diff --git a/backend/src/cell.rs b/backend/src/cell.rs
index d10ccf9..7b8aa32 100644
--- a/backend/src/cell.rs
+++ b/backend/src/cell.rs
@@ -2,7 +2,10 @@ use std::collections::HashSet;
 
 use serde::{Deserialize, Serialize};
 
-use crate::evaluator::*;
+use crate::{
+    common::{LeadErr, LeadErrCode},
+    evaluator::*,
+};
 
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, Hash, PartialEq, Eq)]
 pub struct CellRef {
@@ -93,7 +96,7 @@ impl Cell {
 
 impl CellRef {
     // Zero indexed
-    pub fn new(s: String) -> Result<CellRef, String> {
+    pub fn new(s: String) -> Result<CellRef, LeadErr> {
         let s = s.trim();
         let mut col: usize = 0;
         let mut i = 0;
@@ -111,20 +114,26 @@ impl CellRef {
         }
 
         if col <= 0 {
-            return Err(format!(
-                "Parse error: missing column letters in cell ref: {s}"
-            ));
+            return Err(LeadErr {
+                title: "Parse error.".into(),
+                desc: format!("Missing column letters in cell ref: {s}."),
+                code: LeadErrCode::Syntax,
+            });
         }
 
         let row_part = &s[i..];
         if row_part.is_empty() {
-            return Err(format!(
-                "Parse error: missing column letters in cell ref: {s}"
-            ));
+            return Err(LeadErr {
+                title: "Parse error.".into(),
+                desc: format!("Missing column letters in cell ref: {s}."),
+                code: LeadErrCode::Syntax,
+            });
         } else if !row_part.chars().all(|c| c.is_ascii_digit()) {
-            return Err(format!(
-                "Parse error: row part must be numeric in cell ref: {s}"
-            ));
+            return Err(LeadErr {
+                title: "Parse error.".into(),
+                desc: format!("Row part must be numeric in cell ref: {s}."),
+                code: LeadErrCode::Syntax,
+            });
         }
 
         if let Ok(row) = row_part.parse::<usize>() {
@@ -133,7 +142,11 @@ impl CellRef {
                 col: col - 1,
             })
         } else {
-            Err(format!("Parse error: invalid row number."))
+            Err(LeadErr {
+                title: "Parse error.".into(),
+                desc: format!("Invalid row number."),
+                code: LeadErrCode::Syntax,
+            })
         }
     }
 }
diff --git a/backend/src/common.rs b/backend/src/common.rs
index e69de29..fccf258 100644
--- a/backend/src/common.rs
+++ b/backend/src/common.rs
@@ -0,0 +1,30 @@
+use serde::{Deserialize, Serialize};
+
+// Example usage:
+//
+// title: "Evaluation error."
+// desc: "Function ADD requires numeric type arguments."
+// code: TypeErr
+#[derive(Debug, PartialEq, Clone, Deserialize, Serialize)]
+pub struct LeadErr {
+    pub title: String,
+    pub desc: String,
+    pub code: LeadErrCode,
+}
+
+#[derive(Debug, PartialEq, Clone, Deserialize, Serialize)]
+pub enum LeadErrCode {
+    DivZero,
+    TypeErr,
+    Syntax,
+    Server,
+    Unsupported,
+}
+
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(tag = "type", content = "value")]
+pub enum Literal {
+    Number(f64),
+    Boolean(bool),
+    String(String),
+}
diff --git a/backend/src/evaluator.rs b/backend/src/evaluator.rs
index e72555b..2df45ca 100644
--- a/backend/src/evaluator.rs
+++ b/backend/src/evaluator.rs
@@ -1,9 +1,11 @@
 use serde::{Deserialize, Serialize};
 
 use crate::cell::CellRef;
+use crate::common::LeadErr;
+use crate::common::LeadErrCode;
+use crate::common::Literal;
 use crate::grid::Grid;
 use crate::parser::*;
-use crate::tokenizer::Literal;
 use std::collections::HashSet;
 use std::fmt;
 
@@ -13,6 +15,7 @@ pub enum Eval {
     Literal(Literal),
     CellRef { eval: Box<Eval>, reference: CellRef },
     Range(Vec<Eval>),
+    Err(LeadErr),
     Unset,
 }
 
@@ -23,19 +26,22 @@ impl fmt::Display for Eval {
             Eval::Range(it) => write!(f, "Range({it:?})"),
             Eval::CellRef { eval, reference } => write!(f, "EvalRef({eval:?}, {reference:?})"),
             Eval::Unset => write!(f, "Unset"),
+            Eval::Err(it) => write!(f, "{it:?}"),
         }
     }
 }
 
-pub fn evaluate(str: String, grid: Option<&Grid>) -> Result<(Eval, HashSet<CellRef>), String> {
-    let (expr, _) = parse(&str)?;
+pub fn evaluate(str: String, grid: Option<&Grid>) -> (Eval, HashSet<CellRef>) {
+    match parse(&str) {
+        Ok((expr, _)) => {
+            let mut precs = HashSet::new();
 
-    let mut precs = HashSet::new();
-
-    // Make evaulator adds precs for ranges
-    match evaluate_expr(&expr, &mut precs, grid) {
-        Ok(it) => Ok((it, precs)),
-        Err(it) => Err(it),
+            match evaluate_expr(&expr, &mut precs, grid) {
+                Ok(it) => (it, precs),
+                Err(it) => (Eval::Err(it), precs),
+            }
+        }
+        Err(e) => (Eval::Err(e), HashSet::new()),
     }
 }
 
@@ -43,7 +49,7 @@ fn evaluate_expr(
     expr: &Expr,
     precs: &mut HashSet<CellRef>,
     grid: Option<&Grid>,
-) -> Result<Eval, String> {
+) -> Result<Eval, LeadErr> {
     let res = match expr {
         Expr::Literal(lit) => Eval::Literal(lit.clone()),
         Expr::CellRef(re) => {
@@ -59,7 +65,11 @@ fn evaluate_expr(
                     },
                 }
             } else {
-                return Err("Evaluation error: Found cell reference but no grid.".into());
+                return Err(LeadErr {
+                    title: "Evaluation error.".into(),
+                    desc: "Found cell reference with no grid.".into(),
+                    code: LeadErrCode::Server,
+                });
             }
         }
         Expr::Infix { op, lhs, rhs } => {
@@ -81,7 +91,13 @@ fn evaluate_expr(
                 InfixOp::MUL => eval_mul(&lval, &rval)?,
                 InfixOp::DIV => eval_div(&lval, &rval)?,
                 InfixOp::RANGE => eval_range(&lval, &rval, precs, grid)?,
-                _ => return Err(format!("Evaluation error: Unsupported operator {:?}", op)),
+                _ => {
+                    return Err(LeadErr {
+                        title: "Evaluation error.".into(),
+                        desc: format!("Unsupported operator {:?}", op),
+                        code: LeadErrCode::Unsupported,
+                    });
+                }
             }
         }
         Expr::Prefix { op, expr } => {
@@ -101,9 +117,21 @@ fn evaluate_expr(
         Expr::Group(g) => evaluate_expr(g, precs, grid)?,
         Expr::Function { name, args } => match name.as_str() {
             "AVG" => eval_avg(args, precs, grid)?,
-            it => return Err(format!("Evaluation error: Unsupported function {}.", it)),
+            it => {
+                return Err(LeadErr {
+                    title: "Evaluation error.".into(),
+                    desc: format!("Unsupported function {:?}", it),
+                    code: LeadErrCode::Unsupported,
+                });
+            }
         },
-        it => return Err(format!("Evaluation error: Unsupported expression {:?}", it)),
+        it => {
+            return Err(LeadErr {
+                title: "Evaluation error.".into(),
+                desc: format!("Unsupported expression {:?}", it),
+                code: LeadErrCode::Unsupported,
+            });
+        }
     };
 
     Ok(res)
@@ -114,7 +142,7 @@ fn eval_range(
     rval: &Eval,
     precs: &mut HashSet<CellRef>,
     grid: Option<&Grid>,
-) -> Result<Eval, String> {
+) -> Result<Eval, LeadErr> {
     match (lval, rval) {
         (
             Eval::CellRef {
@@ -139,7 +167,11 @@ fn eval_range(
                     let reference = CellRef { row, col };
 
                     let Some(g) = grid else {
-                        return Err("Evaluation error: Found cell range but no grid.".into());
+                        return Err(LeadErr {
+                            title: "Evaluation error.".into(),
+                            desc: "Found cell range but no grid.".into(),
+                            code: LeadErrCode::Server,
+                        });
                     };
 
                     cells.push(Eval::CellRef {
@@ -157,7 +189,11 @@ fn eval_range(
 
             Ok(Eval::Range(cells))
         }
-        _ => Err("Evaluation error: expected cellref types for RANGE function.".to_string()),
+        _ => Err(LeadErr {
+            title: "Evaluation error.".into(),
+            desc: "Expected cell reference types for RANGE function.".into(),
+            code: LeadErrCode::Unsupported,
+        }),
     }
 }
 
@@ -165,7 +201,7 @@ fn eval_avg(
     args: &Vec<Expr>,
     precs: &mut HashSet<CellRef>,
     grid: Option<&Grid>,
-) -> Result<Eval, String> {
+) -> Result<Eval, LeadErr> {
     let mut res = 0.0;
     let mut count = 0;
 
@@ -178,7 +214,11 @@ fn eval_avg(
             Eval::Range(range) => {
                 for cell in range {
                     let Eval::CellRef { eval, reference: _ } = cell else {
-                        panic!("Found non cellref in evaluation time RANGE!.");
+                        return Err(LeadErr {
+                            title: "Evaluation error.".into(),
+                            desc: "Found non-cellref in RANGE during AVG evaluation.".into(),
+                            code: LeadErrCode::Server,
+                        });
                     };
 
                     if let Eval::Literal(Literal::Number(num)) = *eval {
@@ -187,27 +227,36 @@ fn eval_avg(
                     } else if matches!(*eval, Eval::Unset) {
                         continue;
                     } else {
-                        return Err("Evaluation error: expected numeric types for AVG function."
-                            .to_string());
+                        return Err(LeadErr {
+                            title: "Evaluation error.".into(),
+                            desc: "Expected numeric types for AVG function.".into(),
+                            code: LeadErrCode::Unsupported,
+                        });
                     }
                 }
             }
             _ => {
-                return Err(
-                    "Evaluation error: expected numeric types for AVG function.".to_string()
-                );
+                return Err(LeadErr {
+                    title: "Evaluation error.".into(),
+                    desc: "Expected numeric types for AVG function.".into(),
+                    code: LeadErrCode::Unsupported,
+                });
             }
         }
     }
 
     if count == 0 {
-        Err("Evaluation error: attempted to divide by zero.".to_string())
+        Err(LeadErr {
+            title: "Evaluation error.".into(),
+            desc: "Attempted to divide by zero.".into(),
+            code: LeadErrCode::DivZero,
+        })
     } else {
         Ok(Eval::Literal(Literal::Number(res / count as f64)))
     }
 }
 
-fn eval_add(lval: &Eval, rval: &Eval) -> Result<Eval, String> {
+fn eval_add(lval: &Eval, rval: &Eval) -> Result<Eval, LeadErr> {
     match (lval, rval) {
         (Eval::Literal(a), Eval::Literal(b)) => {
             if let Some(res) = eval_numeric_infix(a, b, |x, y| x + y) {
@@ -221,46 +270,72 @@ fn eval_add(lval: &Eval, rval: &Eval) -> Result<Eval, String> {
                 return Ok(Eval::Literal(Literal::String(res)));
             }
 
-            Err("Evaluation error: expected string or numeric types for ADD function.".to_string())
-        }
-        _ => {
-            Err("Evaluation error: expected string or numeric types for ADD function.".to_string())
+            Err(LeadErr {
+                title: "Evaluation error.".into(),
+                desc: "Expected string or numeric types for ADD function.".into(),
+                code: LeadErrCode::Unsupported,
+            })
         }
+        _ => Err(LeadErr {
+            title: "Evaluation error.".into(),
+            desc: "Expected string or numeric types for ADD function.".into(),
+            code: LeadErrCode::Unsupported,
+        }),
     }
 }
 
-fn eval_sub(lval: &Eval, rval: &Eval) -> Result<Eval, String> {
+fn eval_sub(lval: &Eval, rval: &Eval) -> Result<Eval, LeadErr> {
     match (lval, rval) {
         (Eval::Literal(a), Eval::Literal(b)) => {
             if let Some(res) = eval_numeric_infix(a, b, |x, y| x - y) {
                 return Ok(Eval::Literal(res));
             }
 
-            Err("Evaluation error: expected numeric types for SUB function.".to_string())
+            Err(LeadErr {
+                title: "Evaluation error.".into(),
+                desc: "Expected numeric types for SUB function.".into(),
+                code: LeadErrCode::Unsupported,
+            })
         }
-        _ => Err("Evaluation error: expected numeric types for SUB function.".to_string()),
+        _ => Err(LeadErr {
+            title: "Evaluation error.".into(),
+            desc: "Expected numeric types for SUB function.".into(),
+            code: LeadErrCode::Unsupported,
+        }),
     }
 }
-fn eval_mul(lval: &Eval, rval: &Eval) -> Result<Eval, String> {
+
+fn eval_mul(lval: &Eval, rval: &Eval) -> Result<Eval, LeadErr> {
     match (lval, rval) {
         (Eval::Literal(a), Eval::Literal(b)) => {
             if let Some(res) = eval_numeric_infix(a, b, |x, y| x * y) {
                 return Ok(Eval::Literal(res));
             }
 
-            Err("Evaluation error: expected numeric types for MUL function.".to_string())
+            Err(LeadErr {
+                title: "Evaluation error.".into(),
+                desc: "Expected numeric types for MUL function.".into(),
+                code: LeadErrCode::Unsupported,
+            })
         }
-        _ => Err("Evaluation error: expected numeric types for MUL function.".to_string()),
+        _ => Err(LeadErr {
+            title: "Evaluation error.".into(),
+            desc: "Expected numeric types for MUL function.".into(),
+            code: LeadErrCode::Unsupported,
+        }),
     }
 }
-fn eval_div(lval: &Eval, rval: &Eval) -> Result<Eval, String> {
+
+fn eval_div(lval: &Eval, rval: &Eval) -> Result<Eval, LeadErr> {
     match (lval, rval) {
         (Eval::Literal(a), Eval::Literal(b)) => {
             if let (Literal::Number(_), Literal::Number(y)) = (a, b) {
                 if *y == 0f64 {
-                    return Err(
-                        "Evaluation error: integers attempted to divide by zero.".to_string()
-                    );
+                    return Err(LeadErr {
+                        title: "Evaluation error.".into(),
+                        desc: "Attempted to divide by zero.".into(),
+                        code: LeadErrCode::DivZero,
+                    });
                 }
             }
 
@@ -268,9 +343,50 @@ fn eval_div(lval: &Eval, rval: &Eval) -> Result<Eval, String> {
                 return Ok(Eval::Literal(res));
             }
 
-            Err("Evaluation error: expected numeric types for DIV function.".to_string())
+            Err(LeadErr {
+                title: "Evaluation error.".into(),
+                desc: "Expected numeric types for DIV function.".into(),
+                code: LeadErrCode::Unsupported,
+            })
         }
-        _ => Err("Evaluation error: expected numeric types for DIV  function.".to_string()),
+        _ => Err(LeadErr {
+            title: "Evaluation error.".into(),
+            desc: "Expected numeric types for DIV function.".into(),
+            code: LeadErrCode::Unsupported,
+        }),
+    }
+}
+
+fn eval_pos(val: &Eval) -> Result<Eval, LeadErr> {
+    match val {
+        Eval::Literal(Literal::Number(it)) => Ok(Eval::Literal(Literal::Number(*it))),
+        _ => Err(LeadErr {
+            title: "Evaluation error.".into(),
+            desc: "Expected numeric type for POS function.".into(),
+            code: LeadErrCode::Unsupported,
+        }),
+    }
+}
+
+fn eval_neg(val: &Eval) -> Result<Eval, LeadErr> {
+    match val {
+        Eval::Literal(Literal::Number(it)) => Ok(Eval::Literal(Literal::Number(-it))),
+        _ => Err(LeadErr {
+            title: "Evaluation error.".into(),
+            desc: "Expected numeric type for NEG function.".into(),
+            code: LeadErrCode::Unsupported,
+        }),
+    }
+}
+
+fn eval_not(val: &Eval) -> Result<Eval, LeadErr> {
+    match val {
+        Eval::Literal(Literal::Boolean(it)) => Ok(Eval::Literal(Literal::Boolean(!it))),
+        _ => Err(LeadErr {
+            title: "Evaluation error.".into(),
+            desc: "Expected boolean type for NOT function.".into(),
+            code: LeadErrCode::Unsupported,
+        }),
     }
 }
 
@@ -280,23 +396,3 @@ fn eval_numeric_infix(lhs: &Literal, rhs: &Literal, op: fn(f64, f64) -> f64) ->
         _ => None,
     }
 }
-
-fn eval_pos(val: &Eval) -> Result<Eval, String> {
-    match val {
-        Eval::Literal(Literal::Number(it)) => Ok(Eval::Literal(Literal::Number(*it))),
-        _ => Err("Evaluation error: expected numeric type for POS function.".to_string()),
-    }
-}
-
-fn eval_neg(val: &Eval) -> Result<Eval, String> {
-    match val {
-        Eval::Literal(Literal::Number(it)) => Ok(Eval::Literal(Literal::Number(-it))),
-        _ => Err("Evaluation error: expected numeric type for NEG function.".to_string()),
-    }
-}
-fn eval_not(val: &Eval) -> Result<Eval, String> {
-    match val {
-        Eval::Literal(Literal::Boolean(it)) => Ok(Eval::Literal(Literal::Boolean(!it))),
-        _ => Err("Evaluation error: expected boolean type for NEG function.".to_string()),
-    }
-}
diff --git a/backend/src/grid.rs b/backend/src/grid.rs
index 6f0dff4..b5bef38 100644
--- a/backend/src/grid.rs
+++ b/backend/src/grid.rs
@@ -4,8 +4,8 @@ use log::info;
 
 use crate::{
     cell::{Cell, CellRef},
+    common::Literal,
     evaluator::{Eval, evaluate},
-    tokenizer::Literal,
 };
 
 pub struct Grid {
@@ -36,7 +36,7 @@ impl Grid {
             eval = Eval::Literal(Literal::String(raw_val.to_owned()));
         } else {
             // Evaluate raw expr and get precedents
-            let (res_eval, res_precs) = evaluate(raw_val[1..].to_owned(), Some(&self))?;
+            let (res_eval, res_precs) = evaluate(raw_val[1..].to_owned(), Some(&self));
             eval = res_eval;
             precs = res_precs;
         }
@@ -88,7 +88,7 @@ impl Grid {
         search_set.extend(cell_data.deps().iter());
 
         temp.insert(from);
-        perm.insert(from);
+        perm.insert(from); // Make this inside the inner topo_visit
 
         let mut searched = 1;
 
@@ -249,7 +249,7 @@ impl Grid {
             };
 
             // Now we dropped the borrow of self.cells before this point
-            let (e, _) = evaluate(raw, Some(self))?;
+            let (e, _) = evaluate(raw, Some(self));
 
             if let Some(cell) = self.cells.get_mut(&cell_ref) {
                 cell.set_eval(e);
diff --git a/backend/src/main.rs b/backend/src/main.rs
index 3bf4e67..3f7019f 100644
--- a/backend/src/main.rs
+++ b/backend/src/main.rs
@@ -1,4 +1,5 @@
 mod cell;
+mod common;
 mod evaluator;
 mod grid;
 mod messages;
diff --git a/backend/src/parser.rs b/backend/src/parser.rs
index 5bbf8c2..ae6cbd6 100644
--- a/backend/src/parser.rs
+++ b/backend/src/parser.rs
@@ -1,6 +1,10 @@
 use log::info;
 
-use crate::{cell::CellRef, tokenizer::*};
+use crate::{
+    cell::CellRef,
+    common::{LeadErr, LeadErrCode, Literal},
+    tokenizer::*,
+};
 use std::{collections::HashSet, fmt};
 
 #[derive(Debug, PartialEq, Clone)]
@@ -159,7 +163,7 @@ impl Expr {
     }
 }
 
-pub fn parse(input: &str) -> Result<(Expr, HashSet<CellRef>), String> {
+pub fn parse(input: &str) -> Result<(Expr, HashSet<CellRef>), LeadErr> {
     let mut tokenizer = Tokenizer::new(input)?;
     let mut precs = HashSet::new();
     let expr = _parse(&mut tokenizer, 0, &mut precs)?;
@@ -171,13 +175,17 @@ pub fn _parse(
     input: &mut Tokenizer,
     min_prec: u8,
     precedents: &mut HashSet<CellRef>,
-) -> Result<Expr, String> {
+) -> Result<Expr, LeadErr> {
     let mut lhs = match input.next() {
         Token::Literal(it) => Expr::Literal(it),
         Token::OpenParen => {
             let lhs = _parse(input, 0, precedents)?;
             if input.next() != Token::CloseParen {
-                return Err(format!("Parse error: expected closing paren."));
+                return Err(LeadErr {
+                    title: "Parse error.".into(),
+                    desc: "Expected closing paren.".into(),
+                    code: LeadErrCode::Syntax,
+                });
             }
             Expr::Group(Box::new(lhs))
         }
@@ -186,7 +194,13 @@ pub fn _parse(
                 '+' => PrefixOp::POS,
                 '-' => PrefixOp::NEG,
                 '!' => PrefixOp::NOT,
-                it => return Err(format!("Parse error: unknown prefix operator {:?}.", it)),
+                it => {
+                    return Err(LeadErr {
+                        title: "Parse error.".into(),
+                        desc: format!("Unknown prefix operator {:?}.", it),
+                        code: LeadErrCode::Syntax,
+                    });
+                }
             };
 
             let rhs = _parse(input, prefix_op.prec().1, precedents)?;
@@ -208,10 +222,13 @@ pub fn _parse(
                         input.next();
                         break;
                     } else if nxt != Token::Comma && args.len() != 0 {
-                        return Err(format!(
-                            "Parse error: expected comma while parsing argument of function {:?}.",
-                            id
-                        ));
+                        return Err(LeadErr {
+                            title: "Parse error.".into(),
+                            desc: format!(
+                                "Expected comma while parsing argument of function {id:?}"
+                            ),
+                            code: LeadErrCode::Syntax,
+                        });
                     }
 
                     if args.len() != 0 {
@@ -234,7 +251,13 @@ pub fn _parse(
             }
         },
 
-        it => return Err(format!("Parse error: did not expect token {:?}.", it)),
+        it => {
+            return Err(LeadErr {
+                title: "Parse error.".into(),
+                desc: format!("Unexpected token {:?}.", it),
+                code: LeadErrCode::Syntax,
+            });
+        }
     };
 
     // In the reference article this is a loop with match
@@ -250,7 +273,11 @@ pub fn _parse(
                 '|' => InfixOp::OR,
                 ':' => InfixOp::RANGE,
                 it => {
-                    return Err(format!("Parse error: do not know infix operator {:?}.", it));
+                    return Err(LeadErr {
+                        title: "Parse error.".into(),
+                        desc: format!("Unknown infix operator {:?}.", it),
+                        code: LeadErrCode::Syntax,
+                    });
                 }
             };
 
@@ -270,10 +297,11 @@ pub fn _parse(
             let postfix_op = match op {
                 '%' => PostfixOp::PERCENT,
                 it => {
-                    return Err(format!(
-                        "Parse error: do not know postfix operator {:?}.",
-                        it
-                    ));
+                    return Err(LeadErr {
+                        title: "Parse error.".into(),
+                        desc: format!("Unknown postfix operator {:?}.", it),
+                        code: LeadErrCode::Syntax,
+                    });
                 }
             };
 
diff --git a/backend/src/tokenizer.rs b/backend/src/tokenizer.rs
index 3695053..f051380 100644
--- a/backend/src/tokenizer.rs
+++ b/backend/src/tokenizer.rs
@@ -1,18 +1,11 @@
-use serde::{Deserialize, Serialize};
-
-#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
-#[serde(tag = "type", content = "value")]
-pub enum Literal {
-    Number(f64),
-    Boolean(bool),
-    String(String),
-}
+use crate::common::{LeadErr, LeadErrCode, Literal};
 
 #[derive(Debug, Clone, PartialEq)]
 pub enum Token {
     Identifier(String), // Could be a function
     Literal(Literal),
     Operator(char),
+    Err(LeadErr),
     OpenParen,
     CloseParen,
     Comma,
@@ -26,7 +19,7 @@ pub struct Tokenizer {
 }
 
 impl Tokenizer {
-    pub fn new(input: &str) -> Result<Tokenizer, String> {
+    pub fn new(input: &str) -> Result<Tokenizer, LeadErr> {
         let mut tokens = Vec::new();
         let mut chars = input.chars().peekable();
 
@@ -113,7 +106,11 @@ impl Tokenizer {
                 tokens.push(Token::Comma);
                 chars.next();
             } else {
-                return Err(format!("Encountered unknown token char: {c}"));
+                return Err(LeadErr {
+                    title: "Tokenizer error.".into(),
+                    desc: format!("Encountered unknown token char: {c}"),
+                    code: LeadErrCode::Syntax,
+                });
             }
         }
 
diff --git a/frontend/src/lib/components/grid/cell.svelte b/frontend/src/lib/components/grid/cell.svelte
index 36b88a1..1f01234 100644
--- a/frontend/src/lib/components/grid/cell.svelte
+++ b/frontend/src/lib/components/grid/cell.svelte
@@ -1,13 +1,13 @@
 <script lang="ts">
 	import { Input } from '$lib/components/ui/input/index.js';
 	import clsx from 'clsx';
+	import type { CellT } from './utils';
 
 	let {
 		cla = '',
 		width = '80px',
 		height = '30px',
-		raw_val = $bindable(''),
-		val = undefined,
+		cell = $bindable(undefined),
 		onmousedown = () => {},
 		startediting = () => {},
 		stopediting = () => {},
@@ -17,8 +17,7 @@
 		cla?: string;
 		width?: string;
 		height?: string;
-		raw_val?: string;
-		val?: LiteralValue | undefined;
+		cell?: CellT;
 		onmousedown?: (e: MouseEvent) => void;
 		startediting?: () => void;
 		stopediting?: () => void;
@@ -31,7 +30,7 @@
 		queueMicrotask(() => {
 			const el = node.querySelector('input') as HTMLInputElement | null;
 			if (el !== null) {
-				el.value = raw_val;
+				el.value = cell?.raw_val ?? '';
 				el.focus();
 			}
 		});
@@ -56,7 +55,11 @@
 			class="relative rounded-none p-1 !transition-none delay-0 duration-0
 			focus:z-20 focus:shadow-[0_0_0_1px_var(--color-primary)] focus:outline-none"
 			onblur={(e) => {
-				raw_val = (e.target as HTMLInputElement).value;
+				cell = {
+					isErr: false,
+					val: undefined,
+					raw_val: (e.target as HTMLInputElement).value
+				};
 				stopediting();
 			}}
 		/>
@@ -69,12 +72,12 @@
 		style:height
 		class={clsx('placeholder bg-background p-1', { active }, cla)}
 	>
-		{#if raw_val !== '' || val !== ''}
-			<span class="pointer-events-none select-none">
-				{#if val !== undefined}
-					{val}
+		{#if cell && (cell.raw_val !== '' || cell.val !== '')}
+			<span class={clsx('pointer-events-none select-none', { err: cell.isErr })}>
+				{#if cell.val}
+					{cell.val}
 				{:else}
-					{raw_val}
+					{cell.raw_val}
 				{/if}
 			</span>
 		{/if}
@@ -94,4 +97,21 @@
 		border: 1px solid var(--color-primary);
 		outline: 1px solid var(--color-primary);
 	}
+
+	.active:has(.err),
+	.placeholder:has(.err) {
+		position: relative; /* needed for absolute positioning */
+	}
+
+	.active:has(.err)::after,
+	.placeholder:has(.err)::after {
+		content: '';
+		position: absolute;
+		top: 0;
+		right: 0;
+		width: 0;
+		height: 0;
+		border-top: 12px solid red; /* size & color of the triangle */
+		border-left: 12px solid transparent;
+	}
 </style>
diff --git a/frontend/src/lib/components/grid/grid.svelte b/frontend/src/lib/components/grid/grid.svelte
index 399c228..749d868 100644
--- a/frontend/src/lib/components/grid/grid.svelte
+++ b/frontend/src/lib/components/grid/grid.svelte
@@ -3,7 +3,7 @@
 	import Cell from '$lib/components/grid/cell.svelte';
 	import { onDestroy, onMount } from 'svelte';
 	import CellHeader from './cell-header.svelte';
-	import { colToStr, getEvalLiteral, refToStr, type CellT } from './utils';
+	import { colToStr, getEvalLiteral, isErr, refToStr, type CellT } from './utils';
 	import clsx from 'clsx';
 
 	let {
@@ -44,7 +44,7 @@
 					console.error('Expected cell value for SET msgponse from server.');
 					return;
 				}
-				setCellVal(msg.cell.row, msg.cell.col, getEvalLiteral(msg.eval));
+				setCellVal(msg.cell.row, msg.cell.col, getEvalLiteral(msg.eval), isErr(msg.eval));
 				break;
 			}
 			case 'bulk': {
@@ -107,38 +107,41 @@
 
 	const key = (i: number, j: number) => `${i}:${j}`;
 
-	const getCell = (i: number, j: number) => grid_vals[key(i, j)] ?? undefined;
+	const getCell = (i: number, j: number) => grid_vals[key(i, j)];
 
 	const getCellRaw = (i: number, j: number) => getCell(i, j)?.raw_val ?? '';
 	const setCellRaw = (i: number, j: number, val: string) => {
 		if (grid_vals[key(i, j)] === undefined) {
 			grid_vals[key(i, j)] = {
 				raw_val: val,
+				isErr: false,
 				val: undefined
 			};
 		} else {
 			grid_vals[key(i, j)].raw_val = val;
 		}
 	};
-	const getCellVal = (i: number, j: number) => getCell(i, j)?.val ?? undefined;
-	const setCellVal = (i: number, j: number, val: LiteralValue) => {
+	const getCellVal = (i: number, j: number) => getCell(i, j);
+	const setCellVal = (i: number, j: number, val: LiteralValue, isErr: boolean) => {
 		if (grid_vals[key(i, j)] === undefined) {
 			console.warn('Cell raw value was undefined but recieved eval.');
 		} else {
-			grid_vals[key(i, j)].val = val;
+			let cell = grid_vals[key(i, j)];
+			cell.val = val;
+			cell.isErr = isErr;
 		}
 	};
 
-	const setCell = (row: number, col: number, v: string | undefined) => {
+	const setCell = (row: number, col: number, v: CellT | undefined) => {
 		// ignore “no value” so we don’t create keys on mount
-		if (v == null || v === '') delete grid_vals[key(row, col)];
+		if (v?.raw_val == null || v.raw_val === '') delete grid_vals[key(row, col)];
 		else {
-			setCellRaw(row, col, v);
+			setCellRaw(row, col, v.raw_val);
 
 			let msg: LeadMsg = {
 				msg_type: 'set',
 				cell: { row, col },
-				raw: v
+				raw: v.raw_val
 			};
 
 			socket.send(JSON.stringify(msg));
@@ -237,8 +240,7 @@
 					onmousedown={(e) => {
 						handleCellInteraction(i, j, e);
 					}}
-					bind:raw_val={() => getCellRaw(i, j), (v) => setCell(i, j, v)}
-					val={getCellVal(i, j)}
+					bind:cell={() => getCell(i, j), (v) => setCell(i, j, v)}
 					active={active_cell !== null && active_cell[0] === i && active_cell[1] === j}
 				/>
 			{/each}
diff --git a/frontend/src/lib/components/grid/messages.ts b/frontend/src/lib/components/grid/messages.ts
index bc032cf..794b8d1 100644
--- a/frontend/src/lib/components/grid/messages.ts
+++ b/frontend/src/lib/components/grid/messages.ts
@@ -25,5 +25,16 @@ interface EvalCellRef {
 	reference: CellRef;
 }
 
+interface LeadErr {
+	code: 'DivZero' | 'TypeErr' | 'Syntax' | 'Server' | 'Unsupported';
+	desc: string;
+	title: string;
+}
+
 // Tagged union
-type Eval = { literal: Literal } | { cellref: EvalCellRef } | { range: Range } | 'unset';
+type Eval =
+	| { literal: Literal }
+	| { cellref: EvalCellRef }
+	| { range: Range }
+	| { err: LeadErr }
+	| 'unset';
diff --git a/frontend/src/lib/components/grid/utils.ts b/frontend/src/lib/components/grid/utils.ts
index 4dc24ac..355d441 100644
--- a/frontend/src/lib/components/grid/utils.ts
+++ b/frontend/src/lib/components/grid/utils.ts
@@ -1,6 +1,7 @@
 export interface CellT {
 	raw_val: string;
 	val: LiteralValue | undefined;
+	isErr: boolean;
 }
 
 /**
@@ -47,6 +48,12 @@ export function getEvalLiteral(value: Eval): LiteralValue {
 	if (value === 'unset') return '';
 	if ('literal' in value) return value.literal.value;
 	if ('cellref' in value) return getEvalLiteral(value.cellref.eval);
+	if ('err' in value) return `err: ${value.err.code}`;
 	// if ('range' in value) return 'err';
 	return 'todo!';
 }
+
+export function isErr(value: Eval): boolean {
+	if (value === 'unset') return false;
+	return 'err' in value;
+}