Possible cache bust when Advisor mode changes tool set

[man-vu]

Possible cache bust when Advisor mode changes tool set

I think I may have found a prompt cache bust related to Advisor mode / tool-set changes in Claude Code. It often happens around 5 minutes after the advisor is called.

I built a small live dashboard that leverages quota-status.js / the quota status history JSONL to watch cache reads, cache writes, hit rate, tool diffs, model usage, session totals, and per-request cost in real time. That dashboard surfaced this pattern.

Environment

• Subscription: Claude Max 20
• OS: Windows 11 26200.8037
• Claude Code: 2.1.119
• Cache Fix: 3.2.0
• Executor model: Sonnet 4.6
• Advisor model: Opus 4.7

What I observed

During a long-running session, the prompt cache appeared stable, then completely missed when Advisor disappeared from the available tool set.

Row	Time	Model	Cost	In	Out	Cache Write	Cache Read	Hit %	Body KB	Tools	Tool diff	Msgs
#139	12:39:51 AM	Sonnet 4.6	$0.0339	1	589	252	78,520	100%	266.7	12	none	85
#140	12:40:01 AM	Sonnet 4.6	$0.0314	1	202	791	78,772	99%	270.1	12	none	87
#141	12:43:08 AM	Sonnet 4.6	$0.2967	3	50	78,919	0	0%	269.1	11	-advisor	89
#142	12:44:35 AM	Sonnet 4.6	$0.0305	3	357	211	79,563	100%	271.0	12	+advisor	89
#143	12:44:40 AM	Sonnet 4.6	$0.0290	1	152	458	79,774	99%	272.8	12	none	91

Important transition:

#140 -> #141
same model: Sonnet 4.6
same general request shape/body size: ~270 KB
messages continued: 87 -> 89
tools changed: 12 -> 11
visible tool diff: -advisor

cache read dropped: 78,772 -> 0
cache write jumped: 791 -> 78,919
hit rate dropped: 99% -> 0%
cost jumped: $0.0314 -> $0.2967

#141 -> #142
visible tool diff: +advisor
cache read recovered: 0 -> 79,563
hit rate recovered: 0% -> 100%
cost returned: $0.2967 -> $0.0305

Why I think this matters

This looks like removing advisor from the tool set invalidated the prompt cache. The request did not look like a new session, model switch, or normal subagent cold start. It looks like a tool-list mutation caused the cached prefix to miss.

Since tools are part of the prompt cache prefix, changing the available tool definitions mid-session could force a full cache rewrite. In this case, the cost for the affected request was roughly 9-10x higher than the surrounding requests.

Question

Is this expected behavior when Advisor mode toggles tools?

If yes, it might be worth documenting that enabling/disabling Advisor mode can bust prompt cache. If no, perhaps Claude Code should keep the advisor tool stable in the tool list and simply choose not to call it unless Advisor mode is active.

Raw logs for forensics

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[cnighswonger]

Your diagnosis is technically correct — tools are part of the prompt cache prefix, so removing one (or adding, or reordering) invalidates the cached prefix from that point forward. The cost jump matches: removing advisor triggered a full prefix rebuild.

The hash data in your forensic logs (system block hashes, tools names_sorted_hash, messages.hash) is exactly the right shape for this class of diagnosis — sharper than most users have available.

Two follow-ups from your data:

1. The TTL flip from 1h → 5m on row release: v3.6.2 — quota-bar statusline + Recommended CC operational config #141 is interesting. Beyond the tool-list-changed bust, the ttl_tier shows the cold-cache request also got 5m rather than 1h. That tracks a separate Anthropic-side behavior we documented this week (vsits.co/ttl-fix-verified/) — Anthropic acknowledged the silent 1h→5m downgrade and CC 2.1.129+ now sends ttl: "1h" explicitly. Worth checking whether reproductions on the latest CC still show the 5m flip on bust transitions.

2. Whether CC should stabilize the tool list is the actionable upstream question. If CC kept advisor in the tools array and just didn't call it when Advisor mode is off, the cache prefix would stay stable. Worth filing as a CC issue if one doesn't exist.

Thanks for the dashboard work — this is the kind of forensic detail that makes proxy-layer debugging tractable.

— Chris