diff --git a/internal/internal_task_handlers.go b/internal/internal_task_handlers.go
index f146e5296..f59c71c93 100644
--- a/internal/internal_task_handlers.go
+++ b/internal/internal_task_handlers.go
@@ -179,6 +179,7 @@ type (
 		next           []*historypb.HistoryEvent
 		nextFlags      []sdkFlag
 		binaryChecksum string
+		lastBuildID    string
 		sdkVersion     string
 		sdkName        string
 	}
@@ -197,6 +198,7 @@ type (
 		flags          []sdkFlag
 		msgs           []*protocolpb.Message
 		binaryChecksum string
+		lastBuildID    string
 		sdkVersion     string
 		sdkName        string
 	}
@@ -207,6 +209,7 @@ type (
 		flags          []sdkFlag
 		sdkVersion     string
 		sdkName        string
+		buildID        string
 	}
 )
 
@@ -267,10 +270,16 @@ func (eh *history) isNextWorkflowTaskFailed() (task finishedTask, err error) {
 		nextEventType := nextEvent.GetEventType()
 		isFailed := nextEventType == enumspb.EVENT_TYPE_WORKFLOW_TASK_TIMED_OUT || nextEventType == enumspb.EVENT_TYPE_WORKFLOW_TASK_FAILED
 		var binaryChecksum string
+		buildID := ""
 		var flags []sdkFlag
 		if nextEventType == enumspb.EVENT_TYPE_WORKFLOW_TASK_COMPLETED {
-			binaryChecksum = nextEvent.GetWorkflowTaskCompletedEventAttributes().BinaryChecksum
-			for _, flag := range nextEvent.GetWorkflowTaskCompletedEventAttributes().GetSdkMetadata().GetLangUsedFlags() {
+			completedAttrs := nextEvent.GetWorkflowTaskCompletedEventAttributes()
+			binaryChecksum = completedAttrs.BinaryChecksum
+			workerVersion := completedAttrs.GetWorkerVersion()
+			if workerVersion != nil {
+				buildID = workerVersion.GetBuildId()
+			}
+			for _, flag := range completedAttrs.GetSdkMetadata().GetLangUsedFlags() {
 				f := sdkFlagFromUint(flag)
 				if !f.isValid() {
 					// If a flag is not recognized (value is too high or not defined), it must fail the workflow task
@@ -282,6 +291,7 @@ func (eh *history) isNextWorkflowTaskFailed() (task finishedTask, err error) {
 		return finishedTask{
 			isFailed:       isFailed,
 			binaryChecksum: binaryChecksum,
+			buildID:        buildID,
 			flags:          flags,
 			sdkName:        nextEvent.GetWorkflowTaskCompletedEventAttributes().GetSdkMetadata().GetSdkName(),
 			sdkVersion:     nextEvent.GetWorkflowTaskCompletedEventAttributes().GetSdkMetadata().GetSdkVersion(),
@@ -342,6 +352,7 @@ func (eh *history) nextTask() (*preparedTask, error) {
 
 	result := eh.next
 	checksum := eh.binaryChecksum
+	lastBuildID := eh.lastBuildID
 	sdkFlags := eh.nextFlags
 	sdkName := eh.sdkName
 	sdkVersion := eh.sdkVersion
@@ -366,6 +377,7 @@ func (eh *history) nextTask() (*preparedTask, error) {
 		flags:          sdkFlags,
 		msgs:           msgs,
 		binaryChecksum: checksum,
+		lastBuildID:    lastBuildID,
 		sdkName:        sdkName,
 		sdkVersion:     sdkVersion,
 	}, nil
@@ -986,6 +998,7 @@ ProcessEvents:
 		historyMessages := nextTask.msgs
 		flags := nextTask.flags
 		binaryChecksum := nextTask.binaryChecksum
+		lastBuildID := nextTask.lastBuildID
 		// Check if we are replaying so we know if we should use the messages in the WFT or the history
 		isReplay := len(reorderedEvents) > 0 && reorderedHistory.IsReplayEvent(reorderedEvents[len(reorderedEvents)-1])
 		var msgs *eventMsgIndex
@@ -1016,6 +1029,7 @@ ProcessEvents:
 		} else {
 			w.workflowInfo.BinaryChecksum = binaryChecksum
 		}
+		w.workflowInfo.lastCompletedBuildID = lastBuildID
 		// Reset the mutable side effect markers recorded
 		eventHandler.mutableSideEffectsRecorded = nil
 		// Markers are from the events that are produced from the current workflow task.
diff --git a/internal/workflow.go b/internal/workflow.go
index 6f86bd711..e9aaa9d98 100644
--- a/internal/workflow.go
+++ b/internal/workflow.go
@@ -1005,6 +1005,11 @@ type WorkflowInfo struct {
 	// build-id based versioning, is the explicitly set worker build id. If this is the first worker to operate on the
 	// workflow, it is this worker's current value.
 	BinaryChecksum string
+	// lastCompletedBuildID, if nonempty, contains the BuildID of the worker which last completed a
+	// workflow task for this workflow. This value may change over the lifetime of the workflow run,
+	// but is deterministic and safe to use for branching. The value is empty if the workflow has
+	// never seen a task completed by a worker with a set BuildId.
+	lastCompletedBuildID string
 
 	continueAsNewSuggested bool
 	currentHistorySize     int
@@ -1016,7 +1021,8 @@ type UpdateInfo struct {
 	ID string
 }
 
-// GetBinaryChecksum return binary checksum.
+// GetBinaryChecksum returns the binary checksum of the last worker to complete a task for this
+// workflow, or if this is the first task, this worker's checksum.
 func (wInfo *WorkflowInfo) GetBinaryChecksum() string {
 	if wInfo.BinaryChecksum == "" {
 		return getBinaryChecksum()
@@ -1024,6 +1030,14 @@ func (wInfo *WorkflowInfo) GetBinaryChecksum() string {
 	return wInfo.BinaryChecksum
 }
 
+// GetLastCompletedBuildID returns the BuildID of the worker which last completed a workflow task
+// for this workflow. This value may change over the lifetime of the workflow run, but is
+// deterministic and safe to use for branching. The returned value is empty if the workflow has
+// never seen a task completed by a worker with a set BuildId.
+func (wInfo *WorkflowInfo) GetLastCompletedBuildID() string {
+	return wInfo.lastCompletedBuildID
+}
+
 // GetCurrentHistoryLength returns the current length of history when called.
 // This value may change throughout the life of the workflow.
 func (wInfo *WorkflowInfo) GetCurrentHistoryLength() int {
diff --git a/test/worker_versioning_test.go b/test/worker_versioning_test.go
index d376d34ad..f943f977a 100644
--- a/test/worker_versioning_test.go
+++ b/test/worker_versioning_test.go
@@ -27,6 +27,9 @@ import (
 	"testing"
 	"time"
 
+	"go.temporal.io/api/common/v1"
+	"go.temporal.io/api/workflowservice/v1"
+
 	"github.com/pborman/uuid"
 	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
@@ -149,8 +152,6 @@ func (ts *WorkerVersioningTestSuite) TestTwoWorkersGetDifferentTasks() {
 	ts.NoError(handle12.Get(ctx, nil))
 	ts.NoError(handle21.Get(ctx, nil))
 	ts.NoError(handle22.Get(ctx, nil))
-
-	// TODO: Actually assert they ran on the appropriate workers, once David's changes are ready
 }
 
 func (ts *WorkerVersioningTestSuite) TestReachabilityUnreachable() {
@@ -273,3 +274,63 @@ func (ts *WorkerVersioningTestSuite) TestReachabilityVersions() {
 	ts.Equal(1, len(taskQueueReachability.TaskQueueReachability))
 	ts.Equal([]client.TaskReachability{client.TaskReachabilityNewWorkflows}, taskQueueReachability.TaskQueueReachability)
 }
+
+func (ts *WorkerVersioningTestSuite) TestBuildIDChangesOverWorkflowLifetime() {
+	ctx, cancel := context.WithTimeout(context.Background(), ctxTimeout)
+	defer cancel()
+
+	err := ts.client.UpdateWorkerBuildIdCompatibility(ctx, &client.UpdateWorkerBuildIdCompatibilityOptions{
+		TaskQueue: ts.taskQueueName,
+		Operation: &client.BuildIDOpAddNewIDInNewDefaultSet{
+			BuildID: "1.0",
+		},
+	})
+	ts.NoError(err)
+
+	worker1 := worker.New(ts.client, ts.taskQueueName, worker.Options{BuildID: "1.0", UseBuildIDForVersioning: true})
+	ts.workflows.register(worker1)
+	ts.NoError(worker1.Start())
+
+	// Start workflow
+	wfHandle, err := ts.client.ExecuteWorkflow(ctx, ts.startWorkflowOptions("evolving-wf"), ts.workflows.WaitSignalToStart)
+	// Send it a made up signal to generate a task
+	ts.NoError(ts.client.SignalWorkflow(ctx, wfHandle.GetID(), wfHandle.GetRunID(), "fakesig", ""))
+	var lastBuildID string
+	enval, err := ts.client.QueryWorkflow(ctx, wfHandle.GetID(), wfHandle.GetRunID(), "get-last-build-id", nil)
+	ts.NoError(err)
+	ts.NoError(enval.Get(&lastBuildID))
+	worker1.Stop()
+	ts.Equal("1.0", lastBuildID)
+
+	// Add new compat ver
+	err = ts.client.UpdateWorkerBuildIdCompatibility(ctx, &client.UpdateWorkerBuildIdCompatibilityOptions{
+		TaskQueue: ts.taskQueueName,
+		Operation: &client.BuildIDOpAddNewCompatibleVersion{
+			BuildID:                   "1.1",
+			ExistingCompatibleBuildID: "1.0",
+		},
+	})
+	ts.NoError(err)
+
+	worker11 := worker.New(ts.client, ts.taskQueueName, worker.Options{BuildID: "1.1", UseBuildIDForVersioning: true})
+	ts.workflows.register(worker11)
+	ts.NoError(worker11.Start())
+	defer worker11.Stop()
+
+	_, err = ts.client.WorkflowService().ResetStickyTaskQueue(ctx, &workflowservice.ResetStickyTaskQueueRequest{
+		Namespace: ts.config.Namespace,
+		Execution: &common.WorkflowExecution{
+			WorkflowId: wfHandle.GetID(),
+		},
+	})
+	ts.NoError(err)
+
+	// finish the workflow
+	ts.NoError(ts.client.SignalWorkflow(ctx, wfHandle.GetID(), wfHandle.GetRunID(), "start-signal", ""))
+	ts.NoError(wfHandle.Get(ctx, nil))
+
+	enval, err = ts.client.QueryWorkflow(ctx, wfHandle.GetID(), wfHandle.GetRunID(), "get-last-build-id", nil)
+	ts.NoError(err)
+	ts.NoError(enval.Get(&lastBuildID))
+	ts.Equal("1.1", lastBuildID)
+}
diff --git a/test/workflow_test.go b/test/workflow_test.go
index db88e3302..81b5dd3e9 100644
--- a/test/workflow_test.go
+++ b/test/workflow_test.go
@@ -1877,6 +1877,10 @@ func (w *Workflows) InterceptorCalls(ctx workflow.Context, someVal string) (stri
 }
 
 func (w *Workflows) WaitSignalToStart(ctx workflow.Context) (string, error) {
+	_ = workflow.SetQueryHandler(ctx, "get-last-build-id", func() (string, error) {
+		return workflow.GetInfo(ctx).GetLastCompletedBuildID(), nil
+	})
+
 	var value string
 	workflow.GetSignalChannel(ctx, "start-signal").Receive(ctx, &value)
 	return value, nil