diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2645c59..d9b632c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -55,3 +55,4 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   and install Linux Swift toolchain prerequisites for the WASM CI job.
 - Harden duplicate-key validation to use set membership while preserving exact
   diagnostics.
+- Make validation rules immutable after construction.
diff --git a/Sources/Validation/Rule.swift b/Sources/Validation/Rule.swift
index f4c46e9..f695b82 100644
--- a/Sources/Validation/Rule.swift
+++ b/Sources/Validation/Rule.swift
@@ -1,9 +1,9 @@
 public extension PureYAML.Validation {
     /// A validation rule that can report zero or more path-aware issues.
     struct Rule: Sendable {
-        public var description: String
-        public var predicate: @Sendable (Context) -> Bool
-        public var check: @Sendable (Context) -> [Issue]
+        public let description: String
+        public let predicate: @Sendable (Context) -> Bool
+        public let check: @Sendable (Context) -> [Issue]
 
         public init(
             description: String,