## Motivation and Context
CRuby's `IO#gets` without a limit reads until the next newline with no upper bound.
A peer that never emits a newline can therefore grow a single String until the process
is killed by the OS OOM killer.
This is a denial-of-service exposure (GHSA-7683-3w9x-ch42), identified during
a cross-SDK audit of the same unbounded-buffer pattern across the MCP SDKs.
It is most relevant when a host application spawns a sandboxed or untrusted MCP server
and pipes its stdout into an unconstrained host process, where a memory-capped peer
can exhaust the host.
Each frame is now capped at a configurable `max_line_bytes` (default 4 MiB,
large enough for any realistic frame including base64-embedded images,
and the value recommended in the advisory). A frame that reaches the limit without
a newline is treated as a transport error: the client raises, and the server reports
the error and closes the connection. A short final frame that ends at EOF without
a trailing newline is still accepted, so legitimate traffic is unaffected.
Both constructors validate `max_line_bytes:` as a positive Integer,
since `nil` or a non-positive value makes `IO#gets` read without an upper bound and
would silently disable the protection.
## How Has This Been Tested?
Server tests cover the `open` read loop stopping and reporting on an over-limit frame,
the `send_request` read loop raising on an over-limit response, and a valid final frame
without a trailing newline still being processed.
Client tests cover `read_response` raising on an over-limit response frame and
a normal response still round-tripping within a custom `max_line_bytes`.
Both transports also have tests rejecting invalid `max_line_bytes:` values
(`nil`, zero, negative, non-Integer) with an `ArgumentError`.
The full test suite and RuboCop pass.
## Breaking Changes
None. The default 4 MiB limit is far larger than any realistic JSON-RPC frame,
so existing well-behaved traffic is unaffected. The new `max_line_bytes:` keyword
on both transport constructors is purely additive and optional.
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