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Default missing MCP-Protocol-Version to 2025-03-26 in StreamableHTTPTransport

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## Motivation and Context

MCP 2025-11-25 (basic/transports#protocol-version-header) specifies:

> For backwards compatibility, if the server does not receive an
> `MCP-Protocol-Version` header, and has no other way to identify the
> version - for example, by relying on the protocol version negotiated
> during initialization - the server SHOULD assume protocol version `2025-03-26`.

PR #347 satisfied only the MUST requirement (reject invalid/unsupported
values with 400). The SHOULD requirement to default a missing header to
`2025-03-26` was deferred to this follow-up.

This change brings the Ruby SDK in line with the Python SDK
(`src/mcp/server/streamable_http.py`), which falls back to
`DEFAULT_NEGOTIATED_VERSION` and then revalidates against the supported list.

## Behavior

A new constant `MCP::Configuration::DEFAULT_NEGOTIATED_PROTOCOL_VERSION`
holds `"2025-03-26"`. `validate_protocol_version_header` now substitutes
this value when the `MCP-Protocol-Version` header is absent, then runs
the same supported-version check. Because `"2025-03-26"` is currently in
`SUPPORTED_STABLE_PROTOCOL_VERSIONS`, missing headers continue to pass
through as before. Once `"2025-03-26"` is dropped from the supported
list in a future change, missing headers will start returning 400 with
a message naming the defaulted value.

## How Has This Been Tested?

Existing test `POST request without MCP-Protocol-Version header
succeeds` continues to pass (observable behavior unchanged today).
Added a focused regression test that stubs the supported-version list
to exclude `"2025-03-26"` and verifies that a header-less request is
rejected with the defaulted value reported in the error message;
this exercises the fallback branch directly.

## Breaking Changes

None today. The fallback is future-proofing: when `"2025-03-26"` is
eventually dropped, header-less requests will start returning 400.
That is the intended spec-driven behavior.
This commit is contained in:
Koichi ITO 2026-06-07 10:18:33 +09:00
commit 4322ec0886
2 changed files with 46 additions and 2 deletions
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1
lib/mcp/configuration.rb
View file

@ -6,6 +6,7 @@ module MCP
SUPPORTED_STABLE_PROTOCOL_VERSIONS = [
LATEST_STABLE_PROTOCOL_VERSION, "2025-06-18", "2025-03-26", "2024-11-05",
]
DEFAULT_NEGOTIATED_PROTOCOL_VERSION = "2025-03-26"
attr_writer :exception_reporter, :around_request

3
lib/mcp/server/transports/streamable_http_transport.rb
View file

@ -543,8 +543,7 @@ module MCP
end
def validate_protocol_version_header(request)
header_value = request.env["HTTP_MCP_PROTOCOL_VERSION"]
return if header_value.nil?
header_value = request.env["HTTP_MCP_PROTOCOL_VERSION"] || MCP::Configuration::DEFAULT_NEGOTIATED_PROTOCOL_VERSION
return if MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.include?(header_value)
supported = MCP::Configuration::SUPPORTED_STABLE_PROTOCOL_VERSIONS.join(", ")