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## Motivation and Context An `Accept` header with an empty list member, such as `application/json,,text/event-stream` or one with a leading comma, makes the Streamable HTTP transport answer a POST with 500 and report a `NoMethodError` to the exception reporter, and raise out of a GET, although the required media types are listed (issue modelcontextprotocol#609): `parse_accept_header` splits each member on `;` and an empty member yields nothing to strip. RFC 9110 section 5.6.1.2 has a recipient parse and ignore a reasonable number of empty list elements, and the TypeScript and Python servers accept such a header, the one by substring checks and the other by stripping each member. `parse_accept_header` now skips empty members, so the POST and GET checks see the media types that are there. Fixes modelcontextprotocol#609. ## How Has This Been Tested? Two new tests in `test/mcp/server/transports/streamable_http_transport_test.rb`, a POST with a doubled and with a leading comma and a GET with a leading comma. Against the previous library the POST test fails with a 500 and the GET test raises. ## Breaking Changes None.
atesgoral
approved these changes
Oct 15, 2026
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Motivation and Context
An
Acceptheader with an empty list member, such asapplication/json,,text/event-streamor one with a leading comma, makes the Streamable HTTP transport answer a POST with 500 and report aNoMethodErrorto the exception reporter, and raise out of a GET, although the required media types are listed (issue #609):parse_accept_headersplits each member on;and an empty member yields nothing to strip. RFC 9110 section 5.6.1.2 has a recipient parse and ignore a reasonable number of empty list elements, and the TypeScript and Python servers accept such a header, the one by substring checks and the other by stripping each member.parse_accept_headernow skips empty members, so the POST and GET checks see the media types that are there.Fixes #609.
How Has This Been Tested?
Two new tests in
test/mcp/server/transports/streamable_http_transport_test.rb, a POST with a doubled and with a leading comma and a GET with a leading comma. Against the previous library the POST test fails with a 500 and the GET test raises.Breaking Changes
None.
Types of changes
Checklist