fix(desktop): connect to the WSL backend over localhost first, with the distro IP as fallback

[Ganboo]

Fixes #13738

Scoped to what was asked in #13738 (comment): keep the 0.0.0.0 bind, probe 127.0.0.1 first, fall back to the distro IP when loopback doesn't answer, have the renderer keep using the URL that succeeded, test each order, and leave #9056's bind-host change out.

Problem

In WSL2 NAT mode the desktop connects to the WSL backend only through the distro's hostname -I address. On hosts that filter Windows → WSL NAT-subnet HTTP (a corporate endpoint security client in my case), that address never answers, even though 127.0.0.1 through wslhost forwarding works. The WSL backend runs and is healthy, but the desktop polls a dead URL forever. "Open WSL folder" never appears and WSL projects can't be added.

From Windows	Result
http://127.0.0.1:<port> (wslhost forwarding)	200 in 81ms
http://<wsl-nat-ip>:<port> (current httpBaseUrl)	connection dropped after ~3.7s

VS Code and Cursor Remote-WSL connect through 127.0.0.1 and work on the same machine.

Fix

• resolveWslStartConfig now uses loopback as httpBaseUrl and passes the NAT distro IP as a new optional fallbackHttpBaseUrl. Mirrored mode keeps loopback with no fallback, same as before.
• The readiness probe in runBackendProcess probes both at the same time, and loopback wins whenever it answers. The distro IP is used only after it answers and a fresh 1s probe of loopback still fails. Probing them concurrently rather than one after the other avoids adding a full readiness budget of delay on hosts that need the fallback.
• When readiness settles on a URL, the instance writes it to currentConfig.httpBaseUrl and passes it to onReady, and logs a warning if it was the fallback. Renderer bootstraps, local auth, and the window all read that config, and the renderer already re-registers a secondary when its URL changes, so no web changes are needed.
• Comments that described the distro IP as the advertised URL are updated to match.

The server still binds 0.0.0.0 inside WSL, so both routes stay available. Hosts where wslhost forwarding is unreliable or slow to start (the reason the distro IP was adopted) still connect through the fallback, which is the same URL they use on main today.

This is the loopback-plus-distro-IP probing that #5998 proposed, cut down to just that change on current main. The bind address is intentionally left alone (#9056 / #9066).

Verification

• Two new tests in DesktopBackendManager.test.ts, one per order, both asserting the instance's currentConfig URL (what the renderer reads): only the distro IP answers → distro IP; both answer → loopback. The fallback-order test fails against main's manager. The NAT assertion in DesktopBackendConfiguration.test.ts is updated.
• vp test run src/backend src/wsl src/ipc in apps/desktop: all pass except DesktopWslEnvironment.test.ts ("WSL runtime install script (executed)"), which times out identically on unmodified main on this Windows machine.
• typecheck for @t3tools/desktop, vp lint, and vp fmt on the changed files are clean.
• End to end on the affected machine with vp run dev:desktop (dual mode, NAT): the readiness probe to 127.0.0.1:<port> succeeded and the distro-IP probe was interrupted. The WSL backend then served /.well-known/t3/environment, /oauth/token, /api/auth/websocket-ticket, and /api/orchestration/shell, all from 127.0.0.1.

Summary by CodeRabbit

• Bug Fixes
  • Improved desktop connectivity in WSL by trying localhost first and using the distro address as a fallback when needed.
  • The app now selects the reachable address during startup, helping it connect when localhost is unavailable.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR changes automatic WSL endpoint selection and feeds the selected readiness URL into renderer bootstrapping and local authentication. An unresolved security concern remains around accepting a generic readiness responder before exchanging the desktop bootstrap token, so human review is warranted.

Notes:

• Diff unchanged. Approvability was decided on eligibility alone.

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📝 Walkthrough

Walkthrough

The WSL backend now uses localhost as its primary HTTP URL and can use the distro IP as a readiness fallback. The readiness manager reports and stores the URL that responds.

Changes

WSL backend readiness

Layer / File(s)	Summary
Configure WSL primary and fallback URLs apps/desktop/src/backend/DesktopBackendConfiguration.ts, apps/desktop/src/backend/DesktopBackendConfiguration.test.ts, apps/desktop/src/wsl/DesktopWslEnvironment.ts	WSL configuration uses loopback as the primary HTTP URL. It adds a distro-IP fallback only when the IP is available and is not a Windows-local IPv4 address. Tests and comments reflect the primary and fallback roles.
Probe and record the responding URL apps/desktop/src/backend/DesktopBackendManager.ts, apps/desktop/src/backend/DesktopBackendManager.test.ts	The readiness manager accepts an optional fallback URL and probes it alongside the primary URL. It selects the fallback only if the primary probe still fails, then passes and stores the responding URL. Tests cover primary and fallback responses.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: juliusmarminge

Merge Risk: 🟡 Moderate · up to ef449

A local process that claims the loopback port during startup could receive the desktop bootstrap token. Verify backend ownership before exchanging that token; this risk should be resolved or explicitly accepted before merging.

Security Architecture Review

Security architecture risk: 🔵 Low · up to ef449

The change improves connectivity without adding a network listener, but a local process that answers on a selected route could receive a desktop bootstrap credential. The exposure is limited to the affected desktop and its WSL environment.

Retained concerns

• Low · security · inferred: In non-mirrored WSL mode, a responder on Windows loopback can now be selected in place of the previously preferred distro-IP route. If an unrelated process controls that port, unauthenticated readiness can promote its URL into the configuration used to send the desktop bootstrap credential.

Security review details

Security Blast Radius

• inferred — The added attack opportunity is a locally controlled responder on the selected Windows loopback port in a non-mirrored WSL run. The inspected change does not expand the WSL bind to the LAN or establish cross-device reachability.

Security Findings and Attack Paths

• inferred — If an unrelated process answers on a selected route, HTTP success can mark it ready; the manager then records its URL, and local authentication can submit the desktop bootstrap credential there. The retained finding identifies this credential-exposure path. Whether an attacker can claim the port in a particular Windows/WSL startup state remains unverified here.

Trust Boundaries and Controls

• observed — The fallback comes from the resolved distro IP and is omitted when it matches a Windows-local IPv4 interface. The manager rechecks loopback before accepting fallback and rejects readiness callbacks for a different active run; these are route and run-freshness controls, not responder-identity checks.

Resilience and Maintainability Implications

• inferred — Because bootstrap publication reads currentConfig even when the snapshot is not ready, a stopped or crashed run can leave an endpoint and token visible until configuration is replaced. The inspected stop and exit behavior predates this PR; the route change can alter which endpoint remains recorded.

Hardening Proposals

• proposed — Bind readiness acceptance to the spawned backend before promoting a responding URL to a credential destination; separately, gate bootstrap publication on the current run's readiness or invalidate configuration on terminal transitions.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary change: localhost-first WSL backend connections with the distro IP as a fallback.
Description check	✅ Passed	The description explains the problem, implementation, scope, testing, and verification. It omits the template's Checklist and uses Problem/Fix headings instead of What Changed/Why, but it remains subs…
Linked Issues check	✅ Passed	The PR addresses the coding objective in [#13738]. resolveWslStartConfig sets http://127.0.0.1:<port> as the primary URL and supplies the distro IP as fallbackHttpBaseUrl when appropriate. `runB…
Out of Scope Changes check	✅ Passed	The changes stay within [#13738]. They update WSL URL selection, readiness probing, selected-URL propagation, related comments, and focused tests. The backend bind address remains unchanged. The bind-…
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 5…

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Actionable comments posted: 1

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Inline comments:
In `@apps/desktop/src/backend/DesktopBackendManager.ts`:
- Around line 600-604: Update the readiness flow in DesktopBackendManager so a
generic 2xx response cannot establish the backend URL: require a
backend-specific authenticated challenge before setting
currentConfig.httpBaseUrl or exchanging desktopBootstrapToken, while preserving
the existing port-selection behavior.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 4408bf5 and ef4493f.

📒 Files selected for processing (5)

• apps/desktop/src/backend/DesktopBackendConfiguration.test.ts
• apps/desktop/src/backend/DesktopBackendConfiguration.ts
• apps/desktop/src/backend/DesktopBackendManager.test.ts
• apps/desktop/src/backend/DesktopBackendManager.ts
• apps/desktop/src/wsl/DesktopWslEnvironment.ts

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[maria-rcks]

Note

Written by claude-opus-5-5 on behalf of Maria

Hi! We are cleaning up open PRs, and this one does not say which model or harness was used to create it. If this change is really important, we recommend rebuilding the PR with a newer model and noting the model and harness in the PR description.

[maria-rcks]

Note

Written by claude-opus-5-5 on behalf of Maria

Reopening, this was closed by mistake. Sorry for the noise!

[Ganboo]

[!NOTE]
Written by claude-opus-5-5 on behalf of Maria

Reopening, this was closed by mistake. Sorry for the noise!

No worries I'll update the PR with the details but for now it was written by Opus 5.5 using Claude code through T3 Code