refactor: prune dead session surfaces

packages/core/session/README.md

@@ -35,7 +35,7 @@ Plain class (not a Cordis Service). Create via `ctx.sessions.create()`.
 - `session.append(type, data, opts?): SessionEvent` — synchronous, never blocks on I/O. At this durable boundary, data and surface metadata are lossless-JSON snapshotted and deep-frozen. For an attached session, a reentrant append during dispatch/observer publication rejects, and detach waits for that publication to unwind. Callbacks resolve before the log push; the push is the commit point, after which each observer failure is contained independently. Runtime surface validation covers widened unions and raw seed/load logs.
 - `session.deriveMessages(): Message[]` — the LLM message history, CACHED: each surface node is projected exactly once, when first seen (O(new nodes) per call; a surface rewrite rebuilds via `surface.replaceGeneration`). Returns a fresh array per call over shared, deep-frozen `Message` objects. Each projection reuses the already deep-frozen content in its durable log event, so no second deep clone is needed and a consumer still cannot mutate logged data. The surface is the single source of derived history — there is no raw-log fallback.
 - `session.deriveEventMessage(event): Message | null` — the per-event projection `deriveMessages()` folds: a fresh message wrapper that reuses the event's already frozen content, or `null` when the event produces none (a non-surface event, or an empty-content `assistant/message` hosting only usage). External reconstructors and the dev invariant fold the same function over a log prefix's surface, so no two paths can disagree about what a request's messages were (the reconstructability RFC).
-- `session.surface: SurfaceManager` — the derived surface, lazily rebuilt from `surfaceOp` markers in the log. Processes only new events (delta) on each access — the log is append-only, so prior events never change. `surface.replaceGeneration` is the rewrite signal: bumped by every folded `replace` and by `invalidate()`, never reset, so an incremental consumer comparing generations cannot be fooled.
+- `session.surface: SurfaceManager` — the derived surface, lazily folded from `surfaceOp` markers in the log. Processes only new events (delta) on each access — the log is append-only, so prior events never change. `surface.replaceGeneration` is the rewrite signal: bumped by every folded `replace` and never reset, so an incremental consumer comparing generations cannot be fooled.
 - `session.events` — a cached, frozen array snapshot over deep-frozen events. Repeated reads without an append return the same array; an append invalidates the cache and the next read returns a new snapshot, while earlier snapshots stay unchanged. Neither a cast nor a retained reference can push into the live log or rewrite an accepted event.
 - `session.seq`, `session.id` — current sequence and readonly typed identity.
 - `session.header: SessionHeader` — detached, deep-frozen creation metadata (`version`, `id`, `createdAt`, optional `cwd`/`parentSession`/`seedLength`). Construction validates the durable record and requires its id to match `session.id`.

packages/core/session/src/surface.ts

@@ -73,7 +73,7 @@ export class SurfaceManager {
   private _nodes: SurfaceNode[] = []
   /** Map from event seq → node. */
   private _nodeBySeq = new Map<number, SurfaceNode>()
-  /** The last processed seq. -1 forces a full rebuild on first access. */
+  /** The last processed seq. -1 folds the seeded log on first access. */
   private _lastProcessedSeq = -1
 
   /** Rewrite generation — see {@link replaceGeneration}. */
@@ -82,22 +82,8 @@ export class SurfaceManager {
   constructor(private log: readonly SessionEvent[]) {}
 
   /**
-   * Reset to unprocessed state. Call after the log has been replaced
-   * wholesale (e.g. after Session seed). Not needed for normal appends —
-   * those are picked up incrementally.
-   */
-  invalidate(): void {
-    this._lastProcessedSeq = -1
-    this._nodes = []
-    this._nodeBySeq.clear()
-    // A wholesale rebuild is a rewrite: bump the generation so incremental
-    // consumers (the session's derived-message cache) discard their view.
-    this._replaceGeneration += 1
-  }
-
-  /**
-   * The surface's rewrite generation: bumped by every folded `replace` op and
-   * by {@link invalidate}. A replace is the ONE operation that rewrites the
+   * The surface's rewrite generation, bumped by every folded `replace` op.
+   * A replace is the ONE operation that rewrites the
    * surface non-monotonically, so an incremental consumer of {@link nodes}
    * (the session's derived-message cache) compares this between visits — an
    * unchanged generation guarantees every node it has not seen is a pure tail

packages/core/session/tests/derived-cache.spec.ts

@@ -1,7 +1,7 @@
 /**
  * Derived-message cache tests: the session projects each surface node exactly
- * once (O(new nodes) per call), rebuilds on a surface rewrite (replace /
- * invalidate — the replaceGeneration signal), returns a fresh array snapshot
+ * once (O(new nodes) per call), rebuilds on a surface replace (the
+ * replaceGeneration signal), returns a fresh array snapshot
  * per call over shared frozen messages, and stays deep-equal to a from-scratch
  * replay derivation at every step — the incremental==scratch property the
  * reconstructability RFC's invariant enforces in dev at request time.
@@ -66,17 +66,6 @@ describe('derived-message cache', () => {
     expect(Object.isFrozen(first[0])).toBe(true)
   })
 
-  it('rebuilds after surface.invalidate() (the generation covers wholesale rebuilds too)', () => {
-    const session = new Session(SessionId('cache-invalidate'))
-    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
-    userText(session, 'one')
-    const before = session.deriveMessages()
-    session.surface.invalidate()
-    const after = session.deriveMessages()
-    expect(after).toEqual(before)
-    // A rebuild re-projects: fresh objects, same values.
-    expect(after[0]).not.toBe(before[0])
-  })
 })
 
 describe('Session.deriveEventMessage — the per-event projection', () => {

packages/core/session/tests/surface.spec.ts

@@ -28,14 +28,6 @@ describe('SurfaceManager', () => {
     expect(nodes[1]!.next).toBeNull()
   })
 
-  it('invalidate resets to full rebuild', () => {
-    const s = surfaceSession()
-    expect(s.surface.nodes.length).toBe(2)
-    // After invalidate, the surface should rebuild from scratch on next access.
-    ;(s.surface).invalidate()
-    expect(s.surface.nodes.length).toBe(2) // same result, but rebuilt
-  })
-
   it('empty surface yields empty nodes', () => {
     const s = new Session(SessionId('empty'))
     // Only turn boundaries, no surface nodes.
@@ -336,7 +328,7 @@ describe('surface type guards', () => {
 })
 
 describe('SurfaceManager.replaceGeneration', () => {
-  it('folds the pending log delta on access and counts replaces and invalidations', () => {
+  it('folds the pending log delta on access and counts replaces', () => {
     const s = new Session(SessionId('gen'))
     s.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
     s.append('user/message', { content: [{ type: 'text', text: 'one' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
@@ -350,10 +342,5 @@ describe('SurfaceManager.replaceGeneration', () => {
       content: [{ type: 'text', text: 'summary' }], source: { kind: 'plugin', plugin: 'compact' },
     }, { surfaceOp: { op: 'replace', start: nodes[0]!.seq, end: nodes[1]!.seq }, sourceEventSeqs: [nodes[0]!.seq, nodes[1]!.seq] })
     expect(s.surface.replaceGeneration).toBe(1)
-
-    // invalidate() is a rewrite too: the generation moves forward (and the
-    // refold re-counts the replace), never backwards.
-    s.surface.invalidate()
-    expect(s.surface.replaceGeneration).toBeGreaterThan(1)
   })
 })