Wymcp.
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Single source of truth for MCP protocol version support.
The legacy era (docs/glossary.md, era) accepts exactly one revision:
2025-11-25. The 2025-03-26 and 2025-06-18 revisions were dropped
pre-1.0 — every observed real client requests 2025-11-25, and with one
revision there is nothing left to gate per feature (the supports_*?
predicates and strip_* helpers this module used to carry are gone with
them). The split-endpoint 2024-11-05 revision remains
intentionally unsupported: it predates the Streamable HTTP transport —
accepting the version string would let initialize succeed while every
subsequent request misbehaves.
The modern era serves exactly 2026-07-28 (per-request protocol
fields — no negotiation, no sessions). This module also owns the two
modern protocol-field names — protocol_version_field/0 and
client_capabilities_field/0 — so Wymcp.Plugs.Era and
Wymcp.Plugs.ProtocolFields read one copy of each wire literal.
Counter-proposal
When the client requests an unsupported version, the spec requires the
server to respond with a version it does support (echoing back the same
InitializeResult shape — not a JSON-RPC error). This module does not
implement that policy; Wymcp.Methods.Initialize does. latest/0
exists so that call site has a single canonical fallback.
Summary
Functions
The params._meta key carrying the modern era's client capabilities —
the sibling of protocol_version_field/0, which carries the rationale.
The params._meta key carrying the modern era's protocol version
(docs/glossary.md, protocol fields).
Every version either era serves, modern first — the list
server/discover advertises as supportedVersions and the -32022
error carries as data.supported, so a version-rejected client can see
its legacy fall-back option in the same answer.
Functions
The params._meta key carrying the modern era's 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 modern era's protocol version
(docs/glossary.md, protocol fields).
Both plugs that traffic in the protocol fields read this accessor:
Wymcp.Plugs.Era classifies on the key's presence,
Wymcp.Plugs.ProtocolFields enforces the value. 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 mis-route a compliant client
onto the wrong lane instead of failing loudly.
Examples
iex> Wymcp.ProtocolVersion.protocol_version_field()
"io.modelcontextprotocol/protocolVersion"Every version either era serves, modern first — the list
server/discover advertises as supportedVersions and the -32022
error carries as data.supported, so a version-rejected client can see
its legacy fall-back option in the same answer.
Examples
iex> Wymcp.ProtocolVersion.supported_versions()
["2026-07-28", "2025-11-25"]