tool_choice — auto, explicit, and forced

Key Takeaways

• tool_choice has three modes — auto (Claude decides), any (must call some tool), and explicit named (must call this exact tool) — each maps to a different determinism guarantee you provide to downstream consumers.
• Use explicit named tool_choice when your pipeline has a hard dependency on a specific tool's output, such as a compliance-check or schema-validation step where skipping is not acceptable.
• Reserve auto for exploratory or conversational turns where tool invocation is genuinely optional; never use it when the downstream system assumes a structured tool_result will always be present in the context window.
• Memory anchor — api-gateway-routing-policy: just as a gateway enforces route-level policies (mandatory auth, optional rate-limit), tool_choice enforces invocation policy at the model layer — auto is permissive passthrough, explicit is mandatory route, any is 'must route somewhere'.