Fix unknown-method error code and add a protocol version registry (#2836)

src/mcp/shared/version.py

@@ -1,3 +1,37 @@
+"""Protocol-version registry and comparison helpers.
+
+Date-string protocol revisions happen to sort lexicographically, but versions
+are an enumerated set, not an ordered scalar: future identifiers are not
+guaranteed to be date-shaped, and unrecognized peer strings must compare
+conservatively instead of accidentally (e.g. "zzz" > "2025-11-25"). All
+ordering questions go through KNOWN_PROTOCOL_VERSIONS.
+"""
+
+from typing import Final
+
 from mcp.types import LATEST_PROTOCOL_VERSION
 
+KNOWN_PROTOCOL_VERSIONS: Final[tuple[str, ...]] = (
+    "2024-11-05",
+    "2025-03-26",
+    "2025-06-18",
+    "2025-11-25",
+)
+"""Every released protocol revision, oldest to newest."""
+
 SUPPORTED_PROTOCOL_VERSIONS: list[str] = ["2024-11-05", "2025-03-26", "2025-06-18", LATEST_PROTOCOL_VERSION]
+"""Protocol revisions this SDK can negotiate."""
+
+
+def is_version_at_least(version: str, minimum: str) -> bool:
+    """Return True if `version` is a known revision at least as new as `minimum`.
+
+    Unknown `version` strings return False (treat unrecognized peers
+    conservatively). `minimum` must be a member of KNOWN_PROTOCOL_VERSIONS;
+    passing anything else is programmer error and raises ValueError.
+    """
+    if minimum not in KNOWN_PROTOCOL_VERSIONS:
+        raise ValueError(f"minimum must be a known protocol version, got {minimum!r}")
+    if version not in KNOWN_PROTOCOL_VERSIONS:
+        return False
+    return KNOWN_PROTOCOL_VERSIONS.index(version) >= KNOWN_PROTOCOL_VERSIONS.index(minimum)