codeguard-nix

Nix Codeguard Gate

Goal: turn "looks fine" into repeatable tool evidence, and honestly distinguish pass, fail, unverifiable, and planned.

Start in 30 seconds

1. At the repository root, confirm flake.nix or the target file exists.
2. Probe tools:deadnix --version。
3. First do a read-only check:bash -c 'find . -name '"'"'*.nix'"'"' -type f -print0 | xargs -0 -r deadnix'。
4. Classify results as: code violations, missing configuration, missing tools, tool internal errors.
5. Only perform safe-to-fix items:nixpkgs-fmt '{file}'。
6. Re-run lint, and run the project's existing test/build gates.

You can directly propose:

• "Check this Nix project, do not modify anything first, give me a classification of lint failures."
• "Only fix Nix formatting issues, list semantic issues first."
• "Verify Nix lint, test, and build evidence before committing."

Capability boundaries

✅ Good at handling

1. Based on .nix and project markers to identify Nix workspaces.
2. Execute deadnix '{file}' and retain exit codes, scope, and tool versions.
3. Distinguish formatting, static rules, configuration, dependency, and environment failures.
4. Use nixpkgs-fmt '{file}' to handle reversible issues and re-scan.
5. Report local results in layers with CI/build/test evidence.

⚠️ Requires conditions

1. The target tool must be installed and its version compatible with project constraints.
2. It must be executed from the correct repository or module root directory.
3. If the project has custom configuration, the committed configuration takes precedence.
4. Whether generated code, vendor, and cache directories are scanned must follow repository rules.

❌ Not applicable

1. Do not treat formatter passing as compilation, test, or security audit passing.
2. Do not silently install tools, upgrade dependencies, or modify team rules.
3. Do not use suppress/ignore/raise thresholds to conceal real problems.
4. Do not claim "checks pass" when tools or configuration are missing.

When to use

• The user requests checking or fixing Nix code standards.
• AI modified .nix, needs pre-commit verification.
• CI shows failures related to deadnix '{file}' .
• Need to determine whether an issue can be automatically fixed, or must be handled manually.

When not to use

• When the main task is learning language syntax, framework design, or business modeling, use the corresponding language professional skill instead.
• When the main task is dependency vulnerabilities, security auditing, or license governance, use specialized security skills.
• When the user only asks to explain a single error and there is no project context, first do a local explanation, and do not claim the whole repository is green.

Data and security

This skill does not collect, upload, send, or store user code and credentials. By default it only reads local repository configuration and runs local tools; it does not install dependencies over the network, and does not record tokens, passwords, or private source code. Before any automatic fix, inspect the diff first; destructive or large-scale changes must be explicitly confirmed by the user.

Execution contract

Item	Current facts
Status	stable(since Codeguard V0.3)
File extension	.nix
Project marker	flake.nix
Integration configuration	Based on the repository's existing configuration
Tool preparation	nix-env -iA nixpkgs.nixpkgs-fmt nixpkgs.deadnix
Probe command	使用命令查找与项目配置检查
lint	deadnix '{file}'
format/fix	nixpkgs-fmt '{file}'
Project-level gate	bash -c 'find . -name '"'"'*.nix'"'"' -type f -print0 | xargs -0 -r deadnix'

Standard Workflow

Step 1: Determine the real scope

Confirm the repository root, module roots, generated directories, vendor directories, and user-specified files. For multi-module projects, first list the modules that will be scanned; never guess based on the current directory.

Step 2: Read the project contract

Read the actual commands in "Based on the repository's existing configuration" and CI. Repository configuration takes precedence over the examples in this skill; if the two conflict, report the difference first.

Step 3: Probe tools and versions

Run 对应工具 --version. If a tool is missing, the version is incompatible, or the configuration cannot be loaded, record it as "unable to verify," not as passing.

Step 4: Run read-only checks

Run bash -c 'find . -name '"'"'*.nix'"'"' -type f -print0 | xargs -0 -r deadnix', and record the command, working directory, exit code, tool version, and first actionable error.

Step 5: Failure classification

Type	Typical signal	Handling
Formatting	Indentation, whitespace, import order	Formatter allowed, then check the diff
Static rules	unused, complexity, naming, dangerous API	Minimal semantic fix; no bulk guessing
Configuration	parser/config/schema not found	Fix the integration or report preconditions
Environment	command not found, version conflict	Report installation/version requirements; do not install silently
Tool internal error	crash, timeout, parser exception	Preserve the original evidence and narrow the reproduction scope

Step 6: Controlled fixes

Prefer running nixpkgs-fmt '{file}'. Only handle issues that are certain and reversible; dependency upgrades, rule exemptions, generated files, and business-semantic changes must be explained separately.

Step 7: Verification loop

Re-run the same lint command, then run the project's existing tests and build. Only write "passing" when the command, scope, and exit code are all explicit; otherwise write "not run" or "unable to verify."

Rules

1. Re-verify with the same command: after fixing, you must re-run the original command that triggered the failure.
2. Layered evidence: report lint, format, compile, test, and security separately.
3. Minimal changes: do not refactor along the way, and do not expand a style fix into a business rewrite.
4. Configuration takes precedence: do not override the project's existing ignore, dialect, target, or version constraints.
5. No fabrication: tools not run, configurations not seen, and platforms not verified must be clearly marked.

Output template

Nix Codeguard 结果
- 范围：<仓库/模块/文件>
- 工具：<名称与版本>
- 命令：<实际命令>
- 状态：PASS / FAIL / UNVERIFIED / PLANNED
- 发现：<按格式/规则/配置/环境分类>
- 修改：<文件与原因；无修改写 none>
- 复验：<命令、退出码>
- 未验证：<测试/构建/平台差异>

Gotchas

1. A missing tool is not passing — must output UNVERIFIED and a clear preparation method.
2. A formatter is not a linter — an all-green format check cannot override static rules or compilation errors.
3. The root directory determines the result — Running in the wrong module may miss detections or load the wrong configuration.
4. Placeholders are not literal arguments — {file} and must be replaced with real, authorized paths.
5. Automatic fixes may expand the diff — after fixing, review the diff first, then run the regression gate.
6. Generated and vendor directories require an explicit policy — do not casually scan the entire repository or modify third-party content.

Domain pitfalls and fixes

1. Domain pitfalls — deadnix finds unused expressions, nixpkgs-fmt only formats; you still need to run flake/check or a real build.

When information is insufficient

Do not just say "please provide more information." First give a safe read-only inspection plan, and list the specific items that still need confirmation:

1. repository/module root directory;
2. target tools and versions;
3. project configuration files;
4. the authoritative command in CI;
5. whether automatic fixes are allowed.

FAQ

Q1: If the tool is not installed, can you conclude the code is fine? No. The status must be UNVERIFIED。

Q2: Can you automatically add ignore or suppression? No. Exemption is possible only when the user explicitly accepts it and the reason is recorded.

Q3: Why does it still fail after formatting? The formatter only covers formatting; static rules, configuration, compilation, and tests still need to be handled separately.

Q4: Should generated code be scanned? By default, follow the repository configuration; when there are no rules, exclude generated/vendor first, then explain to the user.

Q5: How can you avoid changing too much at once? First fix in batches by failure category and file; rerun the original command and review the diff for each batch.

Q6: If it passes locally, can you say CI will pass? No. You also need to align the CI's versions, environment variables, operating system, and command scope.

Load resources on demand

• When encountering exit codes, failure classification, or exemption decisions, read references/rules/gate-rules.md。
• When you need to check this language's registry, toolchain, and project configuration, read references/tooling/toolchain.md。
• When you need copyable task examples, read by scenario examples/, no need to load them all at once.