modelcontextprotocol--python-sdk/examples/stories/dual_era

dual-era

One server factory, both protocol eras. A mode="legacy" client runs the initialize handshake; a mode="auto" client probes server/discover and adopts the 2026 stateless era — the same greet tool answers both and reports which era served it via ctx.request_context.protocol_version. Start here when migrating a v1 server: the entry owns the era decision, the server body stays era-agnostic.

Run it

# over HTTP — the same /mcp endpoint serves both eras; the client self-hosts
# the server on a free port, runs, then tears it down
uv run python -m stories.dual_era.client --http
# same, against the lowlevel-API server variant
uv run python -m stories.dual_era.client --http --server server_lowlevel

The bare stdio invocation (uv run python -m stories.dual_era.client) is legacy-only until the SDK's stdio entry can negotiate the era, so the modern leg fails there today — run over --http.

What to look at

• client.py — both connections are visible, against the same targets() factory: Client(targets(), mode=mode) (default "auto", the discover-then-fallback ladder) and Client(targets(), mode="legacy") (forces the initialize handshake). The era decision is one explicit mode= argument at construction; no date strings appear in the body.
• client.py — client.protocol_version / client.server_info / client.server_capabilities are era-neutral: populated by initialize or server/discover, whichever ran.
• server.py — ctx.request_context.protocol_version is the era branch key (lowlevel: ctx.protocol_version directly). Compare against MODERN_PROTOCOL_VERSIONS, never a date literal.
• Where to read the negotiated version. One value, three read paths: client.protocol_version on the client after connect; ctx.protocol_version inside a lowlevel handler; ctx.request_context.protocol_version inside an MCPServer handler.

Caveats

• ctx.request_context.protocol_version is the current way to read the negotiated version; a later release will shorten it to ctx.transport.*.
• Over HTTP the built-in era branch is currently header-only — a 2026 client that omits the MCP-Protocol-Version header is mis-routed to the legacy path. The body-primary classifier lands in a later release.

Spec

• Versioning — backward compatibility
• server/discover

See also

legacy_routing/ (route eras yourself), reconnect/ (persist DiscoverResult for zero-RTT reconnect).