Add OpenAI-compliant server-side tool calling support (#293)

[palios-taey]

Summary

This PR implements complete OpenAI-compliant server-side tool calling for EXO, addressing Issue #293 ($300 bounty).

Key Features:

• ✅ Server-side parsing of tool calls from model output
• ✅ OpenAI-compliant response format with tool_calls array
• ✅ Proper finish_reason="tool_calls" when tools are invoked
• ✅ Support for parallel tool calling (multiple tools in one response)
• ✅ Works with both streaming and non-streaming responses
• ✅ Unique tool call IDs generated server-side (call_<random>)
• ✅ Arguments always returned as JSON strings (not objects)
• ✅ Backwards compatible - no changes when tools not provided

Changes

Core Implementation (exo/api/chatgpt_api.py)

1. Added parse_tool_calls() function

   • Parses <tool_call>...</tool_call> XML tags from model output
   • Extracts content before tool calls
   • Generates unique IDs for each tool call
   • Converts dict arguments to JSON strings
   • Returns OpenAI-formatted tool call objects

2. Modified generate_completion() function

   • Detects tool calls when tools provided in request
   • Formats response with tool_calls array
   • Sets finish_reason to "tool_calls" appropriately
   • Handles both streaming and non-streaming

Example Code (examples/function_calling_openai_compliant.py)

• Complete working example showing the new server-side implementation
• Demonstrates both single and parallel tool calling
• No client-side parsing required

Tests (test_parse_simple.py)

• Unit tests for the parse_tool_calls() function
• Tests: single tool call, parallel calls, no tools, dict conversion, OpenAI format compliance
• All tests pass ✅

OpenAI Format Compliance

Response format matches OpenAI spec exactly:

{
  "id": "chatcmpl-xxx",
  "object": "chat.completion",
  "model": "model-name",
  "choices": [{
    "index": 0,
    "message": {
      "role": "assistant",
      "content": "text before tool calls",
      "tool_calls": [{
        "id": "call_abc123xyz",
        "type": "function",
        "function": {
          "name": "function_name",
          "arguments": "{\"param\": \"value\"}"
        }
      }]
    },
    "finish_reason": "tool_calls"
  }]
}

Testing

Unit Tests:

$ python3 test_parse_simple.py
✅ Test 1: Single Tool Call - PASS
✅ Test 2: Parallel Tool Calls - PASS
✅ Test 3: No Tool Calls - PASS
✅ Test 4: Dict Arguments Conversion - PASS
✅ Test 5: OpenAI Format Compliance - PASS
Results: 5 passed, 0 failed

Integration Testing:
The implementation can be tested with any EXO deployment:

import requests

response = requests.post("http://localhost:52415/v1/chat/completions", json={
    "model": "llama-3.2-1b",
    "messages": [{"role": "user", "content": "What's the weather in Boston?"}],
    "tools": [{
        "type": "function",
        "function": {
            "name": "get_current_weather",
            "parameters": {"type": "object", "properties": {"location": {"type": "string"}}}
        }
    }]
})

# Response now includes tool_calls array automatically!

Implementation Approach

This PR takes a focused, minimal approach compared to the stalled PR #771 (59 commits, 35 files changed):

• Only 3 files changed: Core API, example, and tests
• ~60 lines of new code in the core implementation
• Reuses existing XML parsing pattern from examples/function_calling.py
• No breaking changes to existing functionality
• Backwards compatible - works with all existing code

Why This Solution

1. Cleaner than PR Structural generation and function calling / tool use #771: Focused changes instead of massive refactor
2. Server-side parsing: Matches OpenAI behavior exactly
3. No client changes needed: Existing clients just work
4. Proper format: OpenAI SDK compatibility out of the box
5. Well tested: Comprehensive unit tests included

Closes

Fixes #293

Checklist

• Server-side tool call parsing implemented
• OpenAI-compliant response format
• Streaming and non-streaming support
• Parallel tool calling support
• Unit tests added and passing
• Example code updated
• Backwards compatible
• No breaking changes

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Ready for review and merge! This implementation is production-ready and fully OpenAI-compatible.