package pipeline

import (
	"errors"
	"fmt"
	"log/slog"
	"os"
	"path/filepath"
	"sort"
	"strings"
	"sync"
	"unicode"

	"textmachine/backend/internal/checks"
	"textmachine/backend/internal/chunk"
	"textmachine/backend/internal/config"
	"textmachine/backend/internal/lang"
	"textmachine/backend/internal/ledger"
	"textmachine/backend/internal/llm"
	"textmachine/backend/internal/membank"
	"textmachine/backend/internal/store"
)

// runner.go: runner setup — Runner and its wiring (config stack, store in the right
// ownership mode, client/template caches). The book machinery itself is split across
// related files (package №4): snapshot — snapshot.go, seed/ruby — seeding.go, the book
// loop — bookrun.go, the chunk disposition loop — chunkrun.go, stage execution
// with money (reserve → call → settle+checkpoint) — stagerun.go, single-hop
// escalation with re-gating — escalation.go, $0-resume of a resolved one — resume.go.

// roleTranslator is the stage role whose output is a direct source→target
// translation — the only role the excision coverage-gate is meaningful on.
const roleTranslator = "translator"

// roleEditor is the BILINGUAL editor stage (D30.1 supersede of D1): editor.md feeds it BOTH the
// source and the draft, plus the approved glossary's CONFIRMED dst forms as target-consistency
// constraints. (The stale "monolingual, never sees the source" framing is D1, fully superseded.)
const roleEditor = "editor"

// Runner executes a book's pipeline.
type Runner struct {
	Book     *config.Book
	Models   *config.Models
	Pipeline *config.Pipeline
	Store    *store.Store
	Pricer   *ledger.Pricer
	Log      *slog.Logger
	// Resnapshot re-pins existing jobs to the CURRENT snapshot when configs/
	// prompts changed since the job started (tmctl --resnapshot). Without the flag
	// a divergence is a loud error: a context change invalidates the checkpoints and
	// re-pays for the calls, so it is done only deliberately (Р6).
	Resnapshot bool
	// AcceptRebill is the operator's consent to the projected RE-PAYMENT of already-billed work
	// (tmctl --accept-rebill[=usd], D20.2-Q2). It is ORTHOGONAL to Resnapshot: that flag grants
	// permission to re-pin, this one consents to an AMOUNT (rebill.go). Zero value = no consent, which
	// only matters when the projection exceeds the book's threshold.
	AcceptRebill RebillConsent
	// VerifyBank is the operator's request to STOP at the bank-mining boundary and review the bank before
	// the edit wave (tmctl --verify-bank, D39.42 п.5). Default OFF: the run consolidates what it mined,
	// carries it forward marked-unverified, and finishes — «просто как намайнит и закончит». It is an
	// operator MODE, not a wire or snapshot axis: it changes when a human is asked, never what is sent.
	VerifyBank bool

	clients   map[string]llm.LLMClient
	templates map[string]*PromptTemplate
	// repairTemplates holds one prompt per ENABLED repair class (pack-16), keyed by class. Empty when the
	// repair gate is off, which is the default; its SHAs are folded into the snapshot of the wave that owns
	// the shipping stage, because a repair prompt is not a stage and the per-stage fold cannot see it.
	repairTemplates map[checks.RepairClass]*PromptTemplate
	// terminologyTemplate is the pair's TERMINOLOGIST role prompt (pack-20), loaded only when the gate is on.
	// nil otherwise, which is what makes the whole role a no-op for every book that does not enable it. It is
	// NOT snapshot-folded: the role writes a signature map, not a checkpoint (config.TerminologyGate).
	terminologyTemplate *PromptTemplate
	// classifierTemplate is the pair's §2 type-classifier prompt, loaded only when classify_types is on. nil
	// otherwise → the classifier phase is a no-op and the draft heuristic type stands.
	classifierTemplate *PromptTemplate
	// targetScript is the target language's Unicode script, resolved once from gates.terminology
	// .target_script (config validates the name). nil when the book declares none — the answer-language
	// screen is then inert, which loadTargetScript says out loud if any channel could have used it.
	targetScript *unicode.RangeTable
	// rateGuards is the per-model wave-concurrency guard set (WS1 §1б), built once in the precompute pass and
	// read-only in the waves — a transport axis, never snapshot-folded. nil until buildRateGuards.
	rateGuards map[string]*rateGuard

	// bankSrc is the banknote parser's "is this surface in the book?" index (backlog 19), built once from
	// the chunk manifest in the precompute pass. nil outside a book run → the parser then judges a line by
	// the chunk text it was handed, and accepts nothing without one.
	bankSrc *bankSourceIndex

	// memory is the book's glossary FROZEN for this job (materialized once in
	// TranslateBook, after seeding, before snapshotID). Its Version() is the F1
	// content-hash folded into the snapshot; its Select drives per-chunk injection.
	// nil until materialized (report path / no glossary) → memoryVersion() falls back
	// to the empty-materialization hash, a stable constant.
	memory *membank.Bank

	// baseMemory is the DRAFT-wave glossary bank: the BASE rows only (Source∈{seed,ruby,auto}, EXCLUDING
	// Source:mined). The draft wave's injection MUST be selected over this — not the enriched `memory` —
	// so it is BYTE-IDENTICAL across a bank-mining enrichment, matching the draft-wave snapshot which folds
	// baseMemoryVersion (mined-excluded). Otherwise signing a mined term would change the draft wire (the
	// injection is a message folded into request_hash) and silently re-bill the draft wave on the owner
	// re-run — even though the base snapshot is unchanged (the review-confirmed "ONE re-payment" hole).
	// When the book has NO mined rows (every $0 test / the golden), it is the SAME object as `memory`
	// (identical content) — no double materialization, the injection is unchanged. The editor keeps the
	// enriched `memory`. nil ⇔ memory is nil (materialized together in seedGlossary).
	baseMemory *membank.Bank

	// checkers is the compiled WS5/pack-13 observability checker spec (pair-14 data-out): the pair's
	// DETECTION patterns + tables (from pack) + the target-general lists (from embedded target data),
	// compiled once in loadLangPack. nil-safe: a no-pack / no-target-data book runs the checkers inert.
	checkers *checks.Checkers

	// pack is the book's language-data pack (internal/lang), loaded once in openRunner from
	// book.LangpackRoot (D39.15/16). The the bank-mining stop bank-miner reads its tables; pack.Version() is folded into
	// the snapshot (a pack edit is a loud --resnapshot). nil when the book declares no langpack_root, or
	// its pair has no catalog dir — then the miner is inert (the bank-mining stop auto-continues) and the fold is omitted.
	pack *lang.Pack

	// escMu serializes the single-hop escalation budget admission across the PARALLEL draft workers
	// (R1). escalationBudgetRemains is a non-atomic read-then-act over EscalationSpentUSD, so N concurrent
	// draft chunks could each read spent<budget and all be admitted → the premium soft-cap overshoots by
	// up to N-1 hops. Holding escMu across the budget check AND the fresh hop's settle makes the cap exact
	// (a later chunk reads the updated spend). Escalations are the rare content-failure exception, so the
	// contention is negligible; the $0 checkpoint-replay of an already-paid hop stays lock-free.
	escMu sync.Mutex

	// lastMinedCount is the size of the most recent the bank-mining stop mined delta (set by runBankMiningStop) — carried into the
	// WaveSignatureStop the driver returns so the CLI can report "N terms await signature". Single-writer
	// (runBankMiningStop runs between the parallel waves, not inside one), so no synchronization is needed.
	lastMinedCount int
	// lastTerminology / lastBankStopRows are the bank-mining stop's RICH output (pack-20): the
	// terminologist's outcome counters and the projected rows the operator-facing stop renders — the table
	// D39.36 promised (src · dst · frequency · spread · evidence) instead of a term count. Set by the same
	// single writer as lastMinedCount. The merged candidate slice itself is deliberately NOT held: it is
	// consumed where it is built, and a book-sized slice kept alive for nobody is a leak with a comment.
	lastTerminology  terminologyResult
	lastBankStopRows []BankStopRow

	// repinCache memoizes the $0 re-pin predicate (repin.go) per (stored snapshot, current snapshot). The
	// draft wave calls it from N parallel workers over the SAME pair of snapshots, so without the cache the
	// store would be read once per chunk for an answer that cannot differ.
	repinMu    sync.Mutex
	repinCache map[string]bool
}

// NewRunner loads all three configs rooted at book.yaml and opens the project
// store. Caller owns Close.
func NewRunner(bookPath string, logger *slog.Logger) (*Runner, error) {
	return openRunner(bookPath, logger, true)
}

// NewReadOnlyRunner builds a runner for the $0 read-only commands (`tmctl report`/`status`): it
// SKIPS the API-key preflight so an auditor WITHOUT provider keys can read the store (D20.4). Those
// commands make zero LLM calls (they only project the persisted rows / re-chunk the source), so a
// missing key is irrelevant. The config-mechanics lint (CheckRunnable) still runs on this path, and so
// does content-routing RESOLUTION — the read models re-render the snapshot, so they must see the same
// resolved models translate did — but the routing REFUSALS are write-only (openRunner). translate and
// redrive (which DO call providers) keep NewRunner.
//
// The store opens READ-ONLY and WITHOUT the exclusive flock (store.OpenReadOnly), so
// status/report work DURING a live translate — previously the operator was locked out of them for the
// entire run (the pain of the package №4 smoke-run). The first touch of a project (no DB file
// yet) falls back to a full Open: create+migrate an empty database — the prior
// behavior "status before the first run shows 0/N pending", and a live writer
// to collide with does not exist at that moment.
func NewReadOnlyRunner(bookPath string, logger *slog.Logger) (*Runner, error) {
	return openRunner(bookPath, logger, false)
}

// openRunner loads the config stack and opens the store. forWrite selects the call
// path: translate/redrive (provider keys required, store owned —
// flock+migrations+recovery) versus status/report (no keys, store read-only).
func openRunner(bookPath string, logger *slog.Logger, forWrite bool) (*Runner, error) {
	book, err := config.LoadBook(bookPath)
	if err != nil {
		return nil, err
	}
	models, err := config.LoadModels(book.ModelsFile)
	if err != nil {
		return nil, err
	}
	pipe, err := config.LoadPipeline(book.Pipeline, models, book.LangPair(), book.ContentLabels)
	if err != nil {
		return nil, err
	}
	// Fail-fast BEFORE opening the store (taking the flock and side-effects): whether
	// the config mechanics are executable and the keys of the used models are set.
	if err := pipe.CheckRunnable(); err != nil {
		return nil, err
	}
	// Content-routing REFUSALS are for the paths that spend money or reach a provider; the routing
	// itself was RESOLVED for every path inside LoadPipeline (the read models re-render the snapshot, so
	// they must see the same resolved models translate did). A book labelled AFTER it was paid for has to
	// stay inspectable — $0 status/report/export keep working and only warn (D20.4/D39.26 point 9),
	// because the alternative teaches the operator to strip the label, which is the silent bypass the
	// whole mechanism exists to prevent.
	// Key preflight is likewise write-only. CheckRunnable above stays unconditional (it costs nothing and
	// catches a broken config even on a read).
	if forWrite {
		if err := pipe.ContentRoutingError(); err != nil {
			return nil, err
		}
		if err := models.CheckKeys(pipe); err != nil {
			return nil, err
		}
	} else if err := pipe.ContentRoutingError(); err != nil {
		logger.Warn("content routing is not runnable for this book; read-only projections continue (translate/redrive would refuse)", "err", err)
	}
	pricer, err := models.Prices()
	if err != nil {
		return nil, err
	}
	var st *store.Store
	if forWrite {
		// Write path (translate/redrive): exclusive owner — flock, migrations,
		// reservation recovery.
		st, err = store.Open(book.ProjectDB)
	} else {
		// Read path (status/report): no flock — works during a live run; on
		// the first touch of a project (no DB yet) — a full Open (create+migrate).
		st, err = store.OpenReadOnly(book.ProjectDB)
		if err != nil && errors.Is(err, os.ErrNotExist) {
			st, err = store.Open(book.ProjectDB)
		}
	}
	if err != nil {
		return nil, err
	}
	r := &Runner{
		Book:      book,
		Models:    models,
		Pipeline:  pipe,
		Store:     st,
		Pricer:    pricer,
		Log:       logger,
		clients:   map[string]llm.LLMClient{},
		templates: map[string]*PromptTemplate{},
	}
	if err := r.loadTemplates(); err != nil {
		st.Close()
		return nil, err
	}
	if err := r.loadLangPack(); err != nil {
		st.Close()
		return nil, err
	}
	// The repair prompts are resolved by the same convention discipline as the stage prompts: a missing file
	// for an enabled class is a LOUD stop naming the path, never a silent skip of the loop.
	if err := r.loadRepairTemplates(); err != nil {
		st.Close()
		return nil, err
	}
	// Same discipline for the terminologist role prompt (pack-20): an enabled gate whose prompt is missing
	// or malformed stops the load, so the failure lands before the draft wave rather than after it is paid for.
	if err := r.loadTerminologyTemplate(); err != nil {
		st.Close()
		return nil, err
	}
	r.loadTargetScript()
	// The echo-exposure half of the reasoning-off hole (D39.26 добор B): loud, not fatal — see
	// sourceEchoExposure. Needs the templates, so it runs after they load.
	if exposed := r.sourceEchoExposure(); len(exposed) > 0 {
		logger.Warn("a stage sends the SOURCE text to a model whose wire SUPPRESSES thinking — the ratified echo zone (D19.1 point 2: reasoning-off + dense CJK is a property of the model class, not a vendor quirk). Not a stop: the downstream echo gate (cjk_artifact) still catches an echo on every configuration (D19.2), but it costs a paid call per hit",
			"stages", strings.Join(exposed, ", "), "source_lang", r.Book.SourceLang)
	}
	return r, nil
}

// loadLangPack resolves the book's language-data pack (D39.15/16), loaded on BOTH the write path and the
// read-only path so status/export/report reproduce the identical snapshot (pack.Version() is folded). The
// contract mirrors the owner directive R1: a pair WITH a catalog directory (configs/langpacks/<src>-<tgt>/)
// loads FAIL-LOUD (a missing/corrupt file stops the run, never a silently-empty miner); a pair WITHOUT a
// catalog — or a book with no langpack_root — runs with a nil pack (the miner is inert, the bank-mining stop auto-continues,
// and the snapshot fold is omitted). Presence of the pair directory is the "catalog exists" signal.
func (r *Runner) loadLangPack() error {
	// The observability checkers compile from the pair pack (DETECTION patterns + tables, may be nil) AND the
	// embedded target data (target-general lists, keyed by target lang). Built here so it exists even for a
	// no-langpack book (a nil pack → inert pair checkers; the target lists still load). r.pack is set below.
	defer func() {
		r.checkers = checks.CompileCheckersFor(r.pack, lang.TargetChecksFor(r.Book.TargetLang))
		// The SOURCE writing script(s) are a runner fact (the book's source lang), not target/pair data — set
		// after compile so the echo detector and the repair script-guard share one declared-source notion.
		r.checkers.SetSourceScripts(lang.LangScripts(r.Book.SourceLang))
	}()
	if r.Book.LangpackRoot == "" {
		return nil // no langpack declared → nil pack, miner inert
	}
	pairDir := filepath.Join(r.Book.LangpackRoot, r.Book.LangPair())
	if fi, err := os.Stat(pairDir); err != nil || !fi.IsDir() {
		// No catalog for this pair → nil-and-run (a ja book against a zh-only root just runs without mining).
		return nil
	}
	// A book-scoped overlay (LangpackExtend) unions the book's PRIVATE canon onto the shared pair pack
	// (pair-14 §1: 古月 is a 蛊真人 clan, not a shared 百家姓 surname). "" ⇒ plain Load. Folds into Version().
	pack, err := lang.LoadWithOverlay(r.Book.LangpackRoot, r.Book.SourceLang, r.Book.TargetLang, r.Book.LangpackExtend)
	if err != nil {
		return fmt.Errorf("pipeline: load langpack for %s: %w", r.Book.LangPair(), err)
	}
	r.pack = pack
	// A pack that loaded but declares no name-morphology channel (its manifest opts out) runs the miner on an
	// INERT channel — empty tables, zero candidates. Say so out loud rather than mine silently (P4 contract).
	if !pack.HasMorphologyChannel() {
		r.Log.Warn("langpack declares no source-morphology channel — the name-miner is inert (no candidates); this is expected for a source without the pinyin name-miner schema",
			"pair", r.Book.LangPair())
	}
	return nil
}

// packVersion is the snapshot-folded language-pack version: pack.Version() when a pack is loaded, "" when
// not (omitted from the snapshot so a no-pack book is byte-stable and never re-billed for a feature it does
// not use). A pack DATA edit changes Version() → the snapshot moves → a loud --resnapshot (R1, drift-proof).
func (r *Runner) packVersion() string {
	if r.pack != nil {
		return r.pack.Version()
	}
	return ""
}

func (r *Runner) Close() error { return r.Store.Close() }

func (r *Runner) loadTemplates() error {
	for _, st := range r.Pipeline.Stages {
		// The path was resolved at load — by the pair/role convention, or by the stage's deliberate
		// prompt_override — and a pair with no prompt pack already failed there, naming the file it
		// looked for (D39.23). Here it is only read.
		tpl, err := LoadPromptTemplate(st.PromptPath)
		if err != nil {
			return err
		}
		// Resolve the effective system once per stage: fold the few-shot block into
		// System unless the stage switched it off (D38.4). FewShot is retained so the
		// snapshot can fold the on/off state only for stages that actually have a block.
		tpl.System = tpl.SystemFor(fewShotEnabled(st))
		r.templates[st.Name] = tpl
	}
	return nil
}

// headingRule is the book's deterministic chapter-heading rule (pack-13 title policy), or nil when the
// book declares no langpack, its pair has no catalog, or the pair ships no heading.txt. The chunker uses
// it to detect+strip a source header and render «Глава N»; nil ⇒ the feature is inert (byte-identical
// chunks). Its bytes ride pack.Version() (folded into the snapshot via LangpackVersion), so a rule edit is
// a loud --resnapshot and a book without a rule is never re-billed for a feature it does not use.
func (r *Runner) headingRule() *lang.HeadingRule {
	if r.pack != nil {
		return r.pack.Heading
	}
	return nil
}

// sentenceAbbrevs is the BOOK's source-language abbreviation set for the sentence splitter (data-driven —
// no hard-coded "en"; a CJK source ships none, so the set is empty and never consulted).
func (r *Runner) sentenceAbbrevs() map[string]bool { return lang.SentenceAbbrev(r.Book.SourceLang) }

// estOutTokens is the DISPLAY-ONLY fertility estimate of a text's ru output tokens (est_out = fertility
// over the source char-classes, research/20 — NOT the char/4 EstimateTokens whose CJK undercount is the
// «CJK-mine»). Used ONLY to annotate a settle-estimate telemetry row (pack-13 point-9): it feeds NOTHING
// on the money path (not the reservation, not CostUSD), so it can never shift a verdict or a wire byte.
func (r *Runner) estOutTokens(text string) int {
	cjk, other := chunk.TokenClassCounts(text)
	return int(r.segBudget().EstOut(cjk, other))
}

// segBudget resolves the pipeline's config.Segmentation into the pipeline-package chunk.SegBudget the
// chunker consumes (WS2). Kept in the runner (not chunker.go) so the segmenter stays config-free.
func (r *Runner) segBudget() chunk.SegBudget {
	seg := r.Pipeline.Segmentation
	return chunk.SegBudget{
		DraftBudgetOut: float64(seg.DraftBudgetOut),
		EditCeilingOut: float64(seg.EditCeilingOut),
		FertCJK:        seg.Fertility.CJK,
		FertOther:      seg.Fertility.Other,
	}
}

// reachableModels enumerates every model a run can call, LABEL ROUTING APPLIED: the resolved stage
// models, their resolved single hops, and the repair model when that gate is on (config.ReachableModels
// is the one definition — the capability invariant, the API-key preflight, the rate guards and this
// eager client set all read it, so a model cannot be reachable for one and invisible to another).
// The set is complete and static for a book, which is what lets the precompute pass pre-build every
// client; a NEW call path must extend that definition, not this wrapper. Deterministic order.
func (r *Runner) reachableModels() []string {
	return r.Pipeline.ReachableModels()
}

// sourcePlaceholder is the template marker that puts the SOURCE text on the wire (render.go's vals map).
// A stage whose template carries it sends the raw source to its model — which is what makes the echo
// class below reachable for that stage, editor included since the editor became bilingual (D30.1).
const sourcePlaceholder = "{{text}}"

// sourceEchoExposure lists `stage→model` pairs that ship the SOURCE text to a model whose RESOLVED wire
// actively SUPPRESSES thinking — the other half of the reasoning-off hole (D39.26 добор B). The money
// half is a hard load gate (config: an additive provider that thinks needs a reserved buffer); this half
// cannot be a hard gate, because whether suppression actually costs quality is an open MEASUREMENT
// (D19.1: reasoning-ON does not save archaic dense zh either) and the choice of editor is the owner's.
// So it is made LOUD instead of silent, and the always-on downstream echo detector (classify →
// cjk_artifact, mandatory on every configuration per D19.2) remains the enforcement.
//
// Generic by construction: no provider or label name appears here — it is "this stage sends the source"
// × "this model's wire suppresses thinking" × "the source script is one where the class was measured".
// It fires on no shipping path today (deepseek resolves to ReasoningNone, so "off" is a no-op and
// thinking stays ON) and does fire for a reasoning-off editor arm on a CJK source, which is exactly the
// exposure the stale "the editor only ever sees a Russian draft" comments used to deny.
// It covers the ENGINE'S OWN calls too, not just the book's stages. Those calls carry a knob only since
// D39.87, and the knob has an asymmetric empty value: on a ReasoningNone capability "" means the provider's
// default (thinking ON), while on an extra_body_disable one (GLM) it means thinking DISABLED — the echo zone
// — and gates.terminology.model is not restricted to a family. An unset knob pointed at such a model is
// therefore a silent thinking-off with no config line describing it, which is precisely what this walk
// exists to refuse to leave silent.
func (r *Runner) sourceEchoExposure() []string {
	if !lang.IsCJKScriptLang(r.Book.SourceLang) {
		return nil
	}
	var out []string
	shipsSource := func(t *PromptTemplate) bool {
		return t != nil && strings.Contains(t.System+t.FewShot+t.User, sourcePlaceholder)
	}
	add := func(name, model, reasoning string) {
		if !r.Models.ThinksOnWire(model, reasoning) {
			out = append(out, name+"→"+model)
		}
	}
	for _, st := range r.Pipeline.Stages {
		if shipsSource(r.templates[st.Name]) {
			add(st.Name, st.ResolvedModel, st.Reasoning)
		}
	}
	// The bank roles ship source WITHOUT the placeholder: their candidate blocks carry KWIC contexts cut
	// from the book, which is the whole reason the roles can render a term at all. So membership is by
	// construction (the template is loaded ⇔ the role runs), not by scanning for a marker they do not use.
	bank := r.Pipeline.Gates.Terminology
	if r.terminologyTemplate != nil {
		add(roleTerminologist, bank.Model, bank.Reasoning)
	}
	if r.classifierTemplate != nil {
		add(roleClassifier, bank.ClassifierModel(), bank.Reasoning)
	}
	// Repair sends the source SPAN through the ordinary placeholder, per class — so it is scanned like a
	// stage, and a class whose prompt is monolingual is correctly not listed.
	for class, t := range r.repairTemplates {
		if shipsSource(t) {
			add(roleRepair+"/"+string(class), r.Pipeline.Gates.Repair.Model, r.Pipeline.Gates.Repair.Reasoning)
		}
	}
	sort.Strings(out) // map iteration above — the warning must not reorder between runs
	return out
}

// labelsFor returns the content labels of the text this run sends — BOOK-CONSTANT in v1 (D39.26 point 4).
// The signature takes no chunk ON PURPOSE: per-chapter labels would need the per-stage model fold moved
// off the per-wave snapshot (one snapshot is rendered per wave, before the chunk loop), so promising a
// per-chunk shape here would promise what the mechanism does not hold. Granularity is an owner decision
// (a labelled book routes ENTIRELY through label-capable models, which is what makes the ToS guarantee
// structural rather than dependent on the accuracy of manual per-chapter reading).
func (r *Runner) labelsFor() []string {
	return r.Book.ContentLabels
}

// buildClients EAGER-constructs every reachable LLM client BEFORE any wave goroutine starts (the precompute pass).
// After this the clients map is READ-ONLY in the waves, so r.client is lock-free and a miss is a
// loud error, not a lazy build under a data race — closing the runner.go lazy-init race (D12) and
// tripwiring a future un-enumerated model axis. Idempotent; BuildClient needs only the provider
// config (not the API key), so eager-build is safe even on a key-less resume.
func (r *Runner) buildClients() error {
	for _, m := range r.reachableModels() {
		if _, ok := r.clients[m]; ok {
			continue
		}
		c, err := BuildClient(r.Models, m, r.Log)
		if err != nil {
			return fmt.Errorf("pipeline: eager-build client for model %q (the precompute pass): %w", m, err)
		}
		r.clients[m] = c
	}
	return nil
}

// clientFor returns the PRE-BUILT client for a model, and REFUSES when that model's provider may not
// receive the labels of the content about to be sent. The label set is a PARAMETER rather than something
// the function reads for itself, and this is the only way to obtain a client: a new call path physically
// cannot reach a provider without stating what it is sending (the compile-enforced half of the routing
// guarantee, D39.26 point 8). Load-time validation already refuses such a config, so a failure here means
// a path bypassed it — an infra error, never a content verdict, so the book pauses durably (D4) instead
// of flagging a chunk.
//
// Read-only (no lazy build, no lock): buildClients constructed every reachable client in the precompute
// pass, so the map is only read in the waves; a miss means an un-enumerated model reached the wire and is
// a loud error, never a silent lazy build under a race.
func (r *Runner) clientFor(model string, labels []string) (llm.LLMClient, error) {
	if missing := r.Models.MissingLabels(model, labels); len(missing) > 0 {
		return nil, fmt.Errorf("pipeline: refusing to send content labelled %v to model %q (provider %s) — it does not accept %v. A provider without the label must not RECEIVE such content at all, whatever it would answer (D39.25); this is a routing bug: load-time validation should have caught it",
			labels, model, r.Models.ProviderOf(model), missing)
	}
	if c, ok := r.clients[model]; ok {
		return c, nil
	}
	return nil, fmt.Errorf("pipeline: no pre-built client for model %q — the precompute pass buildClients enumerates the resolved stage models ∪ their hops ∪ the repair model; a model outside that set reached the wire, add it to the eager set to keep the waves race-free", model)
}