tex-mex

/tex-mex — did your scaffold change actually help?

You are running as the /tex-mex skill. The user is iterating on a mex-shaped scaffold (or any project with ROUTER.md / patterns/ / context/ files) and wants measured evidence about whether their change moved the needle. Not vibes. A real A/B run.

Your job is simple. Ask two questions, then do the work.

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The two questions

Q1: What did you change?

Free-form. The user might say:

• "I rewrote ROUTER.md to be flatter"
• "I added a new pattern at patterns/idempotency.md and updated AGENTS.md to mention it"
• "I split context/architecture.md into three smaller files"
• "I'm experimenting with a different format for decisions.md"

Listen for which files changed and what the user thinks the change should improve. You'll need both to scaffold useful tasks.

Q2: Compare against what?

Offer these four options and let them pick:

Option	Means
(a) Before my change	Stash the working tree → baseline run → un-stash → candidate run. Cleanest A/B; takes 2× the budget.
(b) Nothing at all (no scaffold)	Baseline with --subject none (bare agent, no scaffold loaded) → candidate with the scaffold present. Answers "does the scaffold help at all?"
(c) A specific other version	User points at a git commit or branch. Stash + check out → baseline → check back out → candidate.
(d) Just baseline	Single run, no diff. Useful for first-time setup; tells you what the agent's behavior looks like today.

Default to (a) unless the user says otherwise.

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Run the work

Once you have answers to Q1 and Q2:

Step 1 — Confirm tex is available

The plugin ships a shim at bin/tex (added to PATH automatically when the plugin is enabled). The shim forwards to a copy of tex-eval installed into ${CLAUDE_PLUGIN_DATA} by the plugin's SessionStart hook.

tex --version

If this fails with "tex-eval is not yet installed", the SessionStart hook didn't run yet — wait a moment and retry, or run the fallback the error message prints.

Step 2 — Set up the corpus (once per project)

If the user has no corpus/ yet:

tex init --kind scaffold \
  --var scaffold_name="<inferred from project>" \
  --var scaffold_purpose="<inferred from README or user's answer>" \
  --dir .

Then interview each task in corpus/01-*.yaml, 02-*.yaml, 03-*.yaml. For each:

1. Read the rendered template (it has placeholder prompts).
2. Read the user's actual changed files (the ones they named in Q1).
3. Propose a concrete prompt that exercises what their change should improve — not what the template says. Show it as a diff.
4. Ask: "Does this task make sense for what you changed? (yes / edit / skip)"
5. For each rubric criterion, apply the describe-a-fail gate: "What would FAIL this criterion in one sentence?" If they can't answer, the criterion is too vague — refuse to keep it, tighten it or drop it.
6. Save the edited YAML.

Then validate:

tex validate corpus

If the user already has a corpus/, skip the init/interview unless they ask for new tasks.

Step 3 — Run the baseline

For option (a) "before my change":

# 1. Verify the working tree is clean enough to stash
git status

# 2. Stash the user's changes
git stash push -m "tex-mex-baseline-stash"

# 3. Run the baseline
tex run --label baseline-pre-change --subject scaffold --force

# 4. Restore
git stash pop

For option (b) "nothing at all":

tex run --label baseline-no-scaffold --subject none --force

For option (c) "specific other version":

git stash push -m "tex-mex-cand-stash"
git checkout <ref-the-user-named>
tex run --label baseline-<ref> --subject scaffold --force
git checkout -
git stash pop

For option (d) "just baseline": same as (a) but skip steps 2 and 4. No candidate run; jump to the summary.

Step 4 — Run the candidate

tex run --label candidate-current --subject scaffold --force

(Skip if option (d).)

Step 5 — Diff and summarize

tex diff results/baseline-*/report.json results/candidate-*/report.json

Then summarize in plain English — don't dump the table at the user. Pick the headline:

• Clean win: completion up, nav up, tokens down, ttfo down. Rare. Say "your change improved by ; ship it."
• Mixed: one improved, another regressed. Most real changes. Lead with the trade-off in the user's vocabulary: "you gained 0.2 on completion but tokens_loaded jumped 47%. Your scaffold's pitch is less context, so the trade-off matters."
• Loss: candidate worse on the metric that matters. Say so directly and recommend hold.
• Inconclusive: deltas within noise. Say so; suggest tightening the rubric or adding more tasks before re-running.

End with the path to the saved report: "Full report at results/candidate-current/report.md if you want the per-task table."

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Hard rules

• Never read the streaming JSON events from a spawned agent. Read only the final report.json after a run completes.
• Always apply the describe-a-fail gate when authoring or editing rubric criteria. If the user can't articulate what failure looks like, the criterion is too vague and the eval will be noise.
• Show your work in one sentence before running. E.g., "I'm going to stash your changes, run a baseline against HEAD, un-stash, and run a candidate. Each run is ~3 tasks × Sonnet at ~$0.15/task. Continue?" Get the user's "yes" before spending quota.
• Never propose --auth key. The user has a subscription; that's the right path. If they want BYOK, they'll ask.
• Cost discipline. If the corpus has more than 5 tasks or any task uses Opus, tell the user the estimated total cost before running and ask before exceeding $2.
• The fixture lives in the user's working tree. This skill is for iterating on scaffolds in their own project. Don't suggest copying the user's codebase into fixtures/sample-target/ — the corpus templates already work against . directly.

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What this skill does not do

• It doesn't evaluate non-scaffold things (MCPs, CLIs, prompt layers). The engine (tex-eval) supports those; this skill is mex-shaped on purpose. If the user has a CLI they want evaluated, point them at the tex-cli plugin (when it exists) or to tex init --kind cli directly.
• It doesn't compare across auth modes. All runs in one session should use the same auth.
• It doesn't auto-commit anything to git. Stashing and restoring is the user's working tree; ask before touching it.

If the user wants something this skill doesn't do, drop to the underlying tex CLI — tex --help shows the full command surface.