[Bug]: Session-switch data merging / cross-contamination — can't switch chats cleanly

[bgexpert]

Description

When switching between two existing chat sessions in Hermes Desktop, data from one session bleeds into the other ("data merging bug"). It is not possible to switch between chats cleanly / simultaneously — the context or draft/state of one session gets pulled into the other, so the two sessions appear to share or merge state.

This looks related to the session-state integrity class (see #63210 composer draft race and the project-association-loss bug): the per-session state bucket is not being isolated correctly on switch.

Root cause (hypothesis)

The active-session state object is shared/mounted before the target session's state is fully hydrated, so the previous session's state remains attached. If the composer/state API mounts in a stale-session context (same class of bug as getState()-returns-null in #63210, but here the bug is the wrong state rather than null), switching carries the old session's data forward instead of swapping to the new one.

Steps to reproduce

1. Open Hermes Desktop with at least two existing chat sessions that have distinct content/context.
2. Switch from session A to session B (via the sidebar / session list).
3. Observe whether session B shows any data, drafts, or context that belongs to session A, or whether the two appear merged.
4. Switch back and forth — note if the contamination persists or chains.

Expected

Each session switch should fully swap to the target session's isolated state, with no carry-over from the previously active session.

Environment

• OS: Windows 10 64-bit
• Hermes: 0.18.2 (win-unpacked)

Possible fix direction

Ensure the session-state bucket is keyed strictly per-session id and that the switch handler awaits full teardown of the old state before mounting the new one (await hydration before binding the composer/state API, mirroring the fix direction in #63358 but for the switch path, not only new-session).

[bgexpert]

Confirming this bug from real usage (Hermes 0.18.2, Windows 10, Desktop).

Concrete case:

1. Chat A — Topic X. While A is still active/streaming, opened Chat B (Topic Y) in the same Desktop window.
2. Worked in B, then switched back to A.
3. A started answering as if it also had B's context — the two sessions' context had merged. It replied referencing Topic Y while in Chat A (Topic X).

Mitigation I tried: I then asked the model to "forget what isn't its topic and clean up the noise" — it complied, so the merged state was fixable via prompt, but the initial cross-contamination on switch is real.

Code observation (not a fix, just where it likely lives): per-session context is isolated at the agent layer — agent/_session_messages is bound per session in agent/conversation_loop.py (assignments at multiple points, e.g. lines ~1905/1964/4475). So the stored transcript stays isolated; the contamination happens at the UI/switch layer, where the active-session state bucket is not fully isolated on switch (matches the issue's root-cause hypothesis). The corruption is in the live/rendered view, not in state.db (no cross-session rows appear in the DB afterward).

Repro is intermittent (depends on switching while a session is mid-stream), which is probably why it shows as needs-repro. Happy to add more traces if a maintainer points at the exact switch handler to instrument.

[teknium1]

Thanks for the detailed mid-stream reproduction. This is an automated hermes-sweeper review: current main already implements the reported session-isolation guarantee.

• 7dc21f08a188cefe346d12faa45cdcceeb2ca1fa clears the prior $messages transcript on every cold-resume fallback in apps/desktop/src/app/session/hooks/use-session-actions/index.ts:547-555, preventing a prior chat from being carried into the target session.
• da52ffea1481396368c4544bc02cead1c831c817 pins unscoped stream events to the session that received message.start in apps/desktop/src/lib/gateway-events.ts:71-126, so switching chats while A streams cannot route A's deltas or tool events into B.
• apps/desktop/src/lib/gateway-events.test.ts:28-67 covers the A→B switch during an unscoped stream and asserts that A retains ownership through completion.

The report targets Desktop 0.18.2; no release tag containing these commits was found.