mcp-mermaid-validator

Instructions

Implementation Reference

• src/main.ts:22-163 (handler)

  The handler function executes the tool logic by spawning an npx process with @mermaid-js/mermaid-cli to render the Mermaid diagram. It captures output or errors, returns a success message with base64-encoded image (SVG/PNG) if valid, or error details if invalid.
  async ({ diagram, format = "png" }) => { try { try { // Use child_process.spawn to create a process and pipe the diagram to stdin const args = [ "@mermaid-js/mermaid-cli", "-i", "/dev/stdin", "-o", "-", "-e", format, ]; // Add transparent background for PNG format if (format === "png") { args.push("-b", "transparent"); } const mmdc = spawn("npx", args); // Write the diagram to stdin and close it mmdc.stdin.write(diagram); mmdc.stdin.end(); // Capture stdout (image content) and stderr (error messages) const outputChunks: Buffer[] = []; let svgContent = ""; let errorOutput = ""; mmdc.stdout.on("data", (data: Buffer) => { if (format === "svg") { svgContent += data.toString(); } else { outputChunks.push(data); } }); mmdc.stderr.on("data", (data: Buffer) => { errorOutput += data.toString(); }); // Wait for the process to complete await new Promise<void>((resolve, reject) => { mmdc.on("close", (code: number) => { if (code === 0) { resolve(); } else { reject( new Error( `mermaid-cli process exited with code ${code}${errorOutput ? "\n\nError details:\n" + errorOutput : ""}`, ), ); } }); mmdc.on("error", (err: Error) => { reject( new Error( `${err.message}${errorOutput ? "\n\nError details:\n" + errorOutput : ""}`, ), ); }); }); // If we get here, the diagram is valid if (format === "svg") { return { content: [ { type: "text", text: "Mermaid diagram is valid", }, { type: "image", data: Buffer.from(svgContent).toString("base64"), mimeType: "image/svg+xml", }, ], }; } else { const pngBuffer = Buffer.concat(outputChunks); return { content: [ { type: "text", text: "Mermaid diagram is valid", }, { type: "image", data: pngBuffer.toString("base64"), mimeType: "image/png", }, ], }; } } catch (validationError: unknown) { // The diagram is invalid const errorMessage = validationError instanceof Error ? validationError.message : String(validationError); // Split the error message to separate the main error from stderr details const [mainError, ...errorDetails] = errorMessage.split( "\n\nError details:\n", ); const errorContent: Array<{ type: "text"; text: string }> = [ { type: "text", text: "Mermaid diagram is invalid", }, { type: "text", text: mainError, }, ]; // Add stderr output if available if (errorDetails.length > 0) { errorContent.push({ type: "text", text: "Detailed error output:\n" + errorDetails.join("\n"), }); } return { content: errorContent, }; } } catch (error: unknown) { return { content: [ { type: "text", text: `Error processing Mermaid diagram: ${error instanceof Error ? error.message : String(error)}`, }, ], }; } },
• src/main.ts:18-21 (schema)

  Zod schema defining the input parameters for the tool: 'diagram' as a required string, and 'format' as an optional enum (svg or png) defaulting to png.
  { diagram: z.string(), format: z.enum(["svg", "png"]).optional().default("png") },
• src/main.ts:15-17 (registration)

  Registration of the 'validateMermaid' tool with the MCP server, including name, description, schema, and handler.
  server.tool( "validateMermaid", "Validates a Mermaid diagram and returns the rendered image (PNG or SVG) if valid",