fix: return 405 for pre-session GET the server won't serve as SSE

src/mcp/server/streamable_http.py

@@ -656,11 +656,29 @@ async def _handle_get_request(self, request: Request, send: Send) -> None:
         This allows the server to communicate to the client without the client
         first sending data via HTTP POST. The server can send JSON-RPC requests
         and notifications on this stream.
+
+        Per MCP spec: "The server MUST either return Content-Type: text/event-stream
+        in response to this HTTP GET, or else return HTTP 405 Method Not Allowed."
         """
         writer = self._read_stream_writer
         if writer is None:  # pragma: no cover
             raise ValueError("No read stream writer available. Ensure connect() is called first.")
 
+        # Per MCP spec, pre-session GETs that cannot be served as SSE must return
+        # 405 Method Not Allowed. A GET without a session ID in stateful mode
+        # cannot establish an SSE stream because no session exists yet.
+        if self.mcp_session_id and not self._get_session_id(request):
+            headers = {
+                "Allow": "GET, POST, DELETE",
+            }
+            response = self._create_error_response(
+                "Method Not Allowed: GET requires an established session",
+                HTTPStatus.METHOD_NOT_ALLOWED,
+                headers=headers,
+            )
+            await response(request.scope, request.receive, send)
+            return
+
         # Validate Accept header - must include text/event-stream
         _, has_sse = check_accept_headers(request)
 

tests/interaction/_requirements.py

@@ -3094,6 +3094,16 @@ def __post_init__(self) -> None:
         transports=("streamable-http",),
         note="Only observable over HTTP: 405 is an HTTP status code.",
     ),
+    "hosting:http:pre-session-get-405": Requirement(
+        source=f"{SPEC_BASE_URL}/basic/transports#receiving-messages-from-the-server",
+        behavior=(
+            "A GET without a session ID in stateful mode returns 405 Method Not Allowed. "
+            "Per MCP spec: 'The server MUST either return Content-Type: text/event-stream in response "
+            "to this HTTP GET, or else return HTTP 405 Method Not Allowed.'"
+        ),
+        transports=("streamable-http",),
+        note="Only observable over HTTP: 405 is an HTTP status code. Fixes issue #3102.",
+    ),
     "hosting:http:no-broadcast": Requirement(
         source=f"{SPEC_BASE_URL}/basic/transports#multiple-connections",
         behavior=(

tests/interaction/transports/test_hosting_http.py

@@ -95,6 +95,54 @@ async def test_unsupported_http_methods_return_405() -> None:
     assert (patch.status_code, patch.headers.get("allow")) == snapshot((405, "GET, POST, DELETE"))
 
 
+@requirement("hosting:http:pre-session-get-405")
+async def test_pre_session_get_returns_405() -> None:
+    """A GET without a session ID in stateful mode returns 405 Method Not Allowed.
+
+    Per MCP spec: "The server MUST either return Content-Type: text/event-stream in response
+    to this HTTP GET, or else return HTTP 405 Method Not Allowed." A pre-session GET cannot
+    establish an SSE stream, so 405 is the spec-mandated response.
+
+    See: https://github.com/modelcontextprotocol/python-sdk/issues/3102
+    """
+    async with mounted_app(_server()) as (http, _):
+        # Test with various Accept headers - all should return 405 for pre-session GET
+        # Accept: */* (wildcard)
+        response_wildcard = await http.get("/mcp", headers={"accept": "*/*", "mcp-protocol-version": "2025-11-25"})
+        # Accept: application/json (no SSE)
+        response_json = await http.get(
+            "/mcp", headers={"accept": "application/json", "mcp-protocol-version": "2025-11-25"}
+        )
+        # No Accept header at all
+        response_no_accept = await http.get("/mcp", headers={"mcp-protocol-version": "2025-11-25"})
+        # Accept: text/event-stream (correct, but still no session)
+        response_sse = await http.get(
+            "/mcp", headers={"accept": "text/event-stream", "mcp-protocol-version": "2025-11-25"}
+        )
+
+    # All pre-session GETs must return 405 with Allow header listing supported methods
+    assert (response_wildcard.status_code, response_wildcard.headers.get("allow")) == snapshot(
+        (405, "GET, POST, DELETE")
+    )
+    assert "Method Not Allowed" in response_wildcard.json()["error"]["message"]
+    assert response_wildcard.json()["error"]["code"] == -32600  # INVALID_REQUEST
+
+    assert (response_json.status_code, response_json.headers.get("allow")) == snapshot(
+        (405, "GET, POST, DELETE")
+    )
+    assert "Method Not Allowed" in response_json.json()["error"]["message"]
+
+    assert (response_no_accept.status_code, response_no_accept.headers.get("allow")) == snapshot(
+        (405, "GET, POST, DELETE")
+    )
+    assert "Method Not Allowed" in response_no_accept.json()["error"]["message"]
+
+    assert (response_sse.status_code, response_sse.headers.get("allow")) == snapshot(
+        (405, "GET, POST, DELETE")
+    )
+    assert "Method Not Allowed" in response_sse.json()["error"]["message"]
+
+
 @requirement("hosting:http:accept-406")
 async def test_missing_accept_media_types_return_406() -> None:
     """A POST whose Accept header lacks both required types, or a GET lacking text/event-stream, returns 406."""

tests/server/test_streamable_http_security.py

@@ -124,7 +124,8 @@ async def test_streamable_http_security_get_request() -> None:
         assert response.text == "Invalid Host header"
 
         response = await client.get("/", headers={"Accept": "text/event-stream", "Host": "127.0.0.1"})
-        # An allowed host passes security and fails on session validation instead.
-        assert response.status_code == 400
+        # An allowed host passes security but fails because GET requires an established session.
+        # Per MCP spec, pre-session GETs return 405 Method Not Allowed (issue #3102).
+        assert response.status_code == 405
         body = response.json()
-        assert "Missing session ID" in body["error"]["message"]
+        assert "Method Not Allowed" in body["error"]["message"]