[BUG] appendSystemPrompt with per-turn content silently destroys provider prompt caching

Summary

Custom mods that return per-turn dynamic content from the appendSystemPrompt hook
silently destroy provider prompt caching. Everything appendSystemPrompt emits is
prepended as a prefix before the whole conversation, and provider prompt caches
(Anthropic / OpenAI / DeepSeek-style prefix caching) key on the longest common
prefix — so one changed character invalidates the cache and forces the provider to
re-process the entire history (system prompt + every prior turn) on that turn.

There is no mod-side cache API, and nothing in the ModApi documents or enforces
that appendSystemPrompt output must be byte-stable across turns. It is the
natural hook to reach for when a mod "just wants to add context" — and it is the
worst possible place for content that changes.

A related, separate failure mode: splicing dynamic messages one slot too deep in
transformContext (at length - 2 instead of length - 1) spreads a cache miss
across two trailing messages instead of confining it to the final one.

Expected Behavior

• appendSystemPrompt is documented as a static or one-shot hook: output must
  be byte-identical across turns (or stable per discrete state), or injected once
  behind a guard flag and then return undefined.
• Per-turn dynamic context is supported through a first-class, cache-safe mechanism
  (today the de-facto pattern is tail-append via transformContext).
• In dev mode, the harness warns when a registered appendSystemPrompt output
  changes between turns, so mod authors catch cache-breaking prompt churn early.

Actual Behavior

• The appendSystemPrompt contract is silent on cache stability; mod authors
  reasonably use it for per-turn content (round counters, threshold advisories)
  and get near-zero cacheReadTokens with no diagnostic.
• Any single character change in the system-prompt prefix invalidates the cached
  prefix for the entire conversation history on that turn.
• Incorrect message splice depth in transformContext (e.g. length - 2 instead
  of length - 1) spreads a cache miss across two trailing messages instead of
  confining it to the final one — even when the system prompt itself is stable.

Steps to reproduce the issue

1. Install or author a mod whose appendSystemPrompt hook returns content that
   changes each turn (e.g. embed Round N/M in a briefing prompt, or flip
   advisory text at a threshold crossing).
2. Run a multi-turn session against a provider that supports prefix caching.
3. Observe usage on model_request_end — e.g. via a pure observer mod that reads
   cacheReadTokens / cacheWriteTokens and registers no prompt hooks.
4. Compare against a byte-stable system prompt (static / one-shot
   appendSystemPrompt output, dynamic content appended to the message tail
   via transformContext instead).

Result: system-prompt churn correlates with near-zero cache reads; byte-stable
system prompt + tail injection yields consistent cache hits.

Optional related repro: in transformContext, splice a dynamic recall message at
messages.length - 2 instead of length - 1 and observe the cache miss span two
trailing messages rather than one.

Command Code Version

0.1.2 (Desktop)

Operating System

Windows

Terminal/IDE

Command Code Desktop / CLI

Shell

PowerShell

Session file (optional)

No response

Fix prompt (optional)

Document and harden the mod caching contract:

1. Document appendSystemPrompt: output must be byte-identical across turns
   (or stable per discrete state), or one-shot guarded; per-turn content belongs
   on the message tail, not the system prefix.
2. Consider a first-class cache-safe per-turn injection API — standardize the
   tail-append pattern mod authors already use in transformContext.
3. Add a dev-mode diagnostic: serialize-and-compare appendSystemPrompt output
   per turn; warn when it changes.

Relevant harness surface: appendSystemPrompt, transformContext, ModApi docs,
and optional dev-mode diagnostics. Verify with a two-mod setup: one that injects
dynamic system text (should warn / miss cache) and one observer that only reads
cacheReadTokens on model_request_end.

Additional context

Evidence from our mod suite (all occurrences fixed in the mods on 2026-08-23,
not the harness — see command-code-mods CHANGELOG 1.1.0):

1. Per-round counter in a briefing prompt (command-center). A plan-briefing
   state machine embedded Round N/M in its BRIEFING system prompt. It changed
   every round, forcing a full-history re-process per round (up to maxRounds
   times per briefing). Fixed by removing the counter from appendSystemPrompt
   and tail-injecting it via transformContext.
2. Advisory warnings that change at threshold crossings (quality-guards).
   Four guardrail warnings (drift, test-budget, token-budget, run-length) lived
   in appendSystemPrompt. Their text changed at every warning crossing, so the
   system prompt stopped being a stable prefix and forced full-history
   re-processes. Fixed by moving the same thresholds/counters to a
   transformContext tail injection.
3. Recall injection splicing one message too deep (learn-loop, memory-bank).
   Separate from appendSystemPrompt: recall rode transformContext but was
   spliced at length - 2 instead of length - 1, spreading a cache miss across
   two trailing messages instead of confining it to the final one.

A pure observer mod (cache-tracker, added afterward) confirmed the correlation
end-to-end: system-prompt churn → near-zero cache reads; byte-stable system prompt
→ consistent cache hits. Measured interactive sessions on DeepSeek V4 after the
fix showed ~48.7% hit rate over 25 turns (see suite README).

Root cause

• No mod-side cache API; cache performance is entirely at the mercy of provider
  prefix caching.
• appendSystemPrompt contract undocumented on stability requirements.
• No diagnostic when registered prompt output changes between turns.
• No guidance that transformContext splices should stay at the tail
  (length - 1) to confine misses.

Workaround shipped in affected mods

• appendSystemPrompt carries only static / one-shot / per-state-stable content
  (e.g. memory-bank's session-start digest is one-shot behind a guard flag;
  command-center's BRIEFING/COMPILING/REVIEW prompts are byte-stable within each
  state).
• Per-turn dynamic content (round counter, advisories) moved to transformContext
  tail injection so the cached prefix stays byte-identical.
• Recall/injection splices at length - 1 to confine cache misses to a single
  final message.
• Cache observability lives in a separate observer mod that registers no prompt
  hooks, so instrumentation never changes the measured value.

Mods affected: command-center, quality-guards, learn-loop, memory-bank