Addressing PR comments

Author: Evanthx

temporal-sdk/src/main/java/io/temporal/internal/client/RootWorkflowClientInvoker.java

@@ -10,6 +10,7 @@
 import io.grpc.Status;
 import io.grpc.StatusRuntimeException;
 import io.temporal.api.common.v1.*;
+import io.temporal.api.enums.v1.EventType;
 import io.temporal.api.enums.v1.UpdateWorkflowExecutionLifecycleStage;
 import io.temporal.api.enums.v1.WorkflowExecutionStatus;
 import io.temporal.api.errordetails.v1.MultiOperationExecutionFailure;
@@ -105,7 +106,26 @@ public WorkflowStartOutput start(WorkflowStartInput input) {
       if (CurrentNexusOperationContext.isNexusContext()) {
         // Auto-capture the start-workflow backlink so the task handler drains it onto the
         // StartOperationResponse, the same path used for signal/signalWithStart responses.
-        CurrentNexusOperationContext.get().addBacklink(response.getLink());
+        if (response.hasLink()) {
+          CurrentNexusOperationContext.get().addBacklink(response.getLink());
+        } else {
+          // Older servers (pre-1.31) don't return a link on the start response. Fabricate one
+          // pointing at the started workflow's WorkflowExecutionStarted event so the caller still
+          // gets a backlink.
+          CurrentNexusOperationContext.get()
+              .addBacklink(
+                  Link.newBuilder()
+                      .setWorkflowEvent(
+                          Link.WorkflowEvent.newBuilder()
+                              .setNamespace(clientOptions.getNamespace())
+                              .setWorkflowId(execution.getWorkflowId())
+                              .setRunId(execution.getRunId())
+                              .setEventRef(
+                                  Link.WorkflowEvent.EventReference.newBuilder()
+                                      .setEventType(
+                                          EventType.EVENT_TYPE_WORKFLOW_EXECUTION_STARTED)))
+                      .build());
+        }
       }
       return new WorkflowStartOutput(execution);
     }
@@ -171,9 +191,6 @@ public WorkflowSignalWithStartOutput signalWithStart(WorkflowSignalWithStartInpu
     boolean inNexusContext = CurrentNexusOperationContext.isNexusContext();
     if (inNexusContext) {
       requestBuilder.addAllLinks(CurrentNexusOperationContext.get().getNexusOperationLinks());
-    } else {
-      log.debug(
-          "signalWithStart RPC issued outside a Nexus operation context; no link propagation");
     }
     SignalWithStartWorkflowExecutionRequest request = requestBuilder.build();
     SignalWithStartWorkflowExecutionResponse response = genericClient.signalWithStart(request);

temporal-sdk/src/main/java/io/temporal/internal/nexus/InternalNexusOperationContext.java

@@ -92,8 +92,8 @@ public void setNexusOperationLinks(List<Link> links) {
 
   /**
    * Append a backlink returned by an outbound RPC the operation handler issued (e.g. signal,
-   * signalWithStart, etc). The task handler drains the list when building the
-   * operation's StartOperationResponse.
+   * signalWithStart, etc). The task handler drains the list when building the operation's
+   * StartOperationResponse.
    */
   public void addBacklink(Link link) {
     if (link != null) {

temporal-sdk/src/test/java/io/temporal/internal/client/RootWorkflowClientInvokerLinkPropagationTest.java

@@ -13,6 +13,8 @@
 import io.temporal.api.workflowservice.v1.SignalWithStartWorkflowExecutionResponse;
 import io.temporal.api.workflowservice.v1.SignalWorkflowExecutionRequest;
 import io.temporal.api.workflowservice.v1.SignalWorkflowExecutionResponse;
+import io.temporal.api.workflowservice.v1.StartWorkflowExecutionRequest;
+import io.temporal.api.workflowservice.v1.StartWorkflowExecutionResponse;
 import io.temporal.client.WorkflowClient;
 import io.temporal.client.WorkflowClientOptions;
 import io.temporal.client.WorkflowOptions;
@@ -232,6 +234,51 @@ public void mixedSignalAndSignalWithStartAccumulateAllBacklinks() {
         nexusCtx.getBacklinks());
   }
 
+  /**
+   * Flag-enabled server: the start response carries a backlink; the SDK captures it verbatim onto
+   * the operation context (used by the WorkflowRunOperation async path).
+   */
+  @Test
+  public void startUsesServerStartLinkWhenPresent() {
+    Link serverLink =
+        workflowEventLink(
+            WORKFLOW_ID, "target-run", EventType.EVENT_TYPE_WORKFLOW_EXECUTION_STARTED);
+    when(genericClient.start(any(StartWorkflowExecutionRequest.class)))
+        .thenReturn(
+            StartWorkflowExecutionResponse.newBuilder()
+                .setRunId("target-run")
+                .setLink(serverLink)
+                .build());
+
+    invoker.start(newStartInput());
+
+    List<Link> captured = nexusCtx.getBacklinks();
+    Assert.assertEquals("expected the server-provided start link", 1, captured.size());
+    Assert.assertEquals(serverLink, captured.get(0));
+  }
+
+  /**
+   * Older server (pre-1.31): the start response has no link. The SDK must fabricate a backlink
+   * pointing at the started workflow's WorkflowExecutionStarted event so the caller still links to
+   * the callee.
+   */
+  @Test
+  public void startFabricatesStartLinkWhenServerOmitsIt() {
+    when(genericClient.start(any(StartWorkflowExecutionRequest.class)))
+        .thenReturn(StartWorkflowExecutionResponse.newBuilder().setRunId("target-run").build());
+
+    invoker.start(newStartInput());
+
+    List<Link> captured = nexusCtx.getBacklinks();
+    Assert.assertEquals("expected one fabricated backlink", 1, captured.size());
+    Link.WorkflowEvent we = captured.get(0).getWorkflowEvent();
+    Assert.assertEquals(NAMESPACE, we.getNamespace());
+    Assert.assertEquals(WORKFLOW_ID, we.getWorkflowId());
+    Assert.assertEquals("target-run", we.getRunId());
+    Assert.assertEquals(
+        EventType.EVENT_TYPE_WORKFLOW_EXECUTION_STARTED, we.getEventRef().getEventType());
+  }
+
   // ── helpers ──────────────────────────────────────────────────────────────────────────────
 
   private static WorkflowSignalInput newSignalInput() {
@@ -251,6 +298,14 @@ private static WorkflowSignalWithStartInput newSignalWithStartInput() {
         startInput, "test-signal", new Object[] {"signal-payload"});
   }
 
+  private static WorkflowStartInput newStartInput() {
+    // Disable eager execution so start() takes the plain RPC path (no local worker dispatch).
+    WorkflowOptions options =
+        WorkflowOptions.newBuilder().setTaskQueue("tq").setDisableEagerExecution(true).build();
+    return new WorkflowStartInput(
+        WORKFLOW_ID, "TestWorkflow", Header.empty(), new Object[] {}, options);
+  }
+
   private static Link workflowEventLink(String workflowId, String runId, EventType eventType) {
     return Link.newBuilder()
         .setWorkflowEvent(