When an OpenAI Chat Completions client targets an Anthropic-platform group,
ForwardAsChatCompletions converts the request CC → Responses → Anthropic
(ChatCompletionsToResponses → ResponsesToAnthropicRequest) before forwarding it
upstream. The Responses→Anthropic converter emits each function_call as its own
assistant message and each function_call_output as its own user message and
relies solely on mergeConsecutiveMessages to alternate roles. That is not enough
to satisfy Anthropic's tool-pairing invariants, so a trimmed or partial tool
history produces an upstream 400, e.g.:
tool_use_id found in tool_result blocks: call_00_...
Each tool_result block must have a corresponding tool_use block in the
previous message.
The failures this leaves unrepaired:
- orphan tool_result — a client that does sliding-window context management
keeps a recent tool result but drops the assistant tool_calls message that
announced it, so the tool_result has no matching tool_use;
- unanswered/dangling tool_use — a parallel call whose sibling result never
came back, or a call left dangling, which Anthropic also rejects.
Add normalizeAnthropicToolPairing, run between two merge passes: the first merge
groups parallel calls and their results; the pairing pass indexes every
tool_result by its tool_use id, keeps only answered tool_use blocks (dropping
unanswered/dangling calls, and the assistant message entirely when nothing else
remains) and re-emits the matching tool_result blocks as the immediately
following user message; standalone/orphan tool_results are dropped from their
original position; the second merge restores alternation. This mirrors
normalizeChatMessages on the Responses→Chat path.
Tested two ways: responses_to_anthropic_tool_pairing_test.go covers the repair
on direct Responses input (developer message between call and output, parallel
both-answered kept grouped, parallel one-unanswered dropped, orphan tool_result,
dangling call, single-call baseline); responses_to_anthropic_cc_chain_test.go
drives the real ChatCompletionsToResponses → ResponsesToAnthropicRequest chain
and reproduces the production 400 (orphan and unanswered-parallel) — both fail
without the repair and pass with it. The full apicompat suite stays green.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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