graph context: a separate marker message before the first user turn

The map picks no longer glue into the opening prompt as seed text
(owner, 2026-10-04 late). POST /api/chats accepts graphContext
{propositions}; the route resolves the picks against the live graph,
derives the reasoning chains among them (chainsForSelection →
formatChainLine), and writes a graph-context custom marker as
message[0] — before the learner's first user message, in either mode.

- Core: newGraphContextMessage (model-facing content = the
  contextualization framing + proposition lines + chain lines;
  details = the structured pick); toLlmMessages flattens it to a user
  message like every marker, so the model reads it for free.
- View: projects to a user-side data-graph-context part.
- UI: GraphContextCard — minimal, muted, dashed, collapsed by default
  (counts only); expand reveals name — statement rows and the chain
  lines. Same compact-expandable pattern as the practice cards.
- Home: sends graphContext with the creation whenever picks exist;
  the prompt param carries the learner's own words only.

components/chat.tsx

@@ -38,6 +38,7 @@ import type { PlanBlockData } from "@/src/core";
 import { QuestionDock } from "@/components/question-dock";
 import { PracticeDock } from "@/components/practice-dock";
 import { ThinkingCard, type PracticeThinkingData } from "@/components/practice-cards";
+import { GraphContextCard, type GraphContextData } from "@/components/graph-context-card";
 import { TopBar } from "@/components/top-bar";
 import type { ArtifactPartProps, JobRecord, Phase3, QuestionAnswerData, TurnUsage } from "@/src/core";
 import type { LhViewMessage } from "@/src/core";
@@ -1258,6 +1259,13 @@ export function Chat({
 														);
 													case "step-start":
 														return pi > 0 ? <Separator key={pi} className="my-3" /> : null;
+													case "data-graph-context":
+														// The session's map pick (2026-10-04): a minimal
+														// collapsible card — scaffolding above the first
+														// user message, never a bubble.
+														return (
+															<GraphContextCard key={pi} data={part.data as GraphContextData} />
+														);
 													case "data-practice-thinking":
 														// The learner's practice submission (2026-10-04):
 														// user-side — the register (thinking trace /

components/graph-context-card.tsx

@@ -0,0 +1,63 @@
+"use client";
+
+import { useState } from "react";
+import { ChevronRightIcon } from "lucide-react";
+import { cn } from "@/lib/utils";
+
+/**
+ * The graph-context card (owner, 2026-10-04): the session's map pick —
+ * message[0] of a session started with propositions picked from the
+ * graph. Scaffolding, not a turn: minimal, muted, collapsed by default;
+ * expand reveals the picked propositions (name — statement) and the
+ * reasoning chains among them (premises → conclusion lines). The model
+ * reads the same content through the marker's own text (toLlmMessages);
+ * this card is the learner's face of it.
+ */
+
+/** The graph-context marker's details (native-messages.ts). */
+export interface GraphContextData {
+	propositions: { id: string; name: string; statement: string }[];
+	chains: string[];
+}
+
+export function GraphContextCard({ data }: { data: GraphContextData }) {
+	const [open, setOpen] = useState(false);
+	const { propositions = [], chains = [] } = data ?? {};
+	const noun = (n: number, one: string, many: string) => `${n} ${n === 1 ? one : many}`;
+	return (
+		<div data-testid="graph-context-card" className="my-1">
+			<button
+				type="button"
+				data-testid="graph-context-toggle"
+				onClick={() => setOpen((v) => !v)}
+				className="flex w-full cursor-pointer items-center gap-2 rounded-lg border border-dashed border-border/70 px-3 py-2 text-left text-xs text-muted-foreground transition-colors hover:text-foreground"
+			>
+				<ChevronRightIcon aria-hidden="true" className={cn("size-3.5 shrink-0 transition-transform", open && "rotate-90")} />
+				<span className="font-medium">Graph context</span>
+				<span className="min-w-0 flex-1 truncate">
+					{noun(propositions.length, "proposition", "propositions")}
+					{chains.length > 0 ? ` · ${noun(chains.length, "chain", "chains")}` : ""}
+				</span>
+			</button>
+			{open && (
+				<div data-testid="graph-context-body" className="mt-1 space-y-1 rounded-lg border border-dashed border-border/70 px-4 py-3 text-xs">
+					{propositions.map((p) => (
+						<div key={p.id} data-testid={`graph-context-prop-${p.id}`} className="leading-relaxed">
+							<span className="font-medium text-foreground/80">{p.name}</span>
+							<span className="text-muted-foreground"> — {p.statement}</span>
+						</div>
+					))}
+					{chains.length > 0 && (
+						<div className="pt-1">
+							{chains.map((c, i) => (
+								<div key={i} className="font-mono text-[11px] text-muted-foreground">
+									{c}
+								</div>
+							))}
+						</div>
+					)}
+				</div>
+			)}
+		</div>
+	);
+}

components/home.tsx

@@ -112,26 +112,25 @@ export function Home({ initialConfigured }: { initialConfigured: boolean }) {
 		}
 		setBusy(true);
 		try {
-			// The practice seed: the picks ride the OPENING PROMPT (never
-			// the title — the title stays the learner's own words).
-			const seed =
-				mode === "practice" && practiceProps.length > 0
-					? `\n\nPropositions from the map:\n${practiceProps
-							.map((p) => `- ${p.name}: ${p.statement}`)
-							.join("\n")}`
-					: "";
+			// The map context (owner, 2026-10-04): picks in EITHER mode ride
+			// the CREATION as graphContext — the server writes a
+			// graph-context marker as message[0] (a separate custom message
+			// before the learner's first user message, propositions + the
+			// chains among them); the prompt stays the learner's own words.
 			const res = await fetch("/api/chats", {
 				method: "POST",
 				headers: { "Content-Type": "application/json" },
-				body: JSON.stringify(
-					mode === "practice" ? { title: t, kind: "practice" } : { title: t },
-				),
+				body: JSON.stringify({
+					title: t,
+					...(mode === "practice" ? { kind: "practice" } : {}),
+					...(practiceProps.length > 0 ? { graphContext: { propositions: practiceProps } } : {}),
+				}),
 			});
 			const { id } = (await res.json()) as { id: string };
 			// No settings PUT on create (per-session models, owner
 			// 2026-09-28): the SERVER stamps the newborn from the strong
 			// slot — the same default this surface's model control edits.
-			router.push(`/chat/${id}?prompt=${encodeURIComponent(t + seed)}`);
+			router.push(`/chat/${id}?prompt=${encodeURIComponent(t)}`);
 		} catch {
 			setBusy(false);
 		}