feat: add handDrawn look option for sketch-style rendering

Mermaid v11 supports a 'look' setting that switches diagrams to a sketchy,
Excalidraw-style rendering via rough.js. This exposes it as an optional
parameter on the MCP tool so users can request that style from their AI
assistant without having to embed a frontmatter config block.

- Add 'look' field to the Zod schema (enum: classic, handDrawn; default classic)
- Pass it through renderMermaid into mermaidConfig
- Propagate it to the mermaid.ink payload for svg_url / png_url outputs
- Update tool snapshot fixture and add unit tests for createMermaidInkUrl

Backwards compatible: default 'classic' preserves current behavior.

Author: Sébastien MORAND

__tests__/tools/mermaid.json

@@ -15,6 +15,12 @@
         "description": "Theme for the diagram (optional). Default is 'default'.",
         "default": "default"
       },
+      "look": {
+        "type": "string",
+        "enum": ["classic", "handDrawn"],
+        "description": "Visual style for the diagram (optional). 'classic' is the default Mermaid look; 'handDrawn' produces a sketchy, Excalidraw-like rendering (Mermaid v11+).",
+        "default": "classic"
+      },
       "backgroundColor": {
         "type": "string",
         "description": "Background color for the diagram (optional). Default is 'white'.",

__tests__/utils/mermaidUrl.spec.ts

@@ -0,0 +1,42 @@
+import { inflateSync } from "node:zlib";
+import { describe, expect, it } from "vitest";
+import { createMermaidInkUrl } from "../../src/utils/mermaidUrl";
+
+function decodePayload(url: string): {
+  code: string;
+  mermaid: { theme: string; look: string };
+} {
+  const encoded = url.split("pako:")[1];
+  const base64 = encoded.replace(/-/g, "+").replace(/_/g, "/");
+  const padded = base64 + "=".repeat((4 - (base64.length % 4)) % 4);
+  const buffer = Buffer.from(padded, "base64");
+  return JSON.parse(inflateSync(buffer).toString("utf-8"));
+}
+
+describe("createMermaidInkUrl", () => {
+  const sample = "graph TD;A-->B;";
+
+  it("defaults to classic look when not specified", () => {
+    const url = createMermaidInkUrl(sample, "img");
+    expect(url.startsWith("https://mermaid.ink/img/pako:")).toBe(true);
+    const payload = decodePayload(url);
+    expect(payload.mermaid.theme).toBe("default");
+    expect(payload.mermaid.look).toBe("classic");
+    expect(payload.code).toBe(sample);
+  });
+
+  it("propagates handDrawn look into the mermaid.ink payload", () => {
+    const url = createMermaidInkUrl(sample, "svg", "forest", "handDrawn");
+    expect(url.startsWith("https://mermaid.ink/svg/pako:")).toBe(true);
+    const payload = decodePayload(url);
+    expect(payload.mermaid.theme).toBe("forest");
+    expect(payload.mermaid.look).toBe("handDrawn");
+  });
+
+  it("uses the requested variant in the URL path", () => {
+    const svgUrl = createMermaidInkUrl(sample, "svg");
+    const imgUrl = createMermaidInkUrl(sample, "img");
+    expect(svgUrl).toMatch(/^https:\/\/mermaid\.ink\/svg\//);
+    expect(imgUrl).toMatch(/^https:\/\/mermaid\.ink\/img\//);
+  });
+});

src/server.ts

@@ -59,16 +59,23 @@ function setupToolHandlers(server: McpServer): void {
           );
         }
 
-        const { mermaid, theme, backgroundColor, outputType = "base64" } = args;
+        const {
+          mermaid,
+          theme,
+          look,
+          backgroundColor,
+          outputType = "base64",
+        } = args;
         Logger.info(
-          `Rendering diagram (outputType=${outputType}, theme=${theme ?? "default"})`,
+          `Rendering diagram (outputType=${outputType}, theme=${theme ?? "default"}, look=${look ?? "classic"})`,
         );
         const { id, svg, screenshot } = await withRetry(
           () =>
             renderMermaid(
               mermaid as string,
               theme as string,
               backgroundColor as string,
+              look as "classic" | "handDrawn" | undefined,
             ),
           { maxAttempts: 3, delayMs: 500 },
         );
@@ -99,6 +106,7 @@ function setupToolHandlers(server: McpServer): void {
             mermaid as string,
             variant,
             (theme as string) || "default",
+            (look as "classic" | "handDrawn" | undefined) || "classic",
           );
           return {
             content: [

src/tools/index.ts

@@ -15,6 +15,13 @@ C-->D;.`)
     .describe("Theme for the diagram (optional). Default is 'default'.")
     .optional()
     .default("default"),
+  look: z
+    .enum(["classic", "handDrawn"])
+    .describe(
+      "Visual style for the diagram (optional). 'classic' is the default Mermaid look; 'handDrawn' produces a sketchy, Excalidraw-like rendering (Mermaid v11+).",
+    )
+    .optional()
+    .default("classic"),
   backgroundColor: z
     .string()
     .describe(

src/utils/mermaidUrl.ts

@@ -21,8 +21,12 @@ export function createMermaidInkUrl(
   mermaid: string,
   variant: "svg" | "img",
   theme = "default",
+  look: "classic" | "handDrawn" = "classic",
 ): string {
-  const payload = JSON.stringify({ code: mermaid, mermaid: { theme } });
+  const payload = JSON.stringify({
+    code: mermaid,
+    mermaid: { theme, look },
+  });
   const encoded = encodeMermaidToBase64Url(payload);
   return `https://mermaid.ink/${variant}/pako:${encoded}`;
 }

src/utils/render.ts

@@ -20,6 +20,7 @@ export async function renderMermaid(
   mermaid: string,
   theme = "default",
   backgroundColor = "white",
+  look: "classic" | "handDrawn" = "classic",
 ): Promise<RenderResult> {
   if (!renderer) renderer = createMermaidRenderer();
   const cssContent = `svg { background: ${backgroundColor}; }`;
@@ -33,6 +34,8 @@ export async function renderMermaid(
     mermaidConfig: {
       // biome-ignore lint/suspicious/noExplicitAny: <explanation>
       theme: theme as any,
+      // biome-ignore lint/suspicious/noExplicitAny: <explanation>
+      look: look as any,
     },
   });
   const r0 = r[0] as PromiseSettledResult<RenderResult>;