On May 21, 2026, the MCP team locked the 2026-07-28 Release Candidate — the largest revision to the Model Context Protocol since its initial release. The final specification publishes July 28. SDK maintainers have ten weeks to ship support. Production MCP servers have until the same date to comply.

This is not a minor version bump. Six material changes arrive simultaneously. If you run a production MCP server, you need a migration plan.

What the RC Changes

The 2026-07-28 RC delivers on the 2026 MCP roadmap in four areas: a stateless protocol core, the Extensions framework (MCP Apps and Tasks), authorization hardening, and a formal deprecation policy. Three of those four areas introduce breaking changes.

Breaking Change 1 — Sessions Are Gone

The single biggest structural change: the protocol layer no longer has a session concept.

Under the current 2025-11-25 specification, clients establish a session at connection time and the server uses the session ID to retrieve the client’s metadata, capability set, and negotiated protocol version. Every stateful remote MCP server needs a session store to share state across instances, sticky-session routing at the load balancer, and logic to expire or recover sessions.

The 2026-07-28 RC eliminates all of that at the protocol layer. Client metadata, capabilities, and protocol version now travel in the _meta field on every request. Every request is self-contained. A server that previously needed a Redis session store and an L7 load balancer with session affinity can now run behind a plain round-robin load balancer.

For servers already running behind a session store, the migration work is real: you strip the session establishment handshake, ensure the _meta parsing path handles capabilities on every inbound request, and update your infrastructure to remove sticky session requirements.

Local STDIO servers are largely unaffected by this change — sessions were primarily relevant to remote HTTP-based deployments.

Breaking Change 2 — Two New Required HTTP Headers

The Streamable HTTP transport now requires two headers on every request:

  • Mcp-Method — the JSON-RPC method name (e.g., tools/call, resources/read)
  • Mcp-Name — the named operation within the method (e.g., the tool name for a tools/call)

Servers must reject requests where the headers and body disagree. Clients that omit the headers will receive errors.

The practical benefit is routing. Load balancers, API gateways, and rate-limiters can now make routing decisions on the operation type without parsing the request body. A gateway can route tools/call for a computationally expensive tool to a dedicated pool, or apply different rate limits to resources/subscribe versus tools/list, all without DPI.

If you run an MCP gateway or proxy layer, this change is a capability gain. If you run a client that builds raw HTTP requests rather than relying on an SDK, you need to add these headers.

Breaking Change 3 — Error Code Changed

The RC changes at least one error code that client code almost certainly pattern-matches against. The specific code is documented in the RC release notes on blog.modelcontextprotocol.io.

Audit your error handling code for hardcoded numeric error codes from the MCP spec. String-based matching on error messages is similarly fragile. The right fix is to update to named constants from an RC-compatible SDK version once your Tier 1 SDK ships support.

Breaking Change 4 — Caching Semantics for tools/list

The RC introduces caching semantics via new response fields. Servers can now include a ttlMs field in tools/list responses to signal how long clients may cache the tool list before re-fetching.

For most servers, this is additive — omitting ttlMs preserves current behavior where clients re-fetch on every connection. But clients that previously cached aggressively may now have that behavior formalized (or constrained) by this new field. Check whether your client SDK has new caching behavior and whether your server emits ttlMs values consistent with how often your tool list actually changes.

Breaking Change 5 — Distributed Trace Propagation Locked Down

The RC formalizes how distributed trace context (W3C Trace Context headers) propagates through MCP calls. The previous behavior was underspecified, leading to inconsistent implementations.

If your MCP server or client uses distributed tracing — OpenTelemetry, Datadog, or similar — verify that your implementation aligns with the RC’s mandated propagation rules. The specific changes are in the RC release notes and the TokenMix changelog at tokenmix.ai.

Breaking Change 6 — Roots, Sampling, and Logging Deprecated

Three first-class MCP primitives are deprecated in the 2026-07-28 RC:

  • Roots — the capability for servers to declare filesystem or resource roots
  • Sampling — the LLM sampling request primitive (servers requesting the client to run inference)
  • Logging — structured log emission from servers to clients

Nothing is removed yet. The deprecations are annotation-only. All three methods and their capability flags continue to work in this release and in every spec version published within twelve months of the final July 28 spec. You have at least until mid-2027 before they stop working.

But the signal is clear: these primitives will be removed in a future spec version. If your server or client uses Roots, Sampling, or Logging, start planning the migration. The RC release notes are expected to include guidance on what replaces each.

New Features: MCP Apps and the Tasks Extension

The breaking changes land alongside two significant new capabilities.

MCP Apps (SEP-1865)

Servers can now ship interactive HTML interfaces that hosts render in a sandboxed iframe. Tools declare their UI templates ahead of time so hosts can prefetch, cache, and security-review them before anything runs. The rendered UI communicates back to the host over the same JSON-RPC base protocol used everywhere else in MCP.

This is opt-in. Servers that don’t declare UI templates are unaffected. For tools where a visual interface would materially improve usability — configuration forms, data preview panels, approval flows — MCP Apps is the first spec-blessed way to deliver one.

Tasks Extension

The Tasks extension replaces the experimental Tasks feature that shipped as a core feature in 2025-11-25. Production use during the 2025-11-25 cycle surfaced enough redesign needs that Tasks moved from the core spec to an extension.

The new model is built for stateless operation. A server responds to tools/call with a task handle instead of a synchronous result. The client then drives the lifecycle:

  • tasks/get — poll for current state
  • tasks/update — send progress updates or intermediate results
  • tasks/cancel — cancel the running task

For long-running agent operations, this pattern is significantly cleaner than the previous experimental implementation. The stateless model also means task state can live anywhere the server can reach — not in session memory.

Authorization Hardening

The RC aligns MCP authorization more closely with OAuth 2.1 and OpenID Connect deployments. If you run an MCP server with custom auth, review the RC’s authorization section against your current implementation. The changes favor standard token flows over custom session-based schemes — consistent with the broader session elimination in the core protocol.

SDK Tier System and Timeline

The RC uses a tiered SDK system to manage the rollout:

Tier Description Expected Timeline
Tier 1 Official SDKs (Python, TypeScript, others) Must ship RC support within the 10-week window
Tier 2 Community SDKs Best-effort, encouraged
Tier 3 Third-party integrations No formal timeline

The ten-week window from the May 21 lock-date ends approximately July 30, aligning with the July 28 final publication. By the time the final spec is live, Tier 1 SDKs are expected to have shipped RC-compatible versions.

What this means for builders: If you are on a Tier 1 SDK (the official Python or TypeScript MCP SDK), expect RC-compatible releases before July 28. Pin to the RC-compatible version once it ships and run your test suite. Do not wait until July 28 to discover incompatibilities.

If you are on a community SDK, check the SDK’s issue tracker or changelog for RC migration status.

What Your Migration Needs to Cover

For a remote HTTP-based MCP server:

  1. Remove session establishment — strip the session handshake from your server initialization path
  2. Parse _meta on every request — client capabilities, metadata, and protocol version now arrive per-request; update your capability negotiation logic accordingly
  3. Update HTTP layer to accept (and require) Mcp-Method and Mcp-Name headers — your server must reject requests that omit them
  4. Audit error code handling — find every place you emit or match against MCP-defined numeric error codes and update to the RC values
  5. Check your load balancer and gateway configuration — sticky session rules are no longer needed; routing on Mcp-Method is now possible
  6. Update distributed tracing — if using OpenTelemetry or similar, align with the RC’s trace propagation spec
  7. Audit use of Roots, Sampling, and Logging — deprecated, not removed; flag for future migration

For a local STDIO server, the migration is smaller: error code updates and optional ttlMs adoption are the main items.

What Builders Should Do Right Now

The RC is locked. The spec is not going to change before July 28. This is the migration target.

This week:

  • Read the official RC post at blog.modelcontextprotocol.io
  • Identify which Tier 1 SDK you depend on and subscribe to its release notifications
  • Audit your server and client for session handling, hardcoded error codes, and use of deprecated primitives

Before Tier 1 SDK ships RC support:

  • Write tests against the RC behavior for headers, _meta parsing, and error codes so you can run them the moment the SDK ships

Before July 28:

  • Deploy the migrated server to staging, run the full test suite against an RC-compatible client
  • Remove sticky session infrastructure from remote deployments
  • Update production

The Honest Part

The MCP team is shipping six breaking changes in a single spec bump against a hard July 28 deadline. The ten-week window is tight for production deployments that require staging, review, and coordinated rollout.

The rationale for bundling is defensible — the stateless model and the new HTTP headers are architecturally coupled — but the operational burden on server operators is real.

The deprecations of Roots, Sampling, and Logging will be controversial. These are widely-used primitives. The twelve-month grace period before removal is long enough to avoid immediate pain, but the migration paths are not yet fully documented. Watch the official MCP blog for replacement guidance.

The good news: if you have been running a stateless remote MCP server already, or if your usage is purely STDIO, the migration footprint is small. The teams who will feel this most are those running stateful session-based remote servers at scale.

This article was written on June 6, 2026, based on the RC locked May 21, 2026. The final MCP 2026-07-28 specification will publish July 28, 2026. Details may change during the validation window; verify against the official MCP blog before finalizing your migration plan.

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