Improperly formed request. (request_id: 0d949605-489a-493c-90e9-7f89e0e0245e)

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

macOS 26.2.0 (25C56)

### Expected behaviour

I prompted in plan mode but it just shows this error - Improperly formed request. (request_id: 0d949605-489a-493c-90e9-7f89e0e0245e) insteaded it should just work perfectly as usual.

### Actual behaviour

### Steps to reproduce

_No response_

### Environment

```yaml

[q-details]
version = "2.16.2"
hash = "b43b3506cf009fadc655eb9f189dcc2b849db53d"
date = "2026-08-06T04:50:41.03074Z (5d ago)"
variant = "full"

[system-info]
os = "macOS 26.2.0 (25C56)"
chip = "Apple M1"
total-cores = 8
memory = "8.00 GB"

[environment]
cwd = "/Users/USER/Stance"
cli-path = "/Users/USER/Stance"
os = "Mac"
shell-path = "/bin/zsh"
shell-version = "5.9"
terminal = "macOS"
install-method = "unknown"

[env-vars]
DISPLAY = "/private/tmp/com.apple.launchd.TnaFzdjyZu/org.xquartz:0"
PATH = "/Users/USER/.local/bin:/opt/homebrew/share/google-cloud-sdk/bin:/Users/USER/Library/Python/3.9/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.antigravity/antigravity/bin:/Users/USER/.local/bin:/Library/Frameworks/Python.framework/Versions/3.10/bin:/opt/homebrew/bin:/opt/homebrew/sbin:/Library/Frameworks/Python.framework/Versions/3.9/bin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/opt/pmk/env/global/bin:/opt/X11/bin:/Library/Apple/usr/bin:/Library/TeX/texbin:/Users/USER/.local/bin:/Users/USER/.rvm/bin"
QTERM_SESSION_ID = "945fcfa0ec3a4a27ba343c00622e7816"
Q_SET_PARENT_CHECK = "1"
Q_TERM = "2.16.2"
SHELL = "/bin/zsh"
TERM = "xterm-256color"
__CFBundleIdentifier = "com.apple.Terminal"
```

Start by reproducing the failure in plan mode on macOS with version 2.16.2, after running q doctor and q restart. Use request ID 0d949605-489a-493c-90e9-7f89e0e0245e and the attached error output to trace the malformed request; done means plan mode accepts the prompt and behaves normally, with a regression test if the failing path is identified.

Written by the indexing model from the issue text.