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agentsaaifmcp

Migrating a Real MCP Server to the 2026-07-28 Spec

·14 min read·ashwinexe

The 2026-07-28 revision made MCP stateless. Working through what that breaks in a server that already existed: sessions, multi round-trip requests, cacheable lists, and a governed extensions framework.

The 2026-07-28 MCP specification shipped on July 28, replacing 2025-11-25. It is the largest revision since authorization was added. MCP is now a stateless request/response protocol: the initialize handshake is gone, sessions are gone, and server-initiated requests are gone.

The changelog lists all of that. Working out which of your own design decisions were resting on the protocol takes longer.

So this walks the new spec through a server that already existed. Last month I sketched meetup-ops, a community operations server for running meetups, with five tools, one resource, and one prompt, built to prepare follow-up messages for a human to approve rather than send them itself. (That post has the full design; you do not need it to follow this one.) Three weeks later the protocol underneath it changed.

The Web Already Ran This Experiment

MCP was born local. The first spec, November 2024, assumed a desktop app spawning a server as a child process on the same machine and talking to it over stdio. In that world state is free. One client, one server, one long-lived process; the connection and the conversation are the same thing, and the server remembers you between calls because a pipe between two processes can do nothing else.

Remote MCP kept that assumption after it stopped being true. Streamable HTTP arrived in the 2025-03-26 revision and brought the Mcp-Session-Id header with it: a shared identifier that let a stateless protocol impersonate a pipe. It worked, but everything around it had to cooperate. Serving a session-holding server meant sticky routing so every request landed on the one instance that remembered the client, resumable streams so a dropped connection did not lose the conversation, and a shared session store the moment you ran a second instance.

The web ran this exact experiment and published the results. Cookies made HTTP remember; sticky sessions and session-replication clusters followed; and after years of operating those, the industry moved the state out of the connection, into databases and into signed tokens that the client carries and presents back. A server that must remember you cannot be replaced mid-conversation, so past a certain scale, nobody lets servers remember.

The 2026-07-28 revision is MCP landing on the same answer, the web's two decades compressed into twenty months. Every request carries its own context in _meta, so any replica can serve it. Cross-call state is named by handles and lives in a database. Even the new requestState field, an opaque blob the server hands the client to carry between retries, is the signed cookie reinvented: state pushed to the client, sealed so the server can trust it on return.

None of this made MCP servers stateless. My ledger is as stateful as it ever was. What the protocol dropped is the promise to carry that state on my behalf.

The full inventory is in the changelog; the short version is that everything connection-shaped went. The handshake, the session header, server-initiated requests, the GET notification stream, SSE resumability, ping. Tasks left the core for an extension, and every result now declares what kind of result it is. Each of these breaks a server written against 2025-11-25. Here is what they did to mine.

The Handshake Is Gone

Previously a connection began with initialize, the server replied with its capabilities, and both sides carried that agreement for the life of the session. Now each request is self-describing (SEP-2575). Client context travels in _meta:

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "list_checked_in_attendees",
    "arguments": { "eventId": "blr-2026-08" },
    "_meta": {
      "io.modelcontextprotocol/protocolVersion": "2026-07-28",
      "io.modelcontextprotocol/clientInfo": { "name": "goose", "version": "1.9.0" },
      "io.modelcontextprotocol/clientCapabilities": {
        "extensions": { "io.modelcontextprotocol/tasks": {} }
      }
    }
  }
}

Servers identify themselves in each result's _meta under io.modelcontextprotocol/serverInfo. Version mismatches return UnsupportedProtocolVersionError. Clients that want to negotiate up front call server/discover first, which every server MUST implement.

The practical consequence is deployment shape. A meetup-ops server can now sit behind an ordinary round-robin load balancer with no sticky routing and no shared session store. Two consecutive tools/call requests from the same organizer can land on different instances and neither instance needs to have seen the other's traffic.

Sessions Were Doing Work You Now Have to Name

Existing servers break here silently, because nothing in the code says the session was load-bearing.

A session was an implicit place to keep things. If prepare_delivery_outbox stashed the outbox it had just built and get_delivery_ledger read it back, the session made that work without either tool declaring a dependency. That affordance is gone. The spec's replacement is explicit (SEP-2567): servers that need cross-call state use server-minted handles passed as ordinary tool arguments.

So prepare_delivery_outbox now returns an identifier, and the tools that follow take it as input:

{
  "outboxId": "obx_01J9Z4KQ8XN2",
  "eventId": "blr-2026-08",
  "prepared": 47,
  "skipped": 3,
  "exceptions": []
}

get_delivery_ledger takes outboxId. mark_delivery_sent takes outboxId and the external message identifier. Nothing depends on which instance handled the previous call.

I read this as a good change rather than a tax. The original design already required that prepare_delivery_outbox use durable state, so that running it twice could not assign two benefit codes to the same attendee, and that mark_delivery_sent accept the same message identifier repeatedly without creating duplicate state. Statelessness makes that discipline mandatory instead of optional. The state that matters (assignments, delivery status, exception records) was always supposed to be in a durable ledger. The session was never a safe place for it, only a convenient one.

Handles are also attacker-controlled input. An outboxId arrives from the client on every call, so the server must authorize it against the caller rather than trusting that possession implies permission.

Human Approval Is Now a Protocol Pattern

The change I find most interesting for community operations is Multi Round-Trip Requests (MRTR, SEP-2322).

Servers can no longer initiate requests to clients. A server that needs user input mid-call returns an interim result instead. All results now carry a resultType field; "input_required" signals that the server needs something before it can finish:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "resultType": "input_required",
    "inputRequests": {
      "confirm_recipients": {
        "method": "elicitation/create",
        "params": {
          "mode": "form",
          "message": "47 attendees are queued. Approve sending benefit codes?",
          "requestedSchema": {
            "type": "object",
            "properties": { "approved": { "type": "boolean" } },
            "required": ["approved"]
          }
        }
      }
    },
    "requestState": "AEAD-protected blob"
  }
}

The client gathers the input and retries the original request with an inputResponses map keyed to the same identifiers, echoing requestState back verbatim. The retry must use a different JSON-RPC id, because these are two independent requests rather than a continuation. InputRequiredResult is permitted only on tools/call, resources/read, and prompts/get.

requestState is opaque to the client and carries whatever context the server needs to resume. The spec is blunt about the security posture: servers MUST treat it as attacker-controlled, MUST protect its integrity with HMAC or AEAD when it influences authorization or business logic, and SHOULD bind it to the authenticated principal, a short TTL, and a digest of the originating request. Integrity protection alone bounds the replay window but does not guarantee single-use. For a one-time redemption like assigning a benefit code, the server still has to enforce that invariant itself.

An approval token that can be replayed is an approval that can assign a code twice.

I deliberately left a bulk-send tool out of the first version of meetup-ops, because preparing and sending are different risk levels. MRTR does not change that judgment, but it does change what a send tool would look like: the approval gate is now expressible in the protocol, rather than improvised as a second tool call the model is merely instructed to make first.

Long Operations Moved to an Extension

Tasks left the experimental core and became io.modelcontextprotocol/tasks (SEP-2663). The redesign replaces the blocking tasks/result with polling via tasks/get, adds tasks/update for client-to-server input, and removes tasks/list.

A server returns a CreateTaskResult (resultType: "task") containing a taskId, ttlMs, and a suggested pollIntervalMs. The task must be durably created before the response is sent. Statuses are working, input_required, completed, failed, and cancelled, the last three terminal.

This overlaps with MRTR: a task that needs input moves to input_required and surfaces an inputRequests map, which the client answers via tasks/update. The rule of thumb is duration. MRTR handles a pause the client can resolve in seconds. Tasks handle work that outlives the connection, like a batch preparation across a few thousand registrations, or an approval gate waiting on a human who has stepped away from their desk.

Servers MUST NOT return a task to a client that did not declare the extension in its per-request capabilities.

Lists Are Cacheable

tools/list, prompts/list, resources/list, resources/read, and resources/templates/list now return required ttlMs and cacheScope fields (SEP-2549). ttlMs is a freshness hint in milliseconds; cacheScope is "public" or "private" and controls whether shared intermediaries may cache the response.

For meetup://events/{id}/brief the scope is "private". An event brief is internal, and a shared cache in front of the server is not somewhere it belongs. A short TTL is fine, since briefs change but not per-request.

Servers SHOULD also return tools from tools/list in a deterministic order. A stable tool list means a stable prompt prefix, which means LLM prompt cache hits, and sorting your tool registry is close to free.

Two other transport-level details. Streamable HTTP POSTs now require Mcp-Method and Mcp-Name headers, so gateways can route, meter, and rate-limit without parsing the JSON body, which means you can rate-limit prepare_delivery_outbox at the edge. And SSE resumability is gone: no Last-Event-ID, no redelivery. A broken stream loses the in-flight request and the client must re-issue it as a new request with a new id. Idempotent tools survive this. Non-idempotent ones do not.

Extensions Became a Governed System

Extensions were previously an informal convention. They now have reverse-DNS identifiers ({vendor-prefix}/{extension-name}), dedicated ext-* repositories in the MCP GitHub organization, delegated maintainers, and versioning independent of the core spec. Official extensions use io.modelcontextprotocol; a third party uses a reversed domain it owns. Extensions are disabled by default and require explicit opt-in.

Negotiation is symmetric: clients declare extensions in io.modelcontextprotocol/clientCapabilities in each request's _meta, servers advertise theirs in the server/discover response. If one side does not support an extension, the other falls back to core behavior or rejects the request if the extension is mandatory.

Official extensions today cover authorization (OAuth client credentials, Enterprise-Managed Authorization), MCP Apps for server-rendered interactive UI in sandboxed iframes, and Tasks. Experimental work incubates in experimental-ext-* repositories tied to a working group, and graduates through the SEP Extensions Track with at least one reference implementation required before review.

This is the mechanism I care most about, because it is where skills land. SEP-2640, the Skills Extension (identifier io.modelcontextprotocol/skills), is Extensions Track and still in review, from the Skills Over MCP Working Group. Its design serves agent skills as ordinary MCP Resources under skill:// URIs, with the primary content at skill://<skill-path>/SKILL.md and supporting files as siblings. The catalog surface is still being settled between resources/list enumeration, resource templates, and an index document.

Skills work by progressive disclosure: the model learns that a skill exists from a small catalog entry, and reads the full instructions only when it needs them. Skills are resources, resources/read now carries ttlMs and cacheScope, and list results no longer vary per connection, so a skill catalog is cacheable across clients rather than re-fetched per session.

The Revised meetup-ops Interface

PrimitiveNameDoesChanged by this revision
Resourcemeetup://events/{id}/briefEvent context, no attendee recordsReturns ttlMs; cacheScope: "private"
Toollist_checked_in_attendeesReads approved attendees with a check-in timestampNothing; still read-only
Toolprepare_delivery_outboxAssigns each new attendee one benefit code, queues unsent messagesMints and returns outboxId; may return input_required
Toolget_delivery_ledgerReturns assigned, pending, sent, exception countsTakes outboxId as an argument
Toolmark_delivery_sentRecords the external message id after a confirmed sendTakes outboxId; idempotency now load-bearing
Promptprepare_checkin_followupWalks a user through preparing the queueMay return input_required

Tool annotations (readOnlyHint, destructiveHint, idempotentHint, openWorldHint) are unchanged and still hints. The server still enforces its own guarantees.

What I Am Deprecating

The spec adopted a formal feature lifecycle: Active, Deprecated, Removed, with a minimum twelve-month window between deprecation and earliest removal. For features deprecated in this release, that puts the earliest possible removal at July 28, 2027.

DeprecatedSuggested migration
RootsPass directories or files as tool parameters, resource URIs, or server config
SamplingIntegrate directly with an LLM provider API
Loggingstderr on stdio, or OpenTelemetry
HTTP+SSE transportStreamable HTTP
Dynamic Client RegistrationClient ID Metadata Documents (CIMD)

Authorization also hardened: authorization servers SHOULD include the iss parameter per RFC 9207 and clients MUST validate it before redeeming the code; clients MUST key persisted credentials by issuer and re-register when the authorization server changes; and DCR now requires an appropriate application_type to avoid OpenID Connect redirect URI conflicts.

Error codes were also renumbered, which is quieter but will bite. -32000 to -32019 stays implementation-defined, -32020 to -32099 is reserved for the specification, and the codes introduced in this cycle moved accordingly: HeaderMismatch to -32020, MissingRequiredClientCapability to -32021, UnsupportedProtocolVersion to -32022. Resource-not-found also changed from -32002 to -32602 to match JSON-RPC's Invalid Params. If you assert on error codes in tests, they will fail.

A Migration Checklist

  1. Find every place a session was holding state. Replace it with a server-minted handle taken as a tool argument, or move it to the durable ledger where it belonged.
  2. Implement server/discover.
  3. Add resultType to every result. Treat a missing resultType from an older server as "complete".
  4. Convert server-initiated elicitation/create, sampling/createMessage, and roots/list calls to MRTR. Sign and TTL-bound your requestState.
  5. Add ttlMs and cacheScope to all five list-and-read endpoints. Sort tools/list.
  6. Emit Mcp-Method and Mcp-Name on Streamable HTTP POSTs.
  7. Audit non-idempotent tools for stream-loss retries now that SSE resumability is gone.
  8. Update error code assertions.
  9. Declare extensions explicitly. They are off by default.

Tier 1 SDKs (TypeScript, Python, Go, and C#) supported 2026-07-28 at release; Rust is in beta. The MCP Inspector remains the fastest way to verify discovery, schemas, and error handling before wiring a server into a goose recipe.

What Still Belongs to a Human

meetup-ops started from one rule: the server prepares, a human approves, and only then does anything reach an attendee. Open interfaces do not remove accountability. This revision sharpens that point rather than softening it.

MCP now has a real place to put a human decision. input_required is a first-class result, and an approval gate is a protocol state with a signed, expiring token behind it rather than a convention improvised in a system prompt.

The protocol can guarantee that someone was asked. It cannot guarantee that the question was the right one, that the person had enough context to answer it well, or that approving forty-seven recipients in a single form meant anything more than clicking through. A signed requestState proves an approval was not replayed. It proves nothing about whether the approval was considered.

References

Thanks for reading — @ashwinexe

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