Anthropic prompt caching: ephemeral breakpoints on system, tools and the history tail, with cache-hit counters

A computer-use turn sends the same system + tool schemas every step, and the
history is append-only between steps: three breakpoints (system, last tool,
last content block) let steps 2..n of a turn read the cache the first step
wrote. CACHE_STATS counts input/cache_read/cache_creation tokens for make
doctor and the bench (target: cache_read > 70% of input on Claude routes).

Follow-ups in the plan (same item): freeze facts into the request at turn
start, keep the quick-model tool list constant per step (stable prefixes).

src/providers.rs

@@ -1028,23 +1028,65 @@ pub fn anthropic_messages(req: &ChatRequest) -> Vec<Value> {
     messages
 }
 
+/// Prompt caching: a breakpoint on the last block of the last message, so the whole history
+/// prefix is cached too — history is append-only, so each step of a turn reads what the last
+/// step wrote. Anthropic accepts at most four breakpoints; we use three (system, tools, tail).
+fn cache_tail(mut messages: Vec<Value>) -> Vec<Value> {
+    if let Some(last) = messages.last_mut() {
+        match last.get_mut("content") {
+            Some(Value::Array(blocks)) => {
+                if let Some(b) = blocks.last_mut() {
+                    if b.is_object() {
+                        b["cache_control"] = json!({"type": "ephemeral"});
+                    }
+                }
+            }
+            Some(Value::String(text)) => {
+                let t = std::mem::take(text);
+                last["content"] = json!([{"type": "text", "text": t, "cache_control": {"type": "ephemeral"}}]);
+            }
+            _ => {}
+        }
+    }
+    messages
+}
+
+/// (input_tokens, cache_read_input_tokens, cache_creation_input_tokens) summed over the
+/// session's Claude answers: the cache hit rate is the number that tells whether the
+/// breakpoints work (see `anybot` diagnostics / make doctor).
+static CACHE_STATS: std::sync::Mutex<(u64, u64, u64)> = std::sync::Mutex::new((0, 0, 0));
+
+pub fn cache_stats() -> (u64, u64, u64) {
+    *CACHE_STATS.lock().unwrap_or_else(|e| e.into_inner())
+}
+
 fn stream_anthropic(req: &ChatRequest, cancel: &AtomicBool, send: &dyn Fn(StreamEvent)) -> Result<StepOutput, Failure> {
     let mut body = json!({
         "model": req.model,
         "max_tokens": MAX_OUTPUT_TOKENS_ANTHROPIC,
-        "messages": anthropic_messages(req),
+        "messages": [],
         "stream": true,
     });
     let system = system_with_summaries(req);
     if !system.trim().is_empty() {
-        body["system"] = json!(system);
+        // Prompt caching: system + tool schemas are most of every request and stay identical
+        // between the steps of a turn, so both get an ephemeral breakpoint (Anthropic caches
+        // the prefix up to each marked block; the cache is written on the first step of a turn
+        // and read on the rest, which is where a computer-use turn's tokens live).
+        body["system"] = json!([{"type": "text", "text": system, "cache_control": {"type": "ephemeral"}}]);
     }
     if claude_adaptive(&req.model) {
         body["thinking"] = json!({"type": "adaptive", "display": "summarized"});
     }
     if !req.tools.is_empty() {
-        body["tools"] = json!(req.tools.iter().map(|t| json!({"name": t["name"], "description": t["description"], "input_schema": t["parameters"]})).collect::<Vec<_>>());
+        let mut tools: Vec<Value> = req.tools.iter().map(|t| json!({"name": t["name"], "description": t["description"], "input_schema": t["parameters"]})).collect();
+        // One breakpoint on the last tool marks the whole (stable, order-fixed) tools block.
+        if let Some(last) = tools.last_mut() {
+            last["cache_control"] = json!({"type": "ephemeral"});
         }
+        body["tools"] = json!(tools);
+    }
+    body["messages"] = json!(cache_tail(anthropic_messages(req)));
     let mut r = agent()
         .post(&format!("{}/messages", req.base_url))
         .header("Content-Type", "application/json")
@@ -1132,6 +1174,15 @@ fn stream_anthropic(req: &ChatRequest, cancel: &AtomicBool, send: &dyn Fn(Stream
                     }
                 }
             }
+            "message_start" => {
+                if let Some(u) = v["message"]["usage"].as_object() {
+                    let g = |k: &str| u.get(k).and_then(|x| x.as_u64()).unwrap_or(0);
+                    let mut s = CACHE_STATS.lock().unwrap_or_else(|e| e.into_inner());
+                    s.0 += g("input_tokens");
+                    s.1 += g("cache_read_input_tokens");
+                    s.2 += g("cache_creation_input_tokens");
+                }
+            }
             "message_delta" => {
                 if v["delta"]["stop_reason"] == "refusal" {
                     out.text.push_str("\n\n（模型拒絕回答這個要求。）");
@@ -1654,4 +1705,18 @@ mod tests {
         assert_eq!(msgs[i + 1]["role"], "tool");
         assert_eq!(msgs[i + 2]["role"], "tool");
     }
+
+    #[test]
+    fn cache_breakpoints_mark_the_stable_prefix() {
+        // cache_tail marks the last block of the last message, in both content shapes.
+        let m = cache_tail(vec![json!({"role": "user", "content": "hi"})]);
+        assert_eq!(m[0]["role"], "user");
+        assert_eq!(m[0]["content"][0]["cache_control"]["type"], "ephemeral");
+        let m = cache_tail(vec![json!({"role": "user", "content": [{"type": "text", "text": "a"}, {"type": "text", "text": "b"}]})]);
+        assert_eq!(m[0]["content"][1]["cache_control"]["type"], "ephemeral");
+        assert!(m[0]["content"][0]["cache_control"].is_null());
+        assert_eq!(cache_tail(vec![]).len(), 0);
+        let m = cache_tail(vec![json!({"role": "user", "content": []})]);
+        assert!(m[0]["content"].as_array().unwrap().is_empty());
+    }
 }