progress_token is 0 on first tool-call and the return is taken mistakenly

src/mcp/server/fastmcp/server.py

@@ -599,7 +599,7 @@ async def report_progress(
             else None
         )
 
-        if not progress_token:
+        if progress_token is None:
             return
 
         await self.request_context.session.send_progress_notification(

tests/issues/test_176_progress_token.py

@@ -0,0 +1,46 @@
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+from mcp.server.fastmcp import Context
+from mcp.shared.context import RequestContext
+
+pytestmark = pytest.mark.anyio
+
+
+async def test_progress_token_zero_first_call():
+    """Test that progress notifications work when progress_token is 0 on first call."""
+
+    # Create mock session with progress notification tracking
+    mock_session = AsyncMock()
+    mock_session.send_progress_notification = AsyncMock()
+
+    # Create request context with progress token 0
+    mock_meta = MagicMock()
+    mock_meta.progressToken = 0  # This is the key test case - token is 0
+
+    request_context = RequestContext(
+        request_id="test-request", session=mock_session, meta=mock_meta
+    )
+
+    # Create context with our mocks
+    ctx = Context(request_context=request_context, fastmcp=MagicMock())
+
+    # Test progress reporting
+    await ctx.report_progress(0, 10)  # First call with 0
+    await ctx.report_progress(5, 10)  # Middle progress
+    await ctx.report_progress(10, 10)  # Complete
+
+    # Verify progress notifications
+    assert (
+        mock_session.send_progress_notification.call_count == 3
+    ), "All progress notifications should be sent"
+    mock_session.send_progress_notification.assert_any_call(
+        progress_token=0, progress=0.0, total=10.0
+    )
+    mock_session.send_progress_notification.assert_any_call(
+        progress_token=0, progress=5.0, total=10.0
+    )
+    mock_session.send_progress_notification.assert_any_call(
+        progress_token=0, progress=10.0, total=10.0
+    )

tests/shared/test_sse.py

@@ -164,6 +164,7 @@ async def http_client(server, server_url) -> AsyncGenerator[httpx.AsyncClient, N
 async def test_raw_sse_connection(http_client: httpx.AsyncClient) -> None:
     """Test the SSE connection establishment simply with an HTTP client."""
     async with anyio.create_task_group():
+
         async def connection_test() -> None:
             async with http_client.stream("GET", "/sse") as response:
                 assert response.status_code == 200
@@ -211,7 +212,6 @@ async def initialized_sse_client_session(
             yield session
 
 
-
 @pytest.mark.anyio
 async def test_sse_client_happy_request_and_response(
     initialized_sse_client_session: ClientSession,