fix(app): new session uses agent model/variant

packages/app/src/pages/session.tsx

@@ -36,7 +36,7 @@ import { createSessionComposerState, SessionComposerRegion } from "@/pages/sessi
 import { createOpenReviewFile, createSizing } from "@/pages/session/helpers"
 import { MessageTimeline } from "@/pages/session/message-timeline"
 import { type DiffStyle, SessionReviewTab, type SessionReviewTabProps } from "@/pages/session/review-tab"
-import { syncSessionModel } from "@/pages/session/session-model-helpers"
+import { resetSessionModel, syncSessionModel } from "@/pages/session/session-model-helpers"
 import { createScrollSpy } from "@/pages/session/scroll-spy"
 import { SessionMobileTabs } from "@/pages/session/session-mobile-tabs"
 import { SessionSidePanel } from "@/pages/session/session-side-panel"
@@ -424,6 +424,17 @@ export default function Page() {
     ),
   )
 
+  createEffect(
+    on(
+      () => params.id,
+      (id, prev) => {
+        if (id || !prev) return
+        resetSessionModel(local)
+      },
+      { defer: true },
+    ),
+  )
+
   const [store, setStore] = createStore({
     messageId: undefined as string | undefined,
     mobileTab: "session" as "session" | "changes",

packages/app/src/pages/session/session-model-helpers.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test"
 import type { UserMessage } from "@opencode-ai/sdk/v2"
-import { syncSessionModel } from "./session-model-helpers"
+import { resetSessionModel, syncSessionModel } from "./session-model-helpers"
 
 const message = (input?: Partial<Pick<UserMessage, "agent" | "model" | "variant">>) =>
   ({
@@ -20,6 +20,9 @@ describe("syncSessionModel", () => {
     syncSessionModel(
       {
         agent: {
+          current() {
+            return undefined
+          },
           set(value) {
             calls.push(["agent", value])
           },
@@ -54,6 +57,9 @@ describe("syncSessionModel", () => {
     syncSessionModel(
       {
         agent: {
+          current() {
+            return undefined
+          },
           set(value) {
             calls.push(["agent", value])
           },
@@ -81,3 +87,72 @@ describe("syncSessionModel", () => {
     ])
   })
 })
+
+describe("resetSessionModel", () => {
+  test("restores the current agent defaults", () => {
+    const calls: unknown[] = []
+
+    resetSessionModel({
+      agent: {
+        current() {
+          return {
+            model: { providerID: "anthropic", modelID: "claude-sonnet-4" },
+            variant: "high",
+          }
+        },
+        set() {},
+      },
+      model: {
+        set(value) {
+          calls.push(["model", value])
+        },
+        current() {
+          return undefined
+        },
+        variant: {
+          set(value) {
+            calls.push(["variant", value])
+          },
+        },
+      },
+    })
+
+    expect(calls).toEqual([
+      ["model", { providerID: "anthropic", modelID: "claude-sonnet-4" }],
+      ["variant", "high"],
+    ])
+  })
+
+  test("clears the variant when the agent has none", () => {
+    const calls: unknown[] = []
+
+    resetSessionModel({
+      agent: {
+        current() {
+          return {
+            model: { providerID: "anthropic", modelID: "claude-sonnet-4" },
+          }
+        },
+        set() {},
+      },
+      model: {
+        set(value) {
+          calls.push(["model", value])
+        },
+        current() {
+          return undefined
+        },
+        variant: {
+          set(value) {
+            calls.push(["variant", value])
+          },
+        },
+      },
+    })
+
+    expect(calls).toEqual([
+      ["model", { providerID: "anthropic", modelID: "claude-sonnet-4" }],
+      ["variant", undefined],
+    ])
+  })
+})

packages/app/src/pages/session/session-model-helpers.ts

@@ -3,6 +3,12 @@ import { batch } from "solid-js"
 
 type Local = {
   agent: {
+    current():
+      | {
+          model?: UserMessage["model"]
+          variant?: string
+        }
+      | undefined
     set(name: string | undefined): void
   }
   model: {
@@ -19,6 +25,15 @@ type Local = {
   }
 }
 
+export const resetSessionModel = (local: Local) => {
+  const agent = local.agent.current()
+  if (!agent) return
+  batch(() => {
+    local.model.set(agent.model)
+    local.model.variant.set(agent.variant)
+  })
+}
+
 export const syncSessionModel = (local: Local, msg: UserMessage) => {
   batch(() => {
     local.agent.set(msg.agent)