fix(opencode): scope agent variant to model (#11410)

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

@@ -0,0 +1,60 @@
+import { describe, expect, test } from "bun:test"
+import { Instance } from "../../src/project/instance"
+import { Session } from "../../src/session"
+import { SessionPrompt } from "../../src/session/prompt"
+import { tmpdir } from "../fixture/fixture"
+
+describe("session.prompt agent variant", () => {
+  test("applies agent variant only when using agent model", async () => {
+    await using tmp = await tmpdir({
+      git: true,
+      config: {
+        agent: {
+          build: {
+            model: "openai/gpt-5.2",
+            variant: "xhigh",
+          },
+        },
+      },
+    })
+
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const session = await Session.create({})
+
+        const other = await SessionPrompt.prompt({
+          sessionID: session.id,
+          agent: "build",
+          model: { providerID: "opencode", modelID: "kimi-k2.5-free" },
+          noReply: true,
+          parts: [{ type: "text", text: "hello" }],
+        })
+        if (other.info.role !== "user") throw new Error("expected user message")
+        expect(other.info.variant).toBeUndefined()
+
+        const match = await SessionPrompt.prompt({
+          sessionID: session.id,
+          agent: "build",
+          noReply: true,
+          parts: [{ type: "text", text: "hello again" }],
+        })
+        if (match.info.role !== "user") throw new Error("expected user message")
+        expect(match.info.model).toEqual({ providerID: "openai", modelID: "gpt-5.2" })
+        expect(match.info.variant).toBe("xhigh")
+
+        const override = await SessionPrompt.prompt({
+          sessionID: session.id,
+          agent: "build",
+          noReply: true,
+          variant: "high",
+          parts: [{ type: "text", text: "hello third" }],
+        })
+        if (override.info.role !== "user") throw new Error("expected user message")
+        expect(override.info.variant).toBe("high")
+
+        await Session.remove(session.id)
+      },
+    })
+  })
+})