session: cache the derived history — one projection per node, frozen and shared

deriveMessages() no longer re-derives and re-clones the whole surface
per call: each node is projected exactly once, when first seen (O(new
nodes) per access), through the now-public deriveEventMessage — the
per-node pure function external reconstructors and the dev invariant
fold over a log prefix, so no two paths can disagree about a request's
messages. A surface rewrite (replace, invalidate) rebuilds, signalled
by SurfaceManager.replaceGeneration — monotonic, bumped by every folded
replace and by invalidate(), never reset. Callers get a fresh array
snapshot per call over SHARED deep-frozen messages: isolation of the
append-only log moves from per-call cloning to unrepresentability
(mutation throws), which the two isolation tests now pin directly.

docs/cordis-catalog/events.md

@@ -209,7 +209,7 @@ A session was created in the store.
 'session/created'(session: Session): void
 ```
 
-Source: [`packages/core/session/src/index.ts:37`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:38`](../../packages/core/session/src/index.ts)
 
 ### `session/event` — emit
 
@@ -221,7 +221,7 @@ An event was appended to a session log (sync, fire-and-forget). This is the per-
 
 Types: [SessionEvent](../core-data-structures/core.md)
 
-Source: [`packages/core/session/src/index.ts:45`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:46`](../../packages/core/session/src/index.ts)
 
 ### `session/flush` — parallel
 
@@ -231,7 +231,7 @@ Awaited durability checkpoint. The agent loop awaits `ctx.parallel('session/flus
 'session/flush'(session: Session): Promise<void> | void
 ```
 
-Source: [`packages/core/session/src/index.ts:55`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:56`](../../packages/core/session/src/index.ts)
 
 ## `subagent/*`
 

docs/cordis-catalog/services.md

@@ -163,7 +163,7 @@ get(id: SessionId): Session | undefined
 list(): Session[]
 ```
 
-Source: [`packages/core/session/src/index.ts:328`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:345`](../../packages/core/session/src/index.ts)
 
 ## `ctx.subagents` — `SubagentService`
 

docs/core-data-structures/session.md

@@ -189,9 +189,9 @@ export interface SurfaceNode {
 }
 ```
 
-## Derived history: `deriveMessages()`
+## Derived history: `deriveMessages()` and `deriveEventMessage()`
 
-`Session.deriveMessages()` projects the event log into the `Message[]` the model sees. The projection rules:
+`Session.deriveMessages()` projects the event log into the `Message[]` the model sees — cached (each surface node projected once, when first seen; a surface rewrite rebuilds) and frozen (a fresh array per call over shared, deep-frozen messages, so mutating logged history through a projection is unrepresentable). `deriveEventMessage(event)` is the per-node pure function the fold applies — public so external reconstructors and the dev invariant project a log prefix with exactly the same rules and cannot disagree with the cache. The projection rules:
 
 - `user/message` → a user message.
 - `assistant/message` → an assistant message. Raw `assistant/chunk` events are replay/UI data and are **skipped** in derivation (the assembled message is authoritative). An **empty-content** `assistant/message` is also skipped — a max-tokens step cut off with no content still records an `assistant/message` to host its `usage`, but a content-less assistant turn must not enter the provider transcript.

docs/event-producer-consumer.md

@@ -22,9 +22,9 @@ This matrix shows which packages dispatch each harness-owned event and which pac
 | `fs/observed` | `emit` | [`packages/fs/fs/src/index.ts:138`](../packages/fs/fs/src/index.ts) | [`tool-fs`](../packages/fs/tool-fs) (`emit`) | [`fs-policy`](../packages/fs/fs-policy) |
 | `fs/write-intent` | `waterfall` | [`packages/fs/fs/src/index.ts:109`](../packages/fs/fs/src/index.ts) | [`tool-fs`](../packages/fs/tool-fs) (`waterfall`) | [`fs-policy`](../packages/fs/fs-policy) |
 | `llm/stream` | `waterfall` | [`packages/llm/llm/src/index.ts:35`](../packages/llm/llm/src/index.ts) | [`llm`](../packages/llm/llm) (`waterfall`) | [`llm-replay`](../packages/support/llm-replay) |
-| `session/created` | `emit` | [`packages/core/session/src/index.ts:37`](../packages/core/session/src/index.ts) | [`session`](../packages/core/session) (`emit`) | [`invariants`](../packages/support/invariants), [`session-persistence`](../packages/session-persistence/session-persistence) |
-| `session/event` | `emit` | [`packages/core/session/src/index.ts:45`](../packages/core/session/src/index.ts) | [`session`](../packages/core/session) (`emit`) | [`acp`](../packages/ui/acp), [`invariants`](../packages/support/invariants), [`session-persistence`](../packages/session-persistence/session-persistence), [`stdio-agent`](../packages/ui/stdio-agent) |
-| `session/flush` | `parallel` | [`packages/core/session/src/index.ts:55`](../packages/core/session/src/index.ts) | [`agent-loop`](../packages/core/agent-loop) (`parallel`) | [`session-persistence`](../packages/session-persistence/session-persistence) |
+| `session/created` | `emit` | [`packages/core/session/src/index.ts:38`](../packages/core/session/src/index.ts) | [`session`](../packages/core/session) (`emit`) | [`invariants`](../packages/support/invariants), [`session-persistence`](../packages/session-persistence/session-persistence) |
+| `session/event` | `emit` | [`packages/core/session/src/index.ts:46`](../packages/core/session/src/index.ts) | [`session`](../packages/core/session) (`emit`) | [`acp`](../packages/ui/acp), [`invariants`](../packages/support/invariants), [`session-persistence`](../packages/session-persistence/session-persistence), [`stdio-agent`](../packages/ui/stdio-agent) |
+| `session/flush` | `parallel` | [`packages/core/session/src/index.ts:56`](../packages/core/session/src/index.ts) | [`agent-loop`](../packages/core/agent-loop) (`parallel`) | [`session-persistence`](../packages/session-persistence/session-persistence) |
 | `subagent/end` | `emit` | [`packages/subagent/subagent/src/index.ts:98`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`events.dispatch`) | [`hooks-claude`](../packages/hooks/hooks-claude) |
 | `subagent/provider-added` | `emit` | [`packages/subagent/subagent/src/index.ts:72`](../packages/subagent/subagent/src/index.ts) | [`subagent`](../packages/subagent/subagent) (`emit`) | [`tool-subagent`](../packages/subagent/tool-subagent) |
 | `subagent/provider-removed` | `emit` | [`packages/subagent/subagent/src/index.ts:83`](../packages/subagent/subagent/src/index.ts) | - | [`tool-subagent`](../packages/subagent/tool-subagent) |

packages/core/session/README.md

@@ -35,8 +35,9 @@ Creates and holds event-sourced `Session` instances. Persistence is intentionall
 Plain class (not a Cordis Service). Create via `ctx.sessions.create()`.
 
 - `session.append(type, data, opts?): SessionEvent` — synchronous, never blocks on I/O. **Throws** if `data` is not losslessly JSON-serializable (BigInt, function, symbol, undefined, non-finite number, circular ref, or an exotic object like Map/Set/Date) — the event log is the durable source of truth, so this invariant is enforced at the source (exported as `isJsonValue` for backends to reuse on their replay/fork entry points). A third parameter `opts: SurfaceIntent` carries surface metadata: `surfaceOp` controls how the event enters the surface linked list, and `sourceEventSeqs` records provenance (the seq numbers of events this one derives from). It is **required** for the five `SurfaceEventType` events (every message-producing event must declare how it joins the surface) and rejected by the compiler for non-surface types. The marker requirement is enforced two ways: the typed overload makes `opts` mandatory when `type` is a specific `SurfaceEventType` literal, AND `append` **throws** at runtime if a surface-eligible event arrives with no `surfaceOp` — covering the case where `type` widens to the `SessionEventType` union (a caller iterating raw events, where the conditional overload collapses to optional) so a marker-less message event can never silently land in the log and vanish from `deriveMessages()`.
-- `session.deriveMessages(): Message[]` — derive the LLM message history by walking the surface linked list (skipping non-surface events like chunks and boundaries; a `replace` shadows the nodes it covers). The surface is the single source of derived history — there is no raw-log fallback.
-- `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.
+- `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 snapshot per call over SHARED, deep-frozen `Message` objects — cloned once off the log at projection time, so a consumer can never mutate logged data (mutation throws). 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: one event's derived message (an unfrozen clone), or `null` when it 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.events`, `session.seq`, `session.id`
 - `session.header: SessionHeader` — immutable creation metadata (`version`, `id`, `createdAt`, optional `cwd`/`parentSession`/`seedLength`). Kept out of the event log (a storage concern, not replayable state); a minimal header (stamped with the current `SESSION_FORMAT_VERSION`) is synthesized for bare `Session` construction.
 

packages/core/session/src/index.ts

@@ -8,6 +8,7 @@
 
 import { Context, Service } from 'cordis'
 import { isAbsolute } from 'node:path'
+import { deepFreeze } from '@deepseek-ai/dsh-llm'
 import type { ContentBlock, Message, MessageSource } from '@deepseek-ai/dsh-llm'
 import { SESSION_FORMAT_VERSION, SessionId } from './types.ts'
 import type { CreateSessionOptions, SessionEvent, SessionEventMap, SessionEventType, SessionHeader, SurfaceIntent, SurfaceEventType } from './types.ts'
@@ -235,53 +236,68 @@ export class Session {
     return event
   }
 
+  /** The derived-message cache: frozen projections, extended per unseen node. */
+  private derived: Message[] = []
+  /** Surface position (nodes projected) the cache has reached. */
+  private derivedNodes = 0
+  /** {@link SurfaceManager.replaceGeneration} the cache was built under. */
+  private derivedGeneration = 0
+
   /**
    * Derive the LLM message history by walking the session surface — the linked
    * list of message-producing events maintained by `surfaceOp` markers. The
    * surface is the single source of derived history: every message-producing
    * append records its `surfaceOp`, so a raw event with no marker (a chunk, a
    * turn boundary) is correctly absent, and a compaction `replace` deletes the
-   * shadowed nodes from the derivation.
+   * shadowed nodes from the derivation. The projection rules are
+   * {@link deriveEventMessage}, folded per node.
    *
-   * - `user/message` → user message
-   * - `assistant/message` → assistant message (chunks are skipped — they are
-   *   replay/UI data; the assembled message is authoritative for history). An
-   *   EMPTY-content assistant/message is skipped: a max-tokens step cut off with
-   *   no content still records an assistant/message to host its `usage`, but a
-   *   content-less assistant turn must not enter the provider transcript.
-   * - `tool/result` → user message carrying a tool-result block
-   * - `context/message` / `steering/message` → tagged synthetic user messages
-   *   at their chronological position
-   *
-   * The returned `content` is **deep-cloned** off the logged events: the loop
-   * hands these messages into the mutable `agent/request` waterfall and on to
-   * adapters, where mutating the request is sanctioned — but the session log
-   * is append-only by contract. Cloning at this boundary keeps in-flight
-   * mutation from reaching back and rewriting history (which would silently
-   * break replay equivalence). Cost is one structured clone per step,
-   * negligible next to a model call.
+   * CACHED: each surface node is projected exactly once, when first seen — a
+   * call costs O(new nodes), and a surface rewrite (a `replace`;
+   * {@link SurfaceManager.replaceGeneration}) rebuilds. The returned array is
+   * a fresh snapshot per call (later appends never grow an array a caller
+   * already holds); the `Message` objects in it are SHARED and **deep-frozen**
+   * — cloned once off the log at projection time, so consumers can never
+   * mutate logged data, and mutation attempts throw instead of silently
+   * diverging replay from history.
+   * @returns a fresh array of the shared, frozen derived history.
    */
   deriveMessages(): Message[] {
-    const messages: Message[] = []
-    for (const node of this.surface.nodes) {
+    const nodes = this.surface.nodes
+    const generation = this.surface.replaceGeneration
+    if (generation !== this.derivedGeneration) {
+      this.derived = []
+      this.derivedNodes = 0
+      this.derivedGeneration = generation
+    }
+    for (const node of nodes.slice(this.derivedNodes)) {
       // Surface nodes are built from this.log — node.seq is always a valid
       // index by construction. The non-null assertion expresses that invariant.
       // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
-      const msg = this._deriveOneMessage(this.log[node.seq]!)
+      const msg = this.deriveEventMessage(this.log[node.seq]!)
       // A surface node is one of the five message-producing types, but an
       // empty-content assistant/message (a max-tokens step that hosts only
       // usage) derives to null and must not enter the transcript.
-      if (msg) messages.push(msg)
+      if (msg) this.derived.push(deepFreeze(msg))
     }
-    return messages
+    this.derivedNodes = nodes.length
+    return [...this.derived]
   }
 
   /**
-   * Derive a single LLM message from one surface event, or null if it produces
-   * no message (an empty-content assistant/message that exists only to host
-   * usage).
+   * Project a single event into the LLM message it derives to, or null when
+   * it produces none — a non-surface event (chunk, boundary, log-only record)
+   * or an empty-content assistant/message (which exists only to host usage).
+   * The per-node pure function {@link deriveMessages} folds over the surface;
+   * an external reconstructor (or the dev invariant) folds the same function
+   * over a log prefix's surface to rebuild the exact messages any request was
+   * built from (the reconstructability RFC). The returned `content` is
+   * deep-cloned off the logged event: the log is append-only by contract, so
+   * no live reference to logged data leaves this boundary.
+   * @param event - the event to project.
+   * @returns the derived message, or null when the event produces none.
    */
-  private _deriveOneMessage(event: SessionEvent): Message | null {
+  deriveEventMessage(event: SessionEvent): Message | null {
     // Intentionally non-exhaustive: only message-producing events derive
     // history; turn/step boundaries, chunks, usage, and errors are
     // trace/replay data.
@@ -312,8 +328,9 @@ export class Session {
         const { content, source } = event.data
         return { role: 'user', content: renderTagged('steering', structuredClone(content), source) }
       }
-      /* v8 ignore next 2 -- unreachable: only surface nodes (the 5 message-producing types) reach here */
       default:
+        // A non-surface event (boundary, chunk, log-only record) projects to
+        // no message. Merge-extensible union: no assertNever here.
         return null
     }
   }

packages/core/session/src/surface.ts

@@ -72,6 +72,9 @@ export class SurfaceManager {
   /** The last processed seq. -1 forces a full rebuild on first access. */
   private _lastProcessedSeq = -1
 
+  /** Rewrite generation — see {@link replaceGeneration}. */
+  private _replaceGeneration = 0
+
   constructor(private log: readonly SessionEvent[]) {}
 
   /**
@@ -83,6 +86,23 @@ export class SurfaceManager {
     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
+   * 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
+   * append; a changed one means its view must rebuild. Monotonic: it never
+   * moves backwards, so comparisons cannot be fooled by a re-fold.
+   */
+  get replaceGeneration(): number {
+    if (this._lastProcessedSeq < this.log.length - 1) this._processDelta()
+    return this._replaceGeneration
   }
 
   /** The surface nodes in linked-list order (head to tail). */
@@ -155,5 +175,6 @@ export class SurfaceManager {
     if (nextNode) nextNode.prev = newSeq
     this._nodes.splice(startIdx, 0, newNode)
     this._nodeBySeq.set(newSeq, newNode)
+    this._replaceGeneration += 1
   }
 }

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

@@ -0,0 +1,112 @@
+/**
+ * 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
+ * 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.
+ */
+
+import { describe, expect, it } from 'vitest'
+import { Session, SessionId } from '@deepseek-ai/dsh-session'
+
+function userText(session: Session, text: string): void {
+  session.append('user/message', { content: [{ type: 'text', text }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+}
+
+/** From-scratch oracle: replay the log into a fresh session and derive. */
+function scratch(session: Session): unknown {
+  return new Session(SessionId(`${session.id}-scratch-${session.seq}`), [...session.events]).deriveMessages()
+}
+
+describe('derived-message cache', () => {
+  it('stays deep-equal to a from-scratch replay derivation as the log grows', () => {
+    const session = new Session(SessionId('cache-grow'))
+    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    userText(session, 'one')
+    expect(session.deriveMessages()).toEqual(scratch(session))
+    userText(session, 'two')
+    session.append('assistant/message', { turn: 1, step: 1, content: [{ type: 'text', text: 'reply' }] }, { surfaceOp: 'append' })
+    expect(session.deriveMessages()).toEqual(scratch(session))
+    // An empty-content assistant/message (usage host) projects to nothing.
+    session.append('assistant/message', { turn: 1, step: 2, content: [], usage: { inputTokens: 1, outputTokens: 0 } }, { surfaceOp: 'append' })
+    expect(session.deriveMessages()).toEqual(scratch(session))
+  })
+
+  it('rebuilds on a surface replace and still matches scratch', () => {
+    const session = new Session(SessionId('cache-replace'))
+    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    userText(session, 'one')
+    userText(session, 'two')
+    const beforeReplace = session.deriveMessages()
+    expect(beforeReplace).toHaveLength(2)
+
+    const nodes = session.surface.nodes
+    session.append('context/message', {
+      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(session.deriveMessages()).toHaveLength(1)
+    expect(session.deriveMessages()).toEqual(scratch(session))
+    // The array a caller took before the replace is untouched.
+    expect(beforeReplace).toHaveLength(2)
+  })
+
+  it('returns a fresh array per call: later appends never grow a held snapshot', () => {
+    const session = new Session(SessionId('cache-snapshot'))
+    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    userText(session, 'one')
+    const first = session.deriveMessages()
+    userText(session, 'two')
+    const second = session.deriveMessages()
+    expect(first).toHaveLength(1)
+    expect(second).toHaveLength(2)
+    // Shared projection objects: the same frozen message instance, once ever.
+    expect(second[0]).toBe(first[0])
+    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', () => {
+  it('projects one appended event exactly as the full derivation projects its node', () => {
+    const session = new Session(SessionId('per-event'))
+    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    const event = session.append('user/message', { content: [{ type: 'text', text: 'hi' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    // The fold path (deriveMessages) and the per-event path share the
+    // projection, so an external reconstructor cannot disagree with the cache.
+    expect(session.deriveEventMessage(event)).toEqual(session.deriveMessages().at(-1))
+  })
+
+  it('clones content off the log: the projection never aliases the logged event', () => {
+    const session = new Session(SessionId('per-event-clone'))
+    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    const event = session.append('user/message', { content: [{ type: 'text', text: 'orig' }], source: { kind: 'user' } }, { surfaceOp: 'append' })
+    const message = session.deriveEventMessage(event)!
+    expect(message.content).not.toBe(event.data.content)
+    // deriveEventMessage returns an unfrozen clone (the cache freezes ITS
+    // copies); mutating it must not reach the log.
+    ;(message.content[0] as { text: string }).text = 'mutated'
+    expect(session.deriveMessages().at(-1)!.content).toEqual([{ type: 'text', text: 'orig' }])
+  })
+
+  it('projects null for events that produce no message (boundaries, empty assistant)', () => {
+    const session = new Session(SessionId('per-event-null'))
+    session.append('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } })
+    const boundary = session.append('step/start', { turn: 1, step: 1 })
+    expect(session.deriveEventMessage(boundary)).toBeNull()
+    const empty = session.append('assistant/message', { turn: 1, step: 1, content: [] }, { surfaceOp: 'append' })
+    expect(session.deriveEventMessage(empty)).toBeNull()
+  })
+})

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

@@ -109,15 +109,17 @@ describe('Session properties', () => {
     ))
   })
 
-  it('every derived message has a known role and decoupled content', () => {
+  it('every derived message has a known role and is frozen (append-only contract)', () => {
     fc.assert(fc.property(logArb, (events) => {
       const session = build(events)
       const messages = session.deriveMessages()
       const before = structuredClone(session.events)
       for (const m of messages) {
         expect(['user', 'assistant', 'system']).toContain(m.role)
-        // Mutating derived content must not touch the log (append-only).
-        m.content.push({ type: 'text', text: 'mutation' })
+        // Derived messages are frozen shared projections: mutation THROWS
+        // (strict mode) instead of relying on per-call clones for isolation.
+        expect(Object.isFrozen(m)).toBe(true)
+        expect(() => { m.content.push({ type: 'text', text: 'mutation' }) }).toThrow(TypeError)
       }
       expect(session.events).toEqual(before)
     }))

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

@@ -78,19 +78,25 @@ describe('Session', () => {
     }, { surfaceOp: 'append' })
     const before = structuredClone(session.events)
 
-    // A request middleware / adapter mutates the messages it was handed.
+    // A misbehaving consumer tries to mutate the messages it was handed.
+    // Derived messages are frozen shared projections (cloned once off the
+    // log, then deep-frozen): every mutation attempt THROWS in strict mode —
+    // isolation by unrepresentability, not by per-call cloning.
     const messages = session.deriveMessages()
     const userBlock = messages[0]!.content[0]!
-    if (userBlock.type === 'text') userBlock.text = 'HACKED'
+    expect(() => { if (userBlock.type === 'text') userBlock.text = 'HACKED' }).toThrow(TypeError)
     const toolBlock = messages[1]!.content[0]!
-    if (toolBlock.type === 'tool-result') {
-      toolBlock.content.push({ type: 'text', text: 'injected' })
-    }
-    messages[0]!.content.push({ type: 'text', text: 'extra' })
+    expect(() => {
+      if (toolBlock.type === 'tool-result') toolBlock.content.push({ type: 'text', text: 'injected' })
+    }).toThrow(TypeError)
+    expect(() => { messages[0]!.content.push({ type: 'text', text: 'extra' }) }).toThrow(TypeError)
+    // The returned ARRAY is the caller's own snapshot, though — reordering it
+    // is the caller's business and never reaches the cache or the log.
+    messages.reverse()
 
     // The log is unchanged: deep-equal to the snapshot taken before mutation.
     expect(session.events).toEqual(before)
-    // And a fresh derivation still reflects the original content.
+    // And a fresh derivation still reflects the original content and order.
     expect(session.deriveMessages()[0]!.content).toEqual([{ type: 'text', text: 'original' }])
   })