Document Phase 3 as a durable Mongo run queue plus a shared Hugging Face worker Space, so the next implementation can preserve browser reconnect semantics without binding agent execution to SSE lifetimes.

Constraint: Mongo session persistence must be healthy before background workers can own execution.

Rejected: One worker Space per session | shared workers are simpler to operate and Mongo leases already provide coordination.

Confidence: high

Scope-risk: narrow

Directive: Keep browser/SSE as an observer; durable session_runs and workers should own execution in Phase 3.

Tested: git diff --check

Not-tested: Documentation-only change; no runtime tests run.

Add the Phase 3 foundation: Mongo-backed session_runs, an opt-in /api/chat enqueue path, Mongo-polled SSE replay, and a worker runtime that reuses the existing SessionManager and agent loop. The default remains the existing direct execution path until ML_INTERN_BACKGROUND_WORKERS is enabled.

Constraint: External worker Spaces cannot safely execute user-scoped HF operations until token handoff is designed; in-process workers are the safe first rollout.

Rejected: Persist raw user OAuth tokens in Mongo | broadens the trust boundary and should not be the default handoff mechanism.

Rejected: Replay expired running tool calls | non-idempotent tools can duplicate side effects, so expired runs are marked interrupted.

Confidence: medium

Scope-risk: broad

Directive: Keep ML_INTERN_BACKGROUND_WORKERS disabled until prod Mongo is healthy and the worker mode is smoke-tested.

Tested: UV_CACHE_DIR=/tmp/uv-cache uv run --extra dev pytest tests/unit/test_session_persistence.py tests/unit/test_background_worker.py tests/unit/test_session_manager_persistence.py tests/unit/test_user_quotas.py -q

Tested: UV_CACHE_DIR=/tmp/uv-cache uv run python -m py_compile agent/core/session_persistence.py backend/background_worker.py backend/worker_app.py backend/main.py backend/routes/agent.py backend/session_manager.py

Tested: git diff --check

Tested: import smoke for background_worker, worker_app, and main