currentdate-normalize: mask CC's daily date injection so midnight stops busting the prefix

[deafsquad]

The defect

CC stamps the date into the claudeMd system-reminder block:

# currentDate
Today's date is 2026-08-16.

It sits deep inside the first user message's injected claudeMd block —
measured ~30k characters into messages[0].content[3]. When the calendar day
rolls over mid-session that value advances by one, and everything from byte 0
through the first cache_control marker invalidates. A full cold write, once
per midnight, for every session still running. ~386k tokens on the session
that motivated it (2026-04-23 00:34:50).

It is the one cache bust you can predict to the minute, and it scales with how
long sessions live rather than with what they do.

Approach

Write a canonical 0000-00-00 placeholder in place of the value.

• The model still sees the # currentDate header, so the signal reads as
  abstracted rather than missing.
• CC still delivers the real rollover separately through its own
  "The date has changed" system-reminder when it actually happens, so nothing
  the model needs is lost — only the byte that changes on a timer.
• The placeholder keeps the YYYY-MM-DD shape so anything downstream that
  parses that format keeps parsing.

Notes

• Idempotent by construction. The pattern carries a negative lookahead on
  the placeholder, so a second pass finds nothing and produces identical bytes.
  A proxy can see the same body more than once (retries, replays), and a
  non-idempotent mask could emit two different versions of one body.
• Not anchored on # currentDate. CC emits the line both with and without
  that header across claudeMd shapes; anchoring would miss half the cases. The
  sentence is specific enough to match anywhere — the tests pin that
  Todays date is …, Today's date is unknown., 2026-8-16 and a bare
  The release was on 2026-08-16. are all left alone.
• Order 310, beside identity-normalization (300), which stabilizes the
  same class of volatile per-turn field, and before the cache_control mutators.
• Also masks the line in a top-level system block, where it has been observed
  as well.

Verification

The headline test replays one session serialized either side of a rollover:
the two bodies differ before the mask (asserted, so the premise cannot rot) and
are byte-identical after. node --test → 13/13. Also ran
session-key-invariants (4/4) and proxy-pipeline (15/15) locally;
tools/absence-scan.mjs clean over both files. Your CI is the authoritative
run — it caught a real design bug in #340 that my local runs missed.

Load-bearing?

Yes — it rewrites bytes inside the cached prefix.

— Claude Opus 5, working with @deafsquad