.stringify(header)}\n${JSON.stringify(message)}\n${JSON.stringify(toolCallAssistant)}\n${JSON.stringify(plainAssistant)}\n`;
+    await fs.writeFile(file, original, "utf-8");
+
+    const result = await repairSessionFileIfNeeded({ sessionFile: file });
+
+    expect(result.repaired).toBe(true);
+    expect(result.trimmedTrailingAssistantMessages).toBe(1);
+
+    const repaired = await fs.readFile(file, "utf-8");
+    const repairedLines = repaired.trim().split("\n");
+    expect(repairedLines).toHaveLength(3);
+    expect(JSON.parse(repairedLines[2]).id).toBe("msg-asst-tc");
+  });
+
   it("never trims below the session header", async () => {
     const { file } = await createTempSessionPath();
     const { header } = buildSessionHeaderAndMessage();
diff --git a/src/agents/session-file-repair.ts b/src/agents/session-file-repair.ts
@@ -136,15 +136,34 @@ function repairUserEntryWithBlankTextContent(entry: SessionMessageEntry): UserEn
   };
 }
 
-function isAssistantMessageEntry(entry: unknown): boolean {
+function isToolCallBlock(block: unknown): boolean {
+  if (!block || typeof block !== "object") {
+    return false;
+  }
+  const type = (block as { type?: unknown }).type;
+  return type === "toolCall" || type === "toolUse" || type === "functionCall";
+}
+
+/** Trailing assistant without tool calls — safe to trim from disk.
+ * Assistant turns with tool calls are kept so transcript repair can
+ * synthesize missing tool results (mirrors the outbound guard). */
+function isTrimmableTrailingAssistantEntry(entry: unknown): boolean {
   if (!entry || typeof entry !== "object") {
     return false;
   }
   const record = entry as { type?: unknown; message?: unknown };
   if (record.type !== "message" || !record.message || typeof record.message !== "object") {
     return false;
   }
-  return (record.message as { role?: unknown }).role === "assistant";
+  const message = record.message as { role?: unknown; content?: unknown };
+  if (message.role !== "assistant") {
+    return false;
+  }
+  const content = message.content;
+  if (Array.isArray(content) && content.some(isToolCallBlock)) {
+    return false;
+  }
+  return true;
 }
 
 function buildRepairSummaryParts(params: {
@@ -252,7 +271,7 @@ export async function repairSessionFileIfNeeded(params: {
   // thinking is enabled. The outbound path strips per-request, but leaving
   // the file corrupted causes repeated reject cycles across restarts.
   let trimmedTrailingAssistantMessages = 0;
-  while (entries.length > 1 && isAssistantMessageEntry(entries[entries.length - 1])) {
+  while (entries.length > 1 && isTrimmableTrailingAssistantEntry(entries[entries.length - 1])) {
     entries.pop();
     trimmedTrailingAssistantMessages += 1;
   }
