We tested 25 "official" MCP servers for one-click OAuth. Ten failed.

Our connector marketplace has 25 official third-party MCP servers behind it. When we built the one-click "Connect" button, the plan was simple: read the provider's OAuth metadata, and if it advertises dynamic client registration, register ourselves on the spot and skip asking the user for anything. We tested that assumption against all 25 in production. Ten of them failed it.

Bug 1 — a middle dot broke Calendly's registration

Symptom: the Calendly tile's "Connect" button opened a popup, then closed immediately with a generic error. No OAuth screen, no token, nothing in our logs but a failed registration call.

Cause: our client sends a client_name of Command+K · {server name} on every dynamic registration request, so a user connecting Calendly would see "Command+K · Calendly" listed in their Calendly account. Calendly's authorization server enforces a strict character allowlist on that field. The middle dot isn't on it. The registration came back rejected, and our wizard had no idea why — it just surfaced "couldn't connect."

Fix: sanitize client_name to [A-Za-z0-9 -] before it leaves our server, in one place:

• opts.clientName.normalize("NFKD").replace(/[^A-Za-z0-9 -]+/g, " "), then trimmed and capped at 60 characters.
• Any authorization server with a strict charset now sees a name it accepts. Calendly registers with a 201 and connects in one click.

Bug 2 — advertising registration isn't the same as accepting it

Symptom: Intercom, Asana, ClickUp, Vercel, Square, Slack, HubSpot, Salesforce, Figma and Box all publish a registration_endpoint in their OAuth discovery metadata — the field our wizard checked to decide whether a provider was "one-click." All ten failed when we actually tried it: some returned a 403, some redirected to a login page instead of JSON, one enforced a redirect-URI allowlist that a first-time client can't be on. From the outside, the failure looked identical to Bug 1 — a popup that closes with nothing to show for it.

Cause: a registration_endpoint existing in metadata is a claim, not a guarantee. Our discovery probe treated the field's presence as proof that RFC 7591 self-registration would work. It doesn't test that the provider actually accepts a registration from a client it's never seen before — several of these ten gate the endpoint to apps the provider's own team has pre-approved.

Fix: two changes. First, discovery now confirms usability with a real registration POST instead of trusting the advertised field. Second, for the ten providers where that POST fails, we reclassified the connector to a bring-your-own-OAuth-app flow: the wizard shows the callback URL to allowlist, the customer creates their own app on the provider's side, and pastes back a Client ID and Secret. It's an extra step, but it's an honest one — instead of a mysterious failure, the user sees exactly what their provider requires.

A third failure mode: the gate isn't OAuth at all

Freshdesk's MCP server isn't OAuth-based yet — it's an Early Access Program feature, Enterprise plans only, authenticated with a Freshdesk API key. A customer on a lower plan pasting a perfectly valid key still gets rejected, because the gate is on Freshdesk's side, not the credential. We surface that plainly in the connector's setup note rather than let someone debug a "correct" key that will never work.

What we'd tell anyone building a connector marketplace

A provider's OAuth discovery document tells you what it wants you to believe about itself. None of the ten false positives were malicious or even unusual — most enterprise identity teams gate client registration behind manual review, and RFC 7591 doesn't require them not to advertise the endpoint anyway. The only way to know which category a provider is in is to actually register a client and see what comes back, the same way we found the three protocol bugs on the client side of our gateway. Every connector in the Command+K marketplace carries the result of that test today, so the button you click either works in one click or tells you exactly what it needs from you instead.