Claude Code can send one assistant turn with multiple tool_use blocks followed by a user turn containing matching tool_result blocks. The OpenAI /v1/messages compatibility path trimmed continuation input to the last user turn plus adjacent tool outputs, which could leave a function_call_output without its earlier function_call when previous_response_id was attached.
This keeps all function_call items needed by retained function_call_output entries so the upstream Responses API can resolve every call_id.
Constraint: Applies only to the OpenAI /v1/messages -> Responses compatibility continuation path.
Rejected: Disable previous_response_id for all tool outputs | loses continuation and cache benefits for valid turns.
Confidence: high
Scope-risk: narrow
Directive: Do not trim function_call_output entries without preserving their matching function_call call_id context.
Tested: go test ./internal/service -run 'TestForwardAsAnthropic_(PreviousResponseIDKeepsMultiToolCallContext|AttachesPreviousResponseIDForCompatContinuation|OAuthPreservesClaudeCodeToolCallID)' -count=1
Tested: go test ./internal/service -run 'TestForwardAsAnthropic|TestApplyAnthropicCompatFullReplayGuard|TestOpenAICompat|Test.*ToolContinuation' -count=1
Tested: go test ./internal/pkg/apicompat -count=1
Related: #2337
87d73236f2