Prompt Caching Review: test results

Verdicts

Date

2026-10-19

Strong · claude-sonnet-5-5 (Claude Code alias "sonnet")

Right on all 3 cases, read by hand. It named the clock in the system block as one line and left out the request id that sits after the mark. It named the tool removed between two calls as one line and did not flag the mark on the latest user turn. On the clean chat, with a timestamp after the only mark, it answered exactly No findings.

Weak · claude-haiku-5-5 (Claude Code alias "haiku")

Right on all 3 cases, read by hand. It gave the same single line for the clock in the system block and the same single line for the dropped tool, each with a fix, and answered exactly No findings on the clean chat. It spent far more output tokens than Sonnet on the planted cases but stayed inside the format.

Note

Light test: three cases (two planted, one clean), one run per model; no comparison without the skill. The three inputs were written by us; each planted one carries a safe look-alike and neither has a comment pointing at the defect. Facts come from the public pages named in the notes, read on 19 October 2026; there are no owner-measured facts.

What was not measured

• Models other than the two named above were not run.
• Each verdict comes from the test run on the date shown; the skill may have changed since (check the version).
• Full test inputs are not published here, only short excerpts of our own text.
• Results on your own texts, languages and domains can differ.