archy

Architectural sensor for Python codebases - keeps structure honest under AI-assisted development.

pip install archy
archy score .         # one-shot architectural health number
archy hotspots .      # refactor priority = complexity x git churn
archy mcp             # expose 19 tools to Claude Code, Cursor, any MCP client

Free, MIT licensed, no commercial version planned. Built and maintained by Alex Lee.

Status: v0.35.0. Usable today via:

How the score is computed and how to read it: docs/SCORING.md. Benchmarks against pydantic, fastapi, flask, pytest, and archy-on-archy: docs/CASE_STUDIES.md. Design rationale and comparison with sentrux: docs/LEARNINGS.md.

In the wild

archy is used in production by the projects listed in ADOPTERS.md. If you're running archy on a real codebase, please open a PR to add yourself, or file an issue and I'll add you.

Why

I built archy because I kept watching coding agents generate code that passed review but rotted the import graph underneath. The score on a feature change would look fine; six weeks later the cycle count had doubled and nobody noticed until a refactor blew up. I wanted a single number per commit that would have caught it.

AI agents generate code at machine speed. Without a feedback loop on structural health (module coupling, import cycles, layer violations), codebases drift architecturally even when every individual change looks fine in review.

archy watches a Python codebase, builds a live module-dependency graph, and surfaces drift through a single trended score plus a handful of actionable sub-metrics. It's designed to run in CI, in pre-commit, and as an MCP server (archy mcp) so coding agents can read their own architectural impact before committing.

The agent-feedback framing is empirically supported by 2025-2026 research: the Navigation Paradox paper shows large LLM context windows do not eliminate the need for structural graph navigation, LocAgent's ablation finds graph edges materially improve code-localization accuracy, the Constraint Decay paper (arxiv:2605.06445) finds agents lose ~30 points in pass rate as architectural constraints accumulate (Clean Architecture layering alone costs -9.1 points, on the open and mid-tier models tested) and that its ground-truth layer/dependency-direction verifier is essentially archy check, and the coding-agent failure-mode literature names the specific patterns (scope drift, cross-file reasoning failure) that an architectural feedback loop is built to catch. Citations, a failure-mode-to-archy-capability mapping, and the resulting roadmap priorities are in docs/research/RESEARCH_METRICS.md §14c.

Scope

• Python only. The cross-language story belongs to sentrux; that division is settled. archy goes deep on Python (transitive contracts, SDP, NCCD, if TYPE_CHECKING: semantics) rather than broad across languages; see docs/LEARNINGS.md §"Competitive landscape".
• Tree-sitter powered. Robust to in-flight edits and partial files; survives syntax errors that would crash ast.
• Score that trends over time. A single number per commit, persisted, plotted. Trend matters more than the absolute value.
• Rules as YAML. "Layer X cannot import Y." No DSL, no plugins (yet).

Non-goals

• Multi-language analysis
• Replacing linters, type checkers, or test runners
• Generating code or auto-fixing violations

Quick start

Requires Python 3.10+ (archy depends on mcp>=1.27.1 which is 3.10-only). If you only have system Python 3.9 or older, install a newer Python first or use uv which manages versions for you.

pip install archy
# or: uv tool install archy
# or: pipx install archy

Using archy as an MCP server inside an AI coding agent? Skip the manual config and run uvx archy install to wire it into Claude Code, Cursor, Codex, opencode, or Continue automatically. See docs/INSTALL.md.

All examples below use the installed archy command. If you're working from a checkout, prefix them with uv run (e.g. uv run archy graph .).

See docs/SIXTY_SECOND_TOUR.md for the copy-paste path from zero to first score.

Inspect the graph

archy graph path/to/project --internal-only
archy graph path/to/project --format json > graph.json
archy graph path/to/project --format dot | dot -Tsvg > graph.svg

Find import cycles

Tarjan SCCs of size >= 2, plus self-loops (a module importing itself). Use --strict in CI to fail on any cycle.

archy cycles path/to/project
archy cycles path/to/project --format json
archy cycles path/to/project --strict

Enforce layer rules

Reads archy.yaml from the repo root. Exits 1 on any violation. See Layer rules below.

archy check path/to/project
archy check path/to/project --format json
archy check path/to/project --config custom.yaml

Transitive contracts (archy contracts)

archy check only sees direct edges. archy contracts wraps import-linter so the same layer story is enforced transitively (A → B → C still counts as A reaching C). It is the strictness upgrade for projects whose layers leak through indirect paths.

pip install 'archy[contracts]'
archy contracts path/to/project
archy contracts path/to/project --format json

Config resolution. archy contracts reads, in order:

1. The --config argument if passed.
2. .importlinter in the project root: the canonical contracts config.
3. archy.yaml: best-effort fallback. Each forbid: rule becomes one Forbidden contract checked transitively. Emits a UserWarning because this path cannot express ignore_imports, so any legitimate transitive edge (e.g., a service layer reaching psycopg through a sanctioned app.libs.db.* module) will be reported as a violation with no way to whitelist it.

Two configs, one concern each:

• archy.yaml owns layer definitions, direct-edge gating (archy check), sdp:, exclude:, and roots:.
• .importlinter owns transitive contracts: all five contract types (Forbidden, Layers, Independence, Protected, AcyclicSiblings) and ignore_imports whitelists.

Reach for .importlinter as soon as you need transitive enforcement at all; the archy.yaml fallback is a zero-config onramp, not a feature target. See .importlinter in this repo for a real-world example, and the import-linter contract types reference for the full grammar.

Common case: forbid services from reaching psycopg but allow the sanctioned db library to do so:

[importlinter]
root_package = app

[importlinter:contract:services-must-not-reach-psycopg]
name = services must not reach psycopg
type = forbidden
source_modules =
    app.services
forbidden_modules =
    psycopg
ignore_imports =
    app.libs.db.engine -> psycopg

Compute a quality score

Composite of modularity, acyclicity, depth, equality, and complexity (geometric mean of five axes). See docs/SCORING.md for formulas and how to interpret the breakdown. These five axes were chosen after surveying ~15 alternatives from the package-metrics literature (Martin's I/A/D, Lakos's NCCD, MacCormack propagation cost, Structure101 fat/tangle, reflexion models, cognitive complexity, hotspots, logical coupling, dead/duplicate-code detection); Martin's I and the Stable Dependencies Principle check are also shipped as a per-module diagnostic and an archy check rule. See docs/research/RESEARCH_METRICS.md for the full validation, what was shipped, and what was deferred and why.

archy score path/to/project
archy score path/to/project --format json

Track score over time

Persist per-commit scores to .archy/history.jsonl and chart the trend.

archy score path/to/project --record
archy trend path/to/project
archy trend path/to/project --last 30 --format json

Regression gate

Fail if the current score drops more than --strict-tolerance (default 0.02) below the most recent recorded run.

archy score path/to/project --strict
archy score path/to/project --strict --record           # check then record
archy score path/to/project --strict --strict-tolerance 0.0

Blast radius

List internal modules that transitively depend on a given file. Useful before refactoring or removing a module.

archy impact path/to/project --file app/libs/db.py
archy impact path/to/project --file app/libs/db.py --file app/services/auth.py --format json

Affected tests (CI gating)

archy affected is the CI-shaped cousin of archy impact: given changed files, it returns the impacted modules pre-classified into tests and other downstream code, with a depth cap (default 5 hops) so a one-line edit doesn't fan out to thousands of nodes on a monorepo. Pipes naturally from git diff:

git diff --name-only HEAD | archy affected . --stdin
git diff --name-only HEAD | archy affected . --stdin --quiet | xargs pytest
archy affected . src/foo.py --filter "tests/integration/**" --json

Test classification defaults to pytest conventions (test_*.py, *_test.py, anything under a tests/ directory); override with --filter <glob>. Internal modules only; vendored or third-party code is not traced.

Design Structure Matrix (archy dsm)

The DSM puts modules on both axes in a chosen ordering, and cell (row=source, col=target) is non-empty when source imports target. Reading positionally exposes properties any single scalar would hide: block-diagonal cohesion under community grouping, above-diagonal back-edges under topological ordering, off-block layer leakage under layer grouping. Visualization-only (docs/research/DSM_EMPIRICS.md for why no scalar joins the score).

archy dsm path/to/project --group community     # block-diagonal orientation
archy dsm path/to/project --group topological   # back-edges sit above diagonal
archy dsm path/to/project --group layer --weight calls   # cross-layer call traffic
archy dsm path/to/project --focus pkg.module --focus-depth 1   # focal neighborhood
archy dsm path/to/project --format json > .archy/dsm-before.json
# ... edit code ...
archy dsm path/to/project --group topological --diff .archy/dsm-before.json
# prints any new back-edges the edit introduced

archy dsm refuses ASCII rendering for projects larger than --max-nodes (default 80) with an actionable error pointing at --focus, --package, or --format json.

Snapshot and diff (agent feedback loop)

Capture a baseline at the start of an editing session, then diff after edits to see exactly which cycles or layer rules changed. See docs/AGENT_LOOP.md for the full playbook (also available via the MCP server's loop prompt).

archy snapshot path/to/project   # writes .archy/baseline.json
# ... edit code ...
archy diff path/to/project       # risk-weighted summary + score deltas + added/resolved cycles & violations

Run as an MCP server

Stdio transport, so AI agents can call archy directly. See MCP server below.

archy mcp

MCP server (archy mcp)

The server is backed by a persistent parse cache (.archy/index.db): each tool call re-parses only the files whose content changed since the last call, so warm graph builds stay in the low seconds even on very large repos (benchmarked: 21.5s cold to 2.5s warm on Home Assistant's 17,299 modules). The cache is transparent and disposable; deleting .archy/index.db only costs one cold rebuild. The graph is always re-derived from the current files, so a cached result is never stale. archy index sync warms it explicitly; archy index clear removes it.

archy mcp exposes nineteen tools and one prompt to MCP-aware AI agents (Claude Code, the Anthropic API, etc.):

The server also exposes a loop prompt with the agent feedback-loop playbook (snapshot at start, impact before edit, diff after edit). Discoverable via the standard MCP prompts/list call. See docs/AGENT_LOOP.md for the human-readable version.

Tool output contract (structured output)

Every tool declares an outputSchema (JSON Schema, derived from its return model) in tools/list, and every tools/call returns both a structuredContent object (validated against that schema) and a text block with the same JSON, per the 2025-06-18 MCP structured-output spec. All tools are also annotated readOnlyHint: true (closed-domain, idempotent, non-destructive), so trusted clients can auto-approve archy's calls instead of prompting on every read. Sequence returns (archy_cycles, archy_trend) and union returns (archy_diff, archy_graph, archy_dsm) are wrapped under a top-level result key since structuredContent must be a JSON object; for unions every branch (including the in-band *ErrorPayload shapes) is a conforming anyOf member.

Error model (recovery contract)

archy maps failures onto MCP's two error mechanisms with one convention, so an agent has a single recovery contract:

• Usage error → isError: true (a raised exception): an invalid argument value (e.g. response_format="xml", last_n=0), a malformed archy.yaml, or a project over the scan ceiling. The caller must fix the call or the environment.
• Recoverable / advisory → in-band result (isError: false): an expected precondition that isn't met but is recoverable, or a valid-but-degraded result. These are normal results the agent branches on. Either a union variant when there's no usable result (no baseline → DiffErrorPayload, output too large → *TooLargePayload, no config → CheckErrorPayload, no DSM snapshot → DSMErrorPayload), or an advisory field on an otherwise-valid payload (ContractsPayload.available=false, HotspotsPayload.note, WhatToRefactorPayload.git_available). The marker for a "no usable result" variant: a payload with an error field and no success data.
• Protocol error (JSON-RPC): unknown tool or a missing/mistyped required argument, handled by the framework.

Wiring it into your agents

One command detects your installed clients (Claude Code, Cursor, Codex CLI, opencode, Continue) and wires each one up:

uvx archy install        # detect, confirm, register the MCP server in each client
uvx archy uninstall      # the exact inverse; --dry-run to preview

This registers the uvx archy mcp server, drops a short rules file so the agent knows when to call the tools, and (on Claude Code) seeds the permissions.allow allowlist. It does not install a binary or the Claude plugin. The full guide, including the per-client path matrix, the manual stanza for unknown clients, plugin-vs-installer guidance, and troubleshooting, is in docs/INSTALL.md.

The lowest-friction path specifically on Claude Code is the bundled plugin at plugins/claude/: /plugin marketplace add hslee16/archy then /plugin install archy@archy from inside Claude Code (or claude --plugin-dir /path/to/archy/plugins/claude from a checkout). See docs/INSTALL.md for when to prefer it over the installer.

Regression-gate semantics

--strict reads the last row from .archy/history.jsonl and compares the current score against it. Drops beyond the tolerance fail with exit code 1. The default tolerance (0.02) matches the threshold sentrux's gate uses. This gives archy parity with sentrux's regression-gate use case while keeping the long-term JSONL history for archy trend.

CI integration

GitHub Action

archy ships a composite action you can drop into any workflow:

- uses: hslee16/archy@v0.35.0
  with:
    command: score      # score | check | cycles
    path: .
    strict: "true"      # fail on regression (score) or any cycle (cycles)

Inputs (all optional unless noted):

Pre-commit hook

Add to .pre-commit-config.yaml:

repos:
  - repo: https://github.com/hslee16/archy
    rev: v0.35.0
    hooks:
      - id: archy-check          # layer rules from archy.yaml
      - id: archy-score-strict   # regression gate against last recorded score
      - id: archy-cycles         # fail on any import cycle

archy-score-strict reads .archy/history.jsonl; commit a baseline first with archy score . --record.

Layer rules (archy check)

Drop an archy.yaml at the repo root declaring layers and forbidden directions:

layers:
  domain:
    modules:
      - "myapp.domain.**"
  application:
    modules:
      - "myapp.application.**"
  infra:
    modules:
      - "myapp.infra.**"
      - "myapp.adapters.**"

forbid:
  - {from: domain, to: application}
  - {from: domain, to: infra}
  - {from: application, to: infra}

Pattern syntax. Dotted-name globs: * matches one segment, ** matches zero or more. myapp.domain.** covers the package itself and every descendant. Modules must belong to at most one layer.

Excluding directories. Add an optional exclude: list of directory basenames to skip codegen output, vendored code, etc. Each name is matched anywhere in the project tree (same mechanism as the built-in skips for .venv, node_modules, __pycache__):

exclude:
  - baml_client
  - generated

exclude: applies to every analysis (graph, cycles, score, check) and the equivalent MCP tools.

Scan-size guard (max_modules:). archy refuses to start a scan of a tree with more modules than a ceiling, so a stray vendored, cache, or generated directory that the named exclude: skips do not cover cannot silently wedge a scan for minutes. The default (10,000) sits well above the largest real projects; a scan that trips it stops with a message pointing at exclude: / a narrower path. Override or disable it:

max_modules: 25000   # raise the ceiling for a genuinely large monorepo
# max_modules: 0     # disable the guard entirely

Namespace packages (roots:). archy discovers packages by walking __init__.py files. PEP 420 namespace packages (no __init__.py) are invisible by default. Declare them as roots so descendants get qualified names:

roots:
  - app           # `app/main.py` becomes `app.main`
  - src/service   # `src/service/db.py` becomes `service.db`

Without roots:, a project like app/libs/db.py (no app/__init__.py) is either skipped entirely or shows up as a top-level libs.db, which makes layer rules like app.libs.** match nothing.

Discovery. archy check walks PATH upward to find archy.yaml unless --config is given. Exits 1 on violation.

archy enforces its own architecture this way; see archy.yaml at the repo root and the archy check . step in .github/workflows/ci.yml.

Stability check (sdp:). Optionally enable Robert Martin's Stable Dependencies Principle: a module should not import one that is less stable than itself. Stability is I = Ce / (Ce + Ca) where Ce is outgoing internal imports and Ca is incoming, so I = 0 means "depended on, depends on nothing" (most stable) and I = 1 means "depends on lots, nothing depends on this" (least stable).

sdp:
  enabled: true
  tolerance: 0.0   # ignore violations within this I gap; default 0
  mode: error      # 'error' fails the gate (default); 'warn' reports but exits 0

When enabled, archy check flags every internal import edge whose target's I strictly exceeds the source's (plus tolerance). Per-module I is also surfaced in archy graph --format json whether or not sdp: is enabled, so you can audit before turning enforcement on.

Gradual adoption. Existing codebases will often have SDP violations on day one. Set mode: warn to report violations in the output (and archy_check's sdp_violations payload) without failing the gate, then flip to mode: error once the count is at zero. Layer-rule violations always fail the gate regardless of sdp.mode.

Development

uv sync                    # install runtime + dev deps from uv.lock
uv run ruff check          # lint
uv run ruff format         # format
uv run ty check            # type check
uv run pytest              # tests

One pytest case (test_pagerank_matches_networkx_when_available) compares archy's hand-rolled _pagerank against nx.pagerank, which needs numpy/scipy. The dependency is intentionally not in the default install (archy stays scientific-stack free); to run that test locally, sync the optional parity group:

uv sync --group parity     # pulls in numpy + scipy for the parity test
uv run pytest              # the test now runs instead of being skipped

Roadmap

Executive summary below; docs/ROADMAP.md is the canonical Now / Next / Deferred / Rejected view, and docs/FUTURE.md is the long-form list with citations to the literature each idea came from.

Both phases of the index-and-install work have shipped (Phase 1 install-DX in v0.25.0 / v0.26.0, Phase 2 persistent index + watcher in v0.27.0). The core mission is built; the current frontier is adoption and validation, not new features.

Next up (validated, queued, not yet started):

• Per-module score breakdown so an agent can ask "did my edit make this module worse?" rather than "did the project overall regress?". Pairs with archy_diff.
• Change coupling (temporal coupling): rank module pairs that frequently change together in git history but share no import or call edge (a hidden dependency the structural graph can't see). Same Tornhill / CodeScene lineage as archy_hotspots; needs an empirical-validation pass first (co-change is noisy).
• Opt-in agent hooks (archy install --hooks): register a lifecycle hook in the agent client (Claude Stop, Cursor afterFileEdit, ...) that runs the archy gate automatically after edits, so the loop fires whether or not the agent remembers to call the tools. Spec: docs/SPEC_INSTALL_HOOKS.md.
• Duplicate-function detection via AST-shape hashing, and a static fragility proxy (high-instability x high-fan-in) as a git-free hotspot stand-in. Both advisory, not score axes.

Shipped:

Foundations

• Tree-sitter import graph; __init__.py re-export resolution; Tarjan cycle detection.
• YAML layer rules (archy check); composite score (archy score); JSONL history + archy trend.
• MCP server (archy mcp); GitHub Action + pre-commit hooks.

Agent loop

• Blast-radius: archy impact.
• Snapshot/diff: archy snapshot / archy diff + MCP loop prompt.
• Import-linter contract wrap: archy contracts, archy[contracts].
• Graph-navigation MCP tools: archy_graph_focus, archy_graph_summary, archy_graph (design in docs/SPEC_GRAPH_MCP.md).
• Per-module edit_risk composite + archy_high_risk_modules MCP tool: geometric mean of propagation cost, normalized fan-in, and instability; surfaced on every graph payload.
• v0.24, risk-weighted archy_diff summary: additive DiffSummary (headline, top_regressions, top_improvements) ranked by edit_risk so the loop-closer reads one sentence instead of re-ranking raw deltas.
• v0.25, archy affected: depth-capped reverse-impact walk mapping changed files to impacted modules and test files (git diff --name-only HEAD | archy affected . --stdin -q | xargs pytest); CLI + archy_affected MCP tool.
• v0.27, persistent index + file watcher: SQLite parse cache (.archy/index.db) keyed by content hash (7-9x warm-path speedup, byte-identical to a cold build) plus a watchdog observer that keeps the index warm inside archy mcp; new archy_status MCP tool (17th) reports last_synced_at.
• v0.28, causal-framing reframes: archy's output now reads as causal claims and judgment prompts, not just structure. archy_impact returns chains (the shortest import path back to a changed module, with line numbers, explaining why each dependent is impacted); archy_snapshot returns an invariant_brief (declared layers, forbidden edges, the acyclic invariant, baseline score, and load-bearing modules) so an agent is told the constraints before its first edit; and each archy_diff summary item carries a prompt reframing the delta as a reviewer question ("new cycle a -> b; intended, or invert an edge?"). No new tool, axis, or graph; packaging over already-computed data (#152, #153, #154).
• v0.29, archy_simulate (18th tool): counterfactual pre-edit check. Given a proposed import-edge delta (add/remove of {from, to} pairs), it returns the would-be cycles, new back-edges, layer/SDP violations, per-axis score delta, and blast-radius change before any file is written, so an agent can test a refactoring hypothesis and reshape a plan that introduces a cycle before touching code. Mostly composition over the diff/DSM/propagation machinery; empirically validated (oracle 315/315 on real repos, 96% fidelity, SIMULATE_ORACLE_EMPIRICS.md, #156).
• v0.30, archy_what_to_refactor_next (19th tool): one ranked refactor-priority list fusing the behavioral lens (archy_hotspots, CC x churn) and the structural lens (archy_high_risk_modules, edit-risk). The two normalized lens scores are summed into a priority, so a module flagged by both generally outranks a comparable single-lens one, while a dominant single-lens signal (a giant hotspot at the import-graph leaves) can still rank first. Each entry names which lenses fired and carries a one-line rationale; one call replaces two-plus-synthesis. Pure aggregation over the two existing primitives. Honest null: an empty list plus a note when nothing is both complex+churned and nothing is central+fragile above the min_risk floor, rather than manufacturing a phantom #1 (#130).
• v0.35, MCP surface modernization: brought the archy mcp tools up to current MCP best practice (2025-2026 spec) without changing the tool set (still 19, no plugin-pin bump). All tools now declare readOnlyHint / title annotations so trusted clients can auto-approve archy's read-only calls instead of prompting on every read (#225); every tool declares a structured-output outputSchema and returns conforming structuredContent alongside the text block (#228); the token-heavy archy_dsm and archy_graph are concise-by-default with a response_format="summary"|"full" enum and a truncation cap (DSM summary ~89% smaller than the full matrix) (#226); and a single three-tier error model gives agents one recovery contract (isError:true for usage errors, in-band result variants for recoverable conditions like no-baseline / too-large / no-config) (#229). No new tool, axis, or graph; MCP-DX over the existing surface. Tracker #230.

Diagnostics

• v0.16, call-graph edges as a second edge type: kinds, call_lines, call_count on every edge; total_calls / calls_per_edge on archy score; static import-alias resolution per LocAgent's invoke-edge framing.
• v0.17, per-function cyclomatic complexity: per-module function_count / cc_sum / cc_max / cc_mean on every internal node; project-wide aggregates on archy score; tree-sitter McCabe walker in src/archy/complexity.py. Promoted to the complexity score axis in v0.20 (recalibrated /8 in v0.23).
• v0.18, archy hotspots: per-file refactor-priority ranking from cc_sum x git-commit-count; single rename-aware git log --name-status -M pass (folds pre-rename history onto the current path); Tornhill/CodeScene's "Code Red" formulation; filters zero-CC and zero-churn rows. MCP surface (archy_hotspots) followed in v0.19.
• v0.21, call-weighted Newman Q as a parallel diagnostic on archy score (not an axis replacement): the gap between unweighted and weighted Q flags mismatch between import-graph and call-graph community structure (docs/research/CALL_WEIGHTED_Q_EMPIRICS.md).
• v0.22, archy dsm (Design Structure Matrix): CLI + archy_dsm MCP tool with --group=community|layer|topological, --weight=imports|calls, --focus/--package, and --diff for back-edge regression detection. Visualization-only per docs/research/DSM_EMPIRICS.md: no DSM-derived score axis or diagnostic scalar.

Install / distribution

• v0.25, Claude Code plugin (plugins/claude/): bundles the MCP server registration and the canonical archy skill into an installable unit.
• v0.26, agent-detecting installer (archy install / archy uninstall): auto-detects which clients (Claude Code, Cursor, Codex CLI, opencode, Continue) are present, writes each one's MCP stanza and rules file, and seeds Claude's permissions.allow. Adapter registry in src/archy/install/; user docs in docs/INSTALL.md.

Empirically rejected (kept here so they don't get re-proposed): type-hint coverage in any form, calls_per_edge as a 6th axis, HTML output formats, dead-function detection, multi-language analysis. See docs/ROADMAP.md for the evidence behind each.

See docs/FUTURE.md for the longer list and docs/LEARNINGS.md for design notes.

Contributing

See CONTRIBUTING.md for style rules. Notably: no em-dash characters (U+2014) anywhere in the repo.

Reporting security issues

Please report vulnerabilities privately via the Security tab, not as a public issue. See SECURITY.md for scope and response targets.

License

MIT, see LICENSE.