feat(coding-agent): add prompt cache miss tracking (#6427)

Detect prompt cache misses per turn by comparing each assistant message's
cache reads against the previous request's prompt tokens (core/cache-stats.ts).
Significant misses emit a warning-colored transcript notice at the turn they
occur, noting idle gaps past the cache TTL and model switches when relevant.

/session gained cache statistics: a compact token/cache breakdown with hit
rate, a $-prefixed cost section with per-model cost breakdown, and the
cumulative cost re-billed due to cache misses.

packages/coding-agent/src/modes/interactive/components/footer.ts

@@ -20,7 +20,7 @@ function sanitizeStatusText(text: string): string {
 /**
  * Format token counts for compact footer display.
  */
-function formatTokens(count: number): string {
+export function formatTokens(count: number): string {
 	if (count < 1000) return count.toString();
 	if (count < 10000) return `${(count / 1000).toFixed(1)}k`;
 	if (count < 1000000) return `${Math.round(count / 1000)}k`;
@@ -137,7 +137,6 @@ export class FooterComponent implements Component {
 		if ((totalCacheRead > 0 || totalCacheWrite > 0) && latestCacheHitRate !== undefined) {
 			statsParts.push(`CH${latestCacheHitRate.toFixed(1)}%`);
 		}
-
 		// Show cost with "(sub)" indicator if using OAuth subscription
 		const usingSubscription = state.model ? this.session.modelRegistry.isUsingOAuth(state.model) : false;
 		if (totalCost || usingSubscription) {

packages/coding-agent/src/modes/interactive/components/settings-selector.ts

@@ -65,6 +65,7 @@ export interface SettingsConfig {
 	terminalTheme: TerminalTheme;
 	availableThemes: string[];
 	hideThinkingBlock: boolean;
+	showCacheMissNotices: boolean;
 	collapseChangelog: boolean;
 	enableInstallTelemetry: boolean;
 	doubleEscapeAction: "fork" | "tree" | "none";
@@ -95,6 +96,7 @@ export interface SettingsCallbacks {
 	onThemeChange: (theme: string) => void;
 	onThemePreview?: (theme: string) => void;
 	onHideThinkingBlockChange: (hidden: boolean) => void;
+	onShowCacheMissNoticesChange: (shown: boolean) => void;
 	onCollapseChangelogChange: (collapsed: boolean) => void;
 	onEnableInstallTelemetryChange: (enabled: boolean) => void;
 	onDoubleEscapeActionChange: (action: "fork" | "tree" | "none") => void;
@@ -521,6 +523,13 @@ export class SettingsSelectorComponent extends Container {
 				currentValue: config.hideThinkingBlock ? "true" : "false",
 				values: ["true", "false"],
 			},
+			{
+				id: "cache-miss-notices",
+				label: "Cache miss notices",
+				description: "Show transcript notices for significant prompt-cache misses",
+				currentValue: config.showCacheMissNotices ? "true" : "false",
+				values: ["true", "false"],
+			},
 			{
 				id: "collapse-changelog",
 				label: "Collapse changelog",
@@ -764,6 +773,9 @@ export class SettingsSelectorComponent extends Container {
 					case "hide-thinking":
 						callbacks.onHideThinkingBlockChange(newValue === "true");
 						break;
+					case "cache-miss-notices":
+						callbacks.onShowCacheMissNoticesChange(newValue === "true");
+						break;
 					case "collapse-changelog":
 						callbacks.onCollapseChangelogChange(newValue === "true");
 						break;