fix: skip priming events and closeSSEStream for old protocol versions #1233
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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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pcarleton
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Dec 4, 2025
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LGTM! |
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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
closeSSEStreamcallbacks 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?
closeSSEStreamcallbacks are NOT provided for old protocol versionsBreaking 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-versionheaderString comparison works for ISO date format versions (e.g.,
'2025-03-26' < '2025-11-25'→ true).