fix: reject self-mount to prevent infinite recursion (#3925)

* fix: server safety guards for self-mount, duplicate middleware, mount arg order

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Remove duplicate middleware and mount arg order checks

These are runtime type checking, not bugs — a type checker catches them.
Keep only the self-mount guard which is a semantic check.

🤖 Generated with Claude Code

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: strawgate

src/fastmcp/server/server.py

@@ -2047,6 +2047,9 @@ def mount(
 
         from fastmcp.server.providers.fastmcp_provider import FastMCPProvider
 
+        if server is self:
+            raise ValueError("Cannot mount a server onto itself")
+
         # Handle deprecated prefix parameter
         if prefix is not None:
             warnings.warn(

tests/server/test_server_safety.py

@@ -0,0 +1,10 @@
+import pytest
+
+from fastmcp import FastMCP
+
+
+class TestMountSafety:
+    def test_self_mount_raises(self):
+        mcp = FastMCP("test")
+        with pytest.raises(ValueError, match="Cannot mount a server onto itself"):
+            mcp.mount(mcp)