diff --git a/src/memory/__tests__/knowledge-graph.test.ts b/src/memory/__tests__/knowledge-graph.test.ts
index 236242413a..16b3ca2ab4 100644
--- a/src/memory/__tests__/knowledge-graph.test.ts
+++ b/src/memory/__tests__/knowledge-graph.test.ts
@@ -515,4 +515,79 @@ describe('KnowledgeGraphManager', () => {
       expect(result.relations[0]).not.toHaveProperty('type');
     });
   });
-});
+
+  describe('concurrent mutations', () => {
+    // Regression test for #1819: concurrent tool calls each independently
+    // load the graph, mutate their own copy, and write it back. Without
+    // serialization, whichever write lands last silently discards the
+    // other's changes. All mutations below are fired without awaiting each
+    // other first, simulating multiple tool calls landing close together.
+
+    it('should not lose entities created concurrently', async () => {
+      const batch1: Entity[] = Array.from({ length: 10 }, (_, i) => ({
+        name: `batch1-entity-${i}`,
+        entityType: 'test',
+        observations: [],
+      }));
+      const batch2: Entity[] = Array.from({ length: 10 }, (_, i) => ({
+        name: `batch2-entity-${i}`,
+        entityType: 'test',
+        observations: [],
+      }));
+
+      // Fire both concurrently instead of awaiting sequentially.
+      await Promise.all([
+        manager.createEntities(batch1),
+        manager.createEntities(batch2),
+      ]);
+
+      const graph = await manager.readGraph();
+      expect(graph.entities).toHaveLength(20);
+      expect(graph.entities.map(e => e.name).sort()).toEqual(
+        [...batch1, ...batch2].map(e => e.name).sort()
+      );
+    });
+
+    it('should not lose relations created concurrently with entity creation', async () => {
+      await manager.createEntities([
+        { name: 'Alice', entityType: 'person', observations: [] },
+        { name: 'Bob', entityType: 'person', observations: [] },
+        { name: 'Carol', entityType: 'person', observations: [] },
+      ]);
+
+      await Promise.all([
+        manager.createRelations([{ from: 'Alice', to: 'Bob', relationType: 'knows' }]),
+        manager.createRelations([{ from: 'Bob', to: 'Carol', relationType: 'knows' }]),
+        manager.addObservations([
+          { entityName: 'Alice', contents: ['likes coffee'] },
+        ]),
+      ]);
+
+      const graph = await manager.readGraph();
+      expect(graph.relations).toHaveLength(2);
+      expect(graph.entities.find(e => e.name === 'Alice')?.observations).toContain('likes coffee');
+    });
+
+    it('should keep the file valid JSONL after many concurrent mutations', async () => {
+      const operations = Array.from({ length: 25 }, (_, i) =>
+        manager.createEntities([
+          { name: `stress-entity-${i}`, entityType: 'test', observations: [] },
+        ])
+      );
+
+      await Promise.all(operations);
+
+      const raw = await fs.readFile(testFilePath, 'utf-8');
+      const lines = raw.split('\n').filter(line => line.trim() !== '');
+
+      // Every line must parse as valid JSON; a corrupted interleaved write
+      // would produce a truncated or malformed line here.
+      for (const line of lines) {
+        expect(() => JSON.parse(line)).not.toThrow();
+      }
+
+      const graph = await manager.readGraph();
+      expect(graph.entities).toHaveLength(25);
+    });
+  });
+});
\ No newline at end of file
diff --git a/src/memory/index.ts b/src/memory/index.ts
index 9865c5318e..74f2cc1b91 100644
--- a/src/memory/index.ts
+++ b/src/memory/index.ts
@@ -69,6 +69,26 @@ export interface KnowledgeGraph {
 export class KnowledgeGraphManager {
   constructor(private memoryFilePath: string) {}
 
+  // Serializes all read-modify-write graph mutations behind a single queue.
+  // Without this, concurrent tool calls (e.g. multiple mutations dispatched
+  // from one LLM turn) each independently load the graph, mutate their own
+  // copy, and write it back — so whichever write lands last silently
+  // overwrites the other's changes, and interleaved writes to the same file
+  // can corrupt it outright. See #1819.
+  private mutationQueue: Promise<unknown> = Promise.resolve();
+
+  private async withLock<T>(operation: () => Promise<T>): Promise<T> {
+    const result = this.mutationQueue.then(operation, operation);
+    // Always resolve the queue itself, even if this operation failed, so a
+    // single failed mutation doesn't permanently wedge every call after it.
+    // The failure still propagates normally to whoever awaited `result`.
+    this.mutationQueue = result.then(
+      () => undefined,
+      () => undefined,
+    );
+    return result;
+  }
+
   private async loadGraph(): Promise<KnowledgeGraph> {
     try {
       const data = await fs.readFile(this.memoryFilePath, "utf-8");
@@ -118,66 +138,78 @@ export class KnowledgeGraphManager {
   }
 
   async createEntities(entities: Entity[]): Promise<Entity[]> {
-    const graph = await this.loadGraph();
-    const newEntities = entities.filter(e => !graph.entities.some(existingEntity => existingEntity.name === e.name));
-    graph.entities.push(...newEntities);
-    await this.saveGraph(graph);
-    return newEntities;
+    return this.withLock(async () => {
+      const graph = await this.loadGraph();
+      const newEntities = entities.filter(e => !graph.entities.some(existingEntity => existingEntity.name === e.name));
+      graph.entities.push(...newEntities);
+      await this.saveGraph(graph);
+      return newEntities;
+    });
   }
 
   async createRelations(relations: Relation[]): Promise<Relation[]> {
-    const graph = await this.loadGraph();
-    const newRelations = relations.filter(r => !graph.relations.some(existingRelation => 
-      existingRelation.from === r.from && 
-      existingRelation.to === r.to && 
-      existingRelation.relationType === r.relationType
-    ));
-    graph.relations.push(...newRelations);
-    await this.saveGraph(graph);
-    return newRelations;
+    return this.withLock(async () => {
+      const graph = await this.loadGraph();
+      const newRelations = relations.filter(r => !graph.relations.some(existingRelation => 
+        existingRelation.from === r.from && 
+        existingRelation.to === r.to && 
+        existingRelation.relationType === r.relationType
+      ));
+      graph.relations.push(...newRelations);
+      await this.saveGraph(graph);
+      return newRelations;
+    });
   }
 
   async addObservations(observations: { entityName: string; contents: string[] }[]): Promise<{ entityName: string; addedObservations: string[] }[]> {
-    const graph = await this.loadGraph();
-    const results = observations.map(o => {
-      const entity = graph.entities.find(e => e.name === o.entityName);
-      if (!entity) {
-        throw new Error(`Entity with name ${o.entityName} not found`);
-      }
-      const newObservations = o.contents.filter(content => !entity.observations.includes(content));
-      entity.observations.push(...newObservations);
-      return { entityName: o.entityName, addedObservations: newObservations };
+    return this.withLock(async () => {
+      const graph = await this.loadGraph();
+      const results = observations.map(o => {
+        const entity = graph.entities.find(e => e.name === o.entityName);
+        if (!entity) {
+          throw new Error(`Entity with name ${o.entityName} not found`);
+        }
+        const newObservations = o.contents.filter(content => !entity.observations.includes(content));
+        entity.observations.push(...newObservations);
+        return { entityName: o.entityName, addedObservations: newObservations };
+      });
+      await this.saveGraph(graph);
+      return results;
     });
-    await this.saveGraph(graph);
-    return results;
   }
 
   async deleteEntities(entityNames: string[]): Promise<void> {
-    const graph = await this.loadGraph();
-    graph.entities = graph.entities.filter(e => !entityNames.includes(e.name));
-    graph.relations = graph.relations.filter(r => !entityNames.includes(r.from) && !entityNames.includes(r.to));
-    await this.saveGraph(graph);
+    return this.withLock(async () => {
+      const graph = await this.loadGraph();
+      graph.entities = graph.entities.filter(e => !entityNames.includes(e.name));
+      graph.relations = graph.relations.filter(r => !entityNames.includes(r.from) && !entityNames.includes(r.to));
+      await this.saveGraph(graph);
+    });
   }
 
   async deleteObservations(deletions: { entityName: string; observations: string[] }[]): Promise<void> {
-    const graph = await this.loadGraph();
-    deletions.forEach(d => {
-      const entity = graph.entities.find(e => e.name === d.entityName);
-      if (entity) {
-        entity.observations = entity.observations.filter(o => !d.observations.includes(o));
-      }
+    return this.withLock(async () => {
+      const graph = await this.loadGraph();
+      deletions.forEach(d => {
+        const entity = graph.entities.find(e => e.name === d.entityName);
+        if (entity) {
+          entity.observations = entity.observations.filter(o => !d.observations.includes(o));
+        }
+      });
+      await this.saveGraph(graph);
     });
-    await this.saveGraph(graph);
   }
 
   async deleteRelations(relations: Relation[]): Promise<void> {
-    const graph = await this.loadGraph();
-    graph.relations = graph.relations.filter(r => !relations.some(delRelation => 
-      r.from === delRelation.from && 
-      r.to === delRelation.to && 
-      r.relationType === delRelation.relationType
-    ));
-    await this.saveGraph(graph);
+    return this.withLock(async () => {
+      const graph = await this.loadGraph();
+      graph.relations = graph.relations.filter(r => !relations.some(delRelation => 
+        r.from === delRelation.from && 
+        r.to === delRelation.to && 
+        r.relationType === delRelation.relationType
+      ));
+      await this.saveGraph(graph);
+    });
   }
 
   async readGraph(): Promise<KnowledgeGraph> {
@@ -599,4 +631,4 @@ async function main() {
 main().catch((error) => {
   console.error("Fatal error in main():", error);
   process.exit(1);
-});
+});
\ No newline at end of file