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MCP spec 2025-03-26: what's in it

2025-03-26 was the first revision after the original release, published on 26 March 2025. It is a substantial update that hardened the protocol for production use, most notably around authorization and transport. It is no longer current, though: two revisions have shipped since, and against the current 2025-11-25 spec it now counts as outdated, so a server that still negotiates it earns only partial protocol-recency credit in VerifyMCP’s remote endpoint rubric.

What changed versus 2024-11-05

The major changes, as listed in the official changelog, are:

  • An authorization framework based on OAuth 2.1. This gave the protocol a comprehensive, standards-based approach to authenticating clients against servers.
  • The Streamable HTTP transport, which replaced the previous HTTP with Server-Sent Events transport with a more flexible design for remote servers.
  • JSON-RPC batching, adding support for sending multiple requests in a single batch. (This was later removed in 2025-06-18.)
  • Tool annotations, which let a server describe a tool’s behaviour, for example whether it is read-only or potentially destructive.

Alongside those, the revision made several smaller schema additions: a message field on ProgressNotification for descriptive status updates, support for audio content (joining text and image), and a completions capability to explicitly signal support for argument autocompletion.

Why it matters

This revision is where MCP gained the pieces a hosted, multi-tenant server needs: a real authorization story and a transport designed for streaming HTTP. It set the direction that the following revisions refined.

Read the full revision

modelcontextprotocol.io/specification/2025-03-26 (or the changelog).

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Written by Stuart Blackler.