diff --git a/src/main/java/com/google/devtools/build/lib/actions/ExecutionRequirements.java b/src/main/java/com/google/devtools/build/lib/actions/ExecutionRequirements.java
index 7c8f959fdc8677..d6b593b1301764 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/ExecutionRequirements.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/ExecutionRequirements.java
@@ -160,6 +160,8 @@ public String parseIfMatches(String tag) throws ValidationException {
   /** Specify the type of worker protocol the worker uses. */
   public static final String REQUIRES_WORKER_PROTOCOL = "requires-worker-protocol";
 
+  public static final String SUPPORTS_WORKER_CANCELLATION = "supports-worker-cancellation";
+
   /** Denotes what the type of worker protocol the worker uses. */
   public enum WorkerProtocolFormat {
     JSON,
diff --git a/src/main/java/com/google/devtools/build/lib/actions/Spawns.java b/src/main/java/com/google/devtools/build/lib/actions/Spawns.java
index 06e4b613497bc7..640bde6899938c 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/Spawns.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/Spawns.java
@@ -92,6 +92,11 @@ public static boolean supportsMultiplexWorkers(Spawn spawn) {
         .equals(spawn.getExecutionInfo().get(ExecutionRequirements.SUPPORTS_MULTIPLEX_WORKERS));
   }
 
+  public static boolean supportsWorkerCancellation(Spawn spawn) {
+    return "1"
+        .equals(spawn.getExecutionInfo().get(ExecutionRequirements.SUPPORTS_WORKER_CANCELLATION));
+  }
+
   /**
    * Returns which worker protocol format a Spawn claims a persistent worker uses. Defaults to proto
    * if the protocol format is not specified.
diff --git a/src/main/java/com/google/devtools/build/lib/worker/JsonWorkerProtocol.java b/src/main/java/com/google/devtools/build/lib/worker/JsonWorkerProtocol.java
index 8c594515eedd26..2c24073b6241d0 100644
--- a/src/main/java/com/google/devtools/build/lib/worker/JsonWorkerProtocol.java
+++ b/src/main/java/com/google/devtools/build/lib/worker/JsonWorkerProtocol.java
@@ -92,10 +92,12 @@ private WorkResponse parseResponse() throws IOException {
             requestId = reader.nextInt();
             break;
           default:
-            throw new IOException(name + " is an incorrect field in work response");
+            // As per https://docs.bazel.build/versions/master/creating-workers.html#work-responses,
+            // unknown fields are ignored.
+            reader.skipValue();
         }
       }
-    reader.endObject();
+      reader.endObject();
     } catch (MalformedJsonException | EOFException | IllegalStateException e) {
       throw new IOException("Could not parse json work request correctly", e);
     }
diff --git a/src/main/java/com/google/devtools/build/lib/worker/WorkerOptions.java b/src/main/java/com/google/devtools/build/lib/worker/WorkerOptions.java
index dc07ca87e6ca72..dcc21983b55bde 100644
--- a/src/main/java/com/google/devtools/build/lib/worker/WorkerOptions.java
+++ b/src/main/java/com/google/devtools/build/lib/worker/WorkerOptions.java
@@ -178,4 +178,12 @@ public String getTypeDescription() {
           "Currently a no-op. Future: If enabled, workers that support the experimental"
               + " multiplexing feature will use that feature.")
   public boolean workerMultiplex;
+
+  @Option(
+      name = "experimental_worker_cancellation",
+      defaultValue = "false",
+      documentationCategory = OptionDocumentationCategory.UNCATEGORIZED,
+      effectTags = {OptionEffectTag.EXECUTION},
+      help = "If enabled, Bazel may send cancellation requests to workers that support them.")
+  public boolean workerCancellation;
 }
diff --git a/src/main/protobuf/worker_protocol.proto b/src/main/protobuf/worker_protocol.proto
index c628b7eb7a4169..a5f95453be2d82 100644
--- a/src/main/protobuf/worker_protocol.proto
+++ b/src/main/protobuf/worker_protocol.proto
@@ -39,9 +39,19 @@ message WorkRequest {
   // request.
   repeated Input inputs = 2;
 
-  // To support multiplex worker, each WorkRequest must have an unique ID. This
-  // ID should be attached unchanged to the WorkResponse.
+  // Each WorkRequest must have either a unique
+  // request_id or request_id = 0. If request_id is 0, this WorkRequest must be
+  // processed alone, otherwise the worker may process multiple WorkRequests in
+  // parallel (multiplexing). As an exception to the above, if the cancel field
+  // is true, the request_id must be the same as a previously sent WorkRequest.
+  // The request_id must be attached unchanged to the corresponding
+  // WorkResponse.
   int32 request_id = 3;
+
+  // EXPERIMENTAL: When true, this is a cancel request, indicating that a
+  // previously sent WorkRequest with the same request_id should be cancelled.
+  // The arguments and inputs fields must be empty and should be ignored.
+  bool cancel = 4;
 }
 
 // The worker sends this message to Blaze when it finished its work on the
@@ -54,9 +64,17 @@ message WorkResponse {
   // string type here, which gives us UTF-8 encoding.
   string output = 2;
 
-  // To support multiplex worker, each WorkResponse must have an unique ID.
-  // Since worker processes which support multiplex worker will handle multiple
-  // WorkRequests in parallel, this ID will be used to determined which
-  // WorkerProxy does this WorkResponse belong to.
+  // This field must be set to the same request_id as the WorkRequest it is a
+  // response to. Since worker processes which support multiplex worker will
+  // handle multiple WorkRequests in parallel, this ID will be used to
+  // determined which WorkerProxy does this WorkResponse belong to.
   int32 request_id = 3;
+
+  // EXPERIMENTAL When true, indicates that this response was sent due to
+  // receiving a cancel request. The exit_code and output fields should be empty
+  // and will be ignored. Exactly one WorkResponse must be sent for each
+  // non-cancelling WorkRequest received by the worker, but if the worker
+  // received a cancel request, it doesn't matter if it replies with a regular
+  // WorkResponse or with one where was_cancelled = true.
+  bool was_cancelled = 4;
 }
diff --git a/src/test/java/com/google/devtools/build/lib/worker/WorkerTest.java b/src/test/java/com/google/devtools/build/lib/worker/WorkerTest.java
index a7f22586712e97..e67aa06429b678 100644
--- a/src/test/java/com/google/devtools/build/lib/worker/WorkerTest.java
+++ b/src/test/java/com/google/devtools/build/lib/worker/WorkerTest.java
@@ -199,8 +199,16 @@ public void testGetResponse_json_multipleRequestId_fails() throws IOException {
   }
 
   @Test
-  public void testGetResponse_json_incorrectFields_fails() throws IOException {
-    verifyGetResponseFailure(
-        "{\"testField\":0}", "testField is an incorrect field in work response");
+  public void testGetResponse_json_unknownFieldsIgnored() throws IOException, InterruptedException {
+    TestWorker testWorker =
+        createTestWorker(
+            "{\"exitCode\":1,\"output\":\"test output\",\"requestId\":1,\"unknown\":{1:['a']}}"
+                .getBytes(UTF_8),
+            JSON);
+    WorkResponse readResponse = testWorker.getResponse(1);
+    WorkResponse response =
+        WorkResponse.newBuilder().setExitCode(1).setOutput("test output").setRequestId(1).build();
+
+    assertThat(readResponse).isEqualTo(response);
   }
 }