refactor(web): dock chat inline below the summary, integrated view (ADR-027)

The chat was a separate page — opening it left the summary behind, the very
context you're asking about. Now the chat docks open IN PLACE below the summary:
"Dig deeper" reveals the chat section (HTMX, outerHTML over the closed dock) so
the summary stays on screen above it; no navigation. The no-JS fallback renders
the full summary AND the open chat on one page (the same integrated view), so
progressive enhancement holds.

Extracts a shared summaryBody templ so the detail page and the chat page render
one identical summary, not two divergent ones. GET /v/{id}/chat returns just the
open chat section as a fragment for the inline reveal, or the full summary+chat
page for a no-JS navigation; POST swaps the panel inline or re-renders the whole
page. Tests assert summary+chat coexist on the page and the reveal is a fragment.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

DECISIONS.md

@@ -996,6 +996,70 @@ the prior equal-share behaviour. No schema or API change.
 
 ---
 
+## ADR-027 — Chat with a video's stored transcript (deeper-dive, on an already-summarized video)
+
+**Status:** Accepted (2026-06-11). **Consumes ADR-021** (the shared, video-keyed transcript
+store) for the first time beyond summarization; **uses the ADR-022 chain models**; relates to
+ADR-012 (isolation) and ADR-016 (the Stage-0 gate).
+
+**Context — observed demand, not hypothetical.** The maintainer read 10+ real pilot summaries
+and reported the reactions: *many good; some he wanted to dig deeper into; some less useful*
+(the "less useful" split between weak-*model* output and uninteresting-*video* content). The
+middle reaction is the signal: a good summary that makes the reader want *more* is the summary
+succeeding at triage and then hitting a wall — there is nowhere to go deeper short of watching
+the video. That want is the feature. It is also the cheapest possible feature to satisfy
+honestly, because ADR-021 already persists the transcript: the deeper-dive runs entirely on
+stored public-content text + local models, touching **no** caption fetch and **no** YouTube.
+
+**Decision.** Add a per-video chat that lets the user ask questions against a video's stored
+transcript.
+1. **Entry from the summary view only.** A "dig deeper / ask" affordance on a summarized video —
+   the chat lives exactly where the "I want more" reaction happens. No standalone chat surface.
+2. **Stored-transcript-only (load-bearing constraint).** Chat is available **only** for videos
+   that already have a stored transcript. It never triggers a caption fetch, so it cannot touch
+   the rate gate, the 429 surface, or YouTube at all — the entire account-safety constraint that
+   governs the rest of Tapir is satisfied *by construction* here, not by careful gating. (Entry
+   being "from a summary" guarantees the transcript exists.) On-demand fetch for un-stored videos
+   is explicitly deferred.
+3. **Model = the summary's model by default; user-switchable among the ADR-022 chain models**
+   (`phi4-mini` / `gemma4-26b` / `mistral-small` to start). This is deliberate: it doubles as
+   live model-comparison instrumentation — ask the same question of the same transcript under two
+   models and the difference is directly felt. This is the mechanism by which the maintainer
+   learns *which* model is worth defaulting to, and it is the multi-model-analysis direction
+   ADR-021 anticipated, arriving as a user-facing capability.
+   - Chat is a **read-bounded retrieval/QA task** (the user supplies the focus), which is
+     *easier* than summarization (the model must decide what matters). So a model that summarizes
+     mediocrely may chat well — chat is plausibly a partial remedy for the weak-summary case, not
+     an inheritor of it.
+4. **Ephemeral chat (v1).** No persisted history; chat is per-session. Persisting per-user,
+   RLS-scoped history is deferred until there is evidence anyone wants to revisit a conversation.
+5. **Chat-only, trust-the-model (v1) — with a recorded limitation.** The chat does not expose the
+   raw transcript for verification in v1 (kept simple). **Known limitation:** because some
+   summaries were weak-model output, the user has reason not to fully trust a chat answer's
+   fidelity to the transcript, and v1 gives no in-UI way to check. The model-switcher partially
+   compensates (two models disagreeing on the same question is itself a signal). A
+   "show source / view transcript" verification path is the natural **v2** and is *not*
+   foreclosed — ADR-021's stored transcript already makes it cheap. Recorded so v2 is a known
+   next step, not a rediscovery.
+
+**Why this is safe and in-scope.** It adds no caption-fetch surface (stored-only), no new
+non-RLS table (transcripts already shared per ADR-021; ephemeral chat stores nothing), and no
+auth change. It is additive to the read path. The one genuine product expansion — Tapir becomes
+an interactive transcript-QA tool, not only a summarizer — is justified by *observed* demand from
+real reading, which is exactly the kind of evidence the Stage-0 discipline asks for before
+building.
+
+**Relation to the Stage-0 gate.** This is **not** a return-nudge (those stay deferred, ADR-020) —
+it adds nothing that prompts the user to return; it deepens the value *once they are already
+reading*. It does not contaminate the unprompted-return signal. If anything it strengthens the
+"useful to me" case the gate measures, by giving a good summary somewhere to lead.
+
+**Reversibility.** Additive read-path feature over the unchanged engine + the ADR-021 store.
+Removing the summary-view affordance removes the feature; nothing else depends on it. Ephemeral =
+no migration, no stored state to unwind. Spec: `docs/specs/chat-with-transcript.md`.
+
+---
+
 ## Rejected alternatives
 
 Approaches considered during the 2026-06-02 planning + grill session and **deliberately not

cmd/tapir/chat_test.go

@@ -0,0 +1,64 @@
+package main
+
+import (
+	"strings"
+	"testing"
+
+	"gitea.d-ma.be/mathias/tapir/internal/config"
+)
+
+// TestChatModelsReuseTheChainLocalFirst: the switcher offers the ADR-022 chain in
+// order, deduped — primary, local fallback, cloud.
+func TestChatModelsReuseTheChainLocalFirst(t *testing.T) {
+	got := chatModels(config.Config{
+		SummarizerModel:    "koala/phi4-mini",
+		FallbackModel:      "iguana/gemma4-26b",
+		CloudFallbackModel: "berget/mistral-small",
+	})
+	want := []string{"koala/phi4-mini", "iguana/gemma4-26b", "berget/mistral-small"}
+	if strings.Join(got, ",") != strings.Join(want, ",") {
+		t.Fatalf("chatModels = %v, want %v", got, want)
+	}
+}
+
+// TestChatCloudModelAbsentWhenDisabled: the local-first / NDA lever — with the
+// cloud fallback empty (TAPIR_CLOUD_FALLBACK_MODEL=""), no external model is
+// offered in the switcher, so chat content never leaves the local stack (ADR-027,
+// honouring ADR-022's "content stays local" guarantee).
+func TestChatCloudModelAbsentWhenDisabled(t *testing.T) {
+	got := chatModels(config.Config{
+		SummarizerModel:    "koala/phi4-mini",
+		FallbackModel:      "iguana/gemma4-26b",
+		CloudFallbackModel: "",
+	})
+	for _, m := range got {
+		if strings.HasPrefix(m, "berget/") || strings.Contains(m, "mistral") {
+			t.Fatalf("cloud model %q offered though the cloud fallback is disabled", m)
+		}
+	}
+	want := []string{"koala/phi4-mini", "iguana/gemma4-26b"}
+	if strings.Join(got, ",") != strings.Join(want, ",") {
+		t.Fatalf("chatModels = %v, want %v", got, want)
+	}
+}
+
+// TestChatModelsDedup: a config that reuses one alias across slots collapses to a
+// single switcher entry (no duplicate options).
+func TestChatModelsDedup(t *testing.T) {
+	got := chatModels(config.Config{
+		SummarizerModel:    "koala/phi4-mini",
+		FallbackModel:      "koala/phi4-mini",
+		CloudFallbackModel: "",
+	})
+	if len(got) != 1 || got[0] != "koala/phi4-mini" {
+		t.Fatalf("chatModels = %v, want a single deduped entry", got)
+	}
+}
+
+// TestBuildChatNilWithoutGateway: no gateway → no chat backend (routes unmounted,
+// read path unaffected).
+func TestBuildChatNilWithoutGateway(t *testing.T) {
+	if c := buildChat(config.Config{GatewayURL: ""}); c != nil {
+		t.Fatal("buildChat must return nil without a gateway URL")
+	}
+}

cmd/tapir/main.go

@@ -197,6 +197,15 @@ func cmdServe(ctx context.Context, log *slog.Logger) error {
 	secretStore := secrets.NewFileStore(cfg.SecretsFile)
 	app := &web.App{Store: st, Identity: st, Auth: authn, Secrets: secretStore, Log: log, RecencyWindow: cfg.AutoSummarizeWindow}
 
+	// Per-video deeper-dive chat over the STORED transcript (ADR-027). Enabled
+	// whenever a gateway is configured — it needs no YouTube credentials because it
+	// never fetches. Guarded so a typed-nil never lands in the interface field
+	// (which would mount the routes over a nil backend).
+	if c := buildChat(cfg); c != nil {
+		app.Chat = c
+		log.Info("web chat enabled (stored-transcript only)", "models", chatModels(cfg))
+	}
+
 	// User onboarding is handled by the IdP (Authentik invite flow), not Tapir —
 	// the Dex local-password provisioning path was removed (ADR-019). An
 	// authenticated subject with no Tapir user is routed to /register.

cmd/tapir/processor.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"strings"
 
+	"gitea.d-ma.be/mathias/tapir/internal/adapters/chat"
 	"gitea.d-ma.be/mathias/tapir/internal/adapters/llm"
 	"gitea.d-ma.be/mathias/tapir/internal/adapters/secrets"
 	"gitea.d-ma.be/mathias/tapir/internal/adapters/store"
@@ -68,6 +69,49 @@ func buildSummarizer(cfg config.Config) *summarizer.Summarizer {
 	return summarizer.NewChain(eps, cfg.MaxTranscriptChars)
 }
 
+// chatModels is the ordered, local-first set of models offered in the chat
+// switcher (ADR-027), reusing the ADR-022 chain: primary → local fallback →
+// cloud. Empty entries are dropped and duplicates collapsed, so a client/NDA
+// deployment that sets the cloud fallback empty simply has no external model in
+// the switcher — the same local-first lever the summarizer honours.
+func chatModels(cfg config.Config) []string {
+	var models []string
+	add := func(m string) {
+		if m == "" {
+			return
+		}
+		for _, e := range models {
+			if e == m {
+				return
+			}
+		}
+		models = append(models, m)
+	}
+	add(cfg.SummarizerModel)
+	add(cfg.FallbackModel)
+	add(cfg.CloudFallbackModel)
+	return models
+}
+
+// buildChat wires the per-video chat service (ADR-027): a Completer factory over
+// the SAME LiteLLM gateway the summarizer uses (a different alias per model, not a
+// second client config) and the same transcript-truncation budget. It returns nil
+// when no gateway is configured — chat is simply not mounted, the read path is
+// unaffected. It deliberately takes NO YouTube source: chat is stored-only.
+func buildChat(cfg config.Config) *chat.Service {
+	if cfg.GatewayURL == "" {
+		return nil
+	}
+	models := chatModels(cfg)
+	if len(models) == 0 {
+		return nil
+	}
+	newClient := func(model string) chat.Completer {
+		return llm.New(cfg.GatewayURL, cfg.GatewayKey, model, cfg.SummarizerTimeout, llm.WithMaxTokens(cfg.SummaryMaxTokens))
+	}
+	return chat.New(newClient, models, cfg.MaxTranscriptChars)
+}
+
 // providerOf maps a model alias to the domain AIProvider recorded on summaries.
 // A "berget/" alias is an external provider; everything else is the local stack.
 func providerOf(model string) string {

docs/specs/chat-with-transcript.md

@@ -0,0 +1,89 @@
+# Spec — Chat with a video's stored transcript (ADR-027)
+
+**Repo:** tapir · **Size:** medium · **Solo session.** Implements ADR-027. Read CLAUDE.md,
+DECISIONS.md (ADR-021 transcript store, ADR-022 model chain, ADR-012 isolation, ADR-027), and
+`docs/ui-spec.md` first. TBD, conventional commits, `task check` green per commit, `templ
+generate` after view changes.
+
+## FIRST: append ADR-027 to DECISIONS.md
+ADR-027 text is provided separately (planning thread). Insert immediately before the
+`## Rejected alternatives` heading, as the first commit, so the decision precedes the build.
+
+## What this is
+A per-video chat letting the user ask questions against a video's **already-stored** transcript,
+entered from the summary view. Born from observed demand: the maintainer read real summaries and
+some made him want to dig deeper — this gives that "I want more" reaction somewhere to go, without
+watching the video.
+
+## HARD CONSTRAINT — stored-transcript-only (the safety property)
+Chat is available **ONLY** for videos that already have a stored transcript (ADR-021). It must
+**never** trigger a caption fetch, never touch the rate gate, never reach YouTube. Entry being
+"from a summarized video" guarantees the transcript exists. If somehow invoked on a video with no
+stored transcript → show "transcript not available for chat", NO fetch. This is what makes the
+feature safe by construction; do not add an on-demand-fetch path (explicitly deferred).
+
+## 1. Entry point
+- A "Dig deeper" / "Ask about this" affordance on the **summary view** of a summarized video
+  (not the list cards — the detail/summary page). Quiet, consistent with the existing card-state
+  styling.
+- Opens a chat panel/view scoped to that one video, with its stored transcript as context.
+
+## 2. The chat
+- Read the stored transcript for the video (via the ADR-021 `TranscriptStore`, keyed by
+  `(provider, provider_video_id)`). No fetch.
+- Send transcript + the user's question + minimal system framing to the chosen model via the
+  **existing LiteLLM gateway** (the same client the summarizer uses — a chat is a different
+  call, not a new integration).
+- Stream or return the answer; render in the chat panel. HTMX/no-JS ethos — match the existing
+  app (the summarize status uses HTMX polling; chat can use a simple POST-and-render or HTMX
+  streaming if clean).
+- **Transcript truncation:** reuse/respect `TAPIR_MAX_TRANSCRIPT_CHARS` (ADR-022) so a long
+  transcript fits the model context. If truncated, the chat should be honest that it's working
+  from a bounded portion (a quiet note), since answers about the tail of a long video may be
+  incomplete.
+
+## 3. Model selection (the instrumentation win)
+- **Default model = the model that produced this video's summary.** (Store/lookup which chain
+  model summarized it — if not already recorded, this is a small addition; if recording it is
+  non-trivial, default to the chain primary and note the gap.)
+- **User can switch** among the ADR-022 chain models (`phi4-mini`, `gemma4-26b`,
+  `mistral-small` to start) via a simple selector in the chat panel. Switching re-runs against
+  the same transcript — this is deliberate model-comparison instrumentation.
+- Respect the local-first / NDA posture: if `TAPIR_CLOUD_FALLBACK_MODEL=""` (cloud disabled),
+  the external model is NOT offered in the switcher — only local models. Chat must honor the same
+  "content stays local" guarantee as ADR-022.
+
+## 4. Ephemeral (v1)
+- No persisted chat history. Conversation lives for the session/page. No new table, no migration.
+- (Multi-turn within a session is fine — keep the running messages in the request/page state —
+  but nothing is written to the DB.)
+
+## 5. Isolation
+- The transcript is shared/non-RLS (ADR-021) — fine, it's public content. But the chat is invoked
+  by a user about a video **in their feed**; confirm the entry path is reachable only for the
+  requesting user's own videos (the summary view is already RLS-scoped). Chat adds no new
+  user-data surface (ephemeral), so there's nothing new to RLS — but the test should confirm a
+  user can only open chat from their own summary view, not arbitrary video ids.
+
+## Tests
+- Chat on a video with a stored transcript → answer returned; assert NO caption-fetch / no
+  YouTube call occurs (the safety property — this is the key assertion).
+- Chat invoked on a video with no stored transcript → honest "not available", NO fetch.
+- Model switch → re-runs against the same transcript with the selected model; cloud model absent
+  from the switcher when `TAPIR_CLOUD_FALLBACK_MODEL=""`.
+- Truncation honored for a long transcript; the bounded-context note shows.
+- Entry is reachable only from the user's own summary view (isolation).
+
+## Out of scope / deferred (record, don't build)
+- **Persisted chat history** (per-user, RLS-scoped) — deferred until evidence anyone revisits a
+  conversation.
+- **Show-source / transcript-verification UI** — the natural v2 (ADR-027 records it); v1 is
+  chat-only/trust-the-model. ADR-021's stored transcript makes v2 cheap when wanted.
+- **On-demand fetch** for un-stored videos — would reintroduce the caption-fetch surface the
+  stored-only constraint removes. Not now.
+- Anything that nudges the user to return (ADR-020 — gate contamination).
+
+## Boundaries
+Stored-transcript-only (HARD). No rate-gate/fetch surface. No auth changes. No new persisted
+state in v1. Reuse the existing gateway client + truncation config; don't build a new model
+integration.

docs/use-cases/chat_transcript.feature

@@ -0,0 +1,69 @@
+Feature: Chat with a video's stored transcript
+  As a reader whose summary made me want to dig deeper
+  I want to ask questions about the video without watching it
+  So that I can go further on the ones worth it, without leaving the reader
+
+  # ADR-027. The load-bearing constraint is safety-by-construction: chat runs
+  # ONLY against an already-stored transcript (ADR-021) and never fetches captions,
+  # never touches the rate gate, never reaches YouTube. Entry is from the summary
+  # view of one's OWN video; the conversation is ephemeral (no persisted history).
+
+  Background:
+    Given I have a summarized video with a stored transcript
+
+  Scenario: A summary view offers a deeper-dive into the video
+    When I view the summary
+    Then I see a "dig deeper" affordance that opens a chat about this video
+    And it opens the chat in place, below the summary, without leaving the page
+
+  Scenario: The summary and the chat are on one page
+    When I open the chat
+    Then the summary stays visible alongside the chat
+    And I can read the summary while I ask questions
+
+  Scenario: Ask a question answered from the stored transcript
+    When I ask a question in the chat
+    Then the answer is produced from the stored transcript
+    And no caption fetch and no YouTube call occurs
+
+  Scenario: Chat never fetches captions or reaches YouTube
+    When I ask a question in the chat
+    Then Tapir reads only the stored transcript
+    And the caption-fetch and video-fetch paths are never invoked
+
+  Scenario: A video with no stored transcript offers no chat
+    Given a video that has no stored transcript
+    When I open the chat for it
+    Then I am told chat is not available
+    And no fetch is attempted and no model is called
+
+  Scenario: The default model is the summary's model and is switchable
+    When I open the chat
+    Then the model defaults to the model that produced the summary
+    And I can switch among the offered chain models
+
+  Scenario: Switching models re-runs against the same transcript
+    When I ask a question with a different chain model selected
+    Then the chosen model answers
+    And it answers against the same stored transcript
+
+  Scenario: The cloud model is hidden when cloud is disabled
+    Given the cloud fallback model is disabled
+    Then the chat switcher offers only local models
+
+  Scenario: A long transcript is bounded and the chat says so
+    Given the stored transcript is longer than the model budget
+    When I ask a question
+    Then the answer is produced from a bounded portion
+    And the chat notes that it worked from a bounded portion
+
+  Scenario: A multi-turn conversation is ephemeral
+    When I ask a follow-up question
+    Then the prior turn is carried into the answer
+    And nothing about the conversation is written to the database
+
+  Scenario: Chat is reachable only from my own summary view
+    Given another user has a summarized video with a stored transcript
+    When I try to open the chat for their video
+    Then I get a not-found response
+    And no model is called

internal/adapters/chat/chat.go

@@ -0,0 +1,171 @@
+// Package chat implements the per-video deeper-dive chat (ADR-027): a read-only
+// QA over a video's ALREADY-STORED transcript (ADR-021). It is the enforcement
+// point for the feature's load-bearing safety property — stored-transcript-only:
+// the Service has NO VideoSource and NO caption-fetch dependency, only a
+// Completer factory, so it CANNOT reach YouTube or the rate gate by construction.
+// The caller supplies the stored transcript text; chat never fetches.
+//
+// It reuses the same LiteLLM gateway as the summarizer (a chat is a different
+// call, not a new integration) and the same transcript-truncation discipline
+// (TAPIR_MAX_TRANSCRIPT_CHARS) so a long transcript fits a small-context model.
+package chat
+
+import (
+	"context"
+	"fmt"
+	"strings"
+	"unicode/utf8"
+)
+
+// Completer is the minimal LLM chat surface the Service needs. *llm.Client
+// satisfies it; tests use a fake. It is the SAME surface the summarizer uses.
+type Completer interface {
+	Complete(ctx context.Context, system, user string) (string, error)
+}
+
+// Turn is one completed exchange in an ephemeral, session-only conversation
+// (ADR-027 v1: nothing is persisted).
+type Turn struct {
+	Question string
+	Answer   string
+}
+
+// Request is one chat turn: the chosen model, the stored transcript text, the
+// prior turns (for multi-turn context within the session), and the new question.
+type Request struct {
+	Model      string
+	Transcript string
+	History    []Turn
+	Question   string
+}
+
+// Reply is the model's answer plus whether the transcript was bounded to fit the
+// model context (so the UI can be honest that an answer about the tail of a long
+// video may be incomplete).
+type Reply struct {
+	Answer    string
+	Truncated bool
+}
+
+// Service answers questions against a stored transcript via a switchable set of
+// models. models is the ordered, local-first list offered to the user (the cloud
+// model is simply absent when disabled — see cmd wiring); maxChars bounds the
+// transcript sent to any model (0 = unbounded). newClient builds a Completer for
+// a chosen model alias (the same gateway, a different alias).
+type Service struct {
+	newClient func(model string) Completer
+	models    []string
+	maxChars  int
+}
+
+// New constructs a Service. models must be non-empty and already filtered to the
+// offerable set (cloud excluded when disabled) and de-duplicated by the caller.
+func New(newClient func(model string) Completer, models []string, maxChars int) *Service {
+	return &Service{newClient: newClient, models: models, maxChars: maxChars}
+}
+
+// Models returns a copy of the offerable model list (local-first order).
+func (s *Service) Models() []string {
+	out := make([]string, len(s.models))
+	copy(out, s.models)
+	return out
+}
+
+// offers reports whether model is in the offerable set — the guard that keeps an
+// arbitrary, un-offered alias (e.g. a forged form value) from reaching the gateway.
+func (s *Service) offers(model string) bool {
+	for _, m := range s.models {
+		if m == model {
+			return true
+		}
+	}
+	return false
+}
+
+// DefaultModel resolves the model a fresh chat opens with: the summary's own
+// model when it is still an offered option (the ADR-027 default — chat continues
+// in the model that produced the summary), otherwise the first offered model.
+// Returns "" only when no models are configured.
+func (s *Service) DefaultModel(summaryModel string) string {
+	if summaryModel != "" && s.offers(summaryModel) {
+		return summaryModel
+	}
+	if len(s.models) > 0 {
+		return s.models[0]
+	}
+	return ""
+}
+
+// Answer runs one chat turn. The model is forced back to a default if the request
+// names an un-offered alias, so chat can never call the gateway with an arbitrary
+// model. The transcript is truncated up front (reporting whether it was cut) and
+// passed as system context; the running conversation is the user message.
+func (s *Service) Answer(ctx context.Context, req Request) (Reply, error) {
+	if len(s.models) == 0 {
+		return Reply{}, fmt.Errorf("chat: no models configured")
+	}
+	model := req.Model
+	if !s.offers(model) {
+		model = s.DefaultModel("")
+	}
+
+	transcript, truncated := truncate(req.Transcript, s.maxChars)
+	system := buildSystem(transcript, truncated)
+	user := buildUser(req.History, req.Question)
+
+	out, err := s.newClient(model).Complete(ctx, system, user)
+	if err != nil {
+		return Reply{}, fmt.Errorf("chat: %s: %w", model, err)
+	}
+	answer := strings.TrimSpace(out)
+	if answer == "" {
+		return Reply{}, fmt.Errorf("chat: %s returned an empty answer", model)
+	}
+	return Reply{Answer: answer, Truncated: truncated}, nil
+}
+
+const systemPreamble = `You are Tapir, answering questions about ONE video using ONLY the transcript below.
+Ground every answer in the transcript. If the transcript does not contain the answer, say so plainly rather than guessing.`
+
+const truncatedNote = `
+The transcript below is truncated to fit the model — if a question seems to concern something missing, note it may be beyond the available portion.`
+
+// buildSystem frames the model as a transcript-grounded QA assistant and embeds
+// the (possibly truncated) transcript as context.
+func buildSystem(transcript string, truncated bool) string {
+	var b strings.Builder
+	b.WriteString(systemPreamble)
+	if truncated {
+		b.WriteString(truncatedNote)
+	}
+	b.WriteString("\n\nTranscript:\n")
+	b.WriteString(transcript)
+	return b.String()
+}
+
+// buildUser renders the running conversation as the user message: prior turns as
+// Q/A pairs followed by the new question. Folding history into one message keeps
+// the Completer surface (a single system+user call) unchanged — no new llm method.
+func buildUser(history []Turn, question string) string {
+	var b strings.Builder
+	for _, t := range history {
+		fmt.Fprintf(&b, "Q: %s\nA: %s\n\n", t.Question, t.Answer)
+	}
+	fmt.Fprintf(&b, "Q: %s", question)
+	return b.String()
+}
+
+// truncate caps content to max bytes on a UTF-8 rune boundary, reporting whether
+// it cut. It mirrors the summarizer's truncation discipline (ADR-022) but returns
+// the cut flag so the chat UI can be honest about a bounded transcript. A
+// non-positive max (or content already within budget) returns content unchanged.
+func truncate(content string, max int) (string, bool) {
+	if max <= 0 || len(content) <= max {
+		return content, false
+	}
+	cut := max
+	for cut > 0 && !utf8.RuneStart(content[cut]) {
+		cut--
+	}
+	return content[:cut], true
+}

internal/adapters/chat/chat_test.go

@@ -0,0 +1,164 @@
+package chat
+
+import (
+	"context"
+	"errors"
+	"strings"
+	"testing"
+)
+
+// recordingCompleter captures the system+user it was asked with and returns a
+// canned answer (or error). It also records which model alias built it.
+type recordingCompleter struct {
+	model      string
+	lastSystem string
+	lastUser   string
+	answer     string
+	err        error
+	calls      *int
+}
+
+func (c *recordingCompleter) Complete(_ context.Context, system, user string) (string, error) {
+	*c.calls++
+	c.lastSystem = system
+	c.lastUser = user
+	if c.err != nil {
+		return "", c.err
+	}
+	return c.answer, nil
+}
+
+// factory builds a recordingCompleter per model and records the last one built so
+// the test can assert which model alias was actually used for the gateway call.
+type factory struct {
+	answer string
+	err    error
+	calls  int
+	used   *recordingCompleter
+}
+
+func (f *factory) make(model string) Completer {
+	c := &recordingCompleter{model: model, answer: f.answer, err: f.err, calls: &f.calls}
+	f.used = c
+	return c
+}
+
+func TestModelsAreOfferedLocalFirstAndCopied(t *testing.T) {
+	f := &factory{answer: "ok"}
+	s := New(f.make, []string{"koala/phi4-mini", "iguana/gemma4-26b"}, 0)
+
+	got := s.Models()
+	want := []string{"koala/phi4-mini", "iguana/gemma4-26b"}
+	if len(got) != len(want) || got[0] != want[0] || got[1] != want[1] {
+		t.Fatalf("Models() = %v, want %v", got, want)
+	}
+	// Mutating the returned slice must not corrupt the Service's list.
+	got[0] = "tampered"
+	if s.Models()[0] != "koala/phi4-mini" {
+		t.Fatal("Models() leaked its backing slice")
+	}
+}
+
+func TestDefaultModelIsTheSummarysModelWhenOffered(t *testing.T) {
+	f := &factory{answer: "ok"}
+	s := New(f.make, []string{"koala/phi4-mini", "iguana/gemma4-26b", "berget/mistral-small"}, 0)
+
+	if got := s.DefaultModel("iguana/gemma4-26b"); got != "iguana/gemma4-26b" {
+		t.Fatalf("DefaultModel(summary) = %q, want the summary's model", got)
+	}
+	// A summary model no longer offered (e.g. cloud disabled) falls back to first.
+	if got := s.DefaultModel("berget/old-model"); got != "koala/phi4-mini" {
+		t.Fatalf("DefaultModel(un-offered) = %q, want the first offered model", got)
+	}
+	// No summary model recorded → first offered.
+	if got := s.DefaultModel(""); got != "koala/phi4-mini" {
+		t.Fatalf("DefaultModel(\"\") = %q, want the first offered model", got)
+	}
+}
+
+func TestAnswerGroundsOnTranscriptAndCarriesHistory(t *testing.T) {
+	f := &factory{answer: "  The video is about attention.  "}
+	s := New(f.make, []string{"koala/phi4-mini"}, 0)
+
+	reply, err := s.Answer(context.Background(), Request{
+		Model:      "koala/phi4-mini",
+		Transcript: "ATTENTION-TRANSCRIPT-MARKER",
+		History:    []Turn{{Question: "who", Answer: "the host"}},
+		Question:   "what is it about",
+	})
+	if err != nil {
+		t.Fatalf("Answer: %v", err)
+	}
+	if reply.Answer != "The video is about attention." {
+		t.Fatalf("answer not trimmed: %q", reply.Answer)
+	}
+	if reply.Truncated {
+		t.Fatal("short transcript must not report truncated")
+	}
+	// The transcript rides in the system prompt; the conversation in the user msg.
+	if !strings.Contains(f.used.lastSystem, "ATTENTION-TRANSCRIPT-MARKER") {
+		t.Fatal("transcript not grounded into the system prompt")
+	}
+	if !strings.Contains(f.used.lastUser, "Q: who") || !strings.Contains(f.used.lastUser, "A: the host") {
+		t.Fatalf("history not carried into the user message: %q", f.used.lastUser)
+	}
+	if !strings.Contains(f.used.lastUser, "what is it about") {
+		t.Fatal("new question missing from the user message")
+	}
+}
+
+func TestAnswerTruncatesLongTranscriptAndReportsIt(t *testing.T) {
+	f := &factory{answer: "answer"}
+	s := New(f.make, []string{"koala/phi4-mini"}, 10)
+
+	reply, err := s.Answer(context.Background(), Request{
+		Model:      "koala/phi4-mini",
+		Transcript: strings.Repeat("x", 500),
+		Question:   "summarize",
+	})
+	if err != nil {
+		t.Fatalf("Answer: %v", err)
+	}
+	if !reply.Truncated {
+		t.Fatal("a transcript past maxChars must report Truncated")
+	}
+	if strings.Count(f.used.lastSystem, "x") != 10 {
+		t.Fatalf("transcript not bounded to maxChars: got %d x's", strings.Count(f.used.lastSystem, "x"))
+	}
+}
+
+func TestAnswerForcesAnUnofferedModelBackToDefault(t *testing.T) {
+	f := &factory{answer: "answer"}
+	s := New(f.make, []string{"koala/phi4-mini", "iguana/gemma4-26b"}, 0)
+
+	// A forged/un-offered model must never reach the gateway as-is — it is forced
+	// to the default offered model (the cloud-absent guarantee depends on this).
+	_, err := s.Answer(context.Background(), Request{
+		Model:      "berget/secret-cloud-model",
+		Transcript: "t",
+		Question:   "q",
+	})
+	if err != nil {
+		t.Fatalf("Answer: %v", err)
+	}
+	if f.used.model != "koala/phi4-mini" {
+		t.Fatalf("un-offered model reached the gateway as %q, want the default", f.used.model)
+	}
+}
+
+func TestAnswerPropagatesCompleterError(t *testing.T) {
+	f := &factory{err: errors.New("gateway down")}
+	s := New(f.make, []string{"koala/phi4-mini"}, 0)
+
+	_, err := s.Answer(context.Background(), Request{Model: "koala/phi4-mini", Transcript: "t", Question: "q"})
+	if err == nil {
+		t.Fatal("expected the gateway error to propagate")
+	}
+}
+
+func TestAnswerRejectsEmptyModelSet(t *testing.T) {
+	s := New(func(string) Completer { return nil }, nil, 0)
+	if _, err := s.Answer(context.Background(), Request{Question: "q"}); err == nil {
+		t.Fatal("expected an error when no models are configured")
+	}
+}

internal/web/chat.go

@@ -0,0 +1,227 @@
+package web
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"strings"
+
+	"gitea.d-ma.be/mathias/tapir/internal/adapters/chat"
+	"gitea.d-ma.be/mathias/tapir/internal/adapters/store"
+)
+
+// Chatter is the per-video chat backend (ADR-027). *chat.Service satisfies it;
+// tests substitute a fake. It carries NO caption-fetch dependency — the chat
+// handlers reach it only after reading an already-stored transcript, so an
+// enabled chat cannot trigger a fetch, touch the rate gate, or reach YouTube.
+type Chatter interface {
+	// Models returns the offerable models, local-first (cloud absent when disabled).
+	Models() []string
+	// DefaultModel resolves the model a fresh chat opens with given the summary's
+	// model (the ADR-027 default), falling back to the first offered model.
+	DefaultModel(summaryModel string) string
+	// Answer runs one chat turn against the supplied stored transcript text.
+	Answer(ctx context.Context, req chat.Request) (chat.Reply, error)
+}
+
+// maxHistoryTurns bounds the ephemeral conversation carried per request, so a long
+// back-and-forth cannot grow the prompt without limit (the transcript already
+// dominates the budget). Older turns drop off the front.
+const maxHistoryTurns = 8
+
+// chatView is everything the chat templates render: the video identity for links
+// and titles, the model switcher state, the running (ephemeral) conversation, and
+// the honest flags — Available is false when no usable transcript is stored
+// (ADR-027: honest "not available", never a fetch), Truncated when the transcript
+// was bounded to fit the model, Error for a transient model failure.
+type chatView struct {
+	VideoID   string
+	Title     string
+	Available bool
+	Models    []string
+	Selected  string
+	History   []chat.Turn
+	Truncated bool
+	Error     string
+}
+
+// handleChat renders the chat page for a summarized video (GET). Entry is scoped
+// through GetSummaryByVideo, which is RLS/user-scoped: a video that is not the
+// requesting user's own resolves to ErrNotFound → 404, so chat is reachable only
+// from the user's own summary view (ADR-027 isolation). The transcript is read
+// from the SHARED store (ADR-021) — a pure DB read, never a caption fetch; an
+// absent/text-less transcript renders the honest "not available" state, no fetch.
+func (a *App) handleChat(w http.ResponseWriter, r *http.Request) {
+	userID, ok := a.currentUserID(w, r)
+	if !ok {
+		return
+	}
+	row, ok := a.loadOwnedSummary(w, r, userID)
+	if !ok {
+		return
+	}
+	_, hasText, ok := a.readTranscript(w, r, *row)
+	if !ok {
+		return
+	}
+	view := chatView{
+		VideoID:   row.VideoID,
+		Title:     displayTitle(*row),
+		Available: hasText,
+		Models:    a.Chat.Models(),
+		Selected:  a.Chat.DefaultModel(row.AIModel),
+	}
+	// HTMX (the in-place reveal from the summary) gets just the open chat section,
+	// swapped over the closed dock so the summary above it stays put. A no-JS
+	// navigation gets the full page: the whole summary plus the open chat.
+	if isHTMX(r) {
+		a.render(w, r, chatSection(view))
+		return
+	}
+	a.render(w, r, ChatPage(*row, view))
+}
+
+// handleChatMessage answers one question against the stored transcript (POST).
+// It reads the transcript from the store (no fetch), runs the chosen model over
+// it plus the prior turns, appends the answer, and returns the refreshed chat
+// panel (HTMX) or the whole page (no-JS). A model failure is surfaced inline,
+// not as a 500 — the conversation and the question are preserved for a retry.
+func (a *App) handleChatMessage(w http.ResponseWriter, r *http.Request) {
+	userID, ok := a.currentUserID(w, r)
+	if !ok {
+		return
+	}
+	row, ok := a.loadOwnedSummary(w, r, userID)
+	if !ok {
+		return
+	}
+	if err := r.ParseForm(); err != nil {
+		http.Error(w, "bad form", http.StatusBadRequest)
+		return
+	}
+	transcript, hasText, ok := a.readTranscript(w, r, *row)
+	if !ok {
+		return
+	}
+
+	model := a.resolveModel(r.FormValue("model"), row.AIModel)
+	history := parseHistory(r.Form["hq"], r.Form["ha"])
+	question := strings.TrimSpace(r.FormValue("question"))
+
+	view := chatView{
+		VideoID:   row.VideoID,
+		Title:     displayTitle(*row),
+		Available: hasText,
+		Models:    a.Chat.Models(),
+		Selected:  model,
+		History:   history,
+	}
+
+	switch {
+	case !hasText:
+		// Honest "not available" — no fetch, no model call (ADR-027).
+	case question == "":
+		// A model switch with no text just re-renders — no wasted round-trip.
+	default:
+		reply, err := a.Chat.Answer(r.Context(), chat.Request{
+			Model:      model,
+			Transcript: transcript,
+			History:    history,
+			Question:   question,
+		})
+		if err != nil {
+			a.logger().Error("chat answer", "video", row.VideoID, "model", model, "err", err)
+			view.Error = "That model couldn't answer just now. Try again, or switch models."
+		} else {
+			view.History = appendTurn(history, chat.Turn{Question: question, Answer: reply.Answer})
+			view.Truncated = reply.Truncated
+		}
+	}
+	a.renderChatTurn(w, r, *row, view)
+}
+
+// loadOwnedSummary fetches the summary for the path's video scoped to userID, or
+// writes the right response (404 on not-found/not-owned, 500 on error) and reports
+// false. It is the single isolation gate for both chat handlers.
+func (a *App) loadOwnedSummary(w http.ResponseWriter, r *http.Request, userID string) (*store.SummaryRow, bool) {
+	videoID := r.PathValue("videoId")
+	row, err := a.Store.GetSummaryByVideo(r.Context(), userID, videoID)
+	if errors.Is(err, store.ErrNotFound) {
+		http.NotFound(w, r)
+		return nil, false
+	}
+	if err != nil {
+		a.serverError(w, r, "chat get summary", err)
+		return nil, false
+	}
+	return row, true
+}
+
+// readTranscript reads the shared stored transcript for a row (ADR-021) and
+// reports whether it carries usable text. It is a pure DB read — NO caption fetch,
+// the property the whole feature's safety rests on. ok is false only on a store
+// error (after a 500 is written); a missing/text-less transcript is (",", false,
+// true) — the honest "not available" case, handled by the caller, not an error.
+func (a *App) readTranscript(w http.ResponseWriter, r *http.Request, row store.SummaryRow) (content string, hasText, ok bool) {
+	t, found, err := a.Store.GetTranscript(r.Context(), row.Channel, row.ProviderVideoID)
+	if err != nil {
+		a.serverError(w, r, "chat get transcript", err)
+		return "", false, false
+	}
+	if !found || !t.HasText() {
+		return "", false, true
+	}
+	return t.Content, true, true
+}
+
+// renderChatTurn returns the chat panel fragment for an HTMX answer (swapped in
+// place within the open dock), or the full chat page otherwise — the no-JS POST
+// re-renders the whole summary + open chat with the new turn.
+func (a *App) renderChatTurn(w http.ResponseWriter, r *http.Request, row store.SummaryRow, v chatView) {
+	if isHTMX(r) {
+		a.render(w, r, chatPanel(v))
+		return
+	}
+	a.render(w, r, ChatPage(row, v))
+}
+
+// resolveModel keeps the posted model only when it is an offered option; anything
+// else (a forged value, or a model dropped because cloud is disabled) falls back
+// to the default. The chat.Service enforces the same guard before the gateway;
+// this keeps the rendered switcher honest too.
+func (a *App) resolveModel(posted, summaryModel string) string {
+	for _, m := range a.Chat.Models() {
+		if m == posted {
+			return posted
+		}
+	}
+	return a.Chat.DefaultModel(summaryModel)
+}
+
+// parseHistory zips the parallel hidden hq/ha fields back into ordered turns,
+// keeping only the most recent maxHistoryTurns. net/url preserves the submission
+// order of repeated fields, so the pairing is stable.
+func parseHistory(qs, as []string) []chat.Turn {
+	n := len(qs)
+	if len(as) < n {
+		n = len(as)
+	}
+	turns := make([]chat.Turn, 0, n)
+	for i := 0; i < n; i++ {
+		turns = append(turns, chat.Turn{Question: qs[i], Answer: as[i]})
+	}
+	return capHistory(turns)
+}
+
+// appendTurn adds a completed exchange and re-bounds the conversation.
+func appendTurn(history []chat.Turn, t chat.Turn) []chat.Turn {
+	return capHistory(append(history, t))
+}
+
+// capHistory keeps the last maxHistoryTurns turns (drops the oldest).
+func capHistory(turns []chat.Turn) []chat.Turn {
+	if len(turns) <= maxHistoryTurns {
+		return turns
+	}
+	return turns[len(turns)-maxHistoryTurns:]
+}

internal/web/chat_handler_test.go

@@ -0,0 +1,375 @@
+package web_test
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+	"github.com/stretchr/testify/require"
+
+	"gitea.d-ma.be/mathias/tapir/internal/adapters/chat"
+	"gitea.d-ma.be/mathias/tapir/internal/adapters/store"
+	"gitea.d-ma.be/mathias/tapir/internal/domain"
+	"gitea.d-ma.be/mathias/tapir/internal/web"
+)
+
+// videoZ is a video id used by the isolation test for a DIFFERENT user's video.
+const videoZ = "33333333-3333-3333-3333-333333333333"
+
+// --- chat test doubles -----------------------------------------------------
+
+// fakeChatter is a web.Chatter that records the request it received and returns a
+// canned reply. It performs NO network and NO fetch — it stands in for the real
+// chat.Service so the handler behaviour is what's under test.
+type fakeChatter struct {
+	models    []string
+	reply     chat.Reply
+	err       error
+	gotReq    chat.Request
+	callCount int
+}
+
+func (f *fakeChatter) Models() []string { return f.models }
+
+func (f *fakeChatter) DefaultModel(summaryModel string) string {
+	for _, m := range f.models {
+		if m == summaryModel {
+			return summaryModel
+		}
+	}
+	if len(f.models) > 0 {
+		return f.models[0]
+	}
+	return ""
+}
+
+func (f *fakeChatter) Answer(_ context.Context, req chat.Request) (chat.Reply, error) {
+	f.callCount++
+	f.gotReq = req
+	if f.err != nil {
+		return chat.Reply{}, f.err
+	}
+	return f.reply, nil
+}
+
+// tripwireProcessor and tripwireFetcher are the YouTube-reaching collaborators
+// (summarize → caption fetch, and the paste metadata fetch). Wired into the App
+// for the safety test, they fail it the instant chat routes into either — the
+// behavioural proof that chat never triggers a fetch (ADR-027).
+type tripwireProcessor struct{ t *testing.T }
+
+func (p tripwireProcessor) ProcessVideo(context.Context, string, string) error {
+	p.t.Fatal("chat triggered summarization (→ caption fetch) — must never happen (ADR-027)")
+	return nil
+}
+
+type tripwireFetcher struct{ t *testing.T }
+
+func (f tripwireFetcher) FetchVideo(context.Context, string, string) (domain.Video, error) {
+	f.t.Fatal("chat triggered a YouTube video fetch — must never happen (ADR-027)")
+	return domain.Video{}, nil
+}
+
+// --- helpers ---------------------------------------------------------------
+
+// newChatApp builds the App under test as the registered stub user, with a Chat
+// backend wired. Mirrors newApp but adds chat (and any extra wiring via mutate).
+func newChatApp(t *testing.T, chatter web.Chatter, mutate func(*web.App)) *web.App {
+	t.Helper()
+	s := newStore(t)
+	app := &web.App{
+		Store:    s,
+		Identity: s,
+		Auth:     web.StubAuth{U: web.User{Subject: stubSubject}},
+		Chat:     chatter,
+	}
+	if mutate != nil {
+		mutate(app)
+	}
+	return app
+}
+
+// seededProviderVideoID mirrors seedVideo's derivation so the chat path's
+// (provider, providerVideoID) transcript key matches the seeded video row.
+func seededProviderVideoID(videoID string) string { return "pv-" + videoID[:8] }
+
+// seedTranscript stores a shared (provider, providerVideoID) transcript — the
+// ADR-021 stored content the chat reads. Captions source = usable text.
+func seedTranscript(t *testing.T, s *store.Store, videoID, content string) {
+	t.Helper()
+	err := s.SaveTranscript(context.Background(), "youtube", seededProviderVideoID(videoID),
+		domain.Transcript{Source: domain.SourceCaptions, Language: "en", Content: content})
+	require.NoError(t, err)
+}
+
+func getChat(t *testing.T, app *web.App, videoID string) *httptest.ResponseRecorder {
+	t.Helper()
+	return do(t, app, httptest.NewRequest(http.MethodGet, "/v/"+videoID+"/chat", nil))
+}
+
+func postChat(t *testing.T, app *web.App, videoID string, form url.Values, htmx bool) *httptest.ResponseRecorder {
+	t.Helper()
+	req := httptest.NewRequest(http.MethodPost, "/v/"+videoID+"/chat", strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	if htmx {
+		req.Header.Set("HX-Request", "true")
+	}
+	return do(t, app, req)
+}
+
+// --- scenarios -------------------------------------------------------------
+
+// The "dig deeper" affordance appears on a summary detail view only when chat is
+// enabled, and links to that video's chat — the single entry point (ADR-027 §1).
+func TestChatEntryAffordanceOnSummaryView(t *testing.T) {
+	ctx := context.Background()
+	withChat := newChatApp(t, &fakeChatter{models: []string{"phi4-mini"}}, nil)
+	p := rawPool(t)
+	resetDB(t, p)
+	require.NoError(t, deliver(ctx, withChat, videoX, "body x"))
+	seedVideo(t, p, videoX, "X Title", "https://x", time.Time{})
+
+	html := body(t, do(t, withChat, httptest.NewRequest(http.MethodGet, "/v/"+videoX, nil)))
+	require.Contains(t, html, "Dig deeper", "the deeper-dive affordance is shown when chat is enabled")
+	require.Contains(t, html, "/v/"+videoX+"/chat", "it links to this video's chat")
+	require.Contains(t, html, `id="chat-section"`, "the dock lives on the detail page")
+	require.Contains(t, html, `hx-get="/v/`+videoX+`/chat"`, "it opens the chat in place (HTMX), not a navigation")
+
+	// With no chat backend wired the affordance is absent (routes unmounted).
+	noChat := newApp(t)
+	require.NoError(t, deliver(ctx, noChat, videoX, "body x"))
+	html = body(t, do(t, noChat, httptest.NewRequest(http.MethodGet, "/v/"+videoX, nil)))
+	require.NotContains(t, html, "Dig deeper", "no affordance when chat is disabled")
+}
+
+// The summary and the chat live together (the integrated UX): the no-JS chat page
+// renders the full summary alongside the chat, and the HTMX reveal returns just
+// the open chat section as a fragment so it docks in below the summary already on
+// screen — the summary is never navigated away from.
+func TestChatIntegratedWithSummaryOnSamePage(t *testing.T) {
+	ctx := context.Background()
+	chatter := &fakeChatter{models: []string{"phi4-mini", "gemma4-26b"}}
+	app := newChatApp(t, chatter, nil)
+	p := rawPool(t)
+	resetDB(t, p)
+	require.NoError(t, deliver(ctx, app, videoX, "SUMMARY-BODY-MARKER"))
+	seedVideo(t, p, videoX, "X Title", "https://x", time.Time{})
+	seedTranscript(t, app.Store.(*store.Store), videoX, "the transcript")
+
+	// No-JS full page: the summary payload and the chat are on one page.
+	html := body(t, getChat(t, app, videoX))
+	require.Contains(t, html, "SUMMARY-BODY-MARKER", "the summary text is shown on the chat page")
+	require.Contains(t, html, "takeaway one", "takeaways shown alongside the chat")
+	require.Contains(t, html, "highlight one", "highlights shown alongside the chat")
+	require.Contains(t, html, "Ask about this video", "the chat sits on the same page as the summary")
+	require.Contains(t, html, `name="question"`, "the ask form is present")
+
+	// HTMX reveal: the open chat section ONLY (a fragment) — no full-page chrome and
+	// no duplicated summary, so it swaps in below the summary already rendered.
+	req := httptest.NewRequest(http.MethodGet, "/v/"+videoX+"/chat", nil)
+	req.Header.Set("HX-Request", "true")
+	frag := body(t, do(t, app, req))
+	require.NotContains(t, frag, "<html", "the reveal is a fragment, not a full page")
+	require.NotContains(t, frag, "SUMMARY-BODY-MARKER", "the reveal does not re-send the summary (it's already on screen)")
+	require.Contains(t, frag, `id="chat-section"`, "the fragment replaces the dock in place")
+	require.Contains(t, frag, `name="question"`, "the ask form is in the revealed section")
+}
+
+// THE KEY SAFETY ASSERTION (ADR-027): a chat answer is produced entirely from the
+// stored transcript — the model receives the stored text, and neither the
+// summarize→fetch path nor the YouTube fetch path is ever touched. The tripwire
+// collaborators t.Fatal the test if chat reaches them.
+func TestChatAnswersFromStoredTranscriptWithoutAnyFetch(t *testing.T) {
+	ctx := context.Background()
+	const transcript = "STORED-TRANSCRIPT-MARKER: the host explains attention budgets."
+	chatter := &fakeChatter{
+		models: []string{"phi4-mini"},
+		reply:  chat.Reply{Answer: "It is about attention budgets."},
+	}
+	app := newChatApp(t, chatter, func(a *web.App) {
+		a.Processor = tripwireProcessor{t} // fails the test if chat summarizes/fetches
+		a.Fetcher = tripwireFetcher{t}     // fails the test if chat fetches video metadata
+	})
+	p := rawPool(t)
+	resetDB(t, p)
+	require.NoError(t, deliver(ctx, app, videoX, "summary body"))
+	seedVideo(t, p, videoX, "X Title", "https://x", time.Time{})
+	seedTranscript(t, app.Store.(*store.Store), videoX, transcript)
+
+	form := url.Values{"question": {"what is it about?"}, "model": {"phi4-mini"}}
+	rec := postChat(t, app, videoX, form, true)
+
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Contains(t, body(t, rec), "It is about attention budgets.", "the answer is rendered")
+	require.Equal(t, 1, chatter.callCount, "the model was asked exactly once")
+	require.Equal(t, transcript, chatter.gotReq.Transcript,
+		"the model answered from the STORED transcript, not a fetched one")
+	// The tripwires never firing IS the no-fetch / no-YouTube proof.
+}
+
+// A video with no stored transcript yields an honest "not available" — and never
+// a fetch, never a model call (ADR-027 §2: do not add an on-demand-fetch path).
+func TestChatUnavailableWhenNoStoredTranscript(t *testing.T) {
+	ctx := context.Background()
+	chatter := &fakeChatter{models: []string{"phi4-mini"}}
+	app := newChatApp(t, chatter, func(a *web.App) {
+		a.Processor = tripwireProcessor{t}
+		a.Fetcher = tripwireFetcher{t}
+	})
+	p := rawPool(t)
+	resetDB(t, p)
+	require.NoError(t, deliver(ctx, app, videoX, "summary body"))
+	seedVideo(t, p, videoX, "X Title", "https://x", time.Time{})
+	// NB: no seedTranscript — the transcript is absent.
+
+	rec := getChat(t, app, videoX)
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Contains(t, body(t, rec), "isn't available", "honest not-available copy")
+
+	// Asking anyway still triggers nothing: no model call, no fetch.
+	rec = postChat(t, app, videoX, url.Values{"question": {"hi"}, "model": {"phi4-mini"}}, true)
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Equal(t, 0, chatter.callCount, "no model call without a stored transcript")
+}
+
+// Chat is reachable ONLY from the user's own summary view: opening chat for a
+// video that belongs to another user 404s (the summary read is RLS-scoped), so a
+// guessed/arbitrary video id is not a chat surface (ADR-027 §5 isolation).
+func TestChatOnlyReachableForOwnVideo(t *testing.T) {
+	chatter := &fakeChatter{models: []string{"phi4-mini"}}
+	app := newChatApp(t, chatter, func(a *web.App) {
+		a.Processor = tripwireProcessor{t}
+		a.Fetcher = tripwireFetcher{t}
+	})
+	p := rawPool(t)
+	resetDB(t, p)
+
+	// A summarized video with a stored transcript owned by ANOTHER user.
+	const other = "22222222-2222-2222-2222-222222222222"
+	seedForeignSummaryWithTranscript(t, p, other, videoZ, "Foreign Title")
+
+	rec := getChat(t, app, videoZ)
+	require.Equal(t, http.StatusNotFound, rec.Code, "cannot open chat for another user's video")
+	require.Equal(t, 0, chatter.callCount, "no model call for a non-owned video")
+
+	rec = postChat(t, app, videoZ, url.Values{"question": {"hi"}, "model": {"phi4-mini"}}, true)
+	require.Equal(t, http.StatusNotFound, rec.Code, "cannot post chat to another user's video")
+}
+
+// The switcher offers exactly the backend's models, defaults to the summary's own
+// model, and a switch re-runs against the SAME transcript with the chosen model
+// (ADR-027 §3 — model-comparison instrumentation). A model dropped from the offer
+// set (e.g. cloud disabled) is not rendered.
+func TestChatModelSwitcherDefaultAndSwitch(t *testing.T) {
+	ctx := context.Background()
+	// The seeded summary's model is "phi4-mini" (see handlers_test summary()).
+	chatter := &fakeChatter{
+		models: []string{"phi4-mini", "gemma4-26b"}, // note: no cloud model offered
+		reply:  chat.Reply{Answer: "answer"},
+	}
+	app := newChatApp(t, chatter, nil)
+	p := rawPool(t)
+	resetDB(t, p)
+	require.NoError(t, deliver(ctx, app, videoX, "summary body"))
+	seedVideo(t, p, videoX, "X Title", "https://x", time.Time{})
+	seedTranscript(t, app.Store.(*store.Store), videoX, "the transcript text")
+
+	html := body(t, getChat(t, app, videoX))
+	require.Contains(t, html, "gemma4-26b", "every offered model is in the switcher")
+	require.NotContains(t, html, "mistral", "a non-offered (cloud-disabled) model is absent")
+	require.Contains(t, html, `value="phi4-mini" selected`, "defaults to the summary's own model")
+
+	// Switching to gemma re-runs against the same stored transcript.
+	form := url.Values{"question": {"q"}, "model": {"gemma4-26b"}}
+	rec := postChat(t, app, videoX, form, true)
+	require.Equal(t, http.StatusOK, rec.Code)
+	require.Equal(t, "gemma4-26b", chatter.gotReq.Model, "the chosen model answers")
+	require.Equal(t, "the transcript text", chatter.gotReq.Transcript, "against the same transcript")
+}
+
+// A truncated transcript surfaces the honest bounded-context note (ADR-027 §2).
+func TestChatTruncationNoteShown(t *testing.T) {
+	ctx := context.Background()
+	chatter := &fakeChatter{
+		models: []string{"phi4-mini"},
+		reply:  chat.Reply{Answer: "answer", Truncated: true},
+	}
+	app := newChatApp(t, chatter, nil)
+	p := rawPool(t)
+	resetDB(t, p)
+	require.NoError(t, deliver(ctx, app, videoX, "summary body"))
+	seedVideo(t, p, videoX, "X Title", "https://x", time.Time{})
+	seedTranscript(t, app.Store.(*store.Store), videoX, "long transcript")
+
+	rec := postChat(t, app, videoX, url.Values{"question": {"q"}, "model": {"phi4-mini"}}, true)
+	require.Contains(t, body(t, rec), "bounded portion", "the truncation note is shown")
+}
+
+// A multi-turn conversation is carried in the request (hidden fields), not the DB:
+// prior turns ride back into the next answer, and nothing is persisted (ADR-027 §4).
+func TestChatMultiTurnHistoryIsEphemeral(t *testing.T) {
+	ctx := context.Background()
+	chatter := &fakeChatter{models: []string{"phi4-mini"}, reply: chat.Reply{Answer: "second answer"}}
+	app := newChatApp(t, chatter, nil)
+	p := rawPool(t)
+	resetDB(t, p)
+	require.NoError(t, deliver(ctx, app, videoX, "summary body"))
+	seedVideo(t, p, videoX, "X Title", "https://x", time.Time{})
+	seedTranscript(t, app.Store.(*store.Store), videoX, "the transcript")
+
+	// Second turn posts the prior exchange as hidden history fields.
+	form := url.Values{
+		"question": {"follow-up question"},
+		"model":    {"phi4-mini"},
+		"hq":       {"first question"},
+		"ha":       {"first answer"},
+	}
+	rec := postChat(t, app, videoX, form, true)
+	require.Equal(t, http.StatusOK, rec.Code)
+
+	require.Len(t, chatter.gotReq.History, 1, "the prior turn was carried into the request")
+	require.Equal(t, "first question", chatter.gotReq.History[0].Question)
+	require.Equal(t, "first answer", chatter.gotReq.History[0].Answer)
+
+	html := body(t, rec)
+	require.Contains(t, html, "second answer", "the new answer renders")
+	require.Contains(t, html, "first question", "the conversation persists in page state")
+
+	// Nothing was written to the DB — there is no chat table; the summaries/actions
+	// are unchanged by a chat turn.
+	row, err := app.Store.GetSummaryByVideo(ctx, userID, videoX)
+	require.NoError(t, err)
+	require.Equal(t, "summary body", row.Summary, "chat never mutates stored data")
+}
+
+// --- foreign-user seeding (isolation) --------------------------------------
+
+// seedForeignSummaryWithTranscript creates a fully-summarized, transcript-backed
+// video owned by a DIFFERENT user via the raw pool (which bypasses RLS), so the
+// isolation test can confirm the requesting user cannot open chat for it.
+func seedForeignSummaryWithTranscript(t *testing.T, p *pgxpool.Pool, otherUserID, videoID, title string) {
+	t.Helper()
+	ctx := context.Background()
+	_, err := p.Exec(ctx, `INSERT INTO users (id) VALUES ($1) ON CONFLICT (id) DO NOTHING`, otherUserID)
+	require.NoError(t, err)
+	_, err = p.Exec(ctx,
+		`INSERT INTO videos (id, user_id, provider, provider_video_id, title, url)
+		 VALUES ($1, $2, 'youtube', $3, $4, 'https://z')`,
+		videoID, otherUserID, seededProviderVideoID(videoID), title)
+	require.NoError(t, err)
+	_, err = p.Exec(ctx,
+		`INSERT INTO summaries (user_id, video_id, summary, ai_provider, ai_model)
+		 VALUES ($1, $2, 'foreign summary', 'local', 'phi4-mini')`,
+		otherUserID, videoID)
+	require.NoError(t, err)
+	_, err = p.Exec(ctx,
+		`INSERT INTO transcripts (provider, provider_video_id, source, language, content)
+		 VALUES ('youtube', $1, 'captions', 'en', 'foreign transcript')`,
+		seededProviderVideoID(videoID))
+	require.NoError(t, err)
+}

internal/web/handlers.go

@@ -26,6 +26,10 @@ type Store interface {
 	DistinctChannels(ctx context.Context, userID string) ([]string, error)
 	GetSummaryByVideo(ctx context.Context, userID, videoID string) (*store.SummaryRow, error)
 	GetVideoRow(ctx context.Context, userID, videoID string) (*store.SummaryRow, error)
+	// GetTranscript reads the shared, stored transcript keyed by (provider,
+	// providerVideoID) — ADR-021. It is a pure DB read: it never fetches captions,
+	// so the chat path (ADR-027) reaches it without any caption-fetch surface.
+	GetTranscript(ctx context.Context, provider, providerVideoID string) (domain.Transcript, bool, error)
 	ActionsFor(ctx context.Context, userID string, videoIDs []string) (map[string][]string, error)
 	SetAction(ctx context.Context, userID, videoID, action string) error
 	ClearAction(ctx context.Context, userID, videoID, action string) error
@@ -91,6 +95,11 @@ type App struct {
 	// Fetcher, when non-nil, resolves an arbitrary YouTube video id to metadata for
 	// the paste-a-URL flow (Feature 2). Nil = the /paste route is not mounted.
 	Fetcher VideoFetcher
+	// Chat, when non-nil, answers per-video questions against a video's STORED
+	// transcript (ADR-027). Nil = the /v/{id}/chat routes are not mounted and the
+	// summary view shows no "dig deeper" affordance. It holds no caption-fetch
+	// dependency, so an enabled chat cannot reach YouTube or the rate gate.
+	Chat Chatter
 	// Processing tracks in-flight immediate summarizations so the status endpoint
 	// shows the animation until the summary lands. The zero value is ready to use.
 	Processing ProcessingSet
@@ -149,6 +158,12 @@ func (a *App) Router() http.Handler {
 		app.HandleFunc("POST /paste", a.handlePaste)
 	}
 	app.HandleFunc("GET /v/{videoId}/status", a.handleStatus)
+	// Per-video deeper-dive chat over the STORED transcript (ADR-027). Mounted only
+	// when a Chat backend is wired; it never fetches captions.
+	if a.Chat != nil {
+		app.HandleFunc("GET /v/{videoId}/chat", a.handleChat)
+		app.HandleFunc("POST /v/{videoId}/chat", a.handleChatMessage)
+	}
 	app.HandleFunc("GET /register", a.handleRegisterForm)
 	app.HandleFunc("POST /register", a.handleRegister)
 
@@ -265,7 +280,7 @@ func (a *App) handleDetail(w http.ResponseWriter, r *http.Request) {
 		a.serverError(w, r, "get summary", err)
 		return
 	}
-	a.render(w, r, DetailPage(*row))
+	a.render(w, r, DetailPage(*row, a.Chat != nil))
 }
 
 // handleAction toggles one action: re-clicking an active verb clears it, else it

internal/web/view.go

@@ -184,6 +184,12 @@ func statusURL(videoID string) templ.SafeURL {
 	return templ.SafeURL("/v/" + videoID + "/status")
 }
 
+// chatURL builds the per-video chat path (GET renders the page, POST answers) —
+// the deeper-dive over the stored transcript (ADR-027).
+func chatURL(videoID string) templ.SafeURL {
+	return templ.SafeURL("/v/" + videoID + "/chat")
+}
+
 // Charmbracelet-inspired palette for the summarizing animation (TapirSpinner) —
 // a charm purple box, pink tapir, mint snout/eyes/progress. Kept as named consts
 // so the inline span colours and the CSS track/fill share one source of truth.
@@ -725,6 +731,30 @@ a.btn, a.btn:visited { color: var(--accent-fg); }
 .detail ul { margin: 0; padding-left: 1.2rem; line-height: 1.6; }
 .detail li { margin-bottom: var(--s1); }
 
+/* deeper-dive chat (ADR-027) — docks in place below the summary */
+.chat-dock { margin-top: var(--s5); border-top: 1px solid var(--line); padding-top: var(--s4); }
+.chat-dock .chat-open { display: inline-block; }
+.chat-heading { font-size: 1.1rem; margin: 0 0 var(--s2); }
+.chat-scope { margin: 0 0 var(--s3); font-size: .9rem; }
+.chat-panel { display: flex; flex-direction: column; gap: var(--s3); }
+.chat-log { display: flex; flex-direction: column; gap: var(--s3); }
+.chat-turn { border-radius: var(--radius); padding: var(--s2) var(--s3); }
+.chat-turn p { margin: 0; }
+.chat-q { background: var(--accent-weak); color: var(--fg); align-self: flex-end; max-width: 85%; }
+.chat-a { background: var(--card); border: 1px solid var(--line); }
+.chat-a .body { white-space: pre-wrap; line-height: 1.6; }
+.chat-note { margin: 0; font-size: .82rem; font-style: italic; }
+.chat-error { margin: 0; color: #8a1c10; font-size: .9rem; }
+@media (prefers-color-scheme: dark) { .chat-error { color: #f3b5ae; } }
+.chat-form { display: flex; flex-direction: column; gap: var(--s2); margin: var(--s2) 0 0; }
+.chat-model { flex-direction: column; display: flex; gap: var(--s1); font-size: .78rem; text-transform: uppercase; letter-spacing: .04em; color: var(--muted); align-items: flex-start; }
+.chat-model select { font: inherit; text-transform: none; letter-spacing: 0; padding: .4rem .55rem; border: 1px solid var(--line); border-radius: var(--radius); background: var(--card); color: var(--fg); }
+.chat-model-hint { text-transform: none; letter-spacing: 0; font-size: .78rem; }
+.chat-form textarea { font: inherit; padding: .55rem; border: 1px solid var(--line); border-radius: var(--radius); background: var(--card); color: var(--fg); resize: vertical; }
+.chat-form textarea:focus-visible { outline: 2px solid var(--accent); outline-offset: 1px; border-color: var(--accent); }
+.chat-form .btn { align-self: flex-start; }
+.chat-thinking { font-style: italic; }
+
 /* action toggles — watched|skipped form one segmented control (they are mutually
    exclusive), "saved" sits apart as an independent toggle */
 .actions { display: flex; gap: var(--s3); margin: var(--s4) 0; flex-wrap: wrap; align-items: center; }

internal/web/views.templ

@@ -410,69 +410,175 @@ templ noCaptionsCard(r store.SummaryRow) {
 	</li>
 }
 
-// DetailPage is the full summary view: text, highlights, takeaways, metadata,
-// and the action button group.
-templ DetailPage(r store.SummaryRow) {
+// summaryBody is the summary payload shared by the detail page and the no-JS
+// chat page (so the chat page shows the same summary, not a separate view):
+// metadata, embed, source, the action toggles, then the attention-saving order
+// Takeaways → Highlights → Summary (UX review A8).
+templ summaryBody(r store.SummaryRow) {
+	<p class="meta">
+		if detailMeta(r) != "" {
+			<span>{ detailMeta(r) }</span>
+		}
+		if r.FallbackUsed {
+			<span class="badge" title="summarized with the fallback model" aria-label="summarized with the fallback model">fallback</span>
+		}
+	</p>
+	if url, ok := embedURL(r.ProviderVideoID); ok {
+		<div class="embed">
+			<iframe
+				src={ url }
+				title={ displayTitle(r) }
+				loading="lazy"
+				referrerpolicy="strict-origin-when-cross-origin"
+				allow="accelerometer; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
+				allowfullscreen
+			></iframe>
+		</div>
+	}
+	if r.URL != "" {
+		<p class="source"><a href={ externalURL(r.URL) } rel="noopener noreferrer">watch on source ↗</a></p>
+	}
+	@ActionButtons(r.VideoID, actionSet(r.Actions))
+	if len(r.Takeaways) > 0 {
+		<section>
+			<h2>Takeaways</h2>
+			<ul>
+				for _, t := range r.Takeaways {
+					<li>{ t }</li>
+				}
+			</ul>
+		</section>
+	}
+	if len(r.Highlights) > 0 {
+		<section>
+			<h2>Highlights</h2>
+			<ul>
+				for _, h := range r.Highlights {
+					<li>{ h }</li>
+				}
+			</ul>
+		</section>
+	}
+	<section>
+		<h2>Summary</h2>
+		<p class="body">{ r.Summary }</p>
+	</section>
+}
+
+// DetailPage is the full summary view: the summary payload, then (when chat is
+// enabled) the deeper-dive dock (ADR-027) — a reveal that opens the chat IN PLACE
+// below the summary, so the summary stays on screen as the context being asked
+// about rather than being navigated away from.
+templ DetailPage(r store.SummaryRow, chatEnabled bool) {
 	@Layout("Tapir — " + displayTitle(r)) {
 		<article class="detail">
 			<p class="back"><a href="/">← Summaries</a></p>
 			<h1>{ displayTitle(r) }</h1>
-			<p class="meta">
-				if detailMeta(r) != "" {
-					<span>{ detailMeta(r) }</span>
-				}
-				if r.FallbackUsed {
-					<span class="badge" title="summarized with the fallback model" aria-label="summarized with the fallback model">fallback</span>
-				}
-			</p>
-			if url, ok := embedURL(r.ProviderVideoID); ok {
-				<div class="embed">
-					<iframe
-						src={ url }
-						title={ displayTitle(r) }
-						loading="lazy"
-						referrerpolicy="strict-origin-when-cross-origin"
-						allow="accelerometer; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share"
-						allowfullscreen
-					></iframe>
-				</div>
+			@summaryBody(r)
+			if chatEnabled {
+				@chatReveal(r.VideoID)
 			}
-			if r.URL != "" {
-				<p class="source"><a href={ externalURL(r.URL) } rel="noopener noreferrer">watch on source ↗</a></p>
-			}
-			@ActionButtons(r.VideoID, actionSet(r.Actions))
-			// Lead with the attention-saving payload: Takeaways ("is this worth my
-			// time?") first, then Highlights, then the full Summary last (UX review
-			// A8). Takeaways/Highlights are conditional, so a video without them falls
-			// through to the Summary leading naturally.
-			if len(r.Takeaways) > 0 {
-				<section>
-					<h2>Takeaways</h2>
-					<ul>
-						for _, t := range r.Takeaways {
-							<li>{ t }</li>
-						}
-					</ul>
-				</section>
-			}
-			if len(r.Highlights) > 0 {
-				<section>
-					<h2>Highlights</h2>
-					<ul>
-						for _, h := range r.Highlights {
-							<li>{ h }</li>
-						}
-					</ul>
-				</section>
-			}
-			<section>
-				<h2>Summary</h2>
-				<p class="body">{ r.Summary }</p>
-			</section>
 		</article>
 	}
 }
 
+// chatReveal is the CLOSED dock at the foot of the summary: a quiet affordance,
+// not a loud CTA (it deepens value for a reader already here, never nudges). With
+// JS it swaps itself for the open chat section in place (HTMX, summary stays
+// above); without JS the same href navigates to the full chat page, which renders
+// the summary alongside the chat. Either way the summary is never lost.
+templ chatReveal(videoID string) {
+	<section id="chat-section" class="chat-dock">
+		<a
+			class="btn-secondary chat-open"
+			href={ chatURL(videoID) }
+			hx-get={ string(chatURL(videoID)) }
+			hx-target="#chat-section"
+			hx-swap="outerHTML"
+		>
+			Dig deeper — ask about this video →
+		</a>
+	</section>
+}
+
+// chatSection is the OPEN dock: heading + scope note + the chat panel, swapped in
+// over the closed reveal (same #chat-section id, outerHTML). It is the HTMX reveal
+// response AND the inline chat block on the no-JS chat page.
+templ chatSection(v chatView) {
+	<section id="chat-section" class="chat-dock chat-dock-open">
+		<h2 class="chat-heading">Ask about this video</h2>
+		<p class="chat-scope muted">Answers come only from this video's stored transcript — Tapir never fetches anything new here.</p>
+		@chatPanel(v)
+	</section>
+}
+
+// ChatPage is the no-JS full-page render of the chat: the whole summary followed
+// by the open chat dock, so a visitor without JS sees the same integrated view
+// (summary beside the conversation) that JS users get inline via the reveal.
+templ ChatPage(r store.SummaryRow, v chatView) {
+	@Layout("Tapir — " + displayTitle(r)) {
+		<article class="detail">
+			<p class="back"><a href="/">← Summaries</a></p>
+			<h1>{ displayTitle(r) }</h1>
+			@summaryBody(r)
+			@chatSection(v)
+		</article>
+	}
+}
+
+// chatPanel is the conversation + ask form, swapped in place on each answer
+// (HTMX targets #chat-panel, outerHTML). When no transcript is stored it shows the
+// honest "not available" state and no form (ADR-027: never a fetch). The prior
+// turns ride as hidden hq/ha fields so the ephemeral conversation survives the
+// round-trip without any persisted state.
+templ chatPanel(v chatView) {
+	<div id="chat-panel" class="chat-panel">
+		if !v.Available {
+			<p class="chat-unavailable muted">Chat isn't available for this video — its transcript isn't stored, and chat never fetches new captions. Summarize the video first to store its transcript.</p>
+		} else {
+			if len(v.History) > 0 {
+				<div class="chat-log">
+					for _, t := range v.History {
+						<div class="chat-turn chat-q"><p>{ t.Question }</p></div>
+						<div class="chat-turn chat-a"><p class="body">{ t.Answer }</p></div>
+					}
+				</div>
+			}
+			if v.Truncated {
+				<p class="chat-note muted">Working from a bounded portion of a long transcript — answers about the end of the video may be incomplete.</p>
+			}
+			if v.Error != "" {
+				<p class="chat-error" role="alert">{ v.Error }</p>
+			}
+			<form
+				class="chat-form"
+				method="post"
+				action={ chatURL(v.VideoID) }
+				hx-post={ string(chatURL(v.VideoID)) }
+				hx-target="#chat-panel"
+				hx-swap="outerHTML"
+			>
+				for _, t := range v.History {
+					<input type="hidden" name="hq" value={ t.Question }/>
+					<input type="hidden" name="ha" value={ t.Answer }/>
+				}
+				<label class="chat-model">
+					Model
+					<select name="model">
+						for _, m := range v.Models {
+							<option value={ m } selected?={ m == v.Selected }>{ m }</option>
+						}
+					</select>
+					<span class="chat-model-hint muted">Switch models to compare answers on the same transcript.</span>
+				</label>
+				<textarea name="question" rows="3" placeholder="Ask a question about this video…" required aria-label="Your question"></textarea>
+				<button type="submit" class="btn">Ask</button>
+				<span class="htmx-indicator chat-thinking">thinking…</span>
+			</form>
+		}
+	</div>
+}
+
 // RegisterPage is the explicit registration step (ADR-012): an authenticated Dex
 // subject with no tapir user picks a display name to create their account.
 // errMsg, when set, reports a validation problem on the prior POST.

test/acceptance/scenario_coverage_test.go

@@ -63,6 +63,19 @@ var scenarioCoverage = map[string]string{
 	"A returning subject passes straight through":                               "TestRegisteredSubjectPassesThrough",
 	"Deleting an account removes only my data and leaves other users untouched": "TestDeleteAccountWipesDataAndSecretsAndLogsOut",
 
+	// chat_transcript.feature (ADR-027)
+	"A summary view offers a deeper-dive into the video":         "TestChatEntryAffordanceOnSummaryView",
+	"The summary and the chat are on one page":                   "TestChatIntegratedWithSummaryOnSamePage",
+	"Ask a question answered from the stored transcript":         "TestChatAnswersFromStoredTranscriptWithoutAnyFetch",
+	"Chat never fetches captions or reaches YouTube":             "TestChatAnswersFromStoredTranscriptWithoutAnyFetch",
+	"A video with no stored transcript offers no chat":           "TestChatUnavailableWhenNoStoredTranscript",
+	"The default model is the summary's model and is switchable": "TestChatModelSwitcherDefaultAndSwitch",
+	"Switching models re-runs against the same transcript":       "TestChatModelSwitcherDefaultAndSwitch",
+	"The cloud model is hidden when cloud is disabled":           "TestChatCloudModelAbsentWhenDisabled",
+	"A long transcript is bounded and the chat says so":          "TestChatTruncationNoteShown",
+	"A multi-turn conversation is ephemeral":                     "TestChatMultiTurnHistoryIsEphemeral",
+	"Chat is reachable only from my own summary view":            "TestChatOnlyReachableForOwnVideo",
+
 	// summarize_new_video.feature
 	"A subscribed channel posts a video that has captions":         "TestSubscribedVideoWithCaptionsIsSummarizedAndDelivered",
 	"A subscribed channel posts a video with no usable transcript": "TestVideoWithNoTranscriptIsSkipped",