[Feature Request] Streaming per-subagent token usage events during execution

[ArielSmoliar]

Problem

• The SDK provides token usage in two places: RateLimitEvent.utilization (session-level, lagging) and model_usage in ResultMessage (per-agent, but only after completion). During
  execution, there's no way to observe how many tokens a specific subagent has consumed so far.

• This makes it impossible to build real-time resource management on top of the SDK. You can set a static max_budget_usd kill switch, but you can't dynamically adjust agent behavior based on actual consumption mid-execution.

Proposal

Emit a TokenUsageEvent (or extend the existing RateLimitEvent) per subagent during execution:

{
"type": "token_usage",
"agent_id": "researcher-agent",
"input_tokens": 14200,
"output_tokens": 3800,
"cache_read_input_tokens": 600,
"cumulative_cost_usd": 0.042,
"turn": 3
}

This could be emitted after each LLM call within a subagent, similar to how RateLimitEvent is already streamed.

Use case

• I'm building an open-source agent scheduler that sits above agent frameworks and manages resource allocation across concurrent agents.
• The Anthropic SDK's hook architecture (PreToolUse/PostToolUse at zero token cost) is already the most scheduler-friendly integration point across all the major agent platforms I analyzed.
• Streaming per-agent token usage would complete the picture by providing hooks for admission control and usage events for real-time accounting.

Concrete scenarios:

• Throttle a subagent that's consuming disproportionate tokens before it hits max_budget_usd.
• Downgrade a subagent to a cheaper model mid-execution if utilization patterns suggest the task is simpler than expected.
• Rebalance max_turns across concurrent subagents based on actual vs. projected consumption.