diff --git a/app/core/src/engine.ts b/app/core/src/engine.ts
index 61b147a..a0d68a2 100644
--- a/app/core/src/engine.ts
+++ b/app/core/src/engine.ts
@@ -3,7 +3,7 @@
 // a file. Diagrams are capped at 200 KB and the engine refuses ambiguity instead of searching, so a check is fast
 // (0.1–0.4 s for large diagrams).
 
-import { compile, parse, resolve, THEMES } from "../../../engine/src/index.js"
+import { compile, parse, resolve, SourceError, THEMES } from "../../../engine/src/index.js"
 
 export { parse, resolve }
 
@@ -13,8 +13,8 @@ export function check(source: string): string | null {
     compile(source)
     return null
   } catch (e) {
-    const line = e && typeof (e as { line?: number }).line === "number" && (e as { line: number }).line > 0 ? `line ${(e as { line: number }).line}: ` : ""
-    return (line + String(e instanceof Error ? e.message : e)).slice(0, 2000)
+    // the engine's own "line N: …" (SourceError.format), as the app's editor shows it (ui/src/render.ts `refusal`)
+    return (e instanceof SourceError && e.line > 0 ? e.format() : String(e instanceof Error ? e.message : e)).slice(0, 2000)
   }
 }
 
diff --git a/app/core/test/engine.test.ts b/app/core/test/engine.test.ts
new file mode 100644
index 0000000..49e4246
--- /dev/null
+++ b/app/core/test/engine.test.ts
@@ -0,0 +1,16 @@
+import { describe, expect, it } from "vitest"
+import { check } from "../src/engine.js"
+
+// check() is what a refused save, the scheduled drafts and Arch Agent's check_diagram report; the app's editor shows the
+// same words (ui/src/render.ts `refusal`), so a line number reads the same everywhere (#32)
+describe("check", () => {
+  it("is null when the source renders", () => {
+    expect(check('node a "A"\nnode b "B" right of a')).toBeNull()
+  })
+  it("names the line of the mistake", () => {
+    expect(check('node a "A"\nnode b "B" rigth of a')).toBe('line 2: node "b": "rigth" is not a direction')
+  })
+  it("names the line at fault in a statement written over several lines", () => {
+    expect(check('node a "A"\nnode b "B"\n  style: rounded\n  rigth of a')).toBe('line 4: node "b": "rigth" is not a direction')
+  })
+})
diff --git a/app/e2e/app.e2e.ts b/app/e2e/app.e2e.ts
index f313977..42c1935 100644
--- a/app/e2e/app.e2e.ts
+++ b/app/e2e/app.e2e.ts
@@ -135,6 +135,19 @@ async function main() {
       await page.keyboard.press("Control+End")
       await page.keyboard.type('\nnode cache "Menu cache" below db  style: store\n')
       await page.waitForFunction(() => document.querySelector("[data-card=system]")?.textContent?.includes("Menu cache"))
+      // a mistake: the status line and the card name its line, the gutter marks that line, Save waits (#32)
+      await page.keyboard.type('node oops "Oops" rigth of db')
+      await page.waitForFunction(() => /^line \d+: node "oops": "rigth" is not a direction$/.test(document.querySelector("[data-testid=status]")?.textContent?.trim() ?? ""))
+      const said = (await page.locator("[data-testid=status]").textContent())!.trim().match(/^line (\d+):/)![1]
+      const typedOn = String((await ta.inputValue()).split("\n").length)
+      const marked = (await page.locator("[data-error-line]").count()) ? (await page.locator("[data-error-line]").textContent())?.trim() : "no line"
+      if (said !== typedOn || marked !== typedOn) throw new Error(`the mistake is on line ${typedOn}; the status says ${said}, the gutter marks ${marked}`)
+      if (!(await page.locator("[data-card=system]").textContent())?.includes(`line ${said}: node "oops"`)) throw new Error("the card does not name the line")
+      if (await page.isEnabled("[data-testid=save]")) throw new Error("Save is enabled on a source that does not render")
+      await page.keyboard.press("Shift+Home")
+      await page.keyboard.press("Backspace")
+      await page.waitForFunction(() => document.querySelector("[data-testid=status]")?.textContent?.startsWith("Renders"))
+      if (await page.locator("[data-error-line]").count()) throw new Error("the gutter still marks a line after the mistake is gone")
       await page.keyboard.press("Control+s")
       // no git identity: the app asks who commits, then the save goes through
       await page.waitForSelector("[data-testid=identity-dialog]")
diff --git a/app/ui/src/App.tsx b/app/ui/src/App.tsx
index 4d68af6..636f5b1 100644
--- a/app/ui/src/App.tsx
+++ b/app/ui/src/App.tsx
@@ -785,7 +785,7 @@ export default function App() {
                     ✕
                   </button>
                 </div>
-                <Editor value={selSource} onChange={v => (setDrafts(d => ({ ...d, [sel]: v })), current?.sample && tourEvent("source.changed"))} />
+                <Editor value={selSource} errorLine={selCard?.r.line} onChange={v => (setDrafts(d => ({ ...d, [sel]: v })), current?.sample && tourEvent("source.changed"))} />
                 <div className={`border-t border-[var(--line)] px-3 py-2 text-xs ${selCard?.r.error ? "text-[var(--danger)]" : "text-[var(--muted)]"}`} data-testid="status">
                   {selCard?.r.error ?? (current?.source.kind === "github" ? "Renders. ⌘/Ctrl-S saves it as a change waiting for review; the card updates as you type." : "Renders. ⌘/Ctrl-S saves it; the card updates as you type.")}
                 </div>
@@ -1010,7 +1010,8 @@ function ProjectTitleDialog({ project, onClose, onSaved }: { project: Project; o
 }
 
 
-function Editor({ value, onChange }: { value: string; onChange: (v: string) => void }) {
+/** The source editor: a textarea with line numbers; the line the engine refused (`errorLine`) is marked in the gutter. */
+function Editor({ value, errorLine, onChange }: { value: string; errorLine?: number; onChange: (v: string) => void }) {
   const ta = useRef<HTMLTextAreaElement>(null)
   const gutter = useRef<HTMLDivElement>(null)
   const lines = value.split("\n").length
@@ -1018,7 +1019,9 @@ function Editor({ value, onChange }: { value: string; onChange: (v: string) => v
     <div className="relative flex min-h-0 flex-1 font-mono text-[13px] leading-[1.6]">
       <div ref={gutter} className="ad-gutter overflow-hidden py-3 pr-2 pl-3 text-right" aria-hidden>
         {Array.from({ length: lines }, (_, i) => (
-          <div key={i}>{i + 1}</div>
+          <div key={i} className={i + 1 === errorLine ? "ad-gutter-error" : undefined} data-error-line={i + 1 === errorLine || undefined}>
+            {i + 1}
+          </div>
         ))}
       </div>
       <textarea
diff --git a/app/ui/src/index.css b/app/ui/src/index.css
index 196ed75..6379ae9 100644
--- a/app/ui/src/index.css
+++ b/app/ui/src/index.css
@@ -176,10 +176,19 @@ body {
 }
 .ad-gutter {
   color: var(--muted);
-  opacity: 0.6;
   user-select: none;
   min-width: 3em;
 }
+.ad-gutter > div {
+  opacity: 0.6;
+}
+/* the line the engine refused: the status line below says why */
+.ad-gutter > .ad-gutter-error {
+  opacity: 1;
+  color: var(--danger);
+  font-weight: 600;
+  box-shadow: inset -2px 0 0 var(--danger);
+}
 
 .ad-toast {
   padding: 8px 14px;
diff --git a/app/ui/src/render.test.ts b/app/ui/src/render.test.ts
index ecb476b..c2d08a6 100644
--- a/app/ui/src/render.test.ts
+++ b/app/ui/src/render.test.ts
@@ -1,6 +1,37 @@
 // @vitest-environment jsdom
 import { describe, expect, it } from "vitest"
-import { renderSource, sanitize } from "./render"
+import { SourceError } from "@engine"
+import { refusal, renderSource, sanitize } from "./render"
+
+describe("renderSource errors", () => {
+  it("includes the source line when a diagram fails to compile", () => {
+    const result = renderSource('node a "A"\nnode b "B" rigth of a', false)
+    expect(result.error).toBe('line 2: node "b": "rigth" is not a direction')
+    expect(result.line).toBe(2)
+    expect(result.size).toEqual({ w: 560, h: 120 })
+  })
+  it("names the line at fault in a statement written over several lines, not the line it starts on", () => {
+    const result = renderSource('node a "A"\nnode b "B"\n  style: rounded\n  rigth of a', false)
+    expect(result.error).toBe('line 4: node "b": "rigth" is not a direction')
+    expect(result.line).toBe(4)
+  })
+  it("has no error and no line when the source renders", () => {
+    const result = renderSource('node a "A"', false)
+    expect(result.error).toBeUndefined()
+    expect(result.line).toBeUndefined()
+  })
+})
+
+describe("refusal", () => {
+  it("prints a source error the way the engine's command-line tool does", () => {
+    expect(refusal(new SourceError('two placements for "server"', 12))).toEqual({ error: 'line 12: two placements for "server"', line: 12 })
+  })
+  it("keeps errors without a usable line unchanged", () => {
+    expect(refusal(new SourceError("cannot compile diagram", 0))).toEqual({ error: "cannot compile diagram" })
+    expect(refusal(new Error("cannot compile diagram"))).toEqual({ error: "cannot compile diagram" })
+    expect(refusal("cannot compile diagram")).toEqual({ error: "cannot compile diagram" })
+  })
+})
 
 describe("sanitize", () => {
   it("drops the three bypasses the review reproduced", () => {
diff --git a/app/ui/src/render.ts b/app/ui/src/render.ts
index 67d27d6..564d1c3 100644
--- a/app/ui/src/render.ts
+++ b/app/ui/src/render.ts
@@ -1,8 +1,8 @@
 import DOMPurify from "dompurify"
-import { THEMES, compile } from "@engine"
+import { SourceError, THEMES, compile } from "@engine"
 import { svgSize, type Size } from "./model"
 
-export type Rendered = { svg: string; size: Size; error?: undefined } | { svg?: undefined; size: Size; error: string }
+export type Rendered = { svg: string; size: Size; error?: undefined; line?: undefined } | { svg?: undefined; size: Size; error: string; line?: number }
 
 /** Render a source with the engine in a theme. A refusal is returned, not thrown: the card shows it in place. */
 export function renderSource(source: string, dark: boolean): Rendered {
@@ -10,10 +10,17 @@ export function renderSource(source: string, dark: boolean): Rendered {
     const svg = sanitize(compile(source, { theme: dark ? THEMES.dark : THEMES.light }))
     return { svg, size: svgSize(svg) }
   } catch (e) {
-    return { error: e instanceof Error ? e.message : String(e), size: { w: 560, h: 120 } }
+    return { ...refusal(e), size: { w: 560, h: 120 } }
   }
 }
 
+/** The engine's refusal as the user reads it: "line 2: …" in the engine's own words (`SourceError.format`, as its
+ *  command-line tool prints it), and the line on its own so the editor can mark it. No usable line, no prefix. */
+export function refusal(e: unknown): { error: string; line?: number } {
+  if (e instanceof SourceError && e.line > 0) return { error: e.format(), line: e.line }
+  return { error: e instanceof Error ? e.message : String(e) }
+}
+
 const INTERNAL = /^#\/[a-z0-9][a-z0-9-]{0,62}(\/[a-z0-9][a-z0-9-]{0,62})?$/
 const ANCHOR = /^#[A-Za-z][\w.-]*$/
 const WEB = /^https?:\/\/[^\s"'<>]+$/i