diff --git a/plugins/techdocs-backend/src/cache/cacheMiddleware.test.ts b/plugins/techdocs-backend/src/cache/cacheMiddleware.test.ts
new file mode 100644
index 0000000000..5c591bdda4
--- /dev/null
+++ b/plugins/techdocs-backend/src/cache/cacheMiddleware.test.ts
@@ -0,0 +1,142 @@
+/*
+ * Copyright 2021 The Backstage Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { getVoidLogger } from '@backstage/backend-common';
+import express from 'express';
+import request from 'supertest';
+import { createCacheMiddleware, TechDocsCache } from '.';
+
+/**
+ * Mocks cached HTTP response.
+ */
+const getMockHttpResponseFor = (content: string): Buffer => {
+  return Buffer.concat([
+    Buffer.from(`HTTP/1.1 200 OK
+Content-Type: text/plain; charset=utf-8
+Accept-Ranges: bytes
+Cache-Control: public, max-age=0
+Last-Modified: Sat, 1 Jul 2021 12:00:00 GMT
+Date: Sat, 1 Jul 2021 12:00:00 GMT
+Connection: close
+Content-Length: ${content.length}\n\n`),
+    Buffer.from(content),
+  ]);
+};
+
+/**
+ * Wait for the socket to close. Works because, above, we set connection: close
+ */
+const waitForSocketClose = () => new Promise(resolve => setTimeout(resolve, 0));
+
+describe('createCacheMiddleware', () => {
+  let cache: jest.Mocked<TechDocsCache>;
+  let app: express.Express;
+
+  beforeEach(async () => {
+    cache = ({
+      get: jest.fn().mockResolvedValue(undefined),
+      set: jest.fn().mockResolvedValue(undefined),
+      invalidate: jest.fn().mockResolvedValue(undefined),
+      invalidateMultiple: jest.fn().mockResolvedValue(undefined),
+    } as unknown) as jest.Mocked<TechDocsCache>;
+    const router = await createCacheMiddleware({
+      logger: getVoidLogger(),
+      cache,
+    });
+    app = express().use(router);
+    app.use((req, res, next) => {
+      // By default, send cacheable content.
+      if (req.path !== '/api/static/docs/error.png') {
+        res.send('default-response');
+      } else {
+        next(new Error());
+      }
+    });
+  });
+
+  describe('invalidate', () => {
+    it('responds with 400 when no objects are provided', async () => {
+      const response = await request(app).post('/cache/invalidate');
+      expect(response.status).toBe(400);
+    });
+
+    it('responds with 500 if invalidation throws', async () => {
+      cache.invalidateMultiple.mockRejectedValueOnce(new Error());
+      const response = await request(app)
+        .post('/cache/invalidate')
+        .set('Content-Type', 'application/json')
+        .send({
+          objects: ['one/index.html', 'two/index.html'],
+        });
+
+      expect(response.status).toBe(500);
+    });
+
+    it('responds with 204 if invalidation succeeds', async () => {
+      const expectedObjects = ['one/index.html', 'two/index.html'];
+      cache.invalidateMultiple.mockResolvedValue([]);
+      await request(app)
+        .post('/cache/invalidate')
+        .set('Content-Type', 'application/json')
+        .send({
+          objects: expectedObjects,
+        })
+        .expect(204);
+
+      expect(cache.invalidateMultiple).toHaveBeenCalledWith(expectedObjects);
+    });
+  });
+
+  describe('middleware', () => {
+    it('does not apply to non-static/docs paths', async () => {
+      await request(app)
+        .get('/api/static/not-docs')
+        .expect(200, 'default-response');
+
+      expect(cache.set).not.toHaveBeenCalled();
+    });
+
+    it('responds with cached response', async () => {
+      cache.get.mockResolvedValueOnce(getMockHttpResponseFor('xyz'));
+
+      await request(app).get('/api/static/docs/foo.html').expect(200, 'xyz');
+
+      await waitForSocketClose();
+      expect(cache.set).not.toHaveBeenCalled();
+    });
+
+    it('sets cache when content is cacheable', async () => {
+      const expectedPath = 'default/api/xyz/index.html';
+      await request(app)
+        .get(`/api/static/docs/${expectedPath}`)
+        .expect(200, 'default-response');
+
+      await waitForSocketClose();
+      expect(cache.set).toHaveBeenCalled();
+
+      const [actualPath, actualBuffer] = (cache.set as jest.Mock).mock.calls[0];
+      expect(actualPath).toBe(expectedPath);
+      expect(actualBuffer.toString()).toContain('default-response');
+    });
+
+    it('does not set cache on error', async () => {
+      await request(app).get('/api/static/docs/error.png').expect(500);
+
+      await waitForSocketClose();
+      expect(cache.set).not.toHaveBeenCalled();
+    });
+  });
+});
diff --git a/plugins/techdocs-backend/src/cache/cacheMiddleware.ts b/plugins/techdocs-backend/src/cache/cacheMiddleware.ts
index 0c06052ba4..c25d002181 100644
--- a/plugins/techdocs-backend/src/cache/cacheMiddleware.ts
+++ b/plugins/techdocs-backend/src/cache/cacheMiddleware.ts
@@ -114,8 +114,10 @@ export const createCacheMiddleware = ({
     // When a socket is closed, if there were no errors and the data written
     // over the socket should be cached, cache it!
     socket.on('close', hadError => {
-      if (writeToCache && !hadError) {
-        cache.set(reqPath, Buffer.concat(chunks));
+      const content = Buffer.concat(chunks);
+      const head = content.toString('utf8', 0, 12);
+      if (writeToCache && !hadError && head === 'HTTP/1.1 200') {
+        cache.set(reqPath, content);
       }
     });