diff --git a/apps/collab/src/auth.test.ts b/apps/collab/src/auth.test.ts
index 457e5fc..8d9de9e 100644
--- a/apps/collab/src/auth.test.ts
+++ b/apps/collab/src/auth.test.ts
@@ -108,8 +108,12 @@ describe('collab authentication', () => {
       name: 'page-x',
       token: async () => {
         const [h, p, s = ''] = (await signCollabToken(claims, secret, 60)).split('.');
-        // Flip the last character of the signature.
-        const flipped = s.slice(0, -1) + (s.endsWith('A') ? 'B' : 'A');
+        // Flip the FIRST signature character — its bits are all significant.
+        // The last character is not: its low bits are base64url padding that
+        // decoders ignore, so a last-char flip of a signature ending in 'A'
+        // decodes to the same bytes and verifies (jose compares decoded
+        // bytes, unlike the pre-#188 code that compared encoded strings).
+        const flipped = (s.startsWith('A') ? 'B' : 'A') + s.slice(1);
         return `${h}.${p}.${flipped}`;
       },
     },