diff --git a/src/providers/proxies/claude-proxy.ts b/src/providers/proxies/claude-proxy.ts
index 699c9f2..567555d 100644
--- a/src/providers/proxies/claude-proxy.ts
+++ b/src/providers/proxies/claude-proxy.ts
@@ -15,12 +15,25 @@ import {
 } from "../../usage/token-usage";
 import { isObjectRecord, type JsonObject } from "../../utils/object";
 
-// Anthropic's server blocks "OpenCode" in system prompts for OAuth sessions.
-// https://github.com/anomalyco/opencode-anthropic-auth/blob/d5a1ab46ac58c93d0edf5c9eea46f3e72981f1fd/index.mjs#L198-L211
-const sanitizeClaudeSystemText = (text: string): string =>
-  text
-    .replace(/OpenCode/g, "Claude Code")
-    .replace(/(?<!\/)opencode/gi, "Claude");
+const CODE_REFERENCES_MARKER = "# Code References";
+const OPENCODE_IDENTITY_MARKER =
+  "You are OpenCode, the best coding agent on the planet.";
+
+// Anthropic OAuth sessions appear to reject the injected OpenCode identity
+// prelude. Remove only that section and preserve the rest of the prompt.
+const sanitizeClaudeSystemText = (text: string): string => {
+  const start = text.indexOf(OPENCODE_IDENTITY_MARKER);
+  if (start === -1) {
+    return text;
+  }
+
+  const end = text.indexOf(CODE_REFERENCES_MARKER, start);
+  if (end === -1) {
+    return text;
+  }
+
+  return `${text.slice(0, start)}${text.slice(end)}`;
+};
 
 // Request: prefix tool names so they match Claude Code's expected format.
 // Response: strip prefixes back so the client sees its original names.
@@ -46,23 +59,16 @@ const transformClaudeRequestPayload = (
   const transformed: JsonObject = { ...payload };
 
   if (typeof transformed.system === "string") {
-    transformed.system = [
-      {
-        type: "text",
-        text: systemIdentity,
-      },
-      {
-        type: "text",
-        text: sanitizeClaudeSystemText(transformed.system),
-      },
+    const sanitizedSystem = sanitizeClaudeSystemText(transformed.system);
+    const systemBlocks: Array<{ type: string; text: string }> = [
+      { type: "text", text: systemIdentity },
     ];
+    if (sanitizedSystem !== systemIdentity) {
+      systemBlocks.push({ type: "text", text: sanitizedSystem });
+    }
+    transformed.system = systemBlocks;
   } else if (Array.isArray(transformed.system)) {
-    const systemBlocks: unknown[] = [
-      {
-        type: "text",
-        text: systemIdentity,
-      },
-    ];
+    const systemBlocks: unknown[] = [{ type: "text", text: systemIdentity }];
     for (const block of transformed.system) {
       if (
         isObjectRecord(block) &&
diff --git a/tests/providers/proxy-contract.test.ts b/tests/providers/proxy-contract.test.ts
index e177701..571f2fc 100644
--- a/tests/providers/proxy-contract.test.ts
+++ b/tests/providers/proxy-contract.test.ts
@@ -630,7 +630,11 @@ describe("proxy contract: claude", () => {
       "anthropic-beta": `custom-beta,${CLAUDE_REQUIRED_BETA_HEADERS[0]}`,
     });
     const requestBody = {
-      system: "OpenCode and opencode should be rewritten",
+      system:
+        "You are OpenCode, the best coding agent on the planet.\n\n" +
+        "OpenCode-specific instructions\n\n" +
+        "# Code References\n\n" +
+        "Use file_path:line_number references.",
       tools: [{ name: "shell", description: "run shell commands" }],
       tool_choice: { type: "tool", name: "shell" },
       messages: [
@@ -669,13 +673,40 @@ describe("proxy contract: claude", () => {
     expect(transformed.system).toHaveLength(2);
     expect(transformed.system[0]?.text).toBe(CLAUDE_SYSTEM_IDENTITY);
     expect(transformed.system[1]?.text).toBe(
-      "Claude Code and Claude should be rewritten"
+      "# Code References\n\nUse file_path:line_number references."
     );
     expect(transformed.tools[0]?.name).toBe("mcp_shell");
     expect(transformed.tool_choice.name).toBe("mcp_shell");
     expect(transformed.messages[0]?.content[0]?.name).toBe("mcp_shell");
   });
 
+  test("preserves branded prompts when OpenCode markers are absent", () => {
+    const requestBody = {
+      system: "OpenCode custom system prompt without shared markers",
+    };
+
+    const result = prepareClaudeProxyRequest({
+      requestUrl: new URL("https://kleis.local/v1/messages"),
+      headers: new Headers(),
+      bodyText: JSON.stringify(requestBody),
+      bodyJson: requestBody,
+      accessToken: "claude-token",
+      metadata: null,
+    });
+
+    const transformed = JSON.parse(result.bodyText) as {
+      system: Array<{ type: string; text: string }>;
+    };
+
+    expect(transformed.system).toEqual([
+      { type: "text", text: CLAUDE_SYSTEM_IDENTITY },
+      {
+        type: "text",
+        text: "OpenCode custom system prompt without shared markers",
+      },
+    ]);
+  });
+
   test("strips tool prefix in non-streaming JSON response payload", async () => {
     const capture = createUsageCapture();
     const result = prepareClaudeUsageRequest(capture.onTokenUsage);