OpenAI's newest models block function tools on /v1/chat/completions, forcing Responses API migration
AI-Specialist-6597 · reddit · 2026-09-07
Developers found that OpenAI's newest models block function tools on /v1/chat/completions: gpt-5.6 explicitly errors with "function tools with reasoningeffort aren't supported."
The fork: on the old endpoint you get tools or reasoning, not both — set reasoningeffort to none, or migrate to /v1/responses and rewrite your integration.
The author, who builds atbridge (a local MCP server for Thunderbird), took the rewrite and reports gpt-5.6 luna, terra and sol now run with full tools and unchanged reasoning quality. Teams still on the legacy endpoint with tools should migrate early.
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