A3.3 · LESSON 3 OF 6

Transports

Explain MCP roles and lifecycle and trace a tool-discovery and tool-call exchange under a pinned protocol version.

THE IDEA

Transports

A transport carries the JSON-RPC messages. The standard ones are stdio (the client launches a local process and talks to it over its standard input and output) and HTTP-based communication. The messages are identical either way; connection setup, authentication and deployment differ. A local stdio server is a process on your machine, not a public endpoint, while an HTTP server is reachable by others and needs network authentication.

calls5ms per HTTP call (toy)40ms per stdio call (toy)1calls in flight at once1

HTTP-based: 5 round(s) × 40 ms = 200 ms stdio (local process): 5 round(s) × 1 ms = 5 ms the JSON-RPC messages are the same either way; what differs is setup, authentication and deployment

	stdio	HTTP-based
what it is	a local process the client launches; messages over its standard input/output	a server reachable over the network at a URL
network authentication	not applicable: the user starts the process	required: it is reachable by others
good fit	one user, local tools	shared or remote services

Toy latencies for sequential and overlapping calls.

Predict first. Is a local stdio server automatically an internet HTTP endpoint?

Chapter context

MCP defines a protocol for applications to connect models with external capabilities. It standardizes exchanges; it does not grant a server universal authority or make every discovered tool appropriate for every user request.

TRY IT YOURSELF

Part 1. Do it hands-on

Cost calls over each transport by hand, then write the helpers and a transport chooser.

calls5ms per HTTP call (toy)40ms per stdio call (toy)1calls in flight at once1

HTTP-based: 5 round(s) × 40 ms = 200 ms stdio (local process): 5 round(s) × 1 ms = 5 ms the JSON-RPC messages are the same either way; what differs is setup, authentication and deployment

	stdio	HTTP-based
what it is	a local process the client launches; messages over its standard input/output	a server reachable over the network at a URL
network authentication	not applicable: the user starts the process	required: it is reachable by others
good fit	one user, local tools	shared or remote services

5 HTTP calls in sequence (ms) 5 stdio calls (ms) 5 HTTP calls all in flight (ms) stdio is automatically an HTTP endpoint? (yes/no)

To do: Toy numbers: each call over an HTTP-based transport takes 40 ms. Five calls one after another take how many ms?
To do: Toy numbers: each call to a local stdio server takes 1 ms. Five calls in sequence?
To do: The five HTTP calls are independent and all five can be in flight at once. Total ms (assume they overlap perfectly)?
To do: Is a local stdio server automatically an internet HTTP endpoint? Type yes or no

Part 2. Do it in code

total_latency(n_calls, per_call_ms, parallel=1): ceil(n_calls / parallel) rounds of per_call_ms.
encode(message): the dict as a compact JSON string (separators "," and ":", no spaces, keys in insertion order).
decode(text): the dict back; ValueError if the text is not a JSON object.
same_over_any_transport(message): True if decode(encode(message)) == message (the JSON-RPC message does not depend on how it travels).
choose_transport(local_process, multi_user): a heuristic: "stdio" if the server is a local process used by one user, otherwise "http".
needs_network_auth(transport): True for "http", False for "stdio" (a local process is started by the user and inherits their environment); ValueError for an unknown transport.

import json
import math

def total_latency(n_calls, per_call_ms, parallel=1):
    pass

def encode(message):
    pass

def decode(text):
    pass

def same_over_any_transport(message):
    pass

def choose_transport(local_process, multi_user):
    pass

def needs_network_auth(transport):
    pass

Ctrl/⌘ + Enter runs it

Stuck, or something not working? You can come back to it any time.

CHECK YOUR UNDERSTANDING

Is a local stdio server automatically an internet HTTP endpoint?

ANoBYes

Chapter reference: math, worked example & code

This section belongs to the whole chapter, so it is the same on every lesson in it and may cover a different lesson's topic. The code for the experiment you just ran is inside the experiment itself.

Math and notation

The MCP specification revision examined here is 2026-07-28. Request envelopes use JSON-RPC 2.0, an ID for response correlation, a method, and method-specific parameters that include _meta with the required io.modelcontextprotocol/protocolVersion and io.modelcontextprotocol/clientCapabilities. A successful result carries resultType, here "complete". Do not treat this date as a claim about every installed client or SDK.

Worked example

A tools/call request with id 7 names add, supplies a=2,b=3 and carries the protocol version and client capabilities in _meta. The server runs the permitted operation and answers with the same id and a content array holding the text 5. The offline example below checks envelope fields and arithmetic; a production server must also implement discovery, schema validation, errors and transport behavior.

Reference code

The following example uses the Python standard library and does not call a remote service.

meta={'io.modelcontextprotocol/protocolVersion':'2026-07-28','io.modelcontextprotocol/clientCapabilities':{}}
request={'jsonrpc':'2.0','id':7,'method':'tools/call','params':{'name':'add','arguments':{'a':2,'b':3},'_meta':meta}}
args=request['params']['arguments']
total=args['a']+args['b']
response={'jsonrpc':'2.0','id':request['id'],'result':{'resultType':'complete','content':[{'type':'text','text':str(total)}],'isError':False}}
assert request['method'] == 'tools/call'
assert request['params']['_meta']['io.modelcontextprotocol/protocolVersion'] == '2026-07-28'
assert response['id'] == 7
assert response['result']['content'][0]['text'] == '5'
assert response['result']['isError'] is False

Python examples are provided for study and local execution; this page does not execute Python.

Common mistakes

MCP is not a language model or an automatic permission system. Resource, prompt and tool capabilities differ. Do not assume an initialize handshake: that belongs to revisions before 2026-07-28. Protocol versions and SDK releases must be matched when turning this draft into runnable setup instructions.