fix(opencode): preserve Cerebras completion limit (#43736)

Co-authored-by: Aiden <rekram1-node@users.noreply.github.com>
Co-authored-by: ryanl-cerebras <230249923+ryanl-cerebras@users.noreply.github.com>

packages/opencode/test/plugin/cerebras.test.ts

@@ -0,0 +1,47 @@
+import { describe, expect, test } from "bun:test"
+import type { Hooks, PluginInput } from "@opencode-ai/plugin"
+import { CerebrasPlugin } from "../../src/plugin/cerebras"
+
+type ChatParams = NonNullable<Hooks["chat.params"]>
+
+function input(npm: string) {
+  return {
+    model: { api: { npm } },
+  } as Parameters<ChatParams>[0]
+}
+
+function output(options: Record<string, unknown>) {
+  return {
+    maxOutputTokens: 32_000,
+    options,
+  } as Parameters<ChatParams>[1]
+}
+
+describe("CerebrasPlugin", () => {
+  test("omits the generic output cap when max_completion_tokens is configured", async () => {
+    const hook = (await CerebrasPlugin({} as PluginInput))["chat.params"]!
+    const params = output({ max_completion_tokens: 64 })
+
+    await hook(input("@ai-sdk/cerebras"), params)
+
+    expect(params.maxOutputTokens).toBeUndefined()
+  })
+
+  test("preserves the generic output cap without max_completion_tokens", async () => {
+    const hook = (await CerebrasPlugin({} as PluginInput))["chat.params"]!
+    const params = output({})
+
+    await hook(input("@ai-sdk/cerebras"), params)
+
+    expect(params.maxOutputTokens).toBe(32_000)
+  })
+
+  test("does not change other providers", async () => {
+    const hook = (await CerebrasPlugin({} as PluginInput))["chat.params"]!
+    const params = output({ max_completion_tokens: 64 })
+
+    await hook(input("@ai-sdk/openai"), params)
+
+    expect(params.maxOutputTokens).toBe(32_000)
+  })
+})