LargeToolOutputConfig handler is not used in sub agents

[assing]

I am using custom agents with .NET SDK (Copilot SDK Version: 1.0.8). I am passing LargeToolOutputConfig handler as part of session creation. This works fine if any tool was called as part of orchestrator agent and generated large output. Orchestrator agent handles large out as per LargeToolOutputConfig.

But it doesn't work in case of custom agents. Custom agents are not honoring LargeToolOutputConfig. So, tool calls always write data to disk file in case of large output.

How do we pass LargeToolOutputConfig to custom agents as well?

[github-actions]

Investigation Findings

Files reviewed: dotnet/src/Types.cs, dotnet/src/Client.cs, dotnet/test/Unit/SerializationTests.cs

What I Found

The LargeToolOutputConfig (LargeOutput) is only exposed on the session-level config (SessionConfig / ResumeSessionConfig) — it is sent to the CLI runtime at session creation time. It is not a property of CustomAgentConfig.

This means:

1. There is no SDK-level way to configure LargeOutput per custom agent.
2. The CLI runtime appears to not inherit the session-level LargeOutput setting when running custom sub-agents, which is the root cause of the behavior you're seeing.

Classification: Bug

The session-level LargeOutput config should logically propagate to sub-agents running within that session. The fact that custom agents ignore it — and always write large tool outputs to disk — is unintended behavior. I've labeled this as a bug.

Workaround

There is currently no SDK-level workaround to pass LargeToolOutputConfig to custom agents. As a temporary mitigation, you could disable large output handling entirely (Enabled = false) at the session level if you need consistent behavior, but this is not a real fix.

This issue needs to be addressed either at the CLI runtime level (inherit session config for sub-agents) or by exposing a LargeOutput property on CustomAgentConfig in the SDK.

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