Writing a pi-lens ast-grep Rule

Drop path: rules/ast-grep-rules/rules/<id>.yml
Same id as a built-in overrides it. Multiple rules per file: separate with ---.

Minimal template

id: no-foo-bar
language: TypeScript        # PascalCase — see languages below
severity: warning           # error | warning | hint | info — pick by evidence, see below
message: "Avoid foo.bar() — use baz() instead"
note: |
  Longer explanation / fix guidance here.
rule:
  pattern: foo.bar($ARG)

Pick the severity by the evidence behind the rule, not by feel:

• error — only with a documented zero-false-positive audit in the note. Only error maps to semantic blocking and stops a turn.
• warning — a real finding with a known, bounded false-positive rate.
• hint / info — style opinions. They render as advisory text, never block, and lose the report budget to warnings when a report is capped.

Full policy and the error-promotion procedure: AGENTS.md's "Severity policy (#1777)" section. Read it before shipping anything above warning.

Before promoting to error

Three things must all be true, and the rule's note must record them:

1. A multi-corpus false-positive census, not a single-tree count. Run the rule over pi-lens's clients//tests/ plus at least one real external codebase of the kind the rule targets, classify every hit, and put the table in the note.
2. Structural narrowing before exemption. Suppress a legitimate idiom with a relational constraint (inside/has/follows, scoped path globs), not with prose telling readers to ignore the hit. Narrow first — don't reach for a lower tier as the easy way out. no-non-null-assertion was demoted to hint (0124608a), then reverted back to warning once the same false positives were closed with a structural exclusion instead (b3e1fd79).
3. Self-scan wiring. Tag the rule metadata.category: pi-lens-self-scan so npm run astgrep:self-scan holds this tree at zero in CI. An error rule that never runs against pi-lens's own source is an unaudited claim.

If the post-narrowing residual is still tens of legitimate hits, stop and report the numbers instead of shipping at error.

Language values

TypeScript JavaScript Python Go Rust Java C Cpp CSharp Kotlin Ruby Php

Rule conditions

rule:
  pattern: foo($X)          # ast-grep pattern — $X single, $$$ARGS multi
  kind: call_expression     # AST node kind (alternative to pattern)
  regex: "secret|token"     # regex on node text
  has:                      # descendant must match
    pattern: await $$$
  not:
    kind: comment
  any:
    - pattern: foo($X)
    - pattern: bar($X)
  all:
    - pattern: $OBJ.send($$$)
    - not: { kind: await_expression }

Relational & constraint conditions — all supported (native napi, #206)

The runner matches every rule through napi's native engine (root.findAll({rule, constraints})), fed by a faithful js-yaml parse. The full ast-grep grammar works — nest freely; nothing is silently skipped:

rule:
  kind: call_expression
  inside:                     # ancestor must match
    kind: function_declaration
    stopBy: end               # ↑ search ALL ancestors (default is direct parent)
  has:                        # descendant must match (default: DIRECT child)
    field: arguments          # field constraints work
  follows:                    # immediately-preceding sibling
    pattern: const $X = $V
constraints:                  # metavariable regex constraints work
  X:
    regex: "Error$"

⚠ has/inside default to the immediate child/parent (stopBy: neighbor). For a recursive descendant/ancestor search add stopBy: end. This is the #1 migration gotcha — see the has note in reference.md.

⚠ stopBy: end alone is not a boundary. It's a search-depth control — how far the walk searches — not a stop condition; the any: kind list only decides what CAN satisfy the match, not where the walk halts. To scope a relation to the nearest enclosing function (or any other boundary), give stopBy its own rule. See "Scoping to the nearest enclosing X" in reference.md (#1794 F1 — this bug shipped twice in one window).

YAML quoting — REQUIRED (js-yaml will reject the rule otherwise)

The parser is a real YAML parser, so unquoted special chars throw and the rule is silently dropped:

❌ message: !!value to coerce boolean    # `!!` is a YAML tag → js-yaml THROWS, rule dropped
✅ message: "!!value to coerce boolean"
❌ message: foo: bar baz                  # bare `:` → parsed as a nested mapping
✅ message: "foo: bar baz"
   Quote any scalar starting with  ! & * ? | > % @ `  or containing  : #
   Quote keyword-like kinds:  kind: "true"   (bare `true` becomes a boolean → invalid kind)

Gotchas

❌ Overly broad patterns — filtered out automatically
   $VAR  $NAME  $_  $X  $EXPR  (single bare metavar)

❌ PascalCase language is required
   language: typescript  →  language: TypeScript

❌ $VAR inside strings — matches literal "$VAR", not a metavar
   "from $PATH"  →  use tree-sitter or grep instead

✅ Test in playground: https://ast-grep.github.io/playground.html
✅ Schema + autocomplete: rules/ast-grep-rules/rule-schema.json
✅ Docs: docs/custom-rules.md

Reference doc — read before writing a NAPI-runner-specific or hard-to-express rule

reference.md (same directory) covers: ReDoS-safe regex authoring, node-text string-escape quirks, has/inside stopBy defaults (and when to override them), scoping a relation to the nearest enclosing node with a stopBy boundary rule, the -js twin dedup behavior (#657), boolean-parameter matching across the TS/JS grammars, and precision-over-recall heuristics for denylist-shaped rules. Read it when a rule isn't matching (or over-matching) the way you expect, or before shipping a regex/has-heavy rule.

Testing a suppression

Every relational suppression (a has/inside/not exclusion for a legitimate idiom) needs two fixtures per bound metavariable, not one:

1. A same-binding valid case — the idiom the exclusion is meant to suppress, with the metavariable bound consistently (same receiver, same key).
2. A mutation-guard invalid case — the same shape with the binding broken: a different key, a different receiver, or a boundary-crossing lookalike (the guard lives in the wrong scope). This proves the exclusion checks the BINDING, not just the pattern's presence somewhere in the file.

rules/ast-grep-rules/rule-tests/no-non-null-assertion-test.yml is the model: alongside the get-after-has/pop-after-length valid cases, it fixtures a different-key guard, a different-map guard, and an outer-function guard that must NOT suppress an inner closure — each one red if the corresponding binding check is deleted.

Corpus silence is not evidence. A valve with zero corpus hits needs adversarial fixtures MORE, not less — redundant-unsafe-function targets Rust, so it has no pi-lens corpus to census against, and its # Safety valve still shipped an unbounded backward scan that over-suppressed real detections, caught only by adversarial fixtures in review (00284bcc). A documented blind spot (an idiom the rule knowingly can't distinguish from a bug) becomes an invalid-direction fixture with a comment naming the gap, never silence.

Regenerate the catalog doc with npm run docs:rule-catalogs after adding or changing a rule — never hand-edit the generated catalog.

Validating a candidate rule against the REAL engine (not the warm MCP cache)

Live-binary discipline (AGENTS.md shape 16) applies here too: verify parsing and match behavior against a real ast-grep run before you write it into a rule note or a test fixture — a hand-written fixture pins a guess, not a fact.

# inspect how a PATTERN parses → find the node kind you actually need

ast-grep run -p 'x = false' --lang ts --debug-query=cst file.ts

# match by kind (──kind and ──pattern are mutually exclusive in `run`)

ast-grep run --kind required_parameter --lang ts file.ts

# run ONE rule from an sgconfig against a sample

ast-grep scan -c <sgconfig.yml> --filter '^<id>$' sample.ts

# run the fixture harness for one rule

ast-grep test -c rules/ast-grep-rules/.sgconfig.yml --skip-snapshot-tests --filter '<id>'