diff --git a/services/appraisal-api/README.md b/services/appraisal-api/README.md
index 0385a6b..9b1bdfc 100644
--- a/services/appraisal-api/README.md
+++ b/services/appraisal-api/README.md
@@ -71,6 +71,11 @@ A `POST /appraise` body larger than 8192 bytes is rejected with **HTTP 413**
 `{ "error": "payload_too_large", "limitBytes": 8192 }` before the bytes are
 parsed as JSON, so the request never receives a 402 payment quote.
 
+All appraisal quote challenges (`402`), paid appraisal responses (`200`), and
+error responses include `Cache-Control: no-store`, `Pragma: no-cache`,
+`Expires: 0`, and `X-Content-Type-Options: nosniff` so intermediary caches
+never store or replay time-bound payment quotes or paid valuations.
+
 ## Schema fixtures
 
 The canonical request/response shapes and the per-failure-mode payloads live as
diff --git a/services/appraisal-api/src/appraisal.ts b/services/appraisal-api/src/appraisal.ts
index fb3f300..021300f 100644
--- a/services/appraisal-api/src/appraisal.ts
+++ b/services/appraisal-api/src/appraisal.ts
@@ -62,7 +62,7 @@ export interface AppraisalRequest {
 }
 
 export interface Appraisal {
-  model: typeof APPRAISALMODEL;
+  model: typeof APPRAISAL_MODEL;
   itemRef: string;
   inputsHash: string;
   fairValue: number;
@@ -93,7 +93,7 @@ function canonical(value: unknown): string {
     const entries = Object.entries(value as Record<string, unknown>)
       .filter(([, v]) => v !== undefined)
       .sort(([a], [b]) => (a < b ? -1 : a > b ? 1 : 0))
-      .map(([k], v]) => `${JSON.stringify(k)}:${canonical(v)}`);
+      .map(([k, v]) => `${JSON.stringify(k)}:${canonical(v)}`);
     return `{${entries.join(",")}}`;
   }
   return JSON.stringify(value ?? null);
@@ -275,7 +275,7 @@ export function appraise(req: AppraisalRequest): Appraisal {
   const suggestedMaxBid = round2(fairValue * (0.8 + 0.15 * confidence));
 
   const rationale = [
-    `base ${req.basePrice} USDC scaled by quality×${round2(qualityF)}, demand×ä{round2(demandF)}, scarcity×ä{round2(scarcityF)}, risk×ä{round2(riskF)}`,
+    `base ${req.basePrice} USDC scaled by quality×${round2(qualityF)}, demand×${round2(demandF)}, scarcity×${round2(scarcityF)}, risk×${round2(riskF)}`,
     `category '${req.category ?? "none"}' multiplier ×${categoryF}`,
     `${provided}/4 attributes supplied → confidence ${confidence}`,
     `suggested max bid is fair value × ${round2(0.8 + 0.15 * confidence)} to preserve margin`,
diff --git a/services/appraisal-api/src/body-limit.test.ts b/services/appraisal-api/src/body-limit.test.ts
index 7fd4e1d..6ed1a53 100644
--- a/services/appraisal-api/src/body-limit.test.ts
+++ b/services/appraisal-api/src/body-limit.test.ts
@@ -6,10 +6,19 @@ import { once } from "node:events";
 import { PassThrough } from "node:stream";
 import { describe, test } from "node:test";
 
+import { Keypair, Networks } from "@stellar/stellar-sdk";
+import { STELLAR_TESTNET_CAIP2, USDC_TESTNET_ADDRESS } from "@x402/stellar";
+
 import {
   APPRAISAL_BODY_TOO_LARGE,
   takeAppraisalRequestBody,
 } from "./body-limit.js";
+import {
+  APPRAISAL_RESPONSE_SECURITY_HEADERS,
+  buildAppraisalServer,
+  withNonCacheableHeaders,
+} from "./server.js";
+import { validRequest } from "./fixtures/index.js";
 
 function listen(server: http.Server): Promise<number> {
   server.listen(0, "127.0.0.1");
@@ -24,7 +33,7 @@ function listen(server: http.Server): Promise<number> {
 
 function fakeRequest(headers: http.IncomingHttpHeaders, body: Buffer): IncomingMessage {
   const stream = new PassThrough();
-  const req = Object.assign(stream, { headers }) as IncomingMessage;
+  const req = Object.assign(stream, { headers }) as unknown as IncomingMessage;
   queueMicrotask(() => {
     stream.end(body);
   });
@@ -75,6 +84,71 @@ describe("appraisal request body cap", () => {
       });
       assert.equal(allowed.status, 200);
     } finally {
+      server.closeAllConnections?.();
+      server.close();
+    }
+  });
+});
+
+describe("appraisal response non-cacheable headers", () => {
+  test("withNonCacheableHeaders enforces Cache-Control: no-store and safe headers", () => {
+    const merged = withNonCacheableHeaders({
+      "Cache-Control": "public, max-age=3600",
+      "X-Payment-Response": "settled",
+    });
+    assert.equal(merged["cache-control"], "no-store");
+    assert.equal(merged["pragma"], "no-cache");
+    assert.equal(merged["expires"], "0");
+    assert.equal(merged["x-content-type-options"], "nosniff");
+    assert.equal(merged["x-payment-response"], "settled");
+    assert.deepEqual(APPRAISAL_RESPONSE_SECURITY_HEADERS, {
+      "cache-control": "no-store",
+      pragma: "no-cache",
+      expires: "0",
+      "x-content-type-options": "nosniff",
+    });
+  });
+
+  test("402 quote challenges and error responses carry Cache-Control: no-store and safe headers", async () => {
+    const facilitator = Keypair.fromRawEd25519Seed(Buffer.alloc(32, 3));
+    const payee = Keypair.fromRawEd25519Seed(Buffer.alloc(32, 4));
+    const server = await buildAppraisalServer(
+      {
+        facilitatorSecret: facilitator.secret(),
+        payTo: payee.publicKey(),
+        asset: USDC_TESTNET_ADDRESS,
+        price: 0.1,
+        network: STELLAR_TESTNET_CAIP2,
+        networkPassphrase: Networks.TESTNET,
+        rpcUrl: "https://soroban-testnet.stellar.org",
+        port: 0,
+      },
+      { nowSeconds: () => 1_700_000_000 },
+    );
+    const port = await listen(server);
+    try {
+      const challenge = await fetch(`http://127.0.0.1:${port}/appraise`, {
+        method: "POST",
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify(validRequest.value),
+      });
+      assert.equal(challenge.status, 402);
+      assert.equal(challenge.headers.get("cache-control"), "no-store");
+      assert.equal(challenge.headers.get("pragma"), "no-cache");
+      assert.equal(challenge.headers.get("expires"), "0");
+      assert.equal(challenge.headers.get("x-content-type-options"), "nosniff");
+
+      const emptyPost = await fetch(`http://127.0.0.1:${port}/appraise`, {
+        method: "POST",
+        headers: { "content-type": "application/json" },
+        body: "",
+      });
+      assert.equal(emptyPost.status, 400);
+      assert.equal(emptyPost.headers.get("cache-control"), "no-store");
+      assert.equal(emptyPost.headers.get("pragma"), "no-cache");
+      assert.equal(emptyPost.headers.get("x-content-type-options"), "nosniff");
+    } finally {
+      server.closeAllConnections?.();
       server.close();
     }
   });
diff --git a/services/appraisal-api/src/index.ts b/services/appraisal-api/src/index.ts
index 9198c6a..eeebc33 100644
--- a/services/appraisal-api/src/index.ts
+++ b/services/appraisal-api/src/index.ts
@@ -18,7 +18,12 @@ export {
   type AppraisalAttributes,
 } from "./appraisal.js";
 
-export { buildAppraisalServer } from "./server.js";
+export {
+  APPRAISAL_RESPONSE_SECURITY_HEADERS,
+  buildAppraisalServer,
+  withNonCacheableHeaders,
+  type AppraisalServerOptions,
+} from "./server.js";
 export {
   AppraisalConfigError,
   configFromEnv,
diff --git a/services/appraisal-api/src/server.ts b/services/appraisal-api/src/server.ts
index 504f59e..667db96 100644
--- a/services/appraisal-api/src/server.ts
+++ b/services/appraisal-api/src/server.ts
@@ -43,6 +43,39 @@ import type { AppraisalServerConfig } from "./config.js";
 
 const APPRAISE_ROUTE = "POST /appraise";
 
+/** Non-cacheable security headers applied to appraisal quotes and paid responses. */
+export const APPRAISAL_RESPONSE_SECURITY_HEADERS: Readonly<Record<string, string>> =
+  Object.freeze({
+    "cache-control": "no-store",
+    pragma: "no-cache",
+    expires: "0",
+    "x-content-type-options": "nosniff",
+  });
+
+/**
+ * Merge response headers with mandatory non-cacheable security headers.
+ * Case-insensitively prevents upstream headers from re-enabling response caching.
+ */
+export function withNonCacheableHeaders(
+  headers: Record<string, string> = {},
+): Record<string, string> {
+  const normalized: Record<string, string> = {};
+  for (const [key, value] of Object.entries(headers)) {
+    normalized[key.toLowerCase()] = value;
+  }
+  return {
+    ...normalized,
+    ...APPRAISAL_RESPONSE_SECURITY_HEADERS,
+  };
+}
+
+export interface AppraisalServerOptions {
+  /** Optional custom facilitator client (used by deterministic unit tests). */
+  facilitatorClient?: FacilitatorClient;
+  /** Optional wall-clock override returning Unix seconds. */
+  nowSeconds?: () => number;
+}
+
 /** Minimal HTTPAdapter over a Node request + already-read body. */
 function makeAdapter(
   req: http.IncomingMessage,
@@ -73,7 +106,10 @@ function send(
 ) {
   const payload =
     typeof body === "string" ? body : JSON.stringify(body ?? {}, null, 2);
-  res.writeHead(status, { "content-type": "application/json", ...headers });
+  res.writeHead(status, {
+    "content-type": "application/json",
+    ...withNonCacheableHeaders(headers),
+  });
   res.end(payload);
 }
 
@@ -84,7 +120,7 @@ function writeInstructions(res: http.ServerResponse, i: HTTPResponseInstructions
       : JSON.stringify(i.body ?? {}, null, 2);
   res.writeHead(i.status, {
     "content-type": i.isHtml ? "text/html" : "application/json",
-    ...i.headers,
+    ...withNonCacheableHeaders(i.headers),
   });
   res.end(body ?? "");
 }
@@ -92,24 +128,29 @@ function writeInstructions(res: http.ServerResponse, i: HTTPResponseInstructions
 /** Build (and initialize) the x402-gated appraisal HTTP server. */
 export async function buildAppraisalServer(
   config: AppraisalServerConfig,
+  options: AppraisalServerOptions = {},
 ): Promise<http.Server> {
   const rpcConfig = config.rpcUrl ? { url: config.rpcUrl } : undefined;
 
-  // In-process facilitator: verifies + settles on-chain (sponsors fees, submits).
-  const facilitatorSigner = createEd25519Signer(config.facilitatorSecret, config.network);
-  const facilitator = new x402Facilitator().register(
-    config.network,
-    new FacilitatorStellarScheme([facilitatorSigner], { rpcConfig }),
-  );
-  // x402Facilitator already exposes verify/settle/getSupported; adapt the
-  // synchronous getSupported() to the async FacilitatorClient contract so we can
-  // wire it straight into the resource server with no HTTP hop.
-  const facilitatorClient: FacilitatorClient = {
-    verify: (payload, requirements) => facilitator.verify(payload, requirements),
-    settle: (payload, requirements) => facilitator.settle(payload, requirements),
-    getSupported: async (): Promise<SupportedResponse> =>
-      facilitator.getSupported() as unknown as SupportedResponse,
-  };
+  const facilitatorClient: FacilitatorClient =
+    options.facilitatorClient ??
+    (() => {
+      // In-process facilitator: verifies + settles on-chain (sponsors fees, submits).
+      const facilitatorSigner = createEd25519Signer(config.facilitatorSecret, config.network);
+      const facilitator = new x402Facilitator().register(
+        config.network,
+        new FacilitatorStellarScheme([facilitatorSigner], { rpcConfig }),
+      );
+      // x402Facilitator already exposes verify/settle/getSupported; adapt the
+      // synchronous getSupported() to the async FacilitatorClient contract so we can
+      // wire it straight into the resource server with no HTTP hop.
+      return {
+        verify: (payload, requirements) => facilitator.verify(payload, requirements),
+        settle: (payload, requirements) => facilitator.settle(payload, requirements),
+        getSupported: async (): Promise<SupportedResponse> =>
+          facilitator.getSupported() as unknown as SupportedResponse,
+      };
+    })();
 
   // Resource server: builds payment requirements and orchestrates verify/settle.
   const serverScheme = new ServerStellarScheme().registerMoneyParser(async (amount) => ({
@@ -199,13 +240,16 @@ export async function buildAppraisalServer(
       // malformed body never costs the caller.
       let body: unknown;
       try {
+        const nowSeconds = options.nowSeconds
+          ? options.nowSeconds()
+          : Math.floor(globalThis.Date.now() / 1000);
         body = {
           appraisal: appraise(parseAppraisalRequest(parsedBody)),
           quote: buildAppraisalQuote({
             asset: config.asset,
             price: config.price,
             destination: config.payTo,
-            nowSeconds: Math.floor(Date.now() / 1000),
+            nowSeconds,
           }),
         };
       } catch (e) {
@@ -220,7 +264,7 @@ export async function buildAppraisalServer(
         result.paymentPayload as PaymentPayload,
         result.paymentRequirements as PaymentRequirements,
         result.declaredExtensions,
-        { request: ctx, responseBody, responseHeaders: {} },
+        { request: ctx, responseBody, responseHeaders: withNonCacheableHeaders() },
       );
 
       if (!settle.success) {