diff --git a/plugins/config-audit/knowledge/configuration-best-practices.md b/plugins/config-audit/knowledge/configuration-best-practices.md
index dd2cd51..5c0ba02 100644
--- a/plugins/config-audit/knowledge/configuration-best-practices.md
+++ b/plugins/config-audit/knowledge/configuration-best-practices.md
@@ -6,8 +6,8 @@
 
 ## CLAUDE.md
 
-1. **Keep under 200 lines.** Claude's adherence drops on longer files. If the file exceeds 200 lines, extract sections with `@import`.
-2. **Use `@import` for specs/docs.** `@path/to/spec.md` inlines the file at session start. Max 5 hops. Keeps the main file scannable.
+1. **Optimise for prompt-cache stability.** Place stable content in the first 30 lines (cache-friendly prefix); volatile content (timestamps, dynamic counts, rolling activity logs) goes below that threshold or moves to an `@import`-ed file outside the cache prefix. On Opus 4.7 the dominant cost lever is cache reuse, not file length.[^200lines]
+2. **Use `@import` for specs/docs.** `@path/to/spec.md` inlines the file at session start. Max 5 hops, but keep chains ≤ 2 hops — every `@import` boundary fragments the prompt-cache prefix. Keeps the main file scannable.
 3. **Use HTML comments for maintainer notes.** `<!-- Updated 2026-01-01: reason -->` is stripped before context injection — zero token cost.
 4. **Put personal dev notes in `CLAUDE.local.md`**, not `CLAUDE.md`. Add `CLAUDE.local.md` to `.gitignore`. Team members' sandbox URLs should never appear in git.
 5. **Write `~/.claude/CLAUDE.md` for preferences that apply everywhere.** Communication style, preferred tools, output format — not project-specific config.
@@ -91,3 +91,7 @@
 3. **Use `additionalDirectories` for cross-repo work.** If Claude regularly reads `../shared-lib/`, add it: `{"additionalDirectories": ["../shared-lib/"]}`. Otherwise Claude can't access it without prompts.
 4. **Configure `autoMode.environment` before using auto mode.** Without it, Claude's background safety classifier triggers false positives on your org's internal tool names and domains.
 5. **Add `Agent()` deny rules for sensitive agents.** `{"deny": ["Agent(general-purpose)"]}` prevents the most powerful agent from running without explicit permission.
+
+---
+
+[^200lines]: The "keep CLAUDE.md under 200 lines" threshold was a Sonnet-era adherence heuristic — Sonnet's attention quality dropped on longer files, so trimming raw line count was the optimisation lever. Opus 4.7 uses prompt-cache structure as the dominant cost driver: the first 30 lines must stay byte-stable across turns to keep the cache hit, and `@import` boundaries fragment the cached prefix. A 400-line CLAUDE.md with stable structure outperforms a 150-line file whose top contains a daily-rolling activity log. See `knowledge/opus-4.7-patterns.md` for detection IDs (CA-TOK-001..003).