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mcp

ARCP runtime that fronts an MCP server. Inbound tool.invoke envelopes translate to MCP call_tool; the bridge emits the ARCP job lifecycle back to the calling client.

Before ARCP

You either (a) ditch your ARCP-native session/lease/observability story and run MCP straight, losing the runtime layer; or (b) embed MCP into one specific agent that knows how to call it directly, which doesn't compose with the rest of your stack. Wrap one, re-wrap the other.

With ARCP

Per RFC §20:

MCP ARCP
tool schema capability (arcpx.mcp.tool.<name>.v1)
tool call job (tool.invoke → job.completed)
resource stream of kind: event (delegated)

The bridge advertises the upstream server's tools as namespaced capability extensions at session open. Clients that need a specific MCP tool refuse the session if it's not advertised — same shape as any other ARCP capability negotiation.

$mcp = McpClientSession::stdio(upstreamParams());
$mcp->initialize();
$extensions = advertiseFromMcp($mcp);   // MCP tool list -> arcpx.mcp.tool.*.v1
foreach ($inbound as $env) {
    if ($env->payload instanceof ToolInvoke) {
        handleInvoke($send, $mcp, $env);
    }
}

callViaMcp translates MCP errors into canonical ARCP error codes (FAILED_PRECONDITION for result.isError, INTERNAL for unexpected exceptions at the boundary).

ARCP primitives

  • MCP compatibility — RFC §20 (the whole point).
  • tool.invoke / job.accepted / job.started / job.completed / job.failed lifecycle — §6.3, §10.
  • Capability extensions for advertised tools — §7, §21.
  • Canonical error mapping — §18.2.

File tour

  • main.php — the bridge. handleInvoke is the file. Runtime wiring (transport, session manager) is symmetric with Arcp\Runtime\ARCPRuntime and elided.
  • upstream.php — MCP server invocation + stub McpClientSession. No first-class MCP PHP SDK exists yet; vendor a bridge.

Variations

  • Front multiple MCP servers from one ARCP runtime; namespace each set of tools under arcpx.mcp.<server>.tool.<name>.v1.
  • Bridge MCP resources to ARCP streams of kind: event so ARCP observers can subscribe to MCP resource changes.
  • Layer ARCP leases on top: gate tool.invoke for any side-effecting MCP tool through permission.request before forwarding to MCP.