fix: auto-repair and retry on orphan tool_result errors

When the Anthropic API rejects a request with 'unexpected tool_use_id found
in tool_result blocks', this indicates corrupted session history where a
tool_result references a tool_use that doesn't exist in the previous message.

This can happen due to:
- Race conditions in message processing
- Partial saves during crashes
- History truncation that removes tool_use but keeps tool_result

Changes:
- Add isOrphanToolResultError() to detect this specific error pattern
- Add retry logic in runEmbeddedAttempt: when this error is caught,
  repair the transcript using repairToolUseResultPairing() and retry once
- Log repair details (orphans dropped, duplicates dropped, synthetic results added)

This allows sessions to self-recover from corrupted history without requiring
a manual session reset.

Closes #XXXX

src/agents/pi-embedded-helpers.isorphantoolresulterror.test.ts

@@ -0,0 +1,38 @@
+import { describe, it, expect } from "vitest";
+import { isOrphanToolResultError } from "./pi-embedded-helpers.js";
+
+describe("isOrphanToolResultError", () => {
+  it("detects 'unexpected tool_use_id' error message", () => {
+    const error =
+      "LLM request rejected: messages.60.content.1: unexpected tool_use_id found in tool_result blocks: toolu_01KTTwhMaCYW8oiwZMxx6WDt. Each tool_result block must have a corresponding tool_use block in the previous message.";
+    expect(isOrphanToolResultError(error)).toBe(true);
+  });
+
+  it("detects tool_use_id with tool_result in message", () => {
+    const error = "Invalid request: tool_use_id in tool_result does not match any tool_use";
+    expect(isOrphanToolResultError(error)).toBe(true);
+  });
+
+  it("detects 'tool_result does not have corresponding tool_use' pattern", () => {
+    const error =
+      "Error: tool_result block does not have a corresponding tool_use block in previous message";
+    expect(isOrphanToolResultError(error)).toBe(true);
+  });
+
+  it("returns false for unrelated errors", () => {
+    expect(isOrphanToolResultError("rate limit exceeded")).toBe(false);
+    expect(isOrphanToolResultError("authentication failed")).toBe(false);
+    expect(isOrphanToolResultError("context window exceeded")).toBe(false);
+  });
+
+  it("returns false for empty or null input", () => {
+    expect(isOrphanToolResultError("")).toBe(false);
+    expect(isOrphanToolResultError(null as unknown as string)).toBe(false);
+    expect(isOrphanToolResultError(undefined as unknown as string)).toBe(false);
+  });
+
+  it("is case-insensitive", () => {
+    const error = "UNEXPECTED TOOL_USE_ID found in TOOL_RESULT blocks";
+    expect(isOrphanToolResultError(error)).toBe(true);
+  });
+});

src/agents/pi-embedded-helpers.ts

@@ -24,6 +24,7 @@ export {
   isFailoverErrorMessage,
   isImageDimensionErrorMessage,
   isImageSizeError,
+  isOrphanToolResultError,
   isOverloadedErrorMessage,
   isRawApiErrorPayload,
   isRateLimitAssistantError,

src/agents/pi-embedded-helpers/errors.ts

@@ -491,6 +491,24 @@ export function isCloudCodeAssistFormatError(raw: string): boolean {
   return !isImageDimensionErrorMessage(raw) && matchesErrorPatterns(raw, ERROR_PATTERNS.format);
 }
 
+/**
+ * Detects the specific "unexpected tool_use_id found in tool_result blocks" error
+ * from Anthropic. This error indicates orphaned tool_result entries in the transcript
+ * that reference tool_use IDs not present in the previous assistant message.
+ *
+ * When detected, the session history should be repaired using repairToolUseResultPairing()
+ * and the request retried.
+ */
+export function isOrphanToolResultError(raw: string): boolean {
+  if (!raw) return false;
+  const lower = raw.toLowerCase();
+  return (
+    lower.includes("unexpected tool_use_id") ||
+    (lower.includes("tool_use_id") && lower.includes("tool_result")) ||
+    /tool_result.*does not have.*corresponding.*tool_use/i.test(raw)
+  );
+}
+
 export function isAuthAssistantError(msg: AssistantMessage | undefined): boolean {
   if (!msg || msg.stopReason !== "error") return false;
   return isAuthErrorMessage(msg.errorMessage ?? "");

src/agents/pi-embedded-runner/run/attempt.ts

@@ -29,10 +29,12 @@ import { resolveMoltbotDocsPath } from "../../docs-path.js";
 import { resolveModelAuthMode } from "../../model-auth.js";
 import {
   isCloudCodeAssistFormatError,
+  isOrphanToolResultError,
   resolveBootstrapMaxChars,
   validateAnthropicTurns,
   validateGeminiTurns,
 } from "../../pi-embedded-helpers.js";
+import { repairToolUseResultPairing } from "../../session-transcript-repair.js";
 import { subscribeEmbeddedPiSession } from "../../pi-embedded-subscribe.js";
 import {
   ensurePiCompactionReserveTokens,
@@ -778,13 +780,41 @@ export async function runEmbeddedAttempt(
 
           // Only pass images option if there are actually images to pass
           // This avoids potential issues with models that don't expect the images parameter
-          if (imageResult.images.length > 0) {
-            await abortable(activeSession.prompt(effectivePrompt, { images: imageResult.images }));
-          } else {
-            await abortable(activeSession.prompt(effectivePrompt));
+          const runPrompt = async () => {
+            if (imageResult.images.length > 0) {
+              await abortable(activeSession.prompt(effectivePrompt, { images: imageResult.images }));
+            } else {
+              await abortable(activeSession.prompt(effectivePrompt));
+            }
+          };
+
+          try {
+            await runPrompt();
+          } catch (initialErr) {
+            // Check if error is due to orphaned tool_result entries in transcript.
+            // If so, repair the history and retry once before failing.
+            const errMsg = initialErr instanceof Error ? initialErr.message : String(initialErr);
+            if (isOrphanToolResultError(errMsg)) {
+              log.warn(
+                `Orphan tool_result error detected, repairing transcript and retrying: runId=${params.runId} sessionId=${params.sessionId}`,
+              );
+              try {
+                const currentMessages = activeSession.messages;
+                const repaired = repairToolUseResultPairing(currentMessages);
+                if (repaired.messages !== currentMessages) {
+                  activeSession.agent.replaceMessages(repaired.messages);
+                  log.debug(
+                    `Transcript repair applied: dropped=${repaired.droppedOrphanCount} orphans, ${repaired.droppedDuplicateCount} duplicates, added=${repaired.added.length} synthetic results`,
+                  );
+                }
+                await runPrompt();
+              } catch (retryErr) {
+                promptError = retryErr;
+              }
+            } else {
+              promptError = initialErr;
+            }
           }
-        } catch (err) {
-          promptError = err;
         } finally {
           log.debug(
             `embedded run prompt end: runId=${params.runId} sessionId=${params.sessionId} durationMs=${Date.now() - promptStartedAt}`,