docs: add canonical session resolution contract

docs/rfcs/README.md

@@ -46,5 +46,8 @@ First-time contributor RFCs should be discussed in an issue before opening a PR.
   — #2361 consistency rules for keeping transcript, model context, live streams,
   replay, compression, and session metadata coherent during active and recovered
   WebUI runs.
+- [`canonical-session-resolution.md`](canonical-session-resolution.md) — #2361
+  focused contract for resolving URL, query parameter, localStorage, sidebar,
+  and compression-lineage session IDs to one canonical visible chat target.
 - [`turn-journal.md`](turn-journal.md) — Crash-safe WebUI turn journal for
   recovering interrupted chat submissions.

docs/rfcs/canonical-session-resolution.md

@@ -0,0 +1,124 @@
+# Canonical Session Resolution Contract
+
+- **Status:** Proposed
+- **Author:** @ai-ag2026
+- **Created:** 2026-05-25
+- **Tracking issue:** [#2361](https://github.com/nesquena/hermes-webui/issues/2361)
+- **Related architecture:** [#1925](https://github.com/nesquena/hermes-webui/issues/1925), [`webui-run-state-consistency-contract.md`](webui-run-state-consistency-contract.md)
+
+## Problem
+
+WebUI can reach the same conversation through several browser-facing entrypoints:
+
+- a URL route such as `/session/<session_id>`,
+- a query parameter such as `?session=<session_id>` or `?session_id=<session_id>`,
+- the browser's `localStorage` active-session value,
+- sidebar rows built from `/api/sessions`,
+- direct session open actions from links, search, or imported session lists,
+- browser boot restore after reload, auth redirect, or PWA resume.
+
+After automatic compression, those entrypoints can point at different rows in one
+logical conversation lineage. A pre-compression parent snapshot can remain a
+valid archived session while the user-facing conversation tip has moved to a
+newer continuation. If each caller resolves IDs independently, the UI can appear
+to lose the session, reopen an old one-message snapshot, duplicate sidebar rows,
+or prefer the wrong transcript even though durable data is still present.
+
+This contract defines the expected resolution semantics for those entrypoints. It
+is intentionally narrower than the run adapter RFC: this is about choosing the
+correct visible session target, not moving execution ownership.
+
+## Goals
+
+- Define one canonical browser-facing resolution concept for sessions and
+  compression lineage.
+- Make URL, query parameter, localStorage, sidebar, and direct-open behavior use
+  the same mental model.
+- Preserve archived parent snapshots without letting them become the default
+  active target when a continuation exists.
+- Give reviewers a small checklist for future session-routing, sidebar, and
+  compression-lineage changes.
+
+## Non-goals
+
+- Do not delete archived `pre_compression_snapshot` rows.
+- Do not merge or rewrite session files as part of this contract.
+- Do not replace state.db/session sidecar reconciliation.
+- Do not require a new backend endpoint before narrow frontend guards can land.
+- Do not change explicit history browsing when the user deliberately opens an
+  archived snapshot as a record.
+
+## Terms
+
+| Term | Meaning |
+|---|---|
+| Requested session ID | The ID supplied by route, query parameter, localStorage, sidebar click, or direct session open. |
+| Canonical visible session | The session row WebUI should display by default for normal chat navigation. |
+| `canonical_visible_session_id` | Proposed field/name for an API or helper output that identifies the canonical visible session. |
+| Compression snapshot | A preserved archived parent row with `pre_compression_snapshot` set. |
+| Continuation session | The active child/tip created after compression, usually represented by `continuation_session_id`, `_lineage_tip_id`, or newer lineage metadata. |
+| Lineage relation | Links such as `parent_session_id`, `_lineage_root_id`, `_lineage_tip_id`, and `_compression_segment_count` that connect rows belonging to one logical conversation. |
+
+## Resolution Rules
+
+1. **Directly valid non-snapshot IDs stay stable.** If the requested session ID
+   exists and is not a `pre_compression_snapshot`, it should normally resolve to
+   itself.
+2. **Snapshot parents defer to visible continuation tips.** If the requested
+   session ID is a `pre_compression_snapshot` and the session list has a newer
+   non-snapshot continuation in the same lineage, normal chat navigation should
+   resolve to that continuation as the `canonical_visible_session_id`.
+3. **Explicit archive/history inspection remains possible.** A future UI affordance
+   may intentionally open a snapshot as a historical record, but that should be a
+   distinct mode from ordinary boot restore, URL restore, or sidebar continuation.
+4. **Local browser state is advisory.** `localStorage` may remember the last active
+   ID, but browser boot restore must treat it as a requested session ID and still
+   run canonical resolution before rendering.
+5. **Query aliases share the same resolver.** `?session=...`, `?session_id=...`,
+   and `/session/...` should feed the same requested-ID path instead of carrying
+   separate precedence rules.
+6. **Sidebar collapse and session loading agree.** The row chosen as the visible
+   representative for a lineage should match the target opened by `loadSession()`
+   for that lineage during ordinary navigation.
+7. **404 self-heal is separate from lineage resolution.** Missing/deleted sessions
+   should still use the stale-route recovery path. A present archived parent with
+   a live continuation is not a 404; it is a canonicalization problem.
+
+## Entry Point Matrix
+
+| Entry point | Input | Expected resolution |
+|---|---|---|
+| URL route | `/session/<id>` | Treat `<id>` as requested; resolve to canonical visible session before ordinary render. |
+| Query parameter | `?session=<id>` or `?session_id=<id>` | Same as URL route. Query spelling must not change the target semantics. |
+| localStorage | last active session ID | Advisory requested ID during browser boot restore; canonicalize before render. |
+| Sidebar click | visible row ID or lineage representative | Open the same canonical visible session that the row represents. |
+| Direct session open | programmatic call/search/import link | Use the shared requested-ID resolver unless the caller explicitly opts into archive inspection. |
+| Browser boot restore | URL and/or localStorage state after reload/auth/PWA resume | Prefer explicit URL/query input, then localStorage, then canonicalize the requested ID. |
+
+## Review Checklist
+
+For PRs that touch session routing, compression lineage, sidebar collapse, boot
+restore, direct session open, or URL parsing, answer:
+
+- Which entrypoints provide the requested session ID?
+- Does the code path accept both route and query parameter forms where relevant?
+- Does localStorage go through the same canonicalization path as URL restore?
+- Can a `pre_compression_snapshot` become the default active chat when a
+  non-snapshot `continuation_session_id` / `_lineage_tip_id` exists?
+- Do sidebar collapse and `loadSession()` pick the same visible representative?
+- Is missing-session 404 recovery kept distinct from present-but-archived lineage
+  canonicalization?
+- What regression proves route, query parameter, localStorage, and sidebar paths
+  agree for compressed lineage rows?
+
+## Rollout Plan
+
+1. Document this proposed contract and link it from the public contract index.
+2. Keep narrow bugfixes small while referencing the relevant rule they preserve.
+3. Add shared frontend helper coverage for URL/query/localStorage/sidebar
+   requested-ID inputs.
+4. If backend session APIs later expose `canonical_visible_session_id`, make the
+   frontend resolver prefer the backend value while preserving client fallback for
+   older WebUI servers.
+5. If #1925 moves execution/session ownership behind an adapter, carry this
+   contract forward as an adapter-facing session-navigation invariant.