/**
 * Transport factory: creates the appropriate MCP transport based on the
 * plugin's resolved config. Stdio spawns a child process (with credential
 * scrubbing); Streamable HTTP connects to a URL.
 *
 * @module
 */

import type { Transport } from '@modelcontextprotocol/sdk/shared/transport.js'
import { StdioClientTransport } from '@modelcontextprotocol/sdk/client/stdio.js'
import { StreamableHTTPClientTransport } from '@modelcontextprotocol/sdk/client/streamableHttp.js'
import type { Config } from './index.ts'

/**
 * Credential-shaped ambient env vars are NOT forwarded to the child by default
 * (the parent harness's own secrets must not leak into a spawned process
 * implicitly). Same pattern as `dsh-subagent-acp`.
 */
const SENSITIVE_ENV_PATTERN = /KEY|SECRET|TOKEN/i

/** The ambient env minus credential-shaped vars, plus the spec's explicit env. */
function buildChildEnv(extra: Record<string, string>): Record<string, string> {
  const env: Record<string, string> = {}
  for (const [key, value] of Object.entries(process.env)) {
    if (value !== undefined && !SENSITIVE_ENV_PATTERN.test(key)) env[key] = value
  }
  return { ...env, ...extra }
}

/**
 * Create an MCP transport from the resolved plugin config.
 *
 * @param config - Resolved plugin config discriminated on `transport`.
 * @returns A connected-ready MCP Transport (stdio or Streamable HTTP).
 */
export function createTransport(config: Config): Transport {
  switch (config.transport) {
    case 'stdio':
      return new StdioClientTransport({
        command: config.command,
        args: config.args,
        env: buildChildEnv(config.env),
        cwd: config.cwd,
      })
    case 'streamable-http':
      // The MCP SDK's StreamableHTTPClientTransport has optional callback
      // properties typed without `| undefined` (exactOptionalPropertyTypes
      // mismatch with the Transport interface). The cast is safe — the SDK
      // constructed the object, it simply doesn't declare the optionals
      // strictly enough for our tsconfig.
      return new StreamableHTTPClientTransport(
        new URL(config.url),
        { requestInit: { headers: config.headers } },
      ) as Transport
  }
}