Synchronous long tool call: Claude waits, cache dies
A timeline showing a Claude session that initiates a long synchronous tool call (a test suite or build) lasting six minutes. While Claude waits for the result, the prompt cache expires. When the tool finishes, the result-processing turn costs 12.5 times more than the previous turns.
Synchronous long tool call: Claude waits, cache dies
TTL = 5 min
0
1 min
2
3
4
5
6
7
User
Claude
Cache
✕
HOT
test suite / build (sync)
Claude waiting for tool result…
12.5×
test suite / build / heavy analysis (sync)
cache expires while Claude
is still waiting on the tool
single most expensive
turn of the session
User msg
Claude turn
cheap (cache hit)
12.5×
post-miss
tool
sync tool call
TTL ref.