diff --git a/.changeset/event-write-stso-spans.md b/.changeset/event-write-stso-spans.md
new file mode 100644
index 0000000000..7569167b32
--- /dev/null
+++ b/.changeset/event-write-stso-spans.md
@@ -0,0 +1,5 @@
+---
+'@workflow/world-vercel': patch
+---
+
+Add the Workflow client version, step-to-step overhead, and active latency optimizations to terminal event-write client spans for service and version latency analysis.
diff --git a/packages/world-vercel/src/events-v4.ts b/packages/world-vercel/src/events-v4.ts
index 93dd6ba96f..8daf076600 100644
--- a/packages/world-vercel/src/events-v4.ts
+++ b/packages/world-vercel/src/events-v4.ts
@@ -62,12 +62,16 @@ import { deserializeStep, StepWireSchema } from './steps.js';
 import {
   ErrorType,
   NetworkProtocolName,
+  StepLatencyOptimizations,
+  StepStsoMs,
+  WorkflowClientVersion,
   WorkflowEventsTransport,
   WorkflowEventType,
   WorkflowWsRequestId,
   WorkflowWsUrl,
 } from './telemetry.js';
 import { type APIConfig, getHttpConfig, getHttpUrl } from './utils.js';
+import { version } from './version.js';
 import type { WsFrameReply } from './ws-transport.js';
 import { isWsEventsTransportEnabled } from './ws-transport-enabled.js';
 
@@ -99,7 +103,7 @@ async function fetchV4(
   init: { method: string; headers: Headers; body?: Uint8Array },
   config: APIConfig | undefined,
   opName: string,
-  attributes?: Record<string, string | number>
+  attributes?: Record<string, string | number | string[]>
 ): Promise<Response> {
   const dispatcher = getEventsDispatcher(config);
   return instrumentedFetch({
@@ -709,6 +713,11 @@ async function postWorkflowRunEventV4(
     {
       ...WorkflowEventsTransport('http'),
       ...WorkflowEventType(input.eventType),
+      ...WorkflowClientVersion(`@workflow/world-vercel/${version}`),
+      ...(input.stso !== undefined ? StepStsoMs(input.stso) : {}),
+      ...(input.optimizations !== undefined
+        ? StepLatencyOptimizations(input.optimizations)
+        : {}),
     }
   );
 }
@@ -930,6 +939,11 @@ async function postEventFrameOverWs(
       attributes: {
         ...WorkflowEventsTransport('ws'),
         ...WorkflowEventType(input.eventType),
+        ...WorkflowClientVersion(`@workflow/world-vercel/${version}`),
+        ...(input.stso !== undefined ? StepStsoMs(input.stso) : {}),
+        ...(input.optimizations !== undefined
+          ? StepLatencyOptimizations(input.optimizations)
+          : {}),
         ...NetworkProtocolName('websocket'),
         ...WorkflowWsUrl(wsUrl),
       },
diff --git a/packages/world-vercel/src/http-core.ts b/packages/world-vercel/src/http-core.ts
index 9e7b4fd35c..371c728adf 100644
--- a/packages/world-vercel/src/http-core.ts
+++ b/packages/world-vercel/src/http-core.ts
@@ -364,7 +364,7 @@ export interface HttpClientSpanOptions {
    */
   spanName?: string;
   /** Extra attributes merged on top of the standard HTTP attributes. */
-  attributes?: Record<string, string | number>;
+  attributes?: Record<string, string | number | string[]>;
 }
 
 /**
diff --git a/packages/world-vercel/src/telemetry.ts b/packages/world-vercel/src/telemetry.ts
index 0f75669eb2..948edae3dd 100644
--- a/packages/world-vercel/src/telemetry.ts
+++ b/packages/world-vercel/src/telemetry.ts
@@ -282,6 +282,19 @@ export const WorkflowEventType = SemanticConvention<string>(
   'workflow.event.type'
 );
 
+/** Version of the Workflow client package issuing the request. */
+export const WorkflowClientVersion = SemanticConvention<string>(
+  'workflow.client.version'
+);
+
+/** Client-measured step-to-step overhead in milliseconds. */
+export const StepStsoMs = SemanticConvention<number>('step.stso_ms');
+
+/** Runtime optimizations active for the step latency measurement. */
+export const StepLatencyOptimizations = SemanticConvention<string[]>(
+  'step.latency_optimizations'
+);
+
 /**
  * The socket a WS event write actually travelled over
  * (workflow.events.ws.url). `url.full` names the v4 REST endpoint the frame is
diff --git a/packages/world-vercel/src/ws-transport-spans.test.ts b/packages/world-vercel/src/ws-transport-spans.test.ts
index 9576f7eeba..5990a18f5e 100644
--- a/packages/world-vercel/src/ws-transport-spans.test.ts
+++ b/packages/world-vercel/src/ws-transport-spans.test.ts
@@ -46,6 +46,7 @@ import {
   V4_FRAME_CONTENT_TYPE,
 } from './frames.js';
 import { WORKFLOW_SERVER_URL_OVERRIDE } from './utils.js';
+import { version } from './version.js';
 
 vi.mock('@vercel/oidc', () => ({
   getVercelOidcToken: vi.fn().mockRejectedValue(new Error('no OIDC')),
@@ -148,6 +149,8 @@ const input = {
   eventType: 'step_completed',
   specVersion: 2,
   correlationId: 'step_1',
+  stso: 468,
+  optimizations: ['lazyStepStart'],
 } as const;
 
 /** The materialized CBOR body a `step_completed` write answers with. */
@@ -284,6 +287,13 @@ describe('per-write client span', () => {
     expect(span.attributes['network.protocol.name']).toBe('websocket');
     expect(span.attributes['workflow.events.ws.url']).toBe(WS_URL);
     expect(span.attributes['workflow.event.type']).toBe('step_completed');
+    expect(span.attributes['workflow.client.version']).toBe(
+      `@workflow/world-vercel/${version}`
+    );
+    expect(span.attributes['step.stso_ms']).toBe(468);
+    expect(span.attributes['step.latency_optimizations']).toEqual([
+      'lazyStepStart',
+    ]);
   });
 
   it('carries the reqId that joins it to the server log line for the same frame', async () => {
@@ -521,6 +531,13 @@ describe('transport parity', () => {
     expect(span.attributes['http.request.method']).toBe('POST');
     expect(span.attributes['workflow.event.type']).toBe('step_completed');
     expect(span.attributes['workflow.events.transport']).toBe('http');
+    expect(span.attributes['workflow.client.version']).toBe(
+      `@workflow/world-vercel/${version}`
+    );
+    expect(span.attributes['step.stso_ms']).toBe(468);
+    expect(span.attributes['step.latency_optimizations']).toEqual([
+      'lazyStepStart',
+    ]);
     expect(span.attributes['network.protocol.name']).toBeUndefined();
     agent.assertNoPendingInterceptors();
   });