Reject unsupported MCP elicitation forms

src/app-server/protocol/server-requests.ts

@@ -159,6 +159,8 @@ export function appServerMcpElicitationRequest(request: AppServerRequest): Pendi
       },
     };
   }
+  const fields = mcpElicitationFieldsFromRequestedSchema(params.requestedSchema);
+  if (!fields) return null;
   return {
     requestId: request.id,
     params: {
@@ -168,7 +170,7 @@ export function appServerMcpElicitationRequest(request: AppServerRequest): Pendi
       mode: "form",
       message: params.message,
       meta: params.meta,
-      fields: mcpElicitationFieldsFromRequestedSchema(params.requestedSchema),
+      fields,
     },
   };
 }
@@ -604,17 +606,19 @@ function normalizeMcpElicitationParams(params: McpElicitationParams): Normalized
   }
 }
 
-function mcpElicitationFieldsFromRequestedSchema(schema: unknown): PendingMcpElicitationField[] {
+function mcpElicitationFieldsFromRequestedSchema(schema: unknown): PendingMcpElicitationField[] | null {
   const record = asRecordOrNull(schema);
   const properties = asRecordOrNull(record?.["properties"]);
-  if (!properties) return [];
+  if (record?.["type"] !== "object" || !properties) return null;
   const required = new Set(
-    Array.isArray(record?.["required"]) ? record["required"].filter((item): item is string => typeof item === "string") : [],
+    Array.isArray(record["required"]) ? record["required"].filter((item): item is string => typeof item === "string") : [],
   );
-  return Object.entries(properties).flatMap(([id, fieldSchema]) => {
-    if (!isMcpElicitationPrimitiveSchema(fieldSchema)) return [];
-    return [mcpElicitationFieldFromSchema(id, fieldSchema, required.has(id))];
-  });
+  const fields: PendingMcpElicitationField[] = [];
+  for (const [id, fieldSchema] of Object.entries(properties)) {
+    if (!isMcpElicitationPrimitiveSchema(fieldSchema)) return null;
+    fields.push(mcpElicitationFieldFromSchema(id, fieldSchema, required.has(id)));
+  }
+  return fields;
 }
 
 function isMcpElicitationPrimitiveSchema(schema: unknown): schema is AppServerMcpElicitationPrimitiveSchema {

tests/app-server/protocol/server-requests/mcp-elicitation.test.ts

@@ -103,8 +103,12 @@ describe("MCP elicitation request model", () => {
     expect(input.params.fields).toEqual([expect.objectContaining({ id: "title", type: "string" })]);
   });
 
-  it("normalizes malformed schema fields without leaking invalid values", () => {
-    const input = expectPresent(toPendingMcpElicitation(malformedSchemaRequest()));
+  it("rejects a form instead of dropping unsupported schema fields", () => {
+    expect(toPendingMcpElicitation(malformedSchemaRequest())).toBeNull();
+  });
+
+  it("normalizes malformed primitive schema values without leaking them", () => {
+    const input = expectPresent(toPendingMcpElicitation(malformedSchemaRequest({ includeUnsupported: false })));
     if (input.params.mode !== "form") throw new Error("Expected form mode");
 
     expect(input.params.fields.map((field) => field.id)).toEqual(["badDefault", "brokenSelect", "enumSelect", "labels"]);
@@ -217,7 +221,7 @@ function openAiFormRequest(): ServerRequest {
   };
 }
 
-function malformedSchemaRequest(): ServerRequest {
+function malformedSchemaRequest({ includeUnsupported = true }: { includeUnsupported?: boolean } = {}): ServerRequest {
   return {
     id: 44,
     method: "mcpServer/elicitation/request",
@@ -230,8 +234,9 @@ function malformedSchemaRequest(): ServerRequest {
       message: "Provide issue details",
       requestedSchema: {
         type: "object",
+        required: includeUnsupported ? ["unsupported"] : [],
         properties: {
-          unsupported: { type: "object", title: "Unsupported" },
+          ...(includeUnsupported ? { unsupported: { type: "object", title: "Unsupported" } } : {}),
           badDefault: { type: "boolean", title: "Bad default", default: "yes" },
           brokenSelect: { type: "string", title: "Broken select", oneOf: { const: "low", title: "Low" } },
           enumSelect: { type: "string", title: "Enum select", enum: ["low", 1, "high"] },

tests/features/chat/app-server/inbound/routing.test.ts

@@ -119,6 +119,23 @@ describe("chat inbound routing", () => {
     expectRequestRouteKind({ ...request, params: { ...request.params, threadId: "thread-other" } } as ServerRequest, "inactive");
   });
 
+  it("routes MCP forms with unsupported fields to diagnostic rejection", () => {
+    const request = mcpElicitationRequest();
+    const unsupported = {
+      ...request,
+      params: {
+        ...request.params,
+        requestedSchema: {
+          type: "object",
+          required: ["nested"],
+          properties: { nested: { type: "object" } },
+        },
+      },
+    } as ServerRequest;
+
+    expectRequestRouteKind(unsupported, "unsupported");
+  });
+
   it("marks scoped messages inactive when the thread or turn does not match", () => {
     const otherThread = {
       method: "item/agentMessage/delta",