Legacy initialize responses drop capabilities.extensions (2025-11-25 and earlier) #3773

Initial Checks

• I confirm that I'm using the newest release of my line (the latest 2.x, or the latest 1.x if I'm still on v1)
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Release line

2.x (current stable)

Description

What happens. On connections that negotiate 2025-11-25 or earlier, a server's initialize result loses capabilities.extensions. The public ServerCapabilities model exposes extensions, but serialize_server_result("initialize", version, ...) validates against the per-version wire model, and _v2025_11_25.ServerCapabilities has no extensions field and uses extra="ignore". Client advertisements survive, so negotiation is one-sided: the client offers an extension but never sees the server accept it.

Why it matters. Extensions became official in 2026-07-28, but they are widely negotiated through initialize on older versions. MCP Apps' stable spec advertises io.modelcontextprotocol/ui this way, and the TypeScript, Go, C#, Rust, Kotlin, Ruby, PHP, and Java SDKs all carry the field on legacy connections. modelcontextprotocol/modelcontextprotocol#3364 documents this and encourages SDKs that can carry the field to do so. Today, a Python server can't advertise MCP Apps (or any extension) to a 2025-11-25 client.

Expected. capabilities.extensions survives legacy initialize serialization in both directions, as it does for 2026-07-28.

Possible fix. Add an optional extensions: dict[str, dict[str, Any]] | None field to the legacy ClientCapabilities/ServerCapabilities wire models (or carve it out of the sieve), and add an initialize round-trip test per supported legacy version.

Related: #3640, which covers unknown capability keys more broadly. This issue is narrower: extensions is a known field that the SDK already models for 2026-07-28.

Example Code

from mcp_types.methods import serialize_server_result

data = {
    "protocolVersion": "2025-11-25",
    "capabilities": {"tools": {}, "extensions": {"io.modelcontextprotocol/ui": {}}},
    "serverInfo": {"name": "S", "version": "1"},
}
print(serialize_server_result("initialize", "2025-11-25", data)["capabilities"])
# {'tools': {}}  <- extensions dropped