fix: use prompt snapshots for live context diagnostics

docs/logging.md

@@ -198,6 +198,9 @@ diagnostics + the exporter plugin are enabled.
 Model usage:
 
 - `model.usage`: tokens, cost, duration, context, provider/model/channel, session ids.
+  `usage` is provider/turn accounting for cost and telemetry; `context.used`
+  is the current prompt/context snapshot and can be lower than provider
+  `usage.total` when cached input or tool-loop calls are involved.
 
 Message flow:
 

docs/reference/token-use.md

@@ -101,6 +101,13 @@ Assistant transcript entries persist the same normalized usage shape, including
 returns usage metadata. This gives `/usage cost` and transcript-backed session
 status a stable source even after the live runtime state is gone.
 
+OpenClaw keeps provider usage accounting separate from the current context
+snapshot. Provider `usage.total` can include cached input, output, and multiple
+tool-loop model calls, so it is useful for cost and telemetry but can overstate
+the live context window. Context displays and diagnostics use the latest prompt
+snapshot (`promptTokens`, or the last model call when no prompt snapshot is
+available) for `context.used`.
+
 ## Cost estimation (when shown)
 
 Costs are estimated from your model pricing config: