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MCP spec 2025-06-18: what's in it

2025-06-18 was published on 18 June 2025. It refined the authorization model introduced in the previous revision and added several capabilities that enable richer interaction between client and server. It is the revision immediately before the current 2025-11-25 spec, so it is still supported: VerifyMCP gives a server that negotiates it full protocol-recency credit for one release cycle, giving you time to upgrade before it slips to partial credit. See the remote endpoint rubric for the exact window.

What changed versus 2025-03-26

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

  • Removed JSON-RPC batching, reversing the support added in 2025-03-26.
  • Structured tool output, so tools can return structured content rather than only free text.
  • Servers classified as OAuth resource servers, with protected resource metadata so a client can discover the corresponding authorization server. Clients are also required to implement Resource Indicators (RFC 8707) to stop malicious servers obtaining access tokens.
  • Elicitation, letting a server request additional information from the user partway through an interaction.
  • Resource links in tool call results.
  • The MCP-Protocol-Version header, which a client must send on subsequent HTTP requests to declare the negotiated version.
  • A tightening of the lifecycle wording from SHOULD to MUST, plus a new security best practices page.

Smaller schema changes included a _meta field on more interface types, a context field on CompletionRequest, and a title field for human-friendly display names so that name can serve as a stable programmatic identifier.

Why it matters

This revision made the authorization model more robust and gave servers and clients new ways to interact beyond a single request and response, notably elicitation and structured output. The MCP-Protocol-Version header also made the negotiated version explicit on every HTTP request.

Read the full revision

modelcontextprotocol.io/specification/2025-06-18 (or the changelog).

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Written by Stuart Blackler.