diff --git a/apps/web/components/shared/mermaid-utils.test.ts b/apps/web/components/shared/mermaid-utils.test.ts
index ae5c1008e4..1bbda02a49 100644
--- a/apps/web/components/shared/mermaid-utils.test.ts
+++ b/apps/web/components/shared/mermaid-utils.test.ts
@@ -80,4 +80,29 @@ describe("sanitizeMermaidCode", () => {
     const out = sanitizeMermaidCode(input);
     expect(out).toContain(`B["/api/x"]`);
   });
+
+  it("does not match an edge label that spans a newline", () => {
+    const input = `A -->|open\nB | C`;
+    expect(sanitizeMermaidCode(input)).toBe(input);
+  });
+
+  it("does not match a bracket label that spans a newline", () => {
+    const input = `A[open\nB]`;
+    expect(sanitizeMermaidCode(input)).toBe(input);
+  });
+
+  it("does not match a parenthesis label that spans a newline", () => {
+    const input = `A(open\n/path)`;
+    expect(sanitizeMermaidCode(input)).toBe(input);
+  });
+
+  it("does not quote ER diagram cardinality bars as flowchart edge labels", () => {
+    const input = [
+      "erDiagram",
+      "workspaces ||--o{ workflows : owns",
+      "workflows ||--o{ workflow_steps : contains",
+    ].join("\n");
+
+    expect(sanitizeMermaidCode(input)).toBe(input);
+  });
 });
diff --git a/apps/web/components/shared/mermaid-utils.ts b/apps/web/components/shared/mermaid-utils.ts
index b74113fe2c..940d47a45a 100644
--- a/apps/web/components/shared/mermaid-utils.ts
+++ b/apps/web/components/shared/mermaid-utils.ts
@@ -85,7 +85,7 @@ const inAnyRange = (offset: number, ranges: Array<[number, number]>): boolean =>
 export function sanitizeMermaidCode(code: string): string {
   // Quote node labels: [text] or [[text]] etc. Match brackets that aren't already quoted.
   let quoted = findQuotedRanges(code);
-  let result = code.replace(/(\[+)([^\]"]+?)(\]+)/g, (match, open, text, close, offset) => {
+  let result = code.replace(/(\[+)([^\]\n"]+?)(\]+)/g, (match, open, text, close, offset) => {
     if (inAnyRange(offset, quoted)) return match;
     if (BRACKET_TRIGGER_RE.test(text)) {
       return `${open}"${text}"${close}`;
@@ -95,7 +95,7 @@ export function sanitizeMermaidCode(code: string): string {
 
   // Quote edge labels: |text|
   quoted = findQuotedRanges(result);
-  result = result.replace(/\|([^|"]+?)\|/g, (match, text, offset) => {
+  result = result.replace(/\|([^|\n"]+?)\|/g, (match, text, offset) => {
     if (inAnyRange(offset, quoted)) return match;
     if (SPECIAL_CHARS_RE.test(text)) {
       return `|"${text}"|`;
@@ -105,7 +105,7 @@ export function sanitizeMermaidCode(code: string): string {
 
   // Quote parentheses labels: (text) for stadium/circle nodes
   quoted = findQuotedRanges(result);
-  result = result.replace(/(\(+)([^)"]+?)(\)+)/g, (match, open, text, close, offset) => {
+  result = result.replace(/(\(+)([^)\n"]+?)(\)+)/g, (match, open, text, close, offset) => {
     if (inAnyRange(offset, quoted)) return match;
     // Skip if it looks like a subgraph or keyword
     if (/^(subgraph|end|graph|flowchart|sequenceDiagram)\b/.test(text.trim())) {