fix(session-title): serialize provider request writes

packages/session-title/session-title/src/index.ts

@@ -8,7 +8,12 @@ import z from 'schemastery'
 import type { Branded } from '@deepseek-ai/dsh-brand'
 import { deepFreeze, isAgentLoopRequest } from '@deepseek-ai/dsh-llm'
 import type { GenerateOptions } from '@deepseek-ai/dsh-llm'
-import type { Session, SessionEvent } from '@deepseek-ai/dsh-session'
+import type {
+  OutOfBandSessionEventType,
+  Session,
+  SessionEvent,
+  SessionEventMap,
+} from '@deepseek-ai/dsh-session'
 import { fallbackSessionTitle, normalizeSessionTitle } from './normalize.ts'
 
 export { fallbackSessionTitle, normalizeSessionTitle, truncateTitleUtf8 } from './normalize.ts'
@@ -95,6 +100,46 @@ declare module '@deepseek-ai/dsh-session' {
   }
 }
 
+/** Per-session settlement tails for title-capability out-of-band writes. */
+const SESSION_TITLE_WRITE_TAILS = new WeakMap<Session, Promise<void>>()
+
+/** Convert either write outcome into a fulfilled queue tail. */
+function settleSessionTitleWrite(): void {}
+
+/**
+ * Serialize one title-capability out-of-band event with its session peers.
+ * Cancellation is checked when the write reaches the head of the queue; once
+ * the core append starts, its durability contract runs to completion.
+ * @param ctx - context exposing the live session store.
+ * @param session - exact live session that owns the title-capability event.
+ * @param type - plugin-declared log-only title event type.
+ * @param data - typed JSON payload for the event.
+ * @param signal - service or provider lifetime checked before publication starts.
+ * @returns the durably accepted event.
+ */
+export async function appendSessionTitleOutOfBand<T extends OutOfBandSessionEventType>(
+  ctx: Context,
+  session: Session,
+  type: T,
+  data: SessionEventMap[T],
+  signal: AbortSignal,
+): Promise<SessionEvent<T>> {
+  const predecessor = SESSION_TITLE_WRITE_TAILS.get(session)
+  const run = Promise.resolve(predecessor).then(() => {
+    signal.throwIfAborted()
+    return ctx.sessions.appendOutOfBand(session, type, data, { kind: 'session-title' })
+  })
+  const tail = run.then(settleSessionTitleWrite, settleSessionTitleWrite)
+  SESSION_TITLE_WRITE_TAILS.set(session, tail)
+  try {
+    return await run
+  } finally {
+    if (SESSION_TITLE_WRITE_TAILS.get(session) === tail) {
+      SESSION_TITLE_WRITE_TAILS.delete(session)
+    }
+  }
+}
+
 /** One eligible human text message exposed to title providers. */
 export interface SessionTitleUserMessage {
   /** Source `user/message` event seq. */
@@ -481,7 +526,7 @@ export class SessionTitleService extends Service {
       })
       this.assertCurrent(session, work)
       const accepted = this.validateResult(result, messages)
-      await this.ctx.sessions.appendOutOfBand(session, 'session/title', {
+      await appendSessionTitleOutOfBand(this.ctx, session, 'session/title', {
         title: accepted.title,
         messageSeqs: [...accepted.messageSeqs],
         source: {
@@ -489,7 +534,7 @@ export class SessionTitleService extends Service {
           provider: work.registration.provider.id,
           ...accepted.model === undefined ? {} : { model: accepted.model },
         },
-      }, { kind: 'session-title' })
+      }, work.signal)
       return this.get(session)
     } finally {
       const state = this.work.get(session)
@@ -662,11 +707,11 @@ export class SessionTitleService extends Service {
       this.config.fallbackMaxBytes,
     )
     if (title.length === 0) return undefined
-    await this.ctx.sessions.appendOutOfBand(session, 'session/title', {
+    await appendSessionTitleOutOfBand(this.ctx, session, 'session/title', {
       title,
       messageSeqs: [first.seq],
       source: { kind: 'fallback' },
-    }, { kind: 'session-title' })
+    }, this.lifetime.signal)
     return this.get(session)
   }
 }