diff --git a/src/graph/queries.rs b/src/graph/queries.rs
index 7508e45..ee25cc4 100644
--- a/src/graph/queries.rs
+++ b/src/graph/queries.rs
@@ -347,4 +347,92 @@ topic main:
         // At least one edge should exist (the Routes edge from start_agent → main)
         assert!(graph.edge_count() > 0, "Expected at least one edge in the graph");
     }
+
+    /// Source with a topic that defines an action def and a reasoning action that invokes
+    /// it, reads a variable, and writes another variable.
+    fn source_with_invocation() -> &'static str {
+        r#"config:
+   agent_name: "Test"
+
+variables:
+   param_val: mutable string = ""
+   result: mutable string = ""
+
+topic main:
+   description: "Main topic"
+
+   actions:
+      do_something:
+         description: "An action"
+         inputs:
+            param: string
+               description: "Input parameter"
+         outputs:
+            data: string
+               description: "Output data"
+         target: "flow://DoSomething"
+
+   reasoning:
+      instructions: "Handle"
+      actions:
+         invoke_something: @actions.do_something
+            description: "Invoke do_something"
+            with param=@variables.param_val
+            set @variables.result = @outputs.data
+"#
+    }
+
+    #[test]
+    fn test_find_action_invokers() {
+        // A reasoning action referencing @actions.do_something via an Invokes edge should
+        // appear in find_action_invokers(do_something_idx).
+        let graph = parse_and_build(source_with_invocation());
+        let action_def_idx = graph
+            .get_action_def("main", "do_something")
+            .expect("do_something action def not found");
+        let invokers = graph.find_action_invokers(action_def_idx);
+        assert_eq!(invokers.len(), 1, "Expected exactly 1 invoker for do_something");
+    }
+
+    #[test]
+    fn test_find_variable_readers() {
+        // A reasoning action with `with param=@variables.param_val` creates a Reads edge.
+        // find_variable_readers should return that action.
+        let graph = parse_and_build(source_with_invocation());
+        let var_idx = graph
+            .get_variable("param_val")
+            .expect("param_val variable not found");
+        let readers = graph.find_variable_readers(var_idx);
+        assert!(!readers.is_empty(), "Expected at least one reader for param_val");
+    }
+
+    #[test]
+    fn test_find_variable_writers() {
+        // A reasoning action with `set @variables.result = @outputs.data` creates a Writes
+        // edge. find_variable_writers should return that action.
+        let graph = parse_and_build(source_with_invocation());
+        let var_idx = graph
+            .get_variable("result")
+            .expect("result variable not found");
+        let writers = graph.find_variable_writers(var_idx);
+        assert!(!writers.is_empty(), "Expected at least one writer for result");
+    }
+
+    #[test]
+    fn test_get_topic_reasoning_actions() {
+        // get_topic_reasoning_actions should return the reasoning actions declared in a
+        // topic's reasoning block (one in this case: invoke_something).
+        let graph = parse_and_build(source_with_invocation());
+        let actions = graph.get_topic_reasoning_actions("main");
+        assert_eq!(actions.len(), 1, "Expected 1 reasoning action in topic main");
+    }
+
+    #[test]
+    fn test_get_topic_action_defs() {
+        // get_topic_action_defs should return the action definitions declared in a topic's
+        // actions block (one in this case: do_something).
+        let graph = parse_and_build(source_with_invocation());
+        let defs = graph.get_topic_action_defs("main");
+        assert_eq!(defs.len(), 1, "Expected 1 action def in topic main");
+    }
 }
diff --git a/tests/test_serializer_roundtrip.rs b/tests/test_serializer_roundtrip.rs
index cc1c831..e3978db 100644
--- a/tests/test_serializer_roundtrip.rs
+++ b/tests/test_serializer_roundtrip.rs
@@ -216,3 +216,95 @@ topic main:
     assert!(topic.before_reasoning.is_some(), "before_reasoning lost after roundtrip");
     assert!(topic.after_reasoning.is_some(), "after_reasoning lost after roundtrip");
 }
+
+#[test]
+fn test_roundtrip_system_block_with_messages() {
+    // Covers the top-level `system:` block containing both `instructions:` and
+    // `messages:` (welcome + error).  The serializer writes these fields but no
+    // roundtrip test existed — this ensures they survive parse → serialize → parse.
+    let original = r#"config:
+   agent_name: "SupportAgent"
+
+system:
+   instructions: "You are a helpful support agent."
+
+   messages:
+      welcome: "Hello! How can I help you today?"
+      error: "I had trouble processing your request. Please try again."
+
+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("messages:"), "Missing messages block in serialized output");
+
+    let reparsed = parse(&serialized).expect("Failed to reparse serialized");
+    assert!(reparsed.system.is_some(), "system block lost after roundtrip");
+    let system = &reparsed.system.as_ref().unwrap().node;
+    assert!(system.instructions.is_some(), "instructions lost after roundtrip");
+    let messages = system.messages.as_ref().expect("messages 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");
+}
+
+#[test]
+fn test_roundtrip_action_with_various_param_types() {
+    // Covers action defs with non-string parameter types: `id`, `boolean`, `number`.
+    // These types are serialized differently from `string` but no roundtrip test
+    // existed for them.
+    let original = r#"config:
+   agent_name: "TypedAgent"
+
+topic main:
+   description: "Main"
+   actions:
+      lookup_record:
+         description: "Look up a Salesforce record"
+         inputs:
+            record_id: id
+               description: "Salesforce record ID"
+               is_required: True
+            include_details: boolean
+               description: "Whether to include full details"
+               is_required: False
+            max_results: number
+               description: "Maximum number of results to return"
+         outputs:
+            found: boolean
+               description: "Whether the record was found"
+            score: number
+               description: "Confidence score"
+         target: "flow://LookupRecord"
+   reasoning:
+      instructions: "Help the user look up records"
+"#;
+
+    let ast = parse(original).expect("Failed to parse original");
+    let serialized = serialize(&ast);
+
+    assert!(serialized.contains("lookup_record:"), "Missing action def in serialized output");
+    assert!(serialized.contains(": id"), "Missing id param type");
+    assert!(serialized.contains(": boolean"), "Missing boolean param type");
+    assert!(serialized.contains(": number"), "Missing number param type");
+
+    let reparsed = parse(&serialized).expect("Failed to reparse serialized");
+    assert_eq!(reparsed.topics.len(), 1);
+    let topic = &reparsed.topics[0].node;
+    let actions = topic.actions.as_ref().expect("Missing actions block after roundtrip");
+    assert_eq!(actions.node.actions.len(), 1, "Expected 1 action def");
+    let action = &actions.node.actions[0].node;
+    assert_eq!(
+        action.inputs.as_ref().expect("inputs missing").node.len(),
+        3,
+        "Expected 3 inputs"
+    );
+    assert_eq!(
+        action.outputs.as_ref().expect("outputs missing").node.len(),
+        2,
+        "Expected 2 outputs"
+    );
+}