fix(server): allow elicitation string formats from standard schemas

Author: mattzcarey

.changeset/standard-schema-elicitation.md

@@ -4,4 +4,4 @@
 ---
 
 Allow form elicitation requests to accept Standard Schema values such as Zod objects for `requestedSchema`. The server converts these schemas to MCP's restricted elicitation JSON Schema before sending and parses accepted content with the original schema before returning typed
-results.
+results. Zod string formats that map to MCP's supported `email`, `uri`, `date`, or `date-time` formats are accepted; arbitrary regex patterns remain rejected because form elicitation does not carry JSON Schema `pattern`.

docs/migration.md

@@ -703,7 +703,7 @@ server.setRequestHandler('tools/call', async (request, ctx) => {
 });
 ```
 
-Standard Schemas passed to `elicitInput` are converted to MCP's restricted form-elicitation JSON Schema before being sent. They must describe a flat object with primitive properties; accepted responses are parsed with the original schema before `result.content` is returned. With Zod v4, use `.meta({ title: 'Field Label' })` for short form-field labels.
+Standard Schemas passed to `elicitInput` are converted to MCP's restricted form-elicitation JSON Schema before being sent. They must describe a flat object with primitive properties; accepted responses are parsed with the original schema before `result.content` is returned. With Zod v4, use `.meta({ title: 'Field Label' })` for short form-field labels. Zod string helpers that emit the supported `email`, `uri`, `date`, or `date-time` formats are accepted; arbitrary `.regex()` patterns are rejected because form elicitation does not carry JSON Schema `pattern`.
 
 These replace the pattern of calling `server.sendLoggingMessage()`, `server.createMessage()`, and `server.elicitInput()` from within handlers.
 

docs/server.md

@@ -498,7 +498,9 @@ Elicitation lets a tool handler request direct input from the user — form fiel
 > Sensitive information must not be collected via form elicitation; always use URL elicitation or out-of-band flows for secrets.
 
 For form elicitation, pass either the restricted JSON Schema shape used by the MCP wire protocol or a Standard Schema such as a Zod object. Standard Schemas are converted to the restricted elicitation JSON Schema before being sent, so they must describe a flat object with
-primitive properties (`string`, `number`, `integer`, `boolean`, or string enum fields). When the user accepts the form, `result.content` is parsed with the original Standard Schema and is typed as that schema's output. With Zod v4, use `.meta({ title: 'Field Label' })` for short form-field labels; `.describe()` maps to JSON Schema `description`, not `title`.
+primitive properties (`string`, `number`, `integer`, `boolean`, or string enum fields). When the user accepts the form, `result.content` is parsed with the original Standard Schema and is typed as that schema's output. With Zod v4, use `.meta({ title: 'Field Label' })` for short
+form-field labels; `.describe()` maps to JSON Schema `description`, not `title`. Zod string helpers that emit the supported `email`, `uri`, `date`, or `date-time` formats are accepted; arbitrary `.regex()` patterns are rejected because form elicitation does not carry JSON Schema
+`pattern`.
 
 Call `ctx.mcpReq.elicitInput(params)` (from {@linkcode @modelcontextprotocol/server!index.ServerContext | ServerContext}) inside a tool handler:
 

packages/server/src/server/server.ts

@@ -90,6 +90,19 @@ function isJsonObject(value: unknown): value is Record<string, unknown> {
     return typeof value === 'object' && value !== null && !Array.isArray(value);
 }
 
+const ELICITATION_STRING_FORMATS = new Set(['email', 'uri', 'date', 'date-time']);
+
+function isSupportedFormatPattern(original: Record<string, unknown>, parsed: Record<string, unknown>, key: string): boolean {
+    return (
+        key === 'pattern' &&
+        typeof original.pattern === 'string' &&
+        parsed.type === 'string' &&
+        typeof parsed.format === 'string' &&
+        original.format === parsed.format &&
+        ELICITATION_STRING_FORMATS.has(parsed.format)
+    );
+}
+
 function findStrippedJsonSchemaPaths(original: unknown, parsed: unknown, path = ''): string[] {
     if (Array.isArray(original) && Array.isArray(parsed)) {
         return original.flatMap((item, index) => findStrippedJsonSchemaPaths(item, parsed[index], `${path}[${index}]`));
@@ -102,6 +115,9 @@ function findStrippedJsonSchemaPaths(original: unknown, parsed: unknown, path =
     return Object.entries(original).flatMap(([key, value]) => {
         const childPath = path ? `${path}.${key}` : key;
         if (!Object.prototype.hasOwnProperty.call(parsed, key)) {
+            if (isSupportedFormatPattern(original, parsed, key)) {
+                return [];
+            }
             return [childPath];
         }
         return findStrippedJsonSchemaPaths(value, parsed[key], childPath);

packages/server/test/server/jsonSchemaValidatorOverride.test.ts

@@ -124,13 +124,14 @@ describe('server JSON Schema validator overrides', () => {
                 await clientTransport.send({
                     jsonrpc: '2.0',
                     id: message.id,
-                    result: { action: 'accept', content: { count: '5' } }
+                    result: { action: 'accept', content: { count: '5', email: 'user@example.com' } }
                 });
             }
         };
 
         const schema = z.object({
             count: z.coerce.number().min(1).meta({ title: 'Registration Count', description: 'Number of registrations to process' }),
+            email: z.string().email().meta({ title: 'Email', description: 'Email address' }),
             newsletter: z.boolean().default(false)
         });
 
@@ -142,8 +143,8 @@ describe('server JSON Schema validator overrides', () => {
         const result = await server.elicitInput(params);
 
         expectTypeOf(result).toMatchTypeOf<ElicitInputResult<typeof schema>>();
-        expectTypeOf(result.content).toEqualTypeOf<{ count: number; newsletter: boolean } | undefined>();
-        expect(result).toEqual({ action: 'accept', content: { count: 5, newsletter: false } });
+        expectTypeOf(result.content).toEqualTypeOf<{ count: number; email: string; newsletter: boolean } | undefined>();
+        expect(result).toEqual({ action: 'accept', content: { count: 5, email: 'user@example.com', newsletter: false } });
         expect(requestedSchema).toMatchObject({
             type: 'object',
             properties: {
@@ -153,10 +154,18 @@ describe('server JSON Schema validator overrides', () => {
                     title: 'Registration Count',
                     description: 'Number of registrations to process'
                 },
+                email: {
+                    type: 'string',
+                    format: 'email',
+                    title: 'Email',
+                    description: 'Email address'
+                },
                 newsletter: { type: 'boolean', default: false }
             },
-            required: ['count']
+            required: ['count', 'email']
         });
+        const emailSchema = (requestedSchema!.properties as Record<string, Record<string, unknown>>).email!;
+        expect(emailSchema.pattern).toBeUndefined();
         expect(validator.schemas).toEqual([]);
         expect(validator.values).toEqual([]);