diff --git a/packages/cli/src/run/run.ts b/packages/cli/src/run/run.ts
index 0336d55d682a..ba9106ac70e9 100644
--- a/packages/cli/src/run/run.ts
+++ b/packages/cli/src/run/run.ts
@@ -1,5 +1,11 @@
 import { Service, type Endpoint } from "@opencode-ai/client/effect/service"
-import { OpenCode, type OpenCodeClient, type SessionMessageAssistantTool } from "@opencode-ai/client/promise"
+import {
+  OpenCode,
+  type ModelRef,
+  type OpenCodeClient,
+  type SessionInfo,
+  type SessionMessageAssistantTool,
+} from "@opencode-ai/client/promise"
 import { FSUtil } from "@opencode-ai/util/fs-util"
 import { open } from "node:fs/promises"
 import path from "node:path"
@@ -116,8 +122,13 @@ async function execute(input: RunCommandInput, prepared: Prepared, endpoint: End
     return undefined
   })
   if (!target) return
-  const model = target.model ? { providerID: target.model.providerID, modelID: target.model.id } : undefined
-  const variant = target.model?.variant
+  await applyRunSelection({
+    client,
+    sessionID: target.session.id,
+    agent: input.agent,
+    model: target.model,
+    explicit: explicit !== undefined || options.variant !== undefined,
+  })
   if (!target.resume && input.title !== undefined) {
     await client.session.rename({
       sessionID: target.session.id,
@@ -131,9 +142,6 @@ async function execute(input: RunCommandInput, prepared: Prepared, endpoint: End
     location: target.location,
     message: prepared.message,
     files: prepared.files,
-    agent: target.agent,
-    model,
-    variant,
     thinking: input.thinking ?? false,
     format: input.format,
     auto: input.auto ?? false,
@@ -144,6 +152,25 @@ async function execute(input: RunCommandInput, prepared: Prepared, endpoint: End
   }).catch((error) => reportRunError(input, errorMessage(error), target.session.id))
 }
 
+/** @internal Exported for CLI boundary tests. */
+export function applyRunSelection(input: {
+  client: OpenCodeClient
+  sessionID: SessionInfo["id"]
+  agent?: string
+  model?: ModelRef
+  explicit: boolean
+}) {
+  if (input.agent)
+    return input.client.session.select({
+      sessionID: input.sessionID,
+      agent: input.agent,
+      model: input.explicit && input.model ? { type: "explicit", model: input.model } : { type: "configured" },
+    })
+  if (input.explicit && input.model)
+    return input.client.session.switchModel({ sessionID: input.sessionID, model: input.model })
+  return Promise.resolve()
+}
+
 export function mergeInput(message: string | undefined, piped: string | undefined) {
   if (!message) return piped || undefined
   if (!piped) return message
diff --git a/packages/cli/test/run/run.test.ts b/packages/cli/test/run/run.test.ts
new file mode 100644
index 000000000000..bb87106c461c
--- /dev/null
+++ b/packages/cli/test/run/run.test.ts
@@ -0,0 +1,59 @@
+import { afterEach, describe, expect, mock, spyOn, test } from "bun:test"
+import { OpenCode } from "@opencode-ai/client/promise"
+import { applyRunSelection } from "../../src/run/run"
+
+afterEach(() => mock.restore())
+
+describe("run selection", () => {
+  test("resolves the configured model when an agent is explicit", async () => {
+    const client = OpenCode.make({ baseUrl: "https://opencode.test" })
+    const select = spyOn(client.session, "select").mockResolvedValue(undefined)
+
+    await applyRunSelection({ client, sessionID: "ses_test", agent: "modelprobe", explicit: false })
+
+    expect(select).toHaveBeenCalledWith({
+      sessionID: "ses_test",
+      agent: "modelprobe",
+      model: { type: "configured" },
+    })
+  })
+
+  test("selects an explicit agent and model atomically", async () => {
+    const client = OpenCode.make({ baseUrl: "https://opencode.test" })
+    const select = spyOn(client.session, "select").mockResolvedValue(undefined)
+
+    await applyRunSelection({
+      client,
+      sessionID: "ses_test",
+      agent: "modelprobe",
+      model: { providerID: "opencode", id: "deepseek-v4-flash-free" },
+      explicit: true,
+    })
+
+    expect(select).toHaveBeenCalledWith({
+      sessionID: "ses_test",
+      agent: "modelprobe",
+      model: {
+        type: "explicit",
+        model: { providerID: "opencode", id: "deepseek-v4-flash-free" },
+      },
+    })
+  })
+
+  test("switches only the model when no agent is explicit", async () => {
+    const client = OpenCode.make({ baseUrl: "https://opencode.test" })
+    const switchModel = spyOn(client.session, "switchModel").mockResolvedValue(undefined)
+
+    await applyRunSelection({
+      client,
+      sessionID: "ses_test",
+      model: { providerID: "opencode", id: "deepseek-v4-flash-free" },
+      explicit: true,
+    })
+
+    expect(switchModel).toHaveBeenCalledWith({
+      sessionID: "ses_test",
+      model: { providerID: "opencode", id: "deepseek-v4-flash-free" },
+    })
+  })
+})
diff --git a/packages/client/src/effect/api/api.ts b/packages/client/src/effect/api/api.ts
index d596aad0ec09..b65cbfbed6e8 100644
--- a/packages/client/src/effect/api/api.ts
+++ b/packages/client/src/effect/api/api.ts
@@ -154,28 +154,39 @@ export type Endpoint5_7Input = { readonly sessionID: Session.ID; readonly bounda
 export type Endpoint5_7Output = Session.Info
 export type SessionForkOperation<E = never> = (input: Endpoint5_7Input) => Effect.Effect<Endpoint5_7Output, E>
 
-export type Endpoint5_8Input = { readonly sessionID: Session.ID; readonly agent: Agent.ID }
+export type Endpoint5_8Input = {
+  readonly sessionID: Session.ID
+  readonly agent: Agent.ID
+  readonly model:
+    | { readonly type: "preserve" }
+    | { readonly type: "configured" }
+    | { readonly type: "explicit"; readonly model: Model.Ref }
+}
 export type Endpoint5_8Output = void
-export type SessionSwitchAgentOperation<E = never> = (input: Endpoint5_8Input) => Effect.Effect<Endpoint5_8Output, E>
+export type SessionSelectOperation<E = never> = (input: Endpoint5_8Input) => Effect.Effect<Endpoint5_8Output, E>
 
-export type Endpoint5_9Input = { readonly sessionID: Session.ID; readonly model: Model.Ref }
+export type Endpoint5_9Input = { readonly sessionID: Session.ID; readonly agent: Agent.ID }
 export type Endpoint5_9Output = void
-export type SessionSwitchModelOperation<E = never> = (input: Endpoint5_9Input) => Effect.Effect<Endpoint5_9Output, E>
+export type SessionSwitchAgentOperation<E = never> = (input: Endpoint5_9Input) => Effect.Effect<Endpoint5_9Output, E>
 
-export type Endpoint5_10Input = { readonly sessionID: Session.ID; readonly title: string }
+export type Endpoint5_10Input = { readonly sessionID: Session.ID; readonly model: Model.Ref }
 export type Endpoint5_10Output = void
-export type SessionRenameOperation<E = never> = (input: Endpoint5_10Input) => Effect.Effect<Endpoint5_10Output, E>
+export type SessionSwitchModelOperation<E = never> = (input: Endpoint5_10Input) => Effect.Effect<Endpoint5_10Output, E>
 
-export type Endpoint5_11Input = {
+export type Endpoint5_11Input = { readonly sessionID: Session.ID; readonly title: string }
+export type Endpoint5_11Output = void
+export type SessionRenameOperation<E = never> = (input: Endpoint5_11Input) => Effect.Effect<Endpoint5_11Output, E>
+
+export type Endpoint5_12Input = {
   readonly sessionID: Session.ID
   readonly directory: AbsolutePath
   readonly workspaceID?: Workspace.ID | undefined
   readonly delivery?: SessionInbox.Delivery | undefined
 }
-export type Endpoint5_11Output = void
-export type SessionMoveOperation<E = never> = (input: Endpoint5_11Input) => Effect.Effect<Endpoint5_11Output, E>
+export type Endpoint5_12Output = void
+export type SessionMoveOperation<E = never> = (input: Endpoint5_12Input) => Effect.Effect<Endpoint5_12Output, E>
 
-export type Endpoint5_12Input = {
+export type Endpoint5_13Input = {
   readonly sessionID: Session.ID
   readonly id?: SessionMessage.ID | undefined
   readonly text: string
@@ -186,10 +197,10 @@ export type Endpoint5_12Input = {
   readonly delivery?: SessionInbox.Delivery | undefined
   readonly resume?: boolean | undefined
 }
-export type Endpoint5_12Output = SessionInbox.User
-export type SessionPromptOperation<E = never> = (input: Endpoint5_12Input) => Effect.Effect<Endpoint5_12Output, E>
+export type Endpoint5_13Output = SessionInbox.User
+export type SessionPromptOperation<E = never> = (input: Endpoint5_13Input) => Effect.Effect<Endpoint5_13Output, E>
 
-export type Endpoint5_13Input = {
+export type Endpoint5_14Input = {
   readonly sessionID: Session.ID
   readonly id?: SessionMessage.ID | undefined
   readonly command: string
@@ -202,19 +213,19 @@ export type Endpoint5_13Input = {
   readonly delivery?: SessionInbox.Delivery | undefined
   readonly resume?: boolean | undefined
 }
-export type Endpoint5_13Output = SessionInbox.User
-export type SessionCommandOperation<E = never> = (input: Endpoint5_13Input) => Effect.Effect<Endpoint5_13Output, E>
+export type Endpoint5_14Output = SessionInbox.User
+export type SessionCommandOperation<E = never> = (input: Endpoint5_14Input) => Effect.Effect<Endpoint5_14Output, E>
 
-export type Endpoint5_14Input = {
+export type Endpoint5_15Input = {
   readonly sessionID: Session.ID
   readonly id?: SessionMessage.ID | undefined
   readonly skill: Skill.ID
   readonly resume?: boolean | undefined
 }
-export type Endpoint5_14Output = void
-export type SessionSkillOperation<E = never> = (input: Endpoint5_14Input) => Effect.Effect<Endpoint5_14Output, E>
+export type Endpoint5_15Output = void
+export type SessionSkillOperation<E = never> = (input: Endpoint5_15Input) => Effect.Effect<Endpoint5_15Output, E>
 
-export type Endpoint5_15Input = {
+export type Endpoint5_16Input = {
   readonly sessionID: Session.ID
   readonly id?: SessionMessage.ID | undefined
   readonly text: string
@@ -223,97 +234,97 @@ export type Endpoint5_15Input = {
   readonly delivery?: SessionInbox.Delivery | undefined
   readonly resume?: boolean | undefined
 }
-export type Endpoint5_15Output = SessionInbox.Synthetic
-export type SessionSyntheticOperation<E = never> = (input: Endpoint5_15Input) => Effect.Effect<Endpoint5_15Output, E>
+export type Endpoint5_16Output = SessionInbox.Synthetic
+export type SessionSyntheticOperation<E = never> = (input: Endpoint5_16Input) => Effect.Effect<Endpoint5_16Output, E>
 
-export type Endpoint5_16Input = {
+export type Endpoint5_17Input = {
   readonly sessionID: Session.ID
   readonly id?: Event.ID | undefined
   readonly command: string
 }
-export type Endpoint5_16Output = void
-export type SessionShellOperation<E = never> = (input: Endpoint5_16Input) => Effect.Effect<Endpoint5_16Output, E>
+export type Endpoint5_17Output = void
+export type SessionShellOperation<E = never> = (input: Endpoint5_17Input) => Effect.Effect<Endpoint5_17Output, E>
 
-export type Endpoint5_17Input = {
+export type Endpoint5_18Input = {
   readonly sessionID: Session.ID
   readonly id?: SessionMessage.ID | undefined
   readonly delivery?: SessionInbox.Delivery | undefined
 }
-export type Endpoint5_17Output = SessionInbox.Compaction
-export type SessionCompactOperation<E = never> = (input: Endpoint5_17Input) => Effect.Effect<Endpoint5_17Output, E>
+export type Endpoint5_18Output = SessionInbox.Compaction
+export type SessionCompactOperation<E = never> = (input: Endpoint5_18Input) => Effect.Effect<Endpoint5_18Output, E>
 
-export type Endpoint5_18Input = { readonly sessionID: Session.ID }
-export type Endpoint5_18Output = void
-export type SessionWaitOperation<E = never> = (input: Endpoint5_18Input) => Effect.Effect<Endpoint5_18Output, E>
+export type Endpoint5_19Input = { readonly sessionID: Session.ID }
+export type Endpoint5_19Output = void
+export type SessionWaitOperation<E = never> = (input: Endpoint5_19Input) => Effect.Effect<Endpoint5_19Output, E>
 
-export type Endpoint5_19Input = {
+export type Endpoint5_20Input = {
   readonly sessionID: Session.ID
   readonly messageID: SessionMessage.ID
   readonly files?: boolean | undefined
 }
-export type Endpoint5_19Output = Session.Revert
-export type SessionRevertStageOperation<E = never> = (input: Endpoint5_19Input) => Effect.Effect<Endpoint5_19Output, E>
-
-export type Endpoint5_20Input = { readonly sessionID: Session.ID }
-export type Endpoint5_20Output = void
-export type SessionRevertClearOperation<E = never> = (input: Endpoint5_20Input) => Effect.Effect<Endpoint5_20Output, E>
+export type Endpoint5_20Output = Session.Revert
+export type SessionRevertStageOperation<E = never> = (input: Endpoint5_20Input) => Effect.Effect<Endpoint5_20Output, E>
 
 export type Endpoint5_21Input = { readonly sessionID: Session.ID }
 export type Endpoint5_21Output = void
-export type SessionRevertCommitOperation<E = never> = (input: Endpoint5_21Input) => Effect.Effect<Endpoint5_21Output, E>
+export type SessionRevertClearOperation<E = never> = (input: Endpoint5_21Input) => Effect.Effect<Endpoint5_21Output, E>
 
 export type Endpoint5_22Input = { readonly sessionID: Session.ID }
-export type Endpoint5_22Output = ReadonlyArray<SessionMessage.Info>
-export type SessionContextOperation<E = never> = (input: Endpoint5_22Input) => Effect.Effect<Endpoint5_22Output, E>
+export type Endpoint5_22Output = void
+export type SessionRevertCommitOperation<E = never> = (input: Endpoint5_22Input) => Effect.Effect<Endpoint5_22Output, E>
 
 export type Endpoint5_23Input = { readonly sessionID: Session.ID }
-export type Endpoint5_23Output = ReadonlyArray<SessionInbox.Info>
-export type SessionInboxListOperation<E = never> = (input: Endpoint5_23Input) => Effect.Effect<Endpoint5_23Output, E>
+export type Endpoint5_23Output = ReadonlyArray<SessionMessage.Info>
+export type SessionContextOperation<E = never> = (input: Endpoint5_23Input) => Effect.Effect<Endpoint5_23Output, E>
 
-export type Endpoint5_24Input = { readonly sessionID: Session.ID; readonly inboxID: SessionMessage.ID }
-export type Endpoint5_24Output = void
-export type SessionInboxCancelOperation<E = never> = (input: Endpoint5_24Input) => Effect.Effect<Endpoint5_24Output, E>
+export type Endpoint5_24Input = { readonly sessionID: Session.ID }
+export type Endpoint5_24Output = ReadonlyArray<SessionInbox.Info>
+export type SessionInboxListOperation<E = never> = (input: Endpoint5_24Input) => Effect.Effect<Endpoint5_24Output, E>
 
 export type Endpoint5_25Input = { readonly sessionID: Session.ID; readonly inboxID: SessionMessage.ID }
 export type Endpoint5_25Output = void
-export type SessionInboxSteerOperation<E = never> = (input: Endpoint5_25Input) => Effect.Effect<Endpoint5_25Output, E>
+export type SessionInboxCancelOperation<E = never> = (input: Endpoint5_25Input) => Effect.Effect<Endpoint5_25Output, E>
 
 export type Endpoint5_26Input = { readonly sessionID: Session.ID; readonly inboxID: SessionMessage.ID }
 export type Endpoint5_26Output = void
-export type SessionInboxQueueOperation<E = never> = (input: Endpoint5_26Input) => Effect.Effect<Endpoint5_26Output, E>
+export type SessionInboxSteerOperation<E = never> = (input: Endpoint5_26Input) => Effect.Effect<Endpoint5_26Output, E>
+
+export type Endpoint5_27Input = { readonly sessionID: Session.ID; readonly inboxID: SessionMessage.ID }
+export type Endpoint5_27Output = void
+export type SessionInboxQueueOperation<E = never> = (input: Endpoint5_27Input) => Effect.Effect<Endpoint5_27Output, E>
 
-export type Endpoint5_27Input = { readonly sessionID: Session.ID }
-export type Endpoint5_27Output = ReadonlyArray<InstructionEntry.Info>
+export type Endpoint5_28Input = { readonly sessionID: Session.ID }
+export type Endpoint5_28Output = ReadonlyArray<InstructionEntry.Info>
 export type SessionInstructionsEntryListOperation<E = never> = (
-  input: Endpoint5_27Input,
-) => Effect.Effect<Endpoint5_27Output, E>
+  input: Endpoint5_28Input,
+) => Effect.Effect<Endpoint5_28Output, E>
 
-export type Endpoint5_28Input = {
+export type Endpoint5_29Input = {
   readonly sessionID: Session.ID
   readonly key: InstructionEntry.Key
   readonly value: Schema.Json
 }
-export type Endpoint5_28Output = void
-export type SessionInstructionsEntryPutOperation<E = never> = (
-  input: Endpoint5_28Input,
-) => Effect.Effect<Endpoint5_28Output, E>
-
-export type Endpoint5_29Input = { readonly sessionID: Session.ID; readonly key: InstructionEntry.Key }
 export type Endpoint5_29Output = void
-export type SessionInstructionsEntryRemoveOperation<E = never> = (
+export type SessionInstructionsEntryPutOperation<E = never> = (
   input: Endpoint5_29Input,
 ) => Effect.Effect<Endpoint5_29Output, E>
 
-export type Endpoint5_30Input = { readonly sessionID: Session.ID; readonly prompt: string }
-export type Endpoint5_30Output = { readonly text: string }
-export type SessionGenerateOperation<E = never> = (input: Endpoint5_30Input) => Effect.Effect<Endpoint5_30Output, E>
+export type Endpoint5_30Input = { readonly sessionID: Session.ID; readonly key: InstructionEntry.Key }
+export type Endpoint5_30Output = void
+export type SessionInstructionsEntryRemoveOperation<E = never> = (
+  input: Endpoint5_30Input,
+) => Effect.Effect<Endpoint5_30Output, E>
+
+export type Endpoint5_31Input = { readonly sessionID: Session.ID; readonly prompt: string }
+export type Endpoint5_31Output = { readonly text: string }
+export type SessionGenerateOperation<E = never> = (input: Endpoint5_31Input) => Effect.Effect<Endpoint5_31Output, E>
 
-export type Endpoint5_31Input = {
+export type Endpoint5_32Input = {
   readonly sessionID: Session.ID
   readonly after?: Event.Seq | undefined
   readonly follow?: boolean | undefined
 }
-export type Endpoint5_31Output =
+export type Endpoint5_32Output =
   | (
       | {
           readonly id: Event.ID
@@ -902,19 +913,19 @@ export type Endpoint5_31Output =
         }
     )
   | EventLog.Synced
-export type SessionLogOperation<E = never> = (input: Endpoint5_31Input) => Stream.Stream<Endpoint5_31Output, E>
+export type SessionLogOperation<E = never> = (input: Endpoint5_32Input) => Stream.Stream<Endpoint5_32Output, E>
 
-export type Endpoint5_32Input = { readonly sessionID: Session.ID; readonly continue?: boolean | undefined }
-export type Endpoint5_32Output = void
-export type SessionInterruptOperation<E = never> = (input: Endpoint5_32Input) => Effect.Effect<Endpoint5_32Output, E>
-
-export type Endpoint5_33Input = { readonly sessionID: Session.ID }
+export type Endpoint5_33Input = { readonly sessionID: Session.ID; readonly continue?: boolean | undefined }
 export type Endpoint5_33Output = void
-export type SessionBackgroundOperation<E = never> = (input: Endpoint5_33Input) => Effect.Effect<Endpoint5_33Output, E>
+export type SessionInterruptOperation<E = never> = (input: Endpoint5_33Input) => Effect.Effect<Endpoint5_33Output, E>
+
+export type Endpoint5_34Input = { readonly sessionID: Session.ID }
+export type Endpoint5_34Output = void
+export type SessionBackgroundOperation<E = never> = (input: Endpoint5_34Input) => Effect.Effect<Endpoint5_34Output, E>
 
-export type Endpoint5_34Input = { readonly sessionID: Session.ID; readonly messageID: SessionMessage.ID }
-export type Endpoint5_34Output = SessionMessage.Info
-export type SessionMessageOperation<E = never> = (input: Endpoint5_34Input) => Effect.Effect<Endpoint5_34Output, E>
+export type Endpoint5_35Input = { readonly sessionID: Session.ID; readonly messageID: SessionMessage.ID }
+export type Endpoint5_35Output = SessionMessage.Info
+export type SessionMessageOperation<E = never> = (input: Endpoint5_35Input) => Effect.Effect<Endpoint5_35Output, E>
 
 export interface SessionApi<E = never> {
   readonly list: SessionListOperation<E>
@@ -925,6 +936,7 @@ export interface SessionApi<E = never> {
   readonly get: SessionGetOperation<E>
   readonly remove: SessionRemoveOperation<E>
   readonly fork: SessionForkOperation<E>
+  readonly select: SessionSelectOperation<E>
   readonly switchAgent: SessionSwitchAgentOperation<E>
   readonly switchModel: SessionSwitchModelOperation<E>
   readonly rename: SessionRenameOperation<E>
diff --git a/packages/client/src/effect/generated/client.ts b/packages/client/src/effect/generated/client.ts
index 295e76d4ef59..556d19fa94ca 100644
--- a/packages/client/src/effect/generated/client.ts
+++ b/packages/client/src/effect/generated/client.ts
@@ -86,6 +86,8 @@ import type {
   Endpoint5_33Output,
   Endpoint5_34Input,
   Endpoint5_34Output,
+  Endpoint5_35Input,
+  Endpoint5_35Output,
   Endpoint6_0Input,
   Endpoint6_0Output,
   Endpoint7_0Input,
@@ -376,35 +378,43 @@ const Endpoint5_7 = (raw: RawClient["server.session"]) => (input: Endpoint5_7Inp
 
 const Endpoint5_8 = (raw: RawClient["server.session"]) => (input: Endpoint5_8Input) =>
   preserveEffect<Endpoint5_8Output>()(
-    raw["session.switchAgent"]({ params: { sessionID: input["sessionID"] }, payload: { agent: input["agent"] } }).pipe(
-      Effect.mapError(mapClientError),
-    ),
+    raw["session.select"]({
+      params: { sessionID: input["sessionID"] },
+      payload: { agent: input["agent"], model: input["model"] },
+    }).pipe(Effect.mapError(mapClientError)),
   )
 
 const Endpoint5_9 = (raw: RawClient["server.session"]) => (input: Endpoint5_9Input) =>
   preserveEffect<Endpoint5_9Output>()(
-    raw["session.switchModel"]({ params: { sessionID: input["sessionID"] }, payload: { model: input["model"] } }).pipe(
+    raw["session.switchAgent"]({ params: { sessionID: input["sessionID"] }, payload: { agent: input["agent"] } }).pipe(
       Effect.mapError(mapClientError),
     ),
   )
 
 const Endpoint5_10 = (raw: RawClient["server.session"]) => (input: Endpoint5_10Input) =>
   preserveEffect<Endpoint5_10Output>()(
-    raw["session.rename"]({ params: { sessionID: input["sessionID"] }, payload: { title: input["title"] } }).pipe(
+    raw["session.switchModel"]({ params: { sessionID: input["sessionID"] }, payload: { model: input["model"] } }).pipe(
       Effect.mapError(mapClientError),
     ),
   )
 
 const Endpoint5_11 = (raw: RawClient["server.session"]) => (input: Endpoint5_11Input) =>
   preserveEffect<Endpoint5_11Output>()(
+    raw["session.rename"]({ params: { sessionID: input["sessionID"] }, payload: { title: input["title"] } }).pipe(
+      Effect.mapError(mapClientError),
+    ),
+  )
+
+const Endpoint5_12 = (raw: RawClient["server.session"]) => (input: Endpoint5_12Input) =>
+  preserveEffect<Endpoint5_12Output>()(
     raw["session.move"]({
       params: { sessionID: input["sessionID"] },
       payload: { directory: input["directory"], workspaceID: input["workspaceID"], delivery: input["delivery"] },
     }).pipe(Effect.mapError(mapClientError)),
   )
 
-const Endpoint5_12 = (raw: RawClient["server.session"]) => (input: Endpoint5_12Input) =>
-  preserveEffect<Endpoint5_12Output>()(
+const Endpoint5_13 = (raw: RawClient["server.session"]) => (input: Endpoint5_13Input) =>
+  preserveEffect<Endpoint5_13Output>()(
     raw["session.prompt"]({
       params: { sessionID: input["sessionID"] },
       payload: {
@@ -423,8 +433,8 @@ const Endpoint5_12 = (raw: RawClient["server.session"]) => (input: Endpoint5_12I
     ),
   )
 
-const Endpoint5_13 = (raw: RawClient["server.session"]) => (input: Endpoint5_13Input) =>
-  preserveEffect<Endpoint5_13Output>()(
+const Endpoint5_14 = (raw: RawClient["server.session"]) => (input: Endpoint5_14Input) =>
+  preserveEffect<Endpoint5_14Output>()(
     raw["session.command"]({
       params: { sessionID: input["sessionID"] },
       payload: {
@@ -445,16 +455,16 @@ const Endpoint5_13 = (raw: RawClient["server.session"]) => (input: Endpoint5_13I
     ),
   )
 
-const Endpoint5_14 = (raw: RawClient["server.session"]) => (input: Endpoint5_14Input) =>
-  preserveEffect<Endpoint5_14Output>()(
+const Endpoint5_15 = (raw: RawClient["server.session"]) => (input: Endpoint5_15Input) =>
+  preserveEffect<Endpoint5_15Output>()(
     raw["session.skill"]({
       params: { sessionID: input["sessionID"] },
       payload: { id: input["id"], skill: input["skill"], resume: input["resume"] },
     }).pipe(Effect.mapError(mapClientError)),
   )
 
-const Endpoint5_15 = (raw: RawClient["server.session"]) => (input: Endpoint5_15Input) =>
-  preserveEffect<Endpoint5_15Output>()(
+const Endpoint5_16 = (raw: RawClient["server.session"]) => (input: Endpoint5_16Input) =>
+  preserveEffect<Endpoint5_16Output>()(
     raw["session.synthetic"]({
       params: { sessionID: input["sessionID"] },
       payload: {
@@ -471,16 +481,16 @@ const Endpoint5_15 = (raw: RawClient["server.session"]) => (input: Endpoint5_15I
     ),
   )
 
-const Endpoint5_16 = (raw: RawClient["server.session"]) => (input: Endpoint5_16Input) =>
-  preserveEffect<Endpoint5_16Output>()(
+const Endpoint5_17 = (raw: RawClient["server.session"]) => (input: Endpoint5_17Input) =>
+  preserveEffect<Endpoint5_17Output>()(
     raw["session.shell"]({
       params: { sessionID: input["sessionID"] },
       payload: { id: input["id"], command: input["command"] },
     }).pipe(Effect.mapError(mapClientError)),
   )
 
-const Endpoint5_17 = (raw: RawClient["server.session"]) => (input: Endpoint5_17Input) =>
-  preserveEffect<Endpoint5_17Output>()(
+const Endpoint5_18 = (raw: RawClient["server.session"]) => (input: Endpoint5_18Input) =>
+  preserveEffect<Endpoint5_18Output>()(
     raw["session.compact"]({
       params: { sessionID: input["sessionID"] },
       payload: { id: input["id"], delivery: input["delivery"] },
@@ -490,13 +500,13 @@ const Endpoint5_17 = (raw: RawClient["server.session"]) => (input: Endpoint5_17I
     ),
   )
 
-const Endpoint5_18 = (raw: RawClient["server.session"]) => (input: Endpoint5_18Input) =>
-  preserveEffect<Endpoint5_18Output>()(
+const Endpoint5_19 = (raw: RawClient["server.session"]) => (input: Endpoint5_19Input) =>
+  preserveEffect<Endpoint5_19Output>()(
     raw["session.wait"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
   )
 
-const Endpoint5_19 = (raw: RawClient["server.session"]) => (input: Endpoint5_19Input) =>
-  preserveEffect<Endpoint5_19Output>()(
+const Endpoint5_20 = (raw: RawClient["server.session"]) => (input: Endpoint5_20Input) =>
+  preserveEffect<Endpoint5_20Output>()(
     raw["session.revert.stage"]({
       params: { sessionID: input["sessionID"] },
       payload: { messageID: input["messageID"], files: input["files"] },
@@ -506,27 +516,19 @@ const Endpoint5_19 = (raw: RawClient["server.session"]) => (input: Endpoint5_19I
     ),
   )
 
-const Endpoint5_20 = (raw: RawClient["server.session"]) => (input: Endpoint5_20Input) =>
-  preserveEffect<Endpoint5_20Output>()(
-    raw["session.revert.clear"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
-  )
-
 const Endpoint5_21 = (raw: RawClient["server.session"]) => (input: Endpoint5_21Input) =>
   preserveEffect<Endpoint5_21Output>()(
-    raw["session.revert.commit"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
+    raw["session.revert.clear"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
   )
 
 const Endpoint5_22 = (raw: RawClient["server.session"]) => (input: Endpoint5_22Input) =>
   preserveEffect<Endpoint5_22Output>()(
-    raw["session.context"]({ params: { sessionID: input["sessionID"] } }).pipe(
-      Effect.mapError(mapClientError),
-      Effect.map((value) => value.data),
-    ),
+    raw["session.revert.commit"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
   )
 
 const Endpoint5_23 = (raw: RawClient["server.session"]) => (input: Endpoint5_23Input) =>
   preserveEffect<Endpoint5_23Output>()(
-    raw["session.inbox.list"]({ params: { sessionID: input["sessionID"] } }).pipe(
+    raw["session.context"]({ params: { sessionID: input["sessionID"] } }).pipe(
       Effect.mapError(mapClientError),
       Effect.map((value) => value.data),
     ),
@@ -534,58 +536,66 @@ const Endpoint5_23 = (raw: RawClient["server.session"]) => (input: Endpoint5_23I
 
 const Endpoint5_24 = (raw: RawClient["server.session"]) => (input: Endpoint5_24Input) =>
   preserveEffect<Endpoint5_24Output>()(
-    raw["session.inbox.cancel"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
+    raw["session.inbox.list"]({ params: { sessionID: input["sessionID"] } }).pipe(
       Effect.mapError(mapClientError),
+      Effect.map((value) => value.data),
     ),
   )
 
 const Endpoint5_25 = (raw: RawClient["server.session"]) => (input: Endpoint5_25Input) =>
   preserveEffect<Endpoint5_25Output>()(
-    raw["session.inbox.steer"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
+    raw["session.inbox.cancel"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
       Effect.mapError(mapClientError),
     ),
   )
 
 const Endpoint5_26 = (raw: RawClient["server.session"]) => (input: Endpoint5_26Input) =>
   preserveEffect<Endpoint5_26Output>()(
-    raw["session.inbox.queue"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
+    raw["session.inbox.steer"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
       Effect.mapError(mapClientError),
     ),
   )
 
 const Endpoint5_27 = (raw: RawClient["server.session"]) => (input: Endpoint5_27Input) =>
   preserveEffect<Endpoint5_27Output>()(
-    raw["session.instructions.entry.list"]({ params: { sessionID: input["sessionID"] } }).pipe(
+    raw["session.inbox.queue"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
       Effect.mapError(mapClientError),
-      Effect.map((value) => value.data),
     ),
   )
 
 const Endpoint5_28 = (raw: RawClient["server.session"]) => (input: Endpoint5_28Input) =>
   preserveEffect<Endpoint5_28Output>()(
+    raw["session.instructions.entry.list"]({ params: { sessionID: input["sessionID"] } }).pipe(
+      Effect.mapError(mapClientError),
+      Effect.map((value) => value.data),
+    ),
+  )
+
+const Endpoint5_29 = (raw: RawClient["server.session"]) => (input: Endpoint5_29Input) =>
+  preserveEffect<Endpoint5_29Output>()(
     raw["session.instructions.entry.put"]({
       params: { sessionID: input["sessionID"], key: input["key"] },
       payload: { value: input["value"] },
     }).pipe(Effect.mapError(mapClientError)),
   )
 
-const Endpoint5_29 = (raw: RawClient["server.session"]) => (input: Endpoint5_29Input) =>
-  preserveEffect<Endpoint5_29Output>()(
+const Endpoint5_30 = (raw: RawClient["server.session"]) => (input: Endpoint5_30Input) =>
+  preserveEffect<Endpoint5_30Output>()(
     raw["session.instructions.entry.remove"]({ params: { sessionID: input["sessionID"], key: input["key"] } }).pipe(
       Effect.mapError(mapClientError),
     ),
   )
 
-const Endpoint5_30 = (raw: RawClient["server.session"]) => (input: Endpoint5_30Input) =>
-  preserveEffect<Endpoint5_30Output>()(
+const Endpoint5_31 = (raw: RawClient["server.session"]) => (input: Endpoint5_31Input) =>
+  preserveEffect<Endpoint5_31Output>()(
     raw["session.generate"]({ params: { sessionID: input["sessionID"] }, payload: { prompt: input["prompt"] } }).pipe(
       Effect.mapError(mapClientError),
       Effect.map((value) => value.data),
     ),
   )
 
-const Endpoint5_31 = (raw: RawClient["server.session"]) => (input: Endpoint5_31Input) =>
-  preserveStream<Endpoint5_31Output>()(
+const Endpoint5_32 = (raw: RawClient["server.session"]) => (input: Endpoint5_32Input) =>
+  preserveStream<Endpoint5_32Output>()(
     Stream.unwrap(
       raw["session.log"]({
         params: { sessionID: input["sessionID"] },
@@ -597,21 +607,21 @@ const Endpoint5_31 = (raw: RawClient["server.session"]) => (input: Endpoint5_31I
     ),
   )
 
-const Endpoint5_32 = (raw: RawClient["server.session"]) => (input: Endpoint5_32Input) =>
-  preserveEffect<Endpoint5_32Output>()(
+const Endpoint5_33 = (raw: RawClient["server.session"]) => (input: Endpoint5_33Input) =>
+  preserveEffect<Endpoint5_33Output>()(
     raw["session.interrupt"]({
       params: { sessionID: input["sessionID"] },
       query: { continue: input["continue"] },
     }).pipe(Effect.mapError(mapClientError)),
   )
 
-const Endpoint5_33 = (raw: RawClient["server.session"]) => (input: Endpoint5_33Input) =>
-  preserveEffect<Endpoint5_33Output>()(
+const Endpoint5_34 = (raw: RawClient["server.session"]) => (input: Endpoint5_34Input) =>
+  preserveEffect<Endpoint5_34Output>()(
     raw["session.background"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
   )
 
-const Endpoint5_34 = (raw: RawClient["server.session"]) => (input: Endpoint5_34Input) =>
-  preserveEffect<Endpoint5_34Output>()(
+const Endpoint5_35 = (raw: RawClient["server.session"]) => (input: Endpoint5_35Input) =>
+  preserveEffect<Endpoint5_35Output>()(
     raw["session.message"]({ params: { sessionID: input["sessionID"], messageID: input["messageID"] } }).pipe(
       Effect.mapError(mapClientError),
       Effect.map((value) => value.data),
@@ -627,26 +637,27 @@ const adaptGroup5 = (raw: RawClient["server.session"]) => ({
   get: Endpoint5_5(raw),
   remove: Endpoint5_6(raw),
   fork: Endpoint5_7(raw),
-  switchAgent: Endpoint5_8(raw),
-  switchModel: Endpoint5_9(raw),
-  rename: Endpoint5_10(raw),
-  move: Endpoint5_11(raw),
-  prompt: Endpoint5_12(raw),
-  command: Endpoint5_13(raw),
-  skill: Endpoint5_14(raw),
-  synthetic: Endpoint5_15(raw),
-  shell: Endpoint5_16(raw),
-  compact: Endpoint5_17(raw),
-  wait: Endpoint5_18(raw),
-  revert: { stage: Endpoint5_19(raw), clear: Endpoint5_20(raw), commit: Endpoint5_21(raw) },
-  context: Endpoint5_22(raw),
-  inbox: { list: Endpoint5_23(raw), cancel: Endpoint5_24(raw), steer: Endpoint5_25(raw), queue: Endpoint5_26(raw) },
-  instructions: { entry: { list: Endpoint5_27(raw), put: Endpoint5_28(raw), remove: Endpoint5_29(raw) } },
-  generate: Endpoint5_30(raw),
-  log: Endpoint5_31(raw),
-  interrupt: Endpoint5_32(raw),
-  background: Endpoint5_33(raw),
-  message: Endpoint5_34(raw),
+  select: Endpoint5_8(raw),
+  switchAgent: Endpoint5_9(raw),
+  switchModel: Endpoint5_10(raw),
+  rename: Endpoint5_11(raw),
+  move: Endpoint5_12(raw),
+  prompt: Endpoint5_13(raw),
+  command: Endpoint5_14(raw),
+  skill: Endpoint5_15(raw),
+  synthetic: Endpoint5_16(raw),
+  shell: Endpoint5_17(raw),
+  compact: Endpoint5_18(raw),
+  wait: Endpoint5_19(raw),
+  revert: { stage: Endpoint5_20(raw), clear: Endpoint5_21(raw), commit: Endpoint5_22(raw) },
+  context: Endpoint5_23(raw),
+  inbox: { list: Endpoint5_24(raw), cancel: Endpoint5_25(raw), steer: Endpoint5_26(raw), queue: Endpoint5_27(raw) },
+  instructions: { entry: { list: Endpoint5_28(raw), put: Endpoint5_29(raw), remove: Endpoint5_30(raw) } },
+  generate: Endpoint5_31(raw),
+  log: Endpoint5_32(raw),
+  interrupt: Endpoint5_33(raw),
+  background: Endpoint5_34(raw),
+  message: Endpoint5_35(raw),
 })
 
 const Endpoint6_0 = (raw: RawClient["server.message"]) => (input: Endpoint6_0Input) =>
diff --git a/packages/client/src/promise/generated/client.ts b/packages/client/src/promise/generated/client.ts
index 55873f70d61b..ee902f4508ae 100644
--- a/packages/client/src/promise/generated/client.ts
+++ b/packages/client/src/promise/generated/client.ts
@@ -26,6 +26,8 @@ import type {
   SessionRemoveOutput,
   SessionForkInput,
   SessionForkOutput,
+  SessionSelectInput,
+  SessionSelectOutput,
   SessionSwitchAgentInput,
   SessionSwitchAgentOutput,
   SessionSwitchModelInput,
@@ -557,6 +559,18 @@ export function make(options: ClientOptions) {
           },
           requestOptions,
         ).then((value) => value.data),
+      select: (input: SessionSelectInput, requestOptions?: RequestOptions) =>
+        request<SessionSelectOutput>(
+          {
+            method: "POST",
+            path: `/api/session/${encodeURIComponent(input.sessionID)}/selection`,
+            body: { agent: input["agent"], model: input["model"] },
+            successStatus: 204,
+            declaredStatuses: [404, 503, 400, 401],
+            empty: true,
+          },
+          requestOptions,
+        ),
       switchAgent: (input: SessionSwitchAgentInput, requestOptions?: RequestOptions) =>
         request<SessionSwitchAgentOutput>(
           {
diff --git a/packages/client/src/promise/generated/types.ts b/packages/client/src/promise/generated/types.ts
index 8eb68bc84bb8..f770cb3fe52a 100644
--- a/packages/client/src/promise/generated/types.ts
+++ b/packages/client/src/promise/generated/types.ts
@@ -2194,6 +2194,14 @@ export type MessageNotFoundError = {
 export const isMessageNotFoundError = (value: unknown): value is MessageNotFoundError =>
   typeof value === "object" && value !== null && "_tag" in value && value["_tag"] === "MessageNotFoundError"
 
+export type ServiceUnavailableError = {
+  readonly _tag: "ServiceUnavailableError"
+  readonly message: string
+  readonly service?: string | undefined
+}
+export const isServiceUnavailableError = (value: unknown): value is ServiceUnavailableError =>
+  typeof value === "object" && value !== null && "_tag" in value && value["_tag"] === "ServiceUnavailableError"
+
 export type CommandNotFoundError = {
   readonly _tag: "CommandNotFoundError"
   readonly command: string
@@ -2218,14 +2226,6 @@ export type SkillNotFoundError = {
 export const isSkillNotFoundError = (value: unknown): value is SkillNotFoundError =>
   typeof value === "object" && value !== null && "_tag" in value && value["_tag"] === "SkillNotFoundError"
 
-export type ServiceUnavailableError = {
-  readonly _tag: "ServiceUnavailableError"
-  readonly message: string
-  readonly service?: string | undefined
-}
-export const isServiceUnavailableError = (value: unknown): value is ServiceUnavailableError =>
-  typeof value === "object" && value !== null && "_tag" in value && value["_tag"] === "ServiceUnavailableError"
-
 export type SessionBusyError = {
   readonly _tag: "SessionBusyError"
   readonly sessionID: string
@@ -3343,6 +3343,32 @@ export type SessionForkInput = {
 
 export type SessionForkOutput = { data: SessionInfo }["data"]
 
+export type SessionSelectInput = {
+  readonly sessionID: { readonly sessionID: string }["sessionID"]
+  readonly agent: {
+    readonly agent: string
+    readonly model:
+      | { readonly type: "preserve" }
+      | { readonly type: "configured" }
+      | {
+          readonly type: "explicit"
+          readonly model: { readonly id: string; readonly providerID: string; readonly variant?: string }
+        }
+  }["agent"]
+  readonly model: {
+    readonly agent: string
+    readonly model:
+      | { readonly type: "preserve" }
+      | { readonly type: "configured" }
+      | {
+          readonly type: "explicit"
+          readonly model: { readonly id: string; readonly providerID: string; readonly variant?: string }
+        }
+  }["model"]
+}
+
+export type SessionSelectOutput = void
+
 export type SessionSwitchAgentInput = {
   readonly sessionID: { readonly sessionID: string }["sessionID"]
   readonly agent: { readonly agent: string }["agent"]
diff --git a/packages/core/src/session.ts b/packages/core/src/session.ts
index 45b3856fbb35..8394de89c1e1 100644
--- a/packages/core/src/session.ts
+++ b/packages/core/src/session.ts
@@ -204,6 +204,11 @@ export interface Interface {
   }) => Stream.Stream<SessionEvent.DurableEvent | EventLog.Synced, NotFoundError>
   readonly switchAgent: (input: { sessionID: SessionSchema.ID; agent: Agent.ID }) => Effect.Effect<void, NotFoundError>
   readonly switchModel: (input: { sessionID: SessionSchema.ID; model: Model.Ref }) => Effect.Effect<void, NotFoundError>
+  readonly select: (input: {
+    sessionID: SessionSchema.ID
+    agent: Agent.ID
+    model?: Model.Ref
+  }) => Effect.Effect<void, NotFoundError>
   readonly rename: (input: { sessionID: SessionSchema.ID; title: string }) => Effect.Effect<void, NotFoundError>
   readonly move: (input: {
     sessionID: SessionSchema.ID
@@ -701,22 +706,45 @@ const layer = Layer.effect(
             .resume(input.sessionID)
             .pipe(Effect.ignore, Effect.forkIn(scope, { startImmediately: true }), Effect.asVoid)
       }),
-      switchAgent: Effect.fn("Session.switchAgent")(function* (input) {
+      select: Effect.fn("Session.select")(function* (input) {
         const session = yield* result.get(input.sessionID)
-        yield* bus.publish(SessionEvent.AgentSelected, {
-          sessionID: input.sessionID,
-          agent: input.agent,
-          previous: session.agent,
-        })
+        const agent =
+          session.agent === input.agent
+            ? undefined
+            : {
+                sessionID: input.sessionID,
+                agent: input.agent,
+                previous: session.agent,
+              }
+        const model = !input.model || sameModel(session.model, input.model) ? undefined : input.model
+        if (agent && model) {
+          yield* bus.publishAll([
+            [SessionEvent.AgentSelected, agent],
+            [
+              SessionEvent.ModelSelected,
+              {
+                sessionID: input.sessionID,
+                model,
+                previous: session.model,
+              },
+            ],
+          ])
+          return
+        }
+        if (agent) yield* bus.publish(SessionEvent.AgentSelected, agent)
+        if (model)
+          yield* bus.publish(SessionEvent.ModelSelected, {
+            sessionID: input.sessionID,
+            model,
+            previous: session.model,
+          })
+      }),
+      switchAgent: Effect.fn("Session.switchAgent")(function* (input) {
+        yield* result.select({ sessionID: input.sessionID, agent: input.agent })
       }),
       switchModel: Effect.fn("Session.switchModel")(function* (input) {
         const session = yield* result.get(input.sessionID)
-        if (
-          session.model?.providerID === input.model.providerID &&
-          session.model.id === input.model.id &&
-          (session.model.variant ?? "default") === (input.model.variant ?? "default")
-        )
-          return
+        if (sameModel(session.model, input.model)) return
         yield* bus.publish(SessionEvent.ModelSelected, {
           sessionID: input.sessionID,
           model: input.model,
@@ -1084,6 +1112,14 @@ function positiveInt(value: string | null) {
   return Number.isInteger(parsed) && parsed > 0 ? parsed : undefined
 }
 
+function sameModel(left: Model.Ref | undefined, right: Model.Ref) {
+  return (
+    left?.providerID === right.providerID &&
+    left.id === right.id &&
+    (left.variant ?? "default") === (right.variant ?? "default")
+  )
+}
+
 // Mirrors the shell tool's in-memory preview safety limit.
 const SHELL_MAX_CAPTURE_BYTES = 1024 * 1024
 
diff --git a/packages/core/test/session-create.test.ts b/packages/core/test/session-create.test.ts
index 66a115083093..d79384698c43 100644
--- a/packages/core/test/session-create.test.ts
+++ b/packages/core/test/session-create.test.ts
@@ -728,6 +728,34 @@ describe("Session.create", () => {
     }),
   )
 
+  it.effect("atomically selects a different agent and model", () =>
+    Effect.gen(function* () {
+      const session = yield* Session.Service
+      const previous = Model.Ref.make({ id: Model.ID.make("haiku"), providerID: Provider.ID.anthropic })
+      const created = yield* session.create({ location, agent: Agent.ID.make("build"), model: previous })
+      const model = Model.Ref.make({ id: Model.ID.make("sonnet"), providerID: Provider.ID.anthropic })
+
+      yield* session.select({ sessionID: created.id, agent: Agent.ID.make("plan"), model })
+
+      expect(yield* session.get(created.id)).toMatchObject({ agent: "plan", model })
+      expect(
+        Array.from(yield* logEvents(session, created.id).pipe(Stream.runCollect)).map((event) => event.type),
+      ).toEqual(["session.created", "session.agent.selected", "session.model.selected"])
+    }),
+  )
+
+  it.effect("does not emit redundant selection events", () =>
+    Effect.gen(function* () {
+      const session = yield* Session.Service
+      const model = Model.Ref.make({ id: Model.ID.make("sonnet"), providerID: Provider.ID.anthropic })
+      const created = yield* session.create({ location, agent: Agent.ID.make("build"), model })
+
+      yield* session.select({ sessionID: created.id, agent: Agent.ID.make("build"), model })
+
+      expect(Array.from(yield* logEvents(session, created.id).pipe(Stream.runCollect))).toHaveLength(1)
+    }),
+  )
+
   it.effect("rejects an agent switch for a missing Session", () =>
     Effect.gen(function* () {
       const session = yield* Session.Service
diff --git a/packages/protocol/src/groups/session.ts b/packages/protocol/src/groups/session.ts
index 30677d584148..b27d0160010a 100644
--- a/packages/protocol/src/groups/session.ts
+++ b/packages/protocol/src/groups/session.ts
@@ -54,6 +54,12 @@ const SessionsQueryFields = {
   parentID: ParentIDFilter.pipe(Schema.optional),
 }
 
+const SelectionModelPolicy = Schema.Union([
+  Schema.Struct({ type: Schema.Literal("preserve") }),
+  Schema.Struct({ type: Schema.Literal("configured") }),
+  Schema.Struct({ type: Schema.Literal("explicit"), model: Model.Ref }),
+])
+
 const SessionsDirectoryQuery = Schema.Struct({
   ...SessionsQueryFields,
   directory: AbsolutePath,
@@ -250,6 +256,22 @@ export const makeSessionGroup = <I extends HttpApiMiddleware.AnyId, S>(sessionLo
           }),
         ),
     )
+    .add(
+      HttpApiEndpoint.post("session.select", "/api/session/:sessionID/selection", {
+        params: { sessionID: Session.ID },
+        payload: Schema.Struct({ agent: Agent.ID, model: SelectionModelPolicy }),
+        success: HttpApiSchema.NoContent,
+        error: [SessionNotFoundError, ServiceUnavailableError],
+      })
+        .middleware(sessionLocationMiddleware)
+        .annotateMerge(
+          OpenApi.annotations({
+            identifier: "v2.session.select",
+            summary: "Select session agent and model",
+            description: "Atomically select the agent and model policy used by subsequent provider turns.",
+          }),
+        ),
+    )
     .add(
       HttpApiEndpoint.post("session.switchAgent", "/api/session/:sessionID/agent", {
         params: { sessionID: Session.ID },
diff --git a/packages/server/src/handlers/session.ts b/packages/server/src/handlers/session.ts
index 5813663bf549..9915683522bb 100644
--- a/packages/server/src/handlers/session.ts
+++ b/packages/server/src/handlers/session.ts
@@ -1,4 +1,6 @@
 import { Session } from "@opencode-ai/core/session"
+import { Agent } from "@opencode-ai/core/agent"
+import { PluginSupervisor } from "@opencode-ai/core/plugin/supervisor"
 import { SessionTransfer } from "@opencode-ai/core/session/transfer"
 import { InstructionEntry } from "@opencode-ai/core/session/instruction-entry"
 import { DateTime, Effect, Stream } from "effect"
@@ -226,6 +228,41 @@ export const SessionHandler = HttpApiBuilder.group(Api, "server.session", (handl
           }
         }),
       )
+      .handle(
+        "session.select",
+        Effect.fn(function* (ctx) {
+          const model = yield* Effect.gen(function* () {
+            if (ctx.payload.model.type === "preserve") return undefined
+            if (ctx.payload.model.type === "explicit") return ctx.payload.model.model
+            const plugins = yield* PluginSupervisor.Service
+            yield* plugins.flush.pipe(
+              Effect.timeoutOrElse({
+                duration: "5 seconds",
+                orElse: () =>
+                  Effect.fail(
+                    new ServiceUnavailableError({
+                      message: "Agent initialization timed out",
+                      service: "agent.catalog",
+                    }),
+                  ),
+              }),
+            )
+            const agents = yield* Agent.Service
+            return (yield* agents.get(ctx.payload.agent))?.model
+          })
+          yield* session.select({ sessionID: ctx.params.sessionID, agent: ctx.payload.agent, model }).pipe(
+            Effect.catchTag("Session.NotFoundError", (error) =>
+              Effect.fail(
+                new SessionNotFoundError({
+                  sessionID: error.sessionID,
+                  message: `Session not found: ${error.sessionID}`,
+                }),
+              ),
+            ),
+          )
+          return HttpApiSchema.NoContent.make()
+        }),
+      )
       .handle(
         "session.switchAgent",
         Effect.fn(function* (ctx) {
diff --git a/packages/server/test/session.test.ts b/packages/server/test/session.test.ts
new file mode 100644
index 000000000000..7018644f430f
--- /dev/null
+++ b/packages/server/test/session.test.ts
@@ -0,0 +1,119 @@
+import fs from "node:fs/promises"
+import path from "node:path"
+import { expect } from "bun:test"
+import { Effect } from "effect"
+import { HttpServer } from "effect/unstable/http"
+import { tmpdir } from "../../core/test/fixture/tmpdir"
+import { it } from "../../core/test/lib/effect"
+import { ServerProcess } from "../src/process"
+
+it.live("resolves configured agent models after plugin initialization", () =>
+  Effect.acquireUseRelease(
+    Effect.promise(() => tmpdir("opencode-session-endpoint-")),
+    (tmp) =>
+      Effect.gen(function* () {
+        yield* Effect.promise(() =>
+          fs.writeFile(
+            path.join(tmp.path, "opencode.json"),
+            JSON.stringify({
+              agents: {
+                modelprobe: {
+                  description: "Model resolution probe",
+                  mode: "primary",
+                  model: "opencode/nemotron-3.5-lightning-free",
+                },
+                plain: { description: "No configured model", mode: "primary" },
+              },
+            }),
+          ),
+        )
+        const server = yield* ServerProcess.start<never, never>({
+          hostname: "127.0.0.1",
+          port: 0,
+          password: "secret",
+          app: { version: "test-version" },
+          database: { path: ":memory:" },
+          config: { directory: tmp.path },
+          fs: { filewatcher: false },
+        })
+        const base = HttpServer.formatAddress(server.address)
+        const created = yield* request(base, "/api/session", {
+          agent: "modelprobe",
+          location: { directory: tmp.path },
+        })
+        if (!isRecord(created) || !isRecord(created["data"])) throw new Error("Expected a created session")
+        expect(created["data"]["model"]).toBeUndefined()
+        const sessionID = created["data"]["id"]
+        if (typeof sessionID !== "string") throw new Error("Expected a Session ID")
+
+        expect(
+          yield* request(base, `/api/session/${sessionID}/selection`, {
+            agent: "modelprobe",
+            model: { type: "configured" },
+          }),
+        ).toBeUndefined()
+        const configured = yield* get(base, `/api/session/${sessionID}`)
+        if (!isRecord(configured) || !isRecord(configured["data"])) throw new Error("Expected a Session")
+        expect(configured["data"]).toMatchObject({
+          agent: "modelprobe",
+          model: {
+            providerID: "opencode",
+            id: "nemotron-3.5-lightning-free",
+            variant: "default",
+          },
+        })
+
+        expect(
+          yield* request(base, `/api/session/${sessionID}/selection`, {
+            agent: "plain",
+            model: { type: "configured" },
+          }),
+        ).toBeUndefined()
+        const preserved = yield* get(base, `/api/session/${sessionID}`)
+        if (!isRecord(preserved) || !isRecord(preserved["data"])) throw new Error("Expected a Session")
+        expect(preserved["data"]).toMatchObject({ agent: "plain", model: configured["data"]["model"] })
+
+        expect(
+          yield* request(base, `/api/session/${sessionID}/selection`, {
+            agent: "missing",
+            model: { type: "configured" },
+          }),
+        ).toBeUndefined()
+        const missing = yield* get(base, `/api/session/${sessionID}`)
+        if (!isRecord(missing) || !isRecord(missing["data"])) throw new Error("Expected a Session")
+        expect(missing["data"]).toMatchObject({ agent: "missing", model: configured["data"]["model"] })
+      }),
+    (tmp) => Effect.promise(() => tmp[Symbol.asyncDispose]()),
+  ),
+)
+
+function request(base: string, pathname: string, body: unknown) {
+  return Effect.promise(() =>
+    fetch(new URL(pathname, base), {
+      method: "POST",
+      headers: {
+        authorization: `Basic ${btoa("opencode:secret")}`,
+        "content-type": "application/json",
+      },
+      body: JSON.stringify(body),
+    }).then(async (response) => {
+      expect(response.status).toBe(pathname === "/api/session" ? 200 : 204)
+      return response.status === 204 ? undefined : response.json()
+    }),
+  )
+}
+
+function get(base: string, pathname: string) {
+  return Effect.promise(() =>
+    fetch(new URL(pathname, base), {
+      headers: { authorization: `Basic ${btoa("opencode:secret")}` },
+    }).then((response) => {
+      expect(response.status).toBe(200)
+      return response.json()
+    }),
+  )
+}
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null && !Array.isArray(value)
+}