diff --git a/api/mods/index.js b/api/mods/index.js
index 7911774..3ab19b6 100644
--- a/api/mods/index.js
+++ b/api/mods/index.js
@@ -1,37 +1,120 @@
-module.exports = function (context, req) {
-  var https = require("https");
-
-  var domainName = context.bindingData.domainName;
-  var modId = context.bindingData.id;
-  var options = {
-    host: "api.nexusmods.com",
-    path: `/v1/games/${domainName}/mods/${modId}`,
-    headers: { apiKey: process.env.NEXUS_API_KEY },
-    method: "GET",
+const https = require("https");
+const { NodeCache } = require("@cacheable/node-cache");
+
+const CACHE_TTL_SECONDS = 60 * 60;
+const modCache = new NodeCache({
+  stdTTL: CACHE_TTL_SECONDS,
+  checkperiod: 120,
+  maxKeys: 256,
+  useClones: true,
+});
+
+function createResponse(status, body, cacheStatus) {
+  return {
+    status,
+    body,
+    headers: {
+      "Content-Type": "application/json",
+      "X-Cache": cacheStatus,
+    },
   };
+}
 
-  var response = "";
-  const request = https.request(options, (res) => {
-    context.log(`statusCode: ${res.statusCode}`);
+function createHandler({ cache = modCache, request = https.request } = {}) {
+  return function handler(context, req) {
+    const domainName = context.bindingData?.domainName
+      ?.toString()
+      .trim()
+      .toLowerCase();
+    const modId = Number(context.bindingData?.id);
+    let completed = false;
 
-    res.on("data", (d) => {
-      response += d;
-    });
+    const complete = (status, body, cacheStatus = "MISS") => {
+      if (completed) {
+        return;
+      }
 
-    res.on("end", (d) => {
-      context.res = {
-        status: res.statusCode,
-        body: JSON.parse(response),
-        headers: { "Content-Type": "application/json" },
-      };
+      completed = true;
+      context.res = createResponse(status, body, cacheStatus);
       context.done();
-    });
-  });
+    };
+
+    if (!domainName || !Number.isInteger(modId) || modId <= 0) {
+      complete(400, {
+        error: "domainName and a positive integer id are required",
+      });
+      return;
+    }
+
+    const cacheKey = `${domainName}:${modId}`;
+    const cachedResponse = cache.get(cacheKey);
+
+    if (cachedResponse !== undefined) {
+      context.log(`Cache hit: ${cacheKey}`);
+      complete(cachedResponse.status, cachedResponse.body, "HIT");
+      return;
+    }
+
+    context.log(`Cache miss: ${cacheKey}`);
+
+    const options = {
+      host: "api.nexusmods.com",
+      path: `/v1/games/${encodeURIComponent(domainName)}/mods/${modId}`,
+      headers: { apiKey: process.env.NEXUS_API_KEY },
+      method: "GET",
+    };
+
+    try {
+      const upstreamRequest = request(options, (upstreamResponse) => {
+        let responseBody = "";
+
+        context.log(`statusCode: ${upstreamResponse.statusCode}`);
+
+        upstreamResponse.on("data", (chunk) => {
+          responseBody += chunk;
+        });
+
+        upstreamResponse.on("end", () => {
+          let parsedBody;
+
+          try {
+            parsedBody = JSON.parse(responseBody);
+          } catch (error) {
+            context.log.error(error);
+            complete(502, {
+              error: "Nexus Mods API returned invalid JSON",
+            });
+            return;
+          }
+
+          const status = upstreamResponse.statusCode ?? 502;
+
+          if (status === 200) {
+            try {
+              cache.set(cacheKey, { status, body: parsedBody });
+            } catch (error) {
+              context.log.warn(`Unable to cache ${cacheKey}: ${error.message}`);
+            }
+          }
+
+          complete(status, parsedBody);
+        });
+      });
+
+      upstreamRequest.on("error", (error) => {
+        context.log.error(error);
+        complete(502, { error: "Unable to reach Nexus Mods API" });
+      });
+
+      upstreamRequest.end();
+    } catch (error) {
+      context.log.error(error);
+      complete(502, { error: "Unable to reach Nexus Mods API" });
+    }
+  };
+}
 
-  request.on("error", (error) => {
-    context.log.error(error);
-    context.done();
-  });
+const handler = createHandler();
+handler.createHandler = createHandler;
 
-  request.end();
-};
+module.exports = handler;
diff --git a/api/mods/index.test.js b/api/mods/index.test.js
new file mode 100644
index 0000000..5454928
--- /dev/null
+++ b/api/mods/index.test.js
@@ -0,0 +1,182 @@
+const assert = require("node:assert/strict");
+const { EventEmitter } = require("node:events");
+const { test } = require("node:test");
+const { NodeCache } = require("@cacheable/node-cache");
+const modsHandler = require("./index");
+
+function createCache() {
+  return new NodeCache({
+    stdTTL: 3600,
+    checkperiod: 0,
+    maxKeys: 256,
+    useClones: true,
+  });
+}
+
+function createRequestStub(responder) {
+  let callCount = 0;
+
+  const request = (options, callback) => {
+    callCount += 1;
+    const outgoingRequest = new EventEmitter();
+
+    outgoingRequest.end = () => {
+      const response =
+        typeof responder === "function"
+          ? responder(options, callCount)
+          : responder;
+
+      process.nextTick(() => {
+        if (response.error) {
+          outgoingRequest.emit("error", response.error);
+          return;
+        }
+
+        const upstreamResponse = new EventEmitter();
+        upstreamResponse.statusCode = response.status;
+        callback(upstreamResponse);
+        upstreamResponse.emit("data", Buffer.from(response.body));
+        upstreamResponse.emit("end");
+      });
+    };
+
+    return outgoingRequest;
+  };
+
+  request.getCallCount = () => callCount;
+  return request;
+}
+
+function invoke(handler, bindingData) {
+  return new Promise((resolve) => {
+    const logs = [];
+    const log = (...values) => logs.push(["info", ...values]);
+    log.error = (...values) => logs.push(["error", ...values]);
+    log.warn = (...values) => logs.push(["warn", ...values]);
+
+    const context = {
+      bindingData,
+      log,
+      done: () => resolve({ response: context.res, logs }),
+    };
+
+    handler(context, {});
+  });
+}
+
+test("caches a successful response for one hour", async () => {
+  const cache = createCache();
+  const request = createRequestStub({
+    status: 200,
+    body: JSON.stringify({ name: "A mod" }),
+  });
+  const handler = modsHandler.createHandler({ cache, request });
+
+  const first = await invoke(handler, { domainName: "Witcher3", id: 1738 });
+  const second = await invoke(handler, { domainName: "witcher3", id: 1738 });
+
+  assert.equal(request.getCallCount(), 1);
+  assert.equal(first.response.headers["X-Cache"], "MISS");
+  assert.equal(second.response.headers["X-Cache"], "HIT");
+  assert.deepEqual(second.response.body, { name: "A mod" });
+
+  const remainingTtl = cache.getTtl("witcher3:1738") - Date.now();
+  assert.ok(remainingTtl > 3_595_000 && remainingTtl <= 3_600_000);
+});
+
+test("keeps cache entries isolated by domain and mod id", async () => {
+  const cache = createCache();
+  const request = createRequestStub((options) => ({
+    status: 200,
+    body: JSON.stringify({ path: options.path }),
+  }));
+  const handler = modsHandler.createHandler({ cache, request });
+
+  await invoke(handler, { domainName: "witcher3", id: 1738 });
+  await invoke(handler, { domainName: "witcher3", id: 1643 });
+  const cached = await invoke(handler, { domainName: "witcher3", id: 1738 });
+
+  assert.equal(request.getCallCount(), 2);
+  assert.equal(cached.response.headers["X-Cache"], "HIT");
+  assert.equal(cached.response.body.path, "/v1/games/witcher3/mods/1738");
+});
+
+test("does not cache non-200 responses", async () => {
+  const cache = createCache();
+  const request = createRequestStub({
+    status: 404,
+    body: JSON.stringify({ message: "Not found" }),
+  });
+  const handler = modsHandler.createHandler({ cache, request });
+
+  const first = await invoke(handler, { domainName: "witcher3", id: 9999 });
+  const second = await invoke(handler, { domainName: "witcher3", id: 9999 });
+
+  assert.equal(request.getCallCount(), 2);
+  assert.equal(first.response.status, 404);
+  assert.equal(second.response.headers["X-Cache"], "MISS");
+  assert.equal(cache.get("witcher3:9999"), undefined);
+});
+
+test("does not cache malformed upstream responses", async () => {
+  const cache = createCache();
+  const request = createRequestStub({ status: 200, body: "not json" });
+  const handler = modsHandler.createHandler({ cache, request });
+
+  const first = await invoke(handler, { domainName: "witcher3", id: 1738 });
+  const second = await invoke(handler, { domainName: "witcher3", id: 1738 });
+
+  assert.equal(request.getCallCount(), 2);
+  assert.equal(first.response.status, 502);
+  assert.equal(second.response.headers["X-Cache"], "MISS");
+  assert.equal(cache.get("witcher3:1738"), undefined);
+});
+
+test("does not cache network errors", async () => {
+  const cache = createCache();
+  const request = createRequestStub({ error: new Error("network failed") });
+  const handler = modsHandler.createHandler({ cache, request });
+
+  const first = await invoke(handler, { domainName: "witcher3", id: 1738 });
+  const second = await invoke(handler, { domainName: "witcher3", id: 1738 });
+
+  assert.equal(request.getCallCount(), 2);
+  assert.equal(first.response.status, 502);
+  assert.equal(second.response.headers["X-Cache"], "MISS");
+  assert.equal(cache.get("witcher3:1738"), undefined);
+});
+
+test("rejects missing or invalid route values without calling Nexus", async () => {
+  const request = createRequestStub({ status: 200, body: "{}" });
+  const handler = modsHandler.createHandler({ cache: createCache(), request });
+
+  const missingId = await invoke(handler, { domainName: "witcher3" });
+  const invalidId = await invoke(handler, { domainName: "witcher3", id: 0 });
+  const missingDomain = await invoke(handler, { id: 1738 });
+
+  assert.equal(request.getCallCount(), 0);
+  assert.equal(missingId.response.status, 400);
+  assert.equal(invalidId.response.status, 400);
+  assert.equal(missingDomain.response.status, 400);
+});
+
+test("returns successful responses when a bounded cache rejects a write", async () => {
+  const cache = {
+    get: () => undefined,
+    set: () => {
+      throw new Error("cache is full");
+    },
+  };
+  const request = createRequestStub({
+    status: 200,
+    body: JSON.stringify({ name: "A mod" }),
+  });
+  const handler = modsHandler.createHandler({ cache, request });
+
+  const result = await invoke(handler, { domainName: "witcher3", id: 1738 });
+
+  assert.equal(result.response.status, 200);
+  assert.equal(result.response.headers["X-Cache"], "MISS");
+  assert.deepEqual(result.response.body, { name: "A mod" });
+  assert.ok(result.logs.some(([level]) => level === "warn"));
+});
diff --git a/api/package-lock.json b/api/package-lock.json
index 1a19e43..6fa3eb0 100644
--- a/api/package-lock.json
+++ b/api/package-lock.json
@@ -7,7 +7,73 @@
     "": {
       "name": "api",
       "version": "1.0.0",
+      "dependencies": {
+        "@cacheable/node-cache": "3.1.1"
+      },
       "devDependencies": {}
+    },
+    "node_modules/@cacheable/node-cache": {
+      "version": "3.1.1",
+      "resolved": "https://registry.npmjs.org/@cacheable/node-cache/-/node-cache-3.1.1.tgz",
+      "integrity": "sha512-AFW2fw+Z0oupVmhQ0GKZc6v8UasaWrQ7ptOZv7e7avPCos62F2myeucdBbO8riBiCXx4TYfb+sv3hwXbaOwAqA==",
+      "license": "MIT",
+      "dependencies": {
+        "@cacheable/utils": "^2.5.0",
+        "hookified": "^2.1.0",
+        "keyv": "^5.6.0"
+      },
+      "engines": {
+        "node": ">=22"
+      }
+    },
+    "node_modules/@cacheable/utils": {
+      "version": "2.5.0",
+      "resolved": "https://registry.npmjs.org/@cacheable/utils/-/utils-2.5.0.tgz",
+      "integrity": "sha512-buipgOVDkkPXNR5+xBpDw7Zk2n1EvU7qBJCNUcL7rhQ//kfpOXPAvQ511Os0vpLYJ1pZnvudNytkQt2hst3wqA==",
+      "license": "MIT",
+      "dependencies": {
+        "hashery": "^1.5.1",
+        "keyv": "^5.6.0"
+      }
+    },
+    "node_modules/@keyv/serialize": {
+      "version": "1.1.1",
+      "resolved": "https://registry.npmjs.org/@keyv/serialize/-/serialize-1.1.1.tgz",
+      "integrity": "sha512-dXn3FZhPv0US+7dtJsIi2R+c7qWYiReoEh5zUntWCf4oSpMNib8FDhSoed6m3QyZdx5hK7iLFkYk3rNxwt8vTA==",
+      "license": "MIT"
+    },
+    "node_modules/hashery": {
+      "version": "1.5.1",
+      "resolved": "https://registry.npmjs.org/hashery/-/hashery-1.5.1.tgz",
+      "integrity": "sha512-iZyKG96/JwPz1N55vj2Ie2vXbhu440zfUfJvSwEqEbeLluk7NnapfGqa7LH0mOsnDxTF85Mx8/dyR6HfqcbmbQ==",
+      "license": "MIT",
+      "dependencies": {
+        "hookified": "^1.15.0"
+      },
+      "engines": {
+        "node": ">=20"
+      }
+    },
+    "node_modules/hashery/node_modules/hookified": {
+      "version": "1.15.1",
+      "resolved": "https://registry.npmjs.org/hookified/-/hookified-1.15.1.tgz",
+      "integrity": "sha512-MvG/clsADq1GPM2KGo2nyfaWVyn9naPiXrqIe4jYjXNZQt238kWyOGrsyc/DmRAQ+Re6yeo6yX/yoNCG5KAEVg==",
+      "license": "MIT"
+    },
+    "node_modules/hookified": {
+      "version": "2.2.0",
+      "resolved": "https://registry.npmjs.org/hookified/-/hookified-2.2.0.tgz",
+      "integrity": "sha512-p/LgFzRN5FeoD3DLS6bkUapeye6E4SI6yJs6KetENd18S+FBthqYq2amJUWpt5z0EQwwHemidjY5OqJGEKm5uA==",
+      "license": "MIT"
+    },
+    "node_modules/keyv": {
+      "version": "5.6.0",
+      "resolved": "https://registry.npmjs.org/keyv/-/keyv-5.6.0.tgz",
+      "integrity": "sha512-CYDD3SOtsHtyXeEORYRx2qBtpDJFjRTGXUtmNEMGyzYOKj1TE3tycdlho7kA1Ufx9OYWZzg52QFBGALTirzDSw==",
+      "license": "MIT",
+      "dependencies": {
+        "@keyv/serialize": "^1.1.1"
+      }
     }
   }
 }
diff --git a/api/package.json b/api/package.json
index 4342e1b..e02af95 100644
--- a/api/package.json
+++ b/api/package.json
@@ -4,8 +4,9 @@
   "description": "",
   "scripts": {
     "start": "func start",
-    "test": "echo \"No tests yet...\""
+    "test": "node --test mods/index.test.js"
   },
-  "dependencies": {},
-  "devDependencies": {}
-}
\ No newline at end of file
+  "dependencies": {
+    "@cacheable/node-cache": "3.1.1"
+  }
+}