diff --git a/packages/session-persistence-jsonl/src/index.ts b/packages/session-persistence-jsonl/src/index.ts
index f68dde5a0c..7a0c97637c 100644
--- a/packages/session-persistence-jsonl/src/index.ts
+++ b/packages/session-persistence-jsonl/src/index.ts
@@ -25,8 +25,10 @@ import z from 'schemastery'
 import { open, mkdir, readFile, readdir, rename, link, rm, truncate } from 'node:fs/promises'
 import { dirname, resolve } from 'node:path'
 import { randomBytes } from 'node:crypto'
-import { SessionPersistence } from '@deepseek-ai/dsh-session-persistence'
-import { isJsonValue, interruptedTurnClosers } from '@deepseek-ai/dsh-session'
+import {
+  SessionPersistence, assertSerializable, seedCoversPrefix,
+} from '@deepseek-ai/dsh-session-persistence'
+import { interruptedTurnClosers } from '@deepseek-ai/dsh-session'
 import type { Session, SessionEvent, SessionId, SessionMeta, SessionSummary } from '@deepseek-ai/dsh-session'
 import {
   encodeSegment, eventLine, logPath, parseHeaderMeta, scanLog, sessionDir, sidecarPath, toHeaderLine,
@@ -59,44 +61,6 @@ interface SessionState {
   owner?: Session
 }
 
-/**
- * Whether a live session's `seed` reproduces a persisted `prefix` exactly — the
- * prefix is no longer than the seed, and each prefix event DEEP-equals the seed
- * event at the same index. Used to tell a session legitimately continuing a
- * persisted log (HMR re-seeing its own session, or a resume) from a different
- * session that merely reuses the id: the latter would have its already-counted
- * seq 0..prefix-1 events filtered out on flush and its conversation silently
- * grafted onto the old log.
- *
- * The comparison is a full structural equality (via canonical JSON) of each
- * event INCLUDING its `data` payload, not just `seq`/`type`/`time` — a session
- * built from loaded events but with mutated message/tool payloads (same seq/
- * type/time) must NOT be accepted, or the live history and durable log diverge.
- * Both sides are JSON-serializable by contract (Session.append enforces it), so
- * JSON.stringify is a sound canonical form here.
- */
-function seedCoversPrefix(seed: readonly SessionEvent[], prefix: readonly SessionEvent[]): boolean {
-  return prefix.length <= seed.length
-    && prefix.every((e, i) => {
-      const s = seed[i]
-      return s !== undefined && JSON.stringify(s) === JSON.stringify(e)
-    })
-}
-
-/**
- * Reject non-JSON-serializable `event.data`, naming the offending type. Used on
- * the backend's `append(events)` entry point (replay/fork paths that bypass a
- * live `Session`); events that flow through `Session.append` are already
- * validated at the source, so the live write path never needs this.
- */
-function assertSerializable(events: readonly SessionEvent[]): void {
-  for (const event of events) {
-    if (!isJsonValue(event.data)) {
-      throw new Error(`event "${event.type}" carries non-JSON-serializable data (seq ${event.seq})`)
-    }
-  }
-}
-
 /**
  * Whether `error` is a "no such file/directory" (`ENOENT`) failure — the ONLY
  * filesystem error that legitimately means "this session/root is absent" for a
diff --git a/packages/session-persistence-sqlite/src/index.ts b/packages/session-persistence-sqlite/src/index.ts
index 7f59e3dd8a..d28fc8d6f7 100644
--- a/packages/session-persistence-sqlite/src/index.ts
+++ b/packages/session-persistence-sqlite/src/index.ts
@@ -24,8 +24,10 @@ import z from 'schemastery'
 import { DatabaseSync } from 'node:sqlite'
 import { mkdir } from 'node:fs/promises'
 import { dirname, resolve } from 'node:path'
-import { SessionPersistence } from '@deepseek-ai/dsh-session-persistence'
-import { isJsonValue, interruptedTurnClosers } from '@deepseek-ai/dsh-session'
+import {
+  SessionPersistence, assertSerializable, seedCoversPrefix,
+} from '@deepseek-ai/dsh-session-persistence'
+import { interruptedTurnClosers } from '@deepseek-ai/dsh-session'
 import type { Session, SessionEvent, SessionId, SessionMeta, SessionSummary } from '@deepseek-ai/dsh-session'
 import {
   openDatabase, rowToMeta, scanRows, type EventRow, type SessionRow,
@@ -54,32 +56,6 @@ interface SessionState {
   owner?: Session
 }
 
-/**
- * Whether a live session's `seed` reproduces a persisted `prefix` exactly (the
- * prefix is no longer than the seed and each event DEEP-equals the seed event
- * at the same index). Distinguishes a session legitimately continuing a
- * persisted log (HMR re-seeing its own session, or a resume) from a different
- * session that merely reuses the id. Mirrors the JSONL backend's check; both
- * sides are JSON-serializable by contract, so `JSON.stringify` is a sound
- * canonical form.
- */
-function seedCoversPrefix(seed: readonly SessionEvent[], prefix: readonly SessionEvent[]): boolean {
-  return prefix.length <= seed.length
-    && prefix.every((e, i) => {
-      const s = seed[i]
-      return s !== undefined && JSON.stringify(s) === JSON.stringify(e)
-    })
-}
-
-/** Reject non-JSON-serializable `event.data`, naming the offending type. */
-function assertSerializable(events: readonly SessionEvent[]): void {
-  for (const event of events) {
-    if (!isJsonValue(event.data)) {
-      throw new Error(`event "${event.type}" carries non-JSON-serializable data (seq ${event.seq})`)
-    }
-  }
-}
-
 async function settledErrors(promises: Iterable<Promise<unknown>>): Promise<unknown[]> {
   const settled = await Promise.allSettled([...promises])
   const errors: unknown[] = []
diff --git a/packages/session-persistence/src/index.ts b/packages/session-persistence/src/index.ts
index 02e62940d7..0ce1e25146 100644
--- a/packages/session-persistence/src/index.ts
+++ b/packages/session-persistence/src/index.ts
@@ -22,6 +22,7 @@
  */
 
 import { Context, Service } from 'cordis'
+import { isJsonValue } from '@deepseek-ai/dsh-session'
 import type { SessionEvent, SessionId, SessionMeta, SessionSummary } from '@deepseek-ai/dsh-session'
 
 // Re-export the metadata vocabulary so consumers import it from the seam.
@@ -33,6 +34,35 @@ declare module 'cordis' {
   }
 }
 
+/**
+ * Whether a live session's seed reproduces a persisted prefix exactly. Backends
+ * use this collision check to distinguish a legitimate resume/HMR rebind from a
+ * different live session reusing an existing session id.
+ *
+ * The comparison includes the full event payload, not just seq/type/time, so a
+ * mutated seed cannot be grafted onto a durable log with the same envelope.
+ */
+export function seedCoversPrefix(seed: readonly SessionEvent[], prefix: readonly SessionEvent[]): boolean {
+  return prefix.length <= seed.length
+    && prefix.every((event, index) => {
+      const seedEvent = seed[index]
+      return seedEvent !== undefined && JSON.stringify(seedEvent) === JSON.stringify(event)
+    })
+}
+
+/**
+ * Reject non-JSON-serializable event data before a backend serializes a batch.
+ * Live session appends already enforce this; persistence append paths also
+ * accept replay/fork batches that may bypass a live session instance.
+ */
+export function assertSerializable(events: readonly SessionEvent[]): void {
+  for (const event of events) {
+    if (!isJsonValue(event.data)) {
+      throw new Error(`event "${event.type}" carries non-JSON-serializable data (seq ${event.seq})`)
+    }
+  }
+}
+
 /**
  * Abstract durable session-persistence service. Subclass, implement the
  * abstract methods, and load the subclass as a plugin — it registers as
diff --git a/packages/session-persistence/tests/persistence.spec.ts b/packages/session-persistence/tests/persistence.spec.ts
index 32f97a30b5..5c0a29131f 100644
--- a/packages/session-persistence/tests/persistence.spec.ts
+++ b/packages/session-persistence/tests/persistence.spec.ts
@@ -2,7 +2,7 @@ import { describe, expect, it } from 'vitest'
 import { Context } from 'cordis'
 import { SessionId, isJsonValue, interruptedTurnClosers } from '@deepseek-ai/dsh-session'
 import type { SessionEvent, SessionMeta, SessionSummary } from '@deepseek-ai/dsh-session'
-import { SessionPersistence } from '../src/index.ts'
+import { SessionPersistence, assertSerializable, seedCoversPrefix } from '../src/index.ts'
 import { runPersistenceContract, meta, oneTurnLog } from './contract.ts'
 
 /**
@@ -104,3 +104,38 @@ describe('SessionPersistence service registration', () => {
     await fiber.dispose()
   })
 })
+
+describe('shared persistence helpers', () => {
+  it('accepts a seed that reproduces the persisted prefix exactly', () => {
+    const log = oneTurnLog()
+    expect(seedCoversPrefix(log, log.slice(0, 3))).toBe(true)
+    expect(seedCoversPrefix(log, [])).toBe(true)
+  })
+
+  it('rejects a prefix longer than the seed', () => {
+    const log = oneTurnLog()
+    expect(seedCoversPrefix(log.slice(0, 2), log)).toBe(false)
+  })
+
+  it('rejects a same-envelope event with mutated data', () => {
+    const log = oneTurnLog()
+    const tampered = structuredClone(log)
+    const event = tampered[1]!
+    tampered[1] = {
+      ...event,
+      data: { ...event.data, content: [{ type: 'text', text: 'tampered' }] },
+    } as SessionEvent
+    expect(seedCoversPrefix(tampered, log.slice(0, 2))).toBe(false)
+  })
+
+  it('accepts JSON-serializable event data', () => {
+    expect(() => { assertSerializable(oneTurnLog()) }).not.toThrow()
+  })
+
+  it('rejects non-JSON-serializable event data with type and seq context', () => {
+    const bad = [
+      { type: 'user/message', seq: 0, time: 1, data: { content: 1n } },
+    ] as unknown as SessionEvent[]
+    expect(() => { assertSerializable(bad) }).toThrow(/"user\/message".*seq 0/)
+  })
+})