diff --git a/changelog.d/fixes/10601-xai-800-message-limit.md b/changelog.d/fixes/10601-xai-800-message-limit.md
new file mode 100644
index 0000000000..3dcd33ab8e
--- /dev/null
+++ b/changelog.d/fixes/10601-xai-800-message-limit.md
@@ -0,0 +1 @@
+- **fix(xai):** trim Chat Completions `messages` and Responses `input` to xAI's 800-item history cap before dispatch, so long tool loops no longer die on `413 Chat history exceeds the 800-message limit` ([#10601](https://github.com/diegosouzapw/OmniRoute/pull/10601))
diff --git a/open-sse/executors/xai.ts b/open-sse/executors/xai.ts
index de8fcf7425..3b5a5eef03 100644
--- a/open-sse/executors/xai.ts
+++ b/open-sse/executors/xai.ts
@@ -3,6 +3,7 @@ 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";
+import { capXaiRequestHistory } from "../services/xaiMessageCap.ts";
 
 type JsonRecord = Record<string, unknown>;
 
@@ -157,7 +158,8 @@ export class XaiExecutor extends BaseExecutor {
       }
       // Keep model id from the routed request when the translator left it empty.
       if (out.model == null && model) out.model = model;
-      return out;
+      // After chat→Responses expansion, `input` is what xAI counts toward 800.
+      return capXaiRequestHistory(out);
     }
 
     let modelId = typeof out.model === "string" ? out.model : model;
@@ -185,7 +187,7 @@ export class XaiExecutor extends BaseExecutor {
       if (effort) out.reasoning_effort = effort;
     }
 
-    return out;
+    return capXaiRequestHistory(out);
   }
 }
 
diff --git a/open-sse/services/xaiMessageCap.ts b/open-sse/services/xaiMessageCap.ts
new file mode 100644
index 0000000000..92886e68cd
--- /dev/null
+++ b/open-sse/services/xaiMessageCap.ts
@@ -0,0 +1,129 @@
+/**
+ * xAI rejects a request with HTTP 413 when chat history exceeds 800 items:
+ *   "Chat history exceeds the 800-message limit; compact the conversation and retry."
+ *
+ * Token-based compression does not catch this: a long agent loop of tiny
+ * tool calls still fits a 256k–500k window. Cap the arrays xAI actually
+ * counts — Chat Completions `messages` and Responses `input` — at the
+ * executor edge, after any chat→Responses expansion.
+ */
+import {
+  fixToolAdjacency,
+  fixToolPairs,
+  stripTrailingAssistantOrphanToolUse,
+} from "./contextManager.ts";
+
+export const XAI_CHAT_HISTORY_LIMIT = 800;
+
+type HistoryItem = Record<string, unknown>;
+
+function isSystemRole(item: HistoryItem): boolean {
+  return item.role === "system" || item.role === "developer";
+}
+
+function repairChatMessages(messages: HistoryItem[]): HistoryItem[] {
+  let result = fixToolPairs(messages);
+  result = fixToolAdjacency(result);
+  result = fixToolPairs(result);
+  return stripTrailingAssistantOrphanToolUse(result);
+}
+
+/**
+ * Keep system/developer messages plus the newest tail, then drop tool-call
+ * orphans created by the cut. If the repaired list is still over the limit
+ * (lots of system messages), take the newest `limit` items and repair again.
+ */
+export function capXaiChatMessages(
+  messages: HistoryItem[],
+  limit = XAI_CHAT_HISTORY_LIMIT
+): HistoryItem[] {
+  if (!Array.isArray(messages) || messages.length <= limit) return messages;
+
+  const system = messages.filter(isSystemRole);
+  const nonSystem = messages.filter((item) => !isSystemRole(item));
+  const budget = Math.max(2, limit - system.length);
+  let result = repairChatMessages([...system, ...nonSystem.slice(-budget)]);
+
+  if (result.length > limit) {
+    result = repairChatMessages(result.slice(-limit));
+  }
+  return result;
+}
+
+function lastUserIndex(items: HistoryItem[]): number {
+  for (let i = items.length - 1; i >= 0; i--) {
+    if (items[i].role === "user") return i;
+  }
+  return -1;
+}
+
+/**
+ * Responses `input` expands one assistant+tools chat turn into many items
+ * (`function_call` + `function_call_output`). Drop orphans left by a tail cut:
+ * outputs whose call was dropped, and mid-history calls whose output was
+ * dropped. Trailing unmatched `function_call`s (the in-flight turn) stay.
+ */
+export function repairXaiResponsesInput(items: HistoryItem[]): HistoryItem[] {
+  const callIds = new Set<string>();
+  const outputIds = new Set<string>();
+  for (const item of items) {
+    if (typeof item.call_id !== "string") continue;
+    if (item.type === "function_call") callIds.add(item.call_id);
+    if (item.type === "function_call_output") outputIds.add(item.call_id);
+  }
+
+  const lastUser = lastUserIndex(items);
+  return items.filter((item, idx) => {
+    if (item.type === "function_call_output") {
+      return typeof item.call_id === "string" && callIds.has(item.call_id);
+    }
+    if (item.type === "function_call") {
+      if (typeof item.call_id === "string" && outputIds.has(item.call_id)) return true;
+      return lastUser < 0 || idx > lastUser;
+    }
+    return true;
+  });
+}
+
+export function capXaiResponsesInput(
+  input: HistoryItem[],
+  limit = XAI_CHAT_HISTORY_LIMIT
+): HistoryItem[] {
+  if (!Array.isArray(input) || input.length <= limit) return input;
+
+  let result = repairXaiResponsesInput(input.slice(-limit));
+  if (result.length > limit) {
+    result = repairXaiResponsesInput(result.slice(-limit));
+  }
+  return result;
+}
+
+/**
+ * Cap whichever history array the body is using. No-op (same object /
+ * same array refs) when already within the limit.
+ */
+export function capXaiRequestHistory(
+  body: Record<string, unknown>
+): Record<string, unknown> {
+  if (!body || typeof body !== "object") return body;
+
+  const next: Record<string, unknown> = { ...body };
+  let changed = false;
+
+  if (Array.isArray(body.messages)) {
+    const messages = capXaiChatMessages(body.messages as HistoryItem[]);
+    if (messages !== body.messages) {
+      next.messages = messages;
+      changed = true;
+    }
+  }
+  if (Array.isArray(body.input)) {
+    const input = capXaiResponsesInput(body.input as HistoryItem[]);
+    if (input !== body.input) {
+      next.input = input;
+      changed = true;
+    }
+  }
+
+  return changed ? next : body;
+}
diff --git a/tests/unit/xai-message-cap.test.ts b/tests/unit/xai-message-cap.test.ts
new file mode 100644
index 0000000000..d40b4f7236
--- /dev/null
+++ b/tests/unit/xai-message-cap.test.ts
@@ -0,0 +1,194 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import {
+  XAI_CHAT_HISTORY_LIMIT,
+  capXaiChatMessages,
+  capXaiRequestHistory,
+  capXaiResponsesInput,
+  repairXaiResponsesInput,
+} from "../../open-sse/services/xaiMessageCap.ts";
+import { XaiExecutor } from "../../open-sse/executors/xai.ts";
+
+const credentials = { apiKey: "test-key" };
+
+function chatTurn(i: number) {
+  return [
+    { role: "user", content: `u${i}` },
+    { role: "assistant", content: `a${i}` },
+  ];
+}
+
+test("XAI_CHAT_HISTORY_LIMIT matches the upstream 413", () => {
+  assert.equal(XAI_CHAT_HISTORY_LIMIT, 800);
+});
+
+test("capXaiChatMessages is a no-op at or under the limit", () => {
+  const messages = [
+    { role: "system", content: "sys" },
+    ...Array.from({ length: 400 }, (_, i) => chatTurn(i)).flat(),
+  ];
+  assert.equal(messages.length, 801);
+  const under = messages.slice(0, 800);
+  assert.equal(capXaiChatMessages(under), under);
+  assert.equal(capXaiChatMessages(under).length, 800);
+});
+
+test("capXaiChatMessages keeps system plus the newest tail", () => {
+  const messages = [
+    { role: "system", content: "sys" },
+    ...Array.from({ length: 450 }, (_, i) => chatTurn(i)).flat(),
+  ];
+  assert.ok(messages.length > 800);
+  const capped = capXaiChatMessages(messages);
+  assert.ok(capped.length <= 800);
+  assert.equal(capped[0].role, "system");
+  assert.equal(capped[0].content, "sys");
+  assert.equal(capped[capped.length - 1].content, "a449");
+  assert.equal(capped[capped.length - 2].content, "u449");
+  assert.equal(
+    capped.some((m) => m.content === "u0"),
+    false
+  );
+});
+
+test("capXaiChatMessages drops a tool_result whose tool_use was cut", () => {
+  const messages = [
+    { role: "system", content: "sys" },
+    {
+      role: "assistant",
+      content: null,
+      tool_calls: [{ id: "old", type: "function", function: { name: "search" } }],
+    },
+    { role: "tool", tool_call_id: "old", content: "stale" },
+    ...Array.from({ length: 420 }, (_, i) => chatTurn(i)).flat(),
+    {
+      role: "assistant",
+      content: null,
+      tool_calls: [{ id: "kept", type: "function", function: { name: "read" } }],
+    },
+    { role: "tool", tool_call_id: "kept", content: "file" },
+    { role: "user", content: "go" },
+  ];
+  const capped = capXaiChatMessages(messages);
+  assert.ok(capped.length <= 800);
+  const toolIds = capped.filter((m) => m.role === "tool").map((m) => m.tool_call_id);
+  assert.deepEqual(toolIds, ["kept"]);
+});
+
+test("repairXaiResponsesInput drops orphaned function_call_output", () => {
+  const items = [
+    { type: "function_call_output", call_id: "missing", output: "nope" },
+    { role: "user", content: [{ type: "input_text", text: "hi" }] },
+    { type: "function_call", call_id: "ok", name: "read", arguments: "{}" },
+    { type: "function_call_output", call_id: "ok", output: "file" },
+  ];
+  const repaired = repairXaiResponsesInput(items);
+  assert.equal(
+    repaired.some((item) => item.call_id === "missing"),
+    false
+  );
+  assert.equal(repaired.length, 3);
+});
+
+test("repairXaiResponsesInput keeps a trailing in-flight function_call", () => {
+  const items = [
+    { role: "user", content: [{ type: "input_text", text: "hi" }] },
+    { type: "function_call", call_id: "pending", name: "read", arguments: "{}" },
+  ];
+  assert.deepEqual(repairXaiResponsesInput(items), items);
+});
+
+test("capXaiResponsesInput keeps the newest 800 items and repairs the cut", () => {
+  const input = [];
+  for (let i = 0; i < 500; i++) {
+    input.push({ role: "user", content: [{ type: "input_text", text: `u${i}` }] });
+    input.push({ type: "function_call", call_id: `c${i}`, name: "t", arguments: "{}" });
+    input.push({ type: "function_call_output", call_id: `c${i}`, output: `o${i}` });
+  }
+  assert.ok(input.length > 800);
+  const capped = capXaiResponsesInput(input);
+  assert.ok(capped.length <= 800);
+  assert.equal(capped[capped.length - 1].output, "o499");
+  const outputs = capped.filter((item) => item.type === "function_call_output");
+  const calls = new Set(
+    capped.filter((item) => item.type === "function_call").map((item) => item.call_id)
+  );
+  for (const item of outputs) {
+    assert.ok(calls.has(item.call_id), `output ${item.call_id} has no matching call`);
+  }
+});
+
+test("capXaiRequestHistory is a no-op when both arrays already fit", () => {
+  const body = {
+    model: "grok-4.3",
+    messages: [{ role: "user", content: "hi" }],
+  };
+  assert.equal(capXaiRequestHistory(body), body);
+  assert.equal("input" in capXaiRequestHistory(body), false);
+});
+
+test("capXaiRequestHistory caps messages and input independently", () => {
+  const messages = Array.from({ length: 801 }, (_, i) => ({
+    role: i % 2 === 0 ? "user" : "assistant",
+    content: String(i),
+  }));
+  const input = Array.from({ length: 801 }, (_, i) => ({
+    role: "user",
+    content: [{ type: "input_text", text: String(i) }],
+  }));
+  const out = capXaiRequestHistory({ model: "grok-4.3", messages, input });
+  assert.ok((out.messages as unknown[]).length <= 800);
+  assert.ok((out.input as unknown[]).length <= 800);
+  assert.equal((out.messages as { content: string }[]).at(-1)?.content, "800");
+  assert.equal((out.input as { content: { text: string }[] }[]).at(-1)?.content[0].text, "800");
+});
+
+test("XaiExecutor caps Chat Completions history before grok-4.3 leaves the executor", () => {
+  const executor = new XaiExecutor();
+  const messages = [
+    { role: "system", content: "sys" },
+    ...Array.from({ length: 450 }, (_, i) => chatTurn(i)).flat(),
+  ];
+  const out = executor.transformRequest(
+    "grok-4.3",
+    { model: "grok-4.3", messages },
+    false,
+    credentials
+  ) as Record<string, unknown>;
+  assert.ok(Array.isArray(out.messages));
+  assert.ok((out.messages as unknown[]).length <= 800);
+  assert.equal((out.messages as { content: string }[])[0].content, "sys");
+  assert.equal((out.messages as { content: string }[]).at(-1)?.content, "a449");
+});
+
+test("XaiExecutor caps Responses input after expanding a long chat history", () => {
+  const executor = new XaiExecutor();
+  const messages = [];
+  for (let i = 0; i < 300; i++) {
+    messages.push({ role: "user", content: `u${i}` });
+    messages.push({
+      role: "assistant",
+      content: null,
+      tool_calls: [
+        { id: `c${i}a`, type: "function", function: { name: "a", arguments: "{}" } },
+        { id: `c${i}b`, type: "function", function: { name: "b", arguments: "{}" } },
+      ],
+    });
+    messages.push({ role: "tool", tool_call_id: `c${i}a`, content: "ra" });
+    messages.push({ role: "tool", tool_call_id: `c${i}b`, content: "rb" });
+  }
+  // 300 turns × (1 user + 2 function_call + 2 function_call_output) = 1500 input items
+  const out = executor.transformRequest(
+    "grok-4.6",
+    { model: "grok-4.6", messages },
+    false,
+    credentials
+  ) as Record<string, unknown>;
+  assert.equal(out.messages, undefined);
+  assert.ok(Array.isArray(out.input));
+  assert.ok((out.input as unknown[]).length <= 800);
+  const last = (out.input as { type?: string; call_id?: string }[]).at(-1);
+  assert.equal(last?.type, "function_call_output");
+  assert.equal(last?.call_id, "c299b");
+});