fix(sse): emit empty id field to reset Last-Event-ID - #5138
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yusukebe
approved these changes
Jul 21, 2026
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Summary
Emit an SSE
idfield whenSSEMessage.idis an empty string.Problem
SSEStreamingApi.writeSSE()currently uses a truthy check when serializingmessage.id:message.id && \id: ${message.id}``This omits the field when
idis an empty string.In Server-Sent Events, an empty
idfield resets the client's last event ID. After the reset, a subsequent reconnect should not send aLast-Event-IDheader.Hono currently cannot express this behavior through
writeSSE().Specification:
https://html.spec.whatwg.org/multipage/server-sent-events.html#the-last-event-id-header
Change
Use an explicit
undefinedcheck:message.id !== undefined && \id: ${message.id}``This preserves the existing behavior when
idis omitted while allowing an empty string to be serialized as an emptyidfield.Test
Added a regression test using:
writeSSE({ data: 'reset', id: '' })The test verifies the exact output:
Local result: