The shared full-duplex core of the MCP **stdio** transport, used by BOTH the client and the server. The stdio transport is a single newline-delimited JSON-RPC byte stream carrying traffic in both directions at once (basic/transports §stdio), so concurrency is structured as one cooperative vibe read-loop task that demultiplexes inbound lines, plus a serialized writer:
A generous default upper bound (16 MiB) on a single inbound JSON-RPC line for the stdio transport, guarding against unbounded line accumulation: a peer that never sends a newline would otherwise grow the read accumulator without limit (a memory-exhaustion DoS). runStdio and spawnStdioTransport accept an override; pass size_t.max to effectively disable the bound.