fix(opencode): drop max_tokens for OpenAI reasoning models on Cloudflare AI Gateway (#22864)

Co-authored-by: Aiden Cline <[email protected]>

2 files changed, 79 insertions, 0 deletions

diff --git a/packages/opencode/src/plugin/cloudflare.ts b/packages/opencode/src/plugin/cloudflare.ts
index 2ccf5168d..c4bf6bb8e 100644
--- a/packages/opencode/src/plugin/cloudflare.ts
+++ b/packages/opencode/src/plugin/cloudflare.ts
@@ -61,5 +61,16 @@ export async function CloudflareAIGatewayAuthPlugin(_input: PluginInput): Promis
         },
       ],
     },
+    "chat.params": async (input, output) => {
+      if (input.model.providerID !== "cloudflare-ai-gateway") return
+      // The unified gateway routes through @ai-sdk/openai-compatible, which
+      // always emits max_tokens. OpenAI reasoning models (gpt-5.x, o-series)
+      // reject that field and require max_completion_tokens instead, and the
+      // compatible SDK has no way to rename it. Drop the cap so OpenAI falls
+      // back to the model's default output budget.
+      if (!input.model.api.id.toLowerCase().startsWith("openai/")) return
+      if (!input.model.capabilities.reasoning) return
+      output.maxOutputTokens = undefined
+    },
   }
 }
diff --git a/packages/opencode/test/plugin/cloudflare.test.ts b/packages/opencode/test/plugin/cloudflare.test.ts
new file mode 100644
index 000000000..5fa410683
--- /dev/null
+++ b/packages/opencode/test/plugin/cloudflare.test.ts
@@ -0,0 +1,68 @@
+import { expect, test } from "bun:test"
+import { CloudflareAIGatewayAuthPlugin } from "@/plugin/cloudflare"
+
+const pluginInput = {
+  client: {} as never,
+  project: {} as never,
+  directory: "",
+  worktree: "",
+  experimental_workspace: {
+    register() {},
+  },
+  serverUrl: new URL("https://example.com"),
+  $: {} as never,
+}
+
+function makeHookInput(overrides: { providerID?: string; apiId?: string; reasoning?: boolean }) {
+  return {
+    sessionID: "s",
+    agent: "a",
+    provider: {} as never,
+    message: {} as never,
+    model: {
+      providerID: overrides.providerID ?? "cloudflare-ai-gateway",
+      api: { id: overrides.apiId ?? "openai/gpt-5.2-codex", url: "", npm: "ai-gateway-provider" },
+      capabilities: {
+        reasoning: overrides.reasoning ?? true,
+        temperature: false,
+        attachment: true,
+        toolcall: true,
+        input: { text: true, audio: false, image: false, video: false, pdf: false },
+        output: { text: true, audio: false, image: false, video: false, pdf: false },
+        interleaved: false,
+      },
+    } as never,
+  }
+}
+
+function makeHookOutput() {
+  return { temperature: 0, topP: 1, topK: 0, maxOutputTokens: 32_000 as number | undefined, options: {} }
+}
+
+test("omits maxOutputTokens for openai reasoning models on cloudflare-ai-gateway", async () => {
+  const hooks = await CloudflareAIGatewayAuthPlugin(pluginInput)
+  const out = makeHookOutput()
+  await hooks["chat.params"]!(makeHookInput({ apiId: "openai/gpt-5.2-codex", reasoning: true }), out)
+  expect(out.maxOutputTokens).toBeUndefined()
+})
+
+test("keeps maxOutputTokens for openai non-reasoning models", async () => {
+  const hooks = await CloudflareAIGatewayAuthPlugin(pluginInput)
+  const out = makeHookOutput()
+  await hooks["chat.params"]!(makeHookInput({ apiId: "openai/gpt-4-turbo", reasoning: false }), out)
+  expect(out.maxOutputTokens).toBe(32_000)
+})
+
+test("keeps maxOutputTokens for non-openai reasoning models on cloudflare-ai-gateway", async () => {
+  const hooks = await CloudflareAIGatewayAuthPlugin(pluginInput)
+  const out = makeHookOutput()
+  await hooks["chat.params"]!(makeHookInput({ apiId: "anthropic/claude-sonnet-4-5", reasoning: true }), out)
+  expect(out.maxOutputTokens).toBe(32_000)
+})
+
+test("ignores non-cloudflare-ai-gateway providers", async () => {
+  const hooks = await CloudflareAIGatewayAuthPlugin(pluginInput)
+  const out = makeHookOutput()
+  await hooks["chat.params"]!(makeHookInput({ providerID: "openai", apiId: "gpt-5.2-codex", reasoning: true }), out)
+  expect(out.maxOutputTokens).toBe(32_000)
+})