Official MCP SDK still downgrades 2026-07-28 handshakes to the 2025-11-25 era, probe finds

The failure sits in one field of the initialize reply. The client names the era it wants, and the server's answer carries the era it will actually speak ^[4]. A client that requires 2026-07-28 and gets 2025-11-25 back either errors somewhere far from the cause or quietly drops into compatibility mode, according to the post's authors on dev.to ^[4]. Nothing in the server logs marks the moment ^[4]. The Inspector is built to connect, and it does ^[3]. A green local suite shows only that the test client was willing to settle for an older era ^[3].

The pattern first showed up in the official SDK's issue tracker as typescript-sdk#2977 ^[14]. The downgrade on 2.3.1 comes from upstream of any team's own code. The authors did not find the July revision in any of the org's npm packages they probed, including the v2 server line ^[7]. Their check on 2026-10-10 came 74 days after the date the revision carries ^[20]. "Installing the new major does not mean getting the new era," they wrote ^[11].

We think the probe is scoped correctly for a CI gate. It checks compatibility, not conformance, so a server that negotiates 2026-07-28 and then mishandles a tool call would still pass ^[9]. That is the property default clients hide, and the probe tests only that property.

The build is small. It is one file with no dependencies, MIT-licensed, with fixtures that reproduce each behaviour the post describes ^[10]. For each known era it opens a real initialize handshake, records the reply, then calls tools/list to confirm the server is still alive ^[5]. The sample output walks four eras, newest first, from 2026-07-28 down to 2025-03-26 ^[12]. The order is deliberate: a stateful server grants a single handshake, and the authors want it spent on the most informative era ^[17]. Over HTTP the whole run is a handful of small POSTs, with no tool calls and no data read ^[9]. A server that answers initialize but cannot list tools gets its own verdict instead of a pass ^[9].

The DeepWiki result is a single endpoint on a single day ^[18]. The post does not say which SDK that server runs, so it cannot show whether the downgrade there comes from the same package. The authors present it as a snapshot of an ecosystem where, they wrote, "the 2026-07-28 revision barely exists in the wild yet" ^[2].