Working document for Tier 6 axis 6.7. Enumerates every JSON Schema keyword Ajv (the JSON Schema implementation the MCP TypeScript SDK uses) recognises, against cMCP's current validator in src/schema.c. Used to scope the 6.7.2 implementation pass and the 6.7.3 cross-check corpus. Once 6.7 closes, this file is superseded by docs/schema-conformance.md (the "what cMCP supports" reference) and remains here as a record of how the gap was measured.
Reference draft
Ajv defaults to JSON Schema draft 2020-12 for new schemas; it also supports draft-07 in compatibility mode. MCP's inputSchema field is not pinned to a specific draft revision; in practice the TS SDK emits draft-07-compatible shapes via zod-to-json-schema. cMCP targets draft-2020-12 semantics with draft-07 wire compatibility — meaning we implement the 2020-12 keyword names (prefixItems, $defs) and accept the draft-07 aliases (items as tuple, definitions) without emitting our own.
Keyword inventory
Legend:
- ✅ implemented before 6.7 (pre-existing)
- 🟢 implemented in 6.7
- 🟡 implemented in 6.7, partial / documented departure
- ⛔ deliberately not implemented in 6.x — rationale in
schema-conformance.md
- ◻️ deferred to a follow-up (post-6.7)
Validation keywords for any instance type
| Keyword | Status | Notes |
type | ✅ | string OR array-of-strings, all seven canonical tokens. |
enum | ✅ | deep-equality compare. |
const | 🟢 | added 6.7. |
$ref | ◻️ | post-6.7. Requires a bounded ref resolver. |
$defs | ◻️ | paired with $ref. |
definitions (draft-07 alias) | ◻️ | post-6.7. |
$id | ⛔ | cMCP validates at tool-registration time; ids serve composition use cases we don't expose. |
$schema | ⛔ | ignored. Single-draft validator. |
$comment | ⛔ | ignored (it's a comment). |
Validation keywords for numeric instances
| Keyword | Status | Notes |
minimum | ✅ | inclusive. |
maximum | ✅ | inclusive. |
exclusiveMinimum | 🟢 | added 6.7. Draft-2020-12 form (number). |
exclusiveMaximum | 🟢 | added 6.7. Draft-2020-12 form (number). |
multipleOf | 🟢 | added 6.7. Uses fmod on doubles + integer fast-path. |
Validation keywords for strings
| Keyword | Status | Notes |
minLength | ✅ | Unicode code points. |
maxLength | ✅ | Unicode code points. |
pattern | 🟢 | added 6.7. POSIX ERE via <regex.h> — see schema-conformance.md for the regex flavour notes. |
format | ◻️ | post-6.7. Plan covers date-time, email, uri. |
Validation keywords for arrays
| Keyword | Status | Notes |
items (single subschema) | ✅ | applied to every element. |
items (tuple, draft-07) | 🟢 | added 6.7 — array of subschemas, applied positionally. |
prefixItems (draft-2020-12) | 🟢 | added 6.7. Same semantics as draft-07 tuple items. |
additionalItems (draft-07) | 🟢 | added 6.7. Single subschema OR boolean. |
unevaluatedItems | ⛔ | requires evaluation tracking through allOf/anyOf/etc. Out of scope until we'd actually use it. |
minItems | 🟢 | added 6.7. |
maxItems | 🟢 | added 6.7. |
uniqueItems | 🟢 | added 6.7. Deep-equality. |
contains | ◻️ | post-6.7 — needs minContains/maxContains for full semantics. |
minContains | ◻️ | paired with contains. |
maxContains | ◻️ | paired with contains. |
Validation keywords for objects
| Keyword | Status | Notes |
properties | ✅ | recurses into each named property. |
required | ✅ | array of property names. |
additionalProperties: false | ✅ | reject any unlisted property. |
additionalProperties (subschema) | 🟢 | added 6.7. Apply subschema to properties not covered by properties or patternProperties. |
patternProperties | 🟢 | added 6.7. POSIX ERE on each pattern key. |
propertyNames | 🟢 | added 6.7. Subschema applied to each property name (treated as a string). |
minProperties | 🟢 | added 6.7. |
maxProperties | 🟢 | added 6.7. |
unevaluatedProperties | ⛔ | same rationale as unevaluatedItems. |
dependentRequired | ◻️ | post-6.7. |
dependentSchemas | ◻️ | post-6.7. |
dependencies (draft-07) | ◻️ | folded into 6.7 follow-up alongside dependentRequired/dependentSchemas. |
Boolean schema combinators
| Keyword | Status | Notes |
oneOf | 🟢 | added 6.7. Exactly-one semantics. |
anyOf | 🟢 | added 6.7. At-least-one. Short-circuits on first match. |
allOf | 🟢 | added 6.7. Each subschema must validate. |
not | 🟢 | added 6.7. Subschema must NOT validate. |
if / then / else | 🟢 | added 6.7. Conditional. if runs in "schema-only" mode (its failure is silent — only used to choose which branch). |
Schema annotations (non-validating)
These are documentation hints — Ajv ignores them at validation time. cMCP follows suit; they are not in scope for 6.7.
| Keyword | Status | Notes |
title, description | ⛔ | doc-only, surfaced to peers via tools/list. |
default, examples | ⛔ | doc-only. |
readOnly, writeOnly, deprecated | ⛔ | doc-only. |
Boolean schemas
| Schema | Status | Notes |
true (always accept) | 🟢 | added 6.7. |
false (always reject) | 🟢 | added 6.7. |
{} (empty object → always accept) | ✅ | falls out of the existing validator. |
Methodology
Keyword list derived from:
- JSON Schema draft 2020-12 vocabulary (Validation and Applicator vocabularies — annotation keywords listed separately above).
- Ajv's keyword reference for draft-07 and draft-2020-12.
- Manual inspection of the schemas declared by
@modelcontextprotocol/server-everything v2025.11.x and the schemas emitted by zod-to-json-schema in the same install (their dist bundles under conformance/node_modules/).
Deferred to a follow-up (post-6.7)
$ref / $defs / definitions + bounded resolver.format for date-time, email, uri, uuid.dependentRequired / dependentSchemas / dependencies.contains / minContains / maxContains.- Ajv cross-check harness (6.7.3) —
make schema-conformance exercises a corpus of ≥500 (schema, value, expected) tuples comparing cMCP and Ajv outcomes.
Once those land, this file is fully superseded by schema-conformance.md.
