diff --git a/public/viz/viz-hydrate.ts b/public/viz/viz-hydrate.ts
index 7a877d8..bcf884e 100644
--- a/public/viz/viz-hydrate.ts
+++ b/public/viz/viz-hydrate.ts
@@ -20,7 +20,11 @@ function hydrateViz(root: Document | Element = document): void {
 		}
 
 		try {
-			const json = atob(encoded);
+			// Base64 of UTF-8 bytes, as the server emits it; atob alone would
+			// mangle every code point above U+00FF.
+			const json = new TextDecoder().decode(
+				Uint8Array.from(atob(encoded), (ch) => ch.charCodeAt(0)),
+			);
 			const data: unknown = JSON.parse(json);
 			renderer(container, data);
 		} catch (err) {
diff --git a/src/lib/viz/plugin.ts b/src/lib/viz/plugin.ts
index 37d4ddd..8bcb317 100644
--- a/src/lib/viz/plugin.ts
+++ b/src/lib/viz/plugin.ts
@@ -9,6 +9,31 @@ export interface VizPluginEnv {
 	hasViz?: boolean;
 }
 
+function vizError(message: string): string {
+	return `<div class="viz-error">${escapeHtml(message)}</div>\n`;
+}
+
+function renderVizBlock(info: string, content: string, env: unknown): string {
+	const vizType = info.slice(VIZ_PREFIX.length) as VizType;
+	if (!SUPPORTED_TYPES.has(vizType)) {
+		return `<div class="viz-error">Unknown visualization type: <code>${escapeHtml(vizType)}</code></div>\n`;
+	}
+
+	const raw = content.trimEnd();
+	const result = validate(vizType, raw);
+
+	if (!result.ok) {
+		return vizError(`Invalid viz data: ${result.error ?? "unknown error"}`);
+	}
+
+	// Buffer, not btoa: btoa throws on any code point above U+00FF, and the
+	// hydrator decodes these bytes back as UTF-8.
+	const encoded = Buffer.from(raw, "utf8").toString("base64");
+	(env as VizPluginEnv).hasViz = true;
+
+	return `<div class="viz-container" data-viz-type="${vizType}" data-viz="${encoded}"></div>\n`;
+}
+
 export function vizPlugin(md: MarkdownIt): void {
 	const defaultFence =
 		md.renderer.rules["fence"] ??
@@ -24,21 +49,14 @@ export function vizPlugin(md: MarkdownIt): void {
 			return defaultFence(tokens, idx, options, env, self);
 		}
 
-		const vizType = info.slice(VIZ_PREFIX.length) as VizType;
-		if (!SUPPORTED_TYPES.has(vizType)) {
-			return `<div class="viz-error">Unknown visualization type: <code>${escapeHtml(vizType)}</code></div>\n`;
-		}
-
-		const raw = token.content.trimEnd();
-		const result = validate(vizType, raw);
-
-		if (!result.ok) {
-			return `<div class="viz-error">Invalid viz data: ${escapeHtml(result.error ?? "unknown error")}</div>\n`;
+		try {
+			return renderVizBlock(info, token.content, env);
+		} catch (err) {
+			// A throw here escapes renderMarkdown and 500s the note on every
+			// request, so one bad block must never take the page down.
+			return vizError(
+				`Viz render failed: ${err instanceof Error ? err.message : String(err)}`,
+			);
 		}
-
-		const encoded = btoa(raw);
-		(env as VizPluginEnv).hasViz = true;
-
-		return `<div class="viz-container" data-viz-type="${vizType}" data-viz="${encoded}"></div>\n`;
 	};
 }
diff --git a/tests/lib/viz/plugin.test.ts b/tests/lib/viz/plugin.test.ts
index 9a699c8..3ee17b9 100644
--- a/tests/lib/viz/plugin.test.ts
+++ b/tests/lib/viz/plugin.test.ts
@@ -1,6 +1,13 @@
 import { describe, expect, test } from "bun:test";
 import { renderMarkdown } from "../../../src/lib/markdown.ts";
 
+// The server emits base64 of UTF-8 bytes; mirror the hydrator's decode.
+function decodeVizData(html: string): string {
+	const match = html.match(/data-viz="([^"]+)"/);
+	expect(match).not.toBeNull();
+	return Buffer.from(match?.[1] ?? "", "base64").toString("utf8");
+}
+
 describe("viz plugin", () => {
 	test("renders viz-graph block as container div", () => {
 		const { html } = renderMarkdown(
@@ -61,10 +68,32 @@ describe("viz plugin", () => {
 	test("base64 data decodes back to original JSON", () => {
 		const json = '{"nodes":[{"id":"a"}],"links":[]}';
 		const { html } = renderMarkdown(`\`\`\`viz-graph\n${json}\n\`\`\``);
-		const match = html.match(/data-viz="([^"]+)"/);
-		expect(match).not.toBeNull();
-		const decoded = atob(match?.[1] ?? "");
-		expect(decoded).toBe(json);
+		expect(decodeVizData(html)).toBe(json);
+	});
+
+	test("non-Latin-1 content round-trips instead of throwing", () => {
+		const json =
+			'{"nodes":[{"id":"a","label":"\u4e2d\u6587 \ud83c\udf3f \u201cquoted\u201d"}],"links":[]}';
+		const { html, hasViz } = renderMarkdown(`\`\`\`viz-graph\n${json}\n\`\`\``);
+		expect(hasViz).toBe(true);
+		expect(html).not.toContain("viz-error");
+		expect(decodeVizData(html)).toBe(json);
+	});
+
+	test("a throwing renderer degrades to viz-error, not a thrown render", () => {
+		const originalFrom = Buffer.from;
+		(Buffer as { from: unknown }).from = () => {
+			throw new Error("boom");
+		};
+		try {
+			const { html } = renderMarkdown(
+				'```viz-graph\n{"nodes":[{"id":"a"}],"links":[]}\n```',
+			);
+			expect(html).toContain("viz-error");
+			expect(html).toContain("boom");
+		} finally {
+			(Buffer as { from: unknown }).from = originalFrom;
+		}
 	});
 
 	test("multiple viz blocks all render and set hasViz", () => {