diff --git a/src/index.ts b/src/index.ts
index 33b15b9..b640acc 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1561,32 +1561,37 @@ export class ClaudeCodeProxy {
    * Build request headers for a specific provider
    */
   private buildProviderHeaders(provider: 'anthropic' | 'zai' | 'openrouter', reqHeaders: Record<string, string>): Record<string, string> {
-    if (provider === 'zai') {
-      const headers: Record<string, string> = {
-        host: new URL(this.config.zai.baseUrl).host,
-        authorization: `Bearer ${this.config.zai.apiKey}`,
-      };
-      // Copy allowed headers
+    // Headers that must NOT be forwarded to a non-Anthropic upstream. Beyond the
+    // hop-by-hop/auth headers, Anthropic-specific routing headers — especially
+    // `x-api-key` — make OpenRouter route the request into a restricted data-policy
+    // path and return `404: No endpoints available matching your guardrail
+    // restrictions`. `content-length` is recomputed after cleanBody, so drop the
+    // client's value too.
+    const blockedForwardHeaders = [
+      "authorization", "transfer-encoding", "connection", "host", "content-length",
+      "x-api-key", "anthropic-version", "anthropic-beta", "anthropic-dangerous-direct-browser-access",
+    ];
+    const copyAllowed = (headers: Record<string, string>) => {
       for (const [key, value] of Object.entries(reqHeaders)) {
-        if (!["authorization", "transfer-encoding", "connection", "host"].includes(key)) {
+        if (!blockedForwardHeaders.includes(key.toLowerCase())) {
           headers[key] = value;
         }
       }
       return headers;
+    };
+
+    if (provider === 'zai') {
+      return copyAllowed({
+        host: new URL(this.config.zai.baseUrl).host,
+        authorization: `Bearer ${this.config.zai.apiKey}`,
+      });
     } else if (provider === 'openrouter') {
-      const headers: Record<string, string> = {
+      return copyAllowed({
         host: new URL(this.config.openrouter.baseUrl).host,
         authorization: `Bearer ${this.config.openrouter.apiKey}`,
         "HTTP-Referer": "https://claude.ai/code",
         "X-Title": "Claude Code",
-      };
-      // Copy allowed headers
-      for (const [key, value] of Object.entries(reqHeaders)) {
-        if (!["authorization", "transfer-encoding", "connection", "host"].includes(key)) {
-          headers[key] = value;
-        }
-      }
-      return headers;
+      });
     } else {
       // Anthropic headers
       return this.cleanHeaders(reqHeaders);
diff --git a/tests/proxy.test.js b/tests/proxy.test.js
index 0e50c3d..b8ca48c 100644
--- a/tests/proxy.test.js
+++ b/tests/proxy.test.js
@@ -557,10 +557,12 @@ describe("Claude Code Proxy provider request normalization", () => {
     const proxy = createProxy();
     let capturedUrl = "";
     let capturedBody = "";
+    let capturedHeaders = {};
 
     proxy.httpRequest = async (url, options) => {
       capturedUrl = url;
       capturedBody = String(options.body);
+      capturedHeaders = options.headers;
       return {
         status: 200,
         headers: options.headers,
@@ -577,7 +579,14 @@ describe("Claude Code Proxy provider request normalization", () => {
         metadata: { source: "test" },
         extra_field: "should-be-stripped",
       }),
-      { "content-type": "application/json" },
+      {
+        "content-type": "application/json",
+        // Claude Code always sends these Anthropic-specific headers; forwarding
+        // x-api-key to OpenRouter triggers a guardrail 404.
+        "x-api-key": "sk-ant-should-not-leak",
+        "anthropic-version": "2023-06-01",
+        "anthropic-beta": "claude-code-20250219",
+      },
       "/v1/messages?beta=true",
       "POST",
     );
@@ -588,6 +597,14 @@ describe("Claude Code Proxy provider request normalization", () => {
     assert.equal(parsedBody.model, "~anthropic/claude-sonnet-latest");
     assert.equal(parsedBody.extra_field, undefined);
     assert.deepEqual(parsedBody.metadata, { source: "test" });
+
+    // Anthropic-specific headers must NOT reach OpenRouter (cause guardrail 404).
+    assert.equal(capturedHeaders["x-api-key"], undefined);
+    assert.equal(capturedHeaders["anthropic-version"], undefined);
+    assert.equal(capturedHeaders["anthropic-beta"], undefined);
+    // OpenRouter's own auth + attribution headers are present.
+    assert.equal(capturedHeaders.authorization, "Bearer openrouter-test-key");
+    assert.equal(capturedHeaders["HTTP-Referer"], "https://claude.ai/code");
   });
 
   it("normalizes OpenRouter message responses to strict Anthropic shape", async () => {