diff --git a/DEVELOPMENT.md b/DEVELOPMENT.md
index 508eeaba..e4bedc06 100644
--- a/DEVELOPMENT.md
+++ b/DEVELOPMENT.md
@@ -60,3 +60,10 @@ Run the app to apply the migrations.
 If nothing happens, try `cargo clean` and run the app again.
 
 _Note: Development builds use a separate database location from production builds._
+
+## Lezer Grammer Generation
+
+```sh
+# Example
+lezer-generator components/core/Editor/<LANG>/<LANG>.grammar > components/core/Editor/<LANG>/<LANG>.ts
+```
diff --git a/src-tauri/Cargo.lock b/src-tauri/Cargo.lock
index 6e55d2c8..83c90a4e 100644
--- a/src-tauri/Cargo.lock
+++ b/src-tauri/Cargo.lock
@@ -7838,6 +7838,7 @@ dependencies = [
 name = "yaak-templates"
 version = "0.1.0"
 dependencies = [
+ "base64 0.22.1",
  "log",
  "serde",
  "serde_json",
diff --git a/src-tauri/src/lib.rs b/src-tauri/src/lib.rs
index 44e9cfff..4eb486af 100644
--- a/src-tauri/src/lib.rs
+++ b/src-tauri/src/lib.rs
@@ -114,8 +114,8 @@ async fn cmd_metadata(app_handle: AppHandle) -> Result<AppMetaData, ()> {
 }
 
 #[tauri::command]
-async fn cmd_parse_template(template: &str) -> Result<Tokens, String> {
-    Ok(Parser::new(template).parse())
+async fn cmd_parse_template(template: &str) -> YaakResult<Tokens> {
+    Ok(Parser::new(template).parse()?)
 }
 
 #[tauri::command]
diff --git a/src-tauri/yaak-templates/Cargo.toml b/src-tauri/yaak-templates/Cargo.toml
index e6dae6ec..e960475a 100644
--- a/src-tauri/yaak-templates/Cargo.toml
+++ b/src-tauri/yaak-templates/Cargo.toml
@@ -5,9 +5,10 @@ edition = "2021"
 publish = false
 
 [dependencies]
+base64 = "0.22.1"
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
 log = "0.4.22"
-serde = { version = "1.0.208", features = ["derive"] }
-serde_json = "1.0.132"
 thiserror = { workspace = true }
 tokio = { version = "1.39.3", features = ["macros", "rt"] }
 ts-rs = { version = "10.0.0" }
diff --git a/src-tauri/yaak-templates/src/parser.rs b/src-tauri/yaak-templates/src/parser.rs
index f4d6791c..b0f009ee 100644
--- a/src-tauri/yaak-templates/src/parser.rs
+++ b/src-tauri/yaak-templates/src/parser.rs
@@ -1,3 +1,7 @@
+use crate::error::Error::RenderError;
+use crate::error::Result;
+use base64::prelude::BASE64_URL_SAFE_NO_PAD;
+use base64::Engine;
 use serde::{Deserialize, Serialize};
 use std::fmt::Display;
 use ts_rs::TS;
@@ -43,7 +47,13 @@ pub enum Val {
 impl Display for Val {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let str = match self {
-            Val::Str { text } => format!("'{}'", text.to_string().replace("'", "\'")),
+            Val::Str { text } => {
+                if text.chars().all(|c| c.is_alphanumeric() || c == ' ' || c == '_' || c == '_') {
+                    format!("'{}'", text)
+                } else {
+                    format!("b64'{}'", BASE64_URL_SAFE_NO_PAD.encode(text))
+                }
+            }
             Val::Var { name } => name.to_string(),
             Val::Bool { value } => value.to_string(),
             Val::Fn { name, args } => {
@@ -108,13 +118,13 @@ impl Parser {
         }
     }
 
-    pub fn parse(&mut self) -> Tokens {
+    pub fn parse(&mut self) -> Result<Tokens> {
         let start_pos = self.pos;
 
         while self.pos < self.chars.len() {
             if self.match_str("${[") {
                 let start_curr = self.pos;
-                if let Some(t) = self.parse_tag() {
+                if let Some(t) = self.parse_tag()? {
                     self.push_token(t);
                 } else {
                     self.pos = start_curr;
@@ -131,29 +141,29 @@ impl Parser {
         }
 
         self.push_token(Token::Eof);
-        Tokens {
+        Ok(Tokens {
             tokens: self.tokens.clone(),
-        }
+        })
     }
 
-    fn parse_tag(&mut self) -> Option<Token> {
+    fn parse_tag(&mut self) -> Result<Option<Token>> {
         // Parse up to first identifier
         //    ${[ my_var...
         self.skip_whitespace();
 
-        let val = match self.parse_value() {
+        let val = match self.parse_value()? {
             Some(v) => v,
-            None => return None,
+            None => return Ok(None),
         };
 
         // Parse to closing tag
         //    ${[ my_var(a, b, c) ]}
         self.skip_whitespace();
         if !self.match_str("]}") {
-            return None;
+            return Ok(None);
         }
 
-        Some(Token::Tag { val })
+        Ok(Some(Token::Tag { val }))
     }
 
     #[allow(dead_code)]
@@ -167,9 +177,11 @@ impl Parser {
         );
     }
 
-    fn parse_value(&mut self) -> Option<Val> {
-        if let Some((name, args)) = self.parse_fn() {
+    fn parse_value(&mut self) -> Result<Option<Val>> {
+        let v = if let Some((name, args)) = self.parse_fn()? {
             Some(Val::Fn { name, args })
+        } else if let Some(v) = self.parse_string()? {
+            Some(Val::Str { text: v })
         } else if let Some(v) = self.parse_ident() {
             if v == "null" {
                 Some(Val::Null)
@@ -180,38 +192,38 @@ impl Parser {
             } else {
                 Some(Val::Var { name: v })
             }
-        } else if let Some(v) = self.parse_string() {
-            Some(Val::Str { text: v })
         } else {
             None
-        }
+        };
+
+        Ok(v)
     }
 
-    fn parse_fn(&mut self) -> Option<(String, Vec<FnArg>)> {
+    fn parse_fn(&mut self) -> Result<Option<(String, Vec<FnArg>)>> {
         let start_pos = self.pos;
 
         let name = match self.parse_fn_name() {
             Some(v) => v,
             None => {
                 self.pos = start_pos;
-                return None;
+                return Ok(None);
             }
         };
 
-        let args = match self.parse_fn_args() {
+        let args = match self.parse_fn_args()? {
             Some(args) => args,
             None => {
                 self.pos = start_pos;
-                return None;
+                return Ok(None);
             }
         };
 
-        Some((name, args))
+        Ok(Some((name, args)))
     }
 
-    fn parse_fn_args(&mut self) -> Option<Vec<FnArg>> {
+    fn parse_fn_args(&mut self) -> Result<Option<Vec<FnArg>>> {
         if !self.match_str("(") {
-            return None;
+            return Ok(None);
         }
 
         let start_pos = self.pos;
@@ -221,7 +233,7 @@ impl Parser {
         // Fn closed immediately
         self.skip_whitespace();
         if self.match_str(")") {
-            return Some(args);
+            return Ok(Some(args));
         }
 
         while self.pos < self.chars.len() {
@@ -231,7 +243,7 @@ impl Parser {
             self.skip_whitespace();
             self.match_str("=");
             self.skip_whitespace();
-            let value = self.parse_value();
+            let value = self.parse_value()?;
             self.skip_whitespace();
 
             if let (Some(name), Some(value)) = (name.clone(), value.clone()) {
@@ -239,7 +251,7 @@ impl Parser {
             } else {
                 // Didn't find valid thing, so return
                 self.pos = start_pos;
-                return None;
+                return Ok(None);
             }
 
             if self.match_str(")") {
@@ -251,7 +263,7 @@ impl Parser {
             // If we don't find a comma, that's bad
             if !args.is_empty() && !self.match_str(",") {
                 self.pos = start_pos;
-                return None;
+                return Ok(None);
             }
 
             if start_pos == self.pos {
@@ -259,7 +271,7 @@ impl Parser {
             }
         }
 
-        Some(args)
+        Ok(Some(args))
     }
 
     fn parse_ident(&mut self) -> Option<String> {
@@ -319,12 +331,17 @@ impl Parser {
         Some(text)
     }
 
-    fn parse_string(&mut self) -> Option<String> {
+    fn parse_string(&mut self) -> Result<Option<String>> {
         let start_pos = self.pos;
 
         let mut text = String::new();
-        if !self.match_str("'") {
-            return None;
+        let mut is_b64 = false;
+        if self.match_str("b64'") {
+            is_b64 = true;
+        } else if self.match_str("'") {
+            // Nothing
+        } else {
+            return Ok(None);
         }
 
         let mut found_closing = false;
@@ -350,10 +367,21 @@ impl Parser {
 
         if !found_closing {
             self.pos = start_pos;
-            return None;
+            return Ok(None);
         }
 
-        Some(text)
+        let final_text = if is_b64 {
+            let decoded = BASE64_URL_SAFE_NO_PAD
+                .decode(text.clone())
+                .map_err(|_| RenderError(format!("Failed to decode string {text}")))?;
+            let decoded = String::from_utf8(decoded)
+                .map_err(|_| RenderError(format!("Failed to decode utf8 string {text}")))?;
+            decoded
+        } else {
+            text
+        };
+
+        Ok(Some(final_text))
     }
 
     fn skip_whitespace(&mut self) {
@@ -410,14 +438,15 @@ impl Parser {
 
 #[cfg(test)]
 mod tests {
+    use crate::error::Result;
     use crate::Val::Null;
     use crate::*;
 
     #[test]
-    fn var_simple() {
+    fn var_simple() -> Result<()> {
         let mut p = Parser::new("${[ foo ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Var { name: "foo".into() }
@@ -425,13 +454,14 @@ mod tests {
                 Token::Eof
             ]
         );
+        Ok(())
     }
 
     #[test]
-    fn var_dashes() {
+    fn var_dashes() -> Result<()> {
         let mut p = Parser::new("${[ a-b ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Var { name: "a-b".into() }
@@ -439,13 +469,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn var_underscores() {
+    fn var_underscores() -> Result<()> {
         let mut p = Parser::new("${[ a_b ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Var { name: "a_b".into() }
@@ -453,13 +485,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn var_prefixes() {
+    fn var_prefixes() -> Result<()> {
         let mut p = Parser::new("${[ -a ]}${[ 0a ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Raw {
                     // Shouldn't be parsed, because they're invalid
@@ -468,13 +502,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn var_underscore_prefix() {
+    fn var_underscore_prefix() -> Result<()> {
         let mut p = Parser::new("${[ _a ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Var { name: "_a".into() }
@@ -482,13 +518,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn var_boolean() {
+    fn var_boolean() -> Result<()> {
         let mut p = Parser::new("${[ true ]}${[ false ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Bool { value: true },
@@ -499,13 +537,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn var_multiple_names_invalid() {
+    fn var_multiple_names_invalid() -> Result<()> {
         let mut p = Parser::new("${[ foo bar ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Raw {
                     text: "${[ foo bar ]}".into()
@@ -513,13 +553,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn tag_string() {
+    fn tag_string() -> Result<()> {
         let mut p = Parser::new(r#"${[ 'foo \'bar\' baz' ]}"#);
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Str {
@@ -529,13 +571,33 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn var_surrounded() {
+    fn tag_b64_string() -> Result<()> {
+        let mut p = Parser::new(r#"${[ b64'Zm9vICdiYXInIGJheg' ]}"#);
+        assert_eq!(
+            p.parse()?.tokens,
+            vec![
+                Token::Tag {
+                    val: Val::Str {
+                        text: r#"foo 'bar' baz"#.into()
+                    }
+                },
+                Token::Eof
+            ]
+        );
+
+        Ok(())
+    }
+
+    #[test]
+    fn var_surrounded() -> Result<()> {
         let mut p = Parser::new("Hello ${[ foo ]}!");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Raw {
                     text: "Hello ".to_string()
@@ -549,13 +611,15 @@ mod tests {
                 Token::Eof,
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn fn_simple() {
+    fn fn_simple() -> Result<()> {
         let mut p = Parser::new("${[ foo() ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Fn {
@@ -566,13 +630,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn fn_dot_name() {
+    fn fn_dot_name() -> Result<()> {
         let mut p = Parser::new("${[ foo.bar.baz() ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Fn {
@@ -583,13 +649,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn fn_ident_arg() {
+    fn fn_ident_arg() -> Result<()> {
         let mut p = Parser::new("${[ foo(a=bar) ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Fn {
@@ -603,13 +671,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn fn_ident_args() {
+    fn fn_ident_args() -> Result<()> {
         let mut p = Parser::new("${[ foo(a=bar,b = baz, c =qux ) ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Fn {
@@ -633,13 +703,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn fn_mixed_args() {
+    fn fn_mixed_args() -> Result<()> {
         let mut p = Parser::new(r#"${[ foo(aaa=bar,bb='baz \'hi\'', c=qux, z=true ) ]}"#);
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Fn {
@@ -669,13 +741,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn fn_nested() {
+    fn fn_nested() -> Result<()> {
         let mut p = Parser::new("${[ foo(b=bar()) ]}");
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Fn {
@@ -692,13 +766,15 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn fn_nested_args() {
+    fn fn_nested_args() -> Result<()> {
         let mut p = Parser::new(r#"${[ outer(a=inner(a=foo, b='i'), c='o') ]}"#);
         assert_eq!(
-            p.parse().tokens,
+            p.parse()?.tokens,
             vec![
                 Token::Tag {
                     val: Val::Fn {
@@ -730,10 +806,12 @@ mod tests {
                 Token::Eof
             ]
         );
+
+        Ok(())
     }
 
     #[test]
-    fn token_display_var() {
+    fn token_display_var() -> Result<()> {
         assert_eq!(
             Val::Var {
                 name: "foo".to_string()
@@ -741,21 +819,38 @@ mod tests {
             .to_string(),
             "foo"
         );
+
+        Ok(())
     }
 
     #[test]
-    fn token_display_str() {
+    fn token_display_str() -> Result<()> {
+        assert_eq!(
+            Val::Str {
+                text: "Hello You".to_string()
+            }
+            .to_string(),
+            "'Hello You'"
+        );
+
+        Ok(())
+    }
+
+    #[test]
+    fn token_display_complex_str() -> Result<()> {
         assert_eq!(
             Val::Str {
                 text: "Hello 'You'".to_string()
             }
             .to_string(),
-            "'Hello \'You\''"
+            "b64'SGVsbG8gJ1lvdSc'"
         );
+
+        Ok(())
     }
 
     #[test]
-    fn token_null_fn_arg() {
+    fn token_null_fn_arg() -> Result<()> {
         assert_eq!(
             Val::Fn {
                 name: "fn".to_string(),
@@ -775,10 +870,12 @@ mod tests {
             .to_string(),
             r#"fn(a='aaa')"#
         );
+
+        Ok(())
     }
 
     #[test]
-    fn token_display_fn() {
+    fn token_display_fn() -> Result<()> {
         assert_eq!(
             Token::Tag {
                 val: Val::Fn {
@@ -787,7 +884,7 @@ mod tests {
                         FnArg {
                             name: "arg".to_string(),
                             value: Val::Str {
-                                text: "v".to_string()
+                                text: "v 'x'".to_string()
                             }
                         },
                         FnArg {
@@ -800,12 +897,14 @@ mod tests {
                 }
             }
             .to_string(),
-            r#"${[ foo(arg='v', arg2=my_var) ]}"#
+            r#"${[ foo(arg=b64'diAneCc', arg2=my_var) ]}"#
         );
+
+        Ok(())
     }
 
     #[test]
-    fn tokens_display() {
+    fn tokens_display() -> Result<()> {
         assert_eq!(
             Tokens {
                 tokens: vec![
@@ -827,5 +926,7 @@ mod tests {
             .to_string(),
             r#"${[ my_var ]} Some cool text ${[ 'Hello World' ]}"#
         );
+
+        Ok(())
     }
 }
diff --git a/src-tauri/yaak-templates/src/renderer.rs b/src-tauri/yaak-templates/src/renderer.rs
index a9e378a3..989c61a6 100644
--- a/src-tauri/yaak-templates/src/renderer.rs
+++ b/src-tauri/yaak-templates/src/renderer.rs
@@ -1,6 +1,6 @@
 use crate::error::Error::{RenderStackExceededError, VariableNotFound};
 use crate::error::Result;
-use crate::{FnArg, Parser, Token, Tokens, Val};
+use crate::{Parser, Token, Tokens, Val};
 use log::warn;
 use serde_json::json;
 use std::collections::HashMap;
@@ -44,14 +44,14 @@ pub async fn render_json_value_raw<T: TemplateCallback>(
     Ok(v)
 }
 
-async fn parse_and_render_with_depth<T: TemplateCallback>(
+async fn parse_and_render_at_depth<T: TemplateCallback>(
     template: &str,
     vars: &HashMap<String, String>,
     cb: &T,
     depth: usize,
 ) -> Result<String> {
     let mut p = Parser::new(template);
-    let tokens = p.parse();
+    let tokens = p.parse()?;
     render(tokens, vars, cb, depth + 1).await
 }
 
@@ -60,7 +60,7 @@ pub async fn parse_and_render<T: TemplateCallback>(
     vars: &HashMap<String, String>,
     cb: &T,
 ) -> Result<String> {
-    parse_and_render_with_depth(template, vars, cb, 1).await
+    parse_and_render_at_depth(template, vars, cb, 1).await
 }
 
 pub async fn render<T: TemplateCallback>(
@@ -79,7 +79,7 @@ pub async fn render<T: TemplateCallback>(
     for t in tokens.tokens {
         match t {
             Token::Raw { text } => doc_str.push(text),
-            Token::Tag { val } => doc_str.push(render_tag(val, &vars, cb, depth).await?),
+            Token::Tag { val } => doc_str.push(render_value(val, &vars, cb, depth).await?),
             Token::Eof => {}
         }
     }
@@ -87,44 +87,31 @@ pub async fn render<T: TemplateCallback>(
     Ok(doc_str.join(""))
 }
 
-async fn render_tag<T: TemplateCallback>(
+async fn render_value<T: TemplateCallback>(
     val: Val,
     vars: &HashMap<String, String>,
     cb: &T,
     depth: usize,
 ) -> Result<String> {
     let v = match val {
-        Val::Str { text } => text.into(),
+        Val::Str { text } => {
+            let r = Box::pin(parse_and_render_at_depth(&text, vars, cb, depth)).await?;
+            r.to_string()
+        }
         Val::Var { name } => match vars.get(name.as_str()) {
             Some(v) => {
-                let r = Box::pin(parse_and_render_with_depth(v, vars, cb, depth)).await?;
+                let r = Box::pin(parse_and_render_at_depth(v, vars, cb, depth)).await?;
                 r.to_string()
             }
             None => return Err(VariableNotFound(name)),
         },
         Val::Bool { value } => value.to_string(),
         Val::Fn { name, args } => {
-            let empty = "".to_string();
+            // let empty = "".to_string();
             let mut resolved_args: HashMap<String, String> = HashMap::new();
             for a in args {
-                let (k, v) = match a {
-                    FnArg {
-                        name,
-                        value: Val::Str { text },
-                    } => (name.to_string(), text.to_string()),
-                    FnArg {
-                        name,
-                        value: Val::Var { name: var_name },
-                    } => (
-                        name.to_string(),
-                        vars.get(var_name.as_str()).unwrap_or(&empty).to_string(),
-                    ),
-                    FnArg { name, value: val } => {
-                        let r = Box::pin(render_tag(val.clone(), vars, cb, depth)).await?;
-                        (name.to_string(), r)
-                    }
-                };
-                resolved_args.insert(k, v);
+                let v = Box::pin(render_value(a.value, vars, cb, depth)).await?;
+                resolved_args.insert(a.name, v);
             }
             match cb.run(name.as_str(), resolved_args.clone()).await {
                 Ok(s) => s,
@@ -253,6 +240,67 @@ mod parse_and_render_tests {
         Ok(())
     }
 
+    #[tokio::test]
+    async fn render_fn_arg() -> Result<()> {
+        let vars = HashMap::new();
+        let template = r#"${[ upper(foo='bar') ]}"#;
+        let result = r#"BAR"#;
+        struct CB {}
+        impl TemplateCallback for CB {
+            async fn run(&self, fn_name: &str, args: HashMap<String, String>) -> Result<String> {
+                Ok(match fn_name {
+                    "secret" => "abc".to_string(),
+                    "upper" => args["foo"].to_string().to_uppercase(),
+                    _ => "".to_string(),
+                })
+            }
+        }
+
+        assert_eq!(parse_and_render(template, &vars, &CB {}).await?, result.to_string());
+        Ok(())
+    }
+
+    #[tokio::test]
+    async fn render_fn_b64_arg_template() -> Result<()> {
+        let mut vars = HashMap::new();
+        vars.insert("foo".to_string(), "bar".to_string());
+        let template = r#"${[ upper(foo=b64'Zm9vICdiYXInIGJheg') ]}"#;
+        let result = r#"FOO 'BAR' BAZ"#;
+        struct CB {}
+        impl TemplateCallback for CB {
+            async fn run(&self, fn_name: &str, args: HashMap<String, String>) -> Result<String> {
+                Ok(match fn_name {
+                    "upper" => args["foo"].to_string().to_uppercase(),
+                    _ => "".to_string(),
+                })
+            }
+        }
+
+        assert_eq!(parse_and_render(template, &vars, &CB {}).await?, result.to_string());
+        Ok(())
+    }
+
+    #[tokio::test]
+    async fn render_fn_arg_template() -> Result<()> {
+        let mut vars = HashMap::new();
+        vars.insert("foo".to_string(), "bar".to_string());
+        let template = r#"${[ upper(foo='${[ foo ]}') ]}"#;
+        let result = r#"BAR"#;
+        struct CB {}
+        impl TemplateCallback for CB {
+            async fn run(&self, fn_name: &str, args: HashMap<String, String>) -> Result<String> {
+                Ok(match fn_name {
+                    "secret" => "abc".to_string(),
+                    "upper" => args["foo"].to_string().to_uppercase(),
+                    _ => "".to_string(),
+                })
+            }
+        }
+
+        assert_eq!(parse_and_render(template, &vars, &CB {}).await?, result.to_string());
+        Ok(())
+    }
+
     #[tokio::test]
     async fn render_nested_fn() -> Result<()> {
         let vars = HashMap::new();
@@ -277,7 +325,6 @@ mod parse_and_render_tests {
     async fn render_fn_err() -> Result<()> {
         let vars = HashMap::new();
         let template = r#"${[ error() ]}"#;
-        let result = r#""#;
 
         struct CB {}
         impl TemplateCallback for CB {
@@ -286,7 +333,10 @@ mod parse_and_render_tests {
             }
         }
 
-        assert_eq!(parse_and_render(template, &vars, &CB {}).await?, result.to_string());
+        assert_eq!(
+            parse_and_render(template, &vars, &CB {}).await,
+            Err(RenderError("Failed to do it!".to_string()))
+        );
         Ok(())
     }
 }
diff --git a/src-web/components/TemplateFunctionDialog.tsx b/src-web/components/TemplateFunctionDialog.tsx
index 86e77433..443456de 100644
--- a/src-web/components/TemplateFunctionDialog.tsx
+++ b/src-web/components/TemplateFunctionDialog.tsx
@@ -5,10 +5,13 @@ import { useMemo, useState } from 'react';
 import { useDebouncedValue } from '../hooks/useDebouncedValue';
 import { useRenderTemplate } from '../hooks/useRenderTemplate';
 import { useTemplateTokensToString } from '../hooks/useTemplateTokensToString';
+import { useToggle } from '../hooks/useToggle';
 import { Button } from './core/Button';
 import { InlineCode } from './core/InlineCode';
-import { VStack } from './core/Stacks';
+import { HStack, VStack } from './core/Stacks';
 import { DYNAMIC_FORM_NULL_ARG, DynamicForm } from './DynamicForm';
+import { IconButton } from './core/IconButton';
+import { Banner } from './core/Banner';
 
 interface Props {
   templateFunction: TemplateFunction;
@@ -18,6 +21,7 @@ interface Props {
 }
 
 export function TemplateFunctionDialog({ templateFunction, hide, initialTokens, onChange }: Props) {
+  const [showSecretsInPreview, toggleShowSecretsInPreview] = useToggle(false);
   const [argValues, setArgValues] = useState<Record<string, string | boolean>>(() => {
     const initial: Record<string, string> = {};
     const initialArgs =
@@ -77,27 +81,65 @@ export function TemplateFunctionDialog({ templateFunction, hide, initialTokens,
 
   const debouncedTagText = useDebouncedValue(tagText.data ?? '', 200);
   const rendered = useRenderTemplate(debouncedTagText);
-  const tooLarge = (rendered.data ?? '').length > 10000;
+  const tooLarge = rendered.data ? rendered.data.length > 10000 : false;
+  const dataContainsSecrets = useMemo(() => {
+    for (const [name, value] of Object.entries(argValues)) {
+      const isPassword = templateFunction.args.some(
+        (a) => a.type === 'text' && a.password && a.name === name,
+      );
+      if (isPassword && typeof value === 'string' && value && rendered.data?.includes(value)) {
+        return true;
+      }
+    }
+    return false;
+    // Only update this on rendered data change to keep secrets hidden on input change
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [rendered.data]);
 
   return (
     <VStack className="pb-3" space={4}>
       <h1 className="font-mono !text-base">{templateFunction.name}(…)</h1>
       <DynamicForm
+        autocompleteVariables
+        autocompleteFunctions
         inputs={templateFunction.args}
         data={argValues}
         onChange={setArgValues}
         stateKey={`template_function.${templateFunction.name}`}
       />
-      <VStack className="w-full">
-        <div className="text-sm text-text-subtle">Preview</div>
-        <InlineCode
-          className={classNames(
-            'whitespace-pre select-text cursor-text max-h-[10rem] overflow-y-auto hide-scrollbars',
-            tooLarge && 'italic text-danger',
-          )}
-        >
-          {tooLarge ? 'too large to preview' : rendered.data || <>&nbsp;</>}
-        </InlineCode>
+      <VStack className="w-full" space={1}>
+        <HStack space={0.5}>
+          <div className="text-sm text-text-subtle">Rendered Preview</div>
+          <IconButton
+            size="xs"
+            iconSize="sm"
+            icon={showSecretsInPreview ? 'lock' : 'lock_open'}
+            title={showSecretsInPreview ? 'Show preview' : 'Hide preview'}
+            onClick={toggleShowSecretsInPreview}
+            className={classNames(
+              'ml-auto text-text-subtlest',
+              !dataContainsSecrets && 'invisible',
+            )}
+          />
+        </HStack>
+        {rendered.error || tagText.error ? (
+          <Banner color="danger">{`${rendered.error || tagText.error}`}</Banner>
+        ) : (
+          <InlineCode
+            className={classNames(
+              'whitespace-pre select-text cursor-text max-h-[10rem] overflow-y-auto hide-scrollbars',
+              tooLarge && 'italic text-danger',
+            )}
+          >
+            {dataContainsSecrets && !showSecretsInPreview ? (
+              <span className="italic text-text-subtle">------ sensitive values hidden ------</span>
+            ) : tooLarge ? (
+              'too large to preview'
+            ) : (
+              rendered.data || <>&nbsp;</>
+            )}
+          </InlineCode>
+        )}
       </VStack>
       <Button color="primary" onClick={handleDone}>
         Done
diff --git a/src-web/components/core/Editor/Editor.tsx b/src-web/components/core/Editor/Editor.tsx
index f9510fff..0f85626d 100644
--- a/src-web/components/core/Editor/Editor.tsx
+++ b/src-web/components/core/Editor/Editor.tsx
@@ -266,7 +266,7 @@ export const Editor = forwardRef<EditorView | undefined, EditorProps>(function E
     async (fn: TemplateFunction, tagValue: string, startPos: number) => {
       const initialTokens = await parseTemplate(tagValue);
       showDialog({
-        id: 'template-function',
+        id: 'template-function-'+Math.random(), // Allow multiple at once
         size: 'sm',
         title: 'Configure Function',
         description: fn.description,
diff --git a/src-web/components/core/Icon.tsx b/src-web/components/core/Icon.tsx
index e47cfee5..017b5dec 100644
--- a/src-web/components/core/Icon.tsx
+++ b/src-web/components/core/Icon.tsx
@@ -69,6 +69,7 @@ const icons = {
   left_panel_hidden: lucide.PanelLeftOpenIcon,
   left_panel_visible: lucide.PanelLeftCloseIcon,
   lock: lucide.LockIcon,
+  lock_open: lucide.LockOpenIcon,
   magic_wand: lucide.Wand2Icon,
   merge: lucide.MergeIcon,
   minus: lucide.MinusIcon,