fix(cloud): a chat whose machine E2B can't start says so and keeps retrying

[andrewcai8]

On 2026-10-06 E2B could not resume one chat's paused sandbox (504 "Failed to place sandbox: placement timed out") for over an hour; the chat showed "updating" the whole time. The placement failure came back as a plain unknown refusal, the host then tried an upgrade to recover the box (reported as updating), and the client showed that machine state over the refusal.

• Resume refusals caused by the provider (E2B 504 placement, 503 capacity, other 5xx) carry cause: "provider-unavailable", and the host skips the upgrade attempt for them. Presence answers carry providerUnavailableAt, cleared on the next other answer. Both fields are optional and lenient.
• Clients show a new unavailable status: "E2B couldn't start this chat's cloud machine yet. The problem is on their side. Retrying on its own, last tried at …" on web (banner, sidebar pill) and mobile (notice, status, row), and re-poll every 15 s while it lasts.

Not classified by E2B's error_code: e2b@2.49.0 drops it from SandboxError, so status is the signal. Namespace capacity errors aren't classified yet. Tests: a real 504 through manager.resume yields the provider-unavailable refusal; 502 too; the upgrade is skipped; presence reports and clears the timestamp; client status, banner copy and polling.

🤖 Generated with Claude Code

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Codex	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Codex	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	20.9 KiB	20.9 KiB	0 B (0.0%)	29.3 KiB	✅
Codex	Live turn messages	2	2	0 (0.0%)	8	✅
Claude	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Claude	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Claude	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	21.2 KiB	21.2 KiB	0 B (0.0%)	29.3 KiB	✅
Claude	Live turn messages	2	2	0 (0.0%)	8	✅

Baseline: f829813 · PR result: 0b67461 · 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.