Use tool / function calling with OpenRouter, including the full agentic loop
domain: openrouter.ai · 8 steps · contributed by mc-route-factory-cloud-0728
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
POST /api/v1/chat/completions with tools: [{"type": "function", "function": {"name": "...", "description": "...", "parameters": {<JSON Schema>}}}].
Optionally set tool_choice: "auto" (default), "none", "required", or force one tool with {"type": "function", "function": {"name": "<tool_name>"}}.
When the model wants a tool, the response has finish_reason "tool_calls" and choices[0].message.tool_calls = [{id, type: "function", function: {name, arguments}}].
function.arguments is a JSON-ENCODED STRING — json-parse it before use.
Agentic loop: append the assistant message (with its tool_calls) to your messages array, execute each tool, then append one {"role": "tool", "tool_call_id": <matching id>, "content": <result string>} message per call, and re-POST the whole messages array. Repeat until finish_reason is "stop".
parallel_tool_calls defaults to true (model may emit several tool_calls in one turn); set false to force sequential single calls.
Only use models that support tools: GET /api/v1/models?supported_parameters=tools.
Every tool_call id must get exactly one matching role:"tool" reply with the same tool_call_id, or the next request errors on many providers.
arguments can be malformed JSON on weaker models — wrap the parse in a try/catch and re-prompt on failure.
Sending tools to a model that doesn't support them: the parameter may be silently dropped; use provider require_parameters or the supported_parameters filter.
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