Tool Calls

Last updated 2026-10-02

How to declare tools and return tool results across every provider.

Declaring tools

Tool definitions use the OpenAI shape, with Parameters as json.RawMessage so a JSON Schema goes in verbatim:

tools := []llmrouter.Tool{{
    Type: "function",
    Function: llmrouter.ToolFunction{
        Name:        "get_weather",
        Description: "Get the current weather for a city",
        Parameters:  json.RawMessage(`{"type":"object","properties":{"city":{"type":"string"}},"required":["city"]}`),
    },
}}

Returning results

Results come back as Role: "tool" plus ToolCallID. Gemini additionally attaches ToolCall.ThoughtSignature, which must be echoed back verbatim or the next turn is rejected.

Related

• Message Types: Tool, ToolCall, ToolCallFunction fields
• Streaming: tool calls arriving as tool_call events