PRD: add Phase 20 (model matrix, wired) and Phase 21 (context stability,
partial). Phase 21 exists because LLM Team hit this exact wall — running
multi-model ranking on large context silently truncated, rankings
degraded, no pipeline caught it. The stable answer: every agent call
goes through a budget check against the model's declared context_window
minus safety_margin, with a declared overflow_policy when the check
fails.
config/models.json:
- context_window + context_budget per tier
- overflow_policies block: summarize_oldest_tool_results_via_t3,
chunk_lessons_via_cosine_topk, two_pass_map_reduce,
escalate_to_kimi_k2_1t_or_split_decision
- chunking_cache spec (data/_chunk_cache/, corpus-hash keyed)
agent.ts:
- estimateTokens() chars/4 biased safe ~15%
- CONTEXT_WINDOWS table (fallback; prod reads models.json)
- assertContextBudget() — throws on overflow with exact numbers, can
bypass with bypass_budget:true for callers with their own policy
- Wired into generate() and generateCloud() so EVERY call is checked
scenario.ts:
- T3 lesson archive to data/_playbook_lessons/*.json (the old
/vectors/playbook_memory/seed path was silently failing with HTTP 400
because it requires 'fill: Role xN in City, ST' operation shape)
- loadPriorLessons() at scenario start — filters by city/state match,
date-sorted, takes top-3
- prior_lessons.json archived per-run (honest signal for A/B)
- guidanceFor() injects up to 2 prior lessons (≤500 chars each) into
the executor's per-event context
- Retrospective shows explicit "Prior lessons loaded: N" line
Verified: mistral correctly rejects a 150K-char prompt (7532 tokens
over), gpt-oss:120b accepts it with 90K headroom. The enforcement is
in-band on every call now, not an afterthought.
Full chunking service (Rust) remains deferred to the sprint this feeds:
crates/aibridge/src/budget.rs + chunk.rs + storaged/chunk_cache.rs
6e7ca1830e