diff --git a/open-sse/config/providers/registry/xai-oauth/index.ts b/open-sse/config/providers/registry/xai-oauth/index.ts
index 4ec030be28..cd644a707e 100644
--- a/open-sse/config/providers/registry/xai-oauth/index.ts
+++ b/open-sse/config/providers/registry/xai-oauth/index.ts
@@ -18,7 +18,16 @@ export const xai_oauthProvider: RegistryEntry = {
     tokenUrl: "https://auth.x.ai/oauth2/token",
   },
   models: [
-    { id: "grok-4.5", name: "Grok 4.5", contextLength: 500000 },
+    // SuperGrok / xAI OAuth serves grok-4.5 on native /v1/responses. Tag so
+    // chatCore translates OpenAI Chat Completions → Responses (messages→input,
+    // max_tokens→max_output_tokens). Without the tag, some 3.8.50 paths hit
+    // /v1/responses with a chat-shaped body → 422 missing `input` (#10165).
+    {
+      id: "grok-4.5",
+      name: "Grok 4.5",
+      contextLength: 500000,
+      targetFormat: "openai-responses",
+    },
     ...(xaiProvider.models || []),
   ],
 };
diff --git a/open-sse/executors/xai.ts b/open-sse/executors/xai.ts
index f9b92fa1f6..de8fcf7425 100644
--- a/open-sse/executors/xai.ts
+++ b/open-sse/executors/xai.ts
@@ -2,6 +2,7 @@ import { BaseExecutor, type ExecutorLog, type ProviderCredentials } from "./base
 import { PROVIDERS } from "../config/constants.ts";
 import { getModelTargetFormat } from "../config/providerModels.ts";
 import { isResponsesEndpointPath } from "../utils/responsesEndpoint.ts";
+import { chatRequestToXaiResponses } from "@/lib/providers/xai/translators/openai-chat.ts";
 
 type JsonRecord = Record<string, unknown>;
 
@@ -124,12 +125,38 @@ export class XaiExecutor extends BaseExecutor {
     const record = asRecord(cleaned);
     if (!record) return cleaned;
 
-    const out: JsonRecord = { ...record };
+    let out: JsonRecord = { ...record };
     const nativeXaiPassthrough = record._nativeXaiResponsesPassthrough === true;
     delete out._nativeXaiResponsesPassthrough;
     delete out._nativeCodexPassthrough;
 
-    if (nativeXaiPassthrough || getModelTargetFormat(this.provider, model) === "openai-responses") {
+    const useResponses =
+      nativeXaiPassthrough ||
+      getModelTargetFormat(this.provider, model) === "openai-responses" ||
+      isResponsesEndpointPath(credentials?.requestEndpointPath);
+
+    // #10165: chat/completions clients send messages + max_tokens; xAI /v1/responses
+    // requires input + max_output_tokens. Convert at the executor edge so a missed
+    // chatCore translation cannot ship a chat-shaped body to Responses.
+    if (useResponses) {
+      if (Array.isArray(out.messages) && out.input == null) {
+        out = chatRequestToXaiResponses(out as never) as unknown as JsonRecord;
+      } else {
+        if (out.max_completion_tokens != null && out.max_output_tokens == null) {
+          out.max_output_tokens = out.max_completion_tokens;
+          delete out.max_completion_tokens;
+        }
+        if (out.max_tokens != null && out.max_output_tokens == null) {
+          out.max_output_tokens = out.max_tokens;
+          delete out.max_tokens;
+        }
+        if (out.response_format != null && out.text == null) {
+          out.text = { format: out.response_format };
+          delete out.response_format;
+        }
+      }
+      // Keep model id from the routed request when the translator left it empty.
+      if (out.model == null && model) out.model = model;
       return out;
     }
 
diff --git a/open-sse/translator/index.ts b/open-sse/translator/index.ts
index 2db1da2545..a6d93cdee2 100644
--- a/open-sse/translator/index.ts
+++ b/open-sse/translator/index.ts
@@ -97,6 +97,31 @@ function normalizeOpenAIResponsesRequest(body) {
 
   const normalized = promoteStrayReasoningEffort({ ...body });
 
+  // #10165 safety net: if a chat-shaped body reached Responses normalization
+  // without input, promote messages → input and map token/format fields.
+  if (normalized.input == null && Array.isArray(normalized.messages)) {
+    normalized.input = normalized.messages;
+    delete normalized.messages;
+  }
+  if (normalized.max_output_tokens == null) {
+    if (normalized.max_completion_tokens != null) {
+      normalized.max_output_tokens = normalized.max_completion_tokens;
+      delete normalized.max_completion_tokens;
+    } else if (normalized.max_tokens != null) {
+      normalized.max_output_tokens = normalized.max_tokens;
+      delete normalized.max_tokens;
+    }
+  } else {
+    delete normalized.max_tokens;
+    delete normalized.max_completion_tokens;
+  }
+  if (normalized.response_format != null && normalized.text == null) {
+    normalized.text = { format: normalized.response_format };
+    delete normalized.response_format;
+  } else if (normalized.response_format != null) {
+    delete normalized.response_format;
+  }
+
   if (typeof normalized.input === "string") {
     normalized.input = [
       {
diff --git a/tests/unit/executor-xai-chat-to-responses-10165.test.ts b/tests/unit/executor-xai-chat-to-responses-10165.test.ts
new file mode 100644
index 0000000000..6f58303c23
--- /dev/null
+++ b/tests/unit/executor-xai-chat-to-responses-10165.test.ts
@@ -0,0 +1,46 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { XaiExecutor } from "../../open-sse/executors/xai.ts";
+import { getModelTargetFormat } from "../../open-sse/config/providerModels.ts";
+
+test("#10165 xai-oauth grok-4.5 is tagged openai-responses", () => {
+  assert.equal(getModelTargetFormat("xai-oauth", "grok-4.5"), "openai-responses");
+  assert.equal(getModelTargetFormat("xao", "grok-4.5"), "openai-responses");
+});
+
+test("#10165 XaiExecutor converts chat body to Responses fields for grok-4.5", () => {
+  const executor = new XaiExecutor("xai-oauth");
+  const body = {
+    model: "grok-4.5",
+    messages: [{ role: "user", content: "say ok" }],
+    max_tokens: 16,
+    response_format: { type: "json_object" },
+  };
+  const out = executor.transformRequest("grok-4.5", body, false, {
+    accessToken: "test",
+  } as never) as Record<string, unknown>;
+
+  assert.ok(Array.isArray(out.input), "messages must become input");
+  assert.equal(out.messages, undefined);
+  assert.equal(out.max_output_tokens, 16);
+  assert.equal(out.max_tokens, undefined);
+  assert.equal(out.response_format, undefined);
+  assert.ok(out.text && typeof out.text === "object");
+});
+
+test("#10165 XaiExecutor maps max_tokens when input already present", () => {
+  const executor = new XaiExecutor("xai-oauth");
+  const body = {
+    model: "grok-4.5",
+    input: "say ok",
+    max_tokens: 32,
+  };
+  const out = executor.transformRequest("grok-4.5", body, false, {
+    accessToken: "test",
+  } as never) as Record<string, unknown>;
+
+  assert.equal(out.input, "say ok");
+  assert.equal(out.max_output_tokens, 32);
+  assert.equal(out.max_tokens, undefined);
+});