diff --git a/src/graph/queries.rs b/src/graph/queries.rs
index 7508e45..a095b79 100644
--- a/src/graph/queries.rs
+++ b/src/graph/queries.rs
@@ -302,6 +302,144 @@ topic topic_b:
         );
     }
 
+    #[test]
+    fn test_find_variable_readers_via_with_clause() {
+        // A reasoning action that passes a variable via a `with` clause creates a
+        // RefEdge::Reads edge from the reasoning action to the variable.
+        // find_variable_readers() must return that reasoning action.
+        let source = r#"config:
+   agent_name: "Test"
+
+variables:
+   order_id: mutable string = ""
+      description: "Order identifier"
+
+topic main:
+   description: "Main"
+
+   actions:
+      get_order:
+         description: "Retrieve order details"
+         inputs:
+            id: string
+               description: "Order ID"
+         outputs:
+            status: string
+               description: "Order status"
+         target: "flow://GetOrder"
+
+   reasoning:
+      instructions: "Help"
+      actions:
+         fetch_order: @actions.get_order
+            description: "Fetch the order by ID"
+            with id = @variables.order_id
+"#;
+        let graph = parse_and_build(source);
+
+        let var_idx = graph.get_variable("order_id").expect("order_id variable not found");
+        let action_idx = graph
+            .get_reasoning_action("main", "fetch_order")
+            .expect("fetch_order reasoning action not found");
+
+        let readers = graph.find_variable_readers(var_idx);
+        assert_eq!(readers.len(), 1, "Expected exactly 1 reader for order_id");
+        assert_eq!(
+            readers.nodes[0], action_idx,
+            "Expected fetch_order reasoning action to be the reader"
+        );
+    }
+
+    #[test]
+    fn test_find_variable_writers_via_set_clause() {
+        // A reasoning action that writes to a variable via a `set` clause creates a
+        // RefEdge::Writes edge from the reasoning action to the variable.
+        // find_variable_writers() must return that reasoning action.
+        let source = r#"config:
+   agent_name: "Test"
+
+variables:
+   result_status: mutable string = ""
+      description: "Most-recent status value"
+
+topic main:
+   description: "Main"
+
+   actions:
+      get_status:
+         description: "Retrieve status from backend"
+         outputs:
+            status: string
+               description: "Current status"
+         target: "flow://GetStatus"
+
+   reasoning:
+      instructions: "Help"
+      actions:
+         check_status: @actions.get_status
+            description: "Check status and store result"
+            set @variables.result_status = @outputs.status
+"#;
+        let graph = parse_and_build(source);
+
+        let var_idx = graph.get_variable("result_status").expect("result_status variable not found");
+        let action_idx = graph
+            .get_reasoning_action("main", "check_status")
+            .expect("check_status reasoning action not found");
+
+        let writers = graph.find_variable_writers(var_idx);
+        assert_eq!(writers.len(), 1, "Expected exactly 1 writer for result_status");
+        assert_eq!(
+            writers.nodes[0], action_idx,
+            "Expected check_status reasoning action to be the writer"
+        );
+    }
+
+    #[test]
+    fn test_find_action_invokers_returns_reasoning_action() {
+        // A reasoning action whose target is `@actions.<name>` creates a
+        // RefEdge::Invokes edge from the reasoning action to the action definition.
+        // find_action_invokers() must return that reasoning action.
+        let source = r#"config:
+   agent_name: "Test"
+
+topic main:
+   description: "Main"
+
+   actions:
+      lookup_data:
+         description: "Look up data from backend"
+         inputs:
+            id: string
+               description: "Record identifier"
+         outputs:
+            result: string
+               description: "Lookup result"
+         target: "flow://LookupData"
+
+   reasoning:
+      instructions: "Help"
+      actions:
+         do_lookup: @actions.lookup_data
+            description: "Perform the data lookup"
+"#;
+        let graph = parse_and_build(source);
+
+        let action_def_idx = graph
+            .get_action_def("main", "lookup_data")
+            .expect("lookup_data action def not found");
+        let reasoning_idx = graph
+            .get_reasoning_action("main", "do_lookup")
+            .expect("do_lookup reasoning action not found");
+
+        let invokers = graph.find_action_invokers(action_def_idx);
+        assert_eq!(invokers.len(), 1, "Expected exactly 1 invoker for lookup_data");
+        assert_eq!(
+            invokers.nodes[0], reasoning_idx,
+            "Expected do_lookup reasoning action to be the invoker"
+        );
+    }
+
     #[test]
     fn test_stats_counts_nodes_correctly() {
         // Verify that stats() correctly counts topics, action defs, and variables.
diff --git a/tests/integration_test.rs b/tests/integration_test.rs
index f5db09b..6cdbb6e 100644
--- a/tests/integration_test.rs
+++ b/tests/integration_test.rs
@@ -271,4 +271,36 @@ topic main:
             .collect();
         assert!(tokens.iter().any(|t| matches!(t, lexer::Token::Config)));
     }
+
+    #[test]
+    fn test_parse_none_default_on_non_boolean_variable_returns_error() {
+        // `= None` is only a valid default for `boolean` types.  The parser's
+        // validate() callback emits a custom error when it appears on any other type
+        // (e.g. `integer`).  This test verifies that parse_source returns Err and
+        // that at least one error message mentions None or the boolean type
+        // restriction — covering a previously untested parser error path.
+        let source = r#"config:
+   agent_name: "Test"
+
+variables:
+   counter: mutable integer = None
+      description: "Counter — None is not a valid default for integer"
+
+topic main:
+   description: "Main"
+"#;
+        let result = parse_source(source);
+        assert!(
+            result.is_err(),
+            "Expected parse_source to return Err for '= None' on a non-boolean type, got Ok"
+        );
+        let errors = result.unwrap_err();
+        assert!(!errors.is_empty(), "Expected at least one error message");
+        // The validate() callback formats a message that includes "None" and "boolean"
+        assert!(
+            errors.iter().any(|e| e.contains("None") || e.contains("boolean")),
+            "Expected error message referencing None or boolean type, got: {:?}",
+            errors
+        );
+    }
 }
diff --git a/tests/test_serializer_roundtrip.rs b/tests/test_serializer_roundtrip.rs
index cc1c831..eb49ef4 100644
--- a/tests/test_serializer_roundtrip.rs
+++ b/tests/test_serializer_roundtrip.rs
@@ -181,6 +181,78 @@ topic main:
     assert_eq!(reparsed.language.as_ref().unwrap().node.entries.len(), 1);
 }
 
+#[test]
+fn test_roundtrip_system_block_with_messages() {
+    // Covers the `system:` block with both a `messages:` sub-block (welcome + error)
+    // and a top-level `instructions:` entry.  Ensures all three fields survive a
+    // parse → serialize → parse cycle.
+    let original = r#"config:
+   agent_name: "SystemAgent"
+
+system:
+   messages:
+      welcome: "Welcome to our service!"
+      error: "Something went wrong. Please try again."
+   instructions: "You are a helpful assistant."
+
+topic main:
+   description: "Main"
+"#;
+
+    let ast = parse(original).expect("Failed to parse original");
+    let serialized = serialize(&ast);
+
+    assert!(serialized.contains("system:"), "Missing system block in serialized output");
+    assert!(serialized.contains("welcome"), "Missing welcome message key");
+    assert!(serialized.contains("error"), "Missing error message key");
+
+    let reparsed = parse(&serialized).expect("Failed to reparse serialized");
+    let system = reparsed.system.as_ref().expect("system block lost after roundtrip");
+    let messages = system.node.messages.as_ref().expect("messages sub-block lost after roundtrip");
+    assert!(messages.node.welcome.is_some(), "welcome message lost after roundtrip");
+    assert!(messages.node.error.is_some(), "error message lost after roundtrip");
+    assert!(system.node.instructions.is_some(), "instructions lost after roundtrip");
+}
+
+#[test]
+fn test_roundtrip_linked_variable() {
+    // Covers the `linked` variable kind with a `source:` reference — the serializer
+    // handles it but no roundtrip test existed.  Ensures the kind, type, and source
+    // survive a parse → serialize → parse cycle.
+    let original = r#"config:
+   agent_name: "LinkedVarAgent"
+
+variables:
+   user_email: linked string
+      description: "User email injected from conversation context"
+      source: @messagingSession.userEmail
+
+topic main:
+   description: "Main"
+"#;
+
+    let ast = parse(original).expect("Failed to parse original");
+    let serialized = serialize(&ast);
+
+    assert!(serialized.contains("linked"), "Missing 'linked' keyword in serialized output");
+    assert!(serialized.contains("user_email"), "Missing variable name in serialized output");
+
+    let reparsed = parse(&serialized).expect("Failed to reparse serialized");
+    let vars = &reparsed.variables.as_ref().expect("variables block lost after roundtrip").node;
+    assert_eq!(vars.variables.len(), 1, "Expected exactly 1 variable after roundtrip");
+    let var = &vars.variables[0].node;
+    assert_eq!(
+        var.kind,
+        busbar_sf_agentscript::ast::VariableKind::Linked,
+        "Expected Linked variable kind after roundtrip"
+    );
+    assert_eq!(
+        var.ty.node,
+        busbar_sf_agentscript::Type::String,
+        "Expected String type after roundtrip"
+    );
+}
+
 #[test]
 fn test_roundtrip_before_and_after_reasoning() {
     // Covers `before_reasoning:` and `after_reasoning:` directive blocks inside a topic.