Connfirmed this behavior still exist in Claude Code v2.1.92.

[Cojad]

Round A — Header ON（控制組）

Session	Prompt	cache_read	cache_creation	input	output	hit_ratio
1	What is the capital of France? Answer in one word.	11,242	6,073	3	4	.6491
2	List three prime numbers under 20.	11,242	6,069	3	32	.6493
3	Explain what a goroutine is in one sentence.	11,242	6,073	3	70	.6491
4	Name one benefit of TypeScript over JavaScript.	11,242	6,070	3	52	.6492

小計: cache_read 總計 44,968 / cache_creation 總計 24,285 / 平均 hit ratio 64.92%

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Round B — Header OFF（實驗組）

Session	Prompt	cache_read	cache_creation	input	output	hit_ratio
1	What is the capital of France? Answer in one word.	11,242	6,087	3	4	.6486
2	List three prime numbers under 20.	17,311	0	3	11	.9998
3	Explain what a goroutine is in one sentence.	17,315	0	3	65	.9998
4	Name one benefit of TypeScript over JavaScript.	17,312	0	3	52	.9998

小計: cache_read 總計 63,180 / cache_creation 總計 6,087 / 平均 hit ratio 91.20%

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關鍵對比（Session 2–4）

指標	Round A (ON)	Round B (OFF)	差異
平均 cache_read	11,242	17,313	+54%
平均 cache_creation	6,071	0	-100%
平均 hit_ratio	.6492	.9998	+54%

Round B Session 2–4 達成 99.98% cache hit，且 cache_creation 為 0，證實關閉 attribution header 後，跨 session 的 system prompt + tools block 完整命中 cache，不再每次重建。

Thanks for sharing those valuable insights.