diff --git a/protos/local/core_interface.proto b/protos/local/core_interface.proto
index 7bb0f5cff..aa7a91d15 100644
--- a/protos/local/core_interface.proto
+++ b/protos/local/core_interface.proto
@@ -21,3 +21,9 @@ message ActivityHeartbeat {
     string activity_id = 1;
     repeated common.Payload details = 2;
 }
+
+// A request as given to [crate::Core::complete_activity_task]
+message ActivityTaskCompletion {
+    bytes task_token = 1;
+    activity_result.ActivityResult result = 2;
+}
diff --git a/src/lib.rs b/src/lib.rs
index 30aead89b..558ae5b16 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -35,7 +35,7 @@ use crate::{
             activity_task::ActivityTask,
             workflow_activation::WfActivation,
             workflow_completion::{wf_activation_completion, WfActivationCompletion},
-            ActivityHeartbeat,
+            ActivityHeartbeat, ActivityTaskCompletion,
         },
         temporal::api::{
             enums::v1::WorkflowTaskFailedCause, workflowservice::v1::PollWorkflowTaskQueueResponse,
@@ -83,11 +83,7 @@ pub trait Core: Send + Sync {
     async fn complete_workflow_task(&self, completion: WfActivationCompletion) -> Result<()>;
 
     /// Tell the core that an activity has finished executing
-    async fn complete_activity_task(
-        &self,
-        task_token: Vec<u8>,
-        result: ActivityResult,
-    ) -> Result<()>;
+    async fn complete_activity_task(&self, completion: ActivityTaskCompletion) -> Result<()>;
 
     /// Indicate that a long running activity is still making progress
     async fn send_activity_heartbeat(&self, task_token: ActivityHeartbeat) -> Result<()>;
@@ -311,11 +307,16 @@ where
     }
 
     #[instrument(skip(self))]
-    async fn complete_activity_task(
-        &self,
-        task_token: Vec<u8>,
-        result: ActivityResult,
-    ) -> Result<()> {
+    async fn complete_activity_task(&self, completion: ActivityTaskCompletion) -> Result<()> {
+        let task_token = completion.task_token;
+        let result = if let Some(r) = completion.result {
+            r
+        } else {
+            return Err(CoreError::MalformedActivityCompletion {
+                reason: "Activity completion had empty result field".to_owned(),
+                completion: None,
+            });
+        };
         let status = if let Some(s) = result.status {
             s
         } else {
diff --git a/tests/integ_tests/simple_wf_tests.rs b/tests/integ_tests/simple_wf_tests.rs
index 61af33631..eaf6690bc 100644
--- a/tests/integ_tests/simple_wf_tests.rs
+++ b/tests/integ_tests/simple_wf_tests.rs
@@ -3,6 +3,7 @@ use crossbeam::channel::{unbounded, RecvTimeoutError};
 use futures::{channel::mpsc::UnboundedReceiver, future, Future, SinkExt, StreamExt};
 use rand::{self, Rng};
 use std::{collections::HashMap, convert::TryFrom, env, sync::Arc, time::Duration};
+use temporal_sdk_core::protos::coresdk::ActivityTaskCompletion;
 use temporal_sdk_core::{
     protos::coresdk::{
         activity_result::{self, activity_result as act_res, ActivityResult},
@@ -133,10 +134,10 @@ async fn activity_workflow() {
         metadata: Default::default(),
     };
     // Complete activity successfully.
-    core.complete_activity_task(
-        task.task_token,
-        ActivityResult::ok(response_payload.clone()),
-    )
+    core.complete_activity_task(ActivityTaskCompletion {
+        task_token: task.task_token,
+        result: Some(ActivityResult::ok(response_payload.clone())),
+    })
     .await
     .unwrap();
     // Poll workflow task and verify that activity has succeeded.
@@ -192,16 +193,16 @@ async fn activity_non_retryable_failure() {
         non_retryable: true,
         ..Default::default()
     };
-    core.complete_activity_task(
-        task.task_token,
-        ActivityResult {
+    core.complete_activity_task(ActivityTaskCompletion {
+        task_token: task.task_token,
+        result: Some(ActivityResult {
             status: Some(activity_result::activity_result::Status::Failed(
                 activity_result::Failure {
                     failure: Some(failure.clone()),
                 },
             )),
-        },
-    )
+        }),
+    })
     .await
     .unwrap();
     // Poll workflow task and verify that activity has failed.
@@ -256,16 +257,16 @@ async fn activity_retry() {
         non_retryable: false,
         ..Default::default()
     };
-    core.complete_activity_task(
-        task.task_token,
-        ActivityResult {
+    core.complete_activity_task(ActivityTaskCompletion {
+        task_token: task.task_token,
+        result: Some(ActivityResult {
             status: Some(activity_result::activity_result::Status::Failed(
                 activity_result::Failure {
                     failure: Some(failure),
                 },
             )),
-        },
-    )
+        }),
+    })
     .await
     .unwrap();
     // Poll 2nd time
@@ -281,16 +282,10 @@ async fn activity_retry() {
         data: b"hello ".to_vec(),
         metadata: Default::default(),
     };
-    core.complete_activity_task(
-        task.task_token,
-        ActivityResult {
-            status: Some(activity_result::activity_result::Status::Completed(
-                activity_result::Success {
-                    result: Some(response_payload.clone()),
-                },
-            )),
-        },
-    )
+    core.complete_activity_task(ActivityTaskCompletion {
+        task_token: task.task_token,
+        result: Some(ActivityResult::ok(response_payload.clone())),
+    })
     .await
     .unwrap();
     // Poll workflow task and verify activity has succeeded.