Fix race condition in SSE GET request initialization #1212
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The StreamableHttpHandler was flushing HTTP response headers before calling HandleGetRequestAsync, which sets _getHttpRequestStarted = true. This allowed a race where SendNotificationAsync could be called after the client received headers but before _getHttpRequestStarted was set, causing the notification to be silently dropped. Move the flush inside HandleGetRequestAsync so it occurs after _getHttpRequestStarted is set, while still holding the lock. This ensures SendNotificationAsync will either: - Block on the lock until initialization completes, or - See _getHttpRequestStarted = true after acquiring the lock The priming write path already flushes via SseEventWriter.WriteAsync, so the explicit flush is only needed in the non-priming case.
MackinnonBuck
approved these changes
Jan 30, 2026
stephentoub
merged commit f9dec13
into
modelcontextprotocol:main
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The StreamableHttpHandler was flushing HTTP response headers before calling HandleGetRequestAsync, which sets _getHttpRequestStarted = true. This allowed a race where SendNotificationAsync could be called after the client received headers but before _getHttpRequestStarted was set, causing the notification to be silently dropped.
Move the flush inside HandleGetRequestAsync so it occurs after _getHttpRequestStarted is set, while still holding the lock. This ensures SendNotificationAsync will either:
The priming write path already flushes via SseEventWriter.WriteAsync, so the explicit flush is only needed in the non-priming case.
Fixes #1211 (hopefully)