fix(worker): don't double-append streamed answer when completed event carries content

apps/agent-worker/src/runtime/turn-outcome.ts

@@ -42,3 +42,19 @@ export const resolveTurnOutcome = (args: {
   }
   return { text };
 };
+
+// Pure decision for whether the `content` on an `assistant_completed` event
+// should be folded into the accumulated assistant text.
+//
+// Some runtimes (Claude `-p --output-format stream-json`, OpenCode) emit the
+// final answer TWICE: once as streamed `assistant_delta` chunks and again as
+// the `content` on the terminal `assistant_completed`/`result` event. Appending
+// both yields a doubled "<answer><answer>". So only fold in the completed
+// content when nothing has streamed yet (a runtime that emits only a final
+// result). When deltas already carried the answer, skip it. The truly-empty
+// case is still covered by resolveTurnOutcome's recoveredText/finalMessage
+// fallback, so nothing is lost.
+export const shouldAppendCompletedContent = (
+  currentText: string,
+  completedContent?: string,
+): boolean => Boolean(completedContent) && !currentText.trim();

apps/agent-worker/src/worker.ts

@@ -18,7 +18,10 @@ import type { AdapterWorkspace, Claim } from './runtime/provider';
 import type { BoxHandle } from './user-container';
 import type { AgentRuntimeName } from './runtime/agent-runtime';
 import { getAdapter } from './runtime/agent-runtime';
-import { resolveTurnOutcome } from './runtime/turn-outcome';
+import {
+  resolveTurnOutcome,
+  shouldAppendCompletedContent,
+} from './runtime/turn-outcome';
 import './runtime/register';
 import { env } from './env';
 import {
@@ -344,7 +347,7 @@ const handleAgentEvent = async (args: {
     workspace.agentTurnActive = false;
     workspace.resolveTurn?.();
     workspace.resolveTurn = undefined;
-    if (event.content) {
+    if (shouldAppendCompletedContent(assistantContent.value, event.content)) {
       assistantContent.value = truncate(
         `${assistantContent.value}${event.content}`,
         40_000,

apps/agent-worker/tests/unit/turn-outcome.test.ts

@@ -1,6 +1,36 @@
 import { describe, expect, test } from 'vitest';
 
-import { resolveTurnOutcome } from '../../src/runtime/turn-outcome';
+import {
+  resolveTurnOutcome,
+  shouldAppendCompletedContent,
+} from '../../src/runtime/turn-outcome';
+
+describe('shouldAppendCompletedContent', () => {
+  test('does not re-append when deltas already streamed the answer', () => {
+    // Claude/OpenCode emit the final answer as assistant_delta AND as the
+    // content-bearing assistant_completed event. Folding the completed
+    // content in again would produce "<answer><answer>".
+    expect(shouldAppendCompletedContent('the answer', 'the answer')).toBe(
+      false,
+    );
+  });
+
+  test('appends completed content when nothing has streamed yet', () => {
+    expect(shouldAppendCompletedContent('', 'the final result')).toBe(true);
+  });
+
+  test('treats whitespace-only streamed text as empty', () => {
+    expect(shouldAppendCompletedContent('   \n', 'the final result')).toBe(
+      true,
+    );
+  });
+
+  test('does not append when the completed event carries no content', () => {
+    expect(shouldAppendCompletedContent('the answer', undefined)).toBe(false);
+    expect(shouldAppendCompletedContent('', undefined)).toBe(false);
+    expect(shouldAppendCompletedContent('', '')).toBe(false);
+  });
+});
 
 describe('resolveTurnOutcome', () => {
   test('partial streamed text + error is surfaced as failed and text preserved', () => {