2.4 compatibility in Nexa 3.0

This page describes the retained 2.4 language/runtime path. Sources without a version declaration use this path. Start new Open Programs with Nexa 3.0; compare contracts in the migration guide.

Standard library and MCP

Built-in namespaces, custom @tool declarations, and MCP share exposure and approval boundaries. Preserve schemas, execution permissions, and outcome semantics when choosing an integration.

Standard library

src/runtime/stdlib.py defines STD_NAMESPACE_MAP and tool schemas. Common groups include std.fs, std.http, std.json, std.shell, std.db.sqlite, std.auth, std.kv, std.concurrent, std.template, and std.ui. See Standard library reference for functions.

Packaged stdlib/news.nx is an includable Nexa module. Resolution follows the unified compiler.

MCP connections

Install the official Python SDK extra in the Nexa checkout:

uv sync --locked --extra mcp

The current integration uses MCP Python SDK 2.3 and supports stdio and HTTP URLs supported by that SDK. SDK sessions handle discovery, pagination, and shutdown.

from mcp import StdioServerParameters
from src.runtime.agent import NexaAgent
from src.runtime.mcp_client import MCPClient

with MCPClient(target=StdioServerParameters(command="python", args=["server.py"])) as client:
    agent = NexaAgent("Researcher", tools=client.get_tools_schema())
    state = agent.execute("Inspect available information")

This usage requires a real server.py and model configuration. Discovered remote tools require approval by default; server annotations do not authorize execution.

fetch_mcp_tools() accepts a single mcpServers configuration file. Legacy tools JSON retains schema loading with a deprecation warning. SDK/API execution closes sessions on exit. Unknown side effects after an MCP disconnect follow the checkpoint contract.

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Source and validation · stdlib / mcp / tools