DRAFT: SEP-1034: Default values for Elicitation Schemas (#1096)

Author: KKonstantinov

src/client/index.ts

@@ -36,10 +36,41 @@ import {
     SUPPORTED_PROTOCOL_VERSIONS,
     type SubscribeRequest,
     type Tool,
-    type UnsubscribeRequest
+    type UnsubscribeRequest,
+    ElicitResultSchema,
+    ElicitRequestSchema
 } from '../types.js';
 import { AjvJsonSchemaValidator } from '../validation/ajv-provider.js';
 import type { JsonSchemaType, JsonSchemaValidator, jsonSchemaValidator } from '../validation/types.js';
+import { ZodLiteral, ZodObject, z } from 'zod';
+import type { RequestHandlerExtra } from '../shared/protocol.js';
+
+/**
+ * Elicitation default application helper. Applies defaults to the data based on the schema.
+ *
+ * @param schema - The schema to apply defaults to.
+ * @param data - The data to apply defaults to.
+ */
+function applyElicitationDefaults(schema: JsonSchemaType | undefined, data: unknown): void {
+    if (!schema || data === null || typeof data !== 'object') return;
+
+    // Handle object properties
+    if (schema.type === 'object' && schema.properties && typeof schema.properties === 'object') {
+        const obj = data as Record<string, unknown>;
+        const props = schema.properties as Record<string, JsonSchemaType & { default?: unknown }>;
+        for (const key of Object.keys(props)) {
+            const propSchema = props[key];
+            // If missing or explicitly undefined, apply default if present
+            if (obj[key] === undefined && Object.prototype.hasOwnProperty.call(propSchema, 'default')) {
+                obj[key] = propSchema.default;
+            }
+            // Recurse into existing nested objects/arrays
+            if (obj[key] !== undefined) {
+                applyElicitationDefaults(propSchema, obj[key]);
+            }
+        }
+    }
+}
 
 export type ClientOptions = ProtocolOptions & {
     /**
@@ -141,6 +172,64 @@ export class Client<
         this._capabilities = mergeCapabilities(this._capabilities, capabilities);
     }
 
+    /**
+     * Override request handler registration to enforce client-side validation for elicitation.
+     */
+    public override setRequestHandler<
+        T extends ZodObject<{
+            method: ZodLiteral<string>;
+        }>
+    >(
+        requestSchema: T,
+        handler: (
+            request: z.infer<T>,
+            extra: RequestHandlerExtra<ClientRequest | RequestT, ClientNotification | NotificationT>
+        ) => ClientResult | ResultT | Promise<ClientResult | ResultT>
+    ): void {
+        const method = requestSchema.shape.method.value;
+        if (method === 'elicitation/create') {
+            const wrappedHandler = async (
+                request: z.infer<T>,
+                extra: RequestHandlerExtra<ClientRequest | RequestT, ClientNotification | NotificationT>
+            ): Promise<ClientResult | ResultT> => {
+                const validatedRequest = ElicitRequestSchema.safeParse(request);
+                if (!validatedRequest.success) {
+                    throw new McpError(ErrorCode.InvalidParams, `Invalid elicitation request: ${validatedRequest.error.message}`);
+                }
+
+                const result = await Promise.resolve(handler(request, extra));
+
+                const validationResult = ElicitResultSchema.safeParse(result);
+                if (!validationResult.success) {
+                    throw new McpError(ErrorCode.InvalidParams, `Invalid elicitation result: ${validationResult.error.message}`);
+                }
+
+                const validatedResult = validationResult.data;
+
+                if (
+                    this._capabilities.elicitation?.applyDefaults &&
+                    validatedResult.action === 'accept' &&
+                    validatedResult.content &&
+                    validatedRequest.data.params.requestedSchema
+                ) {
+                    try {
+                        applyElicitationDefaults(validatedRequest.data.params.requestedSchema, validatedResult.content);
+                    } catch {
+                        // gracefully ignore errors in default application
+                    }
+                }
+
+                return validatedResult;
+            };
+
+            // Install the wrapped handler
+            return super.setRequestHandler(requestSchema, wrappedHandler as unknown as typeof handler);
+        }
+
+        // Non-elicitation handlers use default behavior
+        return super.setRequestHandler(requestSchema, handler);
+    }
+
     protected assertCapability(capability: keyof ServerCapabilities, method: string): void {
         if (!this._serverCapabilities?.[capability]) {
             throw new Error(`Server does not support ${capability} (required for ${method})`);

src/server/elicitation.test.ts

@@ -609,6 +609,72 @@ function testElicitationFlow(validatorProvider: typeof ajvProvider | typeof cfWo
         });
     });
 
+    test(`${validatorName}: should default missing fields from schema defaults`, async () => {
+        const server = new Server(
+            { name: 'test-server', version: '1.0.0' },
+            {
+                capabilities: {},
+                jsonSchemaValidator: validatorProvider
+            }
+        );
+
+        const client = new Client(
+            { name: 'test-client', version: '1.0.0' },
+            {
+                capabilities: {
+                    elicitation: {
+                        applyDefaults: true
+                    }
+                }
+            }
+        );
+
+        // Client returns no values; SDK should apply defaults automatically (and validate)
+        client.setRequestHandler(ElicitRequestSchema, request => {
+            expect(request.params.requestedSchema).toEqual({
+                type: 'object',
+                properties: {
+                    subscribe: { type: 'boolean', default: true },
+                    nickname: { type: 'string', default: 'Guest' },
+                    age: { type: 'integer', minimum: 0, maximum: 150, default: 18 },
+                    color: { type: 'string', enum: ['red', 'green'], default: 'green' }
+                },
+                required: ['subscribe', 'nickname', 'age', 'color']
+            });
+            return {
+                action: 'accept',
+                content: {}
+            };
+        });
+
+        const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair();
+        await Promise.all([client.connect(clientTransport), server.connect(serverTransport)]);
+
+        const result = await server.elicitInput({
+            message: 'Provide your preferences',
+            requestedSchema: {
+                type: 'object',
+                properties: {
+                    subscribe: { type: 'boolean', default: true },
+                    nickname: { type: 'string', default: 'Guest' },
+                    age: { type: 'integer', minimum: 0, maximum: 150, default: 18 },
+                    color: { type: 'string', enum: ['red', 'green'], default: 'green' }
+                },
+                required: ['subscribe', 'nickname', 'age', 'color']
+            }
+        });
+
+        expect(result).toEqual({
+            action: 'accept',
+            content: {
+                subscribe: true,
+                nickname: 'Guest',
+                age: 18,
+                color: 'green'
+            }
+        });
+    });
+
     test(`${validatorName}: should reject invalid email format`, async () => {
         const server = new Server(
             { name: 'test-server', version: '1.0.0' },

src/spec.types.test.ts

@@ -62,6 +62,19 @@ type MakeUnknownsNotOptional<T> =
                       }
           : T;
 
+// Targeted fix: in spec, treat ClientCapabilities.elicitation?: object as Record<string, unknown>
+type FixSpecClientCapabilities<T> = T extends { elicitation?: object }
+    ? Omit<T, 'elicitation'> & { elicitation?: Record<string, unknown> }
+    : T;
+
+type FixSpecInitializeRequestParams<T> = T extends { capabilities: infer C }
+    ? Omit<T, 'capabilities'> & { capabilities: FixSpecClientCapabilities<C> }
+    : T;
+
+type FixSpecInitializeRequest<T> = T extends { params: infer P } ? Omit<T, 'params'> & { params: FixSpecInitializeRequestParams<P> } : T;
+
+type FixSpecClientRequest<T> = T extends { params: infer P } ? Omit<T, 'params'> & { params: FixSpecInitializeRequestParams<P> } : T;
+
 const sdkTypeChecks = {
     RequestParams: (sdk: SDKTypes.RequestParams, spec: SpecTypes.RequestParams) => {
         sdk = spec;
@@ -75,7 +88,10 @@ const sdkTypeChecks = {
         sdk = spec;
         spec = sdk;
     },
-    InitializeRequestParams: (sdk: SDKTypes.InitializeRequestParams, spec: SpecTypes.InitializeRequestParams) => {
+    InitializeRequestParams: (
+        sdk: SDKTypes.InitializeRequestParams,
+        spec: FixSpecInitializeRequestParams<SpecTypes.InitializeRequestParams>
+    ) => {
         sdk = spec;
         spec = sdk;
     },
@@ -480,23 +496,29 @@ const sdkTypeChecks = {
         sdk = spec;
         spec = sdk;
     },
-    InitializeRequest: (sdk: WithJSONRPCRequest<SDKTypes.InitializeRequest>, spec: SpecTypes.InitializeRequest) => {
+    InitializeRequest: (
+        sdk: WithJSONRPCRequest<SDKTypes.InitializeRequest>,
+        spec: FixSpecInitializeRequest<SpecTypes.InitializeRequest>
+    ) => {
         sdk = spec;
         spec = sdk;
     },
     InitializeResult: (sdk: SDKTypes.InitializeResult, spec: SpecTypes.InitializeResult) => {
         sdk = spec;
         spec = sdk;
     },
-    ClientCapabilities: (sdk: SDKTypes.ClientCapabilities, spec: SpecTypes.ClientCapabilities) => {
+    ClientCapabilities: (sdk: SDKTypes.ClientCapabilities, spec: FixSpecClientCapabilities<SpecTypes.ClientCapabilities>) => {
         sdk = spec;
         spec = sdk;
     },
     ServerCapabilities: (sdk: SDKTypes.ServerCapabilities, spec: SpecTypes.ServerCapabilities) => {
         sdk = spec;
         spec = sdk;
     },
-    ClientRequest: (sdk: RemovePassthrough<WithJSONRPCRequest<SDKTypes.ClientRequest>>, spec: SpecTypes.ClientRequest) => {
+    ClientRequest: (
+        sdk: RemovePassthrough<WithJSONRPCRequest<SDKTypes.ClientRequest>>,
+        spec: FixSpecClientRequest<SpecTypes.ClientRequest>
+    ) => {
         sdk = spec;
         spec = sdk;
     },

src/types.ts

@@ -286,7 +286,17 @@ export const ClientCapabilitiesSchema = z.object({
     /**
      * Present if the client supports eliciting user input.
      */
-    elicitation: AssertObjectSchema.optional(),
+    elicitation: z.intersection(
+        z
+            .object({
+                /**
+                 * Whether the client should apply defaults to the user input.
+                 */
+                applyDefaults: z.boolean().optional()
+            })
+            .optional(),
+        z.record(z.string(), z.unknown()).optional()
+    ),
     /**
      * Present if the client supports listing roots.
      */
@@ -1198,49 +1208,52 @@ export const CreateMessageResultSchema = ResultSchema.extend({
  */
 export const BooleanSchemaSchema = z.object({
     type: z.literal('boolean'),
-    title: z.optional(z.string()),
-    description: z.optional(z.string()),
-    default: z.optional(z.boolean())
+    title: z.string().optional(),
+    description: z.string().optional(),
+    default: z.boolean().optional()
 });
 
 /**
  * Primitive schema definition for string fields.
  */
 export const StringSchemaSchema = z.object({
     type: z.literal('string'),
-    title: z.optional(z.string()),
-    description: z.optional(z.string()),
-    minLength: z.optional(z.number()),
-    maxLength: z.optional(z.number()),
-    format: z.optional(z.enum(['email', 'uri', 'date', 'date-time']))
+    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()
 });
 
 /**
  * Primitive schema definition for number fields.
  */
 export const NumberSchemaSchema = z.object({
     type: z.enum(['number', 'integer']),
-    title: z.optional(z.string()),
-    description: z.optional(z.string()),
-    minimum: z.optional(z.number()),
-    maximum: z.optional(z.number())
+    title: z.string().optional(),
+    description: z.string().optional(),
+    minimum: z.number().optional(),
+    maximum: z.number().optional(),
+    default: z.number().optional()
 });
 
 /**
  * Primitive schema definition for enum fields.
  */
 export const EnumSchemaSchema = z.object({
     type: z.literal('string'),
-    title: z.optional(z.string()),
-    description: z.optional(z.string()),
+    title: z.string().optional(),
+    description: z.string().optional(),
     enum: z.array(z.string()),
-    enumNames: z.optional(z.array(z.string()))
+    enumNames: z.array(z.string()).optional(),
+    default: z.string().optional()
 });
 
 /**
  * Union of all primitive schema definitions.
  */
-export const PrimitiveSchemaDefinitionSchema = z.union([BooleanSchemaSchema, StringSchemaSchema, NumberSchemaSchema, EnumSchemaSchema]);
+export const PrimitiveSchemaDefinitionSchema = z.union([EnumSchemaSchema, BooleanSchemaSchema, StringSchemaSchema, NumberSchemaSchema]);
 
 /**
  * Parameters for an `elicitation/create` request.