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

[chr-hertel]

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

SEPs covered

SEP	Title	Spec PR
SEP-2106	Tools inputSchema/outputSchema conform to JSON Schema 2020-12	#2106
SEP-2164	Standardize resource-not-found error code to -32602	#2164

Sub-issues

SEP-2106 (JSON Schema 2020-12):

• [Schema] SEP-2106: Support JSON Schema 2020-12 vocabulary in SchemaGenerator (oneOf/anyOf/allOf/if-then-else/$ref) #356 — Vocabulary support (oneOf/anyOf/allOf/if-then-else/$ref)
• [Schema] SEP-2106: Allow non-object outputSchema and structuredContent (arrays/primitives) #357 — Allow non-object outputSchema and structuredContent
• [Schema] SEP-2106: Harden SchemaValidator against external $ref SSRF and composition DoS #358 — Harden SchemaValidator against external $ref SSRF and composition DoS

SEP-2164:

• [Schema] SEP-2164: Switch resource-not-found error code to -32602 with protocol-version-aware fallback #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.