fix: skip priming events and close_sse_stream for old protocol versions #1719
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Priming events (SEP-1699) have empty SSE data which older clients cannot
handle - they try to JSON.parse("") and crash. Only send priming events
to clients with protocol version >= 2025-11-25, which includes the fix
for handling empty SSE data.
For the initialize request, the protocol version is extracted from the
request params. For subsequent requests, it's taken from the
mcp-protocol-version header.
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Fixes backwards compatibility issue where SEP-1699 priming events break old clients.
Motivation and Context
SEP-1699 introduced "priming events" - SSE events with an ID but empty data (
data: \n). Old clients (SDK versions before the empty-data fix) try toJSON.parse("")and crash with "Unexpected end of JSON input".Additionally, the
close_sse_streamcallbacks should not be provided to old clients because they can't resume if the stream is closed early (they never received a priming event with an event ID).How Has This Been Tested?
Breaking Changes
None. This is a backwards-compatible fix that makes new servers work correctly with old clients.
Types of changes
Checklist
Additional context
The fix gates on protocol version >= 2025-11-25:
params.protocolVersionmcp-protocol-versionheaderChanges:
_maybe_send_priming_event: renamed from_send_priming_event, now checks protocol version_create_session_message: only providesclose_sse_streamcallbacks when protocol version supports resumabilityString comparison works for ISO date format versions (e.g.,
'2025-03-26' < '2025-11-25'→ true).