diff --git a/packages/session-persistence/session-persistence/src/coordinator.ts b/packages/session-persistence/session-persistence/src/coordinator.ts
index 25ffad5f93..457b19eaa6 100644
--- a/packages/session-persistence/session-persistence/src/coordinator.ts
+++ b/packages/session-persistence/session-persistence/src/coordinator.ts
@@ -117,7 +117,10 @@ async function settledErrors(promises: Iterable<Promise<unknown>>): Promise<unkn
  * Reject a stored/live pair whose immutable policy baselines differ.
  * Adoption and ownerless claims retain the STORED header, so accepting a
  * conflicting pair would let a session run under its live baseline now but
- * resume under the stored one later — a silent policy swap.
+ * resume under the stored one later — a silent policy swap. When either side
+ * carries a baseline, the seed boundary is part of the policy identity too:
+ * `overrideOf()` folds own switches past `seedLength`, so differing
+ * boundaries make the same log resolve different policies across a restart.
  */
 function assertSamePolicyBaselines(id: SessionId, stored: SessionHeader, live: SessionHeader): void {
   if (stored.sandboxMode !== live.sandboxMode || stored.approvalPolicy !== live.approvalPolicy) {
@@ -127,6 +130,13 @@ function assertSamePolicyBaselines(id: SessionId, stored: SessionHeader, live: S
       + `live: ${String(live.sandboxMode)}/${String(live.approvalPolicy)}) (id collision)`,
     )
   }
+  const hasBaseline = stored.sandboxMode !== undefined || stored.approvalPolicy !== undefined
+  if (hasBaseline && (stored.seedLength ?? 0) !== (live.seedLength ?? 0)) {
+    throw new Error(
+      `session "${id}" is already persisted with a different policy seed boundary `
+      + `(persisted seedLength: ${String(stored.seedLength)}, live: ${String(live.seedLength)}) (id collision)`,
+    )
+  }
 }
 
 /** Whether a live session seed reproduces a persisted prefix exactly. */
diff --git a/packages/session-persistence/session-persistence/tests/coordinator-contract.ts b/packages/session-persistence/session-persistence/tests/coordinator-contract.ts
index 4bbb8aa471..a82ef525ec 100644
--- a/packages/session-persistence/session-persistence/tests/coordinator-contract.ts
+++ b/packages/session-persistence/session-persistence/tests/coordinator-contract.ts
@@ -700,6 +700,34 @@ export function runCoordinatorContract(name: string, makeFixture: () => Promise<
       }
     })
 
+    it('a live session with a DIFFERENT seed boundary cannot adopt a stored prefix when a baseline exists', async () => {
+      const fix = await makeFixture()
+      const { ctx, fiber } = await freshCtx(fix)
+      try {
+        // Same wide baseline both sides, but the stored header says event 0 is
+        // seed-carried (seedLength 1) while the live header says it is the
+        // session's OWN (seedLength 0). overrideOf() resolves policy through
+        // that boundary: a read-only switch at event 0 tightens the live
+        // session, yet a restart resumes under the stored header and the wide
+        // baseline silently returns. The boundary is part of the policy
+        // identity whenever a baseline exists.
+        await ctx.sessionPersistence.create({
+          ...meta('seed-boundary-conflict', WORK),
+          sandboxMode: 'danger-full-access',
+          seedLength: 1,
+        })
+        await ctx.sessionPersistence.append(SessionId('seed-boundary-conflict'), oneTurnLog())
+        const live = ctx.sessions.create(SessionId('seed-boundary-conflict'), {
+          seed: oneTurnLog(),
+          meta: { cwd: WORK, sandboxMode: 'danger-full-access' },
+        })
+        await expect(ctx.sessions.flush(live)).rejects.toThrow(/seed boundary|id collision/)
+      } finally {
+        await fiber.dispose()
+        await fix.cleanup()
+      }
+    })
+
     it('a no-cwd ownerless state cannot be claimed by a live session WITH a cwd (cwd scope, undefined side)', async () => {
       const fix = await makeFixture()
       const { ctx, fiber } = await freshCtx(fix)