Overview

The document’s mcpdesc value remains 0.9.0; its exact schema identity is 0.9.0-rc.3. For validators and projection tools, the bare 0.9.0 selector advances with release candidates until final 0.9.0 is released. Select an exact RC supported by your tool when you need to pin a snapshot. Publishing this specification snapshot does not itself add RC.3 support to downstream packages.

Format version: 0.9.0 | Snapshot: rc.3 | Date: November 7, 2026

About this release candidate

These versioned sections, examples, validation rules, and companion guides mirror the immutable RC.3 snapshot. Earlier draft and RC snapshots remain available from the canonical specification repository.

RC.3 extends RC.2’s discriminator-driven validation with union-aware, source-located diagnostics. Disjoint unions with approved selectors use portable guarded conditionals; only the inclusive oauthFlows union retains anyOf. Findings identify the applicable failing rule rather than irrelevant alternatives.

For diagnostic consumers:

Missing-member paths identify the enclosing object; detail.member names the absent member.
Unexpected-member paths identify the member that exists in the source.
Findings reached through $componentRef identify the reference use site and provide a component-definition entry in relatedLocations.
Render recognized detail.kind values and their factual fields; fall back to message for unknown kinds or diagnostics without detail.
Do not depend on raw diagnostic counts or validation-engine schema paths.

Validation result format advances to 1.4.0. Rule format remains 1.3.0, with 85 rules (46 MCP, 1 JSON Schema, and 38 mcpdesc), updated ownership locations and fixture citations, and an expanded unchecked-requirement inventory.

Read the 0.8.0-to-0.9.0 migration guide, including RC.3 diagnostic migration, for compatibility details. The cumulative candidate also retains protocol-version-scoped validation, Effective Protocol Views, merge and conversion profiles, and the non-semantic document ordering profile. The proposals implemented by this candidate remain under review.

Source and schemas

Canonical source - spec/draft/mcp-description.md
Immutable source tag - v0.9.0-rc.3
MCP Description JSON Schema - 0.9.0-rc.3.json
Validation rule schema - validation-rule/1.3.0.json
Validation result schema - validation-result/1.4.0.json
Companion guides - RC.3 guides
How validation works - Protocol-version validation, views, and projection
Validation Rules - Catalog and troubleshooting
Upstream changelog - RC.3 changes
Validation result changelog - Format 1.4.0
Feedback - Discuss the release candidate or report an issue

For AI assistants and tools

Complete specification, single file - /specification/0.9.0-rc.3/mcpdesc.md
Version index - /specification/index.json, including stable and candidate channels and their canonical source and schema URLs
Validation rule catalog - manifest.json for the 85 rules in rules.json, with hash-pinned rule and result schemas; published catalogs are listed in the catalog index