Showcase — Agent Guidance (canonical)

BEFORE touching ANY showcase cell (integration, test, fixture, frontend), these are non-negotiable.

These rules have been re-explained ~20 times because they were written down NOWHERE. This file is the canonical statement. Read it before editing anything under showcase/. The deeper, checklist-form reference is INTEGRATION-CHECKLIST.md.

A showcase "cell" is one (integration × feature) pair rendered on the dashboard. The whole design rests on this invariant: a cell's behavior must be determined by the integration's backend + its fixture, and NOTHING else. Everything shared is shared once. The four iron rules below enforce that; violating any of them is what causes divergence bugs.

The 4 Iron Rules

1. Identical tests — ONE shared probe, run across all integrations

The test that measures a feature (the e2e/probe spec) is byte-identical across every integration. Differences between integrations live ONLY in fixtures, never in the test. For D6/D5 this is a single shared harness probe — e.g. showcase/harness/src/probes/scripts/d5-gen-ui-a2ui-fixed.ts — run against every integration.

What a violation looks like: a per-integration copy of a test/probe, or an if slug === "mastra" branch inside the probe.
How to satisfy it: edit the one shared probe; if a specific integration behaves differently, that difference belongs in its fixture, not in the test.

2. Near-identical frontends — a cell renders the same regardless of backend

The feature UI is a shared / near-identical frontend, so a cell looks and renders the same no matter which backend drives it (e.g. mastra's frontend ≡ langgraph-python's frontend, byte-identical).

What a violation looks like: one integration's frontend component diverging from the others for the same feature.
How to satisfy it: edit the shared frontend source; verify parity by screenshot/diff, don't diverge per-integration.

3. Minimal backends — the thinnest thing that drives the feature

Each integration's backend is the minimal glue needed to drive the feature. No per-integration logic that actually belongs in shared.

What a violation looks like: business/feature logic living in one integration's backend that every other integration re-implements (or should).
How to satisfy it: push shared logic into showcase/shared/...; keep the integration backend as thin wiring.

4. Per-integration fixtures ONLY — the single sanctioned variation

The ONLY sanctioned per-integration variation is the aimock fixture: one per integration, keyed to its slug, under showcase/aimock/d6/<slug>/....

What a violation looks like: encoding an integration's differences anywhere other than its fixture (in the test, frontend, or shared code).
How to satisfy it: put the integration-specific recorded behavior in showcase/aimock/d6/<slug>/; leave everything else shared.

The single-source symlink mechanism (LOAD-BEARING)

Rules 1 and 3 are mechanically enforced by symlinks. This is the part that keeps eroding, so read carefully.

showcase/integrations/*/shared-tools/, */tools/, and */_shared/ are meant to be SYMLINKS to showcase/shared/... — a single source of truth. The build honors this:

stage_shared() in showcase/scripts/cli/_common.sh dereferences the symlinks into real files for the Docker build.
restore_symlinks() (same file) restores them to symlinks afterward.

So: EDIT THE SHARED SOURCE ONLY (showcase/shared/...). A real file (not a symlink) under shared-tools/ / tools/ / _shared/ is a BUG — it means the symlink was clobbered and that copy will drift.

⚠️ This has ERODED on main. Several of these paths are now real committed 100644 files that have DRIFTED from showcase/shared/. That drift IS the root cause of showcase divergence bugs (e.g. the a2ui flat-vs-nested + render_a2ui vs _design_a2ui_surface split fixed in PR #5971).

NEVER "fix all N copies byte-identically." That fights the design and reintroduces drift. If you find a real file where a symlink should be: fix the shared source, then restore the symlink — don't perpetuate the copies.

To check whether a path is still a proper symlink:

ls -l showcase/integrations/<slug>/shared-tools   # should print "-> ../../shared/..."

Preflight checklist (before editing a cell)

Locate the shared source first. Is what you're about to edit a shared probe (rule 1), a shared frontend (rule 2), shared tool logic (rule 3), or a per-integration fixture (rule 4)? Edit the correct layer.
Confirm symlink integrity. If you're editing anything under shared-tools/ / tools/ / _shared/, verify it's a symlink (ls -l). If it's a real file, you're looking at drift — fix the shared source and restore the symlink instead.
Never per-integration-copy a test or frontend. Differences go in the fixture (showcase/aimock/d6/<slug>/) only.
Value-test before merge (mandatory). Run the real probe surface, not unit tests against fakes:
```
bin/showcase test <slug>:<feature> --d6 --direct
```
(run from within showcase/, i.e. showcase/bin/showcase). Observe RED on the failing cell BEFORE your change, then GREEN after — on ≥3 real cells. Unit tests against fakes are NOT sufficient proof.

For the full package/integration checklist (manifest, source files, fixtures, external setup), see INTEGRATION-CHECKLIST.md.

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