Setting `LargeOutput` but if subagent started, it won't inherit the LargeOutput but use the operating system's default tmp directory

[AngryBerryMS]

When using the copilot sdk, we want to specify the LargeOutput dir to limit the large output from Tools be saved to specified directory. It works for the main agent, but sometimes subagents are launched, and the subagents will also call MCP Tools, whose large output will be saved to /tmp in Linux.

This is not ideal, because we want all large output to be saved to the specified directory. Our read tools are not allowed to read any directory in the operainng system, if the large output is saved to the default tmp dir (like /tmp in Linux), some customed defined read tools will fail since we only allow them to read specific directory

[github-actions]

Bug Confirmed ✅

What I investigated

• largeOutput in session config (nodejs/src/types.ts, dotnet/src/Types.cs, etc.): The LargeToolOutputConfig is correctly wired into the main session's create/resume request via toWireLargeOutput().
• SubagentSettingsEntry (generated types in nodejs/src/generated/rpc.ts, dotnet/src/Generated/Rpc.cs, java/src/generated/java/...): This struct only exposes model, effortLevel, and contextTier — there is no largeOutput field.

What I found

When a subagent is launched, it does not inherit the parent session's largeOutput directory, nor is there any way to configure it per-subagent via updateSubagentSettings. As a result, large tool outputs from subagent MCP tool calls fall back to the OS default temp directory (e.g. /tmp on Linux), ignoring the user-specified path.

This is a bug: the intent of largeOutput is clearly to control where large tool outputs are stored, but subagents silently bypass this setting.

Root cause

SubagentSettingsEntry in the RPC schema does not include a largeOutput field, and the CLI server does not propagate the parent session's largeOutput config to spawned subagents.

The fix would require either:

1. Adding a largeOutput override field to SubagentSettingsEntry (for per-subagent control), or
2. Having the CLI server automatically inherit the parent's largeOutput config when spawning subagents.

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