fix(api): require session selection

packages/cli/src/run/run.ts

@@ -94,11 +94,9 @@ async function execute(input: RunCommandInput, prepared: Prepared, endpoint: End
     prepare: async (next) => {
       const selected =
         next.model ??
-        (options.variant
-          ? await client.model
-              .default({ location: { directory: next.location.directory, workspace: next.location.workspaceID } })
-              .then((result) => result.data)
-          : undefined)
+        (await client.model
+          .default({ location: { directory: next.location.directory, workspace: next.location.workspaceID } })
+          .then((result) => result.data))
       const model = selected
         ? {
             providerID: selected.providerID,
@@ -108,7 +106,12 @@ async function execute(input: RunCommandInput, prepared: Prepared, endpoint: End
         : undefined
       if ((options.variant ?? explicit?.variant) && !model)
         throw new RunTargetError("Cannot select a variant before selecting a model", next.session?.id)
-      return { model, agent: next.agent }
+      const agent =
+        next.agent ??
+        (await client.agent
+          .list({ location: { directory: next.location.directory, workspace: next.location.workspaceID } })
+          .then((result) => result.data.find((item) => item.mode !== "subagent" && !item.hidden)?.id))
+      return { model, agent }
     },
   }).catch((error) => {
     if (!(error instanceof RunTargetError)) throw error

packages/cli/src/session-target.ts

@@ -56,13 +56,16 @@ export async function resolveSessionTarget(input: {
     agent: input.agent ?? selected?.agent,
     signal: input.signal,
   })
+  if (!selected && (!prepared.agent || !prepared.model)) {
+    throw new SessionTargetMutationError(new Error("Creating a session requires an agent and model"))
+  }
   const session =
     selected ??
     (await input.client.session
       .create(
         {
-          agent: prepared.agent,
-          model: prepared.model,
+          agent: prepared.agent!,
+          model: prepared.model!,
           location: { directory: location.directory, workspaceID: location.workspaceID },
         },
         ...requestOptions(input.signal),

packages/cli/test/session-target.test.ts

@@ -61,7 +61,11 @@ describe("session target resolver", () => {
     spyOn(client.location, "get").mockResolvedValue(location("/server", "work_1"))
     const create = spyOn(client.session, "create").mockImplementation(async (input) => {
       order.push("create")
-      expect(input).toMatchObject({ agent: "prepared", location: { directory: "/server", workspaceID: "work_1" } })
+      expect(input).toMatchObject({
+        agent: "prepared",
+        model: { providerID: "openai", id: "gpt-5" },
+        location: { directory: "/server", workspaceID: "work_1" },
+      })
       return session("ses_fresh", "/server", "work_1")
     })
 
@@ -71,20 +75,22 @@ describe("session target resolver", () => {
       prepare: async (input) => {
         order.push("prepare")
         expect(input.location.workspaceID).toBe("work_1")
-        return { model: input.model, agent: "prepared" }
+        return { model: { providerID: "openai", id: "gpt-5" }, agent: "prepared" }
       },
     })
     expect(create).toHaveBeenCalledTimes(1)
     expect(order).toEqual(["prepare", "create"])
   })
 
-  test("uses the agent resolved by the server for a fresh Session", async () => {
+  test("requires an explicit agent and model for a fresh Session", async () => {
     const client = OpenCode.make({ baseUrl: "https://opencode.test" })
     spyOn(client.location, "get").mockResolvedValue(location("/project"))
-    spyOn(client.session, "create").mockResolvedValue({ ...session("ses_fresh", "/project"), agent: "review" })
+    const create = spyOn(client.session, "create")
 
-    const target = await resolveSessionTarget({ client, prepare })
-    expect(target.agent).toBe("review")
+    await expect(resolveSessionTarget({ client, prepare })).rejects.toThrow(
+      "Creating a session requires an agent and model",
+    )
+    expect(create).not.toHaveBeenCalled()
   })
 
   test("does not retry an ambiguous Session creation", async () => {

packages/client/src/effect/api/api.ts

@@ -120,12 +120,12 @@ export type SessionListOperation<E = never> = (input?: Endpoint5_0Input) => Effe
 export type Endpoint5_1Input = {
   readonly id?: Session.ID | undefined
   readonly title?: string | undefined
-  readonly agent?: Agent.ID | undefined
-  readonly model?: Model.Ref | undefined
+  readonly agent: Agent.ID
+  readonly model: Model.Ref
   readonly location?: Location.Ref | undefined
 }
 export type Endpoint5_1Output = Session.Info
-export type SessionCreateOperation<E = never> = (input?: Endpoint5_1Input) => Effect.Effect<Endpoint5_1Output, E>
+export type SessionCreateOperation<E = never> = (input: Endpoint5_1Input) => Effect.Effect<Endpoint5_1Output, E>
 
 export type Endpoint5_2Input = {
   readonly info: Session.Info

packages/client/src/effect/generated/client.ts

@@ -305,15 +305,15 @@ const Endpoint5_0 = (raw: RawClient["server.session"]) => (input?: Endpoint5_0In
     }).pipe(Effect.mapError(mapClientError)),
   )
 
-const Endpoint5_1 = (raw: RawClient["server.session"]) => (input?: Endpoint5_1Input) =>
+const Endpoint5_1 = (raw: RawClient["server.session"]) => (input: Endpoint5_1Input) =>
   preserveEffect<Endpoint5_1Output>()(
     raw["session.create"]({
       payload: {
-        id: input?.["id"],
-        title: input?.["title"],
-        agent: input?.["agent"],
-        model: input?.["model"],
-        location: input?.["location"],
+        id: input["id"],
+        title: input["title"],
+        agent: input["agent"],
+        model: input["model"],
+        location: input["location"],
       },
     }).pipe(
       Effect.mapError(mapClientError),

packages/client/src/promise/generated/client.ts

@@ -464,17 +464,17 @@ export function make(options: ClientOptions) {
           },
           requestOptions,
         ),
-      create: (input?: SessionCreateInput, requestOptions?: RequestOptions) =>
+      create: (input: SessionCreateInput, requestOptions?: RequestOptions) =>
         request<{ readonly data: SessionCreateOutput }>(
           {
             method: "POST",
             path: `/api/session`,
             body: {
-              id: input?.["id"],
-              title: input?.["title"],
-              agent: input?.["agent"],
-              model: input?.["model"],
-              location: input?.["location"],
+              id: input["id"],
+              title: input["title"],
+              agent: input["agent"],
+              model: input["model"],
+              location: input["location"],
             },
             successStatus: 200,
             declaredStatuses: [401, 400],

packages/client/src/promise/generated/types.ts

@@ -2436,36 +2436,36 @@ export type SessionCreateInput = {
   readonly id?: {
     readonly id?: string | null
     readonly title?: string | null
-    readonly agent?: string | null
-    readonly model?: { readonly id: string; readonly providerID: string; readonly variant?: string } | null
+    readonly agent: string
+    readonly model: { readonly id: string; readonly providerID: string; readonly variant?: string }
     readonly location?: { readonly directory: string; readonly workspaceID?: string } | null
   }["id"]
   readonly title?: {
     readonly id?: string | null
     readonly title?: string | null
-    readonly agent?: string | null
-    readonly model?: { readonly id: string; readonly providerID: string; readonly variant?: string } | null
+    readonly agent: string
+    readonly model: { readonly id: string; readonly providerID: string; readonly variant?: string }
     readonly location?: { readonly directory: string; readonly workspaceID?: string } | null
   }["title"]
-  readonly agent?: {
+  readonly agent: {
     readonly id?: string | null
     readonly title?: string | null
-    readonly agent?: string | null
-    readonly model?: { readonly id: string; readonly providerID: string; readonly variant?: string } | null
+    readonly agent: string
+    readonly model: { readonly id: string; readonly providerID: string; readonly variant?: string }
     readonly location?: { readonly directory: string; readonly workspaceID?: string } | null
   }["agent"]
-  readonly model?: {
+  readonly model: {
     readonly id?: string | null
     readonly title?: string | null
-    readonly agent?: string | null
-    readonly model?: { readonly id: string; readonly providerID: string; readonly variant?: string } | null
+    readonly agent: string
+    readonly model: { readonly id: string; readonly providerID: string; readonly variant?: string }
     readonly location?: { readonly directory: string; readonly workspaceID?: string } | null
   }["model"]
   readonly location?: {
     readonly id?: string | null
     readonly title?: string | null
-    readonly agent?: string | null
-    readonly model?: { readonly id: string; readonly providerID: string; readonly variant?: string } | null
+    readonly agent: string
+    readonly model: { readonly id: string; readonly providerID: string; readonly variant?: string }
     readonly location?: { readonly directory: string; readonly workspaceID?: string } | null
   }["location"]
 }

packages/client/test/effect.test.ts

@@ -181,6 +181,8 @@ test("session methods retain decoded Effect inputs and outputs", async () => {
     const page = yield* client.session.list({ limit: 10 })
     const active = yield* client.session.active()
     const created = yield* client.session.create({
+      agent: Agent.ID.make("build"),
+      model: Model.Ref.make({ id: "claude", providerID: "anthropic" }),
       location: Location.Ref.make({ directory: AbsolutePath.make("/tmp/project") }),
     })
     yield* client.session.switchAgent({ sessionID: Session.ID.make("ses_test"), agent: Agent.ID.make("build") })

packages/client/test/promise.test.ts

@@ -454,7 +454,11 @@ test("session methods use the public HTTP contract", async () => {
 
   const page = await client.session.list({ limit: 10, order: "desc", parentID: null })
   const active = await client.session.active()
-  const created = await client.session.create({ location: { directory: "/tmp/project" } })
+  const created = await client.session.create({
+    agent: "build",
+    model: { id: "claude", providerID: "anthropic" },
+    location: { directory: "/tmp/project" },
+  })
   await client.session.switchAgent({ sessionID: "ses_test", agent: "build" })
   await client.session.switchModel({
     sessionID: "ses_test",
@@ -528,7 +532,7 @@ test("middleware errors remain declared client errors", async () => {
   })
 
   try {
-    await client.session.create({})
+    await client.session.create({ agent: "build", model: { id: "claude", providerID: "anthropic" } })
     throw new Error("Expected request to fail")
   } catch (error) {
     expect(isUnauthorizedError(error)).toBe(true)

packages/core/src/plugin/host.ts

@@ -340,12 +340,12 @@ export const make = Effect.fn("PluginHost.make")(function* (plugin: import("../p
       hook: (name, callback) => hooks.register("session", name, callback),
       create: (input) =>
         runtime.session.create({
-          id: input?.id,
-          title: input?.title,
-          agent: input?.agent,
-          model: input?.model,
+          id: input.id,
+          title: input.title,
+          agent: input.agent,
+          model: input.model,
           location:
-            input?.location ?? Location.Ref.make({ directory: location.directory, workspaceID: location.workspaceID }),
+            input.location ?? Location.Ref.make({ directory: location.directory, workspaceID: location.workspaceID }),
         }),
       get: (input) => runtime.session.get(input.sessionID),
       prompt: runtime.session.prompt,

packages/core/src/plugin/promise.ts

@@ -269,25 +269,21 @@ export function fromPromise(plugin: Plugin) {
               register(host.session.hook(name, (event) => Effect.promise(() => Promise.resolve(callback(event))))),
             create: (input) =>
               run(
-                host.session.create(
-                  input === undefined
-                    ? undefined
-                    : {
-                        id: input.id == null ? undefined : Session.ID.make(input.id),
-                        agent: input.agent == null ? undefined : Agent.ID.make(input.agent),
-                        model: input.model == null ? undefined : model(input.model),
-                        location:
-                          input.location == null
-                            ? undefined
-                            : Location.Ref.make({
-                                directory: AbsolutePath.make(input.location.directory),
-                                workspaceID:
-                                  input.location.workspaceID === undefined
-                                    ? undefined
-                                    : Workspace.ID.make(input.location.workspaceID),
-                              }),
-                      },
-                ),
+                host.session.create({
+                  id: input.id == null ? undefined : Session.ID.make(input.id),
+                  agent: Agent.ID.make(input.agent),
+                  model: model(input.model),
+                  location:
+                    input.location == null
+                      ? undefined
+                      : Location.Ref.make({
+                          directory: AbsolutePath.make(input.location.directory),
+                          workspaceID:
+                            input.location.workspaceID === undefined
+                              ? undefined
+                              : Workspace.ID.make(input.location.workspaceID),
+                        }),
+                }),
               ),
             get: (input) => run(host.session.get({ sessionID: Session.ID.make(input.sessionID) })),
             prompt: (input) =>

packages/protocol/src/groups/session.ts

@@ -151,8 +151,8 @@ export const makeSessionGroup = <I extends HttpApiMiddleware.AnyId, S>(sessionLo
         payload: Schema.Struct({
           id: Session.ID.pipe(Schema.optional),
           title: Schema.String.pipe(Schema.optional),
-          agent: Agent.ID.pipe(Schema.optional),
-          model: Model.Ref.pipe(Schema.optional),
+          agent: Agent.ID,
+          model: Model.Ref,
           location: Location.Ref.pipe(Schema.optional),
         }),
         success: Schema.Struct({ data: Session.Info }),
@@ -160,7 +160,7 @@ export const makeSessionGroup = <I extends HttpApiMiddleware.AnyId, S>(sessionLo
         OpenApi.annotations({
           identifier: "v2.session.create",
           summary: "Create session",
-          description: "Create a session at the requested location.",
+          description: "Create a session with an explicit agent and model at the requested location.",
         }),
       ),
     )