fix: complete extension usage accounting

closes #6509

packages/coding-agent/docs/extensions.md

@@ -468,6 +468,7 @@ pi.on("session_before_compact", async (event, ctx) => {
       summary: "...",
       firstKeptEntryId: preparation.firstKeptEntryId,
       tokensBefore: preparation.tokensBefore,
+      // usage: summaryResponse.usage, // Optional; included in session totals
     }
   };
 });
@@ -489,7 +490,13 @@ pi.on("session_before_tree", async (event, ctx) => {
   const { preparation, signal } = event;
   return { cancel: true };
   // OR provide custom summary:
-  return { summary: { summary: "...", details: {} } };
+  return {
+    summary: {
+      summary: "...",
+      // usage: summaryResponse.usage, // Optional; included in session totals
+      details: {},
+    },
+  };
 });
 
 pi.on("session_tree", async (event, ctx) => {
@@ -813,7 +820,7 @@ In parallel tool mode, `tool_result` and `tool_execution_end` may interleave in
 `tool_result` handlers chain like middleware:
 - Handlers run in extension load order
 - Each handler sees the latest result after previous handler changes
-- Handlers can return partial patches (`content`, `details`, or `isError`); omitted fields keep their current values
+- Handlers can return partial patches (`content`, `details`, `isError`, or `usage`); omitted fields keep their current values
 
 Use `ctx.signal` for nested async work inside the handler. This lets Esc cancel model calls, `fetch()`, and other abort-aware operations started by the extension.
 
@@ -822,7 +829,7 @@ import { isBashToolResult } from "@earendil-works/pi-coding-agent";
 
 pi.on("tool_result", async (event, ctx) => {
   // event.toolName, event.toolCallId, event.input
-  // event.content, event.details, event.isError
+  // event.content, event.details, event.isError, event.usage
 
   if (isBashToolResult(event)) {
     // event.details is typed as BashToolDetails
@@ -835,7 +842,7 @@ pi.on("tool_result", async (event, ctx) => {
   });
 
   // Modify result:
-  return { content: [...], details: {...}, isError: false };
+  return { content: [...], details: {...}, isError: false, usage: nestedModelUsage };
 });
 ```
 
@@ -1932,6 +1939,7 @@ pi.registerTool({
     return {
       content: [{ type: "text", text: "Done" }],  // Sent to LLM
       details: { data: result },                   // For rendering & state
+      // usage: nestedModelResponse.usage,          // Optional nested LLM usage
       // Optional: stop after this tool batch when every finalized tool result
       // in the batch also returns terminate: true.
       terminate: true,
@@ -1944,6 +1952,8 @@ pi.registerTool({
 });
 ```
 
+**Usage accounting:** If a tool makes nested LLM calls, return their combined `Usage` as `usage`. Pi persists it on the tool result and includes it in footer, `/session`, and RPC session totals. `tool_result` handlers can inspect or replace this value.
+
 **Signaling errors:** To mark a tool execution as failed (sets `isError: true` on the result and reports it to the LLM), throw an error from `execute`. Returning a value never sets the error flag regardless of what properties you include in the return object.
 
 **Early termination:** Return `terminate: true` from `execute()` to hint that the automatic follow-up LLM call should be skipped after the current tool batch. This only takes effect when every finalized tool result in that batch is terminating. See [examples/extensions/structured-output.ts](../examples/extensions/structured-output.ts) for a minimal example where the agent ends on a final structured-output tool call.