diff --git a/.changeset/ninety-clubs-sink.md b/.changeset/ninety-clubs-sink.md
new file mode 100644
index 00000000..9dfd23c1
--- /dev/null
+++ b/.changeset/ninety-clubs-sink.md
@@ -0,0 +1,5 @@
+---
+'@openai/agents-core': patch
+---
+
+fix: #316 developer-friendly message for output type errors
diff --git a/packages/agents-core/src/runImplementation.ts b/packages/agents-core/src/runImplementation.ts
index 2e24ef66..53c832ee 100644
--- a/packages/agents-core/src/runImplementation.ts
+++ b/packages/agents-core/src/runImplementation.ts
@@ -173,6 +173,44 @@ function getApprovalIdentity(approval: ApprovalItemLike): string | undefined {
   }
 }
 
+function formatFinalOutputTypeError(error: unknown): string {
+  // Surface structured output validation hints without echoing potentially large or sensitive payloads.
+  try {
+    if (error instanceof z.ZodError) {
+      const issue = error.issues[0];
+      if (issue) {
+        const issuePathParts = Array.isArray(issue.path) ? issue.path : [];
+        const issuePath =
+          issuePathParts.length > 0
+            ? issuePathParts.map((part) => String(part)).join('.')
+            : '(root)';
+        const message = truncateForDeveloper(issue.message ?? '');
+        return `Invalid output type: final assistant output failed schema validation at "${issuePath}" (${message}).`;
+      }
+      return 'Invalid output type: final assistant output failed schema validation.';
+    }
+
+    if (error instanceof Error && error.message) {
+      return `Invalid output type: ${truncateForDeveloper(error.message)}`;
+    }
+  } catch {
+    // Swallow formatting errors so we can return a generic message below.
+  }
+
+  return 'Invalid output type: final assistant output did not match the expected schema.';
+}
+
+function truncateForDeveloper(message: string, maxLength = 160): string {
+  const trimmed = message.trim();
+  if (!trimmed) {
+    return 'Schema validation failed.';
+  }
+  if (trimmed.length <= maxLength) {
+    return trimmed;
+  }
+  return `${trimmed.slice(0, maxLength - 3)}...`;
+}
+
 /**
  * @internal
  * Walks a raw model response and classifies each item so the runner can schedule follow-up work.
@@ -898,13 +936,14 @@ export async function resolveTurnAfterModelResponse<TContext>(
       );
       const [error] = await safeExecute(() => parser(potentialFinalOutput));
       if (error) {
+        const outputErrorMessage = formatFinalOutputTypeError(error);
         addErrorToCurrentSpan({
-          message: 'Invalid output type',
+          message: outputErrorMessage,
           data: {
             error: String(error),
           },
         });
-        throw new ModelBehaviorError('Invalid output type');
+        throw new ModelBehaviorError(outputErrorMessage);
       }
 
       return new SingleStepResult(
diff --git a/packages/agents-core/test/runImplementation.test.ts b/packages/agents-core/test/runImplementation.test.ts
index 9bb70c61..0e535104 100644
--- a/packages/agents-core/test/runImplementation.test.ts
+++ b/packages/agents-core/test/runImplementation.test.ts
@@ -2317,6 +2317,73 @@ describe('resolveTurnAfterModelResponse', () => {
     }
   });
 
+  it('throws descriptive error when structured output validation fails', async () => {
+    const structuredAgent = new Agent({
+      name: 'StructuredAgent',
+      outputType: z.object({
+        summary: z.string(),
+        sections: z.array(
+          z.object({
+            title: z.string(),
+            points: z.array(
+              z.object({
+                label: z.string(),
+                score: z.number().min(0).max(1),
+              }),
+            ),
+          }),
+        ),
+      }),
+    });
+
+    const response: ModelResponse = {
+      output: [
+        fakeModelMessage(
+          JSON.stringify({
+            summary: 'Example',
+            sections: [
+              {
+                title: 'One',
+                points: [{ label: 42, score: 0.5 }],
+              },
+            ],
+          }),
+        ),
+      ],
+      usage: new Usage(),
+    } as any;
+
+    const processedResponse = processModelResponse(
+      response,
+      structuredAgent,
+      [],
+      [],
+    );
+
+    const structuredState = new RunState(
+      new RunContext(),
+      'test input',
+      structuredAgent,
+      1,
+    );
+
+    await expect(
+      withTrace('test', () =>
+        resolveTurnAfterModelResponse(
+          structuredAgent,
+          'test input',
+          [],
+          response,
+          processedResponse,
+          runner,
+          structuredState,
+        ),
+      ),
+    ).rejects.toThrowError(
+      /Invalid output type: final assistant output failed schema validation at "sections\.0\.points\.0\.label" \(Expected string, received number\)./,
+    );
+  });
+
   it('does not finalize after computer actions in the same turn; runs again', async () => {
     const computerAgent = new Agent({
       name: 'ComputerAgent',