Custos

Custos is a secure-coding helper for AI agents — an MCP server that puts official security guidance in the agent's context while it writes code, enforces that the guidance lands, and turns the result into audit evidence.

Guide → Enforce → Prove

Guide — official secure-coding guidance, in-context, at write time. MCP tools over the full MITRE CWE corpus (944 weaknesses with official mitigations and vulnerable/fixed code examples), NIST 800-53 Rev 5, OWASP ASVS 5.0, NIST SSDF, ISO 27001/27017, and NIST cloud guidance. controls_for_change turns "add password reset" into the exact controls and mitigations before a line is written; cwe_lookup turns any scanner finding into remediation guidance. All text verbatim from the official publications — nothing AI-generated, no network at runtime.

Enforce — the guidance has to land in code. A pre-edit nudge (Claude Code hook) surfaces relevant controls and CWE mitigations — split into design-phase and implementation-phase hints, so the reader picks what fits where they are — the moment a security-touching edit starts, and a pre-commit/CI gate blocks commits to security-sensitive code that carry no citation receipt (// Refs: NIST IA-5(1) inline or in the commit message). Advisory at edit time, enforced at commit, non-bypassable in CI (--strict).

Prove — audit evidence out the other end. custos-evidence walks a repo, collects every citation, resolves NIST → ISO 27001 Annex A through the official NIST OLIR mappings, and emits a COMPLIANCE.md evidence index with file:line pointers.

Related MCP server: Metsuke

Install

Requires Bun.

bun add -g mcp-custos        # or: npm install -g mcp-custos

Claude Code

claude mcp add custos -- mcp-custos

Any other MCP client (Claude Desktop, Cursor, …) — add to its MCP config:

{ "mcpServers": { "custos": { "command": "mcp-custos" } } }

The pre-edit nudge is Claude Code-specific; the tools, commit gate, CI check, and evidence generator work with any agent (or none).

Project setup (optional hooks + server registration)

Two install styles, one command either way:

Team (recommended) — add it as a devDependency so teammates get it with plain pnpm install and the lockfile keeps everyone on the same version:

pnpm add -D mcp-custos                 # or npm i -D / yarn add -D / bun add -d
pnpm exec custos-init .

Solo/global — with the package installed globally:

custos-init /path/to/your/project      # --skip-hooks=husky,ci to skip layers

custos-init detects the install mode (override with --local/--global), registers the custos server in the project's .mcp.json (matched to your package manager), and copies three opt-in layers: .claude/settings.json (pre-edit nudge), .husky/pre-commit (citation gate — requires husky in the project), and a GitHub Actions workflow (the same gate with --strict). Existing files are never overwritten. Humans can git commit --no-verify locally; CI is the backstop.

Generate audit evidence

custos-evidence /path/to/your/repo --out=COMPLIANCE.md

Data and provenance

All datasets are bundled (src/data/) and refreshed from official sources via bun run update-sources: the MITRE cwec catalog, NIST OSCAL (800-53, SSDF), the OWASP ASVS release JSON, NIST OLIR mappings, and verbatim extracts from the NIST cloud SPs. ISO standard text is not shipped (paywalled) — only IDs/titles, with implementation detail coming from the mapped NIST controls. The only project-curated (non-official) data: the CWE→ASVS/NIST starter mappings and the Top 25 overlay, labeled as such inside the dataset.

Development

git clone https://github.com/an0malous/mcp-custos && cd mcp-custos
bun install
bun link            # exposes the mcp-custos / custos-* commands from this checkout
bun run typecheck && bun test

Feature work runs through the Kiro spec flow — see CLAUDE.md.

MIT © an0malous