diff --git a/src/callLifecycle.ts b/src/callLifecycle.ts
index e2e703a..5985421 100644
--- a/src/callLifecycle.ts
+++ b/src/callLifecycle.ts
@@ -7,8 +7,8 @@ import {
   ATTR,
   addPermissionRequestEvent,
   addPermissionResolvedEvent,
-  assistantOutputMessages,
   jsonStr,
+  recordOutput,
 } from './genaiSpans.js';
 import type { SpanParent } from './genaiSpans.js';
 import { VERSION } from './setup.js';
@@ -378,7 +378,7 @@ function finishAgentSpan(
   const output = outcome.ok ? outcome.output : outcome.error;
   if (output !== undefined && output !== null && output !== '') {
     const text = typeof output === 'string' ? output : jsonStr(output);
-    call.span.setAttributes({ [ATTR.OUTPUT_MESSAGES]: assistantOutputMessages([text]) });
+    recordOutput(call.span, [text]);
   }
   if (outcome.ok) {
     call.span.end(endTime ? { endTime } : undefined);
diff --git a/src/genaiSpans.ts b/src/genaiSpans.ts
index 3c2bb52..fe130c9 100644
--- a/src/genaiSpans.ts
+++ b/src/genaiSpans.ts
@@ -5,7 +5,7 @@
 // Attribute-key constants and formatting helpers typed against the `weave` SDK.
 
 import type { Attributes } from '@opentelemetry/api';
-import type { MessagePart, SubAgent, Tool, Turn, Usage } from 'weave';
+import type { Message, MessagePart, SubAgent, Tool, Turn, Usage } from 'weave';
 import { isTextBlock, isThinkingBlock, isRedactedThinkingBlock, isToolUseBlock } from './parser.js';
 import type { UsageSummary } from './parser.js';
 
@@ -107,10 +107,20 @@ export function jsonStr(v: unknown): string {
   }
 }
 
-/** `gen_ai.output.messages` JSON for plain assistant text(s), the shape used on
- *  turn and subagent `invoke_agent` spans (chat spans carry parts instead). */
-export function assistantOutputMessages(texts: string[]): string {
-  return jsonStr(texts.map((content) => ({ role: 'assistant', content })));
+function assistantMessages(texts: string[]): Message[] {
+  return texts.map((content) => ({ role: 'assistant', content }));
+}
+
+/** Record assistant text(s) and model onto a turn or subagent `invoke_agent`
+ *  span; the SDK serializes them at `end()` (chat spans carry parts instead).
+ *  Skipped when there is nothing to record, because `record()` warns once the
+ *  span has already ended. */
+export function recordOutput(span: SpanParent, texts: string[], model?: string): void {
+  if (!texts.length && !model) return;
+  span.record({
+    outputMessages: texts.length ? assistantMessages(texts) : undefined,
+    model,
+  });
 }
 
 /** Parse an ISO timestamp; returns undefined for missing or unparseable input. */
diff --git a/src/hookHandler.ts b/src/hookHandler.ts
index bd84113..49f3271 100644
--- a/src/hookHandler.ts
+++ b/src/hookHandler.ts
@@ -50,7 +50,7 @@ import type {
   TracedCall,
 } from './callLifecycle.js';
 import type { CompactionAttrs } from './genaiSpans.js';
-import { ATTR, assistantOutputMessages, snippet } from './genaiSpans.js';
+import { ATTR, recordOutput, snippet } from './genaiSpans.js';
 import type { SpanParent } from './genaiSpans.js';
 import { parseSessionFd } from './parser.js';
 import { Session } from './session.js';
@@ -852,9 +852,7 @@ export class HookHandler {
         match.call.span.setAttributes({ [ATTR.RESPONSE_MODEL]: transcript.model });
       }
       if (!match.call.toolUseId && text) {
-        match.call.span.setAttributes({
-          [ATTR.OUTPUT_MESSAGES]: assistantOutputMessages([text]),
-        });
+        recordOutput(match.call.span, [text]);
       }
       recordAgentStop(session.calls, match);
     } else if (recovered) {
@@ -862,9 +860,7 @@ export class HookHandler {
         recovered.span.setAttributes({ [ATTR.RESPONSE_MODEL]: transcript.model });
       }
       if (text) {
-        recovered.span.setAttributes({
-          [ATTR.OUTPUT_MESSAGES]: assistantOutputMessages([text]),
-        });
+        recordOutput(recovered.span, [text]);
       }
     }
 
diff --git a/src/session.ts b/src/session.ts
index b500efb..71a56e0 100644
--- a/src/session.ts
+++ b/src/session.ts
@@ -9,9 +9,9 @@ import * as weave from 'weave';
 import { emitChatSpans } from './chatSpans.js';
 import {
   ATTR,
-  assistantOutputMessages,
   buildIntegrationAttrs,
   parseTimestamp,
+  recordOutput,
   setCompactionAttrs,
 } from './genaiSpans.js';
 import type { CompactionAttrs } from './genaiSpans.js';
@@ -449,7 +449,6 @@ export class Session {
     const text = responses.flatMap(response => extractAssistantTextBlocks(response.content));
     if (!text.length && options.lastMessage) text.push(options.lastMessage);
     const attributes: Attributes = {};
-    if (text.length) attributes[ATTR.OUTPUT_MESSAGES] = assistantOutputMessages(text);
     const finishReasons = responses
       .map(response => response.finishReason)
       .filter((reason): reason is string => Boolean(reason));
@@ -457,8 +456,7 @@ export class Session {
     if (options.orphanReason) attributes[ATTR.WEAVE_ORPHAN_REASON] = options.orphanReason;
     if (Object.keys(attributes).length) turn.span.setAttributes(attributes);
 
-    const model = responses.filter(response => response.model).at(-1)?.model;
-    if (model) turn.span.record({ model });
+    recordOutput(turn.span, text, responses.filter(response => response.model).at(-1)?.model);
   }
 
   private recordFinalTurnOutput(
diff --git a/src/teamCoordinator.ts b/src/teamCoordinator.ts
index be926ad..fc4be4a 100644
--- a/src/teamCoordinator.ts
+++ b/src/teamCoordinator.ts
@@ -7,7 +7,7 @@ import type * as weave from 'weave';
 import { deferAgentOutcome, denyCall, finishAgentCall } from './callLifecycle.js';
 import type { ToolResult, TracedAgent } from './callLifecycle.js';
 import { emitChatSpans } from './chatSpans.js';
-import { ATTR, assistantOutputMessages, parseTimestamp } from './genaiSpans.js';
+import { ATTR, parseTimestamp, recordOutput } from './genaiSpans.js';
 import type { ParsedTurn } from './parser.js';
 import type { Session } from './session.js';
 import { VERSION } from './setup.js';
@@ -111,9 +111,7 @@ function emitTeammate(
   });
   try {
     emitChatSpans(span, responses, { agentName: memberName });
-    const output = turns.flatMap(turn => turn.text);
-    if (output.length) span.setAttributes({ [ATTR.OUTPUT_MESSAGES]: assistantOutputMessages(output) });
-    if (model) span.record({ model });
+    recordOutput(span, turns.flatMap(turn => turn.text), model);
     return { model, text: turns.at(-1)?.text.join('\n') || undefined };
   } finally {
     span.end({ endTime: parseTimestamp(responses.at(-1)?.endTime) ?? new Date() });