feat(web): copy an environment's MCP URL for outside agents

[juliusmarminge]

Part 3 of 3 for MCP sign-in from outside T3 Code (access declarations → sign-in → this). Based on #16336. Replaces #15222, which GitHub would not move off its old stack.

Problem

After #16336 an outside agent can sign in to an environment's /mcp, but the user has to work out the right URL themselves. For a T3 Connect environment that address isn't shown anywhere in the UI.

Fix

• Saved environments in Settings → Connections get Copy MCP URL in the row menu. The toast shows the URL and a claude mcp add example.
• The URL comes from environmentMcpUrl in client-runtime (shared with mobile if it ever wants it): the bearer profile's httpBaseUrl, or relay discovery's endpoint.httpBaseUrl for T3 Connect environments. It is offered only for https or loopback addresses, because MCP clients refuse plain-http token endpoints elsewhere, and never for SSH, whose address is a local forward.
• docs/user/remote-access.md gains "Connect an outside agent": add the URL, approve with a pairing code, choose Read only or a mode limit, which routes work, revoking in Connections, the 30-day sign-in.
• Mobile: no UI by design; it's a client of agents, not where you set them up.

Evidence

Same saved environment (a second dev server served over Tailscale https), same viewport, base vs head:

Before (main)	After

After choosing it:

The "Updates Available" toast in the before shot is unrelated dev noise.

Verification

• vp test run src/connection/presentation.test.ts (client-runtime): the https address yields /mcp, loopback http is allowed, and a plain-http tailnet IP yields nothing.
• Typecheck clean for web and client-runtime.
• End to end over a T3 Connect tunnel on a personal relay stage: claude mcp add --transport http t3 https://<tunnel>/mcp (the form this menu copies), then claude mcp login t3, signed in and ran T3 tools, including launching a thread. Details and approval-page screenshots are in feat(server): outside agents sign in to the T3 MCP server with OAuth #16336.

Model: Claude Opus 5.5 (1M context) via T3 Code's Claude Code harness.

🤖 Generated with Claude Code

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[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR adds a new outside-agent MCP workflow with sign-in, pairing, revocation, and potentially supervised/full-access thread control. Although the code is localized and existing connection behavior is preserved, the new authentication-sensitive capability warrants human review.

No code changes detected at 2dfe3a7. Prior analysis still applies.

^{You can add or adjust custom eligibility rules. Learn more.}

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

ℹ️ No successful main baseline artifact is available yet. This run establishes the initial measurement.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	—	5.0 KiB	—	6.8 KiB	✅
Codex	Thread snapshot wire	—	3.8 KiB	—	4.9 KiB	✅
Codex	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Codex	Live turn WebSocket decoded	—	20.9 KiB	—	29.3 KiB	✅
Codex	Live turn messages	—	2	—	8	✅
Claude	Total thread wire	—	5.0 KiB	—	6.8 KiB	✅
Claude	Thread snapshot wire	—	3.8 KiB	—	4.9 KiB	✅
Claude	Live turn WebSocket wire	—	1.2 KiB	—	2.0 KiB	✅
Claude	Live turn WebSocket decoded	—	21.2 KiB	—	29.3 KiB	✅
Claude	Live turn messages	—	1	—	8	✅

Baseline: unavailable · PR result: 2dfe3a7 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 108.5 KiB
• Claude decoded thread snapshot: 108.8 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

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📥 Commits

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📝 Walkthrough

Walkthrough

The client runtime builds MCP URLs for eligible connections. Saved environment menus offer a copy action with success and failure toasts. Remote-access documentation describes agent setup, access modes, accepted addresses, revocation, and sign-in duration.

Changes

MCP URL access

Layer / File(s)	Summary
MCP URL generation and validation packages/client-runtime/src/connection/presentation.ts, packages/client-runtime/src/connection/presentation.test.ts	environmentMcpUrl builds /mcp URLs for eligible connections. Tests cover HTTPS, loopback HTTP, and plain-HTTP LAN addresses.
Saved environment copy flow and access instructions apps/web/src/components/settings/ConnectionsSettings.tsx, docs/user/remote-access.md	Saved environment menus offer a copy action when an MCP URL is available. The UI retains the last discovered relay HTTP base URL and displays copy result toasts. Documentation describes outside-agent access and sign-in rules.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Feature

Suggested reviewers: t3dotgg

Merge Risk: ⚪ Minimal · up to 77a25

This change adds a way to copy an eligible environment’s MCP URL and instructions for outside-agent access. No actionable defect remains in the reviewed material, so no material merge risk is identified.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding a way to copy an environment’s MCP URL for outside agents.
Description check	✅ Passed	The description explains the problem, the change, the scope rationale, and focused verification. It also includes before-and-after screenshots and reports observed test and typecheck results. It refer…

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