fix: harden Standard Schema elicitation conversion

[felixweinberger]

Follow-up hardening for the Standard Schema elicitation support — four contained fixes, sent as a PR onto your branch so you can review and merge it into #2369 directly. No changes to the API shape or the overall design, which look right.

Motivation and Context

1. Unknown root-level keywords passed the flat-primitive gate onto the wire.
The gate works by parsing with ElicitRequestFormParamsSchema and diffing for stripped keys — but that schema's requestedSchema root is .catchall(z.unknown()), so unknown root keys are never stripped and the diff can't fire for them. Concretely, z.strictObject(...) shipped additionalProperties: false in the outgoing request, .catchall(z.object(...)) shipped a nested schema, and a custom Standard Schema could ship $defs. The spec declares a closed root shape for requestedSchema ($schema/type/properties/required), so the fix whitelists the root after conversion — the keyword set is derived from the wire schema so it tracks spec revisions, $schema stays (it's spec-declared and zod emits it on every conversion), annotation-only root keywords (title/description from .meta()) are dropped, and anything else throws the existing "unsupported JSON Schema keyword(s)" error before sending.

One deliberate trade-off to flag: root title/description are now silently dropped rather than sent or rejected — the spec root shape doesn't declare them, so forwarding risks strict-client rejections, and throwing on .describe() felt hostile. Happy to change that if you see it differently.

2. The vendored zod regexes were a time bomb against the ^4.2.0 peer range.
ZOD_ISO_DATE_PATTERN and friends were byte-copies of what one zod version emits. zod is a peer at ^4.2.0: if a future in-range zod tweaked any of those regexes, previously-working z.email() / z.iso.datetime() elicitations would start throwing at user runtime while lockfile-pinned CI stayed green. The reference patterns are now derived from the installed zod at runtime (z.toJSONSchema(z.email(), ...) etc., with the date-time option space enumerated from the pattern under test), so the converter and the reference can't drift apart. Behavior is unchanged — I checked the new derivation against the old matcher's full accepted set under zod 4.3.6 and they're equivalent in both directions, and your customized-pattern rejection tests (z.email({ pattern }), the hand-built date-time case) pass untouched.

Residual limitation worth knowing: if an app resolves two zod copies whose regexes differ, the mismatch rejects (fail closed). That's much rarer than the single-copy upgrade the vendored approach broke on, and zod being a peer dep is exactly what makes installs dedupe.

3. Function-typed Standard Schemas (ArkType) were misrouted past conversion.
The schema-detection guard required typeof schema === 'object', but ArkType schemas are functions — they satisfied the typed overload, then fell through to the raw branch: no conversion, no flat-primitive gate, and the arktype-internal object shipped as requestedSchema. The guard now delegates to isStandardSchema, which accepts functions and actually verifies ~standard.validate (and is the guard normalizeRawShapeSchema already uses for this routing decision — gating on ~standard.jsonSchema would front-run the zod 4.0/4.1 fallback). This also fixes the reverse misroute: a plain JSON requestedSchema containing a literal "~standard" key — wire-legal via the catchall — was routed into conversion and threw; it now stays on the raw branch.

4. inputRequired.elicit now throws TypeError on unsupported schemas.
inputRequired() throws TypeError for authoring mistakes, but elicit with an unsupported schema threw ProtocolError(InvalidParams) — inside a prompts/get or resources/read handler that reaches the client as -32602 on its own request, when the defect is server-side. The builder now rewraps as TypeError (with the original error as cause). elicitInput keeps throwing ProtocolError, where it maps to the outgoing request.

How Has This Been Tested?

• New packages/core-internal/test/shared/elicitation.test.ts unit-testing normalizeElicitInputFormParams directly: root-keyword rejections (strictObject, looseObject, catchall, root default, $defs), $schema/annotation handling, the format acceptance matrix (z.email(), z.url(), z.iso.date(), all z.iso.datetime variants incl. precision −1/0/3), customized-pattern rejections, an installed-zod equivalence case, and the schema-detection cases (function-typed schema converts; literal "~standard" key stays raw).
• A server-level strictObject case asserting the request throws before anything reaches the client.
• Existing suites pass unmodified except the two builder tests updated for the TypeError contract (1,328 core-internal + 381 server tests green).
• Manually drove a real Server over InMemoryTransport through the public package exports: observed the wire requestedSchema (formats kept, no patterns, clean root; a function schema converts rather than shipping vendor internals), the typed/coerced result, pre-send rejections, and the builder error type.

Breaking Changes

None released — this adjusts behavior introduced on this branch. Two in-branch behavior changes: schemas emitting unknown root keywords now reject instead of leaking them onto the wire, and inputRequired.elicit authoring errors are TypeError instead of ProtocolError. The changeset is updated to describe the root handling.

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

The root-keyword set is derived from ElicitRequestFormParamsSchema's declared shape plus $schema, so a future spec revision that adds a root key only needs the schema change. One behavior note on the guard fix: an object carrying a ~standard key that is not a valid Standard Schema (no validate function) now passes through the raw branch onto the wire instead of throwing a vendor error — matching how the same object behaves without this feature. Not touched here (can follow up separately): JSDoc for the new public types/overloads, enum and response-failure test coverage, and the response-validation asymmetry between elicitInput and inputRequired.elicit + acceptedContent.

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@modelcontextprotocol/core-internal	Minor
@modelcontextprotocol/server	Minor
@modelcontextprotocol/client	Minor
@modelcontextprotocol/express	Major
@modelcontextprotocol/fastify	Major
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