diff --git a/web/public/pattern-sandbox.html b/web/public/pattern-sandbox.html
index 07009fbbb..abb0c298e 100644
--- a/web/public/pattern-sandbox.html
+++ b/web/public/pattern-sandbox.html
@@ -19,7 +19,12 @@
 <script>
 // Built by scripts/build-sandbox.ts from src/sandbox/sandbox.ts and the lab runtime in
 // src/lib/pattern/. Do not edit: run `npm run build:sandbox` in web/.
-"use strict";(()=>{var PANEL_GEOMETRIES={"128x64_SINGLE":{id:1,physW:128,physH:64,chain:1,resW:128,resH:64},"64x64_SINGLE":{id:2,physW:64,physH:64,chain:1,resW:64,resH:64},"64x64_CHAIN2":{id:3,physW:64,physH:64,chain:2,resW:128,resH:64}},PANEL_GEOM_DEFAULT="128x64_SINGLE",KNOB_COUNT=4,DETENTS_PER_TURN=24;function wrapKnobDelta(delta){return delta>.5?delta-1:delta<-.5?delta+1:delta}var PATTERN_MATRIX_WIDTH=PANEL_GEOMETRIES[PANEL_GEOM_DEFAULT].resW,PATTERN_MATRIX_HEIGHT=PANEL_GEOMETRIES[PANEL_GEOM_DEFAULT].resH,PATTERN_KNOB_COUNT=KNOB_COUNT,PATTERN_DETENTS_PER_TURN=DETENTS_PER_TURN;function clampByte(value){return Number.isFinite(value)?Math.max(0,Math.min(255,Math.round(value))):0}var RAMP_MODES=["linear","smooth","step","hsvShort","hsvLong","oklab","oklchShort","oklchLong"];function rgbToHsv(r,g,b){let rf=r/255,gf=g/255,bf=b/255,max=Math.max(rf,gf,bf),min=Math.min(rf,gf,bf),delta=max-min,h=0;delta>0&&(max===rf?h=(gf-bf)/delta%6:max===gf?h=(bf-rf)/delta+2:h=(rf-gf)/delta+4,h/=6,h<0&&(h+=1));let s=max===0?0:delta/max;return[h,s,max]}function hsvToRgbTuple(h,s,v){h=h-Math.floor(h);let i=Math.floor(h*6),f=h*6-i,p=v*(1-s),q=v*(1-f*s),t=v*(1-(1-f)*s),r,g,b;switch(i%6){case 0:r=v,g=t,b=p;break;case 1:r=q,g=v,b=p;break;case 2:r=p,g=v,b=t;break;case 3:r=p,g=q,b=v;break;case 4:r=t,g=p,b=v;break;default:r=v,g=p,b=q;break}return[Math.round(r*255),Math.round(g*255),Math.round(b*255)]}function srgbChannelToLinear(c){return c/=255,c<=.04045?c/12.92:Math.pow((c+.055)/1.055,2.4)}function linearChannelToSrgb(c){let v=c<=.0031308?c*12.92:1.055*Math.pow(c,.4166666666666667)-.055;return Math.max(0,Math.min(1,v))*255}function srgbToOklab(r,g,b){let lr=srgbChannelToLinear(r),lg=srgbChannelToLinear(g),lb=srgbChannelToLinear(b),l=Math.cbrt(.4122214708*lr+.5363325363*lg+.0514459929*lb),m=Math.cbrt(.2119034982*lr+.6806995451*lg+.1073969566*lb),s=Math.cbrt(.0883024619*lr+.2817188376*lg+.6299787005*lb);return[.2104542553*l+.793617785*m-.0040720468*s,1.9779984951*l-2.428592205*m+.4505937099*s,.0259040371*l+.7827717662*m-.808675766*s]}function oklabToLinearRgb(L,A,B){let l_=L+.3963377774*A+.2158037573*B,m_=L-.1055613458*A-.0638541728*B,s_=L-.0894841775*A-1.291485548*B,l=l_*l_*l_,m=m_*m_*m_,s=s_*s_*s_;return[4.0767416621*l-3.3077115913*m+.2309699292*s,-1.2684380046*l+2.6097574011*m-.3413193965*s,-.0041960863*l-.7034186147*m+1.707614701*s]}var GAMUT_EPS=1e-4;function inLinearGamut(rgb){return rgb[0]>=-GAMUT_EPS&&rgb[0]<=1+GAMUT_EPS&&rgb[1]>=-GAMUT_EPS&&rgb[1]<=1+GAMUT_EPS&&rgb[2]>=-GAMUT_EPS&&rgb[2]<=1+GAMUT_EPS}function oklabToSrgb(L,A,B){let rgb=oklabToLinearRgb(L,A,B);if(!inLinearGamut(rgb)){let lo=0,hi=1;for(let i=0;i<16;i++){let mid=(lo+hi)/2;inLinearGamut(oklabToLinearRgb(L,A*mid,B*mid))?lo=mid:hi=mid}rgb=oklabToLinearRgb(L,A*lo,B*lo)}return[linearChannelToSrgb(rgb[0]),linearChannelToSrgb(rgb[1]),linearChannelToSrgb(rgb[2])]}function mixRampColors(a,b,t,mode){if(mode==="smooth"&&(t=t*t*(3-2*t)),mode==="oklab"||mode==="oklchShort"||mode==="oklchLong"){let[La,Aa,Ba]=srgbToOklab(a[0],a[1],a[2]),[Lb,Ab,Bb]=srgbToOklab(b[0],b[1],b[2]),L=La+(Lb-La)*t;if(mode==="oklab")return oklabToSrgb(L,Aa+(Ab-Aa)*t,Ba+(Bb-Ba)*t);let CHROMA_EPS=1e-4,TWO_PI=Math.PI*2,Ca=Math.sqrt(Aa*Aa+Ba*Ba),Cb=Math.sqrt(Ab*Ab+Bb*Bb),hueA=Math.atan2(Ba,Aa)/TWO_PI;hueA<0&&(hueA+=1);let hueB=Math.atan2(Bb,Ab)/TWO_PI;hueB<0&&(hueB+=1);let h1=Ca<CHROMA_EPS?Cb<CHROMA_EPS?0:hueB:hueA,dh=(Cb<CHROMA_EPS?h1:hueB)-h1;mode==="oklchShort"?(dh>.5&&(dh-=1),dh<-.5&&(dh+=1)):dh>0&&dh<.5?dh-=1:dh<=0&&dh>-.5&&(dh+=1);let C=Ca+(Cb-Ca)*t,H=(h1+dh*t)*TWO_PI;return oklabToSrgb(L,Math.cos(H)*C,Math.sin(H)*C)}if(mode==="hsvShort"||mode==="hsvLong"){let ha=rgbToHsv(a[0],a[1],a[2]),hb=rgbToHsv(b[0],b[1],b[2]),h1=ha[1]===0?hb[0]:ha[0],dh=(hb[1]===0?h1:hb[0])-h1;return mode==="hsvShort"?(dh>.5&&(dh-=1),dh<-.5&&(dh+=1)):dh>0&&dh<.5?dh-=1:dh<=0&&dh>-.5&&(dh+=1),hsvToRgbTuple(h1+dh*t,ha[1]+(hb[1]-ha[1])*t,ha[2]+(hb[2]-ha[2])*t)}return[a[0]+(b[0]-a[0])*t,a[1]+(b[1]-a[1])*t,a[2]+(b[2]-a[2])*t]}function stopAlpha(stop){let a=stop.alpha;return a===void 0||!Number.isFinite(a)?1:Math.max(0,Math.min(1,a))}function mixStopAlpha(a,b,t,mode){mode==="smooth"&&(t=t*t*(3-2*t));let aa=stopAlpha(a);return aa+(stopAlpha(b)-aa)*t}function sampleSortedRampRGBA(stops,mode,wrap,t){if(stops.length===0)return[255,255,255,1];if(stops.length===1)return[...stops[0].color,stopAlpha(stops[0])];t=Math.max(0,Math.min(1,t));let first=stops[0],last=stops[stops.length-1];if(t<=first.position||t>=last.position){if(!wrap){let edge=t<=first.position?first:last;return[...edge.color,stopAlpha(edge)]}let span=1-last.position+first.position,local=span<=0?0:(t>=last.position?t-last.position:t+1-last.position)/span;return mode==="step"?[...last.color,stopAlpha(last)]:[...mixRampColors(last.color,first.color,local,mode),mixStopAlpha(last,first,local,mode)]}for(let i=0;i<stops.length-1;i++){let a=stops[i],b=stops[i+1];if(t>b.position)continue;if(mode==="step")return[...a.color,stopAlpha(a)];let span=b.position-a.position,local=span<=0?0:(t-a.position)/span;return[...mixRampColors(a.color,b.color,local,mode),mixStopAlpha(a,b,local,mode)]}return[...last.color,stopAlpha(last)]}function buildRampLUTRGBA(ramp){let sorted=[...ramp.stops].sort((a,b)=>a.position-b.position),lut=new Uint8Array(256*4);for(let i=0;i<256;i++){let[r,g,b,a]=sampleSortedRampRGBA(sorted,ramp.mode,ramp.wrap,i/255);lut[i*4]=clampByte(r),lut[i*4+1]=clampByte(g),lut[i*4+2]=clampByte(b),lut[i*4+3]=clampByte(a*255)}return lut}function normalizePatternCode(code){return code.replace(/^\s*import\s+.*?;?\s*$/gm,"").replace(/\bexport\s+function\b/g,"function")}var WRAPPER_LINES_BEFORE_CODE=2,functionPreambleLines=null;function stackLocation(error){let stack=error instanceof Error?error.stack:null;if(!stack)return null;let match=stack.match(/(?:<anonymous>|Function|eval):(\d+):(\d+)/);return match?{line:Number(match[1]),column:Number(match[2])}:null}function getFunctionPreambleLines(){if(functionPreambleLines!==null)return functionPreambleLines;functionPreambleLines=2;try{new Function("throw new Error('probe');")()}catch(error){let location=stackLocation(error);location&&(functionPreambleLines=location.line-1)}return functionPreambleLines}function describePatternError(error,code){let message=error instanceof Error?error.message:String(error);if(!code)return message;let location=stackLocation(error);if(!location)return message;let line=location.line-getFunctionPreambleLines()-WRAPPER_LINES_BEFORE_CODE,sourceLines=code.split(`
+"use strict";(()=>{var PANEL_GEOMETRIES={"128x64_SINGLE":{id:1,physW:128,physH:64,chain:1,resW:128,resH:64},"64x64_SINGLE":{id:2,physW:64,physH:64,chain:1,resW:64,resH:64},"64x64_CHAIN2":{id:3,physW:64,physH:64,chain:2,resW:128,resH:64}},PANEL_GEOM_DEFAULT="128x64_SINGLE",KNOB_COUNT=4,DETENTS_PER_TURN=24;function wrapKnobDelta(delta){return delta>.5?delta-1:delta<-.5?delta+1:delta}var PATTERN_MATRIX_WIDTH=PANEL_GEOMETRIES[PANEL_GEOM_DEFAULT].resW,PATTERN_MATRIX_HEIGHT=PANEL_GEOMETRIES[PANEL_GEOM_DEFAULT].resH,PATTERN_KNOB_COUNT=KNOB_COUNT,PATTERN_DETENTS_PER_TURN=DETENTS_PER_TURN;function clampByte(value){return Number.isFinite(value)?Math.max(0,Math.min(255,Math.round(value))):0}var RAMP_MODES=["linear","smooth","step","hsvShort","hsvLong","oklab","oklchShort","oklchLong"];function rgbToHsv(r,g,b){let rf=r/255,gf=g/255,bf=b/255,max=Math.max(rf,gf,bf),min=Math.min(rf,gf,bf),delta=max-min,h=0;delta>0&&(max===rf?h=(gf-bf)/delta%6:max===gf?h=(bf-rf)/delta+2:h=(rf-gf)/delta+4,h/=6,h<0&&(h+=1));let s=max===0?0:delta/max;return[h,s,max]}function hsvToRgbTuple(h,s,v){h=h-Math.floor(h);let i=Math.floor(h*6),f=h*6-i,p=v*(1-s),q=v*(1-f*s),t=v*(1-(1-f)*s),r,g,b;switch(i%6){case 0:r=v,g=t,b=p;break;case 1:r=q,g=v,b=p;break;case 2:r=p,g=v,b=t;break;case 3:r=p,g=q,b=v;break;case 4:r=t,g=p,b=v;break;default:r=v,g=p,b=q;break}return[Math.round(r*255),Math.round(g*255),Math.round(b*255)]}function srgbChannelToLinear(c){return c/=255,c<=.04045?c/12.92:Math.pow((c+.055)/1.055,2.4)}function linearChannelToSrgb(c){let v=c<=.0031308?c*12.92:1.055*Math.pow(c,.4166666666666667)-.055;return Math.max(0,Math.min(1,v))*255}function srgbToOklab(r,g,b){let lr=srgbChannelToLinear(r),lg=srgbChannelToLinear(g),lb=srgbChannelToLinear(b),l=Math.cbrt(.4122214708*lr+.5363325363*lg+.0514459929*lb),m=Math.cbrt(.2119034982*lr+.6806995451*lg+.1073969566*lb),s=Math.cbrt(.0883024619*lr+.2817188376*lg+.6299787005*lb);return[.2104542553*l+.793617785*m-.0040720468*s,1.9779984951*l-2.428592205*m+.4505937099*s,.0259040371*l+.7827717662*m-.808675766*s]}function oklabToLinearRgb(L,A,B){let l_=L+.3963377774*A+.2158037573*B,m_=L-.1055613458*A-.0638541728*B,s_=L-.0894841775*A-1.291485548*B,l=l_*l_*l_,m=m_*m_*m_,s=s_*s_*s_;return[4.0767416621*l-3.3077115913*m+.2309699292*s,-1.2684380046*l+2.6097574011*m-.3413193965*s,-.0041960863*l-.7034186147*m+1.707614701*s]}var GAMUT_EPS=1e-4;function inLinearGamut(rgb){return rgb[0]>=-GAMUT_EPS&&rgb[0]<=1+GAMUT_EPS&&rgb[1]>=-GAMUT_EPS&&rgb[1]<=1+GAMUT_EPS&&rgb[2]>=-GAMUT_EPS&&rgb[2]<=1+GAMUT_EPS}function oklabToSrgb(L,A,B){let rgb=oklabToLinearRgb(L,A,B);if(!inLinearGamut(rgb)){let lo=0,hi=1;for(let i=0;i<16;i++){let mid=(lo+hi)/2;inLinearGamut(oklabToLinearRgb(L,A*mid,B*mid))?lo=mid:hi=mid}rgb=oklabToLinearRgb(L,A*lo,B*lo)}return[linearChannelToSrgb(rgb[0]),linearChannelToSrgb(rgb[1]),linearChannelToSrgb(rgb[2])]}function mixRampColors(a,b,t,mode){if(mode==="smooth"&&(t=t*t*(3-2*t)),mode==="oklab"||mode==="oklchShort"||mode==="oklchLong"){let[La,Aa,Ba]=srgbToOklab(a[0],a[1],a[2]),[Lb,Ab,Bb]=srgbToOklab(b[0],b[1],b[2]),L=La+(Lb-La)*t;if(mode==="oklab")return oklabToSrgb(L,Aa+(Ab-Aa)*t,Ba+(Bb-Ba)*t);let CHROMA_EPS=1e-4,TWO_PI=Math.PI*2,Ca=Math.sqrt(Aa*Aa+Ba*Ba),Cb=Math.sqrt(Ab*Ab+Bb*Bb),hueA=Math.atan2(Ba,Aa)/TWO_PI;hueA<0&&(hueA+=1);let hueB=Math.atan2(Bb,Ab)/TWO_PI;hueB<0&&(hueB+=1);let h1=Ca<CHROMA_EPS?Cb<CHROMA_EPS?0:hueB:hueA,dh=(Cb<CHROMA_EPS?h1:hueB)-h1;mode==="oklchShort"?(dh>.5&&(dh-=1),dh<-.5&&(dh+=1)):dh>0&&dh<.5?dh-=1:dh<=0&&dh>-.5&&(dh+=1);let C=Ca+(Cb-Ca)*t,H=(h1+dh*t)*TWO_PI;return oklabToSrgb(L,Math.cos(H)*C,Math.sin(H)*C)}if(mode==="hsvShort"||mode==="hsvLong"){let ha=rgbToHsv(a[0],a[1],a[2]),hb=rgbToHsv(b[0],b[1],b[2]),h1=ha[1]===0?hb[0]:ha[0],dh=(hb[1]===0?h1:hb[0])-h1;return mode==="hsvShort"?(dh>.5&&(dh-=1),dh<-.5&&(dh+=1)):dh>0&&dh<.5?dh-=1:dh<=0&&dh>-.5&&(dh+=1),hsvToRgbTuple(h1+dh*t,ha[1]+(hb[1]-ha[1])*t,ha[2]+(hb[2]-ha[2])*t)}return[a[0]+(b[0]-a[0])*t,a[1]+(b[1]-a[1])*t,a[2]+(b[2]-a[2])*t]}function stopAlpha(stop){let a=stop.alpha;return a===void 0||!Number.isFinite(a)?1:Math.max(0,Math.min(1,a))}function mixStopAlpha(a,b,t,mode){mode==="smooth"&&(t=t*t*(3-2*t));let aa=stopAlpha(a);return aa+(stopAlpha(b)-aa)*t}function sampleSortedRampRGBA(stops,mode,wrap,t){if(stops.length===0)return[255,255,255,1];if(stops.length===1)return[...stops[0].color,stopAlpha(stops[0])];t=Math.max(0,Math.min(1,t));let first=stops[0],last=stops[stops.length-1];if(t<=first.position||t>=last.position){if(!wrap){let edge=t<=first.position?first:last;return[...edge.color,stopAlpha(edge)]}let span=1-last.position+first.position,local=span<=0?0:(t>=last.position?t-last.position:t+1-last.position)/span;return mode==="step"?[...last.color,stopAlpha(last)]:[...mixRampColors(last.color,first.color,local,mode),mixStopAlpha(last,first,local,mode)]}for(let i=0;i<stops.length-1;i++){let a=stops[i],b=stops[i+1];if(t>b.position)continue;if(mode==="step")return[...a.color,stopAlpha(a)];let span=b.position-a.position,local=span<=0?0:(t-a.position)/span;return[...mixRampColors(a.color,b.color,local,mode),mixStopAlpha(a,b,local,mode)]}return[...last.color,stopAlpha(last)]}function buildRampLUTRGBA(ramp){let sorted=[...ramp.stops].sort((a,b)=>a.position-b.position),lut=new Uint8Array(256*4);for(let i=0;i<256;i++){let[r,g,b,a]=sampleSortedRampRGBA(sorted,ramp.mode,ramp.wrap,i/255);lut[i*4]=clampByte(r),lut[i*4+1]=clampByte(g),lut[i*4+2]=clampByte(b),lut[i*4+3]=clampByte(a*255)}return lut}function normalizePatternCode(code){return code.replace(/^\s*import\s+.*?;?\s*$/gm,"").replace(/\bexport\s+function\b/g,"function")}var WRAPPER_LINES_BEFORE_CODE=2,functionPreambleLines=null;function stackLocation(error){let stack=error instanceof Error?error.stack:null;if(!stack)return null;let match=stack.match(/(?:<anonymous>|Function|eval):(\d+):(\d+)/);return match?{line:Number(match[1]),column:Number(match[2])}:null}function getFunctionPreambleLines(){if(functionPreambleLines!==null)return functionPreambleLines;functionPreambleLines=2;try{new Function("throw new Error('probe');")()}catch(error){let location=stackLocation(error);location&&(functionPreambleLines=location.line-1)}return functionPreambleLines}var OPENER_OF={")":"(","]":"[","}":"{"},REGEX_AFTER_WORDS=new Set(["return","typeof","instanceof","in","of","new","delete","void","throw","case","do","else","yield","await"]);function regexCanStart(prev,word){return word?REGEX_AFTER_WORDS.has(word):prev===""||"(,=:[!&|?{};+-*%<>~^".includes(prev)}function openAtEnd(code){let stack=[],innermostFirst=inside=>inside?[inside,...stack.slice().reverse()]:stack.slice().reverse(),prev="",word="",i=0;for(;i<code.length;){let c=code[i];if(stack[stack.length-1]==="`"){c==="\\"?i+=2:c==="`"?(stack.pop(),prev=c,word="",i++):c==="$"&&code[i+1]==="{"?(stack.push("${"),prev="{",word="",i+=2):i++;continue}if(/\s/.test(c)){i++;continue}if(/[\w$]/.test(c)){let j=i+1;for(;j<code.length&&/[\w$]/.test(code[j]);)j++;word=code.slice(i,j),prev=code[j-1],i=j;continue}let next=code[i+1];if(c==="/"&&next==="/"){let end=code.indexOf(`
+`,i);if(end<0)break;i=end;continue}if(c==="/"&&next==="*"){let end=code.indexOf("*/",i+2);if(end<0)return innermostFirst("/*");i=end+2;continue}if(c==='"'||c==="'"){let j=i+1;for(;j<code.length&&code[j]!==c;){if(code[j]===`
+`)return null;j+=code[j]==="\\"?2:1}if(j>=code.length)return innermostFirst(c);prev=c,word="",i=j+1;continue}if(c==="/"&&regexCanStart(prev,word)){let j=i+1,inClass=!1;for(;j<code.length&&(inClass||code[j]!=="/");){if(code[j]===`
+`)return null;code[j]==="["?inClass=!0:code[j]==="]"&&(inClass=!1),j+=code[j]==="\\"?2:1}if(j>=code.length)return null;prev=c,word="",i=j+1;continue}if(c==="`"||c==="("||c==="["||c==="{")stack.push(c);else if(c==="}"&&stack[stack.length-1]==="${")stack.pop();else if(c in OPENER_OF&&stack.pop()!==OPENER_OF[c])return null;prev=c,word="",i++}return innermostFirst()}function describeCutOff(code){var _a;let open=openAtEnd(code);if(!open||open.length===0)return null;let counts=new Map;for(let token of open)counts.set(token,((_a=counts.get(token))!=null?_a:0)+1);let parts=[...counts].map(([token,count])=>`${count} ${token}`),list=parts.length>1?`${parts.slice(0,-1).join(", ")} and ${parts[parts.length-1]}`:parts[0],verb=open.length===1?"never closes":"never close",lines=code.split(`
+`),last=lines.length;for(;last>1&&!lines[last-1].trim();)last--;let source=lines[last-1].trim(),quoted=source.length>80?`\u2026${source.slice(-79)}`:source;return`The code stops before it ends: ${list} ${verb}. Was the paste cut off?
+line ${last} (the last): ${quoted}`}function describePatternError(error,code){let message=error instanceof Error?error.message:String(error);if(!code)return message;if(error instanceof Error&&error.name==="SyntaxError"){let cutOff=describeCutOff(code);if(cutOff)return cutOff}let location=stackLocation(error);if(!location)return message;let line=location.line-getFunctionPreambleLines()-WRAPPER_LINES_BEFORE_CODE,sourceLines=code.split(`
 `);if(line<1||line>sourceLines.length)return message;let source=sourceLines[line-1].trim(),quoted=source.length>80?`${source.slice(0,79)}\u2026`:source;return`${message}
 line ${line}: ${quoted}`}function compilePatternCode(code){let wrapper=`
     "use strict";
diff --git a/web/src/components/3d/LedMatrixTexture.test.ts b/web/src/components/3d/LedMatrixTexture.test.ts
index 971a0b864..855183bea 100644
--- a/web/src/components/3d/LedMatrixTexture.test.ts
+++ b/web/src/components/3d/LedMatrixTexture.test.ts
@@ -56,7 +56,7 @@ describe('LedMatrixTexture errors', () => {
     frames(texture, 1);
 
     expect(onError).toHaveBeenCalledTimes(1);
-    expect(onError.mock.calls[0][0]).toMatch(/^Unexpected/);
+    expect(onError.mock.calls[0][0]).toMatch(/^The code stops before it ends: 1 \( and 1 \{ never close\./);
     expect(topLeft(texture)).toEqual([255, 0, 0]);
   });
 
diff --git a/web/src/lib/pattern/harness.test.ts b/web/src/lib/pattern/harness.test.ts
new file mode 100644
index 000000000..a61eff5d9
--- /dev/null
+++ b/web/src/lib/pattern/harness.test.ts
@@ -0,0 +1,104 @@
+// A pasted pattern that was cut off used to be reported as "Unexpected token
+// 'return'" or "Unexpected token ')'": the `return` compilePatternCode puts
+// after the code, and the `)` that `new Function` closes it with. Neither is in
+// the editor. The Lab's Code panel and the Live Editor both show what
+// describePatternError says, through PatternRuntime and LedMatrixTexture.
+
+import { describe, expect, it } from "vitest";
+import { PatternRuntime, describePatternError } from "./harness";
+
+const FULL = `export function setup(params) {
+  params.t = 0;
+}
+
+export function draw(display, params, time) {
+  for (let y = 0; y < display.height; y++) {
+    for (let x = 0; x < display.width; x++) {
+      display.setPixel(x, y, 255, 0, 0);
+    }
+  }
+}
+`;
+
+function loadError(code: string) {
+  const result = new PatternRuntime(8, 8).loadCode(code);
+  expect(result.ok).toBe(false);
+  return result.error;
+}
+
+describe("a pattern cut off partway", () => {
+  it("says the code stops early, and quotes the last line, when cut inside a call", () => {
+    expect(loadError(FULL.slice(0, FULL.indexOf("255")))).toBe(
+      "The code stops before it ends: 1 ( and 3 { never close. Was the paste cut off?\n" +
+        "line 8 (the last): display.setPixel(x, y,",
+    );
+  });
+
+  it("says so when only the last } is missing", () => {
+    expect(loadError(FULL.trimEnd().slice(0, -1))).toBe(
+      "The code stops before it ends: 1 { never closes. Was the paste cut off?\nline 10 (the last): }",
+    );
+  });
+
+  it("names the string, template or comment the code stops inside", () => {
+    expect(loadError(`function draw(display) {\n  const label = "Patt`)).toMatch(
+      /^The code stops before it ends: 1 " and 1 \{ never close\./,
+    );
+    expect(loadError("function draw(display) {\n  const label = `x = ${display.width")).toMatch(
+      /^The code stops before it ends: 1 \$\{, 1 ` and 1 \{ never close\./,
+    );
+    expect(loadError("function draw(display) {\n  /* fill the panel")).toMatch(
+      /^The code stops before it ends: 1 \/\* and 1 \{ never close\./,
+    );
+  });
+
+  it("does not count brackets inside strings, comments, templates or regular expressions", () => {
+    const code = `function draw(display, params) {
+  const a = "(([";
+  const b = '{{';
+  const c = \`\${display.width} ((\`;
+  const d = /[({]+/g.test(a) ? display.width / 2 : display.height / 2;
+  // ((({
+  /* {[( */
+  display.setPixel(d, 0,`;
+    expect(loadError(code)).toBe(
+      "The code stops before it ends: 1 ( and 1 { never close. Was the paste cut off?\n" +
+        "line 8 (the last): display.setPixel(d, 0,",
+    );
+  });
+
+  it("keeps the tail of a long last line, where the paste stopped", () => {
+    const tail = "display.setPixel(x, y, 255, 0, ";
+    const message = loadError(`function draw(display) {\n  ${"const pad = 0; ".repeat(8)}${tail}`);
+    expect(message).toMatch(/\nline 2 \(the last\): …/);
+    expect(message?.endsWith(tail.trim())).toBe(true);
+  });
+
+  it("skips blank lines after the cut when it quotes the last line", () => {
+    expect(loadError("function draw(display) {\n  display.setPixel(0, 0,\n\n\n")).toMatch(
+      /\nline 2 \(the last\): display\.setPixel\(0, 0,$/,
+    );
+  });
+});
+
+describe("a syntax error that is not a cut", () => {
+  it("keeps the engine's message when a ) is missing in the middle", () => {
+    const code = FULL.replace("255, 0, 0)", "255, 0, 0");
+    expect(loadError(code)).not.toMatch(/stops before it ends/);
+  });
+
+  it("keeps the engine's message when a string runs into the next line", () => {
+    const code = FULL.replace("255, 0, 0);", '"red);');
+    expect(loadError(code)).not.toMatch(/stops before it ends/);
+  });
+
+  it("keeps the engine's message when every bracket closes", () => {
+    const code = FULL.replace("let y = 0;", "let y = = 0;");
+    expect(loadError(code)).not.toMatch(/stops before it ends/);
+  });
+
+  it("leaves errors that are not syntax errors alone", () => {
+    const error = new TypeError("display.fill is not a function");
+    expect(describePatternError(error, "function draw(display) {")).toBe("display.fill is not a function");
+  });
+});
diff --git a/web/src/lib/pattern/harness.ts b/web/src/lib/pattern/harness.ts
index 7a7b898f4..f1489e6f3 100644
--- a/web/src/lib/pattern/harness.ts
+++ b/web/src/lib/pattern/harness.ts
@@ -408,15 +408,172 @@ function getFunctionPreambleLines(): number {
   return functionPreambleLines;
 }
 
+// ── Code that stops early ───────────────────────────────────────────────────
+// A paste cut off partway is the commonest syntax error, and the engine blames
+// a token the author never typed. Cut inside `display.setPixel(x, y, ` and the
+// `return` compilePatternCode appends below the code lands inside the call:
+// "Unexpected token 'return'". Lose only the last `}` and it is the `)` that
+// `new Function` closes the body with: "Unexpected token ')'". Neither is in
+// the editor. What is still open where the code ends says it was cut.
+
+const OPENER_OF: Record<string, string> = { ")": "(", "]": "[", "}": "{" };
+
+// After one of these words a `/` starts a regular expression, not a division.
+const REGEX_AFTER_WORDS = new Set([
+  "return", "typeof", "instanceof", "in", "of", "new", "delete", "void",
+  "throw", "case", "do", "else", "yield", "await",
+]);
+
+function regexCanStart(prev: string, word: string) {
+  if (word) return REGEX_AFTER_WORDS.has(word);
+  return prev === "" || "(,=:[!&|?{};+-*%<>~^".includes(prev);
+}
+
+/**
+ * What is still open where the code ends, innermost first: `(`, `[`, `{`, a
+ * template's `${`, and the quote, backtick or `/*` the code stops inside.
+ * Brackets inside strings, comments and regular expressions do not count.
+ * Null when the code is broken in the middle instead (a closer that meets the
+ * wrong opener, a string that runs into a newline): there the engine's own
+ * message is the better guess.
+ */
+function openAtEnd(code: string): string[] | null {
+  // Each entry is "(", "[", "{", "${" or "`"; a "`" on top means the scan is
+  // in template text.
+  const stack: string[] = [];
+  const innermostFirst = (inside?: string) =>
+    inside ? [inside, ...stack.slice().reverse()] : stack.slice().reverse();
+  // The last significant character outside strings and comments, and the word
+  // it ends, to tell a regular expression from a division.
+  let prev = "";
+  let word = "";
+  let i = 0;
+
+  while (i < code.length) {
+    const c = code[i];
+
+    if (stack[stack.length - 1] === "`") {
+      if (c === "\\") i += 2;
+      else if (c === "`") {
+        stack.pop();
+        prev = c;
+        word = "";
+        i++;
+      } else if (c === "$" && code[i + 1] === "{") {
+        stack.push("${");
+        prev = "{";
+        word = "";
+        i += 2;
+      } else i++;
+      continue;
+    }
+
+    if (/\s/.test(c)) {
+      i++;
+      continue;
+    }
+    if (/[\w$]/.test(c)) {
+      let j = i + 1;
+      while (j < code.length && /[\w$]/.test(code[j])) j++;
+      word = code.slice(i, j);
+      prev = code[j - 1];
+      i = j;
+      continue;
+    }
+
+    const next = code[i + 1];
+    if (c === "/" && next === "/") {
+      const end = code.indexOf("\n", i);
+      if (end < 0) break;
+      i = end;
+      continue;
+    }
+    if (c === "/" && next === "*") {
+      const end = code.indexOf("*/", i + 2);
+      if (end < 0) return innermostFirst("/*");
+      i = end + 2;
+      continue;
+    }
+    if (c === '"' || c === "'") {
+      let j = i + 1;
+      while (j < code.length && code[j] !== c) {
+        if (code[j] === "\n") return null;
+        j += code[j] === "\\" ? 2 : 1;
+      }
+      if (j >= code.length) return innermostFirst(c);
+      prev = c;
+      word = "";
+      i = j + 1;
+      continue;
+    }
+    if (c === "/" && regexCanStart(prev, word)) {
+      let j = i + 1;
+      let inClass = false;
+      while (j < code.length && (inClass || code[j] !== "/")) {
+        if (code[j] === "\n") return null;
+        if (code[j] === "[") inClass = true;
+        else if (code[j] === "]") inClass = false;
+        j += code[j] === "\\" ? 2 : 1;
+      }
+      if (j >= code.length) return null;
+      // The flags that follow scan as a word; with none, a `/` after the
+      // literal divides.
+      prev = c;
+      word = "";
+      i = j + 1;
+      continue;
+    }
+
+    if (c === "`" || c === "(" || c === "[" || c === "{") stack.push(c);
+    else if (c === "}" && stack[stack.length - 1] === "${") stack.pop();
+    else if (c in OPENER_OF && stack.pop() !== OPENER_OF[c]) return null;
+    prev = c;
+    word = "";
+    i++;
+  }
+
+  return innermostFirst();
+}
+
+/**
+ * "The code stops before it ends: 1 ( and 3 { never close." with the last line
+ * the code has, or null when nothing is left open at the end.
+ */
+function describeCutOff(code: string): string | null {
+  const open = openAtEnd(code);
+  if (!open || open.length === 0) return null;
+
+  const counts = new Map<string, number>();
+  for (const token of open) counts.set(token, (counts.get(token) ?? 0) + 1);
+  const parts = [...counts].map(([token, count]) => `${count} ${token}`);
+  const list = parts.length > 1 ? `${parts.slice(0, -1).join(", ")} and ${parts[parts.length - 1]}` : parts[0];
+  const verb = open.length === 1 ? "never closes" : "never close";
+
+  const lines = code.split("\n");
+  let last = lines.length;
+  while (last > 1 && !lines[last - 1].trim()) last--;
+  // The end of the line is where the paste stopped, so a long one keeps its tail.
+  const source = lines[last - 1].trim();
+  const quoted = source.length > 80 ? `…${source.slice(-79)}` : source;
+  return `The code stops before it ends: ${list} ${verb}. Was the paste cut off?\nline ${last} (the last): ${quoted}`;
+}
+
 /**
  * Message plus, when the stack allows it, the pattern's own line number and the
- * source of that line. Falls back to the bare message — a syntax error carries
- * no usable frame, and a wrong line number would be worse than none.
+ * source of that line. A syntax error carries no usable frame, so it gets the
+ * bare message, unless the code ends with brackets still open: then it says the
+ * code was cut off and quotes the last line. A wrong line number would be worse
+ * than none.
  */
 export function describePatternError(error: unknown, code: string | null): string {
   const message = error instanceof Error ? error.message : String(error);
   if (!code) return message;
 
+  if (error instanceof Error && error.name === "SyntaxError") {
+    const cutOff = describeCutOff(code);
+    if (cutOff) return cutOff;
+  }
+
   const location = stackLocation(error);
   if (!location) return message;