Parse HTTP client response bodies without Faraday's JSON middleware

[koic]

Motivation and Context

json 3.0, released on 2026-09-07, accepts the options of JSON.parse as keywords only, and Faraday's json response middleware, through 2.14.3, hands them over as a positional Hash, which Ruby 3 no longer converts. With both installed, every JSON body the client receives fails inside that middleware, and MCP::Client::HTTP reports each request as an internal error. Faraday's main branch passes the options as keywords, but no release carries that yet.

The client already parses streamed bodies and buffered SSE bodies itself; only a JSON body delivered whole by an adapter without streaming support still went through the middleware. The client now parses that body too and no longer registers the middleware, so every body takes parse_json_buffer and a malformed one answers :parse_error whichever way it arrived. A JSON middleware added through the Faraday customizer block still works, since a body it has parsed already is used as is.

json 3.0 also declares Ruby 2.7 as its floor, but its load_file forwards arguments after a leading parameter, def load_file(filespec, ...), syntax Ruby 2.7.3 was the first to parse. On Ruby 2.7.0 through 2.7.2, requiring json 3.0.1 is a SyntaxError, so require "mcp" fails before any test runs, and the CI job pinned to 2.7.0, the minimum the gemspec supports, fails the whole suite at load time. The Gemfile keeps json below 3.0 on those three patch levels only. Later 2.7 releases and Ruby 3 parse the syntax and stay on whatever json the resolver picks. The line carries a FIXME to drop it once a json release includes ruby/json#1072, whichever version that turns out to be; excluding only the two broken releases would silently trust a 3.0.2 that may not carry the fix.

How Has This Been Tested?

A new test drives a buffered JSON response through a stand-in JSON.parse with the json 3.0 signature, options as keywords only, so on Ruby 3 it failed with the middleware registered whatever json version is installed and passes now. The existing buffered-body test keeps covering the plain path. The full suite passes on Ruby 3.0.7 with json 3.0.1 and faraday 2.14.3, where five client tests failed before.

gem "json", "3.0.1"; require "json" raises the SyntaxError on Ruby 2.7.0 and loads on 2.7.3 and 2.7.8. With the pin, a fresh bundle install on Ruby 2.7.2 resolves json 2.x and bundle exec rake test passes.

Breaking Changes

None. A malformed JSON body delivered by an adapter without streaming support now raises RequestHandlerError with error_type: :parse_error, as the streamed path already did, instead of :internal_error wrapping a Faraday::ParsingError. The Gemfile is not part of the gem.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

[koic]

I'll merge this due to the CI failure.