Extract is_llm_handler_type() to deduplicate 4 match-arm sites

The pattern Some("agent") | Some("agent_loop") | Some("prompt") |
Some("one_shot") was duplicated across preamble.rs (3x) and
validation/rules.rs (1x). Centralizes into a single function in
graph/types.rs. Also fixes stale doc comment on default_registry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/arc-workflows/src/graph/types.rs

@@ -67,6 +67,12 @@ impl AttrValue {
     }
 }
 
+/// Returns true if the handler type is an LLM-based handler (agent or prompt, including legacy aliases).
+#[must_use]
+pub fn is_llm_handler_type(handler_type: Option<&str>) -> bool {
+    matches!(handler_type, Some("agent") | Some("agent_loop") | Some("prompt") | Some("one_shot"))
+}
+
 /// Maps Graphviz shapes to handler type strings (Section 2.8).
 #[must_use]
 pub fn shape_to_handler_type(shape: &str) -> Option<&'static str> {
@@ -480,6 +486,17 @@ mod tests {
         assert_eq!(shape_to_handler_type("unknown"), None);
     }
 
+    #[test]
+    fn is_llm_handler_type_checks() {
+        assert!(is_llm_handler_type(Some("agent")));
+        assert!(is_llm_handler_type(Some("agent_loop")));
+        assert!(is_llm_handler_type(Some("prompt")));
+        assert!(is_llm_handler_type(Some("one_shot")));
+        assert!(!is_llm_handler_type(Some("command")));
+        assert!(!is_llm_handler_type(Some("human")));
+        assert!(!is_llm_handler_type(None));
+    }
+
     #[test]
     fn node_defaults() {
         let node = Node::new("test");

crates/arc-workflows/src/handler/mod.rs

@@ -114,8 +114,8 @@ impl HandlerRegistry {
 
 /// Build a [`HandlerRegistry`] with all built-in handler types registered.
 ///
-/// The `make_backend` closure is called once per handler that needs a backend
-/// (`AgentHandler` default, explicit `"agent"`, and `"parallel.fan_in"`).
+/// The `make_backend` closure is called for each handler that needs a backend
+/// (default, `"agent"`, `"agent_loop"`, `"prompt"`, `"one_shot"`, and `"parallel.fan_in"`).
 #[must_use]
 pub fn default_registry(
     interviewer: Arc<dyn Interviewer>,

crates/arc-workflows/src/preamble.rs

@@ -2,7 +2,7 @@ use std::collections::{HashMap, HashSet};
 
 use crate::artifact::{artifact_path, format_artifact_reference};
 use crate::context::Context;
-use crate::graph::{Graph, Node};
+use crate::graph::{is_llm_handler_type, Graph, Node};
 use crate::outcome::Outcome;
 
 /// Build a fidelity-appropriate preamble string for non-full context modes.
@@ -157,7 +157,7 @@ fn render_compact_stage_details(
             }
             lines
         }
-        Some("agent") | Some("agent_loop") | Some("prompt") | Some("one_shot") => {
+        h if is_llm_handler_type(h) => {
             let mut lines = Vec::new();
             if let Some(usage) = &outcome.usage {
                 let input = format_token_count(usage.input_tokens);
@@ -234,7 +234,7 @@ fn render_summary_high_stage_section(
                 }
             }
         }
-        Some("agent") | Some("agent_loop") | Some("prompt") | Some("one_shot") => {
+        h if is_llm_handler_type(h) => {
             if let Some(usage) = &outcome.usage {
                 lines.push(format!("- Model: {}", usage.model));
                 lines.push(format!(
@@ -497,7 +497,7 @@ fn build_summary_preamble(
                                     }
                                 }
                             }
-                            Some("agent") | Some("agent_loop") | Some("prompt") | Some("one_shot") => {
+                            h if is_llm_handler_type(h) => {
                                 if let Some(usage) = &outcome.usage {
                                     parts.push(format!("  - Model: {}", usage.model));
                                 }

crates/arc-workflows/src/validation/rules.rs

@@ -2,7 +2,7 @@ use std::collections::{HashSet, VecDeque};
 use std::str::FromStr;
 
 use crate::condition::parse_condition;
-use crate::graph::{AttrValue, Graph};
+use crate::graph::{is_llm_handler_type, AttrValue, Graph};
 
 use super::{Diagnostic, LintRule, Severity};
 
@@ -620,7 +620,7 @@ impl LintRule for PromptOnLlmNodesRule {
     fn apply(&self, graph: &Graph) -> Vec<Diagnostic> {
         let mut diagnostics = Vec::new();
         for node in graph.nodes.values() {
-            if matches!(node.handler_type(), Some("agent") | Some("agent_loop") | Some("prompt") | Some("one_shot")) {
+            if is_llm_handler_type(node.handler_type()) {
                 let has_prompt = node.prompt().is_some_and(|p| !p.is_empty());
                 let has_label = node
                     .attrs