fix: Replace arbitrary sleeps with active server readiness checks in tests (#1527)

Co-authored-by: Claude <noreply@anthropic.com>

tests/server/test_sse_security.py

@@ -3,7 +3,6 @@
 import logging
 import multiprocessing
 import socket
-import time
 
 import httpx
 import pytest
@@ -17,6 +16,7 @@ from mcp.server import Server
 from mcp.server.sse import SseServerTransport
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.types import Tool
+from tests.test_helpers import wait_for_server
 
 logger = logging.getLogger(__name__)
 SERVER_NAME = "test_sse_security_server"
@@ -66,26 +66,6 @@ def run_server_with_settings(port: int, security_settings: TransportSecuritySett
     uvicorn.run(starlette_app, host="127.0.0.1", port=port, log_level="error")
 
 
-def wait_for_server(port: int, timeout: float = 5.0) -> None:
-    """Wait for server to be ready to accept connections.
-
-    Polls the server port until it accepts connections or timeout is reached.
-    This eliminates race conditions without arbitrary sleeps.
-    """
-    start_time = time.time()
-    while time.time() - start_time < timeout:
-        try:
-            with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
-                s.settimeout(0.1)
-                s.connect(("127.0.0.1", port))
-                # Server is ready
-                return
-        except (ConnectionRefusedError, OSError):
-            # Server not ready yet, retry quickly
-            time.sleep(0.01)
-    raise TimeoutError(f"Server on port {port} did not start within {timeout} seconds")
-
-
 def start_server_process(port: int, security_settings: TransportSecuritySettings | None = None):
     """Start server in a separate process."""
     process = multiprocessing.Process(target=run_server_with_settings, args=(port, security_settings))

tests/server/test_streamable_http_security.py

@@ -3,7 +3,6 @@
 import logging
 import multiprocessing
 import socket
-import time
 from collections.abc import AsyncGenerator
 from contextlib import asynccontextmanager
 
@@ -18,6 +17,7 @@ from mcp.server import Server
 from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.types import Tool
+from tests.test_helpers import wait_for_server
 
 logger = logging.getLogger(__name__)
 SERVER_NAME = "test_streamable_http_security_server"
@@ -77,8 +77,8 @@ def start_server_process(port: int, security_settings: TransportSecuritySettings
     """Start server in a separate process."""
     process = multiprocessing.Process(target=run_server_with_settings, args=(port, security_settings))
     process.start()
-    # Give server time to start
-    time.sleep(1)
+    # Wait for server to be ready to accept connections
+    wait_for_server(port)
     return process
 
 

tests/shared/test_sse.py

@@ -32,6 +32,7 @@ from mcp.types import (
     TextResourceContents,
     Tool,
 )
+from tests.test_helpers import wait_for_server
 
 SERVER_NAME = "test_server_for_SSE"
 
@@ -123,19 +124,8 @@ def server(server_port: int) -> Generator[None, None, None]:
     proc.start()
 
     # Wait for server to be running
-    max_attempts = 20
-    attempt = 0
     print("waiting for server to start")
-    while attempt < max_attempts:
-        try:
-            with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
-                s.connect(("127.0.0.1", server_port))
-                break
-        except ConnectionRefusedError:
-            time.sleep(0.1)
-            attempt += 1
-    else:
-        raise RuntimeError(f"Server failed to start after {max_attempts} attempts")
+    wait_for_server(server_port)
 
     yield
 

tests/shared/test_streamable_http.py

@@ -7,7 +7,6 @@ Contains tests for both server and client sides of the StreamableHTTP transport.
 import json
 import multiprocessing
 import socket
-import time
 from collections.abc import Generator
 from typing import Any
 
@@ -43,6 +42,7 @@ from mcp.shared.exceptions import McpError
 from mcp.shared.message import ClientMessageMetadata
 from mcp.shared.session import RequestResponder
 from mcp.types import InitializeResult, TextContent, TextResourceContents, Tool
+from tests.test_helpers import wait_for_server
 
 # Test constants
 SERVER_NAME = "test_streamable_http_server"
@@ -344,18 +344,7 @@ def basic_server(basic_server_port: int) -> Generator[None, None, None]:
     proc.start()
 
     # Wait for server to be running
-    max_attempts = 20
-    attempt = 0
-    while attempt < max_attempts:
-        try:
-            with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
-                s.connect(("127.0.0.1", basic_server_port))
-                break
-        except ConnectionRefusedError:
-            time.sleep(0.1)
-            attempt += 1
-    else:
-        raise RuntimeError(f"Server failed to start after {max_attempts} attempts")
+    wait_for_server(basic_server_port)
 
     yield
 
@@ -391,18 +380,7 @@ def event_server(
     proc.start()
 
     # Wait for server to be running
-    max_attempts = 20
-    attempt = 0
-    while attempt < max_attempts:
-        try:
-            with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
-                s.connect(("127.0.0.1", event_server_port))
-                break
-        except ConnectionRefusedError:
-            time.sleep(0.1)
-            attempt += 1
-    else:
-        raise RuntimeError(f"Server failed to start after {max_attempts} attempts")
+    wait_for_server(event_server_port)
 
     yield event_store, f"http://127.0.0.1:{event_server_port}"
 
@@ -422,18 +400,7 @@ def json_response_server(json_server_port: int) -> Generator[None, None, None]:
     proc.start()
 
     # Wait for server to be running
-    max_attempts = 20
-    attempt = 0
-    while attempt < max_attempts:
-        try:
-            with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
-                s.connect(("127.0.0.1", json_server_port))
-                break
-        except ConnectionRefusedError:
-            time.sleep(0.1)
-            attempt += 1
-    else:
-        raise RuntimeError(f"Server failed to start after {max_attempts} attempts")
+    wait_for_server(json_server_port)
 
     yield
 
@@ -1407,18 +1374,7 @@ def context_aware_server(basic_server_port: int) -> Generator[None, None, None]:
     proc.start()
 
     # Wait for server to be running
-    max_attempts = 20
-    attempt = 0
-    while attempt < max_attempts:
-        try:
-            with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
-                s.connect(("127.0.0.1", basic_server_port))
-                break
-        except ConnectionRefusedError:
-            time.sleep(0.1)
-            attempt += 1
-    else:
-        raise RuntimeError(f"Context-aware server failed to start after {max_attempts} attempts")
+    wait_for_server(basic_server_port)
 
     yield
 

tests/shared/test_ws.py

@@ -26,6 +26,7 @@ from mcp.types import (
     TextResourceContents,
     Tool,
 )
+from tests.test_helpers import wait_for_server
 
 SERVER_NAME = "test_server_for_WS"
 
@@ -110,19 +111,8 @@ def server(server_port: int) -> Generator[None, None, None]:
     proc.start()
 
     # Wait for server to be running
-    max_attempts = 20
-    attempt = 0
     print("waiting for server to start")
-    while attempt < max_attempts:
-        try:
-            with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
-                s.connect(("127.0.0.1", server_port))
-                break
-        except ConnectionRefusedError:
-            time.sleep(0.1)
-            attempt += 1
-    else:
-        raise RuntimeError(f"Server failed to start after {max_attempts} attempts")
+    wait_for_server(server_port)
 
     yield
 

tests/test_helpers.py

@@ -0,0 +1,31 @@
+"""Common test utilities for MCP server tests."""
+
+import socket
+import time
+
+
+def wait_for_server(port: int, timeout: float = 5.0) -> None:
+    """Wait for server to be ready to accept connections.
+
+    Polls the server port until it accepts connections or timeout is reached.
+    This eliminates race conditions without arbitrary sleeps.
+
+    Args:
+        port: The port number to check
+        timeout: Maximum time to wait in seconds (default 5.0)
+
+    Raises:
+        TimeoutError: If server doesn't start within the timeout period
+    """
+    start_time = time.time()
+    while time.time() - start_time < timeout:
+        try:
+            with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
+                s.settimeout(0.1)
+                s.connect(("127.0.0.1", port))
+                # Server is ready
+                return
+        except (ConnectionRefusedError, OSError):
+            # Server not ready yet, retry quickly
+            time.sleep(0.01)
+    raise TimeoutError(f"Server on port {port} did not start within {timeout} seconds")