diff --git a/open-sse/translator/request/claude-to-openai.ts b/open-sse/translator/request/claude-to-openai.ts
index b6ff5563161..94b2a19b922 100644
--- a/open-sse/translator/request/claude-to-openai.ts
+++ b/open-sse/translator/request/claude-to-openai.ts
@@ -5,6 +5,19 @@ import { adjustMaxTokens } from "../helpers/maxTokensHelper.ts";
 type JsonRecord = Record<string, unknown>;
 const TOOL_CHOICE_ANY = ["a", "n", "y"].join("");
 
+/**
+ * Port of decolua/9router commit 0aaa5ab3 (closes prompt-cache instability):
+ * Anthropic injects a dynamic `x-anthropic-billing-header: <value>` line at
+ * the top of some system prompts. When we translate Claude → OpenAI and
+ * forward to a non-Anthropic upstream, that line both leaks into the
+ * assistant prompt and rotates per request, destroying prompt-cache hits.
+ * Strip it from each system entry before assembling the OpenAI request.
+ */
+function stripAnthropicBillingHeader(text: unknown): string {
+  if (typeof text !== "string") return "";
+  return text.replace(/^x-anthropic-billing-header:[^\n]*(?:\r?\n)?/i, "");
+}
+
 /**
  * Normalize tool input schema for OpenAI compatibility.
  * OpenAI strict mode requires `properties: {}` on object-type schemas,
@@ -108,8 +121,11 @@ export function claudeToOpenAIRequest(model, body, stream, credentials: unknown
   // System message
   if (body.system) {
     const systemContent = Array.isArray(body.system)
-      ? body.system.map((s) => s.text || "").join("\n")
-      : body.system;
+      ? body.system
+          .map((s) => stripAnthropicBillingHeader(s.text || ""))
+          .filter(Boolean)
+          .join("\n")
+      : stripAnthropicBillingHeader(body.system);
 
     if (systemContent) {
       result.messages.push({
diff --git a/tests/unit/translator-claude-to-openai-strip-billing-header.test.ts b/tests/unit/translator-claude-to-openai-strip-billing-header.test.ts
new file mode 100644
index 00000000000..2d381c17f76
--- /dev/null
+++ b/tests/unit/translator-claude-to-openai-strip-billing-header.test.ts
@@ -0,0 +1,80 @@
+/**
+ * Port of decolua/9router commit 0aaa5ab3:
+ * The Anthropic upstream injects a dynamic `x-anthropic-billing-header: ...`
+ * line at the top of the Claude system prompt for some routes. When we
+ * translate Claude → OpenAI and forward to an OpenAI-compatible upstream,
+ * that line ends up as part of the assistant's system message — which
+ * (a) confuses non-Anthropic models and (b) keeps changing across requests
+ * and so destroys prompt-cache hits.
+ *
+ * Strip a leading `x-anthropic-billing-header: <value>` line from each
+ * system entry before assembling the OpenAI request.
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { claudeToOpenAIRequest } = await import(
+  "../../open-sse/translator/request/claude-to-openai.ts"
+);
+
+test("Claude -> OpenAI strips x-anthropic-billing-header from array system entries", () => {
+  const result = claudeToOpenAIRequest(
+    "gpt-4o",
+    {
+      system: [
+        { text: "x-anthropic-billing-header: abc123\nReal system rule A" },
+        { text: "Real system rule B" },
+      ],
+      messages: [{ role: "user", content: [{ type: "text", text: "hi" }] }],
+    },
+    true
+  ) as { messages: Array<{ role: string; content: string }> };
+
+  const sys = result.messages.find((m) => m.role === "system");
+  assert.ok(sys, "system message present");
+  assert.equal(
+    sys!.content.includes("x-anthropic-billing-header"),
+    false,
+    "billing header line must be stripped"
+  );
+  assert.equal(
+    sys!.content.includes("Real system rule A"),
+    true,
+    "real rule must survive"
+  );
+  assert.equal(
+    sys!.content.includes("Real system rule B"),
+    true,
+    "second block must survive"
+  );
+});
+
+test("Claude -> OpenAI strips x-anthropic-billing-header from a string system field", () => {
+  const result = claudeToOpenAIRequest(
+    "gpt-4o",
+    {
+      system: "x-anthropic-billing-header: zzz\nFollow these rules.",
+      messages: [{ role: "user", content: [{ type: "text", text: "hi" }] }],
+    },
+    true
+  ) as { messages: Array<{ role: string; content: string }> };
+
+  const sys = result.messages.find((m) => m.role === "system");
+  assert.ok(sys, "system message present");
+  assert.equal(sys!.content, "Follow these rules.");
+});
+
+test("Claude -> OpenAI leaves unrelated system content intact (no false positives)", () => {
+  const result = claudeToOpenAIRequest(
+    "gpt-4o",
+    {
+      system: "Normal system prompt with no billing header.",
+      messages: [{ role: "user", content: [{ type: "text", text: "hi" }] }],
+    },
+    true
+  ) as { messages: Array<{ role: string; content: string }> };
+
+  const sys = result.messages.find((m) => m.role === "system");
+  assert.equal(sys!.content, "Normal system prompt with no billing header.");
+});