Wymcp.
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Single source of truth for MCP protocol version support.
wymcp speaks the modern era of MCP only — exactly 2026-07-28, with its
per-request protocol fields, no negotiation and no sessions. Every earlier
revision is answered -32022 by Wymcp.Plugs.ProtocolFields, naming
this one. This module also owns the two protocol-field names —
protocol_version_field/0 and client_capabilities_field/0 — so
Wymcp.Plugs.ProtocolFields and Wymcp.Plugs.HeaderBinding read one copy
of each wire literal.
Summary
Functions
The params._meta key carrying the client capabilities — the sibling of
protocol_version_field/0, which carries the rationale.
The params._meta key carrying the protocol version
(protocol fields).
Every version wymcp serves — the list server/discover advertises as
supportedVersions and the -32022 error carries as data.supported.
Functions
The params._meta key carrying the client capabilities — the sibling of
protocol_version_field/0, which carries the rationale.
Examples
iex> Wymcp.ProtocolVersion.client_capabilities_field()
"io.modelcontextprotocol/clientCapabilities"The params._meta key carrying the protocol version
(protocol fields).
Both plugs that traffic in the protocol fields read this accessor:
Wymcp.Plugs.ProtocolFields enforces the value, and
Wymcp.Plugs.HeaderBinding compares the enforced value against the
MCP-Protocol-Version header that mirrors it. The literal lives here
rather than in either plug so the reverse-DNS string is typed once — a
drift between two hand-typed copies would refuse a compliant client
instead of failing loudly.
Examples
iex> Wymcp.ProtocolVersion.protocol_version_field()
"io.modelcontextprotocol/protocolVersion"Every version wymcp serves — the list server/discover advertises as
supportedVersions and the -32022 error carries as data.supported.
Examples
iex> Wymcp.ProtocolVersion.supported_versions()
["2026-07-28"]