createmcps.com

MCP-DEP-004

No legacy HTTP+SSE endpoint is served

“New implementations SHOULD NOT adopt it; existing implementations SHOULD migrate to Streamable HTTP.” — spec anchor

How to fix this

Stop serving the legacy HTTP+SSE endpoint and migrate to Streamable HTTP. New implementations should not adopt the old transport at all.

Example: failing and passing

A GET to the endpoint opens a legacy HTTP+SSE stream and sends an endpoint event, the deprecated 2024-11-05 transport.
Violates MCP-DEP-004 · HTTP
GET /mcp HTTP/1.1
Host: mcp.example.com
Accept: application/json, text/event-stream

HTTP/1.1 200 OK
Content-Type: text/event-stream

event: endpoint
data: https://mcp.example.com/messages
GET is refused with 405, so no legacy SSE stream is served.
Complies with MCP-DEP-004 · HTTP
GET /mcp HTTP/1.1
Host: mcp.example.com
Accept: application/json, text/event-stream

HTTP/1.1 405 Method Not Allowed
Allow: POST

How the validator checks this

Probe P5.1, against a streamable-http server on the modern / legacy / dual protocol. What it reports:

pass
No legacy HTTP+SSE stream served on GET
warn
GET returned text/event-stream — that is the deprecated HTTP+SSE transport; new implementations should not adopt it

Quoted from the probe that runs this check, so it cannot drift from what the validator actually reports.

This rule is checked deterministically: a fail here is a certain violation, not an inference.

Checked in the same request as MCP-STL-005.

Check your own server against MCP-DEP-004

Probe P5.1 checks MCP-DEP-004 in the same request as 1 other rule, so one run shows your server's result for this rule next to the rest. Validate a server or read how the validator works.

Background on this topic: the 2026-07-28 migration guide.

Other Deprecated and removed features rules

  • MCP-DEP-001ping is no longer implemented
  • MCP-DEP-002logging/setLevel is no longer implemented
  • MCP-DEP-003resources/subscribe and resources/unsubscribe replaced by subscriptions/listen
  • MCP-DEP-005Roots / Sampling / Logging capabilities are not advertised
  • MCP-DEP-006Server accepts initialize (dual-era support)
  • MCP-DEP-007A modern-only server names its supported versions in the initialize error