MCP

Concept

The Model Context Protocol is how agents connect to tools and data they do not own. Rewyn treats it as a first-class source of tools rather than an integration: an MCP tool becomes a Rewyn Tool, goes through the same permission policy, and emits the same TOOL_CALLED and TOOL_RETURNED events as a local function.

That matters for the same reason the rest of this SDK exists. When a Salesforce MCP server changes its behaviour, you want the run manifest to show which server, which version, and which tool was called.

Minimal example

from rewyn.mcp import connect

async with connect("https://crm.internal/mcp", name="crm") as client:
    for tool in client.tools():
        print(tool.name, tool.description)

connect takes a URL, a command line for stdio, an MCPServerConfig, or an in-process server object.

Production example

from rewyn import Agent
from rewyn.mcp import connect
from rewyn.tools import MaxRiskLevel, RiskLevel

crm = connect(
    "npx -y @acme/salesforce-mcp",
    name="salesforce",
    version="4",
    tool_prefix="salesforce_",  # keeps the origin visible in transcripts
    risk_level=RiskLevel.HIGH,  # third-party tools are not low risk
)

async with Agent(
    model="anthropic:claude-opus-5",
    mcp=[crm],
    permission_policy=MaxRiskLevel(approve_above=RiskLevel.MEDIUM),
) as agent:
    result = await agent.arun("What is Acme's account owner?")

Use tool_prefix. Without it, two servers exposing search collide, and a transcript that says search does not tell you whose data was read.

Exposing your own tools

from rewyn.mcp import run_stdio, serve_tools

server = serve_tools([get_customer, issue_refund], name="billing")
run_stdio(server)  # or connect(server) in-process, for tests

Configuration discovery

from rewyn.mcp import discover_configs

for config in discover_configs():  # reads mcp.json
    print(config.name, config.transport)

rewyn mcp                  # list configured servers
rewyn mcp tools salesforce # connect and list its tools

API reference

rewyn/mcp/client.py for MCPClient, MCPServerConfig and connect. rewyn/mcp/server.py for serve_tools and run_stdio. rewyn/mcp/discovery.py for discover_configs. rewyn/mcp/adapter.py for the tool translation.

Failure modes

MissingDependencyError. MCP support needs the extra: pip install "rewyn[mcp]".

The model calls a tool that does not exist. Almost always the prefix. With tool_prefix="salesforce_" the tool is salesforce_lookup_account, not lookup_account. The run shows unknown tool as a tool error, and by default the agent recovers and carries on, so check for tool errors rather than assuming success.

The connection leaks. Use async with on the client or the agent. An agent only closes connections it opened itself.

A server is slow or hangs. MCP calls are network calls. Set a timeout on the agent (max_time) and treat a third-party server as untrusted for latency as well as content.

Tool schemas are vague. You do not control a third-party server's descriptions. Where they are poor, wrap the tool locally with a better one.