Architecture Drift Guard — E2E Action Skill

Keep a codebase's real structure honest against its documented architecture and fail CI when they diverge. The architecture is declared once in a manifest (architecture.yaml); the Mermaid C4 diagram and the CI gate are both derived from it. The blocking gate is deterministic code — a model is never in its decision path. Same inputs always yield the same exit code; that reproducibility is the entire value.

Scope: this enforces the C4 Component level (packages/modules and their edges) for Python (grimp parses Python imports). Edges are direct, not transitive. Context (L1) and Container (L2) are human-defined and not derivable from code — do not promise drift detection there.

1. Preconditions (input contract)

• An architecture.yaml manifest exists with schema_version, root_packages, and components; dependencies may be empty when bootstrapping.
• The root_packages are importable — installed, or reachable via the manifest's sys_path entries (which the runner prepends to sys.path).
• grimp and pyyaml are installed (pip install grimp pyyaml).

If any precondition fails the runner stops and reports it (exit 2) — it does not improvise around a broken manifest.

2. Procedure (the E2E steps)

Deterministic work lives in scripts/; the library is scripts/adguard/ and the two thin entrypoints call it.

1. Bootstrap the declared edges from reality, then review them WITH the user (undocumented edges are exactly the drift to surface — don't launder them):

   python scripts/drift_check.py --manifest architecture.yaml --emit-actual

   Paste the reviewed dependencies: block into the manifest.
2. Generate the diagram and commit both files:

   python scripts/mermaid_gen.py --manifest architecture.yaml -o architecture.mmd

3. Gate in CI — run both checks on every PR:

   python scripts/drift_check.py --manifest architecture.yaml          # drift gate
   python scripts/mermaid_gen.py --manifest architecture.yaml --check -o architecture.mmd  # freshness gate

3. Output contract (postconditions — what "done" means)

• drift_check.py exits 0 when every actual component edge is declared, 1 when an undocumented edge exists (drift), 2 on a manifest/extraction error. Undocumented edges are printed by name; declared-but-unused edges are a [warn] only and never fail.
• mermaid_gen.py (no --check) writes a deterministic C4 Component diagram. With --check it exits 1 iff the committed diagram differs from a freshly rendered one (after whitespace/newline normalisation).
• --emit-actual prints a deterministic, round-trippable dependencies: block to stdout and exits 0.

4. Failure handling

• Drift (exit 1): classify the new edge WITH the user. If the dependency is intended, add it to dependencies, regenerate architecture.mmd, commit both. If it is a mistake, the code changes, not the manifest — the gate did its job. Never silence the gate by editing the manifest reflexively.
• Stale diagram (exit 1 on --check): regenerate and commit architecture.mmd.
• Error (exit 2): the manifest is missing/malformed or a root is not importable; fix the precondition. The runner leaves no partial artifacts.

5. Validation gate (before declaring success)

You are not done until this exits 0:

python scripts/validate_skill.py --skill . --tier structural,behavioral

The bundled library also has a full unit suite (run from the skill dir):

python -m pytest tests --cov=adguard --cov-report=term-missing --cov-fail-under=90

6. Examples

Example 1 (clean): a manifest whose declared edges match the code → drift_check.py exits 0 with "matches the manifest".

Example 2 (drift, edge case): code has a core -> api back-edge the manifest omits → drift_check.py prints core -> api and exits 1, naming the undocumented dependency rather than silently passing.

Optional: directional rules via import-linter

Graph-equality catches "an edge appeared that isn't declared." It does not express "this layering must never invert." If the user wants named directional rules, add import-linter as a second, separate gate alongside this one — it complements, not replaces, the drift check.

Files

• scripts/drift_check.py — the drift gate (also --emit-actual for bootstrap).
• scripts/mermaid_gen.py — manifest → Mermaid C4, with --check freshness mode.
• scripts/adguard/ — reusable library; extractor.py is the only grimp-bound module, so a future non-Python extractor is an additive change.
• references/manifest-schema.md — full manifest field reference.