diff --git a/open-sse/services/claudeCodeToolRemapper.ts b/open-sse/services/claudeCodeToolRemapper.ts
index 9ab697a100..754ecc978a 100644
--- a/open-sse/services/claudeCodeToolRemapper.ts
+++ b/open-sse/services/claudeCodeToolRemapper.ts
@@ -194,6 +194,12 @@ function toPascalCaseToolName(name: string): string {
 export function needsThirdPartyCloak(name: string): boolean {
   if (!name) return false;
   if (CLAUDE_BUILTIN_TOOL_NAMES.has(name)) return false;
+  // `mcp__<server>__<tool>` names are genuine Claude Code MCP tool names that
+  // Anthropic accepts natively. Cloaking them to PascalCase is unnecessary and,
+  // via round-trip asymmetry (a history tool_use keeping the original name while
+  // tools[] is cloaked), produces "Tool reference 'mcp__…' not found in available
+  // tools" 400s on the native claude OAuth path. Leave the MCP namespace alone.
+  if (name.startsWith("mcp__")) return false;
   return /[a-z]/.test(name.charAt(0)) || name.includes("_") || name.includes("-");
 }
 
diff --git a/src/sse/handlers/chat.ts b/src/sse/handlers/chat.ts
index 58f732adc3..03ef7f5d05 100644
--- a/src/sse/handlers/chat.ts
+++ b/src/sse/handlers/chat.ts
@@ -1013,7 +1013,7 @@ async function handleSingleModelChat(
             );
       preselectedCredentials = null;
 
-      if (!credentials || "allRateLimited" in credentials) {
+      if (!credentials || "allRateLimited" in credentials || !credentials.connectionId) {
         if (credentials?.allRateLimited) {
           const retryDecision = getCooldownAwareRetryDecision({
             retryAfter: credentials.retryAfter,
diff --git a/tests/unit/claude-oauth-tool-cloak.test.ts b/tests/unit/claude-oauth-tool-cloak.test.ts
index 2fdb3c71b5..46bffc1d90 100644
--- a/tests/unit/claude-oauth-tool-cloak.test.ts
+++ b/tests/unit/claude-oauth-tool-cloak.test.ts
@@ -109,6 +109,14 @@ describe("cloakThirdPartyToolNames", () => {
     assert.equal(needsThirdPartyCloak("mixture_of_agents"), true);
   });
 
+  it("needsThirdPartyCloak leaves mcp__ namespace untouched (#4861)", () => {
+    // Genuine Claude Code MCP names Anthropic accepts natively; cloaking them
+    // caused round-trip "Tool reference 'mcp__…' not found" 400s on claude OAuth.
+    assert.equal(needsThirdPartyCloak("mcp__filesystem__read_file"), false);
+    assert.equal(needsThirdPartyCloak("mcp__github__create_issue"), false);
+    assert.equal(needsThirdPartyCloak("mcp__server"), false);
+  });
+
   it("preserves the reserved name of a versioned Anthropic server tool", () => {
     const body: AnyRecord = {
       tools: [{ type: "web_search_20250305", name: "web_search", max_uses: 5 }],