Fix RunUsage.tool_calls being undercounted due to race condition when running tools in parallel

pydantic_ai_slim/pydantic_ai/_agent_graph.py

@@ -426,7 +426,7 @@ async def stream(
 
         model_response = streamed_response.get()
 
-        self._finish_handling(ctx, model_response)
+        await self._finish_handling(ctx, model_response)
         assert self._result is not None  # this should be set by the previous line
 
     async def _make_request(
@@ -439,7 +439,7 @@ async def _make_request(
         model_response = await ctx.deps.model.request(message_history, model_settings, model_request_parameters)
         ctx.state.usage.requests += 1
 
-        return self._finish_handling(ctx, model_response)
+        return await self._finish_handling(ctx, model_response)
 
     async def _prepare_request(
         self, ctx: GraphRunContext[GraphAgentState, GraphAgentDeps[DepsT, NodeRunEndT]]
@@ -481,7 +481,7 @@ async def _prepare_request(
 
         return model_settings, model_request_parameters, message_history, run_context
 
-    def _finish_handling(
+    async def _finish_handling(
         self,
         ctx: GraphRunContext[GraphAgentState, GraphAgentDeps[DepsT, NodeRunEndT]],
         response: _messages.ModelResponse,

pydantic_ai_slim/pydantic_ai/_tool_manager.py

@@ -1,10 +1,12 @@
 from __future__ import annotations
 
+import asyncio
 import json
 from collections.abc import Iterator
 from contextlib import contextmanager
 from contextvars import ContextVar
 from dataclasses import dataclass, field, replace
+from functools import cached_property
 from typing import Any, Generic
 
 from opentelemetry.trace import Tracer
@@ -72,6 +74,11 @@ def tool_defs(self) -> list[ToolDefinition]:
 
         return [tool.tool_def for tool in self.tools.values()]
 
+    @cached_property
+    def _usage_lock(self) -> asyncio.Lock:
+        """Lock to prevent race conditions when incrementing usage.tool_calls from concurrent tool executions."""
+        return asyncio.Lock()
+
     def should_call_sequentially(self, calls: list[ToolCallPart]) -> bool:
         """Whether to require sequential tool calls for a list of tool calls."""
         return _sequential_tool_calls_ctx_var.get() or any(
@@ -234,7 +241,8 @@ async def _call_function_tool(
         ) as span:
             try:
                 tool_result = await self._call_tool(call, allow_partial, wrap_validation_errors)
-                usage.tool_calls += 1
+                async with self._usage_lock:
+                    usage.tool_calls += 1
 
             except ToolRetryError as e:
                 part = e.tool_retry

tests/test_usage_limits.py

@@ -17,6 +17,7 @@
     ModelRequest,
     ModelResponse,
     RunContext,
+    TextPart,
     ToolCallPart,
     ToolReturnPart,
     UsageLimitExceeded,
@@ -355,6 +356,41 @@ def test_deprecated_usage_limits():
         assert UsageLimits(output_tokens_limit=100).response_tokens_limit == 100  # type: ignore
 
 
+async def test_race_condition_parallel_tool_calls():
+    """Test that demonstrates race condition in parallel tool execution.
+
+    This test would fail intermittently on main without the fix because multiple
+    asyncio tasks calling usage.incr() can interleave their read-modify-write operations.
+    """
+    # Run multiple iterations to increase chance of catching race condition
+    for iteration in range(20):
+        call_count = 0
+
+        def parallel_tools_model(messages: list[ModelMessage], info: AgentInfo) -> ModelResponse:
+            nonlocal call_count
+            call_count += 1
+            if call_count == 1:
+                # Return 10 parallel tool calls for more contention
+                return ModelResponse(parts=[ToolCallPart('tool_a', {}, f'call_{i}') for i in range(10)])
+            else:
+                # Return final text response
+                return ModelResponse(parts=[TextPart(content='done')])
+
+        agent = Agent(FunctionModel(parallel_tools_model))
+
+        @agent.tool_plain
+        async def tool_a() -> str:
+            # Add multiple await points to increase chance of task interleaving
+            await asyncio.sleep(0.0001)
+            await asyncio.sleep(0.0001)
+            return 'result'
+
+        result = await agent.run('test')
+        # Without proper synchronization, tool_calls might be undercounted
+        actual = result.usage().tool_calls
+        assert actual == 10, f'Iteration {iteration}: Expected 10 tool calls, got {actual}'
+
+
 async def test_parallel_tool_calls_limit_enforced():
     """Parallel tool calls must not exceed the limit and should raise immediately."""
     executed_tools: list[str] = []