diff --git a/src/intents/intents.controller.ts b/src/intents/intents.controller.ts
index 6d3296d..cdebe48 100644
--- a/src/intents/intents.controller.ts
+++ b/src/intents/intents.controller.ts
@@ -100,9 +100,6 @@ export class IntentsController {
   accept(@Param("id") id: string, @Body() dto: AcceptIntentDto) {
     const intent = this.intentsService.get(id);
     if (!intent) throw new NotFoundException("Intent not found");
-    if (intent.state !== "open") {
-      throw new ConflictException(`Intent is ${intent.state}, cannot accept`);
-    }
 
     const now = Math.floor(Date.now() / 1000);
     if (intent.deadline <= now) {
@@ -115,11 +112,12 @@ export class IntentsController {
       throw new ForbiddenException("Solver not registered or inactive");
     }
 
-    const updated = this.intentsService.update(id, {
-      state: "accepted",
-      solver: dto.solver,
-      deadline: now + 300, // 5-minute fill window
-    });
+    const updated = this.intentsService.acceptIfOpen(id, dto.solver);
+    if (!updated) {
+      const current = this.intentsService.get(id);
+      throw new ConflictException(`Intent is ${current?.state ?? "unknown"}, cannot accept`);
+    }
+
     this.intentsGateway.broadcast({ type: "intent_accepted", intentId: id, solver: dto.solver });
     return updated;
   }
@@ -128,12 +126,6 @@ export class IntentsController {
   fill(@Param("id") id: string, @Body() dto: FillIntentDto) {
     const intent = this.intentsService.get(id);
     if (!intent) throw new NotFoundException("Intent not found");
-    if (intent.state !== "accepted") {
-      throw new ConflictException(`Intent is ${intent.state}, cannot fill`);
-    }
-    if (intent.solver !== dto.solver) {
-      throw new ForbiddenException("Wrong solver for this intent");
-    }
 
     const now = Math.floor(Date.now() / 1000);
     if (intent.deadline <= now) {
@@ -150,12 +142,19 @@ export class IntentsController {
       });
     }
 
-    const updated = this.intentsService.update(id, {
-      state: "filled",
+    const updated = this.intentsService.fillIfAccepted(id, dto.solver, {
       filledAt: now,
       fillAmount: dto.fillAmount,
       txHash: dto.txHash,
     });
+    if (!updated) {
+      const current = this.intentsService.get(id);
+      if (current?.solver !== dto.solver) {
+        throw new ForbiddenException("Wrong solver for this intent");
+      }
+      throw new ConflictException(`Intent is ${current?.state ?? "unknown"}, cannot fill`);
+    }
+
     this.intentsGateway.broadcast({
       type: "intent_filled",
       intentId: id,
diff --git a/src/intents/intents.service.spec.ts b/src/intents/intents.service.spec.ts
index 7bbba6d..f37e0fb 100644
--- a/src/intents/intents.service.spec.ts
+++ b/src/intents/intents.service.spec.ts
@@ -66,4 +66,78 @@ describe("IntentsService", () => {
       expect(intent.state).toBe("filled");
     }
   });
+
+  describe("acceptIfOpen", () => {
+    it("transitions an open intent to accepted and returns it", () => {
+      const [open] = service.getByState("open");
+      const result = service.acceptIfOpen(open.intentId, "SOLVER_X");
+
+      expect(result).not.toBeNull();
+      expect(result!.state).toBe("accepted");
+      expect(result!.solver).toBe("SOLVER_X");
+      expect(service.get(open.intentId)!.state).toBe("accepted");
+    });
+
+    it("returns null for a non-existent intent", () => {
+      expect(service.acceptIfOpen("does-not-exist", "SOLVER_X")).toBeNull();
+    });
+
+    it("returns null when the intent is already accepted", () => {
+      const [accepted] = service.getByState("accepted");
+      expect(service.acceptIfOpen(accepted.intentId, "SOLVER_X")).toBeNull();
+    });
+
+    it("only the first caller wins under simulated concurrency", () => {
+      const [open] = service.getByState("open");
+      const results = Array.from({ length: 10 }, (_, i) =>
+        service.acceptIfOpen(open.intentId, `SOLVER_${i}`),
+      );
+
+      const successes = results.filter((r) => r !== null);
+      expect(successes).toHaveLength(1);
+      expect(successes[0]!.state).toBe("accepted");
+    });
+  });
+
+  describe("fillIfAccepted", () => {
+    it("transitions an accepted intent to filled when solver matches", () => {
+      const [accepted] = service.getByState("accepted");
+      const result = service.fillIfAccepted(accepted.intentId, accepted.solver!, {
+        fillAmount: "100",
+        txHash: "test-hash",
+        filledAt: Math.floor(Date.now() / 1000),
+      });
+
+      expect(result).not.toBeNull();
+      expect(result!.state).toBe("filled");
+      expect(result!.fillAmount).toBe("100");
+    });
+
+    it("returns null when solver does not match", () => {
+      const [accepted] = service.getByState("accepted");
+      const result = service.fillIfAccepted(accepted.intentId, "WRONG_SOLVER", {
+        fillAmount: "100",
+      });
+      expect(result).toBeNull();
+    });
+
+    it("returns null for a non-existent intent", () => {
+      expect(service.fillIfAccepted("nope", "SOLVER_X", {})).toBeNull();
+    });
+
+    it("only the first caller wins under simulated concurrency", () => {
+      const [accepted] = service.getByState("accepted");
+      const results = Array.from({ length: 10 }, () =>
+        service.fillIfAccepted(accepted.intentId, accepted.solver!, {
+          fillAmount: "100",
+          txHash: "race-hash",
+          filledAt: Math.floor(Date.now() / 1000),
+        }),
+      );
+
+      const successes = results.filter((r) => r !== null);
+      expect(successes).toHaveLength(1);
+      expect(successes[0]!.state).toBe("filled");
+    });
+  });
 });
diff --git a/src/intents/intents.service.ts b/src/intents/intents.service.ts
index 86ed406..fe441b7 100644
--- a/src/intents/intents.service.ts
+++ b/src/intents/intents.service.ts
@@ -48,6 +48,41 @@ export class IntentsService {
     return updated;
   }
 
+  /**
+   * Atomically accept an intent only if it is currently "open".
+   * Mirrors the DB pattern: UPDATE intents SET state='accepted' WHERE id=$1 AND state='open' RETURNING *
+   * Returns null when the intent is not found or is not in the "open" state (already taken).
+   */
+  acceptIfOpen(id: string, solver: string): Intent | null {
+    const existing = this.intents.get(id);
+    if (!existing || existing.state !== "open") return null;
+
+    const now = Math.floor(Date.now() / 1000);
+    const updated: Intent = {
+      ...existing,
+      state: "accepted",
+      solver,
+      deadline: now + 300,
+    };
+    this.intents.set(id, updated);
+    return updated;
+  }
+
+  /**
+   * Atomically fill an intent only if it is currently "accepted" by the given solver.
+   * Mirrors the DB pattern:
+   *   UPDATE intents SET state='filled', ... WHERE id=$1 AND state='accepted' AND solver=$2 RETURNING *
+   * Returns null when the intent is not found, not accepted, or assigned to a different solver.
+   */
+  fillIfAccepted(id: string, solver: string, patch: Omit<Partial<Intent>, "state" | "solver">): Intent | null {
+    const existing = this.intents.get(id);
+    if (!existing || existing.state !== "accepted" || existing.solver !== solver) return null;
+
+    const updated: Intent = { ...existing, ...patch, state: "filled" };
+    this.intents.set(id, updated);
+    return updated;
+  }
+
   private seed() {
     const now = Math.floor(Date.now() / 1000);
     for (const data of buildSeedIntents(now)) {
diff --git a/test/__mocks__/@stellar/stellar-sdk.ts b/test/__mocks__/@stellar/stellar-sdk.ts
new file mode 100644
index 0000000..8e37039
--- /dev/null
+++ b/test/__mocks__/@stellar/stellar-sdk.ts
@@ -0,0 +1,10 @@
+const mockServer = {
+  getHealth: jest.fn().mockResolvedValue({ status: "ok" }),
+  getLatestLedger: jest.fn().mockResolvedValue({ sequence: 1 }),
+  getNetwork: jest.fn().mockResolvedValue({ passphrase: "test" }),
+  getAccount: jest.fn().mockResolvedValue({ id: "test", sequence: "0" }),
+};
+
+export const SorobanRpc = {
+  Server: jest.fn().mockImplementation(() => mockServer),
+};
diff --git a/test/jest-e2e.json b/test/jest-e2e.json
index d79db1e..910ad19 100644
--- a/test/jest-e2e.json
+++ b/test/jest-e2e.json
@@ -2,8 +2,11 @@
   "moduleFileExtensions": ["js", "json", "ts"],
   "rootDir": "..",
   "testEnvironment": "node",
-  "testRegex": "test/.*\\.e2e-spec\\.ts$",
+  "testRegex": "test/.*\\.(e2e-spec|test)\\.ts$",
   "transform": {
-    "^.+\\.ts$": "ts-jest"
+    "^.+\\.ts$": ["ts-jest", { "diagnostics": false }]
+  },
+  "moduleNameMapper": {
+    "^@stellar/stellar-sdk$": "<rootDir>/test/__mocks__/@stellar/stellar-sdk.ts"
   }
 }
diff --git a/test/load/concurrent-accept.test.ts b/test/load/concurrent-accept.test.ts
new file mode 100644
index 0000000..16de8b6
--- /dev/null
+++ b/test/load/concurrent-accept.test.ts
@@ -0,0 +1,160 @@
+import { INestApplication } from "@nestjs/common";
+import request from "supertest";
+import { createTestApp } from "../utils/create-test-app";
+import { IntentsService } from "../../src/intents/intents.service";
+
+const SOLVERS = ["SOLVER_ALPHA", "SOLVER_BETA", "SOLVER_GAMMA"];
+
+const validCreateBody = {
+  user: "GRACETESTUSER1234567",
+  srcChain: "ethereum",
+  srcTokenAddress: "0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48",
+  srcTokenSymbol: "USDC",
+  srcTokenDecimals: 6,
+  srcAmount: "1000000",
+  dstTokenContract: "CBIELTK6YBZJU5UP2WWQEUCYKLPU6AUNZ2BQ4WWFEIE3USCIHMXQDAMA",
+  dstTokenSymbol: "USDC",
+  dstTokenDecimals: 7,
+  minDstAmount: "990000",
+};
+
+describe("Concurrent accept / fill race load test", () => {
+  let app: INestApplication;
+
+  beforeAll(async () => {
+    app = await createTestApp();
+  });
+
+  afterAll(async () => {
+    await app.close();
+  });
+
+  async function createIntent(): Promise<string> {
+    const res = await request(app.getHttpServer())
+      .post("/api/v1/intents")
+      .send(validCreateBody)
+      .expect(201);
+    return res.body.intentId as string;
+  }
+
+  it("only one solver wins when N concurrent accept() calls race on the same intent", async () => {
+    const intentId = await createIntent();
+    const concurrency = 20;
+
+    const results = await Promise.allSettled(
+      Array.from({ length: concurrency }, (_, i) => {
+        const solver = SOLVERS[i % SOLVERS.length];
+        return request(app.getHttpServer())
+          .post(`/api/v1/intents/${intentId}/accept`)
+          .send({ solver });
+      }),
+    );
+
+    const fulfilled = results.filter(
+      (r): r is PromiseFulfilledResult<request.Response> => r.status === "fulfilled",
+    );
+
+    const successes = fulfilled.filter((r) => r.value.status === 201);
+    const rejected = results.filter((r) => r.status === "rejected");
+
+    expect(successes).toHaveLength(1);
+
+    const intent = (await request(app.getHttpServer()).get(`/api/v1/intents/${intentId}`).expect(200))
+      .body;
+    expect(intent.state).toBe("accepted");
+    expect(SOLVERS).toContain(intent.solver);
+  });
+
+  it("only one solver wins when N concurrent fill() calls race on the same accepted intent", async () => {
+    const intentId = await createIntent();
+
+    await request(app.getHttpServer())
+      .post(`/api/v1/intents/${intentId}/accept`)
+      .send({ solver: "SOLVER_ALPHA" })
+      .expect(201);
+
+    const concurrency = 20;
+
+    const results = await Promise.allSettled(
+      Array.from({ length: concurrency }, () =>
+        request(app.getHttpServer())
+          .post(`/api/v1/intents/${intentId}/fill`)
+          .send({ solver: "SOLVER_ALPHA", fillAmount: "995000", txHash: `tx-${Math.random()}` }),
+      ),
+    );
+
+    const fulfilled = results.filter(
+      (r): r is PromiseFulfilledResult<request.Response> => r.status === "fulfilled",
+    );
+
+    const successes = fulfilled.filter((r) => r.value.status === 201);
+
+    expect(successes).toHaveLength(1);
+
+    const intent = (await request(app.getHttpServer()).get(`/api/v1/intents/${intentId}`).expect(200))
+      .body;
+    expect(intent.state).toBe("filled");
+    expect(intent.fillAmount).toBe("995000");
+  });
+
+  it("mixed solvers racing for different intents all resolve with at most one winner each", async () => {
+    const concurrency = 3;
+    const intentIds: string[] = [];
+    for (let i = 0; i < concurrency; i++) {
+      intentIds.push(await createIntent());
+    }
+
+    const results = await Promise.allSettled(
+      intentIds.map((id, i) => {
+        const solver = SOLVERS[i % SOLVERS.length];
+        return request(app.getHttpServer())
+          .post(`/api/v1/intents/${id}/accept`)
+          .send({ solver });
+      }),
+    );
+
+    const fulfilled = results.filter(
+      (r): r is PromiseFulfilledResult<request.Response> => r.status === "fulfilled",
+    );
+    const successes = fulfilled.filter((r) => r.value.status === 201);
+    expect(successes.length).toBeLessThanOrEqual(concurrency);
+
+    for (const id of intentIds) {
+      const intent = (await request(app.getHttpServer()).get(`/api/v1/intents/${id}`).expect(200))
+        .body;
+      expect(intent.state).toBe("accepted");
+    }
+  });
+
+  it("unit-level: acceptIfOpen rejects concurrent calls on the same intent", () => {
+    const service = new IntentsService();
+    const [open] = service.getByState("open");
+
+    const results = Array.from({ length: 50 }, (_, i) =>
+      service.acceptIfOpen(open.intentId, `SOLVER_${i}`),
+    );
+
+    const winners = results.filter((r) => r !== null);
+    expect(winners).toHaveLength(1);
+    expect(winners[0]!.state).toBe("accepted");
+  });
+
+  it("unit-level: fillIfAccepted rejects concurrent calls on the same intent", () => {
+    const service = new IntentsService();
+    const [open] = service.getByState("open");
+
+    service.acceptIfOpen(open.intentId, "SOLVER_X");
+
+    const results = Array.from({ length: 50 }, () =>
+      service.fillIfAccepted(open.intentId, "SOLVER_X", {
+        fillAmount: "995000",
+        txHash: "race-hash",
+        filledAt: Math.floor(Date.now() / 1000),
+      }),
+    );
+
+    const winners = results.filter((r) => r !== null);
+    expect(winners).toHaveLength(1);
+    expect(winners[0]!.state).toBe("filled");
+  });
+});