feat(session): make generated titles optional (#39747)

packages/schema/src/session.ts

@@ -42,7 +42,7 @@ export const Info = Schema.Struct({
     updated: DateTimeUtcFromMillis,
     archived: DateTimeUtcFromMillis.pipe(optional),
   }),
-  title: Schema.String,
+  title: Schema.String.pipe(optional),
   location: Location.Ref,
   subpath: RelativePath.pipe(optional),
   revert: Revert.pipe(optional),

packages/schema/src/v1/session.ts

@@ -552,7 +552,7 @@ export const SessionInfo = Schema.Struct({
   cost: optional(Schema.Finite),
   tokens: optional(SessionTokens),
   share: optional(SessionShare),
-  title: Schema.String,
+  title: optional(Schema.String),
   agent: optional(Schema.String),
   model: optional(SessionModel),
   version: Schema.String,

packages/schema/test/contract-hygiene.test.ts

@@ -17,7 +17,7 @@ import { Money } from "../src/money.js"
 import { Skill } from "../src/skill.js"
 import { Shell } from "../src/shell.js"
 import { PersistedRevert } from "../src/session-revert.js"
-import { optional } from "../src/schema.js"
+import { AbsolutePath, optional } from "../src/schema.js"
 
 describe("contract hygiene", () => {
   test("restricts agent colors to six-digit hex values", () => {
@@ -52,6 +52,18 @@ describe("contract hygiene", () => {
         metadata: undefined,
       }),
     ).toEqual({ text: "completed" })
+
+    expect(
+      Schema.encodeSync(Session.Info)({
+        id: Session.ID.make("ses_untitled"),
+        projectID: Project.ID.make("global"),
+        cost: Money.USD.zero,
+        tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+        time: { created: DateTime.makeUnsafe(0), updated: DateTime.makeUnsafe(0) },
+        title: undefined,
+        location: { directory: AbsolutePath.make("/project") },
+      }),
+    ).not.toHaveProperty("title")
   })
 
   test("pending session items omit the internal admission sequence", () => {