fix: stop emitting duplicate tool-call events on trailing-whitespace deltas

[0age]

Summary

The streaming chat handler merge-into-existing-tool-call path emits a tool-call stream event whenever the accumulated function.arguments is parsable JSON. Because JSON.parse accepts trailing whitespace, any subsequent argument delta for the same tool-call index — a stray space, newline, or closing-token chunk — leaves the arguments parsable and re-triggers the emit. The result is a second tool-call event with the same toolCallId as the first.

Downstream tool runners (e.g. Vercel AI SDK streamText) treat the two events as two independent tool calls and execute the tool twice.

Where I observed this

Production traffic using moonshotai/kimi-k2.6 via OpenRouter from a Vercel AI SDK streamText agent loop. The user-visible effect was every outbound message being delivered twice, every shell command running twice, etc. Once the model saw the duplicate in history it would attempt to apologize via the same tool, and that apology was duplicated too — producing a self-reinforcing loop the model could not escape because the duplication was happening downstream of its own output.

The braintrust trace fingerprint:

Tool call (same id)	input
functions.bash:15	' {"command": "cat MEMORY.md | head -30"}'
functions.bash:15	' {"command": "cat MEMORY.md | head -30"} ' ← trailing space

functions.send:1	' {"actions":[…"apologies — duplicated the send there. fixed."}]}'
functions.send:1	' {"actions":[…"apologies — duplicated the send there. fixed."}]} ' ← trailing space

Every duplicate pair we saw differed only by trailing whitespace, which is exactly the signature of merge-path re-emission.

Root cause

In src/chat/index.ts, the streaming chunk handler has two code paths for tool-call deltas:

1. New-path (toolCalls[index] == null) — creates the slot, dedupes the id via seenToolCallIds, emits tool-call if the arguments are already complete, sets toolCall.sent = true.
2. Merge-path (existing slot) — concatenates the incoming arguments and emits tool-call if the result is parsable JSON.

The merge-path never reads toolCall.sent. It only sets it after emitting. So once the new-path has emitted a complete tool call and set sent = true, any subsequent argument delta for the same index whose appended result is still parsable JSON (which trailing whitespace trivially is) re-emits the tool call.

The existing seenToolCallIds dedup at lines 930-933 doesn't help here because (a) it lives in the new-path, and (b) the duplicate emit reuses the same toolCallId deliberately — it is the same logical tool call.

Fix

Gate the merge-path tool-call emit on !toolCall.sent, mirroring the new-path behavior. The flag was already being set; this just teaches the merge-path to read it.

if (
  !toolCall.sent &&
  toolCall.function?.name != null &&
  toolCall.function?.arguments != null &&
  isParsableJson(toolCall.function.arguments)
) {
  // ...emit tool-input-end + tool-call...
  toolCall.sent = true;
}

Test plan

• Added regression test in src/chat/index.test.ts under describe('includeRawChunks') that streams a complete tool call followed by a trailing-whitespace-only argument delta for the same index, and asserts exactly one tool-call event is emitted.
• Confirmed the new test fails on main (1 tool-call event expected, 2 received).
• Confirmed the new test passes with the fix.
• Full pnpm test:node suite passes (423/423, including the existing parallel/multi-chunk tool-call tests).
• pnpm typecheck clean.
• pnpm stylecheck (biome) clean.
• Added a patch changeset.

Suggested review order

1. src/chat/index.ts — one-line guard added (plus a comment explaining the trailing-whitespace gotcha).
2. src/chat/index.test.ts — regression test demonstrating the bug.
3. .changeset/fix-streaming-merge-path-duplicate-tool-call.md.

Notes

• This shouldn't affect any provider that flushes tool-call args in a single chunk or whose final arg-fragment is the closing brace (so parsability becomes true exactly once on the final delta). It will silence a class of duplicate emits for providers that emit a trailing-whitespace or trailing-token chunk after a complete tool call — which appears to include current Moonshot/Kimi routing on OpenRouter.
• I did not file an issue first; happy to do so if preferred. The trace evidence is included in this PR description for visibility.

[robert-j-y]

Cool!