Send max_completion_tokens to OpenAI reasoning models

First-party OpenAI reasoning models reject max_tokens, so the preset must send max_completion_tokens for those models. The requirement is declared per model on the catalog entry rather than inferred from name prefixes, and the quirk follows the first-party endpoint so relays serving the same model names keep max_tokens.

Author: TheGreatAxios

packages/first-class-providers/src/index.ts

@@ -1,5 +1,7 @@
 export {
   FIRST_CLASS_PROVIDERS,
+  OPENAI_API_BASE_URL,
+  OPENAI_API_MAX_COMPLETION_TOKENS_MODELS,
   connectListProviders,
   firstClassPathAsProvider,
   firstClassProviderById,

packages/first-class-providers/src/providers.test.ts

@@ -56,6 +56,22 @@ describe("FIRST_CLASS_PROVIDERS", () => {
     expect(api?.models).toContain(api?.defaultModel);
   });
 
+  test("OpenAI API path declares max_completion_tokens models explicitly", () => {
+    const openai = firstClassProviderById("openai");
+    const api = openai?.paths?.find((p) => p.id === "api");
+    expect(api?.maxCompletionTokensModels).toEqual([
+      "gpt-6-astra",
+      "gpt-5.4",
+      "gpt-5.4-mini",
+      "o3",
+      "o4-mini",
+    ]);
+    for (const model of api?.maxCompletionTokensModels ?? []) {
+      expect(api?.models).toContain(model);
+    }
+    expect(api?.maxCompletionTokensModels).not.toContain("gpt-4.1");
+  });
+
   test("OpenAI API and Zen catalogs include gpt-6-astra without changing defaults", () => {
     const openai = firstClassProviderById("openai");
     const api = openai?.paths?.find((p) => p.id === "api");

packages/first-class-providers/src/providers.ts

@@ -17,6 +17,23 @@ const OPENAI_API_MODELS = [
 ] as const;
 const OPENAI_API_DEFAULT = "gpt-5.4";
 
+/** First-party OpenAI chat-completions endpoint for the API-key path below. */
+export const OPENAI_API_BASE_URL = "https://api.openai.com/v1";
+
+/**
+ * Preset models whose first-party endpoint rejects `max_tokens` and requires
+ * `max_completion_tokens`. Explicit per-model list: adding a model here
+ * declares its own requirement, never inferred from name prefixes. gpt-4.1
+ * is non-reasoning and stays on `max_tokens`.
+ */
+export const OPENAI_API_MAX_COMPLETION_TOKENS_MODELS: readonly string[] = [
+  "gpt-6-astra",
+  "gpt-5.4",
+  "gpt-5.4-mini",
+  "o3",
+  "o4-mini",
+];
+
 /**
  * First-class providers shown in the models-surface Connect list.
  * Tier A order: dual-path OpenAI, OAuth xAI, Go/Zen, Z.AI, big three, Custom.
@@ -38,11 +55,12 @@ export const FIRST_CLASS_PROVIDERS: readonly FirstClassProviderDef[] = [
         id: "api",
         label: "OpenAI API — API key",
         auth: "api-key",
-        baseURL: "https://api.openai.com/v1",
+        baseURL: OPENAI_API_BASE_URL,
         models: OPENAI_API_MODELS,
         defaultModel: OPENAI_API_DEFAULT,
         authHint: "Paste your OpenAI API key (sk-...)",
         providerId: "openai",
+        maxCompletionTokensModels: OPENAI_API_MAX_COMPLETION_TOKENS_MODELS,
       },
     ],
   },
@@ -167,6 +185,9 @@ export function firstClassPathAsProvider(
       ? { defaultModel: path.defaultModel }
       : {}),
     ...(path.authHint !== undefined ? { authHint: path.authHint } : {}),
+    ...(path.maxCompletionTokensModels !== undefined
+      ? { maxCompletionTokensModels: path.maxCompletionTokensModels }
+      : {}),
     ...(def.anthropic === true ? { anthropic: true } : {}),
     ...(def.opencodeGo === true ? { opencodeGo: true } : {}),
     ...(def.billingProduct !== undefined

packages/first-class-providers/src/types.ts

@@ -28,6 +28,15 @@ export interface FirstClassProviderPath {
    * e.g. "codex" for ChatGPT OAuth, "openai" for API key.
    */
   providerId?: string;
+  /**
+   * Models on this path whose endpoint rejects `max_tokens` and requires
+   * `max_completion_tokens` instead (first-party OpenAI reasoning models).
+   * An explicit per-model list: adding a model here declares its own
+   * requirement, never inferred from name prefixes. Relays serving the same
+   * model names through other endpoints are unaffected — the quirk follows
+   * this endpoint, not the bare model name.
+   */
+  maxCompletionTokensModels?: readonly string[];
 }
 
 export interface FirstClassProviderDef {
@@ -57,4 +66,9 @@ export interface FirstClassProviderDef {
    * api-key flow runs against that path's fields / providerId.
    */
   paths?: readonly FirstClassProviderPath[];
+  /**
+   * Carried from a chooser path by firstClassPathAsProvider when the seeded
+   * def originates from a path (see FirstClassProviderPath for semantics).
+   */
+  maxCompletionTokensModels?: readonly string[];
 }

src/config/index.ts

@@ -209,6 +209,7 @@ export function buildOpenAISource(fields: {
   apiKey?: string;
   model: string;
   reasoningEffort?: ReasoningEffort;
+  quirks?: Record<string, unknown>;
 }): InferenceSource {
   const overrides =
     fields.reasoningEffort !== undefined
@@ -226,6 +227,7 @@ export function buildOpenAISource(fields: {
         : KEYLESS_API_KEY,
     model: fields.model,
     defaults: { maxTokens: SOURCE_MAX_TOKENS, ...overrides },
+    ...(fields.quirks !== undefined ? { quirks: fields.quirks } : {}),
   };
 }
 

src/config/inference-sources.test.ts

@@ -12,6 +12,7 @@ import {
   setProviderContextWindowOverrides,
 } from "../provider/context-window.js";
 import { createOpenAICompatibleAdapter } from "../provider/openai-compatible-adapter.js";
+import { firstClassProviderById } from "../../packages/first-class-providers/src/index.js";
 
 const WINDOW = 400_000;
 
@@ -85,3 +86,73 @@ describe("contextWindow / maxTokens split (CL-7784)", () => {
     expect(contextWindowFor("fp:fp-large")).toBe(WINDOW);
   });
 });
+
+describe("OpenAI reasoning max_completion_tokens quirk (CL-7785)", () => {
+  const openaiApi = firstClassProviderById("openai")?.paths?.find(
+    (p) => p.id === "api",
+  );
+  const presetModels = [...(openaiApi?.models ?? [])];
+  const presetBaseURL = openaiApi?.baseURL ?? "";
+  // A relay serving the same model names through the same adapter but taking
+  // max_tokens (per the vendor adapter comment) — the quirk must not follow
+  // the bare model name there.
+  const RELAY_BASE_URL = "https://opencode.ai/zen/v1";
+
+  function wireBody(model: string, baseURL: string): Record<string, unknown> {
+    const entryCatalog: ProviderCatalogEntry[] = [
+      { name: "openai", baseURL, apiKey: "test-key", models: [model] },
+    ];
+    const source = buildInferenceSourceForRef(
+      { provider: "openai", model },
+      { sessionId: "sess-1", catalog: entryCatalog },
+      undefined,
+    );
+    // Mirror the harness: it resolves the adapter with source.quirks.
+    const adapter = createOpenAICompatibleAdapter(
+      source as unknown as Parameters<typeof createOpenAICompatibleAdapter>[0],
+      source?.quirks,
+    );
+    const messages = [
+      { role: "user", content: [{ type: "text", text: "hi" }] },
+    ] as unknown as ConversationTurn[];
+    const built = adapter.buildRequest(messages, model, {
+      maxTokens: source?.defaults?.maxTokens,
+    } as InferenceOptions);
+    return JSON.parse(built.body) as Record<string, unknown>;
+  }
+
+  test("shipped preset declares an explicit per-model requirement", () => {
+    expect(presetModels.length).toBeGreaterThan(0);
+    expect(openaiApi?.maxCompletionTokensModels?.length).toBeGreaterThan(0);
+    for (const model of openaiApi?.maxCompletionTokensModels ?? []) {
+      expect(presetModels).toContain(model);
+    }
+  });
+
+  test("reasoning preset models emit max_completion_tokens, never max_tokens", () => {
+    for (const model of openaiApi?.maxCompletionTokensModels ?? []) {
+      const body = wireBody(model, presetBaseURL);
+      expect(body["max_completion_tokens"]).toBe(SOURCE_MAX_TOKENS);
+      expect("max_tokens" in body).toBe(false);
+    }
+  });
+
+  test("non-reasoning preset models keep max_tokens", () => {
+    const declared = new Set(openaiApi?.maxCompletionTokensModels ?? []);
+    const rest = presetModels.filter((m) => !declared.has(m));
+    expect(rest.length).toBeGreaterThan(0);
+    for (const model of rest) {
+      const body = wireBody(model, presetBaseURL);
+      expect(body["max_tokens"]).toBe(SOURCE_MAX_TOKENS);
+      expect("max_completion_tokens" in body).toBe(false);
+    }
+  });
+
+  test("relay endpoint keeps max_tokens for every preset model", () => {
+    for (const model of presetModels) {
+      const body = wireBody(model, RELAY_BASE_URL);
+      expect(body["max_tokens"]).toBe(SOURCE_MAX_TOKENS);
+      expect("max_completion_tokens" in body).toBe(false);
+    }
+  });
+});

src/config/inference-sources.ts

@@ -9,7 +9,11 @@ import {
   buildXaiSource,
   type ProviderCatalogEntry,
 } from "./index.js";
-import type { Settings } from "./settings.js";
+import {
+  OPENAI_API_BASE_URL,
+  OPENAI_API_MAX_COMPLETION_TOKENS_MODELS,
+} from "../../packages/first-class-providers/src/index.js";
+import { normalizeOpenAICompatibleBaseURL, type Settings } from "./settings.js";
 import {
   resolveSessionEffort,
   type ReasoningEffort,
@@ -36,6 +40,26 @@ function catalogEntry(
   return catalog.find((e) => e.name === provider);
 }
 
+// First-party OpenAI reasoning models reject `max_tokens` and require
+// `max_completion_tokens`. The requirement is declared per model on the
+// first-class OpenAI API-key preset — never inferred from name prefixes —
+// and the quirk attaches to the source actually in use: it follows the
+// first-party endpoint, so relays serving the same model names through the
+// same adapter keep `max_tokens`.
+function openAISourceQuirks(
+  baseURL: string,
+  model: string,
+): Record<string, unknown> | undefined {
+  const normalized = normalizeOpenAICompatibleBaseURL(baseURL);
+  if (normalized !== normalizeOpenAICompatibleBaseURL(OPENAI_API_BASE_URL)) {
+    return undefined;
+  }
+  if (!OPENAI_API_MAX_COMPLETION_TOKENS_MODELS.includes(model)) {
+    return undefined;
+  }
+  return { maxTokensField: "max_completion_tokens" };
+}
+
 export function buildInferenceSourceForRef(
   ref: ProviderRef,
   ctx: BuildSourceContext,
@@ -124,6 +148,7 @@ export function buildInferenceSourceForRef(
     });
   }
 
+  const quirks = openAISourceQuirks(baseURL, ref.model);
   return buildOpenAISource({
     id: ref.provider,
     baseURL,
@@ -134,6 +159,7 @@ export function buildInferenceSourceForRef(
         : {}),
     model: ref.model,
     ...(effort !== undefined ? { reasoningEffort: effort } : {}),
+    ...(quirks !== undefined ? { quirks } : {}),
   });
 }