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docs: Add bug report for duplicate tool_result blocks issueBrowse files

Documents API error where multiple tool_result blocks are sent with
the same tool_use_id when a tool fails. The "Sibling tool call errored"
message is incorrectly being sent for the same tool that failed,
rather than only for other sibling tools in a parallel batch.

Includes:
- Root cause analysis
- Example JSON from reported incidents
- Suggested fix locations and pseudocode
- Related request IDs for debugging

Claude authored on Dec 8, 2025 parent de49a07 · commit 20c5837683b0

1 changed file+123 -0compared with de49a07

A.github/issues/duplicate-tool-result-bug.md

A.github/issues/duplicate-tool-result-bug.md+123 -0

		@@ -1,0 +1,123 @@
	1	+# Bug Report: Duplicate tool_result blocks with same tool_use_id
	2	+
	3	+## Summary
	4	+When a tool call fails (particularly observed with the Tmux tool), two `tool_result` blocks are incorrectly sent to the API with the same `tool_use_id`, causing an API error:
	5	+
	6	+```
	7	+API Error: 400
	8	+{"type":"error","error":{"type":"invalid_request_error","message":"messages.X.content.1: each tool_use must have a single result. Found multiple `tool_result` blocks with id: toolu_XXX"}}
	9	+```
	10	+
	11	+## Root Cause
	12	+The parallel tool execution error handling code sends a "Sibling tool call errored" message even for the tool that actually failed, when it should only send this message for OTHER sibling tools in the parallel batch.
	13	+
	14	+## Reproduction
	15	+1. Execute a tool call that will fail (e.g., Tmux select-pane on a non-existent window)
	16	+2. Observe two tool_result messages being generated:
	17	+ - First: The actual error result ("Window not found...")
	18	+ - Second: A "<tool_use_error>Sibling tool call errored</tool_use_error>" message with the SAME tool_use_id
	19	+
	20	+## Example JSON (from reported incident)
	21	+```json
	22	+// Assistant requests tool use
	23	+{
	24	+ "type": "tool_use",
	25	+ "id": "toolu_01K2K2KFvUrwA9PaHGp652zW",
	26	+ "name": "Tmux",
	27	+ "input": {"args": ["select-pane", "-t", "test-perms3:0.0"]}
	28	+}
	29	+
	30	+// First tool_result (correct - actual error)
	31	+{
	32	+ "type": "user",
	33	+ "message": {
	34	+ "role": "user",
	35	+ "content": [{
	36	+ "tool_use_id": "toolu_01K2K2KFvUrwA9PaHGp652zW",
	37	+ "type": "tool_result",
	38	+ "content": "Window not found. The specified window may not exist.",
	39	+ "is_error": true
	40	+ }]
	41	+ }
	42	+}
	43	+
	44	+// Second tool_result (BUG - same tool_use_id!)
	45	+{
	46	+ "type": "user",
	47	+ "message": {
	48	+ "role": "user",
	49	+ "content": [{
	50	+ "type": "tool_result",
	51	+ "content": "<tool_use_error>Sibling tool call errored</tool_use_error>",
	52	+ "is_error": true,
	53	+ "tool_use_id": "toolu_01K2K2KFvUrwA9PaHGp652zW"
	54	+ }]
	55	+ }
	56	+}
	57	+```
	58	+
	59	+## Expected Behavior
	60	+- When a tool fails, only ONE tool_result should be sent for that tool_use_id
	61	+- The "Sibling tool call errored" message should ONLY be sent for OTHER tool_use_ids in a parallel batch, not for the tool that actually errored
	62	+
	63	+## Suggested Fix Location
	64	+The fix should be in the code that handles parallel tool execution errors:
	65	+
	66	+```typescript
	67	+// Pseudocode for the fix
	68	+function handleParallelToolResults(toolUseIds: string[], failedToolId: string, error: Error) {
	69	+ const results = [];
	70	+
	71	+ for (const id of toolUseIds) {
	72	+ if (id === failedToolId) {
	73	+ // Send actual error for the failed tool
	74	+ results.push({
	75	+ tool_use_id: id,
	76	+ type: "tool_result",
	77	+ content: error.message,
	78	+ is_error: true
	79	+ });
	80	+ } else {
	81	+ // Only send sibling error for OTHER tools
	82	+ results.push({
	83	+ tool_use_id: id,
	84	+ type: "tool_result",
	85	+ content: "<tool_use_error>Sibling tool call errored</tool_use_error>",
	86	+ is_error: true
	87	+ });
	88	+ }
	89	+ }
	90	+
	91	+ return results;
	92	+}
	93	+```
	94	+
	95	+Additionally, a deduplication check before sending to the API would prevent this:
	96	+```typescript
	97	+function deduplicateToolResults(messages: Message[]): Message[] {
	98	+ const seenToolUseIds = new Set<string>();
	99	+ return messages.filter(msg => {
	100	+ if (msg.type === "tool_result") {
	101	+ if (seenToolUseIds.has(msg.tool_use_id)) {
	102	+ return false; // Skip duplicate
	103	+ }
	104	+ seenToolUseIds.add(msg.tool_use_id);
	105	+ }
	106	+ return true;
	107	+ });
	108	+}
	109	+```
	110	+
	111	+## Related CHANGELOG Fixes
	112	+Similar issues have been fixed before:
	113	+- v1.0.84: "Fix tool_use/tool_result id mismatch error when network is unstable"
	114	+- v2.0.0: "Hooks: Reduced PostToolUse 'tool_use' ids were found without 'tool_result' blocks errors"
	115	+- v2.0.22: "Fix bug causing duplicate permission prompts with parallel tool calls"
	116	+
	117	+## Reported By
	118	+Issue reported via Slack with multiple reproductions showing the Tmux tool triggering this bug.
	119	+
	120	+## Request IDs (for debugging)
	121	+- req_011CVt2KXhCGmrL2YN9sSZQP
	122	+- req_011CVtFYJLPxn9oB9FEkCnRr
	123	+- req_011CVtG1PREveTLikakXGEdn