diff --git a/packages/server/src/server/agent/diagnose-6346.real.test.ts b/packages/server/src/server/agent/diagnose-6346.real.test.ts
new file mode 100644
index 0000000000..9907d381dd
--- /dev/null
+++ b/packages/server/src/server/agent/diagnose-6346.real.test.ts
@@ -0,0 +1,84 @@
+import { test, expect, vi } from "vitest";
+import { mkdirSync, mkdtempSync, rmSync } from "node:fs";
+import { join } from "node:path";
+import pino from "pino";
+import { AgentManager } from "./agent-manager.js";
+import { AgentStorage } from "./agent-storage.js";
+import { setupFinishNotification } from "./agent-prompt.js";
+import { ClaudeAgentClient } from "./providers/claude/agent.js";
+import { createTestAgentClient } from "../test-utils/fake-agent-client.js";
+
+// Diagnostic reproduction for #6346. Requires Claude Code; intentionally fails on main.
+// PASEO_DIAGNOSE_6346=1 npx vitest run --config packages/server/vitest.config.ts \
+//   packages/server/src/server/agent/diagnose-6346.real.test.ts
+// The child runs real Claude Code; the parent is an injected recording provider.
+test.runIf(process.env.PASEO_DIAGNOSE_6346 === "1")(
+  "Claude background completion notifies its parent after the initial finish",
+  async () => {
+    const diagnosticRoot = join(process.cwd(), ".dev");
+    mkdirSync(diagnosticRoot, { recursive: true });
+    const root = mkdtempSync(join(diagnosticRoot, "diagnose-6346-"));
+    const logger = pino({ level: "silent" });
+    const storage = new AgentStorage(join(root, "agents"), logger);
+    const received: unknown[] = [];
+    const manager = new AgentManager({
+      registry: storage,
+      logger,
+      paseoToolsEnabled: false,
+      clients: {
+        claude: new ClaudeAgentClient({ logger }),
+        codex: createTestAgentClient("codex", {
+          onStartTurn: (prompt) => {
+            received.push(prompt);
+          },
+        }),
+      },
+    });
+    const ids: string[] = [];
+    let stop: (() => void) | undefined;
+    try {
+      const parent = await manager.createAgent(
+        { provider: "codex", cwd: root, modeId: "full-access" },
+        undefined,
+        { workspaceId: "wks_diagnosis" },
+      );
+      ids.push(parent.id);
+      const child = await manager.createAgent(
+        { provider: "claude", cwd: root, model: "haiku", modeId: "bypassPermissions" },
+        undefined,
+        { workspaceId: "wks_diagnosis" },
+      );
+      ids.push(child.id);
+      stop = setupFinishNotification({
+        agentManager: manager,
+        agentStorage: storage,
+        childAgentId: child.id,
+        callerAgentId: parent.id,
+        logger,
+      });
+      const result = await manager.runAgent(
+        child.id,
+        "Use Bash with run_in_background: true to run exactly: sleep 60 && echo BACKGROUND_DONE_6346. Once it is launched, reply WAITING_6346 and end this turn immediately. Do not wait or poll for it. When its task notification arrives later, report FINAL_6346 with the output.",
+      );
+      expect(result.finalText).toContain("WAITING_6346");
+      await vi.waitFor(() => expect(received).toHaveLength(1));
+      expect(received[0]).toContain("WAITING_6346");
+      await vi.waitFor(
+        async () => {
+          expect(await manager.getLastAssistantMessage(child.id)).toContain("FINAL_6346");
+        },
+        { timeout: 180_000, interval: 500 },
+      );
+      await vi.waitFor(() => expect(manager.getAgent(child.id)?.lifecycle).toBe("idle"), {
+        timeout: 20_000,
+      });
+      await vi.waitFor(() => expect(received).toHaveLength(2));
+      expect(received[1]).toContain("FINAL_6346");
+    } finally {
+      stop?.();
+      for (const id of ids.toReversed()) await manager.closeAgent(id);
+      rmSync(root, { recursive: true, force: true });
+    }
+  },
+  300_000,
+);