MERIDIAN

Tool calling

Function calling with Meridian — define tools, control tool_choice, and parse streaming tool_calls in responses.

Defining tools

Pass a tools array in your chat completion request. Each tool declares a function with a name, description, and JSON Schema parameters.

{
  "tools": [
    {
      "type": "function",
      "function": {
        "name": "get_weather",
        "description": "Get current weather for a city",
        "parameters": {
          "type": "object",
          "properties": {
            "city": {
              "type": "string",
              "description": "City name, e.g. San Francisco"
            },
            "unit": {
              "type": "string",
              "enum": ["celsius", "fahrenheit"]
            }
          },
          "required": ["city"]
        }
      }
    }
  ]
}

Controlling tool_choice

The tool_choice parameter governs whether and which tools the model invokes.

// auto — model decides whether to call a tool
"tool_choice": "auto"

// none — model never calls a tool
"tool_choice": "none"

// required — model must call one of the provided tools
"tool_choice": "required"

// force a specific tool
"tool_choice": { "type": "function", "function": { "name": "get_weather" } }

Parsing tool_calls in the response

When the model decides to call a tool, the streaming delta contains a tool_calls array. Accumulate fragments by index — arguments arrive as JSON fragments across multiple deltas.

{
  "id": "msg_01XzQ2...",
  "choices": [{
    "delta": {
      "tool_calls": [{
        "index": 0,
        "id": "call_abc123",
        "type": "function",
        "function": {
          "name": "get_weather",
          "arguments": "{\"city\":\"Portland\",\"unit\":\"fahrenheit\"}"
        }
      }]
    }
  }]
}

Returning tool results

After executing the function, append a tool role message with the matching tool_call_id. The model uses this to produce its final answer.

{
  "role": "tool",
  "tool_call_id": "call_abc123",
  "content": "{\"temperature\": 58, \"condition\": \"overcast\", \"humidity\": 72}"
}

← Back to docsMeridian