diff --git a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-etag-decorator.js b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-etag-decorator.js
index 21aa6cb..3570081 100644
--- a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-etag-decorator.js
+++ b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-etag-decorator.js
@@ -25,6 +25,14 @@
     this._payloadCache = new Map();
   }
 
+  /**
+   * Get or upgrade fetch options to include an ETag in a request.
+   * @param {string} url The URL being fetched.
+   * @param {!Object=} opt_options Optional options object in which to include
+   *     the ETag request header. If omitted, the result will be a fresh option
+   *     set.
+   * @return {!Object}
+   */
   GrEtagDecorator.prototype.getOptions = function(url, opt_options) {
     const etag = this._etags.get(url);
     if (!etag) {
@@ -36,6 +44,16 @@
     return options;
   };
 
+  /**
+   * Handle a response to a request with ETag headers, potentially incorporating
+   * its result in the payload cache.
+   *
+   * @param {string} url The URL of the request.
+   * @param {!Response} response The response object.
+   * @param {string} payload The raw, unparsed JSON contained in the response
+   *     body. Note: because response.text() cannot be read twice, this must be
+   *     provided separately.
+   */
   GrEtagDecorator.prototype.collect = function(url, response, payload) {
     if (!response ||
         !response.ok ||
@@ -54,19 +72,19 @@
     }
   };
 
+  /**
+   * Get the cached payload for a given URL.
+   * @param {string} url
+   * @return {string|undefined} Returns the unparsed JSON payload from the
+   *     cache.
+   */
   GrEtagDecorator.prototype.getCachedPayload = function(url) {
-    let payload = this._payloadCache.get(url);
-
-    if (typeof payload === 'object') {
-      // Note: For the sake of cache transparency, deep clone the response
-      // object so that cache hits are not equal object references. Some code
-      // expects every network response to deserialize to a fresh object.
-      payload = JSON.parse(JSON.stringify(payload));
-    }
-
-    return payload;
+    return this._payloadCache.get(url);
   };
 
+  /**
+   * Limit the cache size to MAX_CACHE_SIZE.
+   */
   GrEtagDecorator.prototype._truncateCache = function() {
     for (const url of this._etags.keys()) {
       if (this._etags.size <= MAX_CACHE_SIZE) {
diff --git a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-etag-decorator_test.html b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-etag-decorator_test.html
index 40e639e..77edae7 100644
--- a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-etag-decorator_test.html
+++ b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-etag-decorator_test.html
@@ -92,25 +92,5 @@
       etag.collect('/foo', fakeRequest('bar', 200), 'new payload');
       assert.strictEqual(etag.getCachedPayload('/foo'), 'new payload');
     });
-
-    test('getCachedPayload does not preserve object equality', () => {
-      const payload = {foo: 'bar'};
-      etag.collect('/foo', fakeRequest('bar'), payload);
-      assert.deepEqual(etag.getCachedPayload('/foo'), payload);
-      assert.notStrictEqual(etag.getCachedPayload('/foo'), payload);
-      etag.collect('/foo', fakeRequest('bar', 304), {foo: 'baz'});
-      assert.deepEqual(etag.getCachedPayload('/foo'), payload);
-      assert.notStrictEqual(etag.getCachedPayload('/foo'), payload);
-      etag.collect('/foo', fakeRequest('bar', 200), {foo: 'bar baz'});
-      assert.deepEqual(etag.getCachedPayload('/foo'), {foo: 'bar baz'});
-      assert.notStrictEqual(etag.getCachedPayload('/foo'), {foo: 'bar baz'});
-    });
-
-    test('getCachedPayload clones the response deeply', () => {
-      const payload = {foo: {bar: 'baz'}};
-      etag.collect('/foo', fakeRequest('bar'), payload);
-      assert.deepEqual(etag.getCachedPayload('/foo'), payload);
-      assert.notStrictEqual(etag.getCachedPayload('/foo').foo, payload.foo);
-    });
   });
 </script>
diff --git a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.js b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.js
index 684a967..210c42f 100644
--- a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.js
+++ b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.js
@@ -186,17 +186,34 @@
      * @return {?}
      */
     getResponseObject(response) {
+      return this._readResponsePayload(response)
+          .then(payload => payload.parsed);
+    },
+
+    /**
+     * @param {!Object} response
+     * @return {!Object}
+     */
+    _readResponsePayload(response) {
       return response.text().then(text => {
         let result;
         try {
-          result = JSON.parse(text.substring(JSON_PREFIX.length));
+          result = this._parsePrefixedJSON(text);
         } catch (_) {
           result = null;
         }
-        return result;
+        return {parsed: result, raw: text};
       });
     },
 
+    /**
+     * @param {string} source
+     * @return {?}
+     */
+    _parsePrefixedJSON(source) {
+      return JSON.parse(source.substring(JSON_PREFIX.length));
+    },
+
     getConfig() {
       return this._fetchSharedCacheURL('/config/server/info');
     },
@@ -820,8 +837,8 @@
             this._etags.getOptions(urlWithParams))
             .then(response => {
               if (response && response.status === 304) {
-                return Promise.resolve(
-                    this._etags.getCachedPayload(urlWithParams));
+                return Promise.resolve(this._parsePrefixedJSON(
+                    this._etags.getCachedPayload(urlWithParams)));
               }
 
               if (response && !response.ok) {
@@ -834,13 +851,17 @@
               }
 
               const payloadPromise = response ?
-                  this.getResponseObject(response) :
-                  Promise.resolve();
-              payloadPromise.then(payload => {
-                this._etags.collect(urlWithParams, response, payload);
+                  this._readResponsePayload(response) :
+                  Promise.resolve(null);
+
+              return payloadPromise.then(payload => {
+                if (!payload) { return null; }
+
+                this._etags.collect(urlWithParams, response, payload.raw);
                 this._maybeInsertInLookup(payload);
+
+                return payload.parsed;
               });
-              return payloadPromise;
             });
       });
     },
diff --git a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface_test.html b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface_test.html
index b63258f..2058f59 100644
--- a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface_test.html
+++ b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface_test.html
@@ -762,25 +762,90 @@
       });
     });
 
-    test('_getChangeDetail passes params to ETags decorator', () => {
-      const changeNum = 4321;
-      element._projectLookup[changeNum] = 'test';
-      const params = {foo: 'bar'};
-      const expectedUrl = '/changes/test~4321/detail?foo=bar';
-      sandbox.stub(element._etags, 'getOptions');
-      sandbox.stub(element._etags, 'collect');
-      return element._getChangeDetail(changeNum, params).then(() => {
-        assert.isTrue(element._etags.getOptions.calledWithExactly(expectedUrl));
-        assert.equal(element._etags.collect.lastCall.args[0], expectedUrl);
+    suite('_getChangeDetail', () => {
+      test('_getChangeDetail passes params to ETags decorator', () => {
+        const changeNum = 4321;
+        element._projectLookup[changeNum] = 'test';
+        const params = {foo: 'bar'};
+        const expectedUrl = '/changes/test~4321/detail?foo=bar';
+        sandbox.stub(element._etags, 'getOptions');
+        sandbox.stub(element._etags, 'collect');
+        return element._getChangeDetail(changeNum, params).then(() => {
+          assert.isTrue(element._etags.getOptions.calledWithExactly(
+              expectedUrl));
+          assert.equal(element._etags.collect.lastCall.args[0], expectedUrl);
+        });
       });
-    });
 
-    test('_getChangeDetail calls errFn on 500', () => {
-      const errFn = sinon.stub();
-      sandbox.stub(element, '_fetchRawJSON')
-          .returns(Promise.resolve({ok: false, status: 500}));
-      return element._getChangeDetail(123, {}, errFn).then(() => {
-        assert.isTrue(errFn.called);
+      test('_getChangeDetail calls errFn on 500', () => {
+        const errFn = sinon.stub();
+        sandbox.stub(element, '_fetchRawJSON')
+            .returns(Promise.resolve({ok: false, status: 500}));
+        return element._getChangeDetail(123, {}, errFn).then(() => {
+          assert.isTrue(errFn.called);
+        });
+      });
+
+      test('_getChangeDetail populates _projectLookup', () => {
+        sandbox.stub(element, '_fetchRawJSON')
+            .returns(Promise.resolve({ok: true}));
+
+        const mockResponse = {_number: 1, project: 'test'};
+        sandbox.stub(element, '_readResponsePayload').returns(Promise.resolve({
+          parsed: mockResponse,
+          raw: JSON.stringify(mockResponse),
+        }));
+        return element._getChangeDetail(1).then(() => {
+          assert.equal(Object.keys(element._projectLookup).length, 1);
+          assert.equal(element._projectLookup[1], 'test');
+        });
+      });
+
+      suite('_getChangeDetail ETag cache', () => {
+        let requestUrl;
+        let mockResponseSerial;
+        let collectSpy;
+        let getPayloadSpy;
+
+        setup(() => {
+          requestUrl = '/foo/bar';
+          const mockResponse = {foo: 'bar', baz: 42};
+          mockResponseSerial = element.JSON_PREFIX +
+              JSON.stringify(mockResponse);
+          sandbox.stub(element, '_urlWithParams').returns(requestUrl);
+          sandbox.stub(element, 'getChangeActionURL')
+              .returns(Promise.resolve(requestUrl));
+          collectSpy = sandbox.spy(element._etags, 'collect');
+          getPayloadSpy = sandbox.spy(element._etags, 'getCachedPayload');
+        });
+
+        test('contributes to cache', () => {
+          sandbox.stub(element, '_fetchRawJSON').returns(Promise.resolve({
+            text: () => Promise.resolve(mockResponseSerial),
+            status: 200,
+            ok: true,
+          }));
+
+          return element._getChangeDetail(123, {}).then(detail => {
+            assert.isFalse(getPayloadSpy.called);
+            assert.isTrue(collectSpy.calledOnce);
+            const cachedResponse = element._etags.getCachedPayload(requestUrl);
+            assert.equal(cachedResponse, mockResponseSerial);
+          });
+        });
+
+        test('uses cache on HTTP 304', () => {
+          sandbox.stub(element, '_fetchRawJSON').returns(Promise.resolve({
+            text: () => Promise.resolve(mockResponseSerial),
+            status: 304,
+            ok: true,
+          }));
+
+          return element._getChangeDetail(123, {}).then(detail => {
+            assert.isFalse(collectSpy.called);
+            assert.isTrue(getPayloadSpy.calledOnce);
+          });
+        });
       });
     });
 
@@ -858,17 +923,6 @@
       });
     });
 
-    test('getChangeDetail populates _projectLookup', () => {
-      sandbox.stub(element, '_fetchRawJSON')
-          .returns(Promise.resolve({ok: true}));
-      sandbox.stub(element, 'getResponseObject')
-          .returns(Promise.resolve({_number: 1, project: 'test'}));
-      return element._getChangeDetail(1).then(() => {
-        assert.equal(Object.keys(element._projectLookup).length, 1);
-        assert.equal(element._projectLookup[1], 'test');
-      });
-    });
-
     test('_getChangeURLAndFetch', () => {
       element._projectLookup = {1: 'test'};
       const fetchStub = sandbox.stub(element, 'fetchJSON')
@@ -886,5 +940,24 @@
             '/changes/test~1/revisions/1/test'));
       });
     });
+
+    suite('reading responses', () => {
+      test('_readResponsePayload', () => {
+        const mockObject = {foo: 'bar', baz: 'foo'};
+        const serial = element.JSON_PREFIX + JSON.stringify(mockObject);
+        const mockResponse = {text: () => Promise.resolve(serial)};
+        return element._readResponsePayload(mockResponse).then(payload => {
+          assert.deepEqual(payload.parsed, mockObject);
+          assert.equal(payload.raw, serial);
+        });
+      });
+
+      test('_parsePrefixedJSON', () => {
+        const obj = {x: 3, y: {z: 4}, w: 23};
+        const serial = element.JSON_PREFIX + JSON.stringify(obj);
+        const result = element._parsePrefixedJSON(serial);
+        assert.deepEqual(result, obj);
+      });
+    });
   });
 </script>