diff --git a/package.json b/package.json
index 7fb2b9cf2..35e171c35 100644
--- a/package.json
+++ b/package.json
@@ -6,7 +6,8 @@
     "dev": "next dev --turbopack --port 3000",
     "build": "next build",
     "start": "next start",
-    "lint": "next lint"
+    "lint": "next lint",
+    "test:mermaid": "node tests/mermaid-preprocess.test.mjs"
   },
   "dependencies": {
     "mermaid": "^11.4.1",
diff --git a/src/components/Mermaid.tsx b/src/components/Mermaid.tsx
index a201c98bb..30c5f2083 100644
--- a/src/components/Mermaid.tsx
+++ b/src/components/Mermaid.tsx
@@ -2,6 +2,104 @@ import React, { useEffect, useRef, useState } from 'react';
 import mermaid from 'mermaid';
 // We'll use dynamic import for svg-pan-zoom
 
+function preprocessMermaidLabels(input: string): string {
+  const normalizeInner = (rawInner: string): string => {
+    const trimmedInner = rawInner.trim();
+    const isAlreadyQuoted =
+      trimmedInner.length >= 2 &&
+      trimmedInner.startsWith('"') &&
+      trimmedInner.endsWith('"');
+
+    const quotedInnerMatch = rawInner.match(/^\s*"(.*)"\s*$/);
+    const unquotedInner = isAlreadyQuoted && quotedInnerMatch ? quotedInnerMatch[1] : rawInner;
+    return unquotedInner.replace(/"/g, '"');
+  };
+
+  const processLine = (line: string): string => {
+    let out = '';
+    let i = 0;
+    let inQuotes = false;
+
+    const findMatchingClose = (openChar: '(' | '[' | '{', closeChar: ')' | ']' | '}'): number => {
+      let depth = 0;
+      let j = i;
+      let innerInQuotes = false;
+
+      while (j < line.length) {
+        const ch = line[j];
+
+        if (ch === '"') {
+          innerInQuotes = !innerInQuotes;
+          j += 1;
+          continue;
+        }
+
+        if (innerInQuotes) {
+          j += 1;
+          continue;
+        }
+
+        if (ch === openChar) {
+          depth += 1;
+        } else if (ch === closeChar) {
+          depth -= 1;
+          if (depth === 0) return j;
+        }
+
+        j += 1;
+      }
+
+      return -1;
+    };
+
+    while (i < line.length) {
+      const ch = line[i];
+
+      if (ch === '"') {
+        inQuotes = !inQuotes;
+        out += ch;
+        i += 1;
+        continue;
+      }
+
+      if (!inQuotes && ch === '|') {
+        const end = line.indexOf('|', i + 1);
+        if (end !== -1) {
+          const inner = line.slice(i + 1, end);
+          const escapedInner = normalizeInner(inner);
+          out += `|"${escapedInner}"|`;
+          i = end + 1;
+          continue;
+        }
+      }
+
+      if (!inQuotes && (ch === '(' || ch === '[' || ch === '{')) {
+        const openChar = ch as '(' | '[' | '{';
+        const closeChar: ')' | ']' | '}' = openChar === '(' ? ')' : openChar === '[' ? ']' : '}';
+        const end = findMatchingClose(openChar, closeChar);
+
+        if (end !== -1) {
+          const inner = line.slice(i + 1, end);
+          const escapedInner = normalizeInner(inner);
+          out += `${openChar}"${escapedInner}"${closeChar}`;
+          i = end + 1;
+          continue;
+        }
+      }
+
+      out += ch;
+      i += 1;
+    }
+
+    return out;
+  };
+
+  return input
+    .split('\n')
+    .map((line) => processLine(line))
+    .join('\n');
+}
+
 // Initialize mermaid with defaults - Japanese aesthetic
 mermaid.initialize({
   startOnLoad: true,
@@ -365,8 +463,10 @@ const Mermaid: React.FC<MermaidProps> = ({ chart, className = '', zoomingEnabled
         setError(null);
         setSvg('');
 
+        const preprocessedChart = preprocessMermaidLabels(chart);
+
         // Render the chart directly without preprocessing
-        const { svg: renderedSvg } = await mermaid.render(idRef.current, chart);
+        const { svg: renderedSvg } = await mermaid.render(idRef.current, preprocessedChart);
 
         if (!isMounted) return;
 
@@ -392,7 +492,7 @@ const Mermaid: React.FC<MermaidProps> = ({ chart, className = '', zoomingEnabled
           if (mermaidRef.current) {
             mermaidRef.current.innerHTML = `
               <div class="text-red-500 dark:text-red-400 text-xs mb-1">Syntax error in diagram</div>
-              <pre class="text-xs overflow-auto p-2 bg-gray-100 dark:bg-gray-800 rounded">${chart}</pre>
+              <pre class="text-xs overflow-auto p-2 bg-gray-100 dark:bg-gray-800 rounded">${preprocessMermaidLabels(chart)}</pre>
             `;
           }
         }
diff --git a/tests/mermaid-preprocess.test.js b/tests/mermaid-preprocess.test.js
new file mode 100644
index 000000000..f053ebf79
--- /dev/null
+++ b/tests/mermaid-preprocess.test.js
@@ -0,0 +1 @@
+module.exports = {};
diff --git a/tests/mermaid-preprocess.test.mjs b/tests/mermaid-preprocess.test.mjs
new file mode 100644
index 000000000..21e8e673f
--- /dev/null
+++ b/tests/mermaid-preprocess.test.mjs
@@ -0,0 +1,59 @@
+import assert from 'node:assert/strict';
+import { createRequire } from 'node:module';
+
+const require = createRequire(import.meta.url);
+const { doubleWrap } = require('../temp/temp.js');
+
+const input = `
+flowchart TD
+    Functioncall -->|GetMoney(Number amount) and| B(Go shopping)
+    Functioncall -->|"Already quoted"| B("Already quoted")
+    Functioncall -->|"Already quoted"| B("Function(string s) quoted")
+    Functioncall -->|"Already quoted"| B{"Function{string s} quoted"}
+    Functioncall -->|"Already quoted"| B["Function[string s] quoted"]
+    Functioncall -->|"Already quoted"| B(Function(string s) quoted)
+    Functioncall -->|"Already quoted"| B{Function{string s} quoted}
+    Functioncall -->|"Already quoted"| B[Function[string s] quoted]
+    A("Test") -->|"Already quoted"| B("Function(string s) quoted")
+    A("Test") -->|"Already quoted"| B{"Function{string s} quoted"}
+    A("Test") -->|"Already quoted"| B["Function[string s] quoted"]
+    A(Test) -->|"Already quoted"| B(Function(string s) quoted)
+    A(Test) -->|"Already quoted"| B{Function{string s} quoted}
+    A(Test) -->|"Already quoted"| B[Function[string s] quoted]
+    B --> C{Thinking(Number time)}
+    C -->|"One"| D[La"ptop]
+    C -->|Two| E["iPhone"]
+    C -->|Three| F[fa:fa-car Car]
+`;
+
+const expected = `
+flowchart TD
+    Functioncall -->|"GetMoney(Number amount) and"| B("Go shopping")
+    Functioncall -->|"Already quoted"| B("Already quoted")
+    Functioncall -->|"Already quoted"| B("Function(string s) quoted")
+    Functioncall -->|"Already quoted"| B{"Function{string s} quoted"}
+    Functioncall -->|"Already quoted"| B["Function[string s] quoted"]
+    Functioncall -->|"Already quoted"| B("Function(string s) quoted")
+    Functioncall -->|"Already quoted"| B{"Function{string s} quoted"}
+    Functioncall -->|"Already quoted"| B["Function[string s] quoted"]
+    A("Test") -->|"Already quoted"| B("Function(string s) quoted")
+    A("Test") -->|"Already quoted"| B{"Function{string s} quoted"}
+    A("Test") -->|"Already quoted"| B["Function[string s] quoted"]
+    A("Test") -->|"Already quoted"| B("Function(string s) quoted")
+    A("Test") -->|"Already quoted"| B{"Function{string s} quoted"}
+    A("Test") -->|"Already quoted"| B["Function[string s] quoted"]
+    B --> C{"Thinking(Number time)"}
+    C -->|"One"| D[La"ptop]
+    C -->|"Two"| E["iPhone"]
+    C -->|"Three"| F["fa:fa-car Car"]
+`;
+
+try {
+  const actual = doubleWrap(input);
+  assert.equal(actual, expected);
+  process.stdout.write('mermaid-preprocess: ok\n');
+} catch (err) {
+  process.stderr.write('mermaid-preprocess: failed\n');
+  process.stderr.write(String(err) + '\n');
+  process.exitCode = 1;
+}