fix(anthropic): fix duplicate ToolCallStartEvent in streaming tool calls (#3051)

When processing `input_json_delta` events, the Anthropic client was
passing the tool name from the previous `tool_use` event. This caused
ag-ui's `_handle_function_call_content` to emit a `ToolCallStartEvent`
for every streaming chunk (since it triggers on `if content.name:`).

This fix changes the behavior to pass an empty string for `name` in
`input_json_delta` events, matching OpenAI's behavior where streaming
argument chunks have `name=""`. The initial `tool_use` event still
provides the tool name, so only one `ToolCallStartEvent` is emitted.

Co-authored-by: Evan Mattson <35585003+moonbox3@users.noreply.github.com>

python/packages/ag-ui/tests/test_events_comprehensive.py

@@ -152,6 +152,42 @@ async def test_tool_call_streaming_args():
     assert events1[0].tool_call_id == events2[0].tool_call_id == events3[0].tool_call_id
 
 
+async def test_streaming_tool_call_no_duplicate_start_events():
+    """Test that streaming tool calls emit exactly one ToolCallStartEvent.
+
+    This is a regression test for the Anthropic streaming fix where input_json_delta
+    events were incorrectly passing the tool name, causing duplicate ToolCallStartEvents.
+
+    The correct behavior is:
+    - Initial FunctionCallContent with name -> emits ToolCallStartEvent
+    - Subsequent FunctionCallContent with name="" -> emits only ToolCallArgsEvent
+
+    See: https://github.com/microsoft/agent-framework/pull/3051
+    """
+    from agent_framework_ag_ui._events import AgentFrameworkEventBridge
+
+    bridge = AgentFrameworkEventBridge(run_id="test_run", thread_id="test_thread")
+
+    # Simulate streaming tool call: first chunk has name, subsequent chunks have name=""
+    update1 = AgentRunResponseUpdate(contents=[FunctionCallContent(name="get_weather", call_id="call_789")])
+    update2 = AgentRunResponseUpdate(contents=[FunctionCallContent(name="", call_id="call_789", arguments='{"loc":')])
+    update3 = AgentRunResponseUpdate(contents=[FunctionCallContent(name="", call_id="call_789", arguments='"SF"}')])
+
+    events1 = await bridge.from_agent_run_update(update1)
+    events2 = await bridge.from_agent_run_update(update2)
+    events3 = await bridge.from_agent_run_update(update3)
+
+    # Count all ToolCallStartEvents - should be exactly 1
+    all_events = events1 + events2 + events3
+    tool_call_start_count = sum(1 for e in all_events if e.type == "TOOL_CALL_START")
+    assert tool_call_start_count == 1, f"Expected 1 ToolCallStartEvent, got {tool_call_start_count}"
+
+    # Verify event types
+    assert events1[0].type == "TOOL_CALL_START"
+    assert events2[0].type == "TOOL_CALL_ARGS"
+    assert events3[0].type == "TOOL_CALL_ARGS"
+
+
 async def test_tool_result_with_dict():
     """Test FunctionResultContent with dict result."""
     from agent_framework_ag_ui._events import AgentFrameworkEventBridge

python/packages/anthropic/agent_framework_anthropic/_chat_client.py

@@ -832,11 +832,16 @@ class AnthropicClient(BaseChatClient):
                         )
                     )
                 case "input_json_delta":
-                    call_id, name = self._last_call_id_name if self._last_call_id_name else ("", "")
+                    # For streaming argument deltas, only pass call_id and arguments.
+                    # Pass empty string for name - it causes ag-ui to emit duplicate ToolCallStartEvents
+                    # since it triggers on `if content.name:`. The initial tool_use event already
+                    # provides the name, so deltas should only carry incremental arguments.
+                    # This matches OpenAI's behavior where streaming chunks have name="".
+                    call_id, _ = self._last_call_id_name if self._last_call_id_name else ("", "")
                     contents.append(
                         FunctionCallContent(
                             call_id=call_id,
-                            name=name,
+                            name="",
                             arguments=content_block.partial_json,
                             raw_representation=content_block,
                         )

python/packages/anthropic/tests/test_anthropic_client.py

@@ -595,6 +595,53 @@ def test_parse_contents_from_anthropic_tool_use(mock_anthropic_client: MagicMock
     assert result[0].name == "get_weather"
 
 
+def test_parse_contents_from_anthropic_input_json_delta_no_duplicate_name(mock_anthropic_client: MagicMock) -> None:
+    """Test that input_json_delta events have empty name to prevent duplicate ToolCallStartEvents.
+
+    When streaming tool calls, the initial tool_use event provides the name,
+    and subsequent input_json_delta events should have name="" to prevent
+    ag-ui from emitting duplicate ToolCallStartEvents.
+    """
+    chat_client = create_test_anthropic_client(mock_anthropic_client)
+
+    # First, simulate a tool_use event that sets _last_call_id_name
+    tool_use_content = MagicMock()
+    tool_use_content.type = "tool_use"
+    tool_use_content.id = "call_123"
+    tool_use_content.name = "get_weather"
+    tool_use_content.input = {}
+
+    result = chat_client._parse_contents_from_anthropic([tool_use_content])
+    assert len(result) == 1
+    assert isinstance(result[0], FunctionCallContent)
+    assert result[0].call_id == "call_123"
+    assert result[0].name == "get_weather"  # Initial event has name
+
+    # Now simulate input_json_delta events (argument streaming)
+    delta_content_1 = MagicMock()
+    delta_content_1.type = "input_json_delta"
+    delta_content_1.partial_json = '{"location":'
+
+    result = chat_client._parse_contents_from_anthropic([delta_content_1])
+    assert len(result) == 1
+    assert isinstance(result[0], FunctionCallContent)
+    assert result[0].call_id == "call_123"
+    assert result[0].name == ""  # Delta events should have empty name
+    assert result[0].arguments == '{"location":'
+
+    # Another delta
+    delta_content_2 = MagicMock()
+    delta_content_2.type = "input_json_delta"
+    delta_content_2.partial_json = '"San Francisco"}'
+
+    result = chat_client._parse_contents_from_anthropic([delta_content_2])
+    assert len(result) == 1
+    assert isinstance(result[0], FunctionCallContent)
+    assert result[0].call_id == "call_123"
+    assert result[0].name == ""  # Still empty name for subsequent deltas
+    assert result[0].arguments == '"San Francisco"}'
+
+
 # Stream Processing Tests