Session file is corrupted - Invalid literal value

[IeuanWalker]

I'm using the .NET SDK, a few times now, after updating the NuGet version, some (not all) previous sessions become corrupt. This is the error i get when resuming a session -

System.IO.IOException: Communication error with Copilot CLI: Request session.resume failed with message: Session file is corrupted (line 355: ephemeral: Invalid literal value, expected true)
 ---> StreamJsonRpc.RemoteInvocationException: Request session.resume failed with message: Session file is corrupted (line 355: ephemeral: Invalid literal value, expected true)
   at StreamJsonRpc.JsonRpc.InvokeCoreAsync[TResult](RequestId id, String targetName, IReadOnlyList`1 arguments, IReadOnlyList`1 positionalArgumentDeclaredTypes, IReadOnlyDictionary`2 namedArgumentDeclaredTypes, CancellationToken cancellationToken, Boolean isParameterObject)
   at GitHub.Copilot.SDK.CopilotClient.InvokeRpcAsync[T](JsonRpc rpc, String method, Object[] args, StringBuilder stderrBuffer, CancellationToken cancellationToken)
   --- End of inner exception stack trace ---
   at GitHub.Copilot.SDK.CopilotClient.InvokeRpcAsync[T](JsonRpc rpc, String method, Object[] args, StringBuilder stderrBuffer, CancellationToken cancellationToken)
   at GitHub.Copilot.SDK.CopilotClient.InvokeRpcAsync[T](JsonRpc rpc, String method, Object[] args, CancellationToken cancellationToken)
   at GitHub.Copilot.SDK.CopilotClient.ResumeSessionAsync(String sessionId, ResumeSessionConfig config, CancellationToken cancellationToken)
   at App.Features.Sessions.SessionFeature.LoadSession(String sessionId) in D:\a\App\App\src\App\Features\Sessions\SessionFeature.Lifecycle.cs:line 215

[github-actions]

Investigation findings

Thanks for the detailed report. I've investigated this and confirmed it's a bug.

Root cause

The Copilot CLI strictly validates session files against the current event schema when resuming. Certain event types (e.g. session.idle, session.title_changed, assistant.turn_start, assistant.reasoning_delta, assistant.turn_end, and many others) define ephemeral as a required literal true in the schema — meaning those events must always be ephemeral and must carry exactly the value true.

Session files written by an older version of the CLI either omitted the ephemeral field entirely or wrote false for these event types. A newer CLI version (pulled in when you update the NuGet package) now enforces this constraint strictly, causing the validation error:

Session file is corrupted (line 355: ephemeral: Invalid literal value, expected true)

This is a backward-compatibility regression — session files that were perfectly valid under an older schema are now rejected as "corrupted" by the newer CLI. The SDK simply surfaces the raw CLI error with no recovery guidance.

Relevant code paths

• nodejs/src/generated/session-events.ts — defines the schema discriminants; the affected event types carry ephemeral: true (literal, not boolean?)
• dotnet/src/Generated/SessionEvents.cs — .NET side uses bool? Ephemeral everywhere, which was fine for the SDK layer but the CLI enforces the schema on disk at resume time
• dotnet/src/CopilotClient.cs / ResumeSessionAsync — wraps the CLI error in IOException with no way for callers to distinguish a schema-incompatibility failure from other IO errors

Workaround (for now)

Delete the affected session file(s) from ~/.copilot/session-state/<sessionId>/ (or use client.DeleteSessionAsync(sessionId)) and start a fresh session. Unfortunately there is no way to migrate old session files automatically.

What should be fixed

1. The CLI should handle this schema evolution gracefully rather than rejecting old sessions as "corrupted"
2. If strict validation is intentional, the SDK should detect this specific error and throw a typed exception (e.g. SessionIncompatibleException) rather than a generic IOException, so callers can handle it (e.g. delete and recreate)
3. Session format compatibility should be documented in the troubleshooting guide

Labelling as bug since sessions created with one version should remain resumable after updating.

Generated by Bug Handler for issue #1007 · ◷