diff --git a/packages/mermaid/src/diagrams/sequence/parser/sequenceParser.ts b/packages/mermaid/src/diagrams/sequence/parser/sequenceParser.ts
index fa08a8c60..1e7ab8593 100644
--- a/packages/mermaid/src/diagrams/sequence/parser/sequenceParser.ts
+++ b/packages/mermaid/src/diagrams/sequence/parser/sequenceParser.ts
@@ -1,23 +1,69 @@
 // @ts-ignore: JISON doesn't support types
 import jisonParser from './sequenceDiagram.jison';
 
-// Import the ANTLR parser wrapper (safe stub for now)
+// Import the ANTLR parser wrapper
 import antlrParser from './antlr/antlr-parser.js';
 
-// Configuration flag to switch between parsers (same convention as flowcharts)
-const USE_ANTLR_PARSER = process.env.USE_ANTLR_PARSER === 'true';
-
-const newParser: any = Object.assign({}, USE_ANTLR_PARSER ? antlrParser : jisonParser);
-
-newParser.parse = (src: string): unknown => {
-  // Normalize whitespace like flow does to keep parity with Jison behavior
-  const newSrc = src.replace(/}\s*\n/g, '}\n');
-
-  if (USE_ANTLR_PARSER) {
-    return antlrParser.parse(newSrc);
-  } else {
-    return jisonParser.parse(newSrc);
+// Browser-safe environment variable access (same as flowchart parser)
+const getEnvVar = (name: string): string | undefined => {
+  try {
+    if (typeof process !== 'undefined' && process.env) {
+      return process.env[name];
+    }
+  } catch (_e) {
+    // process is not defined in browser, continue to browser checks
   }
+
+  // In browser, check for global variables or default values
+  if (typeof window !== 'undefined' && (window as any).MERMAID_CONFIG) {
+    return (window as any).MERMAID_CONFIG[name];
+  }
+  // Default to ANTLR parser in browser if no config is found
+  if (typeof window !== 'undefined' && name === 'USE_ANTLR_PARSER') {
+    return 'true';
+  }
+  return undefined;
+};
+
+const USE_ANTLR_PARSER = getEnvVar('USE_ANTLR_PARSER') === 'true';
+
+// Force logging to window for debugging
+if (typeof window !== 'undefined') {
+  (window as any).MERMAID_PARSER_DEBUG = {
+    USE_ANTLR_PARSER,
+    env_value: getEnvVar('USE_ANTLR_PARSER'),
+    selected_parser: USE_ANTLR_PARSER ? 'ANTLR' : 'Jison',
+  };
+}
+
+// eslint-disable-next-line no-console
+console.log('🔧 SequenceParser: USE_ANTLR_PARSER =', USE_ANTLR_PARSER);
+// eslint-disable-next-line no-console
+console.log('🔧 SequenceParser: env USE_ANTLR_PARSER =', getEnvVar('USE_ANTLR_PARSER'));
+// eslint-disable-next-line no-console
+console.log('🔧 SequenceParser: Selected parser:', USE_ANTLR_PARSER ? 'ANTLR' : 'Jison');
+
+// Create the appropriate parser instance (same pattern as flowchart)
+let parserInstance;
+if (USE_ANTLR_PARSER) {
+  parserInstance = antlrParser;
+} else {
+  parserInstance = jisonParser;
+}
+
+// Create a wrapper that provides the expected interface (same pattern as flowchart)
+const newParser = {
+  parser: parserInstance,
+  parse: (src: string): unknown => {
+    // Normalize whitespace like flow does to keep parity with Jison behavior
+    const newSrc = src.replace(/}\s*\n/g, '}\n');
+
+    if (USE_ANTLR_PARSER) {
+      return antlrParser.parse(newSrc);
+    } else {
+      return jisonParser.parse(newSrc);
+    }
+  },
 };
 
 export default newParser;