fix(cli): preserve forked session variants

.changeset/fork-session-variant.md

@@ -0,0 +1,5 @@
+---
+"@kilocode/cli": patch
+---
+
+Preserve the selected model reasoning variant when forking a session.

packages/opencode/src/session/prompt.ts

@@ -778,6 +778,7 @@ export const layer = Layer.effect(
           .get(),
       )
       const model = input.model ?? ag.model ?? (yield* currentModel(input.sessionID))
+      const stored = !input.model && !ag.model ? model : undefined
       const same = ag.model && model.providerID === ag.model.providerID && model.modelID === ag.model.modelID
       const full =
         !input.variant && ag.variant && same
@@ -785,7 +786,10 @@ export const layer = Layer.effect(
               .getModel(model.providerID, model.modelID)
               .pipe(Effect.catchIf(Provider.ModelNotFoundError.isInstance, () => Effect.succeed(undefined)))
           : undefined
-      const variant = input.variant ?? (ag.variant && full?.variants?.[ag.variant] ? ag.variant : undefined)
+      const variant =
+        input.variant ??
+        (stored && "variant" in stored && typeof stored.variant === "string" ? stored.variant : undefined) ??
+        (ag.variant && full?.variants?.[ag.variant] ? ag.variant : undefined)
 
       const info: MessageV2.User = {
         id: input.messageID ?? MessageID.ascending(),

packages/opencode/src/session/session.ts

@@ -784,6 +784,7 @@ export const layer: Layer.Layer<
         workspaceID: original.workspaceID,
         title,
         metadata: structuredClone(original.metadata),
+        model: original.model ? { ...original.model } : undefined, // kilocode_change - preserve model + variant from the source session
         sourceID: input.sessionID, // kilocode_change - forks preserve initialized confinement
         sandboxFallback, // kilocode_change - seed confinement from the source session's original directory
       })

packages/opencode/test/session/prompt.test.ts

@@ -2535,6 +2535,39 @@ it.instance(
 
 // Agent variant
 
+noLLMServer.instance(
+  "preserves the session variant through a model-less handoff",
+  () =>
+    Effect.gen(function* () {
+      const prompt = yield* SessionPrompt.Service
+      const sessions = yield* Session.Service
+      const source = yield* sessions.create({
+        model: {
+          id: ModelID.make("test-model"),
+          providerID: ProviderID.make("test"),
+          variant: "high",
+        },
+      })
+      const fork = yield* sessions.fork({ sessionID: source.id })
+      const handoff = yield* prompt.prompt({
+        sessionID: fork.id,
+        noReply: true,
+        parts: [{ type: "text", text: "fork handoff", synthetic: true }],
+      })
+      if (handoff.info.role !== "user") throw new Error("expected user message")
+
+      expect(handoff.info.model).toEqual({
+        providerID: ProviderID.make("test"),
+        modelID: ModelID.make("test-model"),
+        variant: "high",
+      })
+
+      const saved = yield* sessions.get(fork.id)
+      expect(saved.model?.variant).toBe("high")
+    }),
+  { config: cfg },
+)
+
 noLLMServer.instance(
   "applies agent variant only when using agent model",
   () =>

packages/opencode/test/session/session.test.ts

@@ -5,6 +5,7 @@ import { GlobalBus, type GlobalEvent } from "../../src/bus/global"
 import * as Log from "@opencode-ai/core/util/log"
 import { MessageV2 } from "../../src/session/message-v2"
 import { MessageID, PartID, type SessionID } from "../../src/session/schema"
+import { ModelID, ProviderID } from "@/provider/schema"
 import { CrossSpawnSpawner } from "@opencode-ai/core/cross-spawn-spawner"
 import { provideInstance, tmpdirScoped } from "../fixture/fixture"
 import { testEffect } from "../lib/effect"
@@ -215,4 +216,30 @@ describe("Session", () => {
       expect(saved.metadata).toBeUndefined()
     }),
   )
+
+  it.instance("fork preserves model and variant", () =>
+    Effect.gen(function* () {
+      const session = yield* SessionNs.Service
+      const model = {
+        id: ModelID.make("test-model"),
+        providerID: ProviderID.make("test-provider"),
+        variant: "high",
+      }
+      const created = yield* Effect.acquireRelease(
+        session.create({ title: "with-model", model }),
+        (info) => session.remove(info.id).pipe(Effect.ignore),
+      )
+      const saved = yield* session.get(created.id)
+      expect(saved.model).toEqual(model)
+
+      const fork = yield* Effect.acquireRelease(session.fork({ sessionID: created.id }), (info) =>
+        session.remove(info.id).pipe(Effect.ignore),
+      )
+      const forked = yield* session.get(fork.id)
+
+      expect(forked.model).toEqual(model)
+      expect(forked.model?.variant).toBe("high")
+      expect(forked.model).not.toBe(saved.model)
+    }),
+  )
 })