MCP binding

The first A2M binding is an MCP server. Any MCP client that supports remote servers can connect an agent to a market through it.

Transport

• MCP over Streamable HTTP, stateless, JSON responses.
• The agent authenticates the HTTP request with Authorization: Bearer <token>. Tools never take a token as an argument, so the credential never enters the model’s context.
• Without a valid bearer the server answers 401 with APEX_4001 and a WWW-Authenticate header. When OAuth is configured, that header carries resource_metadata pointing at the protected-resource document. See OAuth binding.
• Server instructions tell the model to prefer a2m_place_order, and that a PENDING_APPROVAL result means a human must approve.

Tools

Tool	Input	Annotations	Maps to
a2m_whoami	none	read-only	The agent this token belongs to
a2m_check	Intent	read-only	Dry run of every rule. Reserves nothing; cannot be sent
a2m_place_order	Intent	write, idempotent	Authorize; if ACCEPT or RESIZE, send the signed decision to the executor. Returns the verdict and, when sent, the order. The same intent_id never creates a second order
a2m_submit_decision	{ decision, signature }	write, idempotent	Send an already signed ACCEPT or RESIZE, for example one produced by an approval
a2m_order_status	{ id } (client order id)	read-only	One of this agent’s orders, its fills and cost analysis
a2m_cancel_order	{ id } (client order id)	write	Ask the venue to cancel one of this agent’s working orders
a2m_approval_status	{ id } (approval id)	read-only	Whether a human approved or denied a PENDING_APPROVAL order

The input schema of a2m_check and a2m_place_order is the same intent object the REST API decodes, so the schema an agent sees and the one the authorizer validates cannot drift.

Results

• A refusal from the authorizer or executor (a REJECT, or an HTTP error such as A2M_BAD_SIGNATURE) is returned as a tool result with isError: true and the error body as text, so the model can read it and act on it. It is not an MCP protocol failure.
• a2m_place_order returns verdict, code, message, approval_id, decision, signature and, when the order was sent, order.

Approval flow over MCP

1. a2m_place_order returns PENDING_APPROVAL and an approval_id.
2. Poll a2m_approval_status with the approval_id.
3. When it is approved, pass the signed decision exactly as returned to a2m_submit_decision. Changing any signed field makes it fail with A2M_BAD_SIGNATURE.

REST equivalents

The same objects are available over HTTP: GET /v1/whoami, POST /v1/check, POST /v1/authorize, GET /v1/approvals/{id}, POST /v1/orders, POST /v1/orders/{id}/cancel, GET /v1/agent/orders/{id}, GET /v1/keys. A formal REST binding document is planned.