subagent fails and feedback to gemini is really low quality

### What happened?

gemini invoked a subagent, which worked for quite some time (successfully making tool calls etc), and then eventually failed, crashing, thus wasting the time of everyone involved.

this extremely hostile message was returned, which is clearly not a high quality prompt:
"""
│ x Codebase Investigator Agent {"objective":"We are investigating the `Exo.Runtime.OperatorCli` and `Ex… │
│ │
│ Subagent Failed: codebase_investigator │
│ Error: [{ │
│ "error": { │
│ "code": 400, │
│ "message": "Request contains an invalid argument.", │
│ "errors": [ │
│ { │
│ "message": "Request contains an invalid argument.", │
│ "domain": "global", │
│ "reason": "badRequest" │
│ } │
│ ], │
│ "status": "INVALID_ARGUMENT" │
│ } │
│ } │
│ ] │
╰────────────────────────
"""

first of all, that is clearly not an error. the subagent did not fail, your code failed. if incorrect tool calls cause a system crash that is hardly the subagent's fault.

And since this is a direct communication to the main gemini, I would expect this text to be superb and for the action items/asks to be made clear. if there was a user error on the main gemini's part (I do not think there was), I would expect exact guidance to be provided. if the subagent made an error, I would expect that crashing the entire subagent would not be the most strategic policy here, it is unclear to me why you would design it like this.

### What did you expect to happen?

for the subagent scaffolding to be robust to incorrect tool calls and to provide clear information about the nature of what happened, instead of crashing the world and providing very low quality information. whoever designed the prompt text in this error message completely dropped the ball, or perhaps they did not even realize they were writing a prompt.

I would expect some basic due diligence in trying to implement basic robustness, and for there to be tests to ensure robustness against this obvious class of failures.

### Client information

Client Information

Run `gemini` to enter the interactive CLI, then run the `/about` command.

```console
> /about
│ │
│ CLI Version 0.30.0 │
│ Git Commit 1e3b992f6 │
│ Model gemini-3.1-pro-preview │
│ Sandbox no sandbox │
│ OS darwin │
│ Auth Method Logged in with Google [redacted]
Tier Gemini Code Assist in Google One AI Ultra
```

### Login information

_No response_

### Anything else we need to know?

_No response_