Invalid UTF-8 in MCP-Protocol-Version, Mcp-Method or Mcp-Name gives 500 instead of 400 #663

On the modern path, StreamableHTTPTransport puts raw header values into JSON error bodies:

• the unsupported-version error's data.requested (streamable_http_transport.rb:908)
• the Mcp-Method and Mcp-Name mismatch messages (:998-1026)

Rack gives header values as binary strings. Also, decode_header_value tags a decoded =?base64?...?= value as UTF-8 without checking it (:1586-1592), so an all-ASCII Mcp-Name: =?base64?/w==?= also produces an invalid string. json_rpc_error_response then calls to_json (:2064-2070), which raises JSON::GeneratorError. The rescue at :968-974 reports the exception and answers 500.

Related: #629 (a reply that cannot be generated gets Internal error) and #609 (another header that caused a 500). Open #654 adds a strict UTF-8 check when it decodes Mcp-Param-* values, but it does not change decode_header_value or these error bodies.

Repro

Run from the repository root with bundle exec ruby -I lib repro.rb:

require "mcp"
require "rack"

reported = []
MCP.configure { |config| config.exception_reporter = ->(error, _context) { reported << error.class } }
server = MCP::Server.new(name: "example")
transport = MCP::Server::Transports::StreamableHTTPTransport.new(server)

env = Rack::MockRequest.env_for(
  "http://localhost/",
  method: "POST",
  input: JSON.generate(jsonrpc: "2.0", id: 1, method: "tools/list"),
  "CONTENT_TYPE" => "application/json",
  "HTTP_ACCEPT" => "application/json, text/event-stream",
  "HTTP_MCP_PROTOCOL_VERSION" => "2026-07-28\xFF".b,
)
status, _headers, body = transport.call(env)
p [status, body.join, reported]
transport.close

Expected: 400 with "Unsupported protocol version", and no exception report. For the Mcp-Method and Mcp-Name cases, 400 with the header mismatch error (-32020).

Actual:

[500, "{\"jsonrpc\":\"2.0\",\"error\":{\"code\":-32603,\"message\":\"Internal server error\"}}", [JSON::GeneratorError]]

The same 500 happens with invalid bytes in Mcp-Method or Mcp-Name on a modern tools/call, and with Mcp-Name: =?base64?/w==?=.

Suggested fix

Check header values with valid_encoding? (after tagging them as UTF-8) before using them. Leave invalid values out of error bodies, or scrub them. Make decode_header_value refuse a decoded value that is not valid UTF-8, the same way the codec in #654 does.