feat(core): generate session titles from first prompt

Add SessionTitle service that generates a session title from the
session's sole user message via the customizable "title" agent, then
renames the session. Runs once per session, gated on durable history
having exactly one user message (no default-title string comparison).

Wire it into SessionRunner as a background fork after the first turn's
prompt promotion makes the user message visible; forking before
promotion caused the first-message lookup to see zero rows and silently
no-op.

Closes #34364

packages/core/src/location-services.ts

@@ -31,6 +31,7 @@ import { ReferenceGuidance } from "./reference/guidance"
 import * as SessionRunnerLLM from "./session/runner/llm"
 import { SessionRunnerModel } from "./session/runner/model"
 import { SessionCompaction } from "./session/compaction"
+import { SessionTitle } from "./session/title"
 import { SessionTodo } from "./session/todo"
 import { SkillV2 } from "./skill"
 import { SkillGuidance } from "./skill/guidance"
@@ -86,6 +87,7 @@ export const locationServices = LayerNode.group([
   McpTool.node,
   SessionRunnerModel.node,
   SessionCompaction.node,
+  SessionTitle.node,
   Snapshot.node,
   SessionRunnerLLM.node,
 ])

packages/core/src/session/history.ts

@@ -98,4 +98,22 @@ export const entriesForRunner = Effect.fn("SessionHistory.entriesForRunner")(fun
   )
 })
 
+/** Returns the session's sole user message, or `undefined` once a second one exists. */
+export const firstUserMessageIfOnly = Effect.fn("SessionHistory.firstUserMessageIfOnly")(function* (
+  db: DatabaseService,
+  sessionID: SessionSchema.ID,
+) {
+  const rows = yield* db
+    .select()
+    .from(SessionMessageTable)
+    .where(and(eq(SessionMessageTable.session_id, sessionID), eq(SessionMessageTable.type, "user")))
+    .orderBy(asc(SessionMessageTable.seq))
+    .limit(2)
+    .all()
+    .pipe(Effect.orDie)
+  if (rows.length !== 1) return undefined
+  const message = yield* decodeMessageRow(rows[0]).pipe(Effect.catch(() => Effect.succeed(undefined)))
+  return message?.type === "user" ? message : undefined
+})
+
 export * as SessionHistory from "./history"

packages/core/src/session/runner/llm.ts

@@ -30,6 +30,7 @@ import { SessionHistory } from "../history"
 import { SessionInput } from "../input"
 import { SessionSchema } from "../schema"
 import { SessionStore } from "../store"
+import { SessionTitle } from "../title"
 import { type RunError, Service } from "./index"
 import { SessionRunnerModel } from "./model"
 import { createLLMEventPublisher } from "./publish-llm-event"
@@ -107,6 +108,12 @@ export const layer = Layer.effect(
     const snapshots = yield* Snapshot.Service
     const db = (yield* Database.Service).db
     const compaction = yield* SessionCompaction.Service
+    const title = yield* SessionTitle.Service
+    // Title generation is a side effect of the first turn; it must not delay turn continuation.
+    // Tracked per process so repeated wakes before the second user message arrives don't
+    // re-fire a redundant LLM call; `SessionTitle` itself is idempotent based on durable history.
+    const titleAttempted = new Set<SessionSchema.ID>()
+    const forkTitle = yield* FiberSet.makeRuntime<never, void, never>()
     const getSession = Effect.fn("SessionRunner.getSession")(function* (sessionID: SessionSchema.ID) {
       const session = yield* store.get(sessionID)
       if (!session) return yield* Effect.die(`Session not found: ${sessionID}`)
@@ -394,6 +401,12 @@ export const layer = Layer.effect(
         let step = 1
         while (needsContinuation) {
           const result = yield* runTurn(input.sessionID, promotion, step)
+          // Steer/queue promotion inside runTurn has already made the pending input a visible
+          // user message by this point, so the first-user-message check below is reliable.
+          if (!titleAttempted.has(input.sessionID)) {
+            titleAttempted.add(input.sessionID)
+            forkTitle(title.generateForFirstPrompt(yield* getSession(input.sessionID)).pipe(Effect.ignore))
+          }
           needsContinuation = result.needsContinuation
           step = result.step + 1
           promotion = "steer"
@@ -428,6 +441,7 @@ export const node = makeLocationNode({
     ReferenceGuidance.node,
     McpGuidance.node,
     SessionCompaction.node,
+    SessionTitle.node,
     Snapshot.node,
     Database.node,
   ],

packages/core/src/session/title.ts

@@ -0,0 +1,105 @@
+export * as SessionTitle from "./title"
+
+import { LLM, LLMClient, LLMError, LLMEvent, Message, type LLMRequest } from "@opencode-ai/llm"
+import { Context, DateTime, Effect, Layer, Stream } from "effect"
+import { AgentV2 } from "../agent"
+import { Database } from "../database/database"
+import { EventV2 } from "../event"
+import { makeLocationNode } from "../effect/app-node"
+import { llmClient } from "../effect/app-node-platform"
+import { SessionEvent } from "./event"
+import { SessionHistory } from "./history"
+import { SessionRunnerModel } from "./runner/model"
+import { SessionSchema } from "./schema"
+
+const MAX_LENGTH = 100
+
+type Dependencies = {
+  readonly events: EventV2.Interface
+  readonly llm: {
+    readonly stream: (request: LLMRequest) => Stream.Stream<LLMEvent, LLMError>
+  }
+  readonly agents: AgentV2.Interface
+  readonly models: SessionRunnerModel.Interface
+}
+
+export interface Interface {
+  /** Generates a title from the session's first user message and renames the session. Runs at most once per session. */
+  readonly generateForFirstPrompt: (session: SessionSchema.Info) => Effect.Effect<void>
+}
+
+export class Service extends Context.Service<Service, Interface>()("@opencode/v2/SessionTitle") {}
+
+const truncate = (value: string) => (value.length <= MAX_LENGTH ? value : `${value.slice(0, MAX_LENGTH - 3)}...`)
+
+const make = (dependencies: Dependencies) => {
+  const generateForFirstPrompt = Effect.fn("SessionTitle.generateForFirstPrompt")(function* (
+    db: Database.Interface["db"],
+    session: SessionSchema.Info,
+  ) {
+    if (session.parentID) return
+    const firstUser = yield* SessionHistory.firstUserMessageIfOnly(db, session.id)
+    if (!firstUser) return
+    const agent = yield* dependencies.agents.get(AgentV2.ID.make("title"))
+    if (!agent) return
+    const model = yield* (agent.model
+      ? dependencies.models.resolve({ ...session, model: agent.model })
+      : dependencies.models.resolve(session)
+    ).pipe(Effect.catch(() => Effect.succeed(undefined)))
+    if (!model) return
+    const chunks: string[] = []
+    let failed = false
+    const streamed = yield* dependencies.llm
+      .stream(
+        LLM.request({
+          model,
+          system: agent.system,
+          messages: [Message.user(firstUser.text)],
+          tools: [],
+        }),
+      )
+      .pipe(
+        Stream.runForEach((event) => {
+          if (LLMEvent.is.providerError(event)) failed = true
+          if (LLMEvent.is.textDelta(event)) chunks.push(event.text)
+          return Effect.void
+        }),
+        Effect.as(true),
+        Effect.catchTag("LLM.Error", () => Effect.succeed(false)),
+      )
+    if (!streamed || failed) return
+    const title = chunks
+      .join("")
+      .split("\n")
+      .map((line) => line.trim())
+      .find((line) => line.length > 0)
+    if (!title) return
+    yield* dependencies.events.publish(SessionEvent.Renamed, {
+      sessionID: session.id,
+      timestamp: yield* DateTime.now,
+      title: truncate(title),
+    })
+  })
+  return { generateForFirstPrompt }
+}
+
+export const layer = Layer.effect(
+  Service,
+  Effect.gen(function* () {
+    const events = yield* EventV2.Service
+    const llm = yield* LLMClient.Service
+    const agents = yield* AgentV2.Service
+    const models = yield* SessionRunnerModel.Service
+    const database = yield* Database.Service
+    const title = make({ events, llm, agents, models })
+    return Service.of({
+      generateForFirstPrompt: (session) => title.generateForFirstPrompt(database.db, session),
+    })
+  }),
+)
+
+export const node = makeLocationNode({
+  service: Service,
+  layer,
+  deps: [EventV2.node, llmClient, AgentV2.node, SessionRunnerModel.node, Database.node],
+})