Delegation sub-thread token usage is recorded under model (unknown) — sub-thread cost is unpriced ($0)

[sierragolflima]

Summary

When an agent delegates work to a worker (a sub-thread), the sub-thread's TokenUsage satellite records model: (unknown) instead of the model that actually ran. Because cost is derived from ModelPricing keyed by model, (unknown) matches no price → the sub-thread's tokens are reported at $0 and are invisible in per-model roll-ups. The parent thread records its model correctly; only delegated work is affected.

Observed (two independent runs, 5 sub-threads — 5/5 affected)

Run 1 — orchestrator on a premium model, two delegations:

Thread	_Usage key (model)	Priced?
Parent (orchestrator)	Provider/…/<real-model-A>	✅
Worker sub-thread (write)	(unknown)	❌ $0
Worker sub-thread (verify)	(unknown)	❌ $0

Run 2 — orchestrator on a mid-tier model, three delegations:

Thread	_Usage key (model)	Priced?
Parent (orchestrator)	Provider/…/<real-model-B>	✅
Worker sub-thread (notify)	(unknown)	❌ $0
Worker sub-thread (write)	(unknown)	❌ $0
Worker sub-thread (correction)	(unknown)	❌ $0

Every sub-thread carries real, non-zero token counts but model (unknown) — across different orchestrator models and different tasks.

Impact

• Sub-thread token cost is reported as $0 (no ModelPricing entry matches (unknown)).
• Per-model usage roll-up is broken for all delegated work.
• The headline cost/usage figure undercounts any delegation-heavy task twice over: sub-thread usage lives on a separate _Usage node and is unpriced. In Run 2 above, roughly 13% of the run's total tokens were priced at $0.

Root cause (hypothesis)

TokenUsageNodeType.RecordUsage(...) is called with actualModel ?? request.ModelName in the terminal round handlers (ThreadExecution.cs, ~L2108 / L2199 / L2305). For a delegation sub-thread round, both are null/empty — the sub-thread submission doesn't carry the resolved model id into its RoundParams.ModelName, and the streaming updates don't populate update.ModelId — so RecordUsage falls to the string.IsNullOrWhiteSpace(modelId) ? "(unknown)" fallback (TokenUsage.cs). The parent round populates the model (from update.ModelId); the sub-thread round does not.

Steps to reproduce

1. Configure an agent with delegation (one or more hierarchy worker agents).
2. Run a task that delegates at least one sub-task to a worker sub-thread; let it complete.
3. Read the usage satellites: {parentThread}/_Usage/* and {subThread}/_Usage/*.
4. Observe: the parent has _Usage/{realModel} (priced); each sub-thread has _Usage/(unknown) with real token counts.
5. The cost derived for the sub-thread rounds is $0 (no ModelPricing match for (unknown)).

Expected: each sub-thread's _Usage is keyed by the model that actually executed it, so its tokens price correctly and roll up per model.

Suggested direction

Thread the resolved model id into the delegated sub-thread's round — populate RoundParams.ModelName (or the streaming update.ModelId) for sub-thread execution so the existing actualModel ?? request.ModelName fallback resolves to the real model. Alternatively, have a sub-thread inherit the parent/agent's resolved model when its own submission specifies none.

[sierragolflima]

Additional observation from auditing a further delegation run — the bug has a second, sharper grade:

Cancelled delegation sub-threads record NO usage at all. Two delegation sub-threads in the same run were cancelled mid-execution after the worker had already produced assistant text (its summary shows generated output, i.e. tokens were consumed). Neither sub-thread has any _Usage satellite — not even a (unknown) one. The tokens consumed before cancellation are lost entirely from the accounting.

Likely mechanism: RecordUsage is a documented no-op when both token counts are 0/null, and the Cancelled terminal path for a delegation sub-thread appears to reach it without token counts — so nothing is written.

So the defect has two grades:

1. Completed delegation sub-thread → usage recorded under (unknown) → unpriced ($0) but at least visible (original report above).
2. Cancelled delegation sub-thread → usage not recorded at all → invisible.

A fix should cover both: attribute the resolved model on the sub-thread round (grade 1), and ensure the Cancelled path still records whatever counts the provider returned before cancellation (grade 2).