Follow-up for the reworked Standard Schema elicitation support, as a PR onto this branch so CI and review run before it lands in #2369. One commit: it fixes the outstanding conversion bugs and restructures the gate so this class of bug can't quietly return.

Motivation and Context

The mechanism. The previous gate worked by parsing the converted schema through the wire zod schema (which strips unknown keys) and diffing for what disappeared. That indirection is what made the recent bugs possible: the root catchall meant root keywords never showed up in the diff, zod's format-companion regexes showed up as false positives, and rejection errors were whatever zod's union parse produced (multi-KB dumps naming the offending property once, at the end).

This PR replaces it with an explicit walk of the converted schema against the wire grammar. The grammar — root keys, per-type property key sets, the supported format enum — is derived at module load from the wire zod schemas themselves, so a spec revision updates it automatically. The walk decides each key's fate (keep / drop annotation / reject with its exact path); value validity stays owned by the wire schemas; a residual check rejects key combinations no single wire shape can carry (e.g. enum beside minLength).

The behavior fixes riding on it:

• Natural zod format forms work. z.email(), z.iso.date(), and z.iso.datetime() emit a format-check regex beside the format that the wire can't carry — previously they all threw (masked in tests by z.string().meta({ format }), which emits no pattern and validates nothing). The library's own pattern now drops as redundant. For zod the exact patterns are derived from the resolved zod at runtime — never vendored, so an in-range zod upgrade can't silently break this — and a customized z.email({ pattern }) still rejects rather than silently weakening the advertised constraint. Other vendors' format realizations are unknowable from here, so a pattern beside a matching retained format is trusted as the library's own: ArkType's string.email now converts (pinned with real ArkType and Valibot schemas in the integration suite).
• Unknown root keywords reject instead of leaking onto the wire. The root catchall let z.strictObject()'s additionalProperties: false (and $defs, registry ids, …) ship in requestedSchema; the spec declares a closed root shape, and the converted root is now held to it. Annotation-only root keywords (title/description from .meta()) drop rather than reject.
• Schema detection routes on isStandardSchema rather than isStandardSchemaWithJSON, matching normalizeRawShapeSchema: zod 4.0/4.1 schemas (validate but no ~standard.jsonSchema) now reach the converter's fallback instead of shipping zod internals raw on the wire.
• Rejection errors name the offending paths. z.union([z.literal('a'), z.literal('b')]) now produces a 154-character error naming properties.role, instead of a ~5KB zod union dump. Parsing is scoped to the converted schema, so params-level fields behave exactly as on the raw branch (no more message: 42 blamed on requestedSchema, and unknown params extension keys survive on both branches).
• Corrupt-conversion guard. A converted schema whose required names a property that properties doesn't define (zod's toJSONSchema does this for a __proto__ property) rejects instead of shipping.
• The contract is documented. The elicit JSDoc and the input-required guide state the accepted forms, what rejects, the supported formats, and that responses are validated by re-passing the schema to acceptedContent().

How Has This Been Tested?

• The behavior matrix is pinned through inputRequired.elicit and was written before the mechanism rework — every test passed unchanged across the restructure, which is the behavior-preservation argument: format variants (incl. offset/precision datetimes and the .meta({ format }) style), customized-pattern and literal-union rejections, root keyword rejections and annotation drops, function-typed (ArkType-style) and validate-only schema routing, multi-select/optional/default/z.literal(['a','b']) shapes, params extension-key parity, and the __proto__ guard.
• Real ArkType (type({ email: 'string.email' })) and Valibot conversions run in the integration suite.
• An adversarial equivalence review of the old-vs-new mechanism found the accept-space identical by construction (any key surviving the old union parse lies within the derived key sets, and vice versa); all divergences are error-message composition.
• Full suites green (1,342 core-internal + 439 server + 351 integration) plus root pnpm check:all; manually drove the builder and the 2025 shim leg end-to-end over InMemoryTransport through the public package exports.

Breaking Changes

None released — this adjusts behavior introduced on this branch. In-branch: the natural zod format forms now work instead of throwing; unknown root keywords and corrupt conversions reject instead of shipping; validate-only Standard Schemas route to the converter. The changeset names the accepted and rejected forms.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Known residual, documented in a code comment: the zod reference patterns come from the zod copy this package resolves, so an install with two zod copies whose format regexes diverged fails closed (a false rejection, never a bad wire message). Deferred follow-ups worth separate PRs: a CI leg testing the minimum supported zod peer version, converted-schema coverage in the conformance everything-server, and surfacing schema-validation failures distinctly from missing/declined in acceptedContent.