providerAsSandboxClient prompt() sums usage across events without reading usageMode, so cumulative receipts are counted once per event

[drewstone]

Observation

providerAsSandboxClient(provider).create() returns an instance whose prompt() folds every streamed event with mergeTokenUsage, which adds the counters (src/runtime/environment-provider.ts:1538 and :2501-2532 on origin/main 3b33fe9f).
It never reads AgentEnvironmentEvent.usageMode.
The interface defines cumulative as "the total so far for this turn" (agent-interface environment-runtime.ts:274-278), so each cumulative event restates earlier usage.

Reproduction against agent-runtime 0.219.0, with a stub provider whose stream is a cumulative step total, then result and done carrying the same cumulative receipt:

events: raw {60,6} cumulative, result {100,10} cumulative, done {100,10} cumulative
prompt().usage: { inputTokens: 260, outputTokens: 26 }   // receipt is { 100, 10 }

The executor path is correct: streamProviderExecutor credits the same events through createSandboxUsageLedger, which honors usageMode and marks a falling cumulative total unknown.

Reach today

@tangle-network/agent-provider-tangle cannot reach this path yet, because providerAsSandboxClient always sends providerOptions.sandboxCreateOptions and that provider refuses non-empty create providerOptions.
It becomes reachable for Tangle as soon as that composition works.
The Tangle provider already marks both result and done cumulative, and agent-sdk#327 adds cumulative step totals for OpenCode executions (agent-sdk#324).

Ask

Fold prompt() usage by mode: replace the running total with a cumulative event's usage, add a delta event's usage, and treat usage with no declared mode as it is treated in the ledger.