🤖 fix: add elapsed_ms to bash_output to help detect busy loops (#1046)

## Problem

After restarting the app, agents can get stuck in tight loops calling
`bash_output` repeatedly on processes that no longer exist or have
already exited. The agent has no way to detect that responses are
returning immediately with no useful information.

## Solution

1. **Add `elapsed_ms` to bash_output response** - Shows how long the
tool spent waiting for output. If this is very low (<10ms), it suggests
no real waiting occurred.

2. **Add warning note for suspicious patterns** - When a response is
fast (<10ms), empty, and the process has exited, add a note: "Process
has exited and no new output. Avoid repeated polling of completed
processes."

This gives agents actionable feedback to break out of busy loops.

## Changes

- `src/common/types/tools.ts`: Add `elapsed_ms` field to
`BashOutputToolResult`
- `src/node/services/backgroundProcessManager.ts`: Return `elapsed_ms`
from `getOutput()`
- `src/node/services/tools/bash_output.ts`: Add warning note for fast
empty responses on exited processes
- `src/node/services/backgroundProcessManager.test.ts`: Add test for
`elapsed_ms`

## Testing

All existing tests pass plus new test for `elapsed_ms` field.

_Generated with `mux`_

Author: ammar-agent

src/common/types/tools.ts

@@ -239,6 +239,8 @@ export type BashOutputToolResult =
       output: string;
       exitCode?: number;
       note?: string; // Agent-only message (not displayed in UI)
+      // Time spent waiting for output (helps agent detect/avoid busy loops)
+      elapsed_ms: number;
     }
   | { success: false; error: string };
 

src/node/services/backgroundProcessManager.test.ts

@@ -451,6 +451,25 @@ describe("BackgroundProcessManager", () => {
       expect(output.output).toContain("error message");
     });
 
+    it("should include elapsed_ms in response", async () => {
+      const result = await manager.spawn(runtime, testWorkspaceId, "sleep 0.2; echo done", {
+        cwd: process.cwd(),
+        displayName: "test",
+      });
+
+      expect(result.success).toBe(true);
+      if (!result.success) return;
+
+      // Wait with timeout to ensure blocking
+      const output = await manager.getOutput(result.processId, undefined, 1);
+      expect(output.success).toBe(true);
+      if (!output.success) return;
+
+      // elapsed_ms should be present and reflect the wait time
+      expect(typeof output.elapsed_ms).toBe("number");
+      expect(output.elapsed_ms).toBeGreaterThanOrEqual(0);
+    });
+
     it("should return error for non-existent process", async () => {
       const output = await manager.getOutput("bash_nonexistent");
       expect(output.success).toBe(false);

src/node/services/backgroundProcessManager.ts

@@ -421,6 +421,7 @@ export class BackgroundProcessManager extends EventEmitter<BackgroundProcessMana
         status: "running" | "exited" | "killed" | "failed";
         output: string;
         exitCode?: number;
+        elapsed_ms: number;
       }
     | { success: false; error: string }
   > {
@@ -507,6 +508,7 @@ export class BackgroundProcessManager extends EventEmitter<BackgroundProcessMana
         currentStatus !== "running"
           ? ((await this.getProcess(processId))?.exitCode ?? undefined)
           : undefined,
+      elapsed_ms: Date.now() - startTime,
     };
   }
 

src/node/services/tools/bash_output.ts

@@ -31,7 +31,7 @@ export const createBashOutputTool: ToolFactory = (config: ToolConfiguration) =>
       if (!proc || proc.workspaceId !== config.workspaceId) {
         return {
           success: false,
-          error: `Process not found: ${process_id}`,
+          error: `Process not found: ${process_id}. The process may have exited or the app was restarted. Do not retry - use bash_background_list to see active processes.`,
         };
       }