diff --git a/tests/test_validation.rs b/tests/test_validation.rs
new file mode 100644
index 0000000..40c0043
--- /dev/null
+++ b/tests/test_validation.rs
@@ -0,0 +1,270 @@
+//! Tests for `validate_ast` — the semantic validation layer.
+//!
+//! `validate_ast` enforces five rules that the parser cannot catch because they
+//! require semantic context rather than syntactic structure:
+//!
+//! 1. Mutable variables may not use `integer`, `long`, `datetime`, or `time` types.
+//! 2. Linked variables sourced from `@context.*` may not be of `object` type.
+//! 3. `additional_locales` codes in the `language:` block must be from the allow-list.
+//! 4. `outbound_route_type` in `connection:` blocks must be `"OmniChannelFlow"`.
+//! 5. Action input parameter names must not collide with AgentScript keywords.
+//!
+//! None of these rules had any test coverage before this file was added.
+
+use busbar_sf_agentscript::{parse, validation::{validate_ast, Severity}};
+
+// ============================================================================
+// Rule 1 — Mutable variable type restrictions
+// ============================================================================
+
+#[test]
+fn test_mutable_integer_variable_raises_error() {
+    // `integer` is not a permitted type for mutable variables.  The platform
+    // only accepts: String, Boolean, Number, Currency, Date, Id, Object, Timestamp.
+    let source = r#"config:
+   agent_name: "Test"
+
+variables:
+   counter: mutable integer = 0
+      description: "Turn counter"
+
+topic main:
+   description: "Main"
+   reasoning:
+      instructions: "Help"
+"#;
+    let ast = parse(source).expect("Should parse successfully");
+    let errors = validate_ast(&ast);
+    let rule1_errors: Vec<_> = errors
+        .iter()
+        .filter(|e| e.severity == Severity::Error && e.message.contains("counter"))
+        .collect();
+    assert!(
+        !rule1_errors.is_empty(),
+        "Expected a validation error for mutable integer variable 'counter', got: {:?}",
+        errors
+    );
+}
+
+#[test]
+fn test_mutable_string_variable_has_no_error() {
+    // `string` is a permitted type for mutable variables — no error expected.
+    let source = r#"config:
+   agent_name: "Test"
+
+variables:
+   name: mutable string = ""
+      description: "Customer name"
+
+topic main:
+   description: "Main"
+   reasoning:
+      instructions: "Help"
+"#;
+    let ast = parse(source).expect("Should parse successfully");
+    let errors = validate_ast(&ast);
+    let rule1_errors: Vec<_> = errors
+        .iter()
+        .filter(|e| e.severity == Severity::Error)
+        .collect();
+    assert!(
+        rule1_errors.is_empty(),
+        "Expected no errors for mutable string variable, got: {:?}",
+        errors
+    );
+}
+
+// ============================================================================
+// Rule 2 — Context variable may not be object type
+// ============================================================================
+
+#[test]
+fn test_context_linked_object_variable_raises_error() {
+    // A linked variable whose source namespace is `@context.*` must not use the
+    // `object` type — the platform cannot deserialize a generic object from context.
+    let source = r#"config:
+   agent_name: "Test"
+
+variables:
+   ctx_data: linked object
+      description: "Context payload"
+      source: @context.payload
+
+topic main:
+   description: "Main"
+   reasoning:
+      instructions: "Help"
+"#;
+    let ast = parse(source).expect("Should parse successfully");
+    let errors = validate_ast(&ast);
+    let rule2_errors: Vec<_> = errors
+        .iter()
+        .filter(|e| e.severity == Severity::Error && e.message.contains("ctx_data"))
+        .collect();
+    assert!(
+        !rule2_errors.is_empty(),
+        "Expected error for context-sourced object variable 'ctx_data', got: {:?}",
+        errors
+    );
+}
+
+// ============================================================================
+// Rule 3 — additional_locales must be from the allow-list
+// ============================================================================
+
+#[test]
+fn test_invalid_additional_locale_raises_error() {
+    // "xx_INVALID" is not in the supported locale code list and must be rejected.
+    let source = r#"config:
+   agent_name: "Test"
+
+language:
+   additional_locales: "xx_INVALID"
+
+topic main:
+   description: "Main"
+   reasoning:
+      instructions: "Help"
+"#;
+    let ast = parse(source).expect("Should parse successfully");
+    let errors = validate_ast(&ast);
+    let rule3_errors: Vec<_> = errors
+        .iter()
+        .filter(|e| e.severity == Severity::Error && e.message.contains("xx_INVALID"))
+        .collect();
+    assert!(
+        !rule3_errors.is_empty(),
+        "Expected error for unsupported locale 'xx_INVALID', got: {:?}",
+        errors
+    );
+}
+
+#[test]
+fn test_valid_additional_locale_has_no_error() {
+    // "es_MX" and "fr_CA" are valid locale codes — no errors expected.
+    let source = r#"config:
+   agent_name: "Test"
+
+language:
+   additional_locales: "es_MX,fr_CA"
+
+topic main:
+   description: "Main"
+   reasoning:
+      instructions: "Help"
+"#;
+    let ast = parse(source).expect("Should parse successfully");
+    let errors = validate_ast(&ast);
+    let rule3_errors: Vec<_> = errors
+        .iter()
+        .filter(|e| e.severity == Severity::Error)
+        .collect();
+    assert!(
+        rule3_errors.is_empty(),
+        "Expected no errors for valid locales 'es_MX,fr_CA', got: {:?}",
+        errors
+    );
+}
+
+// ============================================================================
+// Rule 4 — outbound_route_type must be "OmniChannelFlow"
+// ============================================================================
+
+#[test]
+fn test_invalid_outbound_route_type_raises_error() {
+    // Any value other than the literal string "OmniChannelFlow" is invalid.
+    let source = r#"config:
+   agent_name: "Test"
+
+connection live_agent:
+   escalation_message: "Connecting you now."
+   outbound_route_type: "Queue"
+   outbound_route_name: "SupportQueue"
+
+topic main:
+   description: "Main"
+   reasoning:
+      instructions: "Help"
+"#;
+    let ast = parse(source).expect("Should parse successfully");
+    let errors = validate_ast(&ast);
+    let rule4_errors: Vec<_> = errors
+        .iter()
+        .filter(|e| e.severity == Severity::Error && e.message.contains("outbound_route_type"))
+        .collect();
+    assert!(
+        !rule4_errors.is_empty(),
+        "Expected error for outbound_route_type \"Queue\", got: {:?}",
+        errors
+    );
+}
+
+#[test]
+fn test_valid_outbound_route_type_has_no_error() {
+    // "OmniChannelFlow" is the only accepted value and must not produce an error.
+    let source = r#"config:
+   agent_name: "Test"
+
+connection live_agent:
+   escalation_message: "Connecting you now."
+   outbound_route_type: "OmniChannelFlow"
+   outbound_route_name: "SupportQueue"
+
+topic main:
+   description: "Main"
+   reasoning:
+      instructions: "Help"
+"#;
+    let ast = parse(source).expect("Should parse successfully");
+    let errors = validate_ast(&ast);
+    let rule4_errors: Vec<_> = errors
+        .iter()
+        .filter(|e| e.severity == Severity::Error)
+        .collect();
+    assert!(
+        rule4_errors.is_empty(),
+        "Expected no errors for valid outbound_route_type, got: {:?}",
+        errors
+    );
+}
+
+// ============================================================================
+// Rule 5 — Action input parameter name keyword collision
+// ============================================================================
+
+#[test]
+fn test_action_input_named_description_raises_warning() {
+    // An action input named "description" shadows the AgentScript keyword and the
+    // platform may misparse the intent.  The validator should emit a Warning.
+    let source = r#"config:
+   agent_name: "Test"
+
+topic main:
+   description: "Main"
+
+   actions:
+      lookup:
+         description: "Looks up a record"
+         inputs:
+            description: string
+               description: "The record description (bad param name)"
+         outputs:
+            result: string
+               description: "Lookup result"
+         target: "flow://Lookup"
+
+   reasoning:
+      instructions: "Help the user"
+"#;
+    let ast = parse(source).expect("Should parse successfully");
+    let errors = validate_ast(&ast);
+    let rule5_warnings: Vec<_> = errors
+        .iter()
+        .filter(|e| e.severity == Severity::Warning && e.message.contains("description"))
+        .collect();
+    assert!(
+        !rule5_warnings.is_empty(),
+        "Expected a warning for action input named 'description', got: {:?}",
+        errors
+    );
+}