From 241d5c055b05ac2473f1f7bb4398a3407c21fbb4 Mon Sep 17 00:00:00 2001
From: camysidron1 <cam@nebari.ai>
Date: Mon, 27 Jul 2026 17:28:09 -0700
Subject: [PATCH] Default to strict MCP configuration

---
 README.md                     |  9 +++++++++
 src/claude_agent_sdk/types.py |  5 ++++-
 tests/test_transport.py       | 10 +++++-----
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/README.md b/README.md
index 8d8445238..66c7d53e3 100644
--- a/README.md
+++ b/README.md
@@ -184,6 +184,15 @@ options = ClaudeAgentOptions(
 )
 ```
 
+By default, the SDK uses only the MCP servers supplied through `mcp_servers`.
+It does not implicitly load project `.mcp.json`, user/global settings, or
+plugin-provided MCP servers. Applications that intentionally rely on Claude
+Code's ambient MCP discovery can restore that behavior explicitly:
+
+```python
+options = ClaudeAgentOptions(strict_mcp_config=False)
+```
+
 ### Hooks
 
 A **hook** is a Python function that the Claude Code _application_ (_not_ Claude) invokes at specific points of the Claude agent loop. Hooks can provide deterministic processing and automated feedback for Claude. Read more in [Intercept and control agent behavior with hooks](https://platform.claude.com/docs/en/agent-sdk/hooks).
diff --git a/src/claude_agent_sdk/types.py b/src/claude_agent_sdk/types.py
index c5fb86893..fa13de9cb 100644
--- a/src/claude_agent_sdk/types.py
+++ b/src/claude_agent_sdk/types.py
@@ -1800,10 +1800,13 @@ class ClaudeAgentOptions:
     to an MCP config JSON file.
     """
 
-    strict_mcp_config: bool = False
+    strict_mcp_config: bool = True
     """When ``True``, only use MCP servers passed via :attr:`mcp_servers`,
     ignoring all other MCP configurations the CLI would otherwise load (e.g.
     project ``.mcp.json``, user/global settings, plugin-provided servers).
+    Defaults to ``True`` so SDK applications do not implicitly execute MCP
+    servers discovered from their working directory. Set this to ``False`` to
+    retain Claude Code's ambient MCP discovery behavior.
     Maps to the CLI's ``--strict-mcp-config`` flag and matches the TypeScript
     SDK's ``strictMcpConfig`` option."""
 
diff --git a/tests/test_transport.py b/tests/test_transport.py
index 4c1a5abcf..1918bf837 100644
--- a/tests/test_transport.py
+++ b/tests/test_transport.py
@@ -92,13 +92,13 @@ def test_build_command_include_hook_events(self):
         assert "--include-hook-events" not in cmd_off
 
     def test_build_command_strict_mcp_config(self):
-        """Test that --strict-mcp-config is emitted only when enabled."""
-        transport = SubprocessCLITransport(
-            prompt="test", options=make_options(strict_mcp_config=True)
-        )
+        """Test strict MCP isolation is the default and can be disabled."""
+        transport = SubprocessCLITransport(prompt="test", options=make_options())
         assert "--strict-mcp-config" in transport._build_command()
 
-        transport = SubprocessCLITransport(prompt="test", options=make_options())
+        transport = SubprocessCLITransport(
+            prompt="test", options=make_options(strict_mcp_config=False)
+        )
         assert "--strict-mcp-config" not in transport._build_command()
 
     def test_build_command_resume_and_session_id(self):