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Avoid initializing every session event type on first deserialization #2777
Description
SessionEvent.FromJson deserializes through the polymorphic base JsonTypeInfo. On the first event, System.Text.Json configures metadata for every derived session event type before returning the one concrete event that was received.
A cold-process console reproduction that deserializes one user.message event shows these medians across 15 processes:
| Target | Current | Discriminator dispatch prototype |
|---|---|---|
| .NET Framework 4.7.2 | 5,466 ms | 315 ms |
| .NET 8 | 512 ms / 5.09 MB | 139 ms / 0.42 MB |
The prototype reads the existing type discriminator and deserializes with only the matching source-generated concrete JsonTypeInfo. Unknown and missing discriminators continue to produce the base SessionEvent, preserving forward compatibility.
The expected behavior is for first-event deserialization to configure only the metadata required by the received event type, without changing known, unknown, union, or forward-compatible event semantics.
Activity
stephentoub commentedon Sep 28, 2026
If there's a perf issue here with System.Text.Json's polymorphic deserialization, shouldn't we fix that in STJ?
cc: @eiriktsarpalis
eiriktsarpalis commentedon Sep 28, 2026
STJ will generally attempt to configure the entire type graph up front before it attempts any serialization or deserialization operation. We could decide type that a particular shape warrants lazy expansion, but that comes at the expense of serving the user with configuration errors halfway through a serialization operation. Feel free to transfer the issue o dotnet/runtime and we can take a look at possible improvements or if there's an unanticipated performance bug lurking.
patniko commentedon Oct 26, 2026
Thanks for the analysis and the prototype. The discriminator-dispatch approach from #2778 is now in the .NET SDK:
SessionEvent.FromJsonand live event binding read thetypediscriminator and deserialize with only the matching source-generated type metadata instead of configuring every derived event type up front, while unknown or missing discriminators still fall back to the baseSessionEvent. This is available in Copilot SDK v1.0.17 and later and substantially reduces first-event latency, especially on .NET Framework. If you still see the cold-start cost on the latest version, feel free to reopen.Triaged as part of a Copilot SDK backlog review, with help from GitHub Copilot.