diff --git a/src/session.ts b/src/session.ts
index e2e85bd..87075d9 100644
--- a/src/session.ts
+++ b/src/session.ts
@@ -34,6 +34,7 @@ export class StanddownSession {
   readonly #maxAuditEntries: number;
   readonly #selfExemptionScope: 'policy' | 'session';
   readonly #readOnlyAuditLog: AuditEntry[] = [];
+  #stateLock: Promise<unknown> = Promise.resolve();
 
   constructor(
     store: StateStore,
@@ -211,11 +212,33 @@ export class StanddownSession {
   }
 
   async exportAuditLog(): Promise<AuditEntry[]> {
-    const state = await this.#loadState();
-    return trimAuditLog(
-      [...state.auditLog, ...this.#readOnlyAuditLog],
-      this.#maxAuditEntries,
-    ).map(cloneAuditEntry);
+    return this.#serialize(async () => {
+      const state = await this.#loadState();
+      return trimAuditLog(
+        [...state.auditLog, ...this.#readOnlyAuditLog],
+        this.#maxAuditEntries,
+      ).map(cloneAuditEntry);
+    });
+  }
+
+  /**
+   * Serializes every state read-modify-write onto a single FIFO chain.
+   * #withState and #failClosedWithAudit each do an async load -> mutate -> save;
+   * without this, two overlapping evaluations both load the pre-write snapshot
+   * and the second save clobbers the first, so a just-recorded stand-down can be
+   * lost. The adapter's own hooks coalesce via a `pending` flag, but external
+   * callers driving evaluate()/ingest() concurrently are not otherwise
+   * serialized, so this lock is what makes them safe. It also gives
+   * read-after-write consistency: a shouldStandDown queued after an ingest reads
+   * that ingest's committed state.
+   */
+  #serialize<T>(task: () => Promise<T>): Promise<T> {
+    const run = this.#stateLock.then(task, task);
+    this.#stateLock = run.then(
+      () => undefined,
+      () => undefined,
+    );
+    return run;
   }
 
   async #withState(
@@ -228,44 +251,46 @@ export class StanddownSession {
     advertiserHost?: string,
     opts: { persist?: boolean } = {},
   ): Promise<Decision> {
-    let state: StanddownState;
+    return this.#serialize(async () => {
+      let state: StanddownState;
 
-    try {
-      state = await this.#loadState();
-    } catch {
-      return failClosedDecision('store-error');
-    }
+      try {
+        state = await this.#loadState();
+      } catch {
+        return failClosedDecision('store-error');
+      }
 
-    const result = fn(state);
+      const result = fn(state);
 
-    if (this.#auditLog) {
-      const entry = auditEntry({
-        time: now,
-        action,
-        advertiserHost:
-          advertiserHost ?? result.detection?.strongest?.advertiserHost,
-        detection: result.detection,
-        decision: result.decision,
-      });
+      if (this.#auditLog) {
+        const entry = auditEntry({
+          time: now,
+          action,
+          advertiserHost:
+            advertiserHost ?? result.detection?.strongest?.advertiserHost,
+          detection: result.detection,
+          decision: result.decision,
+        });
+
+        if (opts.persist === false) {
+          appendAuditEntry(this.#readOnlyAuditLog, entry, this.#maxAuditEntries);
+        } else {
+          appendAuditEntry(state.auditLog, entry, this.#maxAuditEntries);
+        }
+      }
 
       if (opts.persist === false) {
-        appendAuditEntry(this.#readOnlyAuditLog, entry, this.#maxAuditEntries);
-      } else {
-        appendAuditEntry(state.auditLog, entry, this.#maxAuditEntries);
+        return result.decision;
       }
-    }
-
-    if (opts.persist === false) {
-      return result.decision;
-    }
 
-    try {
-      await this.#store.save(state);
-    } catch {
-      return failClosedDecision('store-error');
-    }
+      try {
+        await this.#store.save(state);
+      } catch {
+        return failClosedDecision('store-error');
+      }
 
-    return result.decision;
+      return result.decision;
+    });
   }
 
   async #failClosedWithAudit(
@@ -281,25 +306,27 @@ export class StanddownSession {
       return decision;
     }
 
-    try {
-      const state = await this.#loadState();
-      appendAuditEntry(
-        state.auditLog,
-        auditEntry({
-          time: now,
-          action,
-          advertiserHost,
-          detection,
-          decision,
-        }),
-        this.#maxAuditEntries,
-      );
-      await this.#store.save(state);
-    } catch {
-      return failClosedDecision('store-error');
-    }
+    return this.#serialize(async () => {
+      try {
+        const state = await this.#loadState();
+        appendAuditEntry(
+          state.auditLog,
+          auditEntry({
+            time: now,
+            action,
+            advertiserHost,
+            detection,
+            decision,
+          }),
+          this.#maxAuditEntries,
+        );
+        await this.#store.save(state);
+      } catch {
+        return failClosedDecision('store-error');
+      }
 
-    return decision;
+      return decision;
+    });
   }
 
   async #loadState(): Promise<StanddownState> {
diff --git a/tests/session.test.ts b/tests/session.test.ts
index 717647b..d6c7141 100644
--- a/tests/session.test.ts
+++ b/tests/session.test.ts
@@ -415,6 +415,33 @@ describe('StanddownSession', () => {
     expect(decision.standDown).toBe(true);
     expect(decision.degraded).toBeUndefined();
   });
+
+  it('serializes concurrent ingests so neither loses the other write', async () => {
+    // A store that yields on every load/save, forcing two overlapping
+    // evaluations to interleave. Without the session's serialization both would
+    // load the same pre-write snapshot and the second save would clobber the
+    // first, dropping one host's stand-down record.
+    const store = new DelayingStore();
+    const session = new StanddownSession(store);
+
+    const [a, b] = await Promise.all([
+      session.ingest(
+        { url: 'https://merchant-a.example/?cjevent=aaa', now: 0 },
+        [cjPolicy],
+      ),
+      session.ingest(
+        { url: 'https://merchant-b.example/?cjevent=bbb', now: 0 },
+        [cjPolicy],
+      ),
+    ]);
+
+    expect(a).toMatchObject({ standDown: true, policyId: 'cj' });
+    expect(b).toMatchObject({ standDown: true, policyId: 'cj' });
+
+    const state = await store.load();
+    expect(state?.sessions['merchant-a.example']).toBeDefined();
+    expect(state?.sessions['merchant-b.example']).toBeDefined();
+  });
 });
 
 class FailingLoadStore implements StateStore {
@@ -437,6 +464,24 @@ class FailingSaveStore implements StateStore {
   }
 }
 
+class DelayingStore implements StateStore {
+  readonly #inner: MemoryStateStore;
+
+  constructor(initialState?: StanddownState) {
+    this.#inner = new MemoryStateStore(initialState);
+  }
+
+  async load(): Promise<StanddownState | undefined> {
+    await Promise.resolve();
+    return this.#inner.load();
+  }
+
+  async save(state: StanddownState): Promise<void> {
+    await Promise.resolve();
+    return this.#inner.save(state);
+  }
+}
+
 class CountingStore implements StateStore {
   readonly #inner: MemoryStateStore;
   saveCount = 0;