From 15e2f240464d115368eeb01317b4a67ce64e8220 Mon Sep 17 00:00:00 2001
From: Tony Gies <tgies@tgies.net>
Date: Sun, 1 Mar 2026 08:58:51 -0600
Subject: [PATCH] fix: skip system messages in OpenRouter cacheAtDepth
 calculation (#5230)

System messages in the OpenRouter messages array were being counted
toward depth and could receive cache_control breakpoints. Since
OpenRouter hoists system messages into Claude's separate system
parameter, this misplaced breakpoints and could prevent caching
entirely if the hoisted content fell below minimum cache size.

Closes #5227
---
 src/prompt-converters.js | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/prompt-converters.js b/src/prompt-converters.js
index 73fc6a23d..f151a6b30 100644
--- a/src/prompt-converters.js
+++ b/src/prompt-converters.js
@@ -1026,6 +1026,11 @@ export function cachingAtDepthForOpenRouterClaude(messages, cachingAtDepth, ttl)
 
         passedThePrefill = true;
 
+        // Skip system messages so they don't affect depth counting or receive cache breakpoints
+        if (messages[i].role === 'system') {
+            continue;
+        }
+
         if (messages[i].role !== previousRoleName) {
             if (depth === cachingAtDepth || depth === cachingAtDepth + 2) {
                 const content = messages[i].content;