MCP

Enable this extension in the conversation's extension picker. Berry has a flat tool namespace, so the three operations are exposed as tools.mcp_list, tools.mcp_describe, and tools.mcp_call.

Configuration

By default, the extension reads ~/.config/berry/mcp.json on the conversation's machine. Select another path in the extension's configuration form; relative paths use the conversation's working directory. A missing file means no configured servers.

{
  "mcpServers": {
    "filesystem": {
      "command": "npx",
      "args": ["-y", "@modelcontextprotocol/server-filesystem", "${WORKSPACE}"]
    },
    "docs": {
      "url": "https://example.com/mcp",
      "headers": { "Authorization": "Bearer ${DOCS_TOKEN}" }
    }
  }
}

Stdio entries support command, args, env, and cwd. Commands use that machine's PATH, inherit its environment, and run in the conversation's working directory unless cwd overrides it. HTTP entries support url and headers; requests originate from that machine too, including DNS and localhost. Only Streamable HTTP is supported, and redirects are refused.

Both transports support enabled: false and timeout in seconds (default 60). Initialization is limited to at most 30 seconds. ${VARIABLE} substitutions use the selected machine's environment; missing variables fail explicitly. No shell or command substitution is performed.

Credentials stay in the configuration file or environment variables, not in configuration events. Protect files containing credentials with appropriate permissions. The file is reread on each tool operation; changed configurations get new connections. Connections are shared only within the same machine, working directory, remote connection lifetime, and resolved server configuration, and close after five idle minutes or when the extension reloads. Replacing an SSH connection opens fresh MCP sessions on the next operation, without replaying calls.

Stdio shutdown force-stops owned process groups and bounds pipe draining, so escaped descendants cannot keep an extension reload waiting indefinitely for inherited stdout or stderr. Startup failures include a bounded stderr tail and the exit code when available.

Tools

text(await tools.mcp_list({}));
text(await tools.mcp_describe({ server: "docs", tool: "search" }));

const result = await tools.mcp_call({
  server: "docs",
  tool: "search",
  arguments: { query: "authentication" },
});
text(result);
for (const block of result.content) {
  if (block.type === "image") image(block);
}

Listing returns each server's status and short tool descriptions; one broken server does not hide others. Pass server to restrict discovery. Describing a server without tool returns its complete tool catalog and instructions.

Calls return the full MCP result, including content, structuredContent, and isError. Tool-level errors remain results; transport and protocol failures reject. Cancellation and progress are forwarded. A failed call is never automatically replayed; a disconnected or expired session reconnects for the next operation.

OAuth sign-in, resources, prompts, sampling, and MCP Apps are not exposed in this initial version. Servers needing authentication must accept credentials supplied in headers or environment variables.

The initial installation also adds streaming Environment.fetch and forceful process termination to Berry's API. Use the header's restart button once to load those changes.