Assisted-by: Claude Opus 4.6 via Claude Code

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+---
+name: ruby-typing
+description: Use when adding type annotations to Ruby code, running Steep or rbs-inline, or troubleshooting type errors in Ruby projects
+---
+
+# Ruby Type Checking with Steep and rbs-inline
+
+## Workflow
+
+1. Add `# rbs_inline: enabled` at the top of the file
+2. Annotate methods and Data.define fields with `#:` comments
+3. Run `rbs-inline --output lib/` to generate `.rbs` files under `sig/generated/`
+4. Run `steep check` to verify
+
+## rbs-inline Annotations
+
+Method signatures use `#:` on the line before `def`:
+
+```ruby
+#: (String, Integer) -> String
+def repeat(str, count)
+  str * count
+end
+
+#: () -> void
+def reset!
+  @count = 0
+end
+```
+
+Data.define fields are annotated inline per member:
+
+```ruby
+MyConfig = Data.define(
+  :name,     #: String
+  :timeout,  #: Integer?
+)
+```
+
+This generates typed `attr_reader`, `self.new`, and `members` in the RBS output.
+
+## Known Limitations
+
+rbs-inline ignores methods defined inside a `Data.define` block. Reopen the class in a separate body:
+
+```ruby
+# Won't work — rbs-inline can't see parse:
+MyData = Data.define(:name) do
+  def self.parse(value) = new(name: value)
+end
+
+# Works — reopen the class:
+MyData = Data.define(:name)
+class MyData
+  #: (String) -> MyData
+  def self.parse(value) = new(name: value)
+end
+```
+
+`String#split` returns `Array[String]`, so destructuring gives `String?` for each element. Use a tuple assertion to tell Steep the exact shape:
+
+```ruby
+login, name = value.split(":", 2) #: [String, String?]
+```
+
+## Steep Tips
+
+`Hash[String, String]` for env-like hashes. Use `String?` values only if the hash actually contains nils — `ENV#fetch` with a block returns `String`, not `String?`.
+
+Steep differentiates `instance` from the class name in return types. `Data.define` constructors return `instance`; your own class methods should return the class name (`MyData`, not `instance`).
+
+## Quick Reference
+
+| Annotation | Meaning |
+|---|---|
+| `#: (String) -> Integer` | Method type (before def) |
+| `#: String` | Field/attr type (inline after member) |
+| `#: [String, String?]` | Tuple type assertion (inline after expression) |
+| `# @rbs skip` | Exclude from RBS generation |
+| `# @rbs override` | Method overrides parent |
+| `# rbs_inline: enabled` | Opt file into RBS generation |