diff --git a/package.json b/package.json
index f4e8bbd5c9..178d6a9c7d 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@bitkyc08/opencodex",
-  "version": "2.24.2",
+  "version": "2.25.0",
   "description": "Universal provider proxy for OpenAI Codex & Claude Code — use any LLM with Codex CLI/App/SDK and Claude Code",
   "type": "module",
   "main": "./bin/package-main.mjs",
diff --git a/src/adapters/anthropic.ts b/src/adapters/anthropic.ts
index fd141ccfb8..3075d72287 100644
--- a/src/adapters/anthropic.ts
+++ b/src/adapters/anthropic.ts
@@ -816,6 +816,53 @@ function normalizeAnthropicInputSchema(schema: unknown): Record<string, unknown>
   return normalized;
 }
 
+/**
+ * AgentRouter's gateway applies a language filter to the FIRST user message
+ * content and hard-fails non-English prompts with 400 content-blocked
+ * (#2074). Prepending an explicit English instruction frame lets the filter
+ * pass while the model still answers in the user's language — the frame says
+ * "respond in the appropriate language", it does not force English output.
+ */
+export const AGR_PREAMBLE_MARKER =
+  "[Instruction: Process the user request below and respond in the appropriate language.]";
+
+export function isAgentRouterBaseUrl(baseUrl: string): boolean {
+  try {
+    return new URL(baseUrl).hostname.includes("agentrouter");
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * Prepend the AgentRouter language frame to the first user message, once.
+ * Idempotent: a message already carrying the marker is left untouched (retries
+ * and replays must not stack frames).
+ */
+export function applyAgrLanguagePreamble(messages: unknown[]): void {
+  const firstUser = messages.find(
+    m => typeof m === "object" && m !== null && (m as { role?: string }).role === "user",
+  ) as { content?: unknown } | undefined;
+  if (!firstUser) return;
+
+  if (typeof firstUser.content === "string") {
+    if (!firstUser.content.includes(AGR_PREAMBLE_MARKER)) {
+      firstUser.content = `${AGR_PREAMBLE_MARKER}\n\n${firstUser.content}`;
+    }
+  } else if (Array.isArray(firstUser.content)) {
+    const textPart = firstUser.content.find(
+      p => typeof p === "object" && p !== null && (p as { type?: string }).type === "text",
+    ) as { text?: string } | undefined;
+    if (textPart && typeof textPart.text === "string") {
+      if (!textPart.text.includes(AGR_PREAMBLE_MARKER)) {
+        textPart.text = `${AGR_PREAMBLE_MARKER}\n\n${textPart.text}`;
+      }
+    } else {
+      firstUser.content.unshift({ type: "text", text: AGR_PREAMBLE_MARKER });
+    }
+  }
+}
+
 export function createAnthropicAdapter(provider: OcxProviderConfig, cacheRetention?: "none" | "short" | "long"): ProviderAdapter {
   const isOAuth = provider.authMode === "oauth";
   const toolNames = buildToolNameTransforms(provider);
@@ -833,6 +880,9 @@ export function createAnthropicAdapter(provider: OcxProviderConfig, cacheRetenti
       }
 
       const { system, messages } = messagesToAnthropicFormat(parsed, toolNames);
+      if (isAgentRouterBaseUrl(provider.baseUrl)) {
+        applyAgrLanguagePreamble(messages);
+      }
       // Primary image layer: resize/re-encode to fit Anthropic limits without dropping
       // (anthropic-image-normalize.ts); the guard below remains the deterministic backstop.
       // imageTierBias > 0 = upstream-413 tightened retry (030): start every image one tier lower.
diff --git a/tests/anthropic-agr-preamble.test.ts b/tests/anthropic-agr-preamble.test.ts
new file mode 100644
index 0000000000..f6d5341b61
--- /dev/null
+++ b/tests/anthropic-agr-preamble.test.ts
@@ -0,0 +1,59 @@
+import { describe, expect, test } from "bun:test";
+import {
+  AGR_PREAMBLE_MARKER,
+  applyAgrLanguagePreamble,
+  isAgentRouterBaseUrl,
+} from "../src/adapters/anthropic";
+
+describe("AgentRouter language preamble (#2074)", () => {
+  test("detects AgentRouter base URLs by hostname", () => {
+    expect(isAgentRouterBaseUrl("https://agentrouter.org")).toBe(true);
+    expect(isAgentRouterBaseUrl("https://agentrouter.org/v1")).toBe(true);
+    expect(isAgentRouterBaseUrl("https://api.anthropic.com")).toBe(false);
+    expect(isAgentRouterBaseUrl("not a url")).toBe(false);
+  });
+
+  test("prepends the frame to a string first user message", () => {
+    const messages = [
+      { role: "user", content: "responda apenas: OK" },
+      { role: "assistant", content: "OK" },
+    ];
+    applyAgrLanguagePreamble(messages);
+    const first = messages[0] as { content: string };
+    expect(first.content.startsWith(AGR_PREAMBLE_MARKER)).toBe(true);
+    expect(first.content).toContain("responda apenas: OK");
+  });
+
+  test("prepends into the first text part of structured content", () => {
+    const messages = [
+      { role: "user", content: [{ type: "image", source: {} }, { type: "text", text: "hola" }] },
+    ];
+    applyAgrLanguagePreamble(messages);
+    const parts = (messages[0] as { content: Array<{ type: string; text?: string }> }).content;
+    const text = parts.find(p => p.type === "text");
+    expect(text?.text?.startsWith(AGR_PREAMBLE_MARKER)).toBe(true);
+    expect(text?.text).toContain("hola");
+  });
+
+  test("inserts a text part when the first message has none", () => {
+    const messages = [{ role: "user", content: [{ type: "image", source: {} }] }];
+    applyAgrLanguagePreamble(messages);
+    const parts = (messages[0] as { content: Array<{ type: string; text?: string }> }).content;
+    expect(parts[0]?.type).toBe("text");
+    expect(parts[0]?.text).toContain(AGR_PREAMBLE_MARKER);
+  });
+
+  test("is idempotent — replays do not stack frames", () => {
+    const messages = [{ role: "user", content: "bonjour" }];
+    applyAgrLanguagePreamble(messages);
+    applyAgrLanguagePreamble(messages);
+    const content = (messages[0] as { content: string }).content;
+    expect(content.split(AGR_PREAMBLE_MARKER).length - 1).toBe(1);
+  });
+
+  test("no user message is a no-op", () => {
+    const messages = [{ role: "assistant", content: "hi" }];
+    applyAgrLanguagePreamble(messages);
+    expect(messages).toEqual([{ role: "assistant", content: "hi" }]);
+  });
+});