diff --git a/src/core/__tests__/structured-output-schema.test.ts b/src/core/__tests__/structured-output-schema.test.ts
index 006f3825..93c801be 100644
--- a/src/core/__tests__/structured-output-schema.test.ts
+++ b/src/core/__tests__/structured-output-schema.test.ts
@@ -25,6 +25,14 @@ function expectStandaloneCompile(schema: JsonObject): void {
   expect(() => ajv.compile(schema)).not.toThrow();
 }
 
+function stripRegistrationResourceEnvelope(schema: JsonObject): JsonObject {
+  const stripped = JSON.parse(JSON.stringify(schema)) as JsonObject;
+  delete stripped.$schema;
+  delete stripped.$id;
+  delete stripped.$defs;
+  return stripped;
+}
+
 describe('structured output schema bundling', () => {
   beforeEach(() => {
     __resetMcpOutputSchemaCacheForTests();
@@ -119,10 +127,20 @@ describe('structured output schema bundling', () => {
       $id: 'https://xcodebuildmcp.com/schemas/structured-output/xcodebuildmcp.output.simulator-list/1.registration.schema.json',
       type: 'object',
       oneOf: [
-        getMcpOutputSchema(ref),
-        getMcpOutputSchema({ schema: 'xcodebuildmcp.output.error', version: '1' }),
+        stripRegistrationResourceEnvelope(getMcpOutputSchema(ref)),
+        stripRegistrationResourceEnvelope(
+          getMcpOutputSchema({ schema: 'xcodebuildmcp.output.error', version: '1' }),
+        ),
       ],
+      $defs: {
+        ...((getMcpOutputSchema(ref).$defs as JsonObject) ?? {}),
+        ...((getMcpOutputSchema({ schema: 'xcodebuildmcp.output.error', version: '1' })
+          .$defs as JsonObject) ?? {}),
+      },
     });
+    expect((jsonSchema.oneOf as JsonObject[])[0].$defs).toBeUndefined();
+    expect((jsonSchema.oneOf as JsonObject[])[0].$id).toBeUndefined();
+    expect((jsonSchema.$defs as JsonObject).errorConsistency).toBeDefined();
     expectNoExternalCommonRefs(jsonSchema);
     expectStandaloneCompile(jsonSchema);
   });
diff --git a/src/core/structured-output-schema.ts b/src/core/structured-output-schema.ts
index 8da1722b..c6ad07b4 100644
--- a/src/core/structured-output-schema.ts
+++ b/src/core/structured-output-schema.ts
@@ -203,6 +203,26 @@ function bundleSchema(rootSchema: JsonObject, commonSchema: JsonObject): JsonObj
   return bundled;
 }
 
+function inlineRegistrationSchemaResource(schema: JsonObject): {
+  schema: JsonObject;
+  defs: JsonObject;
+} {
+  const inlinedSchema = cloneJson(schema);
+  const defsCandidate = inlinedSchema.$defs === undefined ? {} : cloneJson(inlinedSchema.$defs);
+  if (!isRecord(defsCandidate)) {
+    throw new Error('Structured output registration $defs must be an object when present.');
+  }
+
+  delete inlinedSchema.$schema;
+  delete inlinedSchema.$id;
+  delete inlinedSchema.$defs;
+
+  return {
+    schema: inlinedSchema,
+    defs: defsCandidate,
+  };
+}
+
 export function getMcpOutputSchema(ref: StructuredOutputSchemaRef): JsonObject {
   assertSchemaRef(ref);
   const cacheKey = `${ref.schema}@${ref.version}`;
@@ -226,13 +246,30 @@ function getMcpOutputSchemaForRegistrationJson(ref: StructuredOutputSchemaRef):
   if (ref.schema === STRUCTURED_ERROR_SCHEMA_REF.schema) {
     return toolSchema;
   }
+  const errorSchema = getMcpOutputSchema(STRUCTURED_ERROR_SCHEMA_REF);
+  const toolResource = inlineRegistrationSchemaResource(toolSchema);
+  const errorResource = inlineRegistrationSchemaResource(errorSchema);
+  const defs: JsonObject = {};
 
-  return {
+  for (const [name, definition] of Object.entries(toolResource.defs)) {
+    mergeDefinition(defs, name, definition);
+  }
+  for (const [name, definition] of Object.entries(errorResource.defs)) {
+    mergeDefinition(defs, name, definition);
+  }
+
+  const registrationSchema: JsonObject = {
     $schema: 'https://json-schema.org/draft/2020-12/schema',
     $id: `https://xcodebuildmcp.com/schemas/structured-output/${ref.schema}/${ref.version}.registration.schema.json`,
     type: 'object',
-    oneOf: [toolSchema, getMcpOutputSchema(STRUCTURED_ERROR_SCHEMA_REF)],
+    oneOf: [toolResource.schema, errorResource.schema],
   };
+
+  if (Object.keys(defs).length > 0) {
+    registrationSchema.$defs = defs;
+  }
+
+  return registrationSchema;
 }
 
 export function getMcpOutputSchemaForRegistration(ref: StructuredOutputSchemaRef): McpOutputSchema {