diff --git a/typescript/fixtures/broken.cell b/typescript/fixtures/broken.cell
index b59f0fe..3cf51c8 100644
--- a/typescript/fixtures/broken.cell
+++ b/typescript/fixtures/broken.cell
@@ -1,4 +1,4 @@
 cell Broken {
   role: "broken";
-  on(Ping p) { emit Pong; }
+  membrane { accepts: Ping; emits: Pong; }
 }
diff --git a/typescript/transpiler.test.ts b/typescript/transpiler.test.ts
index 9f3ec76..0fa5f28 100644
--- a/typescript/transpiler.test.ts
+++ b/typescript/transpiler.test.ts
@@ -101,6 +101,7 @@ describe('transpiler (file-level)', () => {
     const result = transpileCellFile({ file: bad });
     assert.equal(result.ok, false);
     assert.ok(result.errors.length > 0);
+    assert.match(result.errors[0], /^line 1, column 1: Cell 'Broken' must declare/);
   });
 
   it('returns filesystem error for missing file', () => {
diff --git a/typescript/transpiler.ts b/typescript/transpiler.ts
index 256c45b..cb02548 100644
--- a/typescript/transpiler.ts
+++ b/typescript/transpiler.ts
@@ -33,6 +33,11 @@ export interface TranspileResult {
   signals: string[];
 }
 
+function formatDiagnostic(diagnostic: { message: string; pos?: AST.Position }): string {
+  if (!diagnostic.pos) return diagnostic.message;
+  return `line ${diagnostic.pos.line}, column ${diagnostic.pos.col}: ${diagnostic.message}`;
+}
+
 function tsType(type?: AST.TypeExpr): string {
   if (!type) return 'unknown';
   switch (type.kind) {
@@ -229,7 +234,7 @@ export function transpileCellFile(opts: TranspileFileOptions): TranspileResult {
     };
   }
 
-  const errors = diagnostics.filter(d => d.kind === 'error').map(d => d.message);
+  const errors = diagnostics.filter(d => d.kind === 'error').map(formatDiagnostic);
   if (errors.length > 0) {
     return {
       ok: false,