ported over subagents, minor changes to functionality, refined ui

pi-extension/subagents/session.ts

@@ -178,3 +178,77 @@ export function mergeNewEntries(
   }
   return entries;
 }
+
+export interface SessionStats {
+  model: string | null;
+  toolCount: number;
+  /** Cumulative token usage across all assistant turns. */
+  inputTokens: number;
+  outputTokens: number;
+  cacheReadTokens: number;
+  cacheWriteTokens: number;
+  /** Current context size: the last assistant turn's totalTokens. */
+  contextTokens: number;
+  /** Cumulative cost in USD across all assistant turns. */
+  cost: number;
+}
+
+/**
+ * Parse a completed subagent session JSONL into aggregate stats for display:
+ * model, tool-call count, cumulative token usage + cost, and current context
+ * size. Cumulative usage fields are summed across every assistant turn; the
+ * context size is taken from the last assistant turn's `totalTokens` (the live
+ * context window occupancy). Returns null if the file can't be read.
+ */
+export function summarizeSessionStats(sessionFile: string): SessionStats | null {
+  let entries: SessionEntry[];
+  try {
+    entries = readEntries(sessionFile);
+  } catch {
+    return null;
+  }
+
+  const stats: SessionStats = {
+    model: null,
+    toolCount: 0,
+    inputTokens: 0,
+    outputTokens: 0,
+    cacheReadTokens: 0,
+    cacheWriteTokens: 0,
+    contextTokens: 0,
+    cost: 0,
+  };
+
+  for (const entry of entries) {
+    if (entry.type === "model_change") {
+      const modelId = (entry as { modelId?: unknown }).modelId;
+      if (typeof modelId === "string" && modelId) stats.model = modelId;
+      continue;
+    }
+    if (entry.type !== "message") continue;
+    const msg = (entry as MessageEntry).message;
+    if (msg.role !== "assistant") continue;
+
+    const model = (msg as { model?: unknown }).model;
+    if (typeof model === "string" && model) stats.model = model;
+
+    for (const block of msg.content) {
+      if (block.type === "toolCall") stats.toolCount++;
+    }
+
+    const usage = (msg as { usage?: Record<string, unknown> }).usage;
+    if (usage && typeof usage === "object") {
+      const num = (v: unknown): number => (typeof v === "number" && Number.isFinite(v) ? v : 0);
+      stats.inputTokens += num(usage.input);
+      stats.outputTokens += num(usage.output);
+      stats.cacheReadTokens += num(usage.cacheRead);
+      stats.cacheWriteTokens += num(usage.cacheWrite);
+      const total = num(usage.totalTokens);
+      if (total > 0) stats.contextTokens = total;
+      const cost = usage.cost;
+      if (cost && typeof cost === "object") stats.cost += num((cost as Record<string, unknown>).total);
+    }
+  }
+
+  return stats;
+}