Transports

presa's protocol handling is pure functions over request and response maps; transports carry them. presa.connection is what stdio and the in-process transport build on: one client's session, each request on its own thread, and cancellation. The HTTP server keeps sessions itself.

stdio

presa.transport.stdio/serve! serves a server on stdin and stdout until stdin closes, then waits for requests in flight. Options:

Option	Default	Meaning
:in, :out	stdin, stdout	a Reader and a Writer, for tests
:log	a line on stderr	(fn [& parts]) for diagnostics
:max-line-bytes	2000000	a longer line gets -32600 and is skipped

presa.transport.stdio/command is the client side: it starts a server subprocess and speaks to it. Options :env, :dir, :stderr (:inherit, or a function of each line) and :close-timeout-ms.

(client/connect (stdio/command ["bb" "-m" "my.server"] {:dir "/path/to/project"}))

Streamable HTTP

presa.transport.http/serve! runs an http-kit server; handler returns the ring handler, to mount in another http-kit server.

(require '[presa.client :as client]
         '[presa.server :as server]
         '[presa.transport.http :as http])

(def srv (server/server {:name "web" :version "1"
                         :tools [{:name "ping" :description "Pong" :handler (fn [_ _] "pong")}]}))

(def running (http/serve! srv {:port 0}))   ; 0 picks a free port

(def c (client/connect (http/url (:url running))))

(client/text (client/call-tool c "ping"))
;; => "pong"

(client/close c)
((:stop! running))

What the server enforces, after snodo and the conformance runner:

• POST only, on one path (:path, default /mcp); GET and DELETE only for initialize-era sessions.
• Content-Type: application/json (415 otherwise) and an Accept that lists application/json and text/event-stream (406).
• On 2026-07-28, MCP-Protocol-Version, Mcp-Method and, for tools/call, prompts/get, resources/read and the task methods, Mcp-Name must match the body (400 with -32020). Mcp-Param-* headers must match the arguments a tool's schema mirrors with x-mcp-header.
• Errors get the status the runner expects (-32601 is 404, -32602 400, -32603 500); initialize-era clients get 200, as they treat other statuses as transport failures.
• A response is JSON until the handler sends progress or log messages, then an event stream.

Options of handler and serve!:

Option	Default	Meaning
:host, :port (serve! only)	127.0.0.1, 0	where to listen
:path	/mcp	the endpoint
:allowed-origins	loopback only	a set or predicate of other origins to accept (403 otherwise; guards against DNS rebinding)
:allowed-hosts	any	a set or predicate of Host values to accept
:max-body-bytes	2000000	a larger body gets 413
:keepalive-ms	15000	comments on idle subscription streams, which also find clients that left
:auth	none	a resource-server config; see authorization.md
:cancellation	none	:subject lets a posted cancel stop the same subject's request
:log	none	(fn [& parts]) for diagnostics

presa.transport.http/url is the client transport. It mirrors the protocol headers (base64-wrapping values that are not plain ASCII), sends Mcp-Param-* for annotated arguments, leaves out tools whose annotations are invalid, keeps initialize-era sessions and their GET stream, and resumes a stream that ended early with Last-Event-ID. :headers adds headers to every request, such as Authorization; :token-provider does OAuth (see authorization.md); :connect-timeout-ms (default 10000) and :log are the others.

Initialize-era sessions

Over stdio the connection is the session. Over HTTP, initialize answers with an Mcp-Session-Id that later requests carry; GET opens the session's event stream, where the server sends requests and notifications outside any request; DELETE ends the session. See initialize-era.md.