feat(examples): add prompt caching test script

Standalone TypeScript script (zero deps, native fetch) to verify
Anthropic prompt caching is active. Documents 4 production gotchas
not in official docs: beta header required for Claude 4.x, effective
threshold ~2048+ tokens, cached tokens excluded from input_tokens,
new nested cache_creation response format.

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

examples/scripts/README.md

@@ -26,6 +26,7 @@ Utility scripts for Claude Code power users.
 | `rtk-benchmark.sh` | Benchmark RTK token savings vs raw commands |
 | `sync-claude-config.sh` | Sync Claude config files across machines |
 | `sonnetplan.sh` | Run Claude with Sonnet replacing Opus (cost optimization alias) |
+| `test-prompt-caching.ts` | Verify Anthropic prompt caching is active (no deps, fetch only) |
 
 ---
 

examples/scripts/test-prompt-caching.ts

@@ -0,0 +1,132 @@
+/**
+ * Prompt Caching Test — Anthropic API
+ *
+ * Verifies that prompt caching is active on your Anthropic API key.
+ * Runs 3 identical calls and checks that calls 2-3 read from cache.
+ *
+ * Usage:
+ *   ANTHROPIC_API_KEY=sk-ant-... npx tsx test-prompt-caching.ts
+ *
+ * Requirements:
+ *   - Node 18+ (native fetch)
+ *   - No dependencies required
+ *
+ * Gotchas discovered in production (not in official docs):
+ *   1. The `anthropic-beta: prompt-caching-2024-07-31` header is required
+ *      even for Claude 4.x models — omitting it silently disables caching.
+ *   2. The effective token threshold for Claude 4.x is ~2048+, not the
+ *      documented 1024. Blocks below this threshold return write=0 with no warning.
+ *   3. Cached tokens are excluded from `input_tokens` in the response.
+ *      Track `cache_creation_input_tokens` and `cache_read_input_tokens` instead.
+ *   4. The new API format exposes a nested `cache_creation` object with
+ *      `ephemeral_5m_input_tokens` (5-min TTL) and `ephemeral_1h_input_tokens` (1-hour TTL).
+ */
+
+const API_KEY = process.env.ANTHROPIC_API_KEY;
+if (!API_KEY) {
+  console.error("Error: ANTHROPIC_API_KEY environment variable is not set.");
+  process.exit(1);
+}
+
+// Stable system prompt — must exceed the effective cache threshold (~2048 tokens for Claude 4.x).
+// In real usage: consolidate agent rules, platform context, and static instructions into one block.
+const STABLE_SYSTEM_PROMPT = `
+You are a helpful assistant.
+
+Core rules:
+- Be concise and accurate
+- Cite sources when referencing external content
+- Match the language of the user
+
+Extended context (padding to exceed cache threshold):
+${Array(200).fill("This platform helps users collaborate on tasks, track progress, and organize their workflow efficiently.").join(" ")}
+`.trim();
+
+type Usage = {
+  input_tokens: number;
+  output_tokens: number;
+  cache_creation_input_tokens?: number;
+  cache_read_input_tokens?: number;
+  cache_creation?: {
+    ephemeral_5m_input_tokens?: number;
+    ephemeral_1h_input_tokens?: number;
+  };
+};
+
+async function callAPI(callIndex: number): Promise<Usage> {
+  const response = await fetch("https://api.anthropic.com/v1/messages", {
+    method: "POST",
+    headers: {
+      "x-api-key": API_KEY!,
+      "anthropic-version": "2023-06-01",
+      // Required for all models, including Claude 4.x
+      "anthropic-beta": "prompt-caching-2024-07-31",
+      "content-type": "application/json",
+    },
+    body: JSON.stringify({
+      model: "claude-sonnet-4-6",
+      max_tokens: 50,
+      system: [
+        {
+          type: "text",
+          text: STABLE_SYSTEM_PROMPT,
+          // Mark this block for caching — must be the last or only system block
+          cache_control: { type: "ephemeral" },
+        },
+      ],
+      messages: [
+        { role: "user", content: `Test call ${callIndex} — reply OK` },
+      ],
+    }),
+  });
+
+  if (!response.ok) {
+    const body = await response.text();
+    throw new Error(`API ${response.status}: ${body}`);
+  }
+
+  const data = (await response.json()) as { usage: Usage };
+  return data.usage;
+}
+
+async function main() {
+  console.log("Prompt Caching Test\n");
+
+  let passed = true;
+
+  for (let i = 1; i <= 3; i++) {
+    const usage = await callAPI(i);
+
+    const write = usage.cache_creation_input_tokens ?? 0;
+    const read = usage.cache_read_input_tokens ?? 0;
+
+    let status: string;
+    if (i === 1) {
+      status = write > 0 ? "cache written" : "❌ write failed (check token threshold)";
+      if (write === 0) passed = false;
+    } else {
+      status = read > 0 ? "✅ cache hit" : "❌ cache miss";
+      if (read === 0) passed = false;
+    }
+
+    console.log(
+      `Call ${i}:  write=${String(write).padStart(5)}  read=${String(read).padStart(5)}  input=${String(usage.input_tokens).padStart(4)}  — ${status}`
+    );
+  }
+
+  console.log();
+  if (passed) {
+    console.log("Result: caching is working correctly.");
+  } else {
+    console.log("Result: caching is NOT working. Common causes:");
+    console.log("  - Token threshold not met (try increasing STABLE_SYSTEM_PROMPT)");
+    console.log("  - Missing beta header (anthropic-beta: prompt-caching-2024-07-31)");
+    console.log("  - Account tier does not support caching");
+    process.exit(1);
+  }
+}
+
+main().catch((err) => {
+  console.error(err.message);
+  process.exit(1);
+});