Tool Calling

Function/tool call and result representation

This document describes how tool/function calling is represented in OpenInference spans.

Tool Definitions

Tools available to the LLM are represented using the llm.tools prefix with flattened attributes:

Attribute Pattern

llm.tools.<index>.tool.json_schema

The json_schema contains the complete tool definition as a JSON string, including:

• Tool type (usually “function”)
• Function name
• Function description
• Parameter schema

Example Tool Definition

{
  "llm.tools.0.tool.json_schema": "{\"type\": \"function\", \"function\": {\"name\": \"get_weather\", \"description\": \"Get current weather for a location\", \"parameters\": {\"type\": \"object\", \"properties\": {\"location\": {\"type\": \"string\", \"description\": \"City and state\"}}, \"required\": [\"location\"]}}}"
}

Tool Calls in Messages

When an LLM generates tool calls, they are represented in the output messages:

Attribute Pattern for Tool Calls

llm.output_messages.<messageIndex>.message.tool_calls.<toolCallIndex>.tool_call.<attribute>

Where:

• <messageIndex> is the zero-based index of the message
• <toolCallIndex> is the zero-based index of the tool call within the message
• <attribute> is the specific tool call attribute

Tool Call Attributes

• tool_call.id: Unique identifier for the tool call
• tool_call.function.name: Name of the function being called
• tool_call.function.arguments: JSON string containing the function arguments

Example Tool Call

{
  "llm.output_messages.0.message.role": "assistant",
  "llm.output_messages.0.message.tool_calls.0.tool_call.id": "call_abc123",
  "llm.output_messages.0.message.tool_calls.0.tool_call.function.name": "get_weather",
  "llm.output_messages.0.message.tool_calls.0.tool_call.function.arguments": "{\"location\": \"San Francisco, CA\"}"
}

Multiple Tool Calls

When an LLM makes multiple tool calls in a single response:

{
  "llm.output_messages.0.message.role": "assistant",
  "llm.output_messages.0.message.tool_calls.0.tool_call.id": "call_001",
  "llm.output_messages.0.message.tool_calls.0.tool_call.function.name": "get_weather",
  "llm.output_messages.0.message.tool_calls.0.tool_call.function.arguments": "{\"location\": \"New York\"}",
  "llm.output_messages.0.message.tool_calls.1.tool_call.id": "call_002",
  "llm.output_messages.0.message.tool_calls.1.tool_call.function.name": "get_weather",
  "llm.output_messages.0.message.tool_calls.1.tool_call.function.arguments": "{\"location\": \"London\"}"
}

Tool Results

Tool results are typically represented as input messages with role “tool”:

{
  "llm.input_messages.3.message.role": "tool",
  "llm.input_messages.3.message.content": "{\"temperature\": 72, \"condition\": \"sunny\"}",
  "llm.input_messages.3.message.tool_call_id": "call_abc123",
  "llm.input_messages.3.message.name": "get_weather"
}

The message.tool_call_id links the result back to the original tool call. The message.name attribute MAY be set to identify which function produced the result — it typically matches tool_call.function.name from the corresponding tool call.

Complete Tool Call Flow Example

1. User Request

{
  "llm.input_messages.0.message.role": "user",
  "llm.input_messages.0.message.content": "What's the weather in Boston?"
}

2. Available Tools

{
  "llm.tools.0.tool.json_schema": "{\"type\": \"function\", \"function\": {\"name\": \"get_weather\", \"description\": \"Get current weather\", \"parameters\": {\"type\": \"object\", \"properties\": {\"location\": {\"type\": \"string\"}}}}}"
}

3. LLM Tool Call

{
  "llm.output_messages.0.message.role": "assistant",
  "llm.output_messages.0.message.tool_calls.0.tool_call.id": "call_123",
  "llm.output_messages.0.message.tool_calls.0.tool_call.function.name": "get_weather",
  "llm.output_messages.0.message.tool_calls.0.tool_call.function.arguments": "{\"location\": \"Boston, MA\"}"
}

4. Tool Result (in next request)

{
  "llm.input_messages.2.message.role": "tool",
  "llm.input_messages.2.message.content": "{\"temperature\": 65, \"condition\": \"cloudy\"}",
  "llm.input_messages.2.message.tool_call_id": "call_123"
}

5. Final Response

{
  "llm.output_messages.0.message.role": "assistant",
  "llm.output_messages.0.message.content": "The current weather in Boston is 65°F and cloudy."
}

Legacy Attributes

Some implementations may use legacy attributes for function calling. New implementations should use the tool_calls structure described above.

Legacy attributes:

• message.function_call_name: Function name (deprecated, use tool_calls)
• message.function_call_arguments_json: Function arguments (deprecated, use tool_calls)
• llm.function_call: Complete function call as JSON (deprecated)