Setting `LargeOutput` doesn't seem to have any effect

[aelij]

By default, sessions write large outputs to the $TMP directory (e.g. /tmp/1782206406834-copilot-tool-output-s4fxvs.txt). We tried to set LargeOutput to redirect the files to a different directory but it doesn't seem to be change the target directory:

config.LargeOutput = new LargeToolOutputConfig
{
    Enabled = true,
    MaxSizeBytes = 2_000,
    OutputDirectory = outputDir,
}

[github-actions]

Investigation findings

I investigated this issue by tracing the LargeOutput.OutputDirectory path through the .NET SDK.

What I looked at

• dotnet/src/Types.cs — the LargeToolOutputConfig class definition
• dotnet/src/Client.cs — CreateSessionAsync and ResumeSessionAsync, including the CreateSessionRequest / ResumeSessionRequest record definitions and how they're constructed
• dotnet/test/Unit/SerializationTests.cs — the unit test that explicitly verifies the wire format

What I found

The SDK correctly passes OutputDirectory to the CLI:

1. Serialization is correct. LargeToolOutputConfig.OutputDirectory has [JsonPropertyName("outputDir")], so it serializes to the expected wire field name:

   [JsonPropertyName("outputDir")]
   public string? OutputDirectory { get; set; }

2. Passed in both session create and resume. Both CreateSessionAsync (line 997) and ResumeSessionAsync (line 1198) include LargeOutput: config.LargeOutput when building the RPC request.

3. A unit test verifies this end-to-end. SerializationTests.SessionRequests_CanSerializePluginDirectoriesAndLargeOutput_WithSdkOptions uses the actual production SerializerOptionsForMessageFormatter and asserts:

   Assert.Equal("/tmp/large-output", createLargeOutput.GetProperty("outputDir").GetString());

   This confirms the outputDir key is present in the JSON that gets sent to the CLI.

Conclusion

The SDK is working as designed — the outputDir field is correctly serialized and sent to the CLI runtime in the session.create / session.resume JSON-RPC call. The Node.js and Python SDKs perform the same outputDirectory → outputDir mapping on the wire.

If the files are still being written to $TMP despite setting OutputDirectory, the most likely cause is that the CLI runtime is not honoring the outputDir field it receives. I'd suggest:

1. Enabling CLI debug logs (set LogLevel = CopilotLogLevel.Debug on CopilotClientOptions) to confirm the largeOutput.outputDir field is present in the outgoing JSON-RPC payload.
2. If the field is present but the CLI ignores it, this would be a CLI-side bug rather than an SDK issue.

Reclassifying as a question since the SDK behavior is correct. If you can confirm via logs that the CLI is receiving outputDir but not using it, please reopen and we can escalate to the runtime team.

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Generated by Bug Handler for issue #1788 · sonnet46 4.4M · ◷