Add sse_retry_interval setting to FastMCP

Expose the retry_interval parameter through FastMCP so servers can
configure the SSE retry field in priming events. This allows clients
to know the recommended reconnection interval.

- Add sse_retry_interval to Settings class
- Pass through to StreamableHTTPSessionManager
- Update everything_server to use 3000ms interval for conformance tests

Author: felixweinberger

examples/servers/everything-server/mcp_everything_server/server.py

@@ -88,6 +88,7 @@ async def replay_events_after(
 mcp = FastMCP(
     name="mcp-conformance-test-server",
     event_store=event_store,
+    sse_retry_interval=3000,  # 3 seconds
 )
 
 

src/mcp/server/fastmcp/server.py

@@ -102,6 +102,8 @@ class Settings(BaseSettings, Generic[LifespanResultT]):
     json_response: bool
     stateless_http: bool
     """Define if the server should create a new transport per request."""
+    sse_retry_interval: int | None
+    """SSE retry interval in milliseconds sent in priming event for client reconnection."""
 
     # resource settings
     warn_on_duplicate_resources: bool
@@ -161,6 +163,7 @@ def __init__(  # noqa: PLR0913
         streamable_http_path: str = "/mcp",
         json_response: bool = False,
         stateless_http: bool = False,
+        sse_retry_interval: int | None = None,
         warn_on_duplicate_resources: bool = True,
         warn_on_duplicate_tools: bool = True,
         warn_on_duplicate_prompts: bool = True,
@@ -180,6 +183,7 @@ def __init__(  # noqa: PLR0913
             streamable_http_path=streamable_http_path,
             json_response=json_response,
             stateless_http=stateless_http,
+            sse_retry_interval=sse_retry_interval,
             warn_on_duplicate_resources=warn_on_duplicate_resources,
             warn_on_duplicate_tools=warn_on_duplicate_tools,
             warn_on_duplicate_prompts=warn_on_duplicate_prompts,
@@ -939,6 +943,7 @@ def streamable_http_app(self) -> Starlette:
                 json_response=self.settings.json_response,
                 stateless=self.settings.stateless_http,  # Use the stateless setting
                 security_settings=self.settings.transport_security,
+                retry_interval=self.settings.sse_retry_interval,
             )
 
         # Create the ASGI handler