fix(core): allow extra JSON Schema keywords on elicitation primitive schemas

.changeset/fix-elicit-primitive-passthrough.md

@@ -0,0 +1,9 @@
+---
+'@modelcontextprotocol/core': patch
+---
+
+Allow extra JSON Schema keywords on elicitation primitive schemas (`string`, `number`, `boolean`).
+
+Previously, `BooleanSchemaSchema`, `StringSchemaSchema`, and `NumberSchemaSchema` used strict Zod parsing, so any keyword not explicitly listed (e.g., `pattern`, `exclusiveMinimum`, `exclusiveMaximum`, `const`) caused the schema to be rejected. This broke real-world use cases where servers send valid JSON Schema with standard annotation or validation keywords.
+
+The fix adds `.passthrough()` to each primitive schema so that extra keys are preserved instead of stripped. The corresponding `BooleanSchema`, `StringSchema`, and `NumberSchema` TypeScript interfaces also gain `[key: string]: unknown` index signatures to stay in sync.

packages/core/src/types/schemas.ts

@@ -1717,37 +1717,43 @@ export const CreateMessageResultWithToolsSchema = ResultSchema.extend({
 /**
  * Primitive schema definition for boolean fields.
  */
-export const BooleanSchemaSchema = z.object({
-    type: z.literal('boolean'),
-    title: z.string().optional(),
-    description: z.string().optional(),
-    default: z.boolean().optional()
-});
+export const BooleanSchemaSchema = z
+    .object({
+        type: z.literal('boolean'),
+        title: z.string().optional(),
+        description: z.string().optional(),
+        default: z.boolean().optional()
+    })
+    .passthrough();
 
 /**
  * Primitive schema definition for string fields.
  */
-export const StringSchemaSchema = z.object({
-    type: z.literal('string'),
-    title: z.string().optional(),
-    description: z.string().optional(),
-    minLength: z.number().optional(),
-    maxLength: z.number().optional(),
-    format: z.enum(['email', 'uri', 'date', 'date-time']).optional(),
-    default: z.string().optional()
-});
+export const StringSchemaSchema = z
+    .object({
+        type: z.literal('string'),
+        title: z.string().optional(),
+        description: z.string().optional(),
+        minLength: z.number().optional(),
+        maxLength: z.number().optional(),
+        format: z.enum(['email', 'uri', 'date', 'date-time']).optional(),
+        default: z.string().optional()
+    })
+    .passthrough();
 
 /**
  * Primitive schema definition for number fields.
  */
-export const NumberSchemaSchema = z.object({
-    type: z.enum(['number', 'integer']),
-    title: z.string().optional(),
-    description: z.string().optional(),
-    minimum: z.number().optional(),
-    maximum: z.number().optional(),
-    default: z.number().optional()
-});
+export const NumberSchemaSchema = z
+    .object({
+        type: z.enum(['number', 'integer']),
+        title: z.string().optional(),
+        description: z.string().optional(),
+        minimum: z.number().optional(),
+        maximum: z.number().optional(),
+        default: z.number().optional()
+    })
+    .passthrough();
 
 /**
  * Schema for single-selection enumeration without display titles for options.

packages/core/src/types/spec.types.ts

@@ -2875,6 +2875,7 @@ export type PrimitiveSchemaDefinition = StringSchema | NumberSchema | BooleanSch
  * @category `elicitation/create`
  */
 export interface StringSchema {
+    [key: string]: unknown;
     type: 'string';
     title?: string;
     description?: string;
@@ -2891,6 +2892,7 @@ export interface StringSchema {
  * @category `elicitation/create`
  */
 export interface NumberSchema {
+    [key: string]: unknown;
     type: 'number' | 'integer';
     title?: string;
     description?: string;
@@ -2906,6 +2908,7 @@ export interface NumberSchema {
  * @category `elicitation/create`
  */
 export interface BooleanSchema {
+    [key: string]: unknown;
     type: 'boolean';
     title?: string;
     description?: string;

test/integration/test/server/elicitation.test.ts

@@ -161,7 +161,6 @@ function testElicitationFlow(validatorProvider: typeof ajvProvider | typeof cfWo
                     age: { type: 'integer', minimum: 0, maximum: 150 },
                     street: { type: 'string' },
                     city: { type: 'string' },
-                    // @ts-expect-error - pattern is not a valid property by MCP spec, however it is making use of the Ajv validator
                     zipCode: { type: 'string', pattern: '^[0-9]{5}$' },
                     newsletter: { type: 'boolean' },
                     notifications: { type: 'boolean' }
@@ -280,7 +279,6 @@ function testElicitationFlow(validatorProvider: typeof ajvProvider | typeof cfWo
             requestedSchema: {
                 type: 'object',
                 properties: {
-                    // @ts-expect-error - pattern is not a valid property by MCP spec, however it is making use of the Ajv validator
                     zipCode: { type: 'string', pattern: '^[0-9]{5}$' }
                 },
                 required: ['zipCode']
@@ -290,6 +288,69 @@ function testElicitationFlow(validatorProvider: typeof ajvProvider | typeof cfWo
         await expect(server.elicitInput(formRequestParams)).rejects.toThrow(/does not match requested schema/);
     });
 
+    test(`${validatorName}: should accept extra JSON Schema keys on string primitive (pattern)`, async () => {
+        client.setRequestHandler('elicitation/create', _request => ({
+            action: 'accept',
+            content: { code: 'abc' }
+        }));
+
+        const result = await server.elicitInput({
+            mode: 'form',
+            message: 'Enter a code',
+            requestedSchema: {
+                type: 'object',
+                properties: {
+                    code: { type: 'string', pattern: '^[a-z]+$' }
+                },
+                required: ['code']
+            }
+        });
+
+        expect(result).toEqual({ action: 'accept', content: { code: 'abc' } });
+    });
+
+    test(`${validatorName}: should accept extra JSON Schema keys on number primitive (exclusiveMinimum)`, async () => {
+        client.setRequestHandler('elicitation/create', _request => ({
+            action: 'accept',
+            content: { score: 0.5 }
+        }));
+
+        const result = await server.elicitInput({
+            mode: 'form',
+            message: 'Enter a score',
+            requestedSchema: {
+                type: 'object',
+                properties: {
+                    score: { type: 'number', exclusiveMinimum: 0, exclusiveMaximum: 1 }
+                },
+                required: ['score']
+            }
+        });
+
+        expect(result).toEqual({ action: 'accept', content: { score: 0.5 } });
+    });
+
+    test(`${validatorName}: should accept extra JSON Schema keys on boolean primitive (const)`, async () => {
+        client.setRequestHandler('elicitation/create', _request => ({
+            action: 'accept',
+            content: { agreed: true }
+        }));
+
+        const result = await server.elicitInput({
+            mode: 'form',
+            message: 'Do you agree?',
+            requestedSchema: {
+                type: 'object',
+                properties: {
+                    agreed: { type: 'boolean', const: true }
+                },
+                required: ['agreed']
+            }
+        });
+
+        expect(result).toEqual({ action: 'accept', content: { agreed: true } });
+    });
+
     test(`${validatorName}: should allow decline action without validation`, async () => {
         client.setRequestHandler('elicitation/create', _request => ({
             action: 'decline'