[2026-07-28] Schema modernization & error code standardization

Tracking issue for the MCP Spec 2026-07-28 release — Schema & errors milestone.

SEPs covered

Sub-issues

SEP-2106 (JSON Schema 2020-12):

• #356 — Vocabulary support (oneOf/anyOf/allOf/if-then-else/$ref)
• #357 — Allow non-object outputSchema and structuredContent
• #358 — Harden SchemaValidator against external $ref SSRF and composition DoS

SEP-2164:

• #359 — Switch resource-not-found error code to -32602 with protocol-version-aware fallback

Notes

• Both SEPs are merged.
• SEP-2106 is the largest code-level lift this cycle for the schema layer: full JSON Schema 2020-12 vocabulary (composition, conditionals, $ref), plus non-object outputSchema / structuredContent. Also requires hardening against external $ref SSRF and composition DoS.
• SEP-2164 is mechanically a one-constant flip in src/Schema/JsonRpc/Error.php with a protocol-version-aware fallback.

Start with sub-issues #356–#359 and their SEP links. For the error-code work, inspect src/Schema/JsonRpc/Error.php and the protocol-version fallback described in the issue; for schema work, begin with the SchemaValidator and the listed JSON Schema vocabulary, outputSchema, structuredContent, SSRF, and DoS requirements. Done means all listed sub-issues are completed against the merged SEP requirements.