aish/docs/PHASE8.md

# aish — Phase 8 Manifest

**Project:** aish — AI-augmented conversational shell
**Document:** Phase 8 Requirements, Architecture & Design Decisions
**Status:** Plan (formulate + analyze + baseline complete; tree at `79bd40d`)
**Date:** 2026-05-16

**Analyze findings (2026-05-16):**

A1. **enforce_budget ONLY checks max_turns, not token_budget — major
    scope gap.** `Context:enforce_budget` (context.lua:319) iterates
    `while #self.turns > self.max_turns`; `self.token_budget = 4096`
    is set but NEVER consulted. So even with accurate tokenization,
    eviction decisions are unaffected — the new `estimate_tokens()`
    only feeds the prompt template's `{ctx_used}` display variable
    (repl.lua:630).

    **Resolution**: extend Phase 8 with a NEW pillar 5: make
    `enforce_budget` honor `token_budget` AS WELL AS max_turns —
    evict the oldest pair when EITHER threshold is exceeded. This
    is the real motivation for accurate tokenization; without it
    Phase 8 is largely cosmetic. Folded into §1 (5 pillars now),
    §3 (context.lua row), §9 (new risk row about under-eviction
    becoming over-eviction if tokenize_fn returns a much higher
    number than char/4).

A2. **`ffi.curl.M.post` signature confirmed.** `(body, status)` on
    success, `(nil, err)` on failure. Matches the formulate-time
    sketch. status is the integer HTTP code. The probe checks
    `status == 200 and out` correctly.

A3. **Single caller of `Context:estimate_tokens()` in tree.** Only
    `repl.lua:630` (prompt template `{ctx_used}` substitution) calls
    it. No internal callers in context.lua. This means:
    - The wiring point is ONE line in repl.lua (the prompt template
      already runs `ctx:estimate_tokens()` on every prompt render).
    - With A1's extension, `enforce_budget` becomes a SECOND caller —
      and a more frequent one (per turn, not per prompt render).
    - Per-turn `_tokens` cache becomes important for the
      enforce_budget path (called from ask_ai after every turn).

A4. **Q-T1 RESOLVED**: per-turn `_tokens` cache lives on the turn
    dict. `:reset` clears `ctx.turns` so the cache dies with them.
    New turns get nil `_tokens`; lazy-set on first count. Trivial.

A5. **Q-T2 RESOLVED**: tokenize_fn closure captures `active_cfg` as
    an UPVALUE. Upvalues are resolved at closure call time, not at
    definition time. When the user `:model cloud` switches,
    `active_cfg = config.models[name]` reassigns the local;
    subsequent tokenize_fn calls see the new value. Natural; no
    explicit re-binding needed.

A6. **Q-T3 RESOLVED**: skip the probe entirely when
    `cfg.tokenize.use_endpoint = false` or unset. Don't even call
    `broker.token_count` — repl.lua won't wire `tokenize_fn` to
    Context.new in the first place. context.lua's tokenize_fn-nil
    branch handles it (char/4 fallback).

A7. **Q-T6 RESOLVED (defer to follow-up)**: tools-schema tokens
    are a fixed cost per session (tools_schema doesn't change unless
    `:mcp connect/disconnect` lands a new session). The under-count
    is bounded and predictable. Defer to a future polish; v1
    counts only messages. Document in §8 out-of-scope.

A8. **Per-turn `_tokens` cache invalidation.** Turn `content` is
    immutable after append (we don't mutate stored turns). Cache is
    safe to live forever on the turn. The only invalidation event
    is `:reset` (clears turns wholesale). No other invalidation
    needed.

A9. **Probe latency baseline** (Q-T4 deferred): probed manually
    during formulate — single tokenize call for ~50 char text ran
    in ~50ms locally. For 40 turns × 500 chars cached = 40 × 50ms
    = 2s ONLY on the first estimate after a fresh session. After
    caching, subsequent estimates are O(1) per turn (dict lookups).

A10. **Streaming during `chat_stream` interleaves with tokenize?**
     No — `Context:estimate_tokens()` is called OUTSIDE the streaming
     callback (in the main loop, before/after broker calls). No
     concurrent network competition.

A11. **MCP tool turn content** — `role:"tool"` turns have `content`
     strings too (the tool result). These get tokenized identically;
     no special-case needed. Cache key is the turn dict itself, so
     tool turns get their own `_tokens` slot.

A12. **`include_usage` interaction with tokenize**: orthogonal. The
     tokenize probe uses a separate (non-streaming) `/tokenize`
     endpoint; never sees the chat completion's stream_options.

PHASE0 is the locked substrate; PHASE1-7 are layered on top. This manifest
specifies what Phase 8 adds — **accurate tokenization**: replace the
char/4 heuristic on `Context:estimate_tokens()` with a per-broker
`/tokenize` round-trip where supported, char/4 fallback otherwise.

Resolves Q1 (`PHASE0.md §13`, originally targeted at Phase 3 — deferred
forward across each phase). PHASE0 §11 amendment to add Phase 8 row
lands in the same commit as this formulate doc.

---

## 1. Scope of Phase 8

Five pillars (A1 added pillar 5):

1. **Per-endpoint tokenize probe (cached)** — at first use, send a
   probe to the broker's tokenize endpoint with a tiny payload; if it
   returns `{tokens: [...]}` we mark the endpoint+model as tokenize-
   capable and use the actual count thereafter. If it 404s or errors,
   mark the slot as `tokenize_supported = false` and fall through to
   char/4 silently. Cached per `(endpoint, model)` for the session.

2. **`broker.token_count(model_cfg, text)`** — thin wrapper that
   returns an accurate token count when the (endpoint, model) is
   tokenize-capable, else the char/4 heuristic. Always returns a
   non-negative integer; never errors. The probe + fallback is
   transparent to callers.

3. **`Context:estimate_tokens()` widening** — currently char/4 over
   `system_prompt` + sum of `turn.content`s. The new shape accepts
   an optional `tokenize_fn` (callback) at `Context.new` time and uses
   it when present; falls back to char/4 when nil. `repl.lua` wires
   `tokenize_fn = function(text) return broker.token_count(active_cfg, text) end`.
   This means the active model's tokenizer is used for budgeting
   decisions, which matches the broker the next ask_ai will hit.

4. **`:cost detail` estimated-vs-actual column** — for each
   (model, category) slot in the accumulator, the actual
   `prompt_tokens` from broker usage is already stored. Add an
   estimated column computed via `broker.token_count` on the
   currently-buffered prompt-shape. Disagreement >10% surfaces in a
   tiny `~est=N` annotation so users can see when the heuristic
   diverges from reality. Display-only; no behavior change.

5. **`enforce_budget` consults `token_budget` (A1)** — currently
   `enforce_budget` only iterates `#turns > max_turns`. Extend to
   ALSO check `estimate_tokens() > token_budget`. Eviction fires
   when EITHER threshold is exceeded; the existing summarize-on-
   evict callback (Phase 5) still gets called per evicted pair.
   This is the real motivation for accurate tokenization — without
   it, the new token counts are display-only. Default budget
   (token_budget = 4096) was set at PHASE0 but never enforced;
   Phase 8 closes that gap.

**Phase 8 is done when:**

- A long-running session with the local `qwen-coder-7b-snappy-8k`
  model evicts at the RIGHT moment (token_budget=4096 hit triggers
  eviction via the new pillar 5 path) rather than only when
  max_turns is exceeded.
- `broker.token_count(local_cfg, "hello world")` returns 2 (matches
  the live tokenize result, not the char/4=2 coincidence — verify
  via `:cost detail` against multi-paragraph text).
- `broker.token_count(cloud_cfg, "hello world")` returns 2 (char/4
  fallback when /tokenize 404s, which it does for OpenRouter).
- Cached per-endpoint capability — the probe fires once per
  endpoint per session, not per call.
- Existing configs without `cfg.tokenize` behave like Phase 7 (zero
  behavior change unless opted in via `cfg.tokenize.use_endpoint = true`).
- `:cost detail` shows estimated-vs-actual where disagreement >10%,
  silent otherwise.

---

## 2. Technology Decisions (delta from Phase 7)

| Decision | Choice | Rationale |
|---|---|---|
| Tokenize endpoint path | `<endpoint>/tokenize` (NOT `<endpoint>/v1/tokenize`) | Per real probe against hossenfelder: `/v1/tokenize` returns 404; `/tokenize` returns `{tokens: [...]}`. This is the llama.cpp server convention. |
| Request body shape | `{"content": "<text>", "model": "<model>"}` | Local model echoed via `model`; llama.cpp ignores it but harmless. Probed shape works. |
| Capability detection | Per-call optimistic probe; on 404/non-200, cache `tokenize_supported[endpoint][model] = false` and never retry that session | One round-trip cost on first miss; zero on subsequent. Sessions are short enough that re-probe across restarts is fine. |
| Fallback heuristic | char/4 (Phase 0 §8 convention) | Established; underestimates ~10% on real code/prose per baseline B1, but acceptable when no better signal available. |
| `Context:estimate_tokens` calling convention | Optional `tokenize_fn` callback at Context.new; absent = char/4 (existing behavior) | Backward-compatible; no caller break. Opt-in via repl.lua. |
| Active-model tokenizer | repl.lua wires `tokenize_fn` against `active_cfg` (the currently active model), so eviction decisions match the broker the next call will hit | When the user `:model cloud` switches mid-session, subsequent estimates use cloud's tokenizer (which falls back to char/4 since OpenRouter has no /tokenize). |
| Caching strategy | Endpoint+model capability flag only; NOT per-text token-count cache | Token counts depend on text content; caching adds memory + correctness risk for marginal speed. Probe latency dominates only on first call per endpoint. |
| Per-text timeout cap | 2s for tokenize calls (much tighter than the model's normal timeout_ms) | Tokenize is a small, fast operation; if it doesn't respond in 2s, the endpoint is misbehaving. Bail to char/4. |
| `:cost detail` est-vs-actual | Show only when disagreement >10%; format `(prompt: 558 ~est=508 / completion: 80)` for the disagreement case, `(prompt: 558 / completion: 80)` otherwise | Always-on noise; suppress when heuristic is close. |
| New config key | `cfg.tokenize = { use_endpoint = true }` — default false until user opts in | Network round-trip cost; user-acknowledged behavior change. |

---

## 3. Module Changes

| File | State after Phase 7 | Phase 8 changes |
|---|---|---|
| `broker.lua` | `chat`, `chat_stream`, `build_request` (opts-widened in Phase 7) | New `M.token_count(model_cfg, text)`: tries `<endpoint>/tokenize` once per (endpoint, model); caches capability; returns int. New `M.tokenize_supported(model_cfg)` introspection helper for tests. |
| `context.lua` | `estimate_tokens()` char/4 sum over system_prompt + turn.contents; `enforce_budget()` only checks max_turns | Widen `estimate_tokens` to use `self.tokenize_fn(text)` if present; else char/4. Per-turn `_tokens` cache on each turn dict; lazy-set on first count. Extend `enforce_budget` to ALSO evict when `estimate_tokens() > token_budget` (A1 — pillar 5). |
| `repl.lua` | wires Context.new with summarize_fn, hosts all metas | `tokenize_fn` wired into Context.new when `cfg.tokenize.use_endpoint = true`. `:cost detail` extended with est-vs-actual column. |
| `config.lua` | Phase 7 cost block example | Add commented-out `tokenize = { use_endpoint = true }` block. |
| `docs/PHASE0.md` | §11 lists phases 0-7 | Amendment: add Phase 8 row to §11. |

No new module files.

---

## 4. Pillar 1+2 — `broker.token_count(model_cfg, text)`

```lua
-- Per-endpoint capability cache (session-scoped local in broker.lua)
local _tokenize_capable = {}    -- [endpoint .. "/" .. model] = true | false

local function _cache_key(model_cfg)
    return (model_cfg.endpoint or "") .. "/" .. (model_cfg.model or "")
end

function M.token_count(model_cfg, text)
    text = text or ""
    if text == "" then return 0 end
    if not (model_cfg and model_cfg.endpoint) then
        return math.floor(#text / 4)   -- pure fallback
    end
    local key = _cache_key(model_cfg)
    local cap = _tokenize_capable[key]
    if cap == false then
        return math.floor(#text / 4)
    end
    -- cap == nil OR cap == true; try the endpoint.
    local url = model_cfg.endpoint:gsub("/+$", "") .. "/tokenize"
    local body = json.encode({ content = text, model = model_cfg.model })
    local out, status = curl.post(url, body,
        { "Content-Type: application/json" },
        2000)  -- 2s timeout
    if not (status == 200 and out) then
        _tokenize_capable[key] = false
        return math.floor(#text / 4)
    end
    local doc = json.decode(out)
    local toks = doc and doc.tokens
    if type(toks) ~= "table" then
        _tokenize_capable[key] = false
        return math.floor(#text / 4)
    end
    _tokenize_capable[key] = true
    return #toks
end

function M.tokenize_supported(model_cfg)
    if not model_cfg then return nil end
    return _tokenize_capable[_cache_key(model_cfg)]
end
```

Uses Phase 1's `ffi/curl.M.post` (blocking POST, returns body + status).

---

## 5. Pillar 3 — `Context:estimate_tokens` widening

```lua
function M.new(opts)
    ...
    return setmetatable({
        ...
        -- Phase 8: optional callback that returns an accurate token
        -- count for a given text. Set by repl.lua when cfg.tokenize.
        -- use_endpoint=true, calling broker.token_count(active_cfg, ...).
        -- nil = char/4 fallback (Phase 0 §8 behavior).
        tokenize_fn          = opts.tokenize_fn,
    }, Context)
end

function Context:estimate_tokens()
    if self.tokenize_fn then
        local n = self.tokenize_fn(self.system_prompt)
        for _, t in ipairs(self.turns) do
            n = n + self.tokenize_fn(t.content)
        end
        return n
    end
    -- char/4 fallback (existing behavior)
    local n = #self.system_prompt
    for _, t in ipairs(self.turns) do n = n + #t.content end
    return math.floor(n / 4)
end
```

Performance note: with N turns of average ~500 chars each, `estimate_tokens`
fires N tokenize round-trips. For N=40 turns × 50ms = 2s — too slow for
per-prompt eviction checks. **Mitigation**: per-turn token count cached
ON the turn dict (`turn._tokens`) the first time it's counted; only
re-tokenized if `turn._tokens` is nil. Set at append time when
tokenize_fn is present; otherwise lazily on first estimate.

---

## 6. Pillar 4 — `:cost detail` est-vs-actual

Current `:cost detail` (Phase 7) shows:

```
  anthropic/claude-haiku-4.5 main                 1 calls,    179 /      8 tokens, $0.000219
```

The `179 / 8` is `prompt_tokens / completion_tokens` from the actual
broker usage payload.

Phase 8 extension: for each slot, also compute an estimated count via
`broker.token_count(model_cfg_for_this_slot, ...)` over the
TURNS THAT CONTRIBUTED to this slot. But that's stateful and
expensive — simpler: show the SUM of `prompt_tokens` (actual) and
the SUM of `estimate_tokens()` (heuristic OR endpoint-based, depending
on what tokenize_fn is wired). If disagreement >10%, annotate.

Simplified format:

```
  anthropic/claude-haiku-4.5 main                 1 calls,    179 ~est=164 / 8 tokens, $0.000219
                                                                    ^^^^^^^^^^ shown when |actual-est|/actual > 0.10
```

The `~est=N` annotation only renders when the disagreement exceeds the
10% threshold. Silent otherwise.

---

## 7. UX Surface Summary

| Meta | Behavior change |
|---|---|
| `:cost detail` | Adds `~est=N` annotation per slot when heuristic disagreement >10% |
| (no new metas in v1) | |

| Config | Default | Effect |
|---|---|---|
| `cfg.tokenize.use_endpoint` | false | When true, repl.lua wires `tokenize_fn` so context budgeting uses real token counts |

The `cfg.tokenize` block being opt-in is conservative: enabling it
means every `Context:estimate_tokens()` call may hit the broker. For
local llama.cpp the cost is ~50ms; for cloud-only configurations there
IS no /tokenize endpoint so we silently fall through to char/4 (cached
after one probe). No surprise; document in config example.

---

## 8. Out of Scope (Phase 8)

- **Cost preflight enforcement** — option 2 of the Phase 7 §12
  candidates. The tokenize work here is a PREREQUISITE for accurate
  preflight cost estimation, but the enforcement layer itself
  (cap_at_dollars that REFUSES the call) is its own surface — defer
  to a separate phase.
- **Cross-session cost rollup** — option 1 of Phase 7 §12 candidates.
  Independent of tokenization.
- **Streaming tokenize** — some servers expose streaming tokenize
  endpoints for partial-prompt token counts during generation. Out
  of scope here; we use the blocking /tokenize for batch estimates.
- **Multi-tokenizer support** (e.g. tiktoken for OpenAI compat,
  sentencepiece for HuggingFace) — would require vendoring a C library
  (violates PHASE0 §3) or shelling out to python. Endpoint-based is
  the only substrate-compliant option for accuracy beyond char/4.
- **Tokenization for `:cost detail` rows that span multiple turns**
  — the actual `prompt_tokens` in the accumulator slot is the sum
  ACROSS calls; the estimate for comparison should be over the
  CURRENT ctx content. Show the per-call comparison only.

---

## 9. Risks

| Risk | Mitigation |
|---|---|
| `/tokenize` 404 silently cached as `tokenize_supported = false` for a typo'd endpoint config | Per-session cache; restart re-probes. Acceptable. |
| Tokenize round-trip on every prompt eviction check adds 50ms × N turns latency | `turn._tokens` per-turn cache set at append-time; only re-tokenize on cache miss. |
| Hossenfelder proxy may forward `/tokenize` differently than direct llama.cpp (e.g., adds `/v1/` prefix expected) | B1 confirms `/tokenize` works against hossenfelder; other proxies untested but the design degrades gracefully (char/4 fallback). |
| Cloud models without /tokenize emit no probes after first 404 — fine but `:cost detail` est-vs-actual will always agree (both are char/4 then) | Documented; no fix needed. Display annotation hides when est=actual exactly OR within 10%. |
| `Context:estimate_tokens` callers downstream expect synchronous fast return (currently O(N) string ops); new path is O(N) round-trips | Per-turn cache makes amortized cost O(1) per turn after first count. |
| Endpoint URL handling — currently `endpoint .. "/v1/chat/completions"` is hardcoded; tokenize uses `endpoint .. "/tokenize"` (no /v1) — asymmetric | Document the asymmetry inline; the llama.cpp convention is that completions go through /v1 (OpenAI compat) but server-internal endpoints like /tokenize do not. |
| A1 pillar 5 — accurate tokenization could cause EARLIER eviction than the char/4 heuristic (real counts are higher per baseline). User session that fit in 4096 tokens under char/4 may now spill. | Default `token_budget = 4096` was set in Phase 0; accurate counts mean Phase 8 finally ENFORCES it. Users on `cfg.context.token_budget` defaults may see eviction earlier than before — document as intentional. Users can raise `token_budget` per their model's real context window. |

---

## 10. Open Questions (Phase 8)

| # | Question | Impact | Resolution target |
|---|---|---|---|
| Q-T1 | Per-turn `_tokens` cache across `:reset` | A4 — dies with turns; new turns get nil and lazy-set on first count. Trivial. |
| Q-T2 | `tokenize_fn` re-bind on `:model` switch | A5 — closure captures `active_cfg` upvalue; resolved at call time; follows `:model` switch naturally. No explicit re-binding needed. |
| Q-T3 | Probe respects opt-out | A6 — when `cfg.tokenize.use_endpoint = false`, repl.lua doesn't wire `tokenize_fn`; context.lua's nil branch takes the char/4 fallback. No probe call at all. |
| Q-T4 | Tokenize round-trip latency | A9 — ~50ms per call locally for typical ~500-char turn. With per-turn cache, amortized O(1) per turn after first count. |
| Q-T5 | `/tokenize` honors `model` field | **Baseline** (not yet probed in detail — assumed yes since llama.cpp echoes `model` in completions; needs explicit multi-model probe). |
| Q-T6 | tools-schema tokens | A7 — deferred to follow-up. Tools schema is fixed per session (changes only on :mcp connect/disconnect); under-count is bounded. v1 counts messages only. |

---

## 11. Phase 8 → Phase 9+ Out-of-band

Candidate follow-ups (non-binding):

- **Phase 9**: cost preflight enforcement (Phase 7 §12 option 2) —
  uses Phase 8's accurate token counts to refuse calls that would
  cross `cap_at_dollars`. The accuracy work here is the foundation.
- **Cross-session cost rollup** (Phase 7 §12 option 1) — independent;
  could land in parallel.
- **Phase X**: project-local config overlay (`.aish.lua`) — was the
  alternative scope to Phase 7's cost work. Still valuable but
  independent of any current line.

Phase 8 itself is self-contained — no upstream dependencies.

---

## 13. Implementation Plan (commit-by-commit)

Bottom-up: broker first (the egress capability all callers depend
on), then context (the consumer + the new pillar 5 budget extension),
then repl.lua wiring + display, then config + status bump. Each
commit leaves the tree green (existing tests + load smoke + per-
commit feature smoke).

### Order

1. **`broker.lua` — `M.token_count` helper + per-endpoint capability cache.**
   - Module-local `_tokenize_capable` table keyed by `endpoint .. "/" .. model`.
   - `M.token_count(model_cfg, text)`:
     - empty text -> 0
     - bad cfg (no endpoint) -> char/4 immediately
     - capability cache says `false` for this slot -> char/4
     - otherwise: probe `<endpoint>/tokenize` with `{content, model}` body,
       2s timeout. On `status == 200 + parseable {tokens=[...]}`:
       cache `true`, return `#tokens`. Anything else (non-200, parse
       fail, transport err): cache `false`, char/4.
   - `M.tokenize_supported(model_cfg)` returns the cache slot for
     introspection (tests + future :tokenize meta).
   - Smoke: hand-call `M.token_count(local_cfg, "hello world")` -> 2;
     `M.token_count(cloud_cfg, "hello world")` -> 2 (char/4 fallback;
     cache marks cloud as unsupported on first try).

2. **`context.lua` — estimate_tokens widening + per-turn cache.**
   - Context.new accepts `opts.tokenize_fn` -> stored as `self.tokenize_fn`.
   - `Context:estimate_tokens()`:
     - if `tokenize_fn` is nil: existing char/4 (no behavior change).
     - else: tokenize `system_prompt` (no caching — system prompt
       changes per turn due to dynamic blocks).
       For each turn: if `turn._tokens` is set use it; else
       compute via tokenize_fn AND cache on turn._tokens.
   - No new helper; the change is internal to estimate_tokens.
   - Smoke: synthetic Context with stub tokenize_fn that returns N=42
     for every call; verify estimate sums correctly + cache populates
     turn._tokens.

3. **`context.lua` — enforce_budget honors token_budget (pillar 5).**
   - Existing `while #self.turns > self.max_turns` loop extended to:
     `while #self.turns > self.max_turns OR self:estimate_tokens() > self.token_budget do`.
   - Per-pair eviction otherwise unchanged (summarize callback, status_evictions).
   - The estimate_tokens call inside the loop is potentially expensive
     under tokenize_fn — but commit #2's per-turn cache means each
     iteration is O(#turns) dict-lookups. Acceptable for the eviction
     hot path.
   - Smoke: Context with `token_budget = 100`, max_turns = 100, fill
     with turns until `estimate_tokens() > 100`, then call enforce_budget
     — should evict until under budget. With char/4: eviction happens
     at known char counts. With tokenize_fn stub returning 50/turn:
     evicts down to <= 100/50 = 2 turns.

4. **`repl.lua` — tokenize_fn wiring + :cost detail est-vs-actual.**
   - When `config.tokenize and config.tokenize.use_endpoint`, build
     `ctx_opts.tokenize_fn = function(text)
        return broker.token_count(active_cfg, text)
      end`. The closure captures `active_cfg` upval per A5 — follows
     `:model` switches naturally.
   - `:cost detail` extension: per slot, compute the sum of actual
     `prompt_tokens` (already in usage_totals) AND an "estimated"
     count via... actually this needs a rethink: the accumulator
     doesn't store per-turn data, just running sums. The estimate
     for comparison would need to be over ctx's CURRENT content
     (a single snapshot, not per-call). Simplest: show a SECOND
     line under `:cost detail` with the current `estimate_tokens()`
     value, labeled "[estimated session ctx: N tokens]". Disagreement
     with the accumulator's prompt_tokens TOTAL surfaced inline.
   - Smoke: with use_endpoint=true on a local-only session, observe
     enforce_budget eviction timing vs disable; observe :cost detail
     estimate row.

5. **`config.lua` example block + `docs/PHASE8.md` status bump.**
   - Commented-out `tokenize = { use_endpoint = true }` block in
     config.lua with parity to Phase 1-7 example blocks. Document
     the per-endpoint network cost (one probe per session) and the
     implication: token_budget actually enforces now.
   - PHASE8.md status header -> **Implement**.

### Risk index per commit

| Commit | Risk | Mitigation |
|---|---|---|
| 1 (broker) | Per-endpoint cache leaks across model_cfg deletions (e.g., user removes a model from config mid-session) | Cache is keyed by string; stale entries don't grow without bound (bounded by #configured models × 1). No GC needed. |
| 1 (broker) | /tokenize probe blocks the calling thread for 2s on a misconfigured endpoint | 2s timeout is the cap; one-shot per endpoint per session. |
| 2 (context) | per-turn `_tokens` cache miss on every estimate when no tokenize_fn -> existing perf preserved | Cache check is conditional on tokenize_fn presence; char/4 path untouched. |
| 3 (context) | enforce_budget loop now calls estimate_tokens potentially every iteration; with tokenize_fn that's O(#turns) per iteration -> O(#turns^2) worst case | Per-turn cache makes this O(#turns) amortized after first fill. For typical max_turns=40 + token_budget=4096 sessions: ~40^2 dict lookups = 1600 ops in worst case, microsecond cost. |
| 3 (context) | accurate counts mean token_budget=4096 (Phase 0 default) finally ENFORCES — sessions that fit under char/4 may now evict earlier | Documented in §9; user can raise token_budget to match their model's real context window. |
| 4 (repl) | tokenize_fn closure binding to `active_cfg` upval — if upval somehow gets reassigned wrong, eviction uses wrong tokenizer | Lua upvalues are call-time-resolved; A5 verified. Test by smoke after `:model` switch. |
| 5 (config + status) | none | |

### Tests + smoke per commit

Each commit:
- Pass `luajit test_safety.lua` (87/87) and `luajit test_router_model.lua` (31/31)
- Load cleanly via `luajit -e 'package.path=...; require("repl"); print("ok")'`
- Pass a per-feature smoke (described in each row above)

### Things deliberately NOT split

- New module file for tokenize — small enough to live in broker.lua.
- Per-text token cache (in addition to per-turn): not needed; turn
  content is immutable post-append.
- :tokenize meta for introspecting the cache — `M.tokenize_supported`
  is exported for testing; if a user needs runtime visibility, that's
  a follow-up.

### Open at plan-time (resolve at implement)

- :cost detail layout — how exactly to show "estimated session ctx"
  relative to the existing per-slot rows. Pick at commit 4 (likely
  a single trailing line under the detail table).
- Whether to expose `:tokenize <text>` for direct-probe debugging.
  Nice-to-have; defer unless useful during verify.