## Description
Headroom exposes plenty of *quantitative* compression telemetry (token
counts, ratios, `headroom perf`, `/metrics`) but no way to actually
**see what the compressor changed** in the content. That makes it hard
to trust compression or debug a quality regression ("did it drop
something I cared about?").
This adds a `headroom inspect` command (the issue's Option 1). It reads
the proxy's existing loopback `/transformations/feed` endpoint — which
already carries the pre/post-compression message snapshots when the
proxy runs with `--log-messages` — and renders, per request, the
original vs compressed content for each message with the changed
segments highlighted. No new dependencies (stdlib `difflib`).
```
headroom inspect # inspect the most recent request
headroom inspect --last 5 # the 5 most recent
headroom inspect --full # include unchanged messages
headroom inspect --format json # raw feed for offline tooling
```
Per request it shows the model, per-request token counts + savings, the
transforms applied, and a colorized unified diff of each changed message
(red = removed, green = added). Clear errors when no proxy is reachable
or when the proxy wasn't started with `--log-messages`.
Side-by-side / interactive rendering (the fuller form of Option 1) can
follow as a polish pass; this lands the core "see what changed"
capability on data Headroom already captures.
Closes#1267
## Type of Change
- [x] New feature (non-breaking change that adds functionality)
## Changes Made
- `headroom/cli/inspect.py`: new `inspect` command +
content-flattening/diff-render helpers.
- `headroom/cli/__init__.py`, `headroom/cli/main.py`: register the
command.
- `tests/test_cli_inspect.py`: unit tests (content extraction, the
no-proxy / no-`--log-messages` / empty-feed paths, text render, json
output).
- `wiki/cli.md`: document the command + options.
## Testing
- [x] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check`)
- [x] New tests added
### Test Output
```text
$ pytest tests/test_cli_inspect.py -q
7 passed
$ ruff check headroom/cli/ tests/test_cli_inspect.py
All checks passed!
```
## Real Behavior Proof
- Environment: repo main @ HEAD, local venv
- Exact command / steps: invoked the `inspect` command against a mocked
`/transformations/feed` payload (one request, a user message with a line
removed by SmartCrusher).
- Observed result: header shows `req-1 gpt-4o`, `tokens 100 → 40 (saved
60, 60.0%)`, `transforms: SmartCrusher`, and a unified diff with the
removed line on the original side; no-proxy and missing-`--log-messages`
cases raise actionable errors; `--format json` emits the raw feed.
- Not tested: live end-to-end against a real proxy with `--log-messages`
(the data source — the feed endpoint — is exercised via the mocked
payload that mirrors its shape).
## Review Readiness
- [x] I have performed a self-review
- [x] This PR is ready for human review
## Checklist
- [x] My code follows the project's style guidelines
- [x] I have performed a self-review of my code
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove the feature works
