--- name: Arch Index slug: arch-index category: Automation description: Arch Index builds and validates .arch/index.json, a commit-pinned map from architectural components to the real files that implement them. It is used to find component implementation files and detect when architecture docs drift from the code. github: "https://github.com/coco-research/coco/tree/main/skills/arch-index" language: HTML stars: 218 forks: 11 install: "npx degit https://github.com/coco-research/coco/tree/main/skills/arch-index ~/.claude/skills/arch-index" installs_to: ~/.claude/skills/arch-index source_path: skills/arch-index/SKILL.md collection_size: 25 category_size: 1860 collection_url: "https://dirskills.com/collections/coco-research/coco" added: 2026-09-04T05:25:13.562Z last_synced: 2026-09-04T05:25:13.562Z canonical_url: "https://dirskills.com/skills/arch-index" --- # Arch Index Arch Index builds and validates .arch/index.json, a commit-pinned map from architectural components to the real files that implement them. It is used to find component implementation files and detect when architecture docs drift from the code. **Install:** ```bash npx degit https://github.com/coco-research/coco/tree/main/skills/arch-index ~/.claude/skills/arch-index ``` ## README # arch-index Produces one artifact: `.arch/index.json`, a commit-pinned map from each architectural component of a repository to the real directories and files that implement it. Every claimed path is verified to exist by a script with an exit code, not by a promise in a prompt. **Guiding principle — Paths or it does not exist.** The protocol that consumers follow is `team:architecture.md`. This file is the producer. ## What this skill is for, and what it is not for It is for answering, mechanically: which files implement this component, and has that stopped being true. It is not a design tool. The index is **reverse-only** — it describes code that exists on disk at a known commit. There is no mode for specifying a system that has not been built, because the central check is that every path resolves. Forward design work belongs in `/team think`, the `SI-*-Design` councils, or `/util:create-architecture-documentation`. It is not a codebase-mapping tool either, and it should not act like one. When `.planning/codebase/` exists, `gsd-map-codebase` has already produced seven structured documents covering the stack, integrations, architecture, structure, conventions, testing, and concerns. This skill **reads those and skips crawling entirely.** Crawling a repository that has already been mapped is the exact waste this skill was designed to avoid, and duplicating `gsd-map-codebase` would make it a worse version of a tool that already exists. It is not a work-allocation mechanism. The index sits at C4-Container altitude, so a single component can own thousands of files. Nothing derives per-agent file ownership from it. ## Modes Resolve the mode from the invocation. When none is given, use `drift` if `.arch/pinned-commit` exists and `build` if it does not. ### `build` **Step 1 — find prior art before doing any work.** In this order: 1. If `.planning/codebase/` exists, read `ARCHITECTURE.md`, `STRUCTURE.md`, `STACK.md`, and `INTEGRATIONS.md` from it. Set `source` to `gsd-codebase-map`. Skip to step 2. Do not crawl. 2. Otherwise, if gitnexus is available for this repository, call `mcp__gitnexus__context` and `mcp__gitnexus__group_list` for a deterministic view of module grouping. Set `source` to `gitnexus`. Skip to step 2. A gitnexus index that is behind HEAD is still usable here, and `list_repos` reports exactly how far behind it is. Record that number in `ecosystem.sourceCommitsBehind`. The reason staleness is tolerable is structural rather than optimistic: gitnexus informs *component grouping*, while every path the index claims is checked by the validator against the working tree. A path that moved since gitnexus was indexed fails check 5 and blocks the build. Stale grouping advice can therefore make the decomposition slightly dated, but it cannot introduce a path that does not exist. Still run `scripts/repo_tree.py` for the authoritative current tree. 3. Otherwise crawl. Run `scripts/repo_tree.py --repo-root .` for a filtered, depth-adaptive tree, then perform one exploration pass under `references/tool-frugality.md`, writing `.arch/ANALYSIS.md` against the seven-section skeleton in `references/analysis-taxonomy.md` under its ten-thousand-character cap. Set `source` to `crawl`. If the tree came back truncated, say so at the top of the analysis and set `ecosystem.treeTruncated` to `true` in the index. A truncated tree disables `drift` mode entirely, by design. **Step 2 — synthesise, with no tool use.** Read only the gathered summary plus the tree. Emit `.arch/index.json` per `references/schema.md`, bound by `references/component-rules.md`. This stage does not explore; separating exploration from formatting is why the structured output stays in schema. **Step 3 — validate.** ```bash python3 skills/arch-index/scripts/validate_index.py .arch/index.json --repo-root . ``` Exit zero proceeds. Non-zero loops back to step 2 with the exact violation lines, bounded to three rounds, then stops and reports the remaining violations. **If the validator cannot be executed at all, that is a BLOCK, not a pass** — there is no prose fallback, because a fallback an agent can narrate is the failure this gate exists to remove. **Step 4 — write the derived artifacts,** using `templates/`. Write `.arch/ARCH-EVIDENCE.md` capturing the command, the exit code, and the per-check table. Write `.arch/INDEX.md` for humans. Emit the Mermaid `C4Container` source and render it through `design:mermaid` (beautiful-mermaid), never the standard Mermaid CDN. **Step 5 — advance the pin.** Write `git rev-parse HEAD` to `.arch/pinned-commit` **only after the validator has exited zero.** A failed run leaves the previous pin untouched, so a broken index can never present itself as current. ### `drift` **Deterministic first, and often only.** ```bash python3 skills/arch-index/scripts/arch_drift.py --repo-root . ``` The script reads the pin, diffs it against HEAD with `--name-status` and `--numstat`, tests every primary path, and writes `.arch/DRIFT.json` with a per-component verdict of `REMOVE`, `PRUNE`, or `KEEP`, plus the files added outside every claimed path. It resolves the gate itself: | Gate | Meaning | What to do | |---|---|---| | `NONE` | Every verdict is `KEEP` and nothing unclaimed was added | Report `NO DRIFT` and stop. **Zero model calls.** | | `FULL_REBUILD` | More than 300 files or 50,000 lines changed since the pin | A rebuild is cheaper than a patch. Run `build`. | | `INCREMENTAL` | Anything else | Continue below. | If the script exits 2, the scan could not run. That is never `NO DRIFT`; report why. In the `INCREMENTAL` case the model receives `DRIFT.json` as **settled fact** and answers exactly one open question: do the unclaimed added files constitute a component that should exist? Apply the verdicts without re-deciding them, preserve surviving component identifiers, prune connections to removed components, then validate and advance the pin as in `build` steps 3 through 5. Never preserve a component whose code is gone. See the deletion-first rule in `references/component-rules.md`. ### `check` Runs the validator alone and writes `.arch/ARCH-EVIDENCE.md`. Makes no model calls and never writes `index.json`. This is what `/team verify` and `/team ship` call. ## Artifacts | Path | Committed | Notes | |---|---|---| | `.arch/index.json` | Yes | The contract. Diffable history is the point. | | `.arch/pinned-commit` | Yes | Advanced only on validator exit 0. | | `.arch/INDEX.md` | Yes | Derived; regenerated every build. | | `.arch/ARCH-EVIDENCE.md` | Yes | The captured gate. | | `.arch/ANALYSIS.md` | Yes | Fallback path only; cached against the pin. | | `.arch/DRIFT.json` | No | Derived scratch; gitignored. | `.arch/` is committed deliberately. An untracked file cannot be diffed across runs, does not exist in the clean checkout that `/team ship` verifies from, and would be silently erased by `git clean`, reverting every consumer to a degraded path without saying so. ## Scope limits, stated plainly **Structural drift only.** A commit that swaps a component's datastore, rewrites it end to end, or violates a declared boundary entirely inside already-claimed paths returns `NO DRIFT`. Every artifact carries these words, and no consumer may drop them. **Currency is caller-enforced.** No script compares the pin to HEAD; the caller does, per `team:architecture.md` step 2. An index behind HEAD is `STALE`, and claims resting on it are `UNVERIFIED` rather than clean. **The read budget is not enforced.** There is no transcript API and no way to count a subagent's tool calls from outside it. `references/tool-frugality.md` is guidance, and no artifact reports it as verified. **Monorepos do not fit.** The three-to-eight ladder forces distinct concerns to merge in a repository with dozens of packages. One index per workspace package plus a root index whose components are the packages is the eventual answer, and is out of scope for 1.0. ## Files | File | Purpose | |---|---| | `references/schema.md` | The contract and the twelve checks | | `references/arch-plan.md` | The forward plan contract, and why declare-then-verify works | | `references/component-rules.md` | What qualifies, naming, counting, merging | | `references/tool-frugality.md` | The read budget for the fallback crawl | | `references/analysis-taxonomy.md` | The seven-section exploration skeleton | | `scripts/validate_index.py` | The gate. Twelve checks, stdlib only, exit 0 or 1 | | `scripts/arch_drift.py` | Deterministic drift scan. Zero model calls | | `scripts/verify_arch_plan.py` | Forward plan: `--declare` before a build, `--verify` after | | `scripts/repo_tree.py` | Filtered, depth-adaptive tree | | `scripts/run_fixtures.sh` | Regression suite for the validator | | `templates/` | `INDEX.md` and `ARCH-EVIDENCE.md` skeletons | Regression gate for any change to the validator: ```bash bash skills/arch-index/scripts/run_fixtures.sh ``` ## Attribution The design this skill implements is adapted from **devildev** by lak7, licensed **Apache-2.0**, . No upstream source code was copied; what was taken is prompt text, one schema idea, and four procedures. | Borrowed | Upstream source | |---|---| | Component-to-code ownership taxonomy | `prompts/ReverseArchitecture.ts` | | Component rules, count ladder, naming and purpose formulas, anti-patterns | `prompts/ReverseArchitecture.ts:278-371`, `:493-507` | | Deletion-first reconciliation | `prompts/ReverseArchitecture.ts:578-641` | | Two-stage explore-then-synthesise split | `actions/reverse-architecture.ts:696-805` | | Tool-frugality read budget | `actions/reverse-architecture.ts:715-749` | | Seven-section analysis taxonomy and character cap | `actions/reverse-architecture.ts:771-805` | Per-file modification notices required by Apache-2.0 § 4(b) are recorded at the foot of each reference file. The most significant modification is that upstream stated its exact-path-matching rule emphatically in prose, with a validation checklist of unchecked boxes, and never verified a single path in roughly 1,300 lines of code; here that rule is `scripts/validate_index.py` checks 5 and 6, with a process exit code. Also not carried across: all presentation fields, numeric confidence bands, the celebrity-persona framing, the framework-only admission gate, and the credits economy.