Linked environments: a pairing code created on this machine is sent to the other machine and fails as "unknown, expired, or already used" #30181

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

Linked environments (link request card, peerLinks.link, Settings → Sharing & access)

Steps to reproduce

1. T3 Code Desktop 0.1.5 (stable) on a Mac (environment bcd6…). A headless server 0.1.5 on a Linux host on the tailnet (a3b7…, https://<host>.ts.net:3773).
2. Turn on Settings → Labs → Linked environments on the Mac.
3. A thread on the Mac posts a link request card for the Linux host (environment id given).
4. To get a code, open Settings → Sharing & access → Create link in the desktop app (the Mac's own settings, which is what the app shows) and paste it into the card right away.
5. Approve. Repeat with a second fresh code and a second card.

Expected behavior

Either the card tells me where to create the code (on the machine being linked, with a way to open that machine's Sharing & access), or, when I paste a code this machine issued itself, it says so: "This code was created on MacBook, not on frontlink-dev. Create the link on frontlink-dev."

Actual behavior

Every attempt fails with "That pairing code is unknown, expired, or already used." even though the code is seconds old and unused.

The logs show why. The Mac's auth_pairing_links has both codes (labels "Environments" at 13:22:01Z and "Linked Environments" at 13:24:49Z, neither consumed). The Linux server gets POST /oauth/mcp/register and POST /oauth/mcp/decision, and EnvironmentAuth.consumeMcpApprovalCode fails with ServerAuthMcpApprovalCodeError because the code is not in its own table (the UPDATE auth_pairing_links SET consumed_at matches no row). The Mac reports PeerLinks.signIn → PeerLinkError: That pairing code is unknown, expired, or already used. three times (13:22:34Z, 13:22:42Z, 13:25:18Z).

The error suggests a timing problem, so I retried quickly several times; the real problem was which machine made the code.

Impact

Linking another machine looks broken. With nothing pointing at the cause, it is easy to give up on the feature.

Version or commit

Desktop 0.1.5 (stable, "T3 Code (Beta).app"), server 0.1.5 on Linux (systemd service). The latest stable is 0.1.6; I found no change about this in its notes.

Environment

macOS (arm64) desktop, Linux x64 headless server, both on Tailscale, T3 Connect on.

Notes

Related, not the same: #30088 (the same error message for an MCP sign-in, closed as a code from another server; the same trap, which suggests the message itself should name the cause), #27688 (Linked environments points to the wrong settings page), #26952 (the link card can't be dismissed).