feat(metrics): consume contextSize — current context-usage readout

Backend context-size handoff: re-pin [email protected] / [email protected]
(+ re-mirror .dispatch reference snapshots). Thread the optional contextSize
through core/metrics (done fold + durable + selectCurrentContextSize: latest
turn's defined value, undefined=>unknown never 0, durable-wins-over-live).
Chat store exposes currentContextSize; ContextSizeBadge renders
"N tokens in context" / "context size unknown" above the composer.
GLOSSARY: add context size / context window. 533 tests green.

3 files changed, 31 insertions, 0 deletions

diff --git a/src/features/chat/index.ts b/src/features/chat/index.ts
index 18ed693..adfb670 100644
--- a/src/features/chat/index.ts
+++ b/src/features/chat/index.ts
@@ -6,6 +6,7 @@ export type { ChatStore, ChatStoreDependencies } from "./store.svelte";
 export { createChatStore } from "./store.svelte";
 export { default as ChatView } from "./ui/ChatView.svelte";
 export { default as Composer } from "./ui/Composer.svelte";
+export { default as ContextSizeBadge } from "./ui/ContextSizeBadge.svelte";
 export { default as ModelSelector } from "./ui/ModelSelector.svelte";
 
 /** Public module manifest — aggregated by the shell's "Loaded Modules" view. */
diff --git a/src/features/chat/store.svelte.ts b/src/features/chat/store.svelte.ts
index f4ad07b..6344aec 100644
--- a/src/features/chat/store.svelte.ts
+++ b/src/features/chat/store.svelte.ts
@@ -18,6 +18,7 @@ import {
 	applyDurableMetrics,
 	foldMetricsEvent,
 	initialMetricsState,
+	selectCurrentContextSize,
 	selectOrderedTurnMetrics,
 } from "../../core/metrics";
 import type { ConversationCache } from "../conversation-cache";
@@ -36,6 +37,12 @@ export interface ChatStore {
 	readonly messages: readonly ChatMessage[];
 	readonly chunks: readonly RenderedChunk[];
 	readonly turnMetrics: readonly TurnMetricsEntry[];
+	/**
+	 * The conversation's current context size (tokens occupied) — the latest
+	 * finalized turn's `contextSize`, or `undefined` ("unknown") when none is
+	 * known yet. Never `0` for the unknown case.
+	 */
+	readonly currentContextSize: number | undefined;
 	readonly pendingSync: boolean;
 	readonly error: string | null;
 	readonly model: string | undefined;
@@ -91,6 +98,9 @@ export function createChatStore(deps: ChatStoreDependencies): ChatStore {
 		get turnMetrics(): readonly TurnMetricsEntry[] {
 			return selectOrderedTurnMetrics(metrics);
 		},
+		get currentContextSize(): number | undefined {
+			return selectCurrentContextSize(metrics);
+		},
 		get pendingSync(): boolean {
 			return _pendingSync;
 		},
diff --git a/src/features/chat/ui/ContextSizeBadge.svelte b/src/features/chat/ui/ContextSizeBadge.svelte
new file mode 100644
index 0000000..475d54f
--- /dev/null
+++ b/src/features/chat/ui/ContextSizeBadge.svelte
@@ -0,0 +1,20 @@
+<script lang="ts">
+	import { formatContextSize } from "../../../core/metrics";
+
+	let {
+		contextSize,
+	}: {
+		// The conversation's current context size (tokens occupied), or `undefined`
+		// ("unknown") when no finalized turn has reported one yet. Never `0` for the
+		// unknown case — `formatContextSize` renders a placeholder instead.
+		contextSize: number | undefined;
+	} = $props();
+
+	const label = $derived(formatContextSize(contextSize));
+</script>
+
+<div class="px-4 pb-1 text-xs opacity-60" aria-live="polite">
+	<span title="The model's max context window is not reported yet — current usage only.">
+		{label}
+	</span>
+</div>