diff --git a/static/brush-editor.js b/static/brush-editor.js
index 4696332..6159db6 100644
--- a/static/brush-editor.js
+++ b/static/brush-editor.js
@@ -2,9 +2,9 @@ const defaultBrush = `
 ; This is your brush.
 ; Feel free to edit it to your liking!
 (stroke
-    8                       ; thickness
-    (rgba 0.0 0.0 0.0 1.0)  ; color
-    (vec))                  ; position
+  8                       ; thickness
+  (rgba 0.0 0.0 0.0 1.0)  ; color
+  (vec))                  ; position
 `.trim();
 
 export class BrushEditor extends HTMLElement {
@@ -28,11 +28,51 @@ export class BrushEditor extends HTMLElement {
                 }),
             );
         });
+
+        this.errorHeader = this.appendChild(document.createElement("h1"));
+        this.errorHeader.classList.add("error-header");
+
+        this.errorArea = this.appendChild(document.createElement("pre"));
+        this.errorArea.classList.add("errors");
     }
 
     get code() {
         return this.textArea.textContent;
     }
+
+    resetErrors() {
+        this.errorHeader.textContent = "";
+        this.errorArea.textContent = "";
+    }
+
+    renderHakuResult(phase, result) {
+        this.resetErrors();
+
+        console.log(result);
+
+        if (result.status != "error") return;
+
+        this.errorHeader.textContent = `${phase} failed`;
+
+        if (result.errorKind == "diagnostics") {
+            // This is kind of wooden; I'd prefer if the error spans were rendered inline in text,
+            // but I haven't integrated anything for syntax highlighting yet that would let me
+            // do that.
+            this.errorArea.textContent = result.diagnostics
+                .map(
+                    (diagnostic) => `${diagnostic.start}..${diagnostic.end}: ${diagnostic.message}`,
+                )
+                .join("\n");
+        } else if (result.errorKind == "plain") {
+            this.errorHeader.textContent = result.message;
+        } else if (result.errorKind == "exception") {
+            // TODO: This should show a stack trace.
+            this.errorArea.textContent = `an exception occurred: ${result.message}`;
+        } else {
+            console.warn(`unknown error kind: ${result.errorKind}`);
+            this.errorHeader.textContent = "(unknown error kind)";
+        }
+    }
 }
 
 customElements.define("rkgk-brush-editor", BrushEditor);
diff --git a/static/index.css b/static/index.css
index 29ac5fd..6f27f0f 100644
--- a/static/index.css
+++ b/static/index.css
@@ -60,6 +60,10 @@ button, textarea, input {
     font-family: inherit;
 }
 
+pre, code {
+    font-family: "Fira Code", monospace;    
+}
+
 /* Main container layout */
 
 main {
@@ -215,7 +219,24 @@ rkgk-brush-editor {
         height: 100%;
         margin: 0;
         resize: none;
-        font-family: "Fira Code", monospace;
+        white-space: pre-wrap;
+    }
+
+    &>.errors:empty, &>.error-header:empty {
+        display: none;
+    }
+
+    &>.error-header {
+        margin-top: 1em;
+        margin-bottom: 0;
+        font-size: 1rem;
+        color: var(--color-error);
+    }
+
+    &>.errors {
+        margin: 0;
+        color: var(--color-error);
+        white-space: pre-wrap;
     }
 }
 
diff --git a/static/index.js b/static/index.js
index 0b97a93..1d859da 100644
--- a/static/index.js
+++ b/static/index.js
@@ -183,7 +183,20 @@ function readUrl() {
 
     canvasRenderer.addEventListener(".paint", async (event) => {
         plotQueue.push({ x: event.x, y: event.y });
-        currentUser.renderBrushToChunks(wall, event.x, event.y);
+
+        if (currentUser.isBrushOk) {
+            brushEditor.resetErrors();
+
+            let result = currentUser.renderBrushToChunks(wall, event.x, event.y);
+            console.log(result);
+
+            if (result.status == "error") {
+                brushEditor.renderHakuResult(
+                    result.phase == "eval" ? "Evaluation" : "Rendering",
+                    result.result,
+                );
+            }
+        }
     });
 
     canvasRenderer.addEventListener(".viewportUpdate", () => reticleRenderer.render());
@@ -191,10 +204,10 @@ function readUrl() {
         updateUrl(session, canvasRenderer.viewport),
     );
 
-    currentUser.setBrush(brushEditor.code);
+    brushEditor.renderHakuResult("Compilation", currentUser.setBrush(brushEditor.code));
     brushEditor.addEventListener(".codeChanged", async () => {
         flushPlotQueue();
-        currentUser.setBrush(brushEditor.code);
+        brushEditor.renderHakuResult("Compilation", currentUser.setBrush(brushEditor.code));
         session.sendSetBrush(brushEditor.code);
     });
 
diff --git a/static/online-users.js b/static/online-users.js
index c3caecb..aef3fb4 100644
--- a/static/online-users.js
+++ b/static/online-users.js
@@ -22,7 +22,7 @@ export class User {
     }
 
     renderBrushToChunks(wall, x, y) {
-        this.painter.renderBrushToWall(this.haku, x, y, wall);
+        return this.painter.renderBrushToWall(this.haku, x, y, wall);
     }
 }
 
diff --git a/static/painter.js b/static/painter.js
index b9bc3f4..cf52dc3 100644
--- a/static/painter.js
+++ b/static/painter.js
@@ -5,7 +5,8 @@ export class Painter {
 
     renderBrushToWall(haku, centerX, centerY, wall) {
         let evalResult = haku.evalBrush();
-        if (evalResult.status != "ok") return evalResult;
+        if (evalResult.status != "ok")
+            return { status: "error", phase: "eval", result: evalResult };
 
         let left = centerX - this.paintArea / 2;
         let top = centerY - this.paintArea / 2;
@@ -21,7 +22,10 @@ export class Painter {
                 let chunk = wall.getOrCreateChunk(chunkX, chunkY);
 
                 let renderResult = haku.renderValue(chunk.pixmap, x, y);
-                if (renderResult.status != "ok") return renderResult;
+                if (renderResult.status != "ok") {
+                    haku.resetVm();
+                    return { status: "error", phase: "render", result: renderResult };
+                }
             }
         }
         haku.resetVm();
@@ -32,5 +36,7 @@ export class Painter {
                 chunk.syncFromPixmap();
             }
         }
+
+        return { status: "ok" };
     }
 }