diff --git a/eslint.config.mjs b/eslint.config.mjs
index c1a1719..5f6266b 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -13,6 +13,8 @@ const nodeGlobals = {
   Buffer: "readonly",
   setTimeout: "readonly",
   clearTimeout: "readonly",
+  setInterval: "readonly",
+  clearInterval: "readonly",
 };
 
 const jestGlobals = {
diff --git a/src/errors/ApiError.ts b/src/errors/ApiError.ts
index 0a49575..5440f28 100644
--- a/src/errors/ApiError.ts
+++ b/src/errors/ApiError.ts
@@ -35,6 +35,13 @@ export class ApiError extends Error {
     return new ApiError(429, code, message);
   }
 
+  static idempotencyKeyReuse(
+    message: string,
+    code = "IDEMPOTENCY_KEY_REUSE",
+  ): ApiError {
+    return new ApiError(422, code, message);
+  }
+
   static serviceUnavailable(
     message: string,
     code = "SERVICE_UNAVAILABLE",
diff --git a/src/middleware/idempotency.test.ts b/src/middleware/idempotency.test.ts
index 5fe3378..ffad474 100644
--- a/src/middleware/idempotency.test.ts
+++ b/src/middleware/idempotency.test.ts
@@ -109,4 +109,93 @@ describe("idempotency", () => {
     expect(first.body.counter).toBe(1);
     expect(second.body.counter).toBe(2);
   });
+
+  it("replays cached response when same key and same body", async () => {
+    const app = makeApp();
+
+    const first = await request(app)
+      .post("/mutate")
+      .set("Idempotency-Key", "match-key")
+      .send({ value: "hello" });
+    const second = await request(app)
+      .post("/mutate")
+      .set("Idempotency-Key", "match-key")
+      .send({ value: "hello" });
+
+    expect(first.status).toBe(201);
+    expect(first.body.counter).toBe(1);
+    expect(second.status).toBe(201);
+    expect(second.body.counter).toBe(1);
+  });
+
+  it("returns 422 IDEMPOTENCY_KEY_REUSE when same key and different body", async () => {
+    const app = makeApp();
+
+    const first = await request(app)
+      .post("/mutate")
+      .set("Idempotency-Key", "mismatch-key")
+      .send({ value: "hello" });
+    const second = await request(app)
+      .post("/mutate")
+      .set("Idempotency-Key", "mismatch-key")
+      .send({ value: "world" });
+
+    expect(first.status).toBe(201);
+    expect(first.body.counter).toBe(1);
+    expect(second.status).toBe(422);
+    expect(second.body.error.code).toBe("IDEMPOTENCY_KEY_REUSE");
+  });
+
+  it("replays cached response when same body with different key ordering", async () => {
+    const app = makeApp();
+
+    const first = await request(app)
+      .post("/mutate")
+      .set("Idempotency-Key", "order-key")
+      .send({ a: 1, b: 2 });
+    const second = await request(app)
+      .post("/mutate")
+      .set("Idempotency-Key", "order-key")
+      .send({ b: 2, a: 1 });
+
+    expect(first.status).toBe(201);
+    expect(first.body.counter).toBe(1);
+    expect(second.status).toBe(201);
+    expect(second.body.counter).toBe(1);
+  });
+
+  it("returns 422 when same key used with body then without body", async () => {
+    const app = makeApp();
+
+    const first = await request(app)
+      .post("/mutate")
+      .set("Idempotency-Key", "body-vs-empty")
+      .send({ value: "data" });
+    const second = await request(app)
+      .post("/mutate")
+      .set("Idempotency-Key", "body-vs-empty");
+
+    expect(first.status).toBe(201);
+    expect(first.body.counter).toBe(1);
+    expect(second.status).toBe(422);
+    expect(second.body.error.code).toBe("IDEMPOTENCY_KEY_REUSE");
+  });
+
+  it("replays cached response when body has numeric-like keys in different orders", async () => {
+    const app = makeApp();
+
+    const first = await request(app)
+      .post("/mutate")
+      .set("Idempotency-Key", "numeric-key")
+      .send({ "1": "a", "2": "b" });
+    const second = await request(app)
+      .post("/mutate")
+      .set("Idempotency-Key", "numeric-key")
+      .send({ "2": "b", "1": "a" });
+
+    expect(first.status).toBe(201);
+    expect(first.body.counter).toBe(1);
+    expect(second.status).toBe(201);
+    expect(second.body.counter).toBe(1);
+  });
 });
diff --git a/src/middleware/idempotency.ts b/src/middleware/idempotency.ts
index 3dc18dd..2054085 100644
--- a/src/middleware/idempotency.ts
+++ b/src/middleware/idempotency.ts
@@ -10,19 +10,62 @@
  * unaffected. State lives in a plain `Map` local to the returned middleware,
  * so like the existing rate limiter this is a per-process safeguard and not
  * suitable for multi-instance deployments without a shared store.
+ *
+ * A SHA-256 fingerprint of the canonical JSON request body is stored alongside
+ * the cached response. On key reuse, if the incoming body hash differs, a 422
+ * `IDEMPOTENCY_KEY_REUSE` error is returned instead of replaying the cached
+ * response. Canonical serialization uses stable key ordering so that
+ * semantically identical bodies with different key orderings are treated as
+ * the same request.
  */
 
+import crypto from "crypto";
 import { NextFunction, Request, Response } from "express";
+import { ApiError } from "../errors/ApiError";
 
 const MUTATING_METHODS = new Set(["POST", "PUT", "PATCH", "DELETE"]);
 
 /** Default time a cached response remains eligible for replay. */
 const DEFAULT_TTL_MS = 24 * 60 * 60 * 1000;
 
+/**
+ * Produce a deterministic JSON string for any value. Object keys are sorted
+ * recursively so that `{ a: 1, b: 2 }` and `{ b: 2, a: 1 }` serialize to the
+ * same string. Array element order is preserved (order is semantically
+ * meaningful in arrays). Primitives and null pass through to JSON.stringify.
+ */
+function stableStringify(value: unknown): string {
+  if (value === null || value === undefined) return JSON.stringify(value);
+  if (typeof value !== "object") return JSON.stringify(value);
+  if (Array.isArray(value)) {
+    return "[" + value.map(stableStringify).join(",") + "]";
+  }
+  const sorted = Object.keys(value as Record<string, unknown>)
+    .sort()
+    .map(
+      (k) =>
+        JSON.stringify(k) +
+        ":" +
+        stableStringify((value as Record<string, unknown>)[k]),
+    );
+  return "{" + sorted.join(",") + "}";
+}
+
+/**
+ * SHA-256 hex digest of the canonicalized request body. `undefined` (no body)
+ * hashes the empty string; an explicit `{}` hashes `"{}"`. This distinction
+ * is deliberate: the two are different wire representations.
+ */
+function hashBody(body: unknown): string {
+  const raw = body !== undefined ? stableStringify(body) : "";
+  return crypto.createHash("sha256").update(raw).digest("hex");
+}
+
 interface CachedResponse {
   status: number;
   body: unknown;
   expiresAt: number;
+  bodyHash: string;
 }
 
 export interface IdempotencyOptions {
@@ -45,6 +88,18 @@ export function idempotency(options: IdempotencyOptions = {}) {
     const now = Date.now();
     const cached = cache.get(cacheKey);
     if (cached && cached.expiresAt > now) {
+      // NOTE: concurrent requests with the same key may both pass this check
+      // before either writes to the cache (check-then-set race). This is a
+      // pre-existing limitation of the in-memory Map design, not introduced
+      // by this change. See README for per-process scope.
+      if (cached.bodyHash !== hashBody(req.body)) {
+        next(
+          ApiError.idempotencyKeyReuse(
+            "Idempotency key already used with a different request body",
+          ),
+        );
+        return;
+      }
       res.status(cached.status).json(cached.body);
       return;
     }
@@ -55,6 +110,7 @@ export function idempotency(options: IdempotencyOptions = {}) {
         status: res.statusCode,
         body,
         expiresAt: now + ttlMs,
+        bodyHash: hashBody(req.body),
       });
       return originalJson(body);
     }) as Response["json"];
diff --git a/src/models/settlement.ts b/src/models/settlement.ts
index 1491e9e..52c77d0 100644
--- a/src/models/settlement.ts
+++ b/src/models/settlement.ts
@@ -5,6 +5,22 @@
 /** Lifecycle state of a settlement, mirroring the on-chain contract. */
 export type SettlementStatus = "pending" | "executed" | "cancelled";
 
+const VALID_STATUSES: readonly SettlementStatus[] = [
+  "pending",
+  "executed",
+  "cancelled",
+];
+
+/** Runtime type guard: returns `true` when `value` is a valid {@link SettlementStatus}. */
+export function isSettlementStatus(
+  value: unknown,
+): value is SettlementStatus {
+  return (
+    typeof value === "string" &&
+    (VALID_STATUSES as readonly string[]).includes(value)
+  );
+}
+
 /** A settlement that draws liquidity from a pool to settle a payment. */
 export interface Settlement {
   /** Monotonic identifier assigned by the service. */
diff --git a/src/repositories/settlementRepository.anchorIndex.test.ts b/src/repositories/settlementRepository.anchorIndex.test.ts
index 1c9434c..ed7550e 100644
--- a/src/repositories/settlementRepository.anchorIndex.test.ts
+++ b/src/repositories/settlementRepository.anchorIndex.test.ts
@@ -20,9 +20,9 @@ describe("SettlementRepository Anchor Index", () => {
 
   it("returns settlements for an anchor sorted most recent first", () => {
     const repo = new SettlementRepository();
-    const s1 = repo.create(draft("anchorA", 100)); // id 1
-    const s2 = repo.create(draft("anchorA", 200)); // id 2
-    const s3 = repo.create(draft("anchorB", 300)); // id 3
+    repo.create(draft("anchorA", 100)); // id 1
+    repo.create(draft("anchorA", 200)); // id 2
+    repo.create(draft("anchorB", 300)); // id 3
     const result = repo.byAnchor("anchorA");
     expect(result.map((s) => s.id)).toEqual([2, 1]);
     expect(result).toHaveLength(2);
diff --git a/src/repositories/settlementRepository.test.ts b/src/repositories/settlementRepository.test.ts
index b23648f..12131ae 100644
--- a/src/repositories/settlementRepository.test.ts
+++ b/src/repositories/settlementRepository.test.ts
@@ -1,5 +1,5 @@
 import { SettlementRepository } from "./settlementRepository";
-import { Settlement } from "../models/settlement";
+import { Settlement, isSettlementStatus } from "../models/settlement";
 
 function draft(anchor: string, amount: number): Omit<Settlement, "id"> {
   return {
@@ -49,3 +49,49 @@ describe("SettlementRepository", () => {
     expect(repo.count()).toBe(3);
   });
 });
+
+describe("isSettlementStatus", () => {
+  it("accepts all valid statuses", () => {
+    expect(isSettlementStatus("pending")).toBe(true);
+    expect(isSettlementStatus("executed")).toBe(true);
+    expect(isSettlementStatus("cancelled")).toBe(true);
+  });
+
+  it("rejects invalid strings", () => {
+    expect(isSettlementStatus("unknown")).toBe(false);
+    expect(isSettlementStatus("")).toBe(false);
+  });
+
+  it("rejects near-miss strings (exact match only)", () => {
+    expect(isSettlementStatus("Pending")).toBe(false);
+    expect(isSettlementStatus("pending ")).toBe(false);
+    expect(isSettlementStatus("PENDING")).toBe(false);
+  });
+
+  it("rejects non-string values", () => {
+    expect(isSettlementStatus(123)).toBe(false);
+    expect(isSettlementStatus(null)).toBe(false);
+    expect(isSettlementStatus(undefined)).toBe(false);
+    expect(isSettlementStatus({})).toBe(false);
+  });
+});
+
+describe("SettlementRepository rejects invalid status", () => {
+  it("save throws on invalid status", () => {
+    const repo = new SettlementRepository();
+    const created = repo.create(draft("anchorA", 100));
+    const invalid = { ...created, status: "bogus" } as unknown as Settlement;
+
+    expect(() => repo.save(invalid)).toThrow(/Invalid settlement status/);
+  });
+
+  it("create throws on invalid status", () => {
+    const repo = new SettlementRepository();
+    const invalid = {
+      ...draft("anchorA", 100),
+      status: "bogus",
+    } as unknown as Omit<Settlement, "id">;
+
+    expect(() => repo.create(invalid)).toThrow(/Invalid settlement status/);
+  });
+});
diff --git a/src/repositories/settlementRepository.ts b/src/repositories/settlementRepository.ts
index 40326e1..5e178e4 100644
--- a/src/repositories/settlementRepository.ts
+++ b/src/repositories/settlementRepository.ts
@@ -2,7 +2,7 @@
  * In-memory store for settlements with an auto-incrementing id.
  */
 
-import { Settlement } from "../models/settlement";
+import { Settlement, isSettlementStatus } from "../models/settlement";
 import { InMemoryRepository } from "./inMemoryRepository";
 
 export class SettlementRepository extends InMemoryRepository<number, Settlement> {
@@ -15,6 +15,13 @@ export class SettlementRepository extends InMemoryRepository<number, Settlement>
 
   /** Stores a settlement under a freshly allocated id. */
   create(settlement: Omit<Settlement, "id">): Settlement {
+    if (!isSettlementStatus(settlement.status)) {
+      // Interpolate only the status, not the whole object, to avoid leaking
+      // untrusted input into logs.
+      throw new Error(
+        `Invalid settlement status: ${String(settlement.status)}`,
+      );
+    }
     const id = this.generateId();
     const created: Settlement = { ...settlement, id };
     // Update anchor index
@@ -26,6 +33,11 @@ export class SettlementRepository extends InMemoryRepository<number, Settlement>
 
   /** Replaces an existing settlement (e.g. after a status change). */
   save(settlement: Settlement): Settlement {
+    if (!isSettlementStatus(settlement.status)) {
+      throw new Error(
+        `Invalid settlement status: ${String(settlement.status)}`,
+      );
+    }
     // Ensure anchor index consistency (anchor is immutable)
     const existing = this.getByKey(settlement.id);
     if (existing && existing.anchor !== settlement.anchor) {