[FEATURE] Option to suppress the daily currentDate injection from the system prompt (prompt-cache friendliness)

[idirectships]

Problem

Heavy multi-dispatch automation (fleet orchestration, CI-driven headless sessions) reuses a large, stable system preamble that can qualify for full cache_control coverage. The binary unconditionally injects a Today's date is … block into the system context on every session. Because that string changes every calendar day, it shifts the cache-control boundary and forces a full preamble re-write once per day per session start — even when the caller already supplies date/time context through other means (user turn, tool output, MEMORY.md, etc.).

Observed behaviour

Confirmed against the compiled CLI (v2.1.231): the date block is assembled unconditionally in context setup (no env flag or settings.json key suppresses it). It is distinct from the sections moved by --exclude-dynamic-system-prompt-sections (cwd / env / memory-paths / git-status), which has its own open request in #87282.

Proposed solution

Any one of these would solve it — smallest surface first:

1. Env var — CLAUDE_NO_DATE_INJECTION=1 (or CLAUDE_CODE_OMIT_DATE_FROM_SYSTEM_PROMPT=1, consistent with existing bool toggles).
2. settings.json key — omitDateFromSystemPrompt: true, readable from managed / user / project tiers.
3. Relocate to first user turn — instead of suppressing, move the date to the first human turn so the stable system block is unaffected and the date is still model-visible.

Option 3 preserves the date signal at zero cost to cache coverage. Options 1–2 let callers who inject date/time themselves avoid the redundancy entirely.

Expected impact

Estimated 80 %+ cache-hit preservation on the stable system preamble component across daily session restarts, measured against cache_control ephemeral usage in fleet automation. Related: #87282 (dynamic-sections flag / managed config).

[NubeBuster]

Option 3 would not be ideal because this would make the date part of the context which would then not be dynamic so a resumed session then contains information about the date that is stale, which would require us to reinsert the date again when the date changes.