Function Calling Syntax Across Platforms

Each major AI platform has its own function calling syntax. Understanding these patterns lets you build portable tool integrations and optimize for each platform's strengths.

Claude's Function Calling (Anthropic)

Tool Definition Format

{
  "tools": [
    {
      "name": "get_weather",
      "description": "Get current weather for a location",
      "input_schema": {
        "type": "object",
        "properties": {
          "location": {
            "type": "string",
            "description": "City name or coordinates"
          },
          "units": {
            "type": "string",
            "enum": ["celsius", "fahrenheit"]
          }
        },
        "required": ["location"]
      }
    }
  ]
}

Tool Use Response (Claude Opus 4.5/Sonnet 4.5)

{
  "content": [
    {
      "type": "tool_use",
      "id": "toolu_01ABC123",
      "name": "get_weather",
      "input": {
        "location": "San Francisco",
        "units": "fahrenheit"
      }
    }
  ],
  "stop_reason": "tool_use"
}

Tool Result Format

{
  "role": "user",
  "content": [
    {
      "type": "tool_result",
      "tool_use_id": "toolu_01ABC123",
      "content": "72°F, sunny, humidity 45%"
    }
  ]
}

Claude Code's Internal Format

Claude Code uses structured tags for internal tool calls:

[function_calls]
[invoke name="Read"]
  [parameter name="file_path"]/src/index.ts[/parameter]
[/invoke]
[/function_calls]

[function_results]
Content of the file...
[/function_results]

OpenAI's Function Calling (GPT-5.2)

Tool Definition Format

{
  "tools": [
    {
      "type": "function",
      "function": {
        "name": "get_weather",
        "description": "Get current weather for a location",
        "parameters": {
          "type": "object",
          "properties": {
            "location": {
              "type": "string",
              "description": "City name"
            }
          },
          "required": ["location"]
        }
      }
    }
  ],
  "tool_choice": "auto"
}

Tool Call Response

{
  "choices": [
    {
      "message": {
        "tool_calls": [
          {
            "id": "call_abc123",
            "type": "function",
            "function": {
              "name": "get_weather",
              "arguments": "{\"location\": \"Tokyo\"}"
            }
          }
        ]
      },
      "finish_reason": "tool_calls"
    }
  ]
}

Tool Result Message

{
  "role": "tool",
  "tool_call_id": "call_abc123",
  "content": "22°C, partly cloudy"
}

GPT-5.2 Parallel Tool Calls

GPT-5.2 supports parallel function calls:

{
  "tool_calls": [
    {
      "id": "call_1",
      "function": {"name": "get_weather", "arguments": "{\"location\": \"NYC\"}"}
    },
    {
      "id": "call_2",
      "function": {"name": "get_weather", "arguments": "{\"location\": \"LA\"}"}
    }
  ]
}

Google's Function Calling (Gemini 3)

Tool Definition Format

{
  "tools": [
    {
      "function_declarations": [
        {
          "name": "get_weather",
          "description": "Returns weather for location",
          "parameters": {
            "type": "object",
            "properties": {
              "location": {"type": "string"}
            },
            "required": ["location"]
          }
        }
      ]
    }
  ]
}

Function Call Response

{
  "candidates": [
    {
      "content": {
        "parts": [
          {
            "functionCall": {
              "name": "get_weather",
              "args": {"location": "London"}
            }
          }
        ]
      }
    }
  ]
}

Function Response Format

{
  "parts": [
    {
      "functionResponse": {
        "name": "get_weather",
        "response": {
          "temperature": "15",
          "condition": "rainy"
        }
      }
    }
  ]
}

Comparison Table

Feature	Claude	OpenAI	Gemini
Schema	input_schema	parameters	parameters
Call ID	tool_use_id	tool_call_id	implicit
Result role	user (tool_result)	tool	function_response
Parallel calls	Yes	Yes	Yes
Streaming	Yes	Yes	Yes

MCP (Model Context Protocol)

Claude Code uses MCP for tool extensibility:

{
  "mcpServers": {
    "filesystem": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem"],
      "env": {
        "ALLOWED_PATHS": "/home/user/projects"
      }
    }
  }
}

MCP Tool Discovery

{
  "jsonrpc": "2.0",
  "method": "tools/list",
  "id": 1
}

Response:
{
  "tools": [
    {
      "name": "read_file",
      "description": "Read file contents",
      "inputSchema": {...}
    }
  ]
}

MCP Tool Invocation

{
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "name": "read_file",
    "arguments": {
      "path": "/src/main.ts"
    }
  },
  "id": 2
}

Best Practices for Cross-Platform Tools

1. Use JSON Schema Standard

{
  "type": "object",
  "properties": {
    "query": {
      "type": "string",
      "minLength": 1,
      "maxLength": 1000
    }
  },
  "required": ["query"],
  "additionalProperties": false
}

2. Normalize Response Handling

def handle_tool_response(platform, response):
    if platform == "claude":
        return response["content"][0]["input"]
    elif platform == "openai":
        return json.loads(response["tool_calls"][0]["function"]["arguments"])
    elif platform == "gemini":
        return response["candidates"][0]["content"]["parts"][0]["functionCall"]["args"]

3. Unified Error Format

{
  "error": {
    "type": "tool_error",
    "code": "INVALID_PARAMS",
    "message": "Missing required parameter: location",
    "recoverable": true
  }
}

Tool Calling Modes

Different invocation strategies:

Mode	Description	Use Case
auto	Model decides	General use
required	Must call tool	Data retrieval
none	No tools	Pure generation
specific	Force specific tool	Controlled flows

// Claude: tool_choice
{"type": "auto"}
{"type": "any"}
{"type": "tool", "name": "get_weather"}

// OpenAI: tool_choice
"auto"
"required"
{"type": "function", "function": {"name": "get_weather"}}

// Gemini: tool_config
{"function_calling_config": {"mode": "AUTO"}}

Key Insight: While syntax differs across platforms, the core concepts are identical: define tools with JSON Schema, receive structured calls, return formatted results. Building abstraction layers lets you write tools once and use them everywhere.

Next, we'll explore MCP integration and tool orchestration patterns. :::