fix(tui): distinguish variant switch notices (#35315)

AGENTS.md

@@ -151,6 +151,7 @@ const table = sqliteTable("session", {
 
 ## V2 Session Core
 
+- Keep durable events minimal: record irreducible new facts and do not repeat state derivable by folding the ordered aggregate history. Enrich projections and read models with previous or derived state when consumers need self-contained views.
 - Keep durable prompt admission separate from model execution. `SessionV2.prompt(...)` admits one durable `session_input` row before scheduling advisory `SessionExecution.wake(sessionID)` unless `resume: false` requests admit-only behavior. The serialized runner promotes admitted inputs into visible user messages at safe boundaries.
 - Reusing a Session ID adopts the existing Session. Reusing a prompt message ID reconciles an exact retry only when Session, prompt, and delivery mode match; conflicting reuse fails. Historical projected prompts lazily synthesize promoted inbox records during exact retry.
 - Keep `SessionExecution` process-global and Session-ID based. Its local implementation owns the process-local Session coordinator and discovers placement through `SessionStore` plus `LocationServiceMap.get(session.location)` only when a drain starts; no layer should take a Session ID. V2 interruption targets the active process-local ownership chain for that Session; idle or missing interruption is a no-op.

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

@@ -928,6 +928,7 @@ export type SessionContextOutput = {
         readonly time: { readonly created: number }
         readonly type: "model-switched"
         readonly model: { readonly id: string; readonly providerID: string; readonly variant?: string }
+        readonly previous?: { readonly id: string; readonly providerID: string; readonly variant?: string }
       }
     | {
         readonly id: string
@@ -1621,6 +1622,7 @@ export type SessionMessageOutput = {
         readonly time: { readonly created: number }
         readonly type: "model-switched"
         readonly model: { readonly id: string; readonly providerID: string; readonly variant?: string }
+        readonly previous?: { readonly id: string; readonly providerID: string; readonly variant?: string }
       }
     | {
         readonly id: string
@@ -1820,6 +1822,7 @@ export type MessageListOutput = {
         readonly time: { readonly created: number }
         readonly type: "model-switched"
         readonly model: { readonly id: string; readonly providerID: string; readonly variant?: string }
+        readonly previous?: { readonly id: string; readonly providerID: string; readonly variant?: string }
       }
     | {
         readonly id: string

packages/core/src/session.ts

@@ -540,7 +540,8 @@ const layer = Layer.effect(
             const started = yield* Effect.gen(function* () {
               const shell = yield* Shell.Service
               return yield* shell.create({ command: input.command, cwd: session.location.directory })
-            }).pipe(Effect.provide(locations.get(session.location)))
+            })
+              .pipe(Effect.provide(locations.get(session.location)))
             yield* events.publish(
               SessionEvent.Shell.Started,
               {
@@ -563,8 +564,7 @@ const layer = Layer.effect(
                     .pipe(Effect.catchTag("Shell.NotFoundError", () => Effect.succeed(missingShellOutput())))
                 : missingShellOutput()
               return { shell: terminal.info, output }
-            })
-              .pipe(Effect.provide(locations.get(session.location)))
+            }).pipe(Effect.provide(locations.get(session.location)))
             yield* events.publish(SessionEvent.Shell.Ended, {
               sessionID: input.sessionID,
               shell: completed.shell,

packages/core/src/session/message-updater.ts

@@ -8,6 +8,7 @@ export type MemoryState = {
 }
 
 export interface Adapter {
+  readonly getModel: () => Effect.Effect<SessionMessage.ModelSelected["model"] | undefined, never, never>
   readonly getCurrentAssistant: () => Effect.Effect<SessionMessage.Assistant | undefined, never, never>
   readonly getAssistant: (
     messageID: SessionMessage.ID,
@@ -29,6 +30,15 @@ export function memory(state: MemoryState): Adapter {
   const latestAssistantIndex = () => state.messages.findLastIndex((message) => message.type === "assistant")
 
   return {
+    getModel() {
+      return Effect.sync(
+        () =>
+          state.messages.findLast(
+            (message): message is SessionMessage.ModelSelected | SessionMessage.Assistant =>
+              message.type === "model-switched" || message.type === "assistant",
+          )?.model,
+      )
+    },
     getCurrentAssistant() {
       return Effect.sync(() => {
         const index = latestAssistantIndex()
@@ -115,15 +125,19 @@ export function update(adapter: Adapter, event: SessionEvent.Event) {
         )
       },
       "session.model.selected": (event) => {
-        return adapter.appendMessage(
-          SessionMessage.ModelSelected.make({
-            id: SessionMessage.ID.fromEvent(event.id),
-            type: "model-switched",
-            metadata: event.metadata,
-            model: event.data.model,
-            time: { created: event.created },
-          }),
-        )
+        return Effect.gen(function* () {
+          const previous = yield* adapter.getModel()
+          yield* adapter.appendMessage(
+            SessionMessage.ModelSelected.make({
+              id: SessionMessage.ID.fromEvent(event.id),
+              type: "model-switched",
+              metadata: event.metadata,
+              model: event.data.model,
+              previous,
+              time: { created: event.created },
+            }),
+          )
+        })
       },
       "session.moved": () => Effect.void,
       "session.renamed": () => Effect.void,

packages/core/src/session/projector.ts

@@ -5,6 +5,7 @@ import { DateTime, Effect, Layer, Schema } from "effect"
 import { Database } from "../database/database"
 import { EventV2 } from "../event"
 import { makeGlobalNode } from "../effect/app-node"
+import { ModelV2 } from "../model"
 import { SessionEvent } from "./event"
 import { SessionV1 } from "../v1/session"
 import { WorkspaceTable } from "../control-plane/workspace.sql"
@@ -356,6 +357,17 @@ function run(db: DatabaseService, event: MessageEvent) {
     }
     const appendMessage = (message: SessionMessage.Message) => insertMessage(db, event, message)
     const adapter: SessionMessageUpdater.Adapter = {
+      getModel() {
+        return db
+          .select({ model: SessionTable.model })
+          .from(SessionTable)
+          .where(eq(SessionTable.id, event.data.sessionID))
+          .get()
+          .pipe(
+            Effect.orDie,
+            Effect.map((row) => (row?.model ? Schema.decodeUnknownSync(ModelV2.Ref)(row.model) : undefined)),
+          )
+      },
       getCurrentAssistant() {
         return Effect.gen(function* () {
           // A newer step supersedes stale incomplete rows; never resume an older assistant projection.
@@ -570,13 +582,13 @@ const layer = Layer.effectDiscard(
     )
     yield* events.project(SessionEvent.ModelSelected, (event) =>
       Effect.gen(function* () {
+        yield* run(db, event)
         yield* db
           .update(SessionTable)
           .set({ model: event.data.model, time_updated: DateTime.toEpochMillis(event.created) })
           .where(eq(SessionTable.id, event.data.sessionID))
           .run()
           .pipe(Effect.orDie)
-        yield* run(db, event)
       }),
     )
     yield* events.project(SessionEvent.Renamed, (event) =>

packages/core/test/session-create.test.ts

@@ -562,9 +562,9 @@ describe("SessionV2.create", () => {
       yield* session.switchModel({ sessionID: created.id, model })
 
       expect(yield* session.get(created.id)).toMatchObject({ model })
-      expect(
-        Array.from(yield* logEvents(session, created.id, true).pipe(Stream.take(1), Stream.runCollect)),
-      ).toMatchObject([{ type: "session.model.selected", data: { model } }])
+      const events = Array.from(yield* logEvents(session, created.id, true).pipe(Stream.take(1), Stream.runCollect))
+      expect(events).toMatchObject([{ type: "session.model.selected" }])
+      expect(events[0]?.data).toEqual({ sessionID: created.id, model })
     }),
   )
 

packages/core/test/session-projector.test.ts

@@ -34,6 +34,7 @@ const sessionsLayer = AppNodeBuilder.build(SessionV2.node, [[SessionExecution.no
 const sessionID = SessionV2.ID.make("ses_projector_test")
 const created = DateTime.makeUnsafe(0)
 const model = { id: ModelV2.ID.make("model"), providerID: ProviderV2.ID.make("provider") }
+const previousModel = { ...model, variant: ModelV2.VariantID.make("medium") }
 const encodeMessage = Schema.encodeSync(SessionMessage.Message)
 
 const assistantRow = (
@@ -238,6 +239,7 @@ describe("SessionProjector", () => {
           directory: "/project",
           title: "test",
           version: "test",
+          model: previousModel,
         })
         .run()
         .pipe(Effect.orDie)
@@ -337,6 +339,7 @@ describe("SessionProjector", () => {
         text: "synthetic context",
         metadata: { source: "projector-test" },
       })
+      expect(messages.find((message) => message.type === "model-switched")).toMatchObject({ previous: previousModel })
       expect(messages.find((message) => message.type === "shell")).toMatchObject({
         shell: { command: "pwd", status: "exited", exit: 0 },
         output: { output: "/project", truncated: false },

packages/schema/src/session-message.ts

@@ -44,6 +44,7 @@ export const ModelSelected = Schema.Struct({
   ...Base,
   type: Schema.Literal("model-switched"),
   model: Model.Ref,
+  previous: Model.Ref.pipe(optional),
 }).annotate({ identifier: "Session.Message.ModelSelected" })
 
 export interface User extends Schema.Schema.Type<typeof User> {}

packages/sdk/js/src/v2/gen/types.gen.ts

@@ -4310,6 +4310,7 @@ export type SessionMessageModelSelected = {
   }
   type: "model-switched"
   model: ModelRef
+  previous?: ModelRef
 }
 
 export type SessionMessageUser = {
@@ -7985,6 +7986,7 @@ export type SessionMessageModelSelected2 = {
   }
   type: "model-switched"
   model: ModelRef2
+  previous?: ModelRef2
 }
 
 export type SessionMessageUser2 = {

packages/tui/src/context/data.tsx

@@ -235,6 +235,7 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
         case "session.model.selected":
           if (store.session.info[event.data.sessionID])
             setStore("session", "info", event.data.sessionID, "model", event.data.model)
+          if (!store.session.message[event.data.sessionID]) break
           message.update(event.data.sessionID, (draft, index) => {
             message.append(draft, index, {
               id: messageIDFromEvent(event.id),
@@ -243,6 +244,16 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
               time: { created: event.created },
             })
           })
+          void sdk.api.session
+            .message({ sessionID: event.data.sessionID, messageID: messageIDFromEvent(event.id) })
+            .then((item) => {
+              message.update(event.data.sessionID, (draft, index) => {
+                const position = index.get(item.id)
+                if (position === undefined) return message.append(draft, index, mutable(item))
+                draft[position] = mutable(item)
+              })
+            })
+            .catch((error) => console.error("Failed to load projected model switch message", error))
           break
         case "session.renamed":
           if (store.session.info[event.data.sessionID])

packages/tui/src/routes/session/index.tsx

@@ -1231,7 +1231,8 @@ function SessionSwitchMessageV2(props: { message: SessionMessage }) {
   const { theme } = useTheme()
   const text = () => {
     if (props.message.type === "agent-switched") return `Switched agent to ${props.message.agent}`
-    if (props.message.type === "model-switched") return switchLabel(props.message.model, ctx.models())
+    if (props.message.type === "model-switched")
+      return switchLabel(props.message.model, ctx.models(), props.message.previous)
     return ""
   }
   return <text fg={theme.textMuted}>{text()}</text>

packages/tui/src/util/model.ts

@@ -34,11 +34,12 @@ export function formatRef(model: { providerID: string; id: string; variant?: str
 export function switchLabel(
   model: { providerID: string; id: string; variant?: string },
   models?: readonly { providerID: string; id: string; name: string }[],
+  previous?: { providerID: string; id: string; variant?: string },
 ) {
+  if (previous?.providerID === model.providerID && previous.id === model.id)
+    return `Switched variant to ${model.variant ?? "default"}`
   const display = models?.find((item) => item.providerID === model.providerID && item.id === model.id)?.name
   if (display === undefined) return `Switched model to ${formatRef(model)}`
-  // Variant-only switches publish the same model id; without the variant the
-  // notice would look like a redundant model switch.
   const variant = model.variant && model.variant !== "default" ? ` (${model.variant})` : ""
   return `Switched model to ${display}${variant}`
 }

packages/tui/test/cli/tui/data.test.tsx

@@ -767,7 +767,18 @@ test("adds and dismisses question requests from live events", async () => {
 
 test("settles pending tools when a live failure arrives", async () => {
   const events = createEventStream()
-  const calls = createFetch(undefined, events)
+  const calls = createFetch((url) => {
+    if (url.pathname === "/api/session/session-1/message/msg_model_1")
+      return json({
+        data: {
+          id: "msg_model_1",
+          type: "model-switched",
+          previous: { id: "model-1", providerID: "provider-1", variant: "medium" },
+          model: { id: "model-1", providerID: "provider-1", variant: "high" },
+          time: { created: 0 },
+        },
+      })
+  }, events)
   let sync!: ReturnType<typeof useData>
   let ready!: () => void
   const mounted = new Promise<void>((resolve) => {
@@ -808,7 +819,7 @@ test("settles pending tools when a live failure arrives", async () => {
       durable: durable("session-1", 1),
       data: {
         sessionID: "session-1",
-        model: { id: "model-1", providerID: "provider-1" },
+        model: { id: "model-1", providerID: "provider-1", variant: "high" },
       },
     })
     emitEvent(events, {
@@ -895,6 +906,11 @@ test("settles pending tools when a live failure arrives", async () => {
       "model-switched",
       "assistant",
     ])
+    expect(sync.session.message.get("session-1", "msg_model_1")).toMatchObject({
+      type: "model-switched",
+      previous: { id: "model-1", providerID: "provider-1", variant: "medium" },
+      model: { id: "model-1", providerID: "provider-1", variant: "high" },
+    })
   } finally {
     app.renderer.destroy()
   }

packages/tui/test/util/model.test.ts

@@ -34,4 +34,16 @@ describe("util.model", () => {
       "Switched model to removed/gone/high",
     )
   })
+
+  test("distinguishes variant-only switches from model switches", () => {
+    const previous = { providerID: "openai", id: "gpt-5.5", variant: "medium" }
+
+    expect(switchLabel({ ...previous, variant: "high" }, undefined, previous)).toBe("Switched variant to high")
+    expect(switchLabel({ providerID: "openai", id: "gpt-5.5" }, undefined, previous)).toBe(
+      "Switched variant to default",
+    )
+    expect(switchLabel({ providerID: "anthropic", id: "sonnet", variant: "high" }, undefined, previous)).toBe(
+      "Switched model to anthropic/sonnet/high",
+    )
+  })
 })