MCP revisions × features

created:      2026-09-30T08:00:00-05:00
last-updated: 2026-09-30T08:00:00-05:00
increment:    1
status:       standard
tags:         area:compare, area:protocol
description:  every method, notification, capability field, content type,
              error code and HTTP header, against the five published
              revisions of the Model Context Protocol.

What the specification defines, revision by revision. This page is about the specification only; what mcpx does with each row is in matrix-mcpx.md and, per difference, in the register. Every row names one source; sources are at modelcontextprotocol/modelcontextprotocol commit 046fa30 (conventions in README.md).

Columns: 24-11 = 2024-11-05, 25-03 = 2025-03-26, 25-06 = 2025-06-18, 25-11 = 2025-11-25, 26-07 = 2026-07-28.

Legend: ✓ defined · — absent · dep defined but deprecated (still works; scheduled for removal under the 2026 feature-lifecycle policy) · rem removed (was defined earlier) · ext only through the io.modelcontextprotocol/tasks extension · MRTR exists only as an input request embedded in an InputRequiredResult, never as a JSON-RPC request of its own · (s) schema only · (p) prose only.

Three things the tables make visible that no single changelog says:

• 2026-07-28 is not an increment. It removes the handshake, ping, sessions, the GET stream, resumability, logging/setLevel, resources/subscribe, every server-to-client request and every unsolicited notification (list changes now arrive only on an opted-in subscriptions/listen stream), and it adds required things to every exchange: _meta version and capabilities on every request, resultType on every result, ttlMs/cacheScope on every cacheable result, and on HTTP the Mcp-Method header (plus Mcp-Name for tools/call, resources/read and prompts/get).
• Deprecated is not removed. Roots, Sampling and Logging are deprecated in 2026-07-28 but still defined and still usable, now only through input_required (roots, sampling) or per-request logLevel (logging).
• Batching lived for one revision. Added in 2025-03-26, removed in 2025-06-18. A server that claims 2025-03-26 is on the hook for it.

1. Base protocol

2. Methods

initialize and ping are in §1.

3. Notifications

4. Capability fields

5. Content types

5.1 Where each content type may appear

T = text, I = image, A = audio, R = embedded resource, L = resource_link, U = tool_use, X = tool_result.

Per-revision sources for the non-2026 cells: schema/2024-11-05/schema.ts:656 “content: (TextContent | ImageContent | EmbeddedResource)[];”; schema/2025-03-26/schema.ts:699 “content: (TextContent | ImageContent | AudioContent | EmbeddedResource)[];”; schema/2025-06-18/schema.ts:823 “content: ContentBlock[];”; schema/2024-11-05/schema.ts:607 “content: TextContent | ImageContent | EmbeddedResource;”; schema/2025-03-26/schema.ts:645 “content: TextContent | ImageContent | AudioContent | EmbeddedResource;”; schema/2025-06-18/schema.ts:750 “content: ContentBlock;”; schema/2024-11-05/schema.ts:812 “content: TextContent | ImageContent;”; schema/2025-03-26/schema.ts:918 “content: TextContent | ImageContent | AudioContent;”; schema/2025-11-25/schema.ts:1683 “content: SamplingMessageContentBlock | SamplingMessageContentBlock[];”; schema/2025-11-25/schema.ts:1884 “content: ContentBlock[];”

Annotations (audience, priority, and from 2025-06-18 lastModified) are allowed on T, I, A, R, L (L via Resource) in every revision the type exists; never on U or X. _meta is on T, I, A, R, L from 2025-06-18 and on U, X from their introduction.

6. Selected fields and types

7. Error codes

8. HTTP headers