cline-fix-volatile-msg

name	cline-fix-volatile-msg
description	Stop Cline from burning a cache breakpoint on the volatile current user message every turn.
target_harness	Cline
target_repo	cline/cline
target_files	["src/core/api/transform/anthropic-format.ts"]
target_commit	65e9727c (verify current at apply time)
estimated_savings	~30% wasted cache write premium eliminated per turn

Cline: fix volatile-message cache thrash

Target

src/core/api/transform/anthropic-format.ts in cline/cline.

Permalink: https://github.com/cline/cline/blob/65e9727c/src/core/api/transform/anthropic-format.ts

Symptom

Cline marks the LAST TWO USER MESSAGES with cache_control: ephemeral. The last user message is the CURRENT user turn — it's different on every request by definition. Caching content that changes every request pays the 1.25x write premium on those tokens every turn for zero cache reads. Net result: one of Cline's 3 breakpoints is burned producing nothing.

You can confirm on the wire: in two consecutive identical-prompt turns, the response usage.cache_creation_input_tokens stays > 0 on turn 2 instead of dropping to ~0.

Fix

Replace the "last 2 user messages" logic with "last STABLE message" (typically the previous assistant turn or tool_result):

--- a/src/core/api/transform/anthropic-format.ts
+++ b/src/core/api/transform/anthropic-format.ts
@@
-  const userMsgIndices = clineMessages.reduce((acc, msg, index) => {
-    if (msg.role === "user") acc.push(index)
-    return acc
-  }, [] as number[])
-  const lastUserMsgIndex = userMsgIndices.at(-1)
-  const secondLastMsgUserIndex = userMsgIndices.at(-2)
+  // Cache the LAST STABLE message — the message BEFORE the current
+  // user turn. The current user turn changes every request and would
+  // pay the 1.25x cache-write premium for zero subsequent reads.
+  // The previous turn (assistant message or tool_result) is stable
+  // across the request → response cycle.
+  const lastStableIdx = clineMessages.length >= 2
+    ? clineMessages.length - 2
+    : -1
@@
-    if (supportCache && (index === lastUserMsgIndex || index === secondLastMsgUserIndex)) {
+    if (supportCache && index === lastStableIdx) {
       return addCacheControl(anthropicMsg)
     }

This frees one breakpoint. Optionally use that freed breakpoint to cache the tools array independently — see audits/cline.md for the extended pattern.

Verify

1. Start mitmproxy: mitmdump -p 8090 -w /tmp/cline.flow
2. Configure Cline to route through proxy (Settings → Anthropic Base URL → http://127.0.0.1:8090)
3. Run the same prompt twice in a row in the same Cline task ("list files in this repo")
4. Inspect the second-turn response usage:
   • Before fix: cache_creation_input_tokens > 0 every turn (bug)
   • After fix: cache_creation_input_tokens ≈ 0, cache_read_input_tokens covers system + tools + previous turn

Hit rate (turn 2): should be ≥80% of input tokens.

Background

See docs/gotchas.md #1 ("Putting cache_control on volatile content"). Anthropic caches the entire prefix up to and including the marked block; marking a block that mutates per request means the cache is invalidated every request.

Full audit: audits/cline.md.