diff --git a/packages/agent-loop/src/agent.ts b/packages/agent-loop/src/agent.ts
index 7705f8c3ed..64576186c3 100644
--- a/packages/agent-loop/src/agent.ts
+++ b/packages/agent-loop/src/agent.ts
@@ -93,29 +93,51 @@ export class LoopAgent implements Agent {
     // open injection turn that would corrupt later turns/replay. (If the
     // turn/start append throws BEFORE pushing — non-serializable trigger, which
     // can't happen for our fixed trigger — no turn was opened and none is owed.)
-    let turnRecorded = false
     try {
       this.session.append('turn/start', { turn, trigger: { kind: 'injection', source } })
       this.session.append('context/message', { content, source })
     } finally {
-      // A turn was recorded iff turn/start made it into the log. Close it and
-      // mark it for the durability checkpoint below — which must run even when
-      // an append's listener threw (the turn is balanced and in memory, so it
-      // still needs a flush or a crash before the next turn/dispose loses it).
+      // Close the turn if turn/start made it into the log. Contain a throwing
+      // turn/end listener: Session.append pushes before notifying, so a throw
+      // here still leaves turn/end in the log (the turn is balanced) — swallow
+      // it so it neither replaces the original exception nor skips the flush
+      // decision below. (It surfaces through the flush path is not needed; the
+      // turn-balance contract is what matters and it holds.)
       if (isTurnOpen(this.session)) {
-        this.session.append('turn/end', { turn, reason: { kind: 'completed' } })
-        turnRecorded = true
+        try {
+          this.session.append('turn/end', { turn, reason: { kind: 'completed' } })
+        } catch {
+          // turn/end is already in the log (pushed before the listener threw),
+          // so the turn is balanced; the throw is the listener's bug.
+        }
       }
+      // Decide the durability checkpoint from the LOG, not a flag: a turn was
+      // recorded iff this turn's turn/start is logged (it may have been closed
+      // by a throwing-listener turn/end above, which still counts). A
+      // `turnRecorded` boolean set after append('turn/end') would be skipped by
+      // a throwing turn/end listener, losing the flush for a balanced in-memory
+      // turn (crash before the next turn/dispose would drop the idle injection).
+      const turnRecorded = this.session.events.some(e => e.type === 'turn/start' && e.data.turn === turn)
       // Checkpoint the one-shot turn for durability, exactly as the loop does at
       // every turn/end. The loop is NOT running (we are idle), so nothing else
       // will flush this turn. Fire-and-forget with error containment: inject()
       // is synchronous, and a persistence backend failing must not throw into
       // the caller (e.g. a tool-bash task-done callback). Disposal still drains
-      // independently, so a slow flush is safe. In the finally so it also runs
-      // when an append's listener threw (the turn is still balanced + durable).
+      // independently, so a slow flush is safe. A flush failure is reported via
+      // agent/error (step 0 — the idle-injection convention, there is no real
+      // step) AND the logger, mirroring the loop's post-turn/end flush path so
+      // plugins monitoring agent/error see idle-injection persistence failures
+      // too. A throwing agent/error listener is contained.
       if (turnRecorded) {
         void Promise.resolve(this.ctx.parallel('session/flush', this.session)).catch((error: unknown) => {
-          this.ctx.logger.warn(`agent "${this.id}": flush after idle injection failed: ${String(error)}`)
+          const err = error instanceof Error ? error : new Error(String(error))
+          this.ctx.logger.warn(`agent "${this.id}": flush after idle injection failed: ${err.message}`)
+          try {
+            this.ctx.emit('agent/error', this, turn, 0, err)
+          } catch {
+            // contained: the failure is already logged; a throwing agent/error
+            // listener must not escape this fire-and-forget catch.
+          }
         })
       }
     }
diff --git a/packages/agent-loop/src/loop.ts b/packages/agent-loop/src/loop.ts
index 71b78d2e58..3497892c1c 100644
--- a/packages/agent-loop/src/loop.ts
+++ b/packages/agent-loop/src/loop.ts
@@ -170,7 +170,6 @@ async function runTurn(ctx: Context, agent: LoopAgent, handle: LoopHandle, turn:
 
   let reason: TurnEndReason = { kind: 'completed' }
   let step = 0
-  let turnStarted = false
   let turnEnded = false
   let stepOpen = false
   let errorReported = false
@@ -236,9 +235,10 @@ async function runTurn(ctx: Context, agent: LoopAgent, handle: LoopHandle, turn:
 
   try {
     // --- Turn boundary. Once turn/start is appended, a turn/end is owed no
-    // matter what throws below; the catch + closeTurn guarantee it.
+    // matter what throws below; the catch + closeTurn guarantee it (the catch
+    // decides "owed" from the log via isTurnOpen, so even a throwing turn/start
+    // listener — append pushes before notifying — still gets its turn/end).
     session.append('turn/start', { turn, trigger })
-    turnStarted = true
     // Record the queued user messages INSIDE the turn (after turn/start), so
     // every event in the log is turn-enclosed. turn/end is now owed, so a throw
     // while appending these is caught below and the turn is still closed.
@@ -320,12 +320,20 @@ async function runTurn(ctx: Context, agent: LoopAgent, handle: LoopHandle, turn:
     // Normal / inline-error loop exit: close the turn and notify.
     closeTurn(true)
   } catch (error: unknown) {
-    // A pre-turn throw (turn/start append) is owed no turn/end — rethrow to
-    // the backstop. Otherwise a boundary emit (turn-start, step-start, the
-    // normal-path turn-end) or other unhandled throw escaped: close any open
-    // step, choose the reason (disposal wins only if no error was reported),
-    // record the error, and close the turn WITHOUT re-emitting agent/turn-end.
-    if (!turnStarted) throw error
+    // Decide whether this turn was ever opened from the LOG, not a flag.
+    // Session.append pushes the event BEFORE notifying session/event listeners,
+    // so a throwing listener on the `turn/start` append leaves turn/start in the
+    // log even though execution never reached the lines after that append.
+    // Gating on a "turn started" boolean would skip turn/end and leave a
+    // permanently OPEN turn that poisons the next turn/replay (ADR 0017). We
+    // check the log for THIS turn's turn/start: present means a turn/end is owed
+    // (or was already appended — closeTurn/failTurn are idempotent, so running
+    // them again is a safe no-op that still preserves the disposed/error reason
+    // chosen below). Absent means the turn/start append threw BEFORE its push (a
+    // non-serializable trigger — impossible for our fixed trigger); nothing was
+    // opened, so rethrow to the runLoop backstop.
+    const turnStartLogged = session.events.some(e => e.type === 'turn/start' && e.data.turn === turn)
+    if (!turnStartLogged) throw error
     closeStep()
     // Choose the close reason. Disposal wins only if no error was already
     // reported: a turn disposed mid-step sets reason=disposed in the step-error
diff --git a/packages/agent-loop/tests/agent.spec.ts b/packages/agent-loop/tests/agent.spec.ts
index 32b28be74a..6ec8d7d8f5 100644
--- a/packages/agent-loop/tests/agent.spec.ts
+++ b/packages/agent-loop/tests/agent.spec.ts
@@ -141,6 +141,50 @@ describe('LoopAgent', () => {
     expect(flushes).toBe(1) // checkpoint fired despite the throw
   })
 
+  it('idle inject() still checkpoints when a listener throws on the synthetic turn/end', async () => {
+    const adapter = new MockAdapter([textResponse('ok')])
+    const ctx = await harness(adapter)
+    const agent = ctx.agentLoop.create('a1', { model: 'mock' })
+    let flushes = 0
+    ctx.on('session/flush', () => { flushes += 1 })
+    // A session/event listener that throws on the synthetic turn/end. Append
+    // pushes before notifying, so turn/end is in the log (turn balanced) but the
+    // throw must NOT skip the durability checkpoint — the flush decision is made
+    // from the log, not a flag set after the (throwing) append.
+    let threw = false
+    ctx.on('session/event', (_s, event) => {
+      if (!threw && event.type === 'turn/end') { threw = true; throw new Error('boom turn/end') }
+    })
+
+    expect(() => { agent.inject([{ type: 'text', text: 'notice' }], { source: { kind: 'plugin', plugin: 'p' } }) }).not.toThrow()
+    const types = agent.session.events.map(e => e.type)
+    expect(types).toEqual(['turn/start', 'context/message', 'turn/end']) // balanced
+    await new Promise(r => setTimeout(r, 10))
+    expect(flushes).toBe(1) // checkpoint fired despite the throwing turn/end listener
+  })
+
+  it('idle inject() reports a failing flush via agent/error (step 0) AND the logger', async () => {
+    const adapter = new MockAdapter([textResponse('ok')])
+    const ctx = await harness(adapter)
+    // A non-Error rejection exercises the String() normalization branch.
+    ctx.on('session/flush', () => { throw 'disk gone' })
+    const warn = vi.spyOn(ctx.logger, 'warn').mockImplementation(() => undefined)
+    const agent = ctx.agentLoop.create('a1', { model: 'mock' })
+    const errors: { turn: number; step: number; message: string }[] = []
+    ctx.on('agent/error', (_a, turn, step, error) => void errors.push({ turn, step, message: error.message }))
+
+    agent.inject([{ type: 'text', text: 'notice' }], { source: { kind: 'plugin', plugin: 'p' } })
+    await new Promise(r => setTimeout(r, 20)) // let the contained flush settle
+
+    // Reported via agent/error (step 0 — the idle-injection convention) so
+    // plugins monitoring agent/error see idle-injection persistence failures,
+    // mirroring the loop's post-turn/end flush path. A non-Error throw is
+    // normalized to an Error.
+    expect(errors).toEqual([{ turn: 1, step: 0, message: 'disk gone' }])
+    expect(warn).toHaveBeenCalledWith(expect.stringContaining('flush after idle injection failed'))
+    warn.mockRestore()
+  })
+
   it('idle inject() with a non-serializable source opens no turn (nothing to close)', async () => {
     const adapter = new MockAdapter([textResponse('ok')])
     const ctx = await harness(adapter)
diff --git a/packages/agent-loop/tests/coverage-edges.spec.ts b/packages/agent-loop/tests/coverage-edges.spec.ts
index 260c521de1..90b87d02aa 100644
--- a/packages/agent-loop/tests/coverage-edges.spec.ts
+++ b/packages/agent-loop/tests/coverage-edges.spec.ts
@@ -35,9 +35,11 @@ function send(agent: LoopAgent, text: string) {
   agent.send([{ type: 'text', text }])
 }
 
-describe('loop backstop catch', () => {
-  it('a throwing turn-start listener is caught by the backstop and loop survives', async () => {
-    // The first turn will abort before the model call (turn-start throw).
+describe('turn boundary listener throws (handled in-turn, loop survives)', () => {
+  it('a throwing agent/turn-start listener surfaces via agent/error and the loop survives', async () => {
+    // The agent/turn-start emit happens AFTER turn/start is appended to the log,
+    // so a throwing listener is handled inside runTurn (the turn is balanced and
+    // closed via failTurn → agent/error), NOT rethrown to the runLoop backstop.
     // The second turn should proceed normally and consume the first script entry.
     const adapter = new MockAdapter([textResponse('turn 2')])
     const ctx = await harness(adapter)
@@ -57,6 +59,9 @@ describe('loop backstop catch', () => {
     send(agent, 'first')
     await waitForIdle(ctx, agent)
     expect(errors.map(e => e.message)).toEqual(['broken turn-start listener'])
+    // The turn is balanced: its turn/start was logged, so a turn/end was owed
+    // and appended (decided from the log, not a flag).
+    expect(agent.session.events.at(-1)?.type).toBe('turn/end')
 
     // loop survives: second turn works fine and makes the model call
     send(agent, 'second')
@@ -65,7 +70,7 @@ describe('loop backstop catch', () => {
     expect(adapter.requests[0]!.messages.some(m => m.content.some(b => 'text' in b && b.text === 'second'))).toBe(true)
   })
 
-  it('a throwing turn-end listener is caught by the backstop and loop survives', async () => {
+  it('a throwing agent/turn-end listener surfaces via agent/error and the loop survives', async () => {
     const adapter = new MockAdapter([textResponse('turn 1'), textResponse('turn 2')])
     const ctx = await harness(adapter)
     const agent = ctx.agentLoop.create('a1', { model: 'mock' })
@@ -84,7 +89,8 @@ describe('loop backstop catch', () => {
     send(agent, 'first')
     await waitForIdle(ctx, agent)
     // The turn-end throw happens after the model call is complete, so turn 1's
-    // request is consumed. The error is surfaced by the backstop.
+    // request is consumed. turn/end is already in the log (append pushes before
+    // notifying), so the turn is balanced; the error is surfaced via agent/error.
     expect(errors.map(e => e.message)).toEqual(['broken turn-end listener'])
 
     // loop survives: second turn works fine
@@ -92,6 +98,35 @@ describe('loop backstop catch', () => {
     await waitForIdle(ctx, agent)
     expect(adapter.requests).toHaveLength(2)
   })
+
+  it('a pre-push turn/start failure (non-serializable source) is rethrown to the runLoop backstop', async () => {
+    // A non-serializable message source makes the turn/start append throw BEFORE
+    // the event is pushed (Session.append validates before push), so turn/start
+    // never enters the log. runTurn sees no logged turn/start and rethrows; the
+    // runLoop backstop reports via agent/error (step 0) + the logger and the
+    // driver survives. This is the ONLY path that reaches the backstop.
+    const adapter = new MockAdapter([textResponse('turn 2')])
+    const ctx = await harness(adapter)
+    const agent = ctx.agentLoop.create('a1', { model: 'mock' })
+
+    const errors: { turn: number; step: number; message: string }[] = []
+    ctx.on('agent/error', (_a, turn, step, error) => void errors.push({ turn, step, message: error.message }))
+
+    // A non-serializable source (BigInt) on the queued message.
+    agent.send([{ type: 'text', text: 'first' }], { source: { kind: 'plugin', plugin: 'p', bad: 1n } as never })
+    await waitForIdle(ctx, agent)
+
+    expect(errors).toHaveLength(1)
+    expect(errors[0]!.step).toBe(0)
+    expect(errors[0]!.message).toMatch(/non-JSON-serializable/)
+    // No turn boundary was written (the turn/start append threw before push).
+    expect(agent.session.events.some(e => e.type === 'turn/start')).toBe(false)
+
+    // loop survives: a well-formed second turn runs normally.
+    send(agent, 'second')
+    await waitForIdle(ctx, agent)
+    expect(adapter.requests).toHaveLength(1)
+  })
 })
 
 describe('tool JSON parse', () => {
@@ -157,7 +192,7 @@ describe('tool JSON parse', () => {
 })
 
 describe('toError normalization', () => {
-  it('normalizes non-Error throws from turn-start listeners via toError in the backstop', async () => {
+  it('normalizes non-Error throws from turn-start listeners via toError', async () => {
     const adapter = new MockAdapter([textResponse('ok')])
     const ctx = await harness(adapter)
     const agent = ctx.agentLoop.create('a1', { model: 'mock' })
@@ -166,7 +201,7 @@ describe('toError normalization', () => {
     ctx.on('agent/turn-start', () => {
       if (!threwOnce) {
         threwOnce = true
-        throw 'naked string error' // non-Error throw, goes through backstop's toError
+        throw 'naked string error' // non-Error throw, normalized via toError
       }
     })
 
diff --git a/packages/agent-loop/tests/review-fixes.spec.ts b/packages/agent-loop/tests/review-fixes.spec.ts
index d6fa3870b9..438e246009 100644
--- a/packages/agent-loop/tests/review-fixes.spec.ts
+++ b/packages/agent-loop/tests/review-fixes.spec.ts
@@ -822,13 +822,15 @@ describe('P1-5: a started turn (and any open step) is always closed on a boundar
     expect(errorEmits).toHaveLength(0)
   })
 
-  it('a throw at the turn/start append (before turnStarted) is rethrown to the runLoop backstop', async () => {
-    // A session/event listener that throws specifically on the turn/start
-    // event makes session.append('turn/start') throw while turnStarted is
-    // still false. runTurn must NOT try to close a turn it never opened — it
-    // rethrows, and the runLoop backstop records the error and survives.
-    // (Uses the plain harness — NOT the invariants oracle — because the
-    // throwing listener is itself a session/event subscriber.)
+  it('a throwing session/event listener on the turn/start append still balances the turn', async () => {
+    // Session.append pushes the event BEFORE notifying session/event listeners,
+    // so a listener throwing on turn/start leaves turn/start IN THE LOG. The
+    // loop must therefore still owe (and append) a turn/end — deciding "owed"
+    // from the log via isTurnOpen, not a "turn started" flag that the throw
+    // skipped. Otherwise the turn stays permanently open and poisons the next
+    // turn/replay (ADR 0017). (Uses the plain harness — NOT the invariants
+    // oracle — because the throwing listener is itself a session/event
+    // subscriber.)
     const adapter = new MockAdapter([textResponse('turn 2')])
     const ctx = await harness(adapter)
     const agent = ctx.agentLoop.create('a-preturn', { model: 'mock' })
@@ -843,10 +845,18 @@ describe('P1-5: a started turn (and any open step) is always closed on a boundar
     send(agent, 'go')
     await waitForIdle(ctx, agent)
 
-    // The backstop logged exactly one error for the failed pre-turn append.
+    // The error was surfaced exactly once via agent/error.
     expect(errors.map(e => e.message)).toEqual(['boom turn/start append'])
-    // No turn/end was appended (none is owed — the turn never opened).
-    expect([...agent.session.events].some(e => e.type === 'turn/end')).toBe(false)
+    // The turn is BALANCED: turn/start is in the log (it was pushed before the
+    // listener threw), so a turn/end was owed and appended — no open turn. The
+    // last turn-boundary event being turn/end is exactly the loop's isTurnOpen
+    // check (no open turn remains).
+    const types = [...agent.session.events].map(e => e.type)
+    expect(types.filter(t => t === 'turn/start')).toHaveLength(1)
+    expect(types.filter(t => t === 'turn/end')).toHaveLength(1)
+    const lastBoundary = [...agent.session.events].reverse().find(e => e.type === 'turn/start' || e.type === 'turn/end')
+    expect(lastBoundary?.type).toBe('turn/end')
+    expect(agent.session.events.at(-1)?.type).toBe('turn/end')
 
     // loop survives: a second turn runs normally.
     send(agent, 'second')
diff --git a/packages/agent/src/types.ts b/packages/agent/src/types.ts
index d6adc4760e..cee1e02666 100644
--- a/packages/agent/src/types.ts
+++ b/packages/agent/src/types.ts
@@ -68,7 +68,9 @@ export interface Agent {
    * an inject while idle wraps its `context/message` in a one-shot `injection`
    * turn (`turn/start` → `context/message` → `turn/end`) and checkpoints it for
    * durability, so every event stays inside a turn and a persistence backend
-   * never loses a between-turn notice.
+   * never loses a between-turn notice. The idle checkpoint is fire-and-forget
+   * (inject is synchronous): a failing flush is reported via `agent/error`
+   * (step `0`) and the logger, never thrown into the caller.
    *
    * TODO(review): exact envelope/rendering rules live in dsh-session and need
    * review once a real adapter exists.
diff --git a/packages/invariants/tests/invariants.spec.ts b/packages/invariants/tests/invariants.spec.ts
index 13f908b451..b96d172c22 100644
--- a/packages/invariants/tests/invariants.spec.ts
+++ b/packages/invariants/tests/invariants.spec.ts
@@ -277,12 +277,15 @@ describe('dev-freeze', () => {
     // mutable. deepFreeze must descend into the already-frozen object and
     // freeze the descendant, not short-circuit on the frozen container —
     // otherwise dev-mode misses exactly the history mutation ADR 0012 catches.
+    // `append` snapshots `data`, so the freeze applies to the LOGGED clone, not
+    // the caller's input — read the event back and assert on its data.
     const innerContent: { type: 'text'; text: string }[] = [{ type: 'text', text: 'inner' }]
     const block = Object.freeze({ type: 'tool-result' as const, toolCallId: CallId('c1'), content: innerContent, isError: false })
-    session.append('user/message', { content: [block], source: { kind: 'user' } })
-    expect(Object.isFrozen(block.content)).toBe(true)
-    expect(Object.isFrozen(block.content[0])).toBe(true)
-    expect(() => { block.content.push({ type: 'text', text: 'mutation' }) }).toThrow()
+    const event = session.append('user/message', { content: [block], source: { kind: 'user' } })
+    const logged = event.data.content[0] as { content: { type: 'text'; text: string }[] }
+    expect(Object.isFrozen(logged.content)).toBe(true)
+    expect(Object.isFrozen(logged.content[0])).toBe(true)
+    expect(() => { logged.content.push({ type: 'text', text: 'mutation' }) }).toThrow()
   })
 
   it('terminates on a cyclic event datum (WeakSet guard)', async () => {
diff --git a/packages/session/README.md b/packages/session/README.md
index 705e3d5b39..7cf443fa71 100644
--- a/packages/session/README.md
+++ b/packages/session/README.md
@@ -8,7 +8,7 @@ Creates and holds event-sourced `Session` instances. Persistence is intentionall
 
 ### Public API
 
-- `ctx.sessions.create(id?: string, options?: { seed?: SessionEvent[]; meta?: { cwd?: string; parentSession?: SessionId } }): Session` — Create a session. `options.seed` replays/forks an existing event log; `options.meta` attaches creation metadata (validated absolute `cwd`, `parentSession` lineage) as the immutable `SessionHeader` (the store fills `version`/`id`/`createdAt`). Disposed with the calling fiber.
+- `ctx.sessions.create(id?: string, options?: { seed?: SessionEvent[]; meta?: { cwd?: string; parentSession?: SessionId; createdAt?: number } }): Session` — Create a session. `options.seed` replays/forks an existing event log; `options.meta` attaches creation metadata (validated absolute `cwd`, `parentSession` lineage) as the immutable `SessionHeader`. The store fills `version`/`id` and defaults `createdAt` to now; a caller reconstructing a persisted session passes the original `createdAt` to preserve it. Disposed with the calling fiber.
 - `ctx.sessions.get(id: string): Session | undefined`
 - `ctx.sessions.list(): Session[]`
 
diff --git a/packages/session/src/index.ts b/packages/session/src/index.ts
index 62cce89c7b..8fe54cb97b 100644
--- a/packages/session/src/index.ts
+++ b/packages/session/src/index.ts
@@ -90,7 +90,15 @@ export class Session {
           throw new Error(`seed event "${event.type}" (seq ${event.seq}) carries non-JSON-serializable data`)
         }
       })
-      this.log = [...seed]
+      // Deep-clone each seed event, NOT just the array: the seed events and
+      // their `data` are still owned by the caller (or the source session of a
+      // fork), so keeping the references would let a post-create mutation of the
+      // original rewrite this session's durable log — or reintroduce a
+      // non-JSON-serializable value AFTER the validation above. Snapshotting at
+      // the boundary makes `session.events` independent and keeps it equal to
+      // what was validated. Serializability is guaranteed by the check above, so
+      // structuredClone can never hit a non-cloneable value here.
+      this.log = seed.map(event => structuredClone(event))
     }
     this.header = header ?? { version: 1, id, createdAt: Date.now() }
   }
@@ -120,7 +128,16 @@ export class Session {
     if (!isJsonValue(data)) {
       throw new Error(`session event "${type}" carries non-JSON-serializable data`)
     }
-    const event = { type, seq: this.log.length, time: Date.now(), data } as SessionEvent<T>
+    // Snapshot `data` into the log, NOT the caller's reference: the validation
+    // above proves it is JSON-serializable AT THIS MOMENT, but the caller still
+    // owns the object and could mutate it afterwards (before a persistence
+    // flush, or permanently in the in-memory history) — making `session.events`
+    // diverge from the value that passed validation, or reintroducing a
+    // non-serializable value. Cloning here keeps the log equal to what was
+    // validated. structuredClone is safe because serializability was just
+    // checked. The returned event carries the SAME snapshot, so a caller reading
+    // back `event.data` sees the logged value, not its own mutable input.
+    const event = { type, seq: this.log.length, time: Date.now(), data: structuredClone(data) } as SessionEvent<T>
     this.log.push(event)
     this.onAppend?.(event)
     return event
diff --git a/packages/session/src/json.ts b/packages/session/src/json.ts
index ebb0c00c64..99fe80ea80 100644
--- a/packages/session/src/json.ts
+++ b/packages/session/src/json.ts
@@ -22,6 +22,14 @@
  * convert lossily. Sparse arrays are rejected too: a hole serializes to `null`,
  * so `[1, , 3]` would not round-trip. Detects circular references (which would
  * throw) and reports them as non-serializable rather than propagating the throw.
+ *
+ * Scope — matches `JSON.stringify` exactly: only an object's OWN ENUMERABLE
+ * STRING-keyed properties are inspected (`Object.values`). Symbol-keyed and
+ * non-enumerable properties are NOT examined, because `JSON.stringify` likewise
+ * drops them — they never reach the durable form, so a non-serializable value
+ * hiding under a symbol/non-enumerable key cannot make the round-trip lossy.
+ * Getters are invoked during the check (again as `JSON.stringify` would), so the
+ * contract is for plain data records, not objects with side-effecting accessors.
  */
 export function isJsonValue(value: unknown, seen: Set<object> = new Set()): boolean {
   if (value === null) return true
diff --git a/packages/session/tests/session.spec.ts b/packages/session/tests/session.spec.ts
index 39d29872ee..50dc9d5907 100644
--- a/packages/session/tests/session.spec.ts
+++ b/packages/session/tests/session.spec.ts
@@ -139,6 +139,39 @@ describe('Session', () => {
     const session = new Session(SessionId('seed-ok'), goodSeed)
     expect(session.events).toHaveLength(3)
   })
+
+  it('snapshots the seed: mutating the original after construction does not affect session.events', () => {
+    const seed = [
+      { type: 'turn/start' as const, seq: 0, time: 1, data: { turn: 1, trigger: { kind: 'message' as const, source: { kind: 'user' as const } } } },
+      { type: 'user/message' as const, seq: 1, time: 2, data: { content: [{ type: 'text' as const, text: 'original' }], source: { kind: 'user' as const } } },
+      { type: 'turn/end' as const, seq: 2, time: 3, data: { turn: 1, reason: { kind: 'completed' as const } } },
+    ] as SessionEvent[]
+    const session = new Session(SessionId('seed-snapshot'), seed)
+    // Mutate the ORIGINAL seed objects after construction: a shared reference
+    // would let this rewrite the forked log (or reintroduce non-serializable
+    // data past validation). The snapshot must shield session.events.
+    const um = seed[1]!
+    ;(um.data as { content: { type: 'text'; text: string }[] }).content[0]!.text = 'HACKED'
+    ;(um.data as Record<string, unknown>)['injected'] = 1n // would have failed validation
+    const logged = session.events[1]!
+    expect(logged.type === 'user/message' && (logged.data.content[0] as { text: string }).text).toBe('original')
+    expect((logged.data as Record<string, unknown>)['injected']).toBeUndefined()
+  })
+
+  it('snapshots append data: mutating the passed object after append does not affect session.events', () => {
+    const session = new Session(SessionId('append-snapshot'))
+    const data = { content: [{ type: 'text' as const, text: 'original' }], source: { kind: 'user' as const } }
+    const event = session.append('user/message', data)
+    // Mutate the caller's object after append returns. A shared reference would
+    // make session.events diverge from the value that passed validation.
+    data.content[0]!.text = 'HACKED'
+    ;(data as Record<string, unknown>)['injected'] = 1n
+    const logged = session.events[0]!
+    expect(logged.type === 'user/message' && (logged.data.content[0] as { text: string }).text).toBe('original')
+    expect((logged.data as Record<string, unknown>)['injected']).toBeUndefined()
+    // The returned event carries the same snapshot, not the caller's input.
+    expect((event.data.content[0] as { text: string }).text).toBe('original')
+  })
 })