diff --git a/openapi/openapiv2.json b/openapi/openapiv2.json
index 61d98c55a..b1b88f950 100644
--- a/openapi/openapiv2.json
+++ b/openapi/openapiv2.json
@@ -12173,6 +12173,10 @@
         "startDelay": {
           "type": "string",
           "description": "Time to wait before dispatching the first activity task. This delay is not applied to retry attempts."
+        },
+        "idReusePolicyReturnExistingOutcomeOnReject": {
+          "type": "boolean",
+          "description": "Include the outcome in a success response if the activity has completed, and\nid_reuse_policy would have caused a failure response otherwise.\nThis is applicable only when the id_reuse_policy is ACTIVITY_ID_REUSE_POLICY_REJECT_DUPLICATE,\nor the activity succeeded and id_reuse_policy is ACTIVITY_ID_REUSE_POLICY_ALLOW_DUPLICATE_FAILED_ONLY."
         }
       }
     },
@@ -18946,7 +18950,7 @@
       "properties": {
         "runId": {
           "type": "string",
-          "description": "The run ID of the activity that was started - or used (via ACTIVITY_ID_CONFLICT_POLICY_USE_EXISTING)."
+          "description": "The run ID of the activity that was started - or used (via ACTIVITY_ID_CONFLICT_POLICY_USE_EXISTING),\nor was already completed when request's id_reuse_policy_return_existing_outcome_on_reject\ncauses an outcome to be included in this response."
         },
         "started": {
           "type": "boolean",
@@ -18954,7 +18958,11 @@
         },
         "link": {
           "$ref": "#/definitions/v1Link",
-          "description": "Link to the started activity."
+          "description": "Link to the started activity - or an already completed activity when request's\nid_reuse_policy_return_existing_outcome_on_reject causes an outcome to be included in this."
+        },
+        "outcome": {
+          "$ref": "#/definitions/v1ActivityExecutionOutcome",
+          "description": "Only set if the activity is already completed and id_reuse_policy_return_existing_outcome_on_reject\nwas true in the request. In that case it also populates the run_id and link for that activity."
         }
       }
     },
diff --git a/openapi/openapiv3.yaml b/openapi/openapiv3.yaml
index f7053f93a..331faa86c 100644
--- a/openapi/openapiv3.yaml
+++ b/openapi/openapiv3.yaml
@@ -16377,19 +16377,37 @@ components:
           pattern: ^-?(?:0|[1-9][0-9]{0,11})(?:\.[0-9]{1,9})?s$
           type: string
           description: Time to wait before dispatching the first activity task. This delay is not applied to retry attempts.
+        idReusePolicyReturnExistingOutcomeOnReject:
+          type: boolean
+          description: |-
+            Include the outcome in a success response if the activity has completed, and
+             id_reuse_policy would have caused a failure response otherwise.
+             This is applicable only when the id_reuse_policy is ACTIVITY_ID_REUSE_POLICY_REJECT_DUPLICATE,
+             or the activity succeeded and id_reuse_policy is ACTIVITY_ID_REUSE_POLICY_ALLOW_DUPLICATE_FAILED_ONLY.
     StartActivityExecutionResponse:
       type: object
       properties:
         runId:
           type: string
-          description: The run ID of the activity that was started - or used (via ACTIVITY_ID_CONFLICT_POLICY_USE_EXISTING).
+          description: |-
+            The run ID of the activity that was started - or used (via ACTIVITY_ID_CONFLICT_POLICY_USE_EXISTING),
+             or was already completed when request's id_reuse_policy_return_existing_outcome_on_reject
+             causes an outcome to be included in this response.
         started:
           type: boolean
           description: If true, a new activity was started.
         link:
           allOf:
             - $ref: '#/components/schemas/Link'
-          description: Link to the started activity.
+          description: |-
+            Link to the started activity - or an already completed activity when request's
+             id_reuse_policy_return_existing_outcome_on_reject causes an outcome to be included in this.
+        outcome:
+          allOf:
+            - $ref: '#/components/schemas/ActivityExecutionOutcome'
+          description: |-
+            Only set if the activity is already completed and id_reuse_policy_return_existing_outcome_on_reject
+             was true in the request. In that case it also populates the run_id and link for that activity.
     StartBatchOperationRequest:
       type: object
       properties:
diff --git a/temporal/api/workflowservice/v1/request_response.proto b/temporal/api/workflowservice/v1/request_response.proto
index a4098d9b1..93fee05a1 100644
--- a/temporal/api/workflowservice/v1/request_response.proto
+++ b/temporal/api/workflowservice/v1/request_response.proto
@@ -3169,15 +3169,27 @@ message StartActivityExecutionRequest {
     temporal.api.common.v1.OnConflictOptions on_conflict_options = 21;
     // Time to wait before dispatching the first activity task. This delay is not applied to retry attempts.
     google.protobuf.Duration start_delay = 22;
+
+    // Include the outcome in a success response if the activity has completed, and
+    // id_reuse_policy would have caused a failure response otherwise.
+    // This is applicable only when the id_reuse_policy is ACTIVITY_ID_REUSE_POLICY_REJECT_DUPLICATE,
+    // or the activity succeeded and id_reuse_policy is ACTIVITY_ID_REUSE_POLICY_ALLOW_DUPLICATE_FAILED_ONLY.
+    bool id_reuse_policy_return_existing_outcome_on_reject = 23;
 }
 
 message StartActivityExecutionResponse {
-    // The run ID of the activity that was started - or used (via ACTIVITY_ID_CONFLICT_POLICY_USE_EXISTING).
+    // The run ID of the activity that was started - or used (via ACTIVITY_ID_CONFLICT_POLICY_USE_EXISTING),
+    // or was already completed when request's id_reuse_policy_return_existing_outcome_on_reject
+    // causes an outcome to be included in this response.
     string run_id = 1;
     // If true, a new activity was started.
     bool started = 2;
-    // Link to the started activity.
+    // Link to the started activity - or an already completed activity when request's
+    // id_reuse_policy_return_existing_outcome_on_reject causes an outcome to be included in this.
     temporal.api.common.v1.Link link = 3;
+    // Only set if the activity is already completed and id_reuse_policy_return_existing_outcome_on_reject
+    // was true in the request. In that case it also populates the run_id and link for that activity.
+    temporal.api.activity.v1.ActivityExecutionOutcome outcome = 4;
 }
 
 message DescribeActivityExecutionRequest {