fix(app): model sticks to session

2 files changed, 116 insertions, 0 deletions

diff --git a/packages/app/src/pages/session/session-model-helpers.test.ts b/packages/app/src/pages/session/session-model-helpers.test.ts
new file mode 100644
index 000000000..ae8700ccd
--- /dev/null
+++ b/packages/app/src/pages/session/session-model-helpers.test.ts
@@ -0,0 +1,83 @@
+import { describe, expect, test } from "bun:test"
+import type { UserMessage } from "@opencode-ai/sdk/v2"
+import { syncSessionModel } from "./session-model-helpers"
+
+const message = (input?: Partial<Pick<UserMessage, "agent" | "model" | "variant">>) =>
+  ({
+    id: "msg",
+    sessionID: "session",
+    role: "user",
+    time: { created: 1 },
+    agent: input?.agent ?? "build",
+    model: input?.model ?? { providerID: "anthropic", modelID: "claude-sonnet-4" },
+    variant: input?.variant,
+  }) as UserMessage
+
+describe("syncSessionModel", () => {
+  test("restores the last message model and variant", () => {
+    const calls: unknown[] = []
+
+    syncSessionModel(
+      {
+        agent: {
+          set(value) {
+            calls.push(["agent", value])
+          },
+        },
+        model: {
+          set(value) {
+            calls.push(["model", value])
+          },
+          current() {
+            return { id: "claude-sonnet-4", provider: { id: "anthropic" } }
+          },
+          variant: {
+            set(value) {
+              calls.push(["variant", value])
+            },
+          },
+        },
+      },
+      message({ variant: "high" }),
+    )
+
+    expect(calls).toEqual([
+      ["agent", "build"],
+      ["model", { providerID: "anthropic", modelID: "claude-sonnet-4" }],
+      ["variant", "high"],
+    ])
+  })
+
+  test("skips variant when the model falls back", () => {
+    const calls: unknown[] = []
+
+    syncSessionModel(
+      {
+        agent: {
+          set(value) {
+            calls.push(["agent", value])
+          },
+        },
+        model: {
+          set(value) {
+            calls.push(["model", value])
+          },
+          current() {
+            return { id: "gpt-5", provider: { id: "openai" } }
+          },
+          variant: {
+            set(value) {
+              calls.push(["variant", value])
+            },
+          },
+        },
+      },
+      message({ variant: "high" }),
+    )
+
+    expect(calls).toEqual([
+      ["agent", "build"],
+      ["model", { providerID: "anthropic", modelID: "claude-sonnet-4" }],
+    ])
+  })
+})
diff --git a/packages/app/src/pages/session/session-model-helpers.ts b/packages/app/src/pages/session/session-model-helpers.ts
new file mode 100644
index 000000000..ab181df59
--- /dev/null
+++ b/packages/app/src/pages/session/session-model-helpers.ts
@@ -0,0 +1,33 @@
+import type { UserMessage } from "@opencode-ai/sdk/v2"
+import { batch } from "solid-js"
+
+type Local = {
+  agent: {
+    set(name: string | undefined): void
+  }
+  model: {
+    set(model: UserMessage["model"] | undefined): void
+    current():
+      | {
+          id: string
+          provider: { id: string }
+        }
+      | undefined
+    variant: {
+      set(value: string | undefined): void
+    }
+  }
+}
+
+export const syncSessionModel = (local: Local, msg: UserMessage) => {
+  batch(() => {
+    local.agent.set(msg.agent)
+    local.model.set(msg.model)
+  })
+
+  const model = local.model.current()
+  if (!model) return
+  if (model.provider.id !== msg.model.providerID) return
+  if (model.id !== msg.model.modelID) return
+  local.model.variant.set(msg.variant)
+}