From c9939063f5143614e3638960898cd2dfa7c19d27 Mon Sep 17 00:00:00 2001
From: Kira <kira@ariaki.me>
Date: Fri, 30 Jan 2026 03:05:56 -0500
Subject: [PATCH] fix: skip tool calls from aborted assistant messages in
 transcript repair

When a streaming response is aborted mid-tool-call, the tool call may be
incomplete (indicated by partialJson in the message). The transcript repair
logic was still extracting tool calls from these aborted messages and
inserting synthetic tool_result blocks for them.

When sent to Anthropic API, the aborted tool_use block may be
invalid/incomplete, causing the API to reject the request with:
'unexpected tool_use_id found in tool_result blocks'

This permanently corrupts the session until manually repaired.

Fix: Check for stopReason === 'aborted' in extractToolCallsFromAssistant
and return early with an empty array, skipping synthetic result insertion.

Fixes #4475
---
 src/agents/session-transcript-repair.test.ts | 34 ++++++++++++++++++++
 src/agents/session-transcript-repair.ts      |  5 +++
 2 files changed, 39 insertions(+)

diff --git a/src/agents/session-transcript-repair.test.ts b/src/agents/session-transcript-repair.test.ts
index ccc63ec7f..b4effd736 100644
--- a/src/agents/session-transcript-repair.test.ts
+++ b/src/agents/session-transcript-repair.test.ts
@@ -109,4 +109,38 @@ describe("sanitizeToolUseResultPairing", () => {
     expect(out.some((m) => m.role === "toolResult")).toBe(false);
     expect(out.map((m) => m.role)).toEqual(["user", "assistant"]);
   });
+
+  it("skips tool calls from aborted assistant messages", () => {
+    // When a streaming response is aborted mid-tool-call, the tool call may be incomplete.
+    // Inserting synthetic results for aborted tool calls causes API validation failures
+    // because the original tool_use block may be malformed.
+    const input = [
+      {
+        role: "assistant",
+        content: [{ type: "toolCall", id: "call_aborted", name: "process", arguments: {} }],
+        stopReason: "aborted",
+      },
+      { role: "user", content: "next message" },
+    ] as AgentMessage[];
+
+    const out = sanitizeToolUseResultPairing(input);
+    // Should NOT insert a synthetic tool result for the aborted tool call
+    expect(out.some((m) => m.role === "toolResult")).toBe(false);
+    expect(out.map((m) => m.role)).toEqual(["assistant", "user"]);
+  });
+
+  it("processes tool calls normally when stopReason is not aborted", () => {
+    const input = [
+      {
+        role: "assistant",
+        content: [{ type: "toolCall", id: "call_1", name: "read", arguments: {} }],
+        stopReason: "toolUse",
+      },
+      { role: "user", content: "next" },
+    ] as AgentMessage[];
+
+    const out = sanitizeToolUseResultPairing(input);
+    // Should insert synthetic result for the missing tool result
+    expect(out.filter((m) => m.role === "toolResult")).toHaveLength(1);
+  });
 });
diff --git a/src/agents/session-transcript-repair.ts b/src/agents/session-transcript-repair.ts
index 2728bb89c..406824eaf 100644
--- a/src/agents/session-transcript-repair.ts
+++ b/src/agents/session-transcript-repair.ts
@@ -8,6 +8,11 @@ type ToolCallLike = {
 function extractToolCallsFromAssistant(
   msg: Extract<AgentMessage, { role: "assistant" }>,
 ): ToolCallLike[] {
+  // Skip extracting tool calls from aborted messages - they may be incomplete
+  // and inserting synthetic results for them causes API validation failures
+  const stopReason = (msg as { stopReason?: unknown }).stopReason;
+  if (stopReason === "aborted") return [];
+
   const content = msg.content;
   if (!Array.isArray(content)) return [];