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[release/0.43] Guard reasoning effort parameter against unsupported models#5011
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When a subagent inherits the parent agent's reasoningEffort but is routed to a model that doesn't support the effort parameter (e.g. claude-haiku-4.5 via search subagent), the API returns a 400 error. Fix: only include the effort parameter in the request body when the endpoint declares supportsReasoningEffort with at least one value. Affects both Messages API (Anthropic) and Responses API (OpenAI). Includes unit tests for the Messages API effort guard.
Yoyokrazy
approved these changes
Apr 6, 2026
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Cherry-pick of #5010 into release/0.43.
Fixes microsoft/vscode#308078
Problem
When a subagent inherits the parent agent's
reasoningEffortbut is routed to a model that doesn't support theeffortparameter (e.g.claude-haiku-4.5via search subagent), the API returns a 400 error:"This model does not support the effort parameter."Fix
messagesApi.ts): Only set effort inoutput_configwhenendpoint.supportsReasoningEffort?.lengthis truthyresponsesApi.ts): Same guard — only include effort when the model declares supportcreateMessagesRequestBody