Return HTTP 405 for GET and DELETE on @mcp/protocol

Streamable HTTP clients treat 404 as a terminated session. The MCP spec requires 405 when the server does not offer a GET SSE stream.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: rboixaderg

CHANGELOG.rst

@@ -12,6 +12,9 @@ CHANGELOG
   [rboixaderg]
 - Chore: restrict MCP version for Python 3.10 and above to ensure compatibility
   [rboixaderg]
+- MCP: return HTTP 405 on GET and DELETE ``@mcp/protocol`` so Streamable HTTP
+  clients treat the endpoint as JSON-only instead of a missing session.
+  [rboixaderg]
 
 
 7.1.3 (2026-06-24)

docs/source/contrib/mcp.md

@@ -64,6 +64,8 @@ Accept: application/json, text/event-stream
 
 Without the `Accept` header, the transport may respond with **406 Not Acceptable**.
 
+The endpoint is JSON-only and does not open a GET SSE stream. Per the MCP Streamable HTTP spec, `GET` and `DELETE` on `@mcp/protocol` return **405 Method Not Allowed** with `Allow: POST`, so clients continue with POST/JSON instead of treating a **404** as a dead session.
+
 Example: list tools via JSON-RPC:
 
 ```bash

guillotina/contrib/mcp/services.py

@@ -4,7 +4,26 @@
 from guillotina.contrib.mcp.interfaces import IMCPToolRegistry
 from guillotina.contrib.mcp.security import require_access_content
 from guillotina.interfaces import IResource
-from guillotina.response import HTTPNotFound, HTTPServiceUnavailable, Response
+from guillotina.response import HTTPMethodNotAllowed, HTTPNotFound, HTTPServiceUnavailable, Response
+
+
+_MCP_PROTOCOL_POST_METHODS = ["POST"]
+
+
+def _reject_non_post_protocol(context, request, method: str):
+    action = request.matchdict.get("action", "")
+    if action != "protocol":
+        raise HTTPNotFound(content={"reason": f"Unknown MCP {method} action: {action}"})
+    require_access_content(context)
+    if method == "DELETE":
+        reason = "MCP endpoint does not support session termination"
+    else:
+        reason = "MCP endpoint does not offer an SSE stream"
+    raise HTTPMethodNotAllowed(
+        method,
+        _MCP_PROTOCOL_POST_METHODS,
+        content={"reason": reason},
+    )
 
 
 def _get_registry():
@@ -72,3 +91,29 @@ async def _handle_protocol(self):
         resp._prepared = True
         resp._eof_sent = True
         return resp
+
+
+@configure.service(
+    method="GET",
+    context=IResource,
+    name="@mcp/{action}",
+    permission="guillotina.MCPExecute",
+    summary="MCP Streamable HTTP GET is not offered (JSON-only)",
+    allow_access=True,
+)
+class MCPActionGetService(Service):
+    async def __call__(self):
+        _reject_non_post_protocol(self.context, self.request, "GET")
+
+
+@configure.service(
+    method="DELETE",
+    context=IResource,
+    name="@mcp/{action}",
+    permission="guillotina.MCPExecute",
+    summary="MCP Streamable HTTP session termination is not offered",
+    allow_access=True,
+)
+class MCPActionDeleteService(Service):
+    async def __call__(self):
+        _reject_non_post_protocol(self.context, self.request, "DELETE")

guillotina/tests/mcp/test_mcp.py

@@ -899,6 +899,33 @@ async def test_protocol_unknown_action_returns_404(container_requester):
         assert status == 404
 
 
+def _assert_protocol_method_not_allowed(status, headers):
+    assert status == 405
+    allow = headers.get("Allow") or headers.get("allow") or ""
+    assert "POST" in allow.upper()
+
+
+@pytest.mark.app_settings(MCP_SETTINGS)
+async def test_protocol_get_returns_405_without_sse(container_requester):
+    async with container_requester as requester:
+        _, status, headers = await requester.make_request(
+            "GET",
+            "/db/guillotina/@mcp/protocol",
+            accept="text/event-stream",
+        )
+        _assert_protocol_method_not_allowed(status, headers)
+
+
+@pytest.mark.app_settings(MCP_SETTINGS)
+async def test_protocol_delete_returns_405(container_requester):
+    async with container_requester as requester:
+        _, status, headers = await requester.make_request(
+            "DELETE",
+            "/db/guillotina/@mcp/protocol",
+        )
+        _assert_protocol_method_not_allowed(status, headers)
+
+
 @pytest.mark.app_settings(MCP_SETTINGS)
 async def test_protocol_resource_registry_matches_defaults(container_requester):
     async with container_requester as requester: