Ch 3: Transports — Under the Hood

Ch 3 — Transports Under the Hood

stdio byte framing, HTTP request/response lifecycle, SSE wire format, session tokens, and reconnection

Index ← High Level

Under the Hood

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Astdio InternalsProcess spawning, pipe wiring, and newline-delimited JSON

terminal

Spawn Processfork + exec with
stdin/stdout pipes

wire

cable

Pipe Pairhost.stdin → server
server → host.stdout

frame

wrap_text

NDJSONNewline-delimited
JSON framing

arrow_downward stdio edge cases: buffering, encoding, and stderr

Bstdio Edge CasesBuffering, UTF-8, stderr, and EOF detection

speed

Flush BuffersMust flush after
every message

translate

UTF-8 OnlyAll text must be
valid UTF-8

report

stderr = LogsNever mix logs
with MCP messages

arrow_downward HTTP POST: sending requests to the server

CHTTP POST Request FlowClient sends JSON-RPC, server responds with JSON or SSE

upload

POST /mcpContent-Type:
application/json

rule

Server DecidesJSON response
or SSE stream?

200 OK

download

Responseapplication/json
or text/event-stream

arrow_downward SSE wire format: event types and data framing

DSSE Wire FormatHow Server-Sent Events carry MCP messages

code

event: messageEvent type for
all MCP messages

data_object

data: JSONFull JSON-RPC
message on one line

tag

id: fieldLast-Event-ID for
reconnection

arrow_downward Session management: tokens, headers, and lifecycle

ESession Token LifecycleHow Mcp-Session-Id flows through HTTP requests

InitializeNo session ID yet
first POST

response

key

Get Session IDServer sets header
Mcp-Session-Id

reuse

repeat

All RequestsClient sends ID
on every request

arrow_downward Reconnection and session resumption

FReconnection & ResumptionRecovering from network drops without re-initializing

wifi_off

Connection LostNetwork drop or
server restart

retry

sync

ReconnectGET /mcp with
same session ID

resume

check_circle

ResumedServer replays
missed events