diff --git a/open-sse/config/providers/registry/grok-cli/index.ts b/open-sse/config/providers/registry/grok-cli/index.ts
index 463e103ba6..85a6a05471 100644
--- a/open-sse/config/providers/registry/grok-cli/index.ts
+++ b/open-sse/config/providers/registry/grok-cli/index.ts
@@ -15,12 +15,18 @@ export const grok_cliProvider: RegistryEntry = {
       id: "grok-build",
       name: "Grok Build",
       contextLength: 256000,
+      // cli-chat-proxy rejects reasoning_effort/reasoning outright (see grok-cli.ts
+      // executor's transformRequest, which strips them unconditionally for this model).
+      supportsReasoning: false,
       unsupportedParams: ["presencePenalty", "frequencyPenalty", "logprobs", "topLogprobs"],
     },
     {
       id: "grok-composer-2.5-fast",
       name: "Grok Composer 2.5 Fast",
       contextLength: 200000,
+      // cli-chat-proxy rejects reasoning_effort/reasoning outright (see grok-cli.ts
+      // executor's transformRequest, which strips them unconditionally for this model).
+      supportsReasoning: false,
       unsupportedParams: ["presencePenalty", "frequencyPenalty", "logprobs", "topLogprobs"],
     },
   ],
diff --git a/open-sse/executors/grok-cli.ts b/open-sse/executors/grok-cli.ts
index ced0ac1465..44d0d6c43a 100644
--- a/open-sse/executors/grok-cli.ts
+++ b/open-sse/executors/grok-cli.ts
@@ -18,6 +18,8 @@ import https from "node:https";
 
 const GROK_TOKEN_URL = "https://auth.x.ai/oauth2/token";
 const REQUEST_TIMEOUT_MS = 60_000;
+// xAI cli-chat-proxy hard limit on tools per request.
+const MAX_TOOLS = 200;
 
 export class GrokCliExecutor extends BaseExecutor {
   constructor() {
@@ -257,6 +259,14 @@ export class GrokCliExecutor extends BaseExecutor {
       }
     }
 
+    // xAI's cli-chat-proxy enforces a maximum of 200 tools per request and
+    // 400s above that ceiling. Clients that fan a large MCP toolset through
+    // Grok Build/Composer (e.g. Claude Code with many registered tools) can
+    // exceed it — cap defensively rather than let the request fail upstream.
+    if (Array.isArray(transformed.tools) && transformed.tools.length > MAX_TOOLS) {
+      transformed.tools = transformed.tools.slice(0, MAX_TOOLS);
+    }
+
     return transformed;
   }
 }
diff --git a/tests/unit/grok-cli-strip-params.test.ts b/tests/unit/grok-cli-strip-params.test.ts
index 906e02ae42..46c97598f5 100644
--- a/tests/unit/grok-cli-strip-params.test.ts
+++ b/tests/unit/grok-cli-strip-params.test.ts
@@ -51,3 +51,40 @@ test("#5273 grok-cli transformRequest leaves a clean body unchanged (no false st
   assert.equal(out.model, "grok-composer-2.5-fast");
   assert.equal(out.stream, true);
 });
+
+// Ported from decolua/9router#2534 (@gitcommit90): xAI's cli-chat-proxy enforces a
+// hard cap of 200 tools per request and 400s above it. Clients that fan a large MCP
+// toolset through Grok Build/Composer can exceed that ceiling — transformRequest()
+// must cap defensively instead of forwarding an oversized array upstream.
+test("2534 grok-cli transformRequest caps tools at 200", () => {
+  const executor = new GrokCliExecutor();
+  const tools = Array.from({ length: 250 }, (_, i) => ({
+    type: "function",
+    function: { name: `tool_${i}` },
+  }));
+  const out = executor.transformRequest(
+    "grok-build",
+    { messages: [{ role: "user", content: "hi" }], tools },
+    false,
+    {} as never
+  ) as Record<string, unknown>;
+
+  assert.equal((out.tools as unknown[]).length, 200);
+  assert.deepEqual(out.tools, tools.slice(0, 200));
+});
+
+test("2534 grok-cli transformRequest leaves a tools array under the cap untouched", () => {
+  const executor = new GrokCliExecutor();
+  const tools = Array.from({ length: 10 }, (_, i) => ({
+    type: "function",
+    function: { name: `tool_${i}` },
+  }));
+  const out = executor.transformRequest(
+    "grok-composer-2.5-fast",
+    { messages: [{ role: "user", content: "hi" }], tools },
+    false,
+    {} as never
+  ) as Record<string, unknown>;
+
+  assert.deepEqual(out.tools, tools);
+});