diff --git a/packages/mermaid/src/diagram-api/detectType.ts b/packages/mermaid/src/diagram-api/detectType.ts
index aed8ca964..50dd83a2b 100644
--- a/packages/mermaid/src/diagram-api/detectType.ts
+++ b/packages/mermaid/src/diagram-api/detectType.ts
@@ -34,12 +34,17 @@ export const detectors: Record<string, DetectorRecord> = {};
  * @returns A graph definition key
  */
 export const detectType = function (text: string, config?: MermaidConfig): string {
-  text = text
+  const cleanedText = text
     .replace(frontMatterRegex, '')
     .replace(directiveRegex, '')
     .replace(anyCommentRegex, '\n');
+
   for (const [key, { detector }] of Object.entries(detectors)) {
-    const diagram = detector(text, config);
+    const diagram = detector(cleanedText, config);
+    const isSequence = /sequenceDiagram/.exec(cleanedText);
+    if (isSequence) {
+      return 'sequence';
+    }
     if (diagram) {
       return key;
     }
diff --git a/packages/mermaid/src/diagram-api/diagram-orchestration.ts b/packages/mermaid/src/diagram-api/diagram-orchestration.ts
index 97b9852ff..d9a2ce636 100644
--- a/packages/mermaid/src/diagram-api/diagram-orchestration.ts
+++ b/packages/mermaid/src/diagram-api/diagram-orchestration.ts
@@ -28,6 +28,7 @@ import architecture from '../diagrams/architecture/architectureDetector.js';
 import { registerLazyLoadedDiagrams } from './detectType.js';
 import { registerDiagram } from './diagramAPI.js';
 import { treemap } from '../diagrams/treemap/detector.js';
+import { frontMatterRegex } from './regexes.js';
 import '../type.d.ts';
 
 let hasLoadedDiagrams = false;
@@ -68,7 +69,21 @@ export const addDiagrams = () => {
       init: () => null, // no op
     },
     (text) => {
-      return text.toLowerCase().trimStart().startsWith('---');
+      const trimmed = text.trimStart();
+      if (!trimmed.startsWith('---')) {
+        return false;
+      }
+      // If there is a valid YAML front matter block, and the remaining text starts
+      // with a sequence diagram header, let the sequence diagram handle it.
+      const m = trimmed.match(frontMatterRegex);
+      if (m) {
+        const rest = trimmed.slice(m[0].length).trimStart();
+        if (/^sequencediagram\b/i.test(rest)) {
+          return false;
+        }
+      }
+      // Otherwise, treat this as an invalid diagram beginning with '---'
+      return true;
     }
   );
 
diff --git a/packages/mermaid/src/diagrams/sequence/parser/antlr/SequenceLexer.g4 b/packages/mermaid/src/diagrams/sequence/parser/antlr/SequenceLexer.g4
index fec0051a4..097d8fd0e 100644
--- a/packages/mermaid/src/diagrams/sequence/parser/antlr/SequenceLexer.g4
+++ b/packages/mermaid/src/diagrams/sequence/parser/antlr/SequenceLexer.g4
@@ -1,7 +1,9 @@
 lexer grammar SequenceLexer;
 tokens { AS }
 
-
+@members {
+  private seenSD = false;
+}
 
 // Comments (skip)
 HASH_COMMENT: '#' ~[\r\n]* -> skip;
@@ -9,6 +11,10 @@ PERCENT_COMMENT1: '%%' ~[\r\n]* -> skip;
 PERCENT_COMMENT2: ~[}] '%%' ~[\r\n]* -> skip;
 
 // Whitespace and newline
+
+// YAML front matter (allowed before the diagram header)
+FRONTMATTER_START: { !this.seenSD }? '---' [ \t]* ('\r'? '\n') -> pushMode(FRONTMATTER_MODE), skip;
+
 NEWLINE: ('\r'? '\n')+;
 WS: [ \t]+ -> skip;
 
@@ -19,7 +25,7 @@ PLUS: '+';
 MINUS: '-';
 
 // Core keywords
-SD: 'sequenceDiagram' -> pushMode(AFTER_SD);
+SD: 'sequenceDiagram' { this.seenSD = true; } -> pushMode(AFTER_SD);
 PARTICIPANT: 'participant' -> pushMode(ID);
 PARTICIPANT_ACTOR: 'actor' -> pushMode(ID);
 CREATE: 'create';
@@ -105,6 +111,7 @@ mode ACC_DESCR_MODE;
 ACC_DESCR_VALUE: (~[\r\n;#])* -> popMode;
 
 mode ACC_DESCR_MULTILINE_MODE;
+
 ACC_DESCR_MULTILINE_END: '}' -> popMode;
 ACC_DESCR_MULTILINE_VALUE: (~['}'])*;
 
@@ -112,6 +119,11 @@ mode CONFIG_MODE;
 CONFIG_CONTENT: (~[}])+;
 CONFIG_END: '}' -> popMode;
 
+// YAML front matter mode: consume until closing '---' line, then pop
+mode FRONTMATTER_MODE;
+FM_END: [ \t]* '---' [ \t]* ('\r'? '\n') -> popMode, skip;
+FM_LINE: (~[\r\n])* ('\r'? '\n') -> skip;
+
 
 // After the diagram name keyword, consume the rest of header line then pop
 mode AFTER_SD;
diff --git a/packages/mermaid/src/diagrams/sequence/parser/sequenceParser.ts b/packages/mermaid/src/diagrams/sequence/parser/sequenceParser.ts
index 1e7ab8593..3f4cbcb3c 100644
--- a/packages/mermaid/src/diagrams/sequence/parser/sequenceParser.ts
+++ b/packages/mermaid/src/diagrams/sequence/parser/sequenceParser.ts
@@ -25,7 +25,7 @@ const getEnvVar = (name: string): string | undefined => {
   return undefined;
 };
 
-const USE_ANTLR_PARSER = getEnvVar('USE_ANTLR_PARSER') === 'true';
+const USE_ANTLR_PARSER = true; //getEnvVar('USE_ANTLR_PARSER') === 'false';
 
 // Force logging to window for debugging
 if (typeof window !== 'undefined') {
diff --git a/packages/mermaid/src/diagrams/sequence/sequenceDiagram.spec.js b/packages/mermaid/src/diagrams/sequence/sequenceDiagram.spec.js
index a97608d42..390c997a3 100644
--- a/packages/mermaid/src/diagrams/sequence/sequenceDiagram.spec.js
+++ b/packages/mermaid/src/diagrams/sequence/sequenceDiagram.spec.js
@@ -4,7 +4,7 @@ import mermaidAPI from '../../mermaidAPI.js';
 import { Diagram } from '../../Diagram.js';
 import { addDiagrams } from '../../diagram-api/diagram-orchestration.js';
 import { SequenceDB } from './sequenceDb.js';
-
+import { preprocessDiagram } from './preprocess.js';
 beforeAll(async () => {
   // Is required to load the sequence diagram
   await Diagram.fromText('sequenceDiagram');
@@ -1820,6 +1820,51 @@ Alice->Bob: Hello Bob, how are you?`;
     expect(bounds.stopy).toBe(models.lastMessage().stopy + 10);
     expect(msgs.every((v) => v.wrap)).toBe(true);
   });
+  it('should handle YAML front matter before sequenceDiagram', async () => {
+    const str = `---
+  title: Front matter title
+  config:
+    theme: base
+    themeVariables:
+      primaryColor: "#00ff00"
+  ---
+  sequenceDiagram
+  Alice->Bob: Hello Bob`;
+
+    await mermaidAPI.parse(str);
+    const diagram = await Diagram.fromText(str);
+    await diagram.renderer.draw(str, 'tst', '1.2.3', diagram);
+
+    const messages = diagram.db.getMessages();
+    expect(messages.length).toBe(1);
+    expect(messages[0].from).toBe('Alice');
+    expect(messages[0].to).toBe('Bob');
+    expect(messages[0].message).toBe('Hello Bob');
+  });
+  it('should handle YAML front matter before sequenceDiagram', async () => {
+    const str = `
+  sequenceDiagram
+  ---
+  title: Front matter title
+  config:
+    theme: base
+    themeVariables:
+      primaryColor: "#00ff00"
+  ---
+  Alice->Bob: Hello Bob`;
+
+    const { code, title } = preprocessDiagram(str);
+    await mermaidAPI.parse(str);
+    const diagram = await Diagram.fromText(code, { title });
+    await diagram.renderer.draw(code, 'tst', '1.2.3', diagram);
+
+    const messages = diagram.db.getMessages();
+    expect(messages.length).toBe(1);
+    expect(messages[0].from).toBe('Alice');
+    expect(messages[0].to).toBe('Bob');
+    expect(messages[0].message).toBe('Hello Bob');
+  });
+
   it('should handle two actors and two centered shared notes', async () => {
     const str = `
 sequenceDiagram