feat(cache): Anthropic prompt caching, usage telemetry, and Cache Rate view

- send prompt-caching + oauth anthropic-beta headers on the Claude OAuth provider
- restructure the OAuth request body (billing header, identity split, relocate
  third-party system prompt to the first user message) to match Claude Code
- apply rolling cache_control breakpoints and group a turn's tool results into a
  single role:tool message for correct breakpoint placement
- emit per-step usage events (cache read/write split) and add the Cache Rate
  sidebar panel
- dedup byte-identical tool calls within a single batch

2 files changed, 30 insertions, 15 deletions

diff --git a/packages/core/src/credentials/anthropic-betas.ts b/packages/core/src/credentials/anthropic-betas.ts
new file mode 100644
index 0000000..f2ca7a6
--- /dev/null
+++ b/packages/core/src/credentials/anthropic-betas.ts
@@ -0,0 +1,24 @@
+// ─── Anthropic Beta Headers ───────────────────────────────────
+//
+// The set of `anthropic-beta` features the official Claude Code CLI sends on
+// every request. Kept in a dependency-free module (no DB / `bun:sqlite`
+// import) so the LLM provider layer (`llm/provider.ts`) can pull the list in
+// without dragging the whole credentials/DB stack into its import graph.
+//
+// `prompt-caching-scope-2026-01-05` is load-bearing for cost: without it the
+// Anthropic API silently ignores every `cache_control` breakpoint we place,
+// producing a 0% cache hit rate (see claude-report.md). `oauth-2025-04-20`
+// gates the Bearer/OAuth flow used by Claude Pro/Max subscriptions.
+
+const BASE_BETAS = [
+	"claude-code-20250219",
+	"oauth-2025-04-20",
+	"interleaved-thinking-2025-05-14",
+	"prompt-caching-scope-2026-01-05",
+	"context-management-2025-06-27",
+	"advisor-tool-2026-03-01",
+];
+
+export function getAnthropicBetas(): string[] {
+	return [...BASE_BETAS];
+}
diff --git a/packages/core/src/credentials/claude.ts b/packages/core/src/credentials/claude.ts
index 6018207..168d544 100644
--- a/packages/core/src/credentials/claude.ts
+++ b/packages/core/src/credentials/claude.ts
@@ -10,8 +10,14 @@ import {
 import { homedir } from "node:os";
 import { basename, dirname, join } from "node:path";
 import { getDatabase } from "../db/index.js";
+import { getAnthropicBetas } from "./anthropic-betas.js";
 import { getStoredCredentials, listStoredCredentials, updateStoredTokens } from "./store.js";
 
+// Re-exported for backward compatibility — `getAnthropicBetas` historically
+// lived here and is surfaced through `credentials/index.ts`. The definition
+// now lives in the dependency-free `anthropic-betas.ts` module.
+export { getAnthropicBetas };
+
 export interface ClaudeCredentials {
 	accessToken: string;
 	refreshToken: string;
@@ -367,21 +373,6 @@ export function buildBillingHeaderValue(
 
 export const SYSTEM_IDENTITY = "You are Claude Code, Anthropic's official CLI for Claude.";
 
-// ─── Anthropic Beta Headers ───────────────────────────────────
-
-const BASE_BETAS = [
-	"claude-code-20250219",
-	"oauth-2025-04-20",
-	"interleaved-thinking-2025-05-14",
-	"prompt-caching-scope-2026-01-05",
-	"context-management-2025-06-27",
-	"advisor-tool-2026-03-01",
-];
-
-export function getAnthropicBetas(): string[] {
-	return [...BASE_BETAS];
-}
-
 // ─── Anthropic Request Headers ────────────────────────────────
 
 export function getAnthropicHeaders(accessToken: string): Record<string, string> {