From 06b9b98b6ce81a67dfeaefc71c799171924fef9b Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Thu, 22 May 2025 11:22:45 -0400
Subject: [PATCH] Raise an error if a Client is created with no servers in
 config

---
 src/fastmcp/client/transports.py |  6 +++++-
 tests/client/test_client.py      | 12 ++++++++++++
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/fastmcp/client/transports.py b/src/fastmcp/client/transports.py
index e2ae60816..e2e8a7eb5 100644
--- a/src/fastmcp/client/transports.py
+++ b/src/fastmcp/client/transports.py
@@ -528,8 +528,12 @@ class MCPConfigTransport(ClientTransport):
             config = MCPConfig.from_dict(config)
         self.config = config
 
+        # if there are no servers, raise an error
+        if len(self.config.mcpServers) == 0:
+            raise ValueError("No MCP servers defined in the config")
+
         # if there's exactly one server, create a client for that server
-        if len(self.config.mcpServers) == 1:
+        elif len(self.config.mcpServers) == 1:
             self.transport = list(self.config.mcpServers.values())[0].to_transport()
 
         # otherwise create a composite client
diff --git a/tests/client/test_client.py b/tests/client/test_client.py
index 62bddf0ab..e85ca4fc5 100644
--- a/tests/client/test_client.py
+++ b/tests/client/test_client.py
@@ -673,6 +673,18 @@ class TestInferTransport:
         assert transport.transport.command == "echo"
         assert transport.transport.args == ["hello"]
 
+    def test_config_with_no_servers(self):
+        """Test that an empty MCPConfig raises a ValueError."""
+        config = {"mcpServers": {}}
+        with pytest.raises(ValueError, match="No MCP servers defined in the config"):
+            infer_transport(config)
+
+    def test_mcpconfigtransport_with_no_servers(self):
+        """Test that MCPConfigTransport raises a ValueError when initialized with an empty config."""
+        config = {"mcpServers": {}}
+        with pytest.raises(ValueError, match="No MCP servers defined in the config"):
+            MCPConfigTransport(config=config)
+
     def test_infer_composite_client(config):
         config = {
             "mcpServers": {