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ARC Now Supports the MCP 2026-07-28 Specification

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ARC has supported MCP (Model Context Protocol) for some time. Local daemons, the arc mcp bridge, and exposing the AFS tool surface to models were already part of how agents reached our stack.

What is new is that our MCP surface now follows the 2026-07-28 specification's deploy and auth model. Blocklets on ARC can be discovered and called the way that revision expects. Claude Code, Codex, and similar clients have completed direct-connect checks on local, staging, and production paths.

What MCP is

MCP is an open convention for giving models one way to reach tools and data, instead of a custom integration for every product. If an agent can discover tools and invoke them, it can talk to different services in the same shape.

For us, MCP has long been one of the protocol faces agents use to touch AFS and Blocklet capabilities, not a one-off demo endpoint.

What the 2026-07-28 revision changes

The MCP revision published on 28 July 2026 pushes harder on deployability:

  • A more stateless transport face: less dependence on long-lived session or handshake state, so request/response shapes fit multi-instance and restart-friendly deployments.
  • Auth in a standard OAuth direction: protected resources can declare authorization servers; clients complete discovery and negotiation from metadata, instead of vendor-private "paste a token" flows.

Several runtimes and gateways in the industry have been updating along that line. "We already speak MCP" is not the same as shipping the new deploy model. The difference is whether stateless transport and standard authorization discovery are actually in place.

How we support the new specification

On top of the MCP surface we already had, this upgrade lands the pieces that match 2026-07-28:

  • Stateless streamable HTTP: production and local paths share a stateless /mcp endpoint. The server does not need a long-lived session just to keep talking. A process restart no longer surfaces to clients as "not initialized."
  • Standard authorization discovery: write paths use protected-resource metadata (for example /.well-known/oauth-protected-resource) and a Bearer path. A client can complete "discover → register → user grants in the browser → receive a credential" without pasting a key first. Public reads may stay anonymous; write / delete / exec require authorization.
  • Machine-readable discovery: each deployed Blocklet exposes a card such as /.well-known/mcp.json next to /mcp, so an agent can start from a hostname.
  • Direct connect first: third-party code agents have been verified to reach online Blocklets without a mandatory CLI SSE bridge. Clients that only speak stdio can still use arc mcp as a local compatibility bridge.

On verification: Claude Code and Codex CLI completed direct connect on local, staging, and production; sampled production Blocklets returned a usable MCP surface. Full probe data lives in the internal trackers that closed this work.

How to use it

Discover a site's MCP entry points:

text
https://<your-blocklet-host>/.well-known/mcp.json
https://<your-blocklet-host>/mcp

Example (an ARC-hosted site instance):

text
https://new.arcblock.io/.well-known/mcp.json
https://new.arcblock.io/mcp

In a client that supports streamable HTTP, add that MCP URL as a remote server. For a local daemon:

bash
arc mcp

Docs: arc mcp. Glossary: MCP.

Known limitations

  • MCP clients do not all behave the same way. Claude Code and Codex try anonymous read first, then authorize if refused. Some clients, when they see OAuth support advertised, insist on authorizing before any call, so the public-read path may look "unreachable" to them. That is a client policy choice; we will not hide auth discovery to paper over one vendor.
  • One Claude Desktop UI confirmation still needs a human click on the machine. Command-line connect paths already work.
  • A few internal call sites (for example elicitation during provider mount) still follow the prior implementation and will tighten as the official SDK continues to ship; that does not remove the external stateless transport and standard auth discovery described above.

Links

Referenced here

Products

  • ARC active

    The runtime for Blocklets. It gives a developer a place to run an application described as a Blocklet, together with the resources that Blocklet declares it needs.

Terms

  • MCP

    Model Context Protocol: an open way for a model to reach tools and data through one declared interface instead of a bespoke integration each time. It matters here because it is the same problem AFS answers from the other side; what a task is allowed to see.