diff --git a/src/main/native-chat/agent-session-journal/journal-store.test.ts b/src/main/native-chat/agent-session-journal/journal-store.test.ts
index 52603248426..15ecbf0ea6d 100644
--- a/src/main/native-chat/agent-session-journal/journal-store.test.ts
+++ b/src/main/native-chat/agent-session-journal/journal-store.test.ts
@@ -145,6 +145,22 @@ describe('replay', () => {
     expect(reopened.snapshot()).toEqual(live)
   })
 
+  it('refuses rows whose JSON form would change the live render model', async () => {
+    const journal = await open()
+    for (const input of [undefined, Number.NaN]) {
+      await expect(
+        journal.appendItem(
+          item(0),
+          { kind: 'tool-call', name: 'bash', input, state: 'running' },
+          { fence: 1 }
+        )
+      ).rejects.toMatchObject({ code: 'journal_invalid_row' })
+    }
+
+    await journal.appendItem(item(0), body('persisted'), { fence: 1 })
+    expect((await open()).snapshot()).toEqual(journal.snapshot())
+  })
+
   it('serves a resume from a cursor and refuses one from a stale epoch', async () => {
     const journal = await open()
     await journal.appendItem(item(0), body('a'), { fence: 1 })
diff --git a/src/main/native-chat/agent-session-journal/journal-store.ts b/src/main/native-chat/agent-session-journal/journal-store.ts
index f6881290d21..cc7ee6d8fc9 100644
--- a/src/main/native-chat/agent-session-journal/journal-store.ts
+++ b/src/main/native-chat/agent-session-journal/journal-store.ts
@@ -30,12 +30,7 @@ import {
   renderJournalState,
   type JournalReducerState
 } from './journal-reducer'
-import {
-  type AgentJournalEpochReason,
-  journalRowBase,
-  journalRowByteLength,
-  type JournalRow
-} from './journal-row-schema'
+import { type AgentJournalEpochReason, journalRowBase, type JournalRow } from './journal-row-schema'
 import type {
   AgentSessionJournalOptions,
   JournalAppendResult,
@@ -44,6 +39,7 @@ import type {
 } from './journal-store-contracts'
 import {
   assertJournalFence,
+  assertJournalRowPersistable,
   assertNewSubmission,
   assertJournalWritable,
   JournalAppendBudget
@@ -342,11 +338,12 @@ export class AgentSessionJournal {
       const ts = this.now()
       const row = build(this.state.lastSequence + 1, ts)
       assertJournalFence(row.fence, this.state.highestFence)
-      this.budget.assert(row, ts, this.sizeBytes)
+      const rowSizeBytes = assertJournalRowPersistable(row)
+      this.budget.assert(rowSizeBytes, ts, this.sizeBytes)
       await appendJournalRows(this.journalDir, [row])
       applyJournalRow(this.state, row)
       this.tailRows.push(row)
-      this.sizeBytes += journalRowByteLength(row)
+      this.sizeBytes += rowSizeBytes
       return row
     })
   }
diff --git a/src/main/native-chat/agent-session-journal/journal-write-guards.ts b/src/main/native-chat/agent-session-journal/journal-write-guards.ts
index 0274cecea6a..240455fc537 100644
--- a/src/main/native-chat/agent-session-journal/journal-write-guards.ts
+++ b/src/main/native-chat/agent-session-journal/journal-write-guards.ts
@@ -1,10 +1,11 @@
 // Guards an append clears before it becomes durable.
 //
-// All four refuse loudly rather than degrade: a silent drop here is a message
+// These guards refuse loudly rather than degrade: a silent drop here is a message
 // missing from the transcript with nothing to explain it.
 
+import { isDeepStrictEqual } from 'node:util'
 import type { JournalPayloadLimits } from './journal-payload-bounds'
-import { journalRowByteLength, type JournalRow } from './journal-row-schema'
+import { parseJournalRow, serializeJournalRow, type JournalRow } from './journal-row-schema'
 
 export class AgentSessionJournalError extends Error {
   constructor(
@@ -12,6 +13,7 @@ export class AgentSessionJournalError extends Error {
       | 'journal_read_only'
       | 'journal_stale_fence'
       | 'journal_duplicate_submission'
+      | 'journal_invalid_row'
       | 'journal_bound_exceeded'
       | 'journal_rate_exceeded',
     message: string
@@ -21,6 +23,25 @@ export class AgentSessionJournalError extends Error {
   }
 }
 
+/** Refuse values JSON would drop or coerce, keeping live state identical to replay. */
+export function assertJournalRowPersistable(row: JournalRow): number {
+  let serialized = ''
+  let parsed: ReturnType<typeof parseJournalRow>
+  try {
+    serialized = serializeJournalRow(row)
+    parsed = parseJournalRow(serialized)
+  } catch {
+    parsed = { ok: false, unreadable: false }
+  }
+  if (!parsed.ok || !isDeepStrictEqual(parsed.row, row)) {
+    throw new AgentSessionJournalError(
+      'journal_invalid_row',
+      `journal ${row.kind} row contains a value that cannot be persisted losslessly`
+    )
+  }
+  return Buffer.byteLength(serialized, 'utf8') + 1
+}
+
 export function assertNewSubmission(exists: boolean, clientMessageId: string): void {
   if (exists) {
     throw new AgentSessionJournalError(
@@ -62,8 +83,8 @@ export class JournalAppendBudget {
     private readonly limits: JournalPayloadLimits
   ) {}
 
-  assert(row: JournalRow, ts: number, sizeBytes: number): void {
-    if (sizeBytes + journalRowByteLength(row) > this.limits.maxSessionBytes) {
+  assert(rowSizeBytes: number, ts: number, sizeBytes: number): void {
+    if (sizeBytes + rowSizeBytes > this.limits.maxSessionBytes) {
       throw new AgentSessionJournalError(
         'journal_bound_exceeded',
         `agent-session journal for ${this.sessionId} reached its ${this.limits.maxSessionBytes}-byte bound`