fix(preview): sessions stay on the desktop that owns the tab

[mkiera]

Fixes #13540.

What Changed

When a host disconnects, for example after a timed-out request, its lease no longer disappears. A lease that had a tab keeps that tab as its target, and the next call goes only to a host that reports owning the tab in liveTabs. Until one does, the call fails with PreviewAutomationNoAvailableHostError. That error now names the tab and tells the agent to retry after the desktop reconnects, or to start a new session with preview_open and reuseExistingTab: false.

• Opening a first tab is unchanged. With no target tab, any host is eligible and the focused one wins, as in fix(preview): use the visible browser for new agent sessions #13064.
• Live leases stay pinned, and timed-out actions are still not replayed.
• preview_open with reuseExistingTab: false and no tabId drops a retained target whose host is gone, so the agent can start over deliberately.

Why

This follows the fix direction in the #13540 triage. Before, a disconnect deleted the lease, the next call could go to any client in the environment, and that choice stayed pinned after the original desktop reconnected. A second desktop on another machine then loaded the tab on its own network.

Validation

• Replaced the two broker tests that required the old failover (fails over a pinned provider session only after its host disconnects and evicts an unanswered host and lets later calls use a healthy runtime). Their replacements require the session to keep its tab while the owner disconnects and reconnects, and require a timed-out host's tab never to reach another runtime. A replacement stream must also report the retained tab before it receives the call.
• Updated two McpHttpServer fixtures: the mock host now reports the tab it owns, and the snapshot case without an owner expects the new error text.
• vp test run apps/server/src/mcp: 92 passed, 1 failed. The failing test, saves the snapshot PNG on request and reports its path, fails the same way on unmodified main on Windows. It looks for a raw Windows path inside JSON, where the backslashes are escaped.
• tsc --noEmit is clean for apps/server and packages/contracts. Targeted lint is clean.
• Live check on two Windows 11 machines, a desktop and a VM connected through T3 Connect, using a private build of this change plus PR 2 and the open fix(server): keep preview hosts when optional page metadata times out #12279, fix(server): keep preview hosts that report a waitFor miss at the deadline #12899, fix(desktop): preview snapshots no longer time out while the T3 window is hidden #13600 and fix(desktop): account for main-window zoom in preview_resize #12718. After the desktop app restarted, calls for its old tab returned the new error instead of moving to the VM. After a Ctrl+R reload of the desktop window, with the VM as the last focused window, the session stayed on the desktop's tab.

Not included: the triage also suggests keeping one client's LoadFailed out of the shared snapshot. That is a separate change.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes (no UI change)
• I included a video for animation/interaction changes (n/a)

Model: GPT-6 Astra wrote the fix in Codex, and Claude Opus 5.5 split it out and verified it in Claude Code, both running in T3 Code.

Summary by CodeRabbit

• Bug Fixes
  • Preview automation now routes requests for a specific tab only to a connected host that reports owning that tab. If the host disconnects, requests for the retained tab wait until it reconnects and reports the tab.
  • Opening a new browser session no longer reuses a retained tab unless a tab is explicitly specified.
  • When no host is available for a requested tab, the error explains how to reconnect the desktop that owns it or start a new browser session.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This production change alters preview-session routing after desktop disconnects, including suppressing automation until the original tab owner reconnects and reports ownership. The behavior is well-tested and focused, but the broker now gates significant browser work on runtime tab-ownership state, warranting human review.

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📝 Walkthrough

Walkthrough

The broker retains tab targets after host disconnection and requires a host to report ownership of a requested tab before routing to it. Tab-specific no-host errors now explain how to reconnect the desktop or start a new browser session.

Changes

Tab-aware preview automation routing

Layer / File(s)	Summary
Retain tab targets and enforce host ownership apps/server/src/mcp/PreviewAutomationBroker.ts, apps/server/src/mcp/PreviewAutomationBroker.test.ts, apps/server/src/mcp/McpHttpServer.test.ts	Assignments retain their tab when a host disconnects. Routing requires a host to report ownership of a requested tab. Tests cover routing, reconnection, replacement, and eviction.
Explain tab-specific host errors packages/contracts/src/previewAutomation.ts, apps/server/src/mcp/McpHttpServer.test.ts	When a requested tab has no connected host, the error identifies that tab and explains how to reconnect its desktop or start a new browser session.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: juliusmarminge

Merge Risk: 🔵 Low · up to 25904

Preview requests may briefly report that no host is available during connection or reconnection, even when the desktop owns the tab. This is a bounded risk to address or accept before merging.

Security Architecture Review

Security architecture risk: 🟡 Moderate · up to 25904

Tab-aware routing reduces the chance of sending a session to the wrong desktop. However, disconnected sessions can now leave tab targets in server memory without a visible expiry, creating an availability risk as sessions accumulate. The routing protection also depends on desktops accurately reporting which tabs they own.

Retained concerns

• Medium · reliability · inferred: Tab-bearing assignments survive host disconnection without a visible expiry or provider-session cleanup. Accumulation across abandoned sessions can increase shared broker memory and routing work, affecting preview availability.

Security review details

Security Blast Radius

• inferred — The new ownership check reduces unintended routing to another desktop in the same environment after disconnect. Retained assignments reside in broker-wide state, so excessive accumulation could affect other preview sessions served by that broker.

Security Findings and Attack Paths

• inferred — If a caller can repeatedly establish distinct provider sessions with tab-bearing assignments and disconnect their hosts, retained entries have no observed reclaim path. Their accumulation could consume broker resources; no exploitation or practical exhaustion threshold was established.

Trust Boundaries and Controls

• observed — Preview host connection, response and focus calls pass through operation-scope authorization. Broker connection-generation checks limit stale updates, but the inspected RPC handoff checks scope rather than binding reported tab ownership to an authenticated desktop identity.

Resilience and Maintainability Implications

• observed — Pending requests are removed on disconnection, and an unanswered timed-out action invalidates its connection rather than being replayed on another desktop.

Hardening Proposals

• proposed — Bound retained assignments and reclaim them through provider-session termination, tab closure or a defined expiry, while preserving the intended reconnection window.
• proposed — If tab ownership must be a security boundary against another authorized desktop, bind host identity and ownership reports to the authenticated connection rather than relying solely on reported fields.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: keeping preview sessions on the desktop that owns the tab.
Description check	✅ Passed	The description includes the required What Changed, Why, and Checklist sections. It also provides validation details and clarifies that no UI changes require screenshots or video.
Linked Issues check	✅ Passed	The changes satisfy issue #13540. PreviewAutomationBroker retains the target tab after its host disconnects, removes disconnected host identity, and routes later requests only to a host that reports…
Out of Scope Changes check	✅ Passed	The changed broker logic, error message, and tests directly support issue #13540. The tests provide automated coverage for the retained-tab routing and clear-error behavior. No unrelated change is dem…
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 4…

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Actionable comments posted: 1

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🤖 Prompt to fix review comments

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Inline comments:
In `@apps/server/src/mcp/PreviewAutomationBroker.ts`:
- Around line 525-526: Update the target-tab routing filter using
supportsOperation and ownsTargetTab so a connection registered by
acquireConnection is not rejected before its initial liveTabs report arrives.
Track that report as connection readiness or defer ownsTargetTab filtering until
it has been received, while preserving tab-ownership checks afterward.

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📥 Commits

Reviewing files that changed from the base of the PR and between a21b42c and 2590402.

📒 Files selected for processing (4)

• apps/server/src/mcp/McpHttpServer.test.ts
• apps/server/src/mcp/PreviewAutomationBroker.test.ts
• apps/server/src/mcp/PreviewAutomationBroker.ts
• packages/contracts/src/previewAutomation.ts

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