diff --git a/.agents/skills/xtend-to-java/SKILL.md b/.agents/skills/xtend-to-java/SKILL.md
index a2f4238f07..2703e81652 100644
--- a/.agents/skills/xtend-to-java/SKILL.md
+++ b/.agents/skills/xtend-to-java/SKILL.md
@@ -90,6 +90,8 @@ Use this table for quick mechanical transforms. Full details in the rule files.
 | `var x = expr` | `ExplicitType x = expr;` |
 | `typeof(MyClass)` | `MyClass.class` |
 | `def dispatch method(Type1 x)` | Keep as `_method(Type1 x)` with `@SuppressWarnings` |
+| `@Inject extension Foo _foo` (field) | `private Foo foo;` — strip `_` prefix; names must match `[a-z][a-zA-Z0-9]*` |
+| Field `my_field` / `_my_field` | Rename to `myField` (camelCase, no underscores) |
 
 ### Operators and null handling
 
diff --git a/.agents/skills/xtend-to-java/rules/02-variables.md b/.agents/skills/xtend-to-java/rules/02-variables.md
index ef70f6bce6..b68be59af3 100644
--- a/.agents/skills/xtend-to-java/rules/02-variables.md
+++ b/.agents/skills/xtend-to-java/rules/02-variables.md
@@ -35,7 +35,44 @@ Always add explicit visibility (`private` unless a wider scope is genuinely need
 
 Always add explicit visibility.
 
-## 2.5 No leading underscore on non-dispatch fields
+## 2.5 Field member names must match `[a-z][a-zA-Z0-9]*`
 
-Non-dispatch private fields that got `_fieldName` from the converter: rename to `fieldName`.
-The `_` prefix is reserved for dispatch methods only (see [`rules/09-misc-syntax.md`](./09-misc-syntax.md)).
+The Checkstyle `MemberName` rule (default pattern `[a-z][a-zA-Z0-9]*`) is enforced on all
+non-constant instance and static-non-final fields. Every field name **must**:
+
+- Start with a lowercase letter.
+- Contain only ASCII letters and digits — **no underscores anywhere**.
+
+### Leading underscore from Xtend extension fields
+
+Xtend generates `_fieldName` for extension-injected fields. Strip the prefix:
+
+```java
+// Bad — Xtend-generated name
+private MyHelper _myHelper;
+
+// Good
+private MyHelper myHelper;
+```
+
+### Underscores inside the name — convert to camelCase
+
+Any underscore inside a name (rare in Xtend source but can appear when porting
+older code) must be removed by camelCasing the following word:
+
+| Original | Renamed |
+|----------|---------|
+| `my_helper` | `myHelper` |
+| `some_long_name` | `someLongName` |
+| `_some_field` | `someField` |
+
+### Scope of this rule
+
+Applies to **all non-constant member fields** — instance fields and non-final static
+fields. It does **not** apply to:
+
+- `static final` constants, which are validated by Checkstyle `ConstantName`
+  (pattern `[A-Z][A-Z0-9]*(_[A-Z0-9]+)*`, e.g. `LOGGER`, `DEFAULT_SIZE`).
+- Dispatch method parameters / local variables in dispatch methods (the `_` prefix
+  on dispatch methods themselves is intentional — see
+  [`rules/09-misc-syntax.md`](./09-misc-syntax.md)).
diff --git a/.agents/skills/xtend-to-java/workflow/validation-checklist.md b/.agents/skills/xtend-to-java/workflow/validation-checklist.md
index db5a25d469..4a353b1ece 100644
--- a/.agents/skills/xtend-to-java/workflow/validation-checklist.md
+++ b/.agents/skills/xtend-to-java/workflow/validation-checklist.md
@@ -20,7 +20,7 @@ Run through this list before declaring a conversion done. Every item is a hard g
 | 3 | No `val`/`var` | Always use explicit types. `var` is banned. |
 | 4 | No unnecessary boxing | `Integer.valueOf(i)` only when signature requires boxed type. |
 | 14 | LooseCoupling | Interface types (`List`, `Map`, `Set`, `EList`) in fields/params/returns, not `ArrayList`/`HashMap`/`BasicEList`. |
-| 16 | No leading underscore on non-dispatch fields | Rename `_fieldName` → `fieldName`. |
+| 16 | Field names match `[a-z][a-zA-Z0-9]*` | Rename `_fieldName` → `fieldName`. Convert any underscore inside a name to camelCase (`my_field` → `myField`). Applies to all non-constant member fields; `static final` constants are exempt (they follow `ConstantName`). |
 
 ### String building