# ADR-0002: Streamable HTTP as the sole MCP transport

**Status:** Accepted (2026-08-08)

## Context

MCP defines stdio and Streamable HTTP transports; an older HTTP+SSE transport is deprecated in
favor of Streamable HTTP. Bridle is a networked gateway intended to be reachable from agents over
WireGuard, which rules out stdio as the primary transport. A local stdio shim bridging to the HTTP
endpoint was considered for clients with weak Streamable HTTP support, as was continuing to serve
the deprecated SSE transport.

## Decision

Serve **Streamable HTTP only**, at `/mcp`. No stdio shim and no deprecated SSE endpoint at v1.

## Consequences

- Matches the per-agent bearer token model (ADR-0003) directly — the token rides the
  `Authorization` header.
- Works over WireGuard without extra components.
- One transport to implement, test, and secure.
- If a target agent turns out to have unusable Streamable HTTP support, a `bridle-stdio` bridging
  binary is the fallback. It is not in scope until a specific agent demonstrably needs it.