Problem

Non-streaming sub-agent paths (like the spawn_agent tool) still emit only start/complete logs with no progress events. PR #1428 made the activity sink mandatory by creating a discard channel, but nobody actually reads from it — events go into the void.

Current Behavior

Sub-agents spawned via spawn_agent (non-streaming) emit:

• 1 log at start
• 1 log per LLM turn
• 1 log at completion

Missing: tool execution progress, streaming delta events, any intermediate activity.

Root Cause

SubAgentSpawner.SpawnAsync creates a discard channel when activitySink is null:

var discardChannel = Channel.CreateUnbounded<ToolActivityUpdate>();
var activitySinkToUse = activitySink ?? discardChannel.Writer;

This is passed to SubAgentActor.RunSubAgent.ActivitySink, which uses it for progress events. But the discard channel's reader is never consumed — events pile up silently.

Proposed Fix

Either:

1. Consume the discard channel and log events — spawn a task that reads from the channel and logs each ToolActivityUpdate via _logger
2. Write progress events directly to the logger — skip the discard channel for non-streaming paths and emit ILogger events directly in SubAgentActor

Option 2 is simpler and doesn't require managing a dead channel reader.

References

• PR fix(subagents): session-correlated sub-agent log observability #1428 (parent) — made activitySink mandatory
• SubAgentActor.cs — EmitActivity() method, SubAgentStreamPing handler
• SubAgentSpawner.cs — SpawnAsync discard channel creation