diff --git a/backend/src/common.rs b/backend/src/common.rs
index fccf258..e3ed112 100644
--- a/backend/src/common.rs
+++ b/backend/src/common.rs
@@ -19,6 +19,7 @@ pub enum LeadErrCode {
     Syntax,
     Server,
     Unsupported,
+    Invalid,
 }
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
diff --git a/backend/src/evaluator.rs b/backend/src/evaluator.rs
index 2df45ca..5b217fe 100644
--- a/backend/src/evaluator.rs
+++ b/backend/src/evaluator.rs
@@ -7,6 +7,7 @@ use crate::common::Literal;
 use crate::grid::Grid;
 use crate::parser::*;
 use std::collections::HashSet;
+use std::f64;
 use std::fmt;
 
 #[derive(Debug, PartialEq, Clone, Deserialize, Serialize)]
@@ -117,6 +118,15 @@ 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)?,
+            "EXP" => eval_single_arg_numeric(args, precs, grid, |x| x.exp(), "EXP".into())?,
+            "SIN" => eval_single_arg_numeric(args, precs, grid, |x| x.sin(), "SIN".into())?,
+            "COS" => eval_single_arg_numeric(args, precs, grid, |x| x.cos(), "COS".into())?,
+            "TAN" => eval_single_arg_numeric(args, precs, grid, |x| x.tan(), "TAN".into())?,
+            "ASIN" => eval_single_arg_numeric(args, precs, grid, |x| x.asin(), "ASIN".into())?,
+            "ACOS" => eval_single_arg_numeric(args, precs, grid, |x| x.acos(), "ACOS".into())?,
+            "ATAN" => eval_single_arg_numeric(args, precs, grid, |x| x.atan(), "ATAN".into())?,
+            "PI" => eval_const(args, Eval::Literal(Literal::Number(f64::consts::PI)))?,
+            "TAU" => eval_const(args, Eval::Literal(Literal::Number(f64::consts::TAU)))?,
             it => {
                 return Err(LeadErr {
                     title: "Evaluation error.".into(),
@@ -256,6 +266,49 @@ fn eval_avg(
     }
 }
 
+fn eval_single_arg_numeric(
+    args: &Vec<Expr>,
+    precs: &mut HashSet<CellRef>,
+    grid: Option<&Grid>,
+    func: fn(f64) -> f64,
+    func_name: String,
+) -> Result<Eval, LeadErr> {
+    if args.len() != 1 {
+        return Err(LeadErr {
+            title: "Evaluation error.".into(),
+            desc: format!("{func_name} function requires a single argument."),
+            code: LeadErrCode::Invalid,
+        });
+    }
+
+    let err = LeadErr {
+        title: "Evaluation error.".into(),
+        desc: format!("{func_name} function requires a numeric argument."),
+        code: LeadErrCode::TypeErr,
+    };
+
+    match evaluate_expr(&args[0], precs, grid)? {
+        Eval::Literal(Literal::Number(num)) => Ok(Eval::Literal(Literal::Number(func(num)))),
+        Eval::CellRef { eval, .. } => match *eval {
+            Eval::Literal(Literal::Number(n)) => Ok(Eval::Literal(Literal::Number(func(n)))),
+            _ => Err(err),
+        },
+        _ => Err(err),
+    }
+}
+
+fn eval_const(args: &Vec<Expr>, value: Eval) -> Result<Eval, LeadErr> {
+    if args.len() != 0 {
+        return Err(LeadErr {
+            title: "Evaluation error.".into(),
+            desc: format!("PI function requires no arguments."),
+            code: LeadErrCode::Invalid,
+        });
+    }
+
+    Ok(value)
+}
+
 fn eval_add(lval: &Eval, rval: &Eval) -> Result<Eval, LeadErr> {
     match (lval, rval) {
         (Eval::Literal(a), Eval::Literal(b)) => {
diff --git a/backend/src/main.rs b/backend/src/main.rs
index 3f7019f..c6993da 100644
--- a/backend/src/main.rs
+++ b/backend/src/main.rs
@@ -79,17 +79,23 @@ async fn accept_connection(stream: TcpStream) {
                                     }
                                 }
 
-                                let msg = LeadMsg {
-                                    cell: None,
-                                    raw: None,
-                                    eval: None,
-                                    bulk_msgs: Some(msgs),
-                                    msg_type: MsgType::Bulk,
-                                };
+                                if msgs.len() == 1 {
+                                    let _ = write
+                                        .send(serde_json::to_string(&msgs.get(0)).unwrap().into())
+                                        .await;
+                                } else if msgs.len() > 1 {
+                                    let msg = LeadMsg {
+                                        cell: None,
+                                        raw: None,
+                                        eval: None,
+                                        bulk_msgs: Some(msgs),
+                                        msg_type: MsgType::Bulk,
+                                    };
 
-                                let _ = write
-                                    .send(serde_json::to_string(&msg).unwrap().into())
-                                    .await;
+                                    let _ = write
+                                        .send(serde_json::to_string(&msg).unwrap().into())
+                                        .await;
+                                }
                             }
                             Err(e) => {
                                 let res = LeadMsg {
diff --git a/backend/src/messages.rs b/backend/src/messages.rs
index a07dab6..bba7c6c 100644
--- a/backend/src/messages.rs
+++ b/backend/src/messages.rs
@@ -2,7 +2,7 @@ use serde::{Deserialize, Serialize};
 
 use crate::{cell::CellRef, evaluator::Eval};
 
-#[derive(Serialize, Deserialize)]
+#[derive(Serialize, Deserialize, Debug)]
 #[serde(rename_all = "lowercase")]
 pub enum MsgType {
     Set,
@@ -11,7 +11,7 @@ pub enum MsgType {
     Bulk,
 }
 
-#[derive(Serialize, Deserialize)]
+#[derive(Serialize, Deserialize, Debug)]
 pub struct LeadMsg {
     pub msg_type: MsgType,
     pub cell: Option<CellRef>,
diff --git a/frontend/src/lib/components/grid/cell.svelte b/frontend/src/lib/components/grid/cell.svelte
index 1f01234..dbcf0e4 100644
--- a/frontend/src/lib/components/grid/cell.svelte
+++ b/frontend/src/lib/components/grid/cell.svelte
@@ -1,7 +1,8 @@
 <script lang="ts">
 	import { Input } from '$lib/components/ui/input/index.js';
 	import clsx from 'clsx';
-	import type { CellT } from './utils';
+	import { getErrDesc, getErrTitle, getEvalLiteral, isErr, type CellT } from './utils';
+	import * as HoverCard from '$lib/components/ui/hover-card/index.js';
 
 	let {
 		cla = '',
@@ -56,15 +57,30 @@
 			focus:z-20 focus:shadow-[0_0_0_1px_var(--color-primary)] focus:outline-none"
 			onblur={(e) => {
 				cell = {
-					isErr: false,
-					val: undefined,
+					val: cell?.val,
 					raw_val: (e.target as HTMLInputElement).value
 				};
 				stopediting();
 			}}
 		/>
 	</div>
+{:else if cell && isErr(cell.val)}
+	<HoverCard.Root openDelay={500} closeDelay={500}>
+		<HoverCard.Trigger>
+			{@render InnerCell()}
+		</HoverCard.Trigger>
+		<HoverCard.Content side="right">
+			<h2 class="text-md font-semibold tracking-tight transition-colors">
+				{getErrTitle(cell.val)}
+			</h2>
+			{getErrDesc(cell.val)}
+		</HoverCard.Content>
+	</HoverCard.Root>
 {:else}
+	{@render InnerCell()}
+{/if}
+
+{#snippet InnerCell()}
 	<div
 		ondblclick={startediting}
 		{onmousedown}
@@ -72,17 +88,17 @@
 		style:height
 		class={clsx('placeholder bg-background p-1', { active }, cla)}
 	>
-		{#if cell && (cell.raw_val !== '' || cell.val !== '')}
-			<span class={clsx('pointer-events-none select-none', { err: cell.isErr })}>
+		{#if cell && (cell.raw_val !== '' || getEvalLiteral(cell.val) !== '')}
+			<span class={clsx('pointer-events-none select-none', { err: isErr(cell.val) })}>
 				{#if cell.val}
-					{cell.val}
+					{getEvalLiteral(cell.val)}
 				{:else}
 					{cell.raw_val}
 				{/if}
 			</span>
 		{/if}
 	</div>
-{/if}
+{/snippet}
 
 <style>
 	.placeholder {
@@ -101,6 +117,7 @@
 	.active:has(.err),
 	.placeholder:has(.err) {
 		position: relative; /* needed for absolute positioning */
+		color: red;
 	}
 
 	.active:has(.err)::after,
diff --git a/frontend/src/lib/components/grid/grid.svelte b/frontend/src/lib/components/grid/grid.svelte
index 749d868..d811d5e 100644
--- a/frontend/src/lib/components/grid/grid.svelte
+++ b/frontend/src/lib/components/grid/grid.svelte
@@ -1,10 +1,12 @@
 <script lang="ts">
 	import { toast } from 'svelte-sonner';
+	import { Pencil } from '@lucide/svelte';
 	import Cell from '$lib/components/grid/cell.svelte';
 	import { onDestroy, onMount } from 'svelte';
 	import CellHeader from './cell-header.svelte';
-	import { colToStr, getEvalLiteral, isErr, refToStr, type CellT } from './utils';
+	import { colToStr, refToStr, type CellT } from './utils';
 	import clsx from 'clsx';
+	import { Input } from '../ui/input';
 
 	let {
 		socket,
@@ -44,7 +46,12 @@
 					console.error('Expected cell value for SET msgponse from server.');
 					return;
 				}
-				setCellVal(msg.cell.row, msg.cell.col, getEvalLiteral(msg.eval), isErr(msg.eval));
+
+				grid_vals[key(msg.cell.row, msg.cell.col)] = {
+					raw_val: msg.raw ?? '',
+					val: msg.eval
+				};
+
 				break;
 			}
 			case 'bulk': {
@@ -108,44 +115,24 @@
 	const key = (i: number, j: number) => `${i}:${j}`;
 
 	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 setCell = (row: number, col: number, v: CellT) => {
+		if (v?.raw_val == null || v.raw_val === '') {
+			delete grid_vals[key(row, col)];
+			return;
 		}
-	};
-	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 {
-			let cell = grid_vals[key(i, j)];
-			cell.val = val;
-			cell.isErr = isErr;
-		}
-	};
 
-	const setCell = (row: number, col: number, v: CellT | undefined) => {
-		// ignore “no value” so we don’t create keys on mount
-		if (v?.raw_val == null || v.raw_val === '') delete grid_vals[key(row, col)];
-		else {
-			setCellRaw(row, col, v.raw_val);
+		grid_vals[key(row, col)] = {
+			raw_val: v.raw_val,
+			val: v.val
+		};
 
-			let msg: LeadMsg = {
-				msg_type: 'set',
-				cell: { row, col },
-				raw: v.raw_val
-			};
+		let msg: LeadMsg = {
+			msg_type: 'set',
+			cell: { row, col },
+			raw: v.raw_val
+		};
 
-			socket.send(JSON.stringify(msg));
-		}
+		socket.send(JSON.stringify(msg));
 	};
 
 	function handleCellInteraction(i: number, j: number, e: MouseEvent) {
@@ -180,6 +167,33 @@
 		active_cell = [i, j];
 	}
 
+	function getActiveCell(): CellT {
+		if (active_cell != null && grid_vals[key(active_cell[0], active_cell[1])])
+			return grid_vals[key(active_cell[0], active_cell[1])];
+		else
+			return {
+				raw_val: '',
+				val: undefined
+			};
+	}
+
+	function setActiveCellRaw(raw: string): void {
+		if (active_cell == null) return;
+
+		if (grid_vals[key(active_cell[0], active_cell[1])]) {
+			let cell = grid_vals[key(active_cell[0], active_cell[1])];
+			setCell(active_cell[0], active_cell[1], {
+				raw_val: raw,
+				val: cell.val
+			});
+		} else {
+			setCell(active_cell[0], active_cell[1], {
+				raw_val: raw,
+				val: undefined
+			});
+		}
+	}
+
 	onMount(() => {
 		const handler = (e: MouseEvent) => {
 			// optional: check if click target is outside grid container
@@ -192,6 +206,15 @@
 	});
 </script>
 
+<div class="mb-5 ml-5 flex items-center gap-5">
+	<Pencil />
+	<Input
+		bind:value={() => getActiveCell().raw_val, (raw) => setActiveCellRaw(raw)}
+		class="relative w-[200px] 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"
+	></Input>
+</div>
+
 <div
 	class={clsx('grid-wrapper relative h-full min-h-0 max-w-full min-w-0 overflow-auto', className)}
 >
diff --git a/frontend/src/lib/components/grid/utils.ts b/frontend/src/lib/components/grid/utils.ts
index 355d441..d3aeea7 100644
--- a/frontend/src/lib/components/grid/utils.ts
+++ b/frontend/src/lib/components/grid/utils.ts
@@ -1,7 +1,6 @@
 export interface CellT {
 	raw_val: string;
-	val: LiteralValue | undefined;
-	isErr: boolean;
+	val?: Eval;
 }
 
 /**
@@ -44,16 +43,38 @@ export function refToStr(row: number, col: number): string {
 	return colToStr(col) + (row + 1).toString();
 }
 
-export function getEvalLiteral(value: Eval): LiteralValue {
+export function getEvalLiteral(value: Eval | undefined): LiteralValue {
+	if (value === undefined) return '';
 	if (value === 'unset') return '';
-	if ('literal' in value) return value.literal.value;
+	if ('literal' in value) {
+		if (value.literal.value == null) return 'NaN';
+		return value.literal.value;
+	}
 	if ('cellref' in value) return getEvalLiteral(value.cellref.eval);
-	if ('err' in value) return `err: ${value.err.code}`;
+	if ('err' in value) return `#${value.err.code.toUpperCase()}`;
 	// if ('range' in value) return 'err';
 	return 'todo!';
 }
 
-export function isErr(value: Eval): boolean {
+export function isErr(value: Eval | undefined): boolean {
+	if (value === undefined) return false;
 	if (value === 'unset') return false;
+	if ('cellref' in value) return isErr(value.cellref.eval);
 	return 'err' in value;
 }
+
+export function getErrTitle(value: Eval | undefined): string {
+	if (value === undefined) return '';
+	if (value === 'unset') return '';
+	if ('cellref' in value) return getErrTitle(value.cellref.eval);
+	if (!('err' in value)) return '';
+	return value.err.title;
+}
+
+export function getErrDesc(value: Eval | undefined): string {
+	if (value === undefined) return '';
+	if (value === 'unset') return '';
+	if ('cellref' in value) return getErrDesc(value.cellref.eval);
+	if (!('err' in value)) return '';
+	return value.err.desc;
+}
diff --git a/frontend/src/lib/components/ui/hover-card/hover-card-content.svelte b/frontend/src/lib/components/ui/hover-card/hover-card-content.svelte
new file mode 100644
index 0000000..0a7e4f3
--- /dev/null
+++ b/frontend/src/lib/components/ui/hover-card/hover-card-content.svelte
@@ -0,0 +1,29 @@
+<script lang="ts">
+	import { LinkPreview as HoverCardPrimitive } from 'bits-ui';
+	import { cn } from '$lib/utils.js';
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		align = 'center',
+		sideOffset = 4,
+		portalProps,
+		...restProps
+	}: HoverCardPrimitive.ContentProps & {
+		portalProps?: HoverCardPrimitive.PortalProps;
+	} = $props();
+</script>
+
+<HoverCardPrimitive.Portal {...portalProps}>
+	<HoverCardPrimitive.Content
+		bind:ref
+		data-slot="hover-card-content"
+		{align}
+		{sideOffset}
+		class={cn(
+			'z-[200] mt-3 w-64 rounded-md border bg-popover p-4 text-popover-foreground shadow-md outline-hidden outline-none data-[side=bottom]:slide-in-from-top-2 data-[side=left]:slide-in-from-right-2 data-[side=right]:slide-in-from-left-2 data-[side=top]:slide-in-from-bottom-2 data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=closed]:zoom-out-95 data-[state=open]:animate-in data-[state=open]:fade-in-0 data-[state=open]:zoom-in-95',
+			className
+		)}
+		{...restProps}
+	/>
+</HoverCardPrimitive.Portal>
diff --git a/frontend/src/lib/components/ui/hover-card/hover-card-trigger.svelte b/frontend/src/lib/components/ui/hover-card/hover-card-trigger.svelte
new file mode 100644
index 0000000..322172b
--- /dev/null
+++ b/frontend/src/lib/components/ui/hover-card/hover-card-trigger.svelte
@@ -0,0 +1,7 @@
+<script lang="ts">
+	import { LinkPreview as HoverCardPrimitive } from "bits-ui";
+
+	let { ref = $bindable(null), ...restProps }: HoverCardPrimitive.TriggerProps = $props();
+</script>
+
+<HoverCardPrimitive.Trigger bind:ref data-slot="hover-card-trigger" {...restProps} />
diff --git a/frontend/src/lib/components/ui/hover-card/index.ts b/frontend/src/lib/components/ui/hover-card/index.ts
new file mode 100644
index 0000000..85f3949
--- /dev/null
+++ b/frontend/src/lib/components/ui/hover-card/index.ts
@@ -0,0 +1,14 @@
+import { LinkPreview as HoverCardPrimitive } from "bits-ui";
+import Content from "./hover-card-content.svelte";
+import Trigger from "./hover-card-trigger.svelte";
+
+const Root = HoverCardPrimitive.Root;
+
+export {
+	Root,
+	Content,
+	Trigger,
+	Root as HoverCard,
+	Content as HoverCardContent,
+	Trigger as HoverCardTrigger,
+};
diff --git a/frontend/src/routes/+page.svelte b/frontend/src/routes/+page.svelte
index e412480..f233bcf 100644
--- a/frontend/src/routes/+page.svelte
+++ b/frontend/src/routes/+page.svelte
@@ -21,7 +21,9 @@
 <div class="absolute left-0 min-h-0 w-full">
 	<div class="flex h-[100vh] flex-col">
 		<div class="h-[60px] w-full p-3">
-			<Sidebar.Trigger />
+			<div class="flex items-center gap-5">
+				<Sidebar.Trigger />
+			</div>
 		</div>
 
 		<div class="grid-wrapper min-h-0 w-full flex-1">
@@ -32,6 +34,6 @@
 
 <style>
 	.grid-wrapper {
-		border-top: 2px solid var(--color-input);
+		/* border-top: 2px solid var(--color-input); */
 	}
 </style>