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

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) =>