fix(core-internal): make zod toJSONSchema conversion wire-truthful for tool schemas

.changeset/zod-tojsonschema-wire-truthful.md

@@ -0,0 +1,34 @@
+---
+'@modelcontextprotocol/core-internal': patch
+'@modelcontextprotocol/server': patch
+---
+
+Make zod-to-JSON-Schema conversion wire-truthful for tool schemas. A `z.date()` (or another
+unrepresentable type such as `z.bigint()`) in a registered tool's schema no longer throws
+during conversion and fails the entire `tools/list` response — dates are advertised as
+`{type: 'string', format: 'date-time'}` (the shape `JSON.stringify` actually produces), and
+other unrepresentable types degrade to an unconstrained schema. (BigInt values embedded as
+defaults or metadata, e.g. `.default(0n)`, still fail conversion — JSON cannot carry them —
+and so do dynamic catch values, `.catch(ctx => …)`; the `.catch()` degrade covers static
+fallback values only.)
+Output schemas no longer advertise constraints the server doesn't enforce on the raw
+`structuredContent` it ships: fields that may be legitimately absent (`.default()`,
+undefined-accepting types) are dropped from `required` — on objects and enum-keyed records —
+and `additionalProperties: false` is dropped for plain `z.object()` (kept for
+`z.strictObject()`), so validating clients no longer reject legitimate tool results for
+these schema shapes. (Output schemas containing `.transform()`/`.pipe()`/`z.coerce` still
+advertise the post-transform shape while the server ships the raw pre-transform value — a
+pre-existing gap this change does not address. And on zod 4.0–4.2.x, `toJSONSchema` skips
+the sanitization hook on a schema reused both bare and via a `.describe()`/`.meta()` clone
+in the same conversion; full per-node sanitization requires zod >=4.3.0. And the
+`z.date()` advertisement assumes a serializing transport: `InMemoryTransport` passes the
+raw `Date` by reference, so a validating client rejects it over that testing transport.
+On the input side, a required tool/prompt argument of a type JSON cannot carry makes the
+tool listed yet uncallable: `z.date()` is advertised as `string`/`date-time` and other
+unrepresentable types (`z.bigint()`, `z.map()`, `z.set()`, `z.symbol()`) as an
+unconstrained `{}`, but input validation still runs the raw zod schema, which rejects
+every JSON payload — use a JSON-representable type such as `z.iso.date()`/
+`z.iso.datetime()`, `z.number()`, `z.record(...)`, or `z.array(...)`, or make the field
+optional.) Elicitation is unaffected:
+`inputRequired.elicit()` keeps throwing on schemas its restricted form grammar cannot
+round-trip, including `z.date()`.

packages/core-internal/src/shared/elicitation.ts

@@ -28,7 +28,12 @@ function isJsonObject(value: unknown): value is Record<string, unknown> {
 
 function convertStandardElicitationSchema(schema: StandardSchemaWithJSON): Record<string, unknown> {
     try {
-        return standardSchemaToJsonSchema(schema, 'input');
+        // `unrepresentable: 'throw'`: the restricted form grammar must reject shapes it
+        // cannot round-trip. A `z.date()` rewritten to `string`/`date-time` would pass the
+        // wire checks, but the accepted response (a JSON string) could never satisfy the
+        // same `z.date()` schema on handler re-entry — keep the documented loud failure
+        // (`z.iso.date()`/`z.iso.datetime()` are the supported ways to elicit dates).
+        return standardSchemaToJsonSchema(schema, 'input', { unrepresentable: 'throw' });
     } catch (error) {
         const detail = error instanceof Error ? error.message : String(error);
         throw new ProtocolError(