feat: show total session cost including subagents in Visible Context panel

Compute subagentCostUsd from sessionDetail.processes[].metrics and
display total cost (parent + subagents) with breakdown in the header.
When subagents exist, shows e.g. "$24.70 ($16.85 parent + $7.84 subagents)"
with a link to the Session Report for full details.

Replaces the "(parent only)" disclaimer since cost is now accurate.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/renderer/components/chat/ChatHistory.tsx

@@ -102,6 +102,14 @@ export const ChatHistory = ({ tabId }: ChatHistoryProps): JSX.Element => {
     sessionDetail,
   } = tabData;
 
+  // Compute combined subagent cost from process metrics
+  const subagentCostUsd = useMemo(() => {
+    const processes = sessionDetail?.processes;
+    if (!processes || processes.length === 0) return undefined;
+    const total = processes.reduce((sum, p) => sum + (p.metrics.costUsd ?? 0), 0);
+    return total > 0 ? total : undefined;
+  }, [sessionDetail?.processes]);
+
   // State for Context button hover (local state OK - doesn't need per-tab isolation)
   const [isContextButtonHovered, setIsContextButtonHovered] = useState(false);
 
@@ -874,6 +882,7 @@ export const ChatHistory = ({ tabId }: ChatHistoryProps): JSX.Element => {
               onNavigateToUserGroup={handleNavigateToUserGroup}
               totalSessionTokens={lastAiGroupTotalTokens}
               sessionMetrics={sessionDetail?.metrics}
+              subagentCostUsd={subagentCostUsd}
               onViewReport={effectiveTabId ? () => openSessionReport(effectiveTabId) : undefined}
               phaseInfo={sessionPhaseInfo ?? undefined}
               selectedPhase={selectedContextPhase}

src/renderer/components/chat/SessionContextPanel/components/SessionContextHeader.tsx

@@ -28,6 +28,7 @@ interface SessionContextHeaderProps {
   totalTokens: number;
   totalSessionTokens?: number;
   sessionMetrics?: SessionMetrics;
+  subagentCostUsd?: number;
   onClose?: () => void;
   onViewReport?: () => void;
   phaseInfo?: ContextPhaseInfo;
@@ -42,6 +43,7 @@ export const SessionContextHeader = ({
   totalTokens,
   totalSessionTokens,
   sessionMetrics,
+  subagentCostUsd,
   onClose,
   onViewReport,
   phaseInfo,
@@ -132,24 +134,30 @@ export const SessionContextHeader = ({
             <div className="col-span-2">
               <span style={{ color: COLOR_TEXT_MUTED }}>Session Cost: </span>
               <span className="font-medium tabular-nums" style={{ color: COLOR_TEXT_SECONDARY }}>
-                {formatCostUsd(sessionMetrics.costUsd)}
-              </span>
-              <span style={{ color: COLOR_TEXT_MUTED }}>
-                {' (parent only'}
-                {onViewReport && (
-                  <>
-                    {' · '}
-                    <button
-                      onClick={onViewReport}
-                      className="underline"
-                      style={{ color: COLOR_TEXT_SECONDARY }}
-                    >
-                      view full cost
-                    </button>
-                  </>
-                )}
-                {')'}
+                {formatCostUsd(sessionMetrics.costUsd + (subagentCostUsd ?? 0))}
               </span>
+              {subagentCostUsd !== undefined && subagentCostUsd > 0 && (
+                <span style={{ color: COLOR_TEXT_MUTED }}>
+                  {' ('}
+                  {formatCostUsd(sessionMetrics.costUsd)}
+                  {' parent + '}
+                  {formatCostUsd(subagentCostUsd)}
+                  {' subagents'}
+                  {onViewReport && (
+                    <>
+                      {' · '}
+                      <button
+                        onClick={onViewReport}
+                        className="underline"
+                        style={{ color: COLOR_TEXT_SECONDARY }}
+                      >
+                        details
+                      </button>
+                    </>
+                  )}
+                  {')'}
+                </span>
+              )}
             </div>
           )}
         </div>

src/renderer/components/chat/SessionContextPanel/index.tsx

@@ -49,6 +49,7 @@ export const SessionContextPanel = ({
   onNavigateToUserGroup,
   totalSessionTokens,
   sessionMetrics,
+  subagentCostUsd,
   onViewReport,
   phaseInfo,
   selectedPhase,
@@ -193,6 +194,7 @@ export const SessionContextPanel = ({
         totalTokens={totalTokens}
         totalSessionTokens={totalSessionTokens}
         sessionMetrics={sessionMetrics}
+        subagentCostUsd={subagentCostUsd}
         onClose={onClose}
         onViewReport={onViewReport}
         phaseInfo={phaseInfo}

src/renderer/components/chat/SessionContextPanel/types.ts

@@ -27,6 +27,8 @@ export interface SessionContextPanelProps {
   totalSessionTokens?: number;
   /** Full session metrics (input, output, cache tokens, cost) */
   sessionMetrics?: SessionMetrics;
+  /** Combined cost of all subagent processes */
+  subagentCostUsd?: number;
   /** Open the Session Report to see full cost breakdown */
   onViewReport?: () => void;
   /** Phase information for phase selector */