diff --git a/src/validation/wire-schema.test.ts b/src/validation/wire-schema.test.ts
index 98fc4d7b..d3510475 100644
--- a/src/validation/wire-schema.test.ts
+++ b/src/validation/wire-schema.test.ts
@@ -117,6 +117,49 @@ describe('wireSchemaErrors', () => {
     ).toEqual([]);
   });
 
+  it('accepts extension result types through the generic result envelope', () => {
+    expect(
+      wireSchemaErrors(
+        DRAFT_PROTOCOL_VERSION,
+        {
+          jsonrpc: '2.0',
+          id: 3,
+          result: {
+            resultType: 'task',
+            taskId: 'task-1',
+            status: 'working'
+          }
+        },
+        'tools/call'
+      )
+    ).toEqual([]);
+  });
+
+  it('names an unrecognised resultType when the result then fails its typed definition', () => {
+    const errors = wireSchemaErrors(
+      DRAFT_PROTOCOL_VERSION,
+      { jsonrpc: '2.0', id: 3, result: { resultType: 'complet' } },
+      'tools/call'
+    );
+    expect(errors.length).toBeGreaterThan(0);
+    expect(errors[0]).toContain('CallToolResult');
+    expect(errors[0]).toContain("resultType 'complet' is not a core value");
+  });
+
+  it('accepts an unrecognised resultType whose result satisfies the typed definition (open discriminator)', () => {
+    expect(
+      wireSchemaErrors(
+        DRAFT_PROTOCOL_VERSION,
+        {
+          jsonrpc: '2.0',
+          id: 3,
+          result: { resultType: 'x-acme/streamed', content: [] }
+        },
+        'tools/call'
+      )
+    ).toEqual([]);
+  });
+
   it('accepts a JSON-RPC batch under 2025-03-26 and reports per-element errors', () => {
     expect(
       wireSchemaErrors('2025-03-26', [
diff --git a/src/validation/wire-schema.ts b/src/validation/wire-schema.ts
index 8b45849d..1cdbed09 100644
--- a/src/validation/wire-schema.ts
+++ b/src/validation/wire-schema.ts
@@ -61,6 +61,18 @@ const NON_CANONICAL_DEFS = new Set([
   'ServerMessage'
 ]);
 
+/** resultType values defined by the core schema (SEP-2322). */
+const CORE_RESULT_TYPES: ReadonlySet<string> = new Set([
+  'complete',
+  'input_required'
+]);
+/**
+ * resultType values defined by known protocol extensions whose result schema
+ * is not vendored here yet; validated against the generic result envelope.
+ * - 'task': io.modelcontextprotocol/tasks CreateTaskResult (SEP-2663).
+ */
+const EXTENSION_RESULT_TYPES: ReadonlySet<string> = new Set(['task']);
+
 interface CompiledSpec {
   defsKey: '$defs' | 'definitions';
   defs: Record<string, Record<string, unknown>>;
@@ -261,18 +273,37 @@ export function wireSchemaErrors(
 
   if (msg.result !== undefined) {
     // SEP-2322 (MRTR): any request may be answered with an InputRequiredResult
-    // instead of its method's result type; discriminate on resultType.
+    // instead of its method's result type; discriminate on resultType. Results
+    // introduced by a known extension (e.g. the tasks extension's
+    // CreateTaskResult, resultType "task") use the same open discriminator and
+    // are checked against the generic result envelope until their schema is
+    // vendored here. Any other resultType is validated as the method's own
+    // result (the discriminator is open, so it may be a private extension),
+    // and a failure names the unrecognised value so the cause is obvious.
+    const resultType = (msg.result as Record<string, unknown> | null)
+      ?.resultType;
     const inputRequired =
-      (msg.result as Record<string, unknown> | null)?.resultType ===
-        'input_required' && 'InputRequiredResult' in spec.defs;
-    const resultDefName = inputRequired
-      ? 'InputRequiredResult'
-      : requestMethod !== undefined
-        ? spec.resultDefs.get(requestMethod)
+      resultType === 'input_required' && 'InputRequiredResult' in spec.defs;
+    const extensionResult =
+      typeof resultType === 'string' && EXTENSION_RESULT_TYPES.has(resultType);
+    const unrecognisedResultType =
+      typeof resultType === 'string' &&
+      !CORE_RESULT_TYPES.has(resultType) &&
+      !extensionResult
+        ? resultType
         : undefined;
+    let resultDefName: string | undefined;
+    if (inputRequired) {
+      resultDefName = 'InputRequiredResult';
+    } else if (!extensionResult && requestMethod !== undefined) {
+      resultDefName = spec.resultDefs.get(requestMethod);
+    }
     if (resultDefName) {
+      const hint = unrecognisedResultType
+        ? `; resultType '${unrecognisedResultType}' is not a core value or a known extension result (${[...EXTENSION_RESULT_TYPES].map((t) => `'${t}'`).join(', ')}), so it was validated as ${resultDefName}`
+        : '';
       const typed = validateAgainst(resultDefName, msg.result).map(
-        (e) => `${e} (result of '${requestMethod}')`
+        (e) => `${e} (result of '${requestMethod}')${hint}`
       );
       if (typed.length > 0) return typed;
     }