MCP 2026-07-28 Spec: JSON Schema 2020-12, Stateless Protocol, and What Changed¶

The MCP specification released on July 28, 2026 introduced significant changes: full JSON Schema 2020-12 support for tool schemas, a stateless protocol, structured output, and renamed primitives. It is still the current protocol revision listed on modelcontextprotocol.io as of October 30, 2026, alongside the final 2025 revisions (2025-11-25, 2025-06-18, 2025-03-26). This page summarizes what changed and what it means for server and client developers.

Tool schemas: full JSON Schema 2020-12¶

Tool inputSchema and outputSchema now support the full JSON Schema 2020-12 vocabulary (SEP-2106).

Input schemas keep an object root type restriction
Output schemas have no root-type restriction
structuredContent may now be any valid JSON value, not only an object
Schemas default to JSON Schema 2020-12 when no explicit $schema is provided

For CorpusIQ, this makes the AnswerSpec contract host-validatable: the tool declares its output schema, and clients can validate the structured content against it before rendering.

Stateless protocol¶

The 2026-07-28 release removed protocol-level sessions. The handshake is stateless. Each request declares its version in _meta under the key io.modelcontextprotocol/protocolVersion, and on Streamable HTTP the same value also travels in the MCP-Protocol-Version header instead of a session handshake. The server accepts or rejects each request independently, answering with an UnsupportedProtocolVersionError when it does not support the requested version. server/discover returns a server's supported versions, capabilities, and identity in one request, so a client can pick a version up front rather than fail first. This simplifies horizontally scaled deployments.

Structured output¶

CallToolResult now includes a structuredContent field. Servers return machine-readable structured data alongside or instead of text content. Clients can use the declared outputSchema to validate the structured content.

Renamed and deprecated primitives¶

Several primitives moved or were renamed in this release. Servers and clients should check the migration notes in the official specification before upgrading. The protocol version string for this release is 2026-07-28.

Migration notes¶

JSON Schema draft upgraded to 2020-12 (breaking for schemas written against older drafts)
Error code -32002 changed to -32602
Update $schema declarations to https://json-schema.org/draft/2020-12/schema
Validate structuredContent against the tool outputSchema before consuming it