Tool calling

	Chat Completions	Responses	Messages
Declare	`tools: [{"type": "function", "function": {"name", "description", "parameters"}}]`	`tools: [{"type": "function", "name", "description", "parameters"}]`	`tools: [{"name", "description", "input_schema"}]`
The model calls	`message.tool_calls[]` with `id` and `function.arguments` as a JSON string	Output item `function_call` with `call_id` and `arguments`	Content block `tool_use` with `id` and `input` as an object
The turn stops with	`finish_reason: "tool_calls"`	The response ends after the `function_call` items	`stop_reason: "tool_use"`
You answer with	A `role: "tool"` message with `tool_call_id`	An input item `function_call_output` with `call_id`	A `user` message with a `tool_result` block and `tool_use_id`

Rules

Every call needs its result, and every result its call. A tool call without a matching result, or a result without its call (tool_output_unmatched), fails with 400 before the request reaches a provider.
Responses keeps no state. Send the earlier function_call items back together with their function_call_output items. See Conversation state.
Models without tool support lose the tools. If tools is missing from a model’s supported_parameters, NagaAI removes the tools from the request and the model answers in text.
Some models accept only tool_choice: "auto". For claude-opus-5.5, claude-fable-5.1 and Mythos 5.1, NagaAI turns required or a named tool into auto, so the model may answer in text.
NagaAI refuses legacy fields. functions, the function role and function_call on a message fail with 400, and NagaAI drops a top-level function_call. Use tools, tool_choice, tool_calls and the tool role.
Function tools and web search only. NagaAI supports your own function tools and web search. NagaAI drops other server-side tools, see Differences.