diff --git a/CHANGELOG.md b/CHANGELOG.md
index 82ed6a04d1..1f70a9eb8f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,10 @@
 
 _In development — bullets added per PR; finalized at release._
 
+### 🔧 Bug Fixes
+
+- **command-code:** omit `max_tokens` when the client omits it so the upstream applies the model's native default, fixing `400 "expected <=200000"` on `/alpha/generate` for high-cap models; an explicit oversized client value is clamped to the 200k endpoint ceiling (#5221 — thanks @adivekar-utexas)
+
 ---
 
 ## [3.8.39] — 2026-06-28
diff --git a/open-sse/executors/commandCode.ts b/open-sse/executors/commandCode.ts
index 76fd7af3b8..a18100780c 100644
--- a/open-sse/executors/commandCode.ts
+++ b/open-sse/executors/commandCode.ts
@@ -6,6 +6,12 @@ import { BaseExecutor, mergeUpstreamExtraHeaders, type ExecuteInput } from "./ba
 type JsonRecord = Record<string, unknown>;
 
 export const COMMAND_CODE_VERSION = process.env.COMMAND_CODE_VERSION?.trim() || "0.33.2";
+// Hard server-side ceiling enforced by Command Code's /alpha/generate endpoint:
+// any request with params.max_tokens > 200_000 is rejected with a 400
+// "Too big: expected number to be <=200000 at params.max_tokens". We only use
+// this to clamp a CLIENT-SUPPLIED max_tokens down to a value the endpoint will
+// accept; we never fabricate this number for requests that omit the field (see
+// clampMaxTokens / buildCommandCodeBody).
 const MAX_COMMAND_CODE_TOKENS = 200_000;
 const encoder = new TextEncoder();
 
@@ -140,23 +146,16 @@ function convertMessages(messages: unknown): { system: string; messages: unknown
   return { system: system.join("\n\n"), messages: out };
 }
 
-function clampMaxTokens(value: unknown, cap: number = MAX_COMMAND_CODE_TOKENS): number {
-  const numeric = numberValue(value) ?? cap;
-  return Math.max(1, Math.min(Math.floor(numeric), cap));
-}
-
-// Resolve the per-model max_tokens cap for a given CommandCode model id.
-// Falls back to MAX_COMMAND_CODE_TOKENS when the model isn't registered or
-// when the registry entry omits maxOutputTokens. Without this, GLM-5.x
-// requests get capped at 200_000 and rejected with "限制数值范围[1,131072]".
-function getModelMaxTokensCap(modelId: string): number {
-  const entry = REGISTRY["command-code"];
-  if (!entry) return MAX_COMMAND_CODE_TOKENS;
-  const model = entry.models?.find((m: { id: string }) => m.id === modelId);
-  const registryCap = (model as { maxOutputTokens?: number } | undefined)?.maxOutputTokens;
-  return typeof registryCap === "number" && registryCap > 0
-    ? registryCap
-    : MAX_COMMAND_CODE_TOKENS;
+// Clamp a client-supplied max_tokens to the endpoint ceiling, mirroring the
+// provider-driven clamp in antigravity.ts: we only intervene when the value is
+// present AND would otherwise be rejected (> 200_000). A valid value is
+// returned floored; anything absent or non-numeric returns undefined so the
+// caller can OMIT the field entirely and let Command Code's upstream apply the
+// model's own native default (rather than us inventing a number).
+function clampMaxTokens(value: unknown): number | undefined {
+  const numeric = numberValue(value);
+  if (numeric === undefined) return undefined;
+  return Math.max(1, Math.min(Math.floor(numeric), MAX_COMMAND_CODE_TOKENS));
 }
 
 // Reasoning/thinking fields that payload rules or clients may inject and that
@@ -188,13 +187,20 @@ function buildCommandCodeBody(model: string, body: unknown, stream = false): Jso
     messages: converted.messages,
     tools: convertTools(input.tools),
     system,
-    max_tokens: clampMaxTokens(
-      input.max_tokens ?? input.max_completion_tokens,
-      getModelMaxTokensCap(resolvedModel)
-    ),
     stream: true,
   };
 
+  // Only forward max_tokens when the client actually supplied one. Omitting it
+  // lets Command Code's upstream apply the model's own native default, so we
+  // never invent a value (the old behavior, which sent the wrong number and got
+  // DeepSeek V4 rejected with "Too big: expected number to be <=200000"). When
+  // present, it is clamped to the endpoint ceiling so an oversized client value
+  // degrades gracefully instead of 400ing.
+  const maxTokens = clampMaxTokens(input.max_tokens ?? input.max_completion_tokens);
+  if (maxTokens !== undefined) {
+    params.max_tokens = maxTokens;
+  }
+
   for (const field of COMMAND_CODE_PASSTHROUGH_FIELDS) {
     const value = input[field];
     if (value !== undefined && value !== null) {
diff --git a/tests/unit/command-code-executor.test.ts b/tests/unit/command-code-executor.test.ts
index a52f59b62b..35ad263257 100644
--- a/tests/unit/command-code-executor.test.ts
+++ b/tests/unit/command-code-executor.test.ts
@@ -294,40 +294,60 @@ test("Command Code executor surfaces upstream and streamed errors", async () =>
   }, /boom/);
 });
 
-test("Command Code executor caps max_tokens to the registered per-model limit (GLM-5.x)", async () => {
+test("Command Code executor omits max_tokens when the client does not supply one (GLM-5.x)", async () => {
   const calls: FetchCall[] = [];
   globalThis.fetch = async (url, init = {}) => {
     calls.push({ url: String(url), init, body: JSON.parse(String(init.body)) });
     return commandCodeStream([{ type: "text-delta", text: "ok" }, { type: "finish" }]);
   };
 
-  // GLM-5 and GLM-5.1 are registered with maxOutputTokens: 131072.
-  // Without per-model capping, the upstream rejects with
-  // "限制数值范围[1,131072]".
+  // No client max_tokens: we must NOT fabricate one. Omitting the field lets
+  // Command Code's upstream apply the model's own native default.
   await getExecutor("command-code").execute({
     model: "zai-org/GLM-5.1",
     stream: false,
     credentials: { apiKey: "cc_test_key" },
     body: { messages: [{ role: "user", content: "Hi" }] },
   });
-  assert.equal(calls[0].body.params.max_tokens, 131072);
+  assert.ok(!("max_tokens" in calls[0].body.params));
 });
 
-test("Command Code executor caps max_tokens to the registered per-model limit (DeepSeek v4)", async () => {
+test("Command Code executor omits max_tokens for DeepSeek v4 when the client does not supply one", async () => {
   const calls: FetchCall[] = [];
   globalThis.fetch = async (url, init = {}) => {
     calls.push({ url: String(url), init, body: JSON.parse(String(init.body)) });
     return commandCodeStream([{ type: "text-delta", text: "ok" }, { type: "finish" }]);
   };
 
-  // DeepSeek v4 pro is registered with maxOutputTokens: 384000.
+  // Regression: previously the executor invented max_tokens from the registry
+  // (384000), which /alpha/generate rejects with a 400
+  // "Too big: expected number to be <=200000". With no client value we now omit
+  // the field entirely, so the request succeeds and upstream picks the default.
   await getExecutor("command-code").execute({
     model: "deepseek/deepseek-v4-pro",
     stream: false,
     credentials: { apiKey: "cc_test_key" },
     body: { messages: [{ role: "user", content: "Hi" }] },
   });
-  assert.equal(calls[0].body.params.max_tokens, 384000);
+  assert.ok(!("max_tokens" in calls[0].body.params));
+});
+
+test("Command Code executor clamps an oversized client-supplied max_tokens to the endpoint ceiling", async () => {
+  const calls: FetchCall[] = [];
+  globalThis.fetch = async (url, init = {}) => {
+    calls.push({ url: String(url), init, body: JSON.parse(String(init.body)) });
+    return commandCodeStream([{ type: "text-delta", text: "ok" }, { type: "finish" }]);
+  };
+
+  // A client asking for more than the 200000 endpoint ceiling is clamped down
+  // (not 400'd), mirroring the provider-driven clamp in antigravity.ts.
+  await getExecutor("command-code").execute({
+    model: "deepseek/deepseek-v4-pro",
+    stream: false,
+    credentials: { apiKey: "cc_test_key" },
+    body: { messages: [{ role: "user", content: "Hi" }], max_tokens: 500000 },
+  });
+  assert.equal(calls[0].body.params.max_tokens, 200000);
 });
 
 test("Command Code executor honors a smaller client-provided max_tokens under the per-model cap", async () => {