Add $$ escape mechanism for variable expansion

Allows literal $ signs in prompts and DOT files by writing $$. Also
refactors VariableExpansionTransform to use expand_vars instead of
string replace, fixing a substring-matching bug with $goal.

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

crates/arc-workflows/src/cli/run_config.rs

@@ -189,7 +189,10 @@ pub fn expand_vars(source: &str, vars: &HashMap<String, String>) -> anyhow::Resu
     while i < len {
         if bytes[i] == b'$' {
             let start = i + 1;
-            if start < len && (bytes[start].is_ascii_alphabetic() || bytes[start] == b'_') {
+            if start < len && bytes[start] == b'$' {
+                result.push('$');
+                i = start + 1;
+            } else if start < len && (bytes[start].is_ascii_alphabetic() || bytes[start] == b'_') {
                 let mut end = start + 1;
                 while end < len && (bytes[end].is_ascii_alphanumeric() || bytes[end] == b'_') {
                     end += 1;
@@ -315,6 +318,27 @@ language = "python"
         assert_eq!(expand_vars("costs $5", &vars).unwrap(), "costs $5");
     }
 
+    #[test]
+    fn expand_escaped_dollar() {
+        let vars = HashMap::from([("name".to_string(), "world".to_string())]);
+        assert_eq!(
+            expand_vars("literal $$name here", &vars).unwrap(),
+            "literal $name here"
+        );
+    }
+
+    #[test]
+    fn expand_escaped_dollar_at_end() {
+        let vars = HashMap::new();
+        assert_eq!(expand_vars("trailing $$", &vars).unwrap(), "trailing $");
+    }
+
+    #[test]
+    fn expand_escaped_dollar_before_non_ident() {
+        let vars = HashMap::new();
+        assert_eq!(expand_vars("price is $$5", &vars).unwrap(), "price is $5");
+    }
+
     #[test]
     fn parse_toml_with_sandbox() {
         let toml = r#"

crates/arc-workflows/src/transform.rs

@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 use crate::graph::{AttrValue, Edge, Graph, Node};
 use crate::stylesheet::{apply_stylesheet, parse_stylesheet};
 
@@ -12,12 +14,14 @@ pub struct VariableExpansionTransform;
 impl Transform for VariableExpansionTransform {
     fn apply(&self, graph: &mut Graph) {
         let goal = graph.goal().to_string();
+        let vars = HashMap::from([("goal".to_string(), goal)]);
         for node in graph.nodes.values_mut() {
             if let Some(AttrValue::String(prompt)) = node.attrs.get("prompt") {
-                if prompt.contains("$goal") {
-                    let expanded = prompt.replace("$goal", &goal);
-                    node.attrs
-                        .insert("prompt".to_string(), AttrValue::String(expanded));
+                if let Ok(expanded) = crate::cli::run_config::expand_vars(prompt, &vars) {
+                    if expanded != *prompt {
+                        node.attrs
+                            .insert("prompt".to_string(), AttrValue::String(expanded));
+                    }
                 }
             }
         }
@@ -192,6 +196,32 @@ mod tests {
         assert!(!graph.nodes["plan"].attrs.contains_key("prompt"));
     }
 
+    #[test]
+    fn variable_expansion_escaped_dollar_goal() {
+        let mut graph = Graph::new("test");
+        graph.attrs.insert(
+            "goal".to_string(),
+            AttrValue::String("Fix bugs".to_string()),
+        );
+
+        let mut node = Node::new("plan");
+        node.attrs.insert(
+            "prompt".to_string(),
+            AttrValue::String("literal $$goal here".to_string()),
+        );
+        graph.nodes.insert("plan".to_string(), node);
+
+        let transform = VariableExpansionTransform;
+        transform.apply(&mut graph);
+
+        let prompt = graph.nodes["plan"]
+            .attrs
+            .get("prompt")
+            .and_then(AttrValue::as_str)
+            .unwrap();
+        assert_eq!(prompt, "literal $goal here");
+    }
+
     #[test]
     fn stylesheet_transform_empty_stylesheet() {
         let mut graph = Graph::new("test");

docs/workflows/variables.mdx

@@ -42,7 +42,15 @@ When launched with `arc run start run.toml`, Arc replaces `$repo_name`, `$repo_u
 
 If a `$variable` in the DOT file has no matching entry in `[vars]`, Arc raises an error. This catches typos early — a misspelled `$langauge` fails immediately rather than passing a literal `$langauge` to the LLM.
 
-A bare `$` not followed by an identifier character (e.g. `costs $5`) is left as-is.
+### Escaping `$`
+
+To include a literal `$` in the output, write `$$`:
+
+```dot
+test [prompt="The env var is $$HOME"]
+```
+
+This produces `The env var is $HOME` without treating `$HOME` as a variable reference. A bare `$` not followed by an identifier character (e.g. `costs $5`) does not need escaping.
 
 ## The `$goal` variable