diff --git a/components/chat.tsx b/components/chat.tsx
index 274ae5c..0e779fa 100644
--- a/components/chat.tsx
+++ b/components/chat.tsx
@@ -200,12 +200,47 @@ export function Chat({
 	useEffect(() => {
 		void syncTurns();
 	}, [syncTurns]);
-	// After the learner's own turn ends the log has a fresh entry — refetch.
+	// After the learner's own turn ends the log has a fresh entry — and the
+	// persisted transcript may have grown server-authored parts the live
+	// stream can't carry (plan blocks attach at persist). Adopt the record
+	// the moment the server agrees the turn is done: endTurn flips ONLY
+	// after persist, so a not-in-flight record is a complete one — the same
+	// invariant the wake-adoption poll below rides. Poll briefly (the flip
+	// trails the client's stream-end by milliseconds), then adopt once.
 	const prevStatusRef = useRef(status);
 	useEffect(() => {
-		if (prevStatusRef.current !== "ready" && status === "ready") void syncTurns();
+		if (prevStatusRef.current !== "ready" && status === "ready") {
+			// immediate: turns-only refetch — the stats bar updates even while
+			// the server's persist/endTurn lags the client by milliseconds
+			void syncTurns();
+			// deferred: adopt the full record once the server agrees the turn
+			// is done (plan blocks and any other persist-authored parts)
+			let cancelled = false;
+			let timer: ReturnType<typeof setTimeout> | undefined;
+			const adopt = async (attempt: number) => {
+				if (cancelled) return;
+				try {
+					const res = await fetch(`/api/chats/${chatId}`);
+					if (res.ok) {
+						const record = (await res.json()) as { turnInFlight?: boolean };
+						if (record.turnInFlight !== true) {
+							void syncFromServer();
+							return;
+						}
+					}
+				} catch {
+					/* transient — keep polling */
+				}
+				if (attempt < 8 && !cancelled) timer = setTimeout(() => void adopt(attempt + 1), 500);
+			};
+			void adopt(0);
+			return () => {
+				cancelled = true;
+				if (timer) clearTimeout(timer);
+			};
+		}
 		prevStatusRef.current = status;
-	}, [status, syncTurns]);
+	}, [status, chatId, syncFromServer, syncTurns]);
 
 	// ── Central job observability (contracts.ts "THE ASYNC PROTOCOL" §4) ──
 	// ONE poll of /api/jobs drives every job surface (transcript dispatch
diff --git a/src/agent/teacher-agent.ts b/src/agent/teacher-agent.ts
index 3c0c75b..971db29 100644
--- a/src/agent/teacher-agent.ts
+++ b/src/agent/teacher-agent.ts
@@ -1,6 +1,6 @@
 import * as fs from "node:fs";
 import * as path from "node:path";
-import { ToolLoopAgent, stepCountIs, type LanguageModel, type ToolSet } from "ai";
+import { ToolLoopAgent, stepCountIs, type LanguageModel, type LanguageModelUsage, type ToolSet } from "ai";
 import { PATHS } from "../../config.ts";
 import { modelFor } from "./models.ts";
 import { questionTools } from "./tools.ts";
@@ -58,7 +58,16 @@ function withGraphContext(graphContext: string | null | undefined): string {
 	return `${instructions}\n\n---\n\n${graphContext}`;
 }
 
-export function createTeacherAgent(model?: LanguageModel, tools?: ToolSet, graphContext?: string | null) {
+export function createTeacherAgent(
+	model?: LanguageModel,
+	tools?: ToolSet,
+	graphContext?: string | null,
+	// Money channel (wave 4): the chat-handler passes a per-turn collector;
+	// every model call's usage lands in the chat record's turns log
+	// (ChatStore.appendTurnUsage). The step carries far more than usage —
+	// the narrow param keeps the seam honest about what we consume.
+	onStepFinish?: (step: { usage: LanguageModelUsage }) => void,
+) {
 	return new ToolLoopAgent({
 		// Model choice + thinking budget live in config.ts (MODELS.teacher).
 		// Reasoning tokens stream as reasoning parts into the UI's collapsible block.
@@ -74,6 +83,7 @@ export function createTeacherAgent(model?: LanguageModel, tools?: ToolSet, graph
 		// job tools resolve immediately (fire-and-forget ack). stepCountIs is
 		// just the safety net.
 		stopWhen: stepCountIs(20),
+		...(onStepFinish ? { onStepFinish } : {}),
 	});
 }
 
diff --git a/src/core/plan-blocks.ts b/src/core/plan-blocks.ts
new file mode 100644
index 0000000..48baf7f
--- /dev/null
+++ b/src/core/plan-blocks.ts
@@ -0,0 +1,169 @@
+/**
+ * Plan-block emission (wave 4, the seam the dock/popup render against):
+ * the PURE builder that turns a plan tree + transcript into the persisted
+ * `data-plan-block` parts the chat renders in flow. Emission happens ONCE,
+ * at persist time (turn-service seam) — blocks are frozen transcript
+ * content (user decision, 2026-09-11), never derived at render:
+ *
+ * - FULL block: attached to the assistant message carrying a successful
+ *   plan_init call — the whole DAG, snapshot statuses. Always emitted on
+ *   (re)authoring, even an identical rewrite: authoring is the event.
+ * - NEIGHBORHOOD block: attached to the LAST assistant message when the
+ *   derived plan state MOVED since the newest existing block — current
+ *   shifted or a check passed (done grew). A resolved FAILED check also
+ *   moves: done doesn't grow, but the check is no longer out, so current
+ *   falls back to the first eligible node — the block reflects that
+ *   standing. The as-of state is read from the newest block's FROZEN
+ *   SNAPSHOT (not re-derived from the transcript, which the client
+ *   re-posts with answered quizzes mutated in place) and compared with
+ *   the state derived now — no extra state channel exists.
+ *
+ * Idempotent by construction: a message that already carries a plan-block
+ * part is never given another (persist replays / retry paths are safe).
+ */
+
+import type { PlanBlockData, PlanNode, PlanTree } from "./contracts.ts";
+import { derivePlanStatuses } from "./contracts.ts";
+import type { UIMessage } from "ai";
+
+/** The message part the chat renderer consumes (TeacherUIDataParts key
+ *  "plan-block" — the SDK strips the `data-` prefix from the name). */
+export interface PlanBlockPart {
+	type: "data-plan-block";
+	id: string;
+	data: PlanBlockData;
+}
+
+/** Nodes/edges of the one-level neighborhood: current + direct parents +
+ *  direct children, and the edges among exactly those nodes. */
+export function neighborhood(tree: PlanTree, current: string): { nodes: PlanNode[]; edges: Array<{ from: string; to: string }> } {
+	const parents = tree.edges.filter((e) => e.to === current).map((e) => e.from);
+	const children = tree.edges.filter((e) => e.from === current).map((e) => e.to);
+	const ids = new Set([current, ...parents, ...children]);
+	return {
+		nodes: tree.nodes.filter((n) => ids.has(n.id)),
+		edges: tree.edges.filter((e) => ids.has(e.from) && ids.has(e.to)),
+	};
+}
+
+/** Does this message carry a successful plan_init (the full-block trigger)? */
+function hasSuccessfulPlanInit(message: UIMessage): boolean {
+	for (const part of message.parts) {
+		if (part.type !== "tool-plan_init") continue;
+		const ok = (part as { output?: { ok?: unknown } }).output?.ok;
+		if (ok === true) return true;
+	}
+	return false;
+}
+
+/** The plan state as derived from a message slice — the diff key. `checked`
+ *  = every node with a tagged quiz out or answered (a new check IS a plan
+ *  event: the block's glowing current follows it). */
+function planState(messages: readonly UIMessage[], tree: PlanTree) {
+	const statuses = derivePlanStatuses(tree, messages);
+	const checked = new Set<string>();
+	for (const m of messages) {
+		for (const part of m.parts) {
+			if (part.type !== "tool-quiz") continue;
+			const nodeId = (part as { input?: { nodeId?: unknown } }).input?.nodeId;
+			if (typeof nodeId === "string" && nodeId !== "") checked.add(nodeId);
+		}
+	}
+	return {
+		current: statuses.current,
+		done: [...statuses.done].sort().join(","),
+		checked: [...checked].sort().join(","),
+	};
+}
+
+/** The messages up to and including the newest plan-block carrier — the
+ *  "as of last block" slice. Null when no block exists yet. */
+function sliceAsOfLastBlock(messages: readonly UIMessage[]): UIMessage[] | null {
+	for (let i = messages.length - 1; i >= 0; i--) {
+		if (messages[i]!.parts.some((p) => p.type === "data-plan-block")) return messages.slice(0, i + 1);
+	}
+	return null;
+}
+
+/** The as-of state read from the newest existing block's FROZEN SNAPSHOT —
+ *  not re-derived from the transcript (which the client re-posts with quiz
+ *  parts mutated in place: an answered check's part flips from
+ *  input-available to output-available, so a re-derivation would read the
+ *  post-answer state and the move would be invisible). The block is the
+ *  as-of record; that is what frozen snapshots are for. */
+function stateAsOfLastBlock(messages: readonly UIMessage[]): { current: string | null; done: string } | null {
+	for (let i = messages.length - 1; i >= 0; i--) {
+		for (const part of messages[i]!.parts) {
+			if (part.type !== "data-plan-block") continue;
+			const data = (part as { data?: PlanBlockData }).data;
+			if (!data) return null;
+			return {
+				current: data.current,
+				done: data.nodes
+					.filter((n) => n.status === "done")
+					.map((n) => n.id)
+					.sort()
+					.join(","),
+			};
+		}
+	}
+	return null;
+}
+
+function snapshot(kind: PlanBlockData["kind"], tree: PlanTree, messages: readonly UIMessage[]): PlanBlockData | null {
+	const statuses = derivePlanStatuses(tree, messages);
+	if (kind === "neighborhood" && statuses.current === null) return null;
+	const scope = kind === "full" || statuses.current === null ? tree : neighborhood(tree, statuses.current);
+	return {
+		kind,
+		at: new Date().toISOString(),
+		current: statuses.current,
+		nodes: scope.nodes.map((n) => ({
+			id: n.id,
+			title: n.title,
+			status: statuses.current === n.id ? "current" : statuses.done.has(n.id) ? "done" : "upcoming",
+		})),
+		edges: scope.edges.map((e) => ({ from: e.from, to: e.to })),
+	};
+}
+
+/**
+ * Enrich a message list with plan-block parts — THE emission seam's pure
+ * half. `messages` is the deduped list persistTurn is about to write;
+ * `tree` is the record's current tree (the plan tool saved it during the
+ * turn). Messages already carrying a block are never touched.
+ */
+export function withPlanBlocks(messages: UIMessage[], tree: PlanTree | null | undefined): UIMessage[] {
+	if (!tree || tree.nodes.length === 0 || messages.length === 0) return messages;
+
+	const lastAssistant = [...messages].reverse().find((m) => m.role === "assistant");
+	if (!lastAssistant) return messages;
+
+	// neighborhood needed? the plan state moved since the newest existing block
+	let moved = false;
+	const asOf = stateAsOfLastBlock(messages);
+	if (asOf === null) {
+		moved = planState(messages, tree).checked !== ""; // first block: any plan event ever
+	} else {
+		const now = planState(messages, tree);
+		moved = asOf.current !== now.current || asOf.done !== now.done;
+	}
+
+	const blockId = `pb_${Date.now().toString(36)}_${crypto.randomUUID().slice(0, 8)}`;
+	let fullAttached = false;
+	return messages.map((m) => {
+		if (m.role !== "assistant" || m.parts.some((p) => p.type === "data-plan-block")) return m;
+		if (hasSuccessfulPlanInit(m)) {
+			const data = snapshot("full", tree, messages);
+			if (!data) return m;
+			fullAttached = true;
+			return { ...m, parts: [...m.parts, { type: "data-plan-block", id: `${blockId}_f`, data } satisfies PlanBlockPart] };
+		}
+		if (moved && !fullAttached && m.id === lastAssistant.id) {
+			const data = snapshot("neighborhood", tree, messages);
+			if (!data) return m;
+			return { ...m, parts: [...m.parts, { type: "data-plan-block", id: `${blockId}_n`, data } satisfies PlanBlockPart] };
+		}
+		return m;
+	});
+}
diff --git a/src/server/chat-handler.ts b/src/server/chat-handler.ts
index 8f7fa69..4e9809f 100644
--- a/src/server/chat-handler.ts
+++ b/src/server/chat-handler.ts
@@ -1,12 +1,15 @@
-import { createAgentUIStreamResponse, InvalidToolInputError, NoSuchToolError, type UIMessage } from "ai";
+import { createAgentUIStreamResponse, InvalidToolInputError, NoSuchToolError, type LanguageModelUsage, type UIMessage } from "ai";
 import { createTeacherAgent, type TeacherAgent } from "../agent/teacher-agent.ts";
 import { createJobRunner } from "../agent/subagents/runner.ts";
 import { createExtractionPipeline } from "../agent/subagents/extraction.ts";
 import { createTeacherTools } from "../agent/tools.ts";
+import { modelFor } from "../agent/models.ts";
+import { MODELS } from "../../config.ts";
 import { claimSteers, deliverJobResult, setJobRespawn, setTurnRunner } from "./jobs.ts";
 import { stores } from "./stores.ts";
 import { beginTurn, endTurn } from "./turn-liveness.ts";
 import { buildTeacherContext, persistTurn, triggerExtraction } from "./turn-service.ts";
+import type { CallUsage } from "../core/contracts.ts";
 
 /**
  * One chat turn: run the teacher agent over the client's message list and
@@ -93,6 +96,21 @@ export async function handleChatRequest(req: Request, agent?: TeacherAgent): Pro
 		console.error(`[chat ${chatId}] write-ahead persist failed:`, err);
 	}
 
+	// MONEY CHANNEL (wave 4): one collector per request, fed by the agent's
+	// step callback. Every model call's usage lands in the chat record's
+	// append-only turns log at onFinish — idempotent per response message id,
+	// so abort replays and wake retries append exactly once.
+	const steps: CallUsage[] = [];
+	const collectUsage = (step: { usage: LanguageModelUsage }) => {
+		steps.push({
+			inputTokens: step.usage.inputTokens,
+			outputTokens: step.usage.outputTokens,
+			cacheReadTokens: step.usage.inputTokenDetails?.cacheReadTokens,
+			cacheWriteTokens: step.usage.inputTokenDetails?.cacheWriteTokens,
+			reasoningTokens: step.usage.outputTokenDetails?.reasoningTokens,
+		});
+	};
+
 	try {
 		return await createAgentUIStreamResponse({
 		agent:
@@ -106,6 +124,7 @@ export async function handleChatRequest(req: Request, agent?: TeacherAgent): Pro
 					spawn: (job, input) => void jobRunner.runJob(job.id, input),
 				}),
 				buildTeacherContext(stores.graphStore, chatId),
+				collectUsage,
 			),
 		uiMessages: outgoing,
 		// Tool-call validation failures are messages OUR schemas authored (the
@@ -121,6 +140,17 @@ export async function handleChatRequest(req: Request, agent?: TeacherAgent): Pro
 		},
 			onFinish: ({ messages, isAborted }) => {
 				persistTurn(stores.chatStore, chatId, messages as UIMessage[], isAborted);
+				// usage log append (idempotent per response message id) — the
+				// stats bar refolds this on the client after the turn settles
+				const response = [...messages].reverse().find((m) => m.role === "assistant");
+				if (response && steps.length > 0) {
+					stores.chatStore.appendTurnUsage(chatId, {
+						messageId: response.id,
+						at: new Date().toISOString(),
+						model: MODELS.teacher.model,
+						steps,
+					});
+				}
 				triggerExtraction(stores, extraction, chatId);
 				// LAST: a poller that sees not-in-flight must read an already-persisted
 				// record — endTurn only after persistTurn has done its job.
diff --git a/src/server/turn-service.ts b/src/server/turn-service.ts
index e999669..eca8b1b 100644
--- a/src/server/turn-service.ts
+++ b/src/server/turn-service.ts
@@ -17,6 +17,7 @@
  */
 
 import type { UIMessage } from "ai";
+import { withPlanBlocks } from "../core/plan-blocks.ts";
 import { buildGraphContext } from "../core/graph-context.ts";
 import { getGraph, pendingQuizAnswers } from "../core/graph.ts";
 import type { ChatStore, GraphStore } from "../core/ports.ts";
@@ -76,7 +77,12 @@ export function withMessageIds(messages: UIMessage[]): UIMessage[] {
 
 export function persistTurn(chatStore: ChatStore, chatId: string, messages: UIMessage[], isAborted: boolean): void {
 	try {
-		chatStore.saveMessages(chatId, withMessageIds(messages));
+		const deduped = withMessageIds(messages);
+		// Plan-block emission (wave 4): blocks are frozen transcript content —
+		// attached here, once, at the canonical state write. The tree was
+		// saved by the plan tool during the turn; snapshots derive now.
+		const tree = chatStore.getChat(chatId)?.tree;
+		chatStore.saveMessages(chatId, withPlanBlocks(deduped, tree));
 	} catch (err) {
 		console.error(`[chat ${chatId}] persist failed:`, err, { isAborted });
 	}
diff --git a/tests/plan-blocks.test.ts b/tests/plan-blocks.test.ts
new file mode 100644
index 0000000..d54cdf5
--- /dev/null
+++ b/tests/plan-blocks.test.ts
@@ -0,0 +1,191 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import type { UIMessage } from "ai";
+import { neighborhood, withPlanBlocks, type PlanBlockPart } from "../src/core/plan-blocks.ts";
+import type { PlanTree } from "../src/core/contracts.ts";
+
+/** The 5-node DAG the neighborhood tests slice:
+ *  A → B → C, B → D → E (B's children C,D; A's only child is B). */
+function tree(): PlanTree {
+	return {
+		nodes: [
+			{ id: "a", title: "Foundations" },
+			{ id: "b", title: "Core" },
+			{ id: "c", title: "Applied" },
+			{ id: "d", title: "Deep dive" },
+			{ id: "e", title: "Frontier" },
+		],
+		edges: [
+			{ from: "a", to: "b" },
+			{ from: "b", to: "c" },
+			{ from: "b", to: "d" },
+			{ from: "d", to: "e" },
+		],
+		currentNodeId: null,
+		doneNodeIds: [],
+		version: 1,
+	};
+}
+
+function msg(id: string, role: "user" | "assistant", parts: UIMessage["parts"] = []): UIMessage {
+	return { id, role, parts };
+}
+
+function blocksOf(m: UIMessage): PlanBlockPart[] {
+	return m.parts.filter((p): p is PlanBlockPart => p.type === "data-plan-block") as PlanBlockPart[];
+}
+
+function allBlocks(messages: UIMessage[]): PlanBlockPart[] {
+	return messages.flatMap((m) => blocksOf(m));
+}
+
+/** tool-plan_init part in terminal state (the authoring call succeeded). */
+function planInitPart(tree: PlanTree, ok = true) {
+	return { type: "tool-plan_init", state: "output-available", input: { mermaid: "graph TD" }, output: { ok, tree } } as unknown as UIMessage["parts"][number];
+}
+
+/** tool-quiz part tagged with a node. `answered` appends the learner's output. */
+function quizPart(nodeId: string, answered?: { correct: boolean }) {
+	const part = {
+		type: "tool-quiz",
+		state: answered ? "output-available" : "input-available",
+		input: { nodeId, question: "…", options: [] },
+		...(answered ? { output: { correct: answered.correct } } : {}),
+	} as unknown as UIMessage["parts"][number];
+	return part;
+}
+
+describe("neighborhood (pure scope)", () => {
+	it("includes current + direct parents + direct children, nothing further", () => {
+		const { nodes, edges } = neighborhood(tree(), "b");
+		assert.deepEqual(
+			nodes.map((n) => n.id).sort(),
+			["a", "b", "c", "d"],
+		);
+		assert.deepEqual(edges.map((e) => `${e.from}>${e.to}`).sort(), ["a>b", "b>c", "b>d"]);
+	});
+});
+
+describe("withPlanBlocks", () => {
+	it("attaches a FULL block to the assistant message with a successful plan_init", () => {
+		const t = tree();
+		const out = withPlanBlocks(
+			[msg("m1", "user", [{ type: "text", text: "teach me" } as never]), msg("m2", "assistant", [planInitPart(t)])],
+			t,
+		);
+		const blocks = allBlocks(out);
+		assert.equal(blocks.length, 1);
+		assert.equal(blocks[0]!.data.kind, "full");
+		assert.equal(blocks[0]!.data.current, "a"); // first prereq-free node
+		assert.equal(blocks[0]!.data.nodes.length, 5);
+		assert.equal(blocksOf(out[1]!).length, 1); // on the plan_init message itself
+	});
+
+	it("ignores a failed plan_init (ok: false)", () => {
+		const t = tree();
+		const out = withPlanBlocks([msg("m1", "user"), msg("m2", "assistant", [planInitPart(t, false)])], t);
+		assert.equal(allBlocks(out).length, 0);
+	});
+
+	it("attaches a NEIGHBORHOOD block on the last assistant message when a check goes out", () => {
+		const t = tree();
+		const out = withPlanBlocks(
+			[
+				msg("m1", "user"),
+				msg("m2", "assistant", [quizPart("a", { correct: true })]), // a done earlier
+				msg("m3", "assistant", [quizPart("b")]), // check on b now out
+			],
+			t,
+		);
+		const blocks = allBlocks(out);
+		assert.equal(blocks.length, 1);
+		assert.equal(blocks[0]!.data.kind, "neighborhood");
+		assert.equal(blocks[0]!.data.current, "b");
+		assert.deepEqual(
+			blocks[0]!.data.nodes.map((n) => n.id).sort(),
+			["a", "b", "c", "d"], // e excluded: grandchild of b via d
+		);
+		assert.equal(blocksOf(out[2]!).length, 1); // on the LAST assistant message
+	});
+
+	it("does not re-emit when the plan state has not moved", () => {
+		const t = tree();
+		const turn1 = withPlanBlocks([msg("m1", "user"), msg("m2", "assistant", [quizPart("b")])], t);
+		assert.equal(allBlocks(turn1).length, 1);
+		// next turn: plain exchange, plan untouched — the previous block IS the as-of slice
+		const turn2 = withPlanBlocks(
+			[...turn1, msg("m3", "user"), msg("m4", "assistant", [{ type: "text", text: "ok" } as never])],
+			t,
+		);
+		assert.equal(allBlocks(turn2).length, 1); // nothing new
+		assert.equal(blocksOf(turn2[3]!).length, 0);
+	});
+
+	it("re-emits when a check resolves (done grows / current falls back)", () => {
+		const t = tree();
+		const asked = withPlanBlocks([msg("m1", "user"), msg("m2", "assistant", [quizPart("b")])], t);
+		// learner answers correctly: done grows, no check out → current falls to a
+		const answered = withPlanBlocks(
+			[...asked, msg("m3", "user"), msg("m4", "assistant", [{ type: "text", text: "good" } as never])].map((m) =>
+				m.id === "m2" ? { ...m, parts: m.parts.map((p) => (p.type === "tool-quiz" ? quizPart("b", { correct: true }) : p)) } : m,
+			),
+			t,
+		);
+		const blocks = allBlocks(answered);
+		assert.equal(blocks.length, 2);
+		const latest = blocks.at(-1)!.data;
+		assert.equal(latest.kind, "neighborhood");
+		assert.deepEqual(
+			latest.nodes.find((n) => n.id === "b")!.status,
+			"done",
+		);
+		assert.equal(latest.current, "a"); // b done, a never checked → first eligible
+	});
+
+	it("re-emits when a failed check resets the standing (no done growth)", () => {
+		const t = tree();
+		const asked = withPlanBlocks([msg("m1", "user"), msg("m2", "assistant", [quizPart("b")])], t);
+		// learner answers WRONG: done doesn't grow, but no check is out anymore
+		// → current falls back to the first eligible node
+		const failed = withPlanBlocks(
+			[...asked, msg("m3", "user"), msg("m4", "assistant", [{ type: "text", text: "let's review" } as never])].map((m) =>
+				m.id === "m2" ? { ...m, parts: m.parts.map((p) => (p.type === "tool-quiz" ? quizPart("b", { correct: false }) : p)) } : m,
+			),
+			t,
+		);
+		const blocks = allBlocks(failed);
+		assert.equal(blocks.length, 2);
+		const latest = blocks.at(-1)!.data;
+		assert.equal(latest.current, "a");
+		assert.equal(latest.nodes.find((n) => n.id === "b")!.status, "upcoming"); // not done
+	});
+
+	it("is idempotent when the same list is persisted twice (retry replay)", () => {
+		const t = tree();
+		const once = withPlanBlocks([msg("m1", "user"), msg("m2", "assistant", [quizPart("b")])], t);
+		const twice = withPlanBlocks(once, t);
+		assert.equal(allBlocks(twice).length, 1);
+		assert.equal(twice[1]!.parts.length, once[1]!.parts.length);
+	});
+
+	it("attaches a new FULL block on a plan rewrite, keeping the old block frozen", () => {
+		const t = tree();
+		const authored = withPlanBlocks([msg("m1", "user"), msg("m2", "assistant", [planInitPart(t)])], t);
+		const rewrite = withPlanBlocks(
+			[...authored, msg("m3", "user"), msg("m4", "assistant", [planInitPart(t)])],
+			t,
+		);
+		const blocks = allBlocks(rewrite);
+		assert.equal(blocks.length, 2);
+		assert.deepEqual(blocks.map((b) => b.data.kind), ["full", "full"]);
+		assert.equal(blocksOf(rewrite[3]!).length, 1); // new block on the NEW authoring message
+	});
+
+	it("no-ops on null/empty tree and on messages without assistant turns", () => {
+		assert.equal(withPlanBlocks([msg("m1", "user")], tree()).length, 1);
+		const empty = withPlanBlocks([msg("m1", "user"), msg("m2", "assistant")], { ...tree(), nodes: [] });
+		assert.equal(allBlocks(empty).length, 0);
+		const noTree = withPlanBlocks([msg("m1", "user"), msg("m2", "assistant")], null);
+		assert.equal(allBlocks(noTree).length, 0);
+	});
+});