diff --git a/apps/api/app/routes/machine.ts b/apps/api/app/routes/machine.ts
index 60ee408a..09adae39 100644
--- a/apps/api/app/routes/machine.ts
+++ b/apps/api/app/routes/machine.ts
@@ -1,10 +1,11 @@
 import { Actor } from '@nestri/core/actor';
+import { Box } from '@nestri/core/box/index';
 import { ErrorCodes, VisibleError } from '@nestri/core/error';
 import { Examples } from '@nestri/core/examples';
 import { Identifier } from '@nestri/core/id';
 import { Machine } from '@nestri/core/machine/index';
-import { Member } from '@nestri/core/team/member';
 import { Team } from '@nestri/core/team/index';
+import { Member } from '@nestri/core/team/member';
 import { Hono } from 'hono';
 import { describeRoute } from 'hono-openapi';
 import { z } from 'zod';
@@ -260,6 +261,82 @@ export namespace MachineApi {
 				});
 			}
 		)
+		.post(
+			'/report',
+			machineOnly,
+			describeRoute({
+				tags: ['Machine'],
+				summary: 'Say what the host is running',
+				description:
+					'Records one full snapshot of the boxes on the calling host. Separate from the beat because the two have different loss tolerance: a dropped report is corrected by the next one, where a dropped beat moves a host towards offline. Send one when a box changes lifecycle, and send one anyway every so often so a single lost snapshot cannot leave this record permanently wrong. Never send a delta — a retrying agent cannot promise ordering, and out-of-order deltas describe a host that never existed.',
+				responses: {
+					200: {
+						content: { 'application/json': { schema: Result(Box.ReportOutcome) } },
+						description: 'The snapshot was recorded'
+					},
+					400: ErrorResponses[400],
+					403: ErrorResponses[403],
+					404: ErrorResponses[404]
+				}
+			}),
+			validator(
+				'json',
+				z
+					.object({
+						agentPid: z.number().int().meta({
+							description: 'The reporting agent’s own process id, in its own namespace'
+						}),
+						boxesKnown: z.number().int().meta({ description: 'How many boxes the host holds' }),
+						boxesRunning: z.number().int().meta({
+							description: 'How many of them are running'
+						}),
+						boxes: z.array(Box.Reported).meta({
+							description: 'Every box the host holds. A full snapshot, never a delta'
+						})
+					})
+					// Strict, so a field this cannot act on is a validation error a
+					// host operator sees rather than one quietly dropped. Capacity
+					// belongs here eventually and it has no honest fields yet;
+					// refusing the ones nobody measures is how it stays that way.
+					.strict()
+			),
+			async (c) => {
+				const machine = await Machine.fromID(Actor.machineID);
+				if (!machine) {
+					// Same answer as the beat gives, for the same reason: the
+					// credentials authenticated but the row is gone, and a host
+					// must re-register rather than keep reporting into nothing.
+					throw new VisibleError(
+						'not_found',
+						ErrorCodes.NotFound.RESOURCE_NOT_FOUND,
+						'This machine no longer exists'
+					);
+				}
+
+				const { boxes } = c.req.valid('json');
+				const outcome = await Box.applyHostReport({ machineId: Actor.machineID, boxes });
+
+				// `agentPid`, `boxesKnown` and `boxesRunning` are read and not
+				// stored. They are a summary of the list that follows them, and a
+				// stored copy is a second answer to a question the list already
+				// answers — one that goes stale the first time the two disagree.
+				// They stay on the wire because a host that cannot enumerate its
+				// boxes can still say how many it has.
+				if (outcome.unknown.length > 0) {
+					// Loudly, per the contract this endpoint is built to: a host
+					// holding boxes nobody placed there is a bug to surface, not a
+					// state to reconcile quietly. Nothing is created for them.
+					// eslint-disable-next-line no-console
+					console.warn(
+						'host report named boxes that are not placed here:',
+						Actor.machineID,
+						outcome.unknown.join(', ')
+					);
+				}
+
+				return c.json({ data: outcome });
+			}
+		)
 		.get(
 			'/me',
 			machineOnly,
diff --git a/apps/api/test/report.test.ts b/apps/api/test/report.test.ts
new file mode 100644
index 00000000..39550997
--- /dev/null
+++ b/apps/api/test/report.test.ts
@@ -0,0 +1,183 @@
+import { afterAll, describe, expect, test } from 'bun:test';
+
+import { Box } from '@nestri/core/box/index';
+import { Fixtures } from '@nestri/core/db/fixtures';
+import { testDb } from '@nestri/core/db/test';
+import { Identifier } from '@nestri/core/id';
+import { Machine } from '@nestri/core/machine/index';
+
+import { app } from '../app/index';
+import './setup';
+
+const sql = testDb();
+
+const createdUserIds: string[] = [];
+
+async function registeredHost(label: string) {
+	const owner = await Fixtures.owner(label);
+	createdUserIds.push(owner.userId);
+	const registered = await Machine.register({
+		id: Identifier.ascending('machine'),
+		ownerUserId: owner.userId,
+		teamId: owner.teamId,
+		label
+	});
+	return {
+		id: registered.id,
+		userId: owner.userId,
+		headers: {
+			'x-nestri-machine-id': registered.id,
+			'x-nestri-machine-secret': registered.secret,
+			'content-type': 'application/json'
+		}
+	};
+}
+
+async function boxOn(host: { id: string; userId: string }, label: string) {
+	return Box.create({
+		id: Identifier.ascending('box'),
+		userId: host.userId,
+		machineId: host.id,
+		label,
+		tier: 'sm'
+	});
+}
+
+function report(host: { headers: Record<string, string> }, body: unknown) {
+	return app.request('/machine/report', {
+		method: 'POST',
+		headers: host.headers,
+		body: JSON.stringify(body)
+	});
+}
+
+afterAll(async () => {
+	if (createdUserIds.length > 0) {
+		await sql`delete from "box" where user_id in ${sql(createdUserIds)}`;
+		await sql`delete from "user" where id in ${sql(createdUserIds)}`;
+		createdUserIds.length = 0;
+	}
+});
+
+describe('POST /machine/report', () => {
+	test('the body is the shape the agent sends, flat and camelCased', async () => {
+		const host = await registeredHost('report-shape');
+		const running = await boxOn(host, 'one');
+		const idle = await boxOn(host, 'two');
+
+		// Written from the contract and not from the handler: host fields flat at
+		// the top, boxes under `boxes`, the lifecycle tag flattened into each box,
+		// and `uptimeS` rather than `uptime_s`. A rename on either side of this
+		// seam produces a host that reports, is understood by nothing, and is
+		// told it succeeded.
+		const res = await report(host, {
+			agentPid: 4711,
+			boxesKnown: 2,
+			boxesRunning: 1,
+			boxes: [
+				{ boxId: running.id, tier: 'sm', state: 'running', pid: 1234, uptimeS: 45 },
+				{ boxId: idle.id, tier: 'sm', state: 'created' }
+			]
+		});
+
+		expect(res.status).toBe(200);
+		const body = (await res.json()) as any;
+		expect(body.data.recorded).toBe(2);
+		expect(body.data.unknown).toEqual([]);
+		expect((await Box.fromID(running.id))!.state).toBe('running');
+	});
+
+	test('a request is not wrapped in an envelope, but the response is', async () => {
+		const host = await registeredHost('report-envelope');
+
+		const wrapped = await report(host, {
+			data: { agentPid: 1, boxesKnown: 0, boxesRunning: 0, boxes: [] }
+		});
+		expect(wrapped.status).toBe(400);
+
+		const flat = await report(host, {
+			agentPid: 1,
+			boxesKnown: 0,
+			boxesRunning: 0,
+			boxes: []
+		});
+		expect(flat.status).toBe(200);
+		expect(await flat.json()).toHaveProperty('data');
+	});
+
+	test('a stop carries its reason all the way to the row', async () => {
+		const host = await registeredHost('report-stop');
+		const box = await boxOn(host, 'faulty');
+
+		await report(host, {
+			agentPid: 1,
+			boxesKnown: 1,
+			boxesRunning: 0,
+			boxes: [
+				{ boxId: box.id, tier: 'sm', state: 'stopped', reason: 'guest exited 1', clean: false }
+			]
+		});
+
+		const row = (await Box.fromID(box.id))!;
+		expect(row.state).toBe('stopped');
+		expect(row.stopReason).toBe('guest exited 1');
+		expect(row.stopClean).toBe(false);
+	});
+
+	test('a host cannot report on another host’s boxes', async () => {
+		// The access rule that matters here. A machine credential is a long-lived
+		// secret on hardware in somebody's living room, so the scope is in the
+		// query and not in the agent asking politely about its own boxes.
+		const mine = await registeredHost('report-mine');
+		const theirs = await registeredHost('report-theirs');
+		const victim = await boxOn(theirs, 'not yours');
+		await Box.setState({ id: victim.id, state: 'running' });
+
+		const res = await report(mine, {
+			agentPid: 1,
+			boxesKnown: 1,
+			boxesRunning: 1,
+			boxes: [{ boxId: victim.id, tier: 'sm', state: 'stopped', reason: 'mine now', clean: false }]
+		});
+
+		expect(res.status).toBe(200);
+		expect(((await res.json()) as any).data.unknown).toEqual([victim.id]);
+		expect((await Box.fromID(victim.id))!.state).toBe('running');
+	});
+
+	test('a field nobody can act on is refused rather than dropped', async () => {
+		const host = await registeredHost('report-strict');
+
+		// Capacity belongs on this call eventually and has no honest fields yet.
+		// Refusing an invented one is how an unmeasured number stays out of a
+		// record a placement decision will one day read.
+		const res = await report(host, {
+			agentPid: 1,
+			boxesKnown: 0,
+			boxesRunning: 0,
+			boxes: [],
+			gpusFree: 4
+		});
+		expect(res.status).toBe(400);
+	});
+
+	test('wrong credentials are indistinguishable from none', async () => {
+		const host = await registeredHost('report-wrongsecret');
+		const body = JSON.stringify({ agentPid: 1, boxesKnown: 0, boxesRunning: 0, boxes: [] });
+
+		const none = await app.request('/machine/report', {
+			method: 'POST',
+			headers: { 'content-type': 'application/json' },
+			body
+		});
+		const wrong = await app.request('/machine/report', {
+			method: 'POST',
+			headers: { ...host.headers, 'x-nestri-machine-secret': 'msk_wrong' },
+			body
+		});
+
+		expect(wrong.status).toBe(403);
+		expect(none.status).toBe(403);
+		expect(await wrong.json()).toEqual(await none.json());
+	});
+});
diff --git a/packages/core/src/box/box.test.ts b/packages/core/src/box/box.test.ts
index e28039b5..533598f9 100644
--- a/packages/core/src/box/box.test.ts
+++ b/packages/core/src/box/box.test.ts
@@ -139,3 +139,137 @@ describe('Box', () => {
 		expect((await Box.listByMachine(machineB)).map((b) => b.label)).toEqual(['b1']);
 	});
 });
+
+describe('Box.applyHostReport', () => {
+	async function placed(label: string, count: number) {
+		const owner = await newOwner(label);
+		const machineId = await Fixtures.machine(owner);
+		const boxes = [];
+		for (let i = 0; i < count; i++) {
+			boxes.push(
+				await Box.create({
+					id: Identifier.ascending('box'),
+					userId: owner.userId,
+					machineId,
+					label: `${label}-${i}`,
+					tier: 'sm'
+				})
+			);
+		}
+		return { owner, machineId, boxes };
+	}
+
+	test('a snapshot moves the boxes it names', async () => {
+		const { machineId, boxes } = await placed('report-moves', 2);
+
+		const outcome = await Box.applyHostReport({
+			machineId,
+			boxes: [
+				{ boxId: boxes[0]!.id, tier: 'sm', state: 'running', pid: 1234, uptimeS: 45 },
+				{ boxId: boxes[1]!.id, tier: 'sm', state: 'stopped', reason: 'guest exited 0', clean: true }
+			]
+		});
+
+		expect(outcome.recorded).toBe(2);
+		expect(outcome.unknown).toEqual([]);
+		expect(outcome.markedStopped).toEqual([]);
+
+		expect((await Box.fromID(boxes[0]!.id))!.state).toBe('running');
+		const stopped = (await Box.fromID(boxes[1]!.id))!;
+		expect(stopped.state).toBe('stopped');
+		expect(stopped.stopReason).toBe('guest exited 0');
+		expect(stopped.stopClean).toBe(true);
+	});
+
+	test('pid and uptime are accepted and not stored', async () => {
+		const { machineId, boxes } = await placed('report-drops', 1);
+
+		await Box.applyHostReport({
+			machineId,
+			boxes: [{ boxId: boxes[0]!.id, tier: 'sm', state: 'running', pid: 4711, uptimeS: 900 }]
+		});
+
+		// The columns do not exist, so what this pins is that the row is still
+		// readable and carries nothing invented in the two nullable columns it
+		// does have.
+		const row = (await Box.fromID(boxes[0]!.id))!;
+		expect(row.state).toBe('running');
+		expect(row.stopReason).toBeNull();
+		expect(row.stopClean).toBeNull();
+	});
+
+	test('a running box the snapshot omits is stopped, and says why', async () => {
+		const { machineId, boxes } = await placed('report-omits', 2);
+		await Box.setState({ id: boxes[0]!.id, state: 'running' });
+		await Box.setState({ id: boxes[1]!.id, state: 'running' });
+
+		const outcome = await Box.applyHostReport({
+			machineId,
+			boxes: [{ boxId: boxes[0]!.id, tier: 'sm', state: 'running', uptimeS: 5 }]
+		});
+
+		expect(outcome.markedStopped).toEqual([boxes[1]!.id]);
+		const gone = (await Box.fromID(boxes[1]!.id))!;
+		expect(gone.state).toBe('stopped');
+		expect(gone.stopReason).toBe(Box.OMITTED_FROM_REPORT);
+		expect(gone.stopClean).toBe(false);
+	});
+
+	test('a created box the snapshot omits is left alone', async () => {
+		// The ordinary path, not a divergence: a person creates a box here before
+		// its host has been told anything about it.
+		const { machineId, boxes } = await placed('report-created', 1);
+
+		const outcome = await Box.applyHostReport({ machineId, boxes: [] });
+
+		expect(outcome.markedStopped).toEqual([]);
+		expect((await Box.fromID(boxes[0]!.id))!.state).toBe('created');
+	});
+
+	test('an empty snapshot from a host holding nothing stops only what was running', async () => {
+		const { machineId, boxes } = await placed('report-empty', 2);
+		await Box.setState({ id: boxes[0]!.id, state: 'running' });
+
+		const outcome = await Box.applyHostReport({ machineId, boxes: [] });
+
+		expect(outcome.recorded).toBe(0);
+		expect(outcome.markedStopped).toEqual([boxes[0]!.id]);
+		expect((await Box.fromID(boxes[1]!.id))!.state).toBe('created');
+	});
+
+	test('a box the snapshot names that is not placed here is never created', async () => {
+		const { machineId } = await placed('report-unknown', 0);
+		const invented = Identifier.ascending('box');
+
+		const outcome = await Box.applyHostReport({
+			machineId,
+			boxes: [{ boxId: invented, tier: 'lg', state: 'running', uptimeS: 1 }]
+		});
+
+		expect(outcome.unknown).toEqual([invented]);
+		expect(outcome.recorded).toBe(0);
+		expect(await Box.fromID(invented)).toBeNull();
+	});
+
+	test('a snapshot cannot move a box placed on another host', async () => {
+		const mine = await placed('report-mine', 1);
+		const theirs = await placed('report-theirs', 1);
+		await Box.setState({ id: theirs.boxes[0]!.id, state: 'running' });
+
+		const outcome = await Box.applyHostReport({
+			machineId: mine.machineId,
+			boxes: [
+				{
+					boxId: theirs.boxes[0]!.id,
+					tier: 'sm',
+					state: 'stopped',
+					reason: 'mine now',
+					clean: false
+				}
+			]
+		});
+
+		expect(outcome.unknown).toEqual([theirs.boxes[0]!.id]);
+		expect((await Box.fromID(theirs.boxes[0]!.id))!.state).toBe('running');
+	});
+});
diff --git a/packages/core/src/box/index.ts b/packages/core/src/box/index.ts
index bfcd36c7..906421e7 100644
--- a/packages/core/src/box/index.ts
+++ b/packages/core/src/box/index.ts
@@ -1,4 +1,4 @@
-import { and, eq, isNull, sql } from 'drizzle-orm';
+import { and, eq, isNull, notInArray, sql } from 'drizzle-orm';
 import z from 'zod';
 
 import { Database } from '../db/index.js';
@@ -189,10 +189,157 @@ export namespace Box {
 
 	export const remove = fn(Info.shape.id, async (id) => {
 		await Database.use(async (tx) => {
-			await tx.update(BoxTable).set({ timeDeleted: sql`now()` }).where(eq(BoxTable.id, id));
+			await tx
+				.update(BoxTable)
+				.set({ timeDeleted: sql`now()` })
+				.where(eq(BoxTable.id, id));
 		});
 	});
 
+	/**
+	 * What a host says about one box, in the shape the wire carries.
+	 *
+	 * Flat rather than nested: the agent's own type is an internally-tagged
+	 * enum, and flattening it here is what lets one box be one object on both
+	 * sides. `pid` and `uptimeS` are accepted and deliberately not stored —
+	 * a pid is a number in another machine's namespace and means nothing here,
+	 * and uptime is derivable from the run's `timeStarted`, which is already
+	 * kept and already trustworthy. Storing a plausible number instead of a
+	 * measured one is how a scheduler learns to trust a field nobody produced.
+	 */
+	export const Reported = z.discriminatedUnion('state', [
+		z.object({
+			boxId: z.string(),
+			tier: z.enum(BoxTier.enumValues),
+			state: z.literal('created')
+		}),
+		z.object({
+			boxId: z.string(),
+			tier: z.enum(BoxTier.enumValues),
+			state: z.literal('running'),
+			pid: z.number().int().optional(),
+			uptimeS: z.number().int()
+		}),
+		z.object({
+			boxId: z.string(),
+			tier: z.enum(BoxTier.enumValues),
+			state: z.literal('stopped'),
+			reason: z.string(),
+			clean: z.boolean()
+		})
+	]);
+
+	export type Reported = z.infer<typeof Reported>;
+
+	/** What a snapshot changed, and what it disagreed with us about. */
+	export const ReportOutcome = z.object({
+		recorded: z.number().meta({ description: 'Boxes in the snapshot that we know and updated' }),
+		unknown: z.array(z.string()).meta({
+			description: 'Boxes the host is holding that are not placed here. Recorded, never created'
+		}),
+		markedStopped: z.array(z.string()).meta({
+			description: 'Boxes placed here that the snapshot did not mention, and are now stopped'
+		})
+	});
+
+	export type ReportOutcome = z.infer<typeof ReportOutcome>;
+
+	/**
+	 * Written on a box its host did not mention.
+	 *
+	 * A sentence rather than a code because it is read by whoever is looking at
+	 * a box that stopped for no reason they can see, and "the host stopped
+	 * mentioning it" is the fact they need.
+	 */
+	export const OMITTED_FROM_REPORT = 'not in its host’s last report';
+
+	/**
+	 * Record one full snapshot of what a host is running.
+	 *
+	 * Scoped to the calling machine in the `where` clause and not by trusting
+	 * the ids in the body: a machine credential is a long-lived secret on
+	 * hardware in somebody's living room, and a snapshot naming another host's
+	 * boxes must move nothing.
+	 *
+	 * Three rules, and the second two are why this is one function rather than
+	 * a loop of `setState` in a route:
+	 *
+	 * - A box we know, that the snapshot names, takes the reported state.
+	 * - A box we know, that the snapshot omits, is stopped — absence inside a
+	 *   received snapshot is information. (Silence from the host is not, and is
+	 *   not this function's input at all: no report means this is never called.)
+	 * - A box the snapshot names that is not placed here is **not created**. An
+	 *   agent that can conjure a row is an agent that can mint owned resources,
+	 *   and a box belongs to somebody this snapshot cannot name. It is returned
+	 *   as a divergence to be surfaced loudly instead.
+	 *
+	 * One transaction, because a snapshot is one observation: applying half of
+	 * it leaves a state the host was never in.
+	 */
+	export const applyHostReport = fn(
+		z.object({ machineId: Info.shape.machineId, boxes: z.array(Reported) }),
+		async (input): Promise<ReportOutcome> => {
+			return Database.transaction(async (tx) => {
+				const placed = await tx
+					.select({ id: BoxTable.id })
+					.from(BoxTable)
+					.where(and(eq(BoxTable.machineId, input.machineId), isNull(BoxTable.timeDeleted)))
+					.then((rows) => new Set(rows.map((row) => row.id)));
+
+				const seen: string[] = [];
+				const unknown: string[] = [];
+				for (const box of input.boxes) {
+					if (!placed.has(box.boxId)) {
+						unknown.push(box.boxId);
+						continue;
+					}
+					seen.push(box.boxId);
+					const stopped = box.state === 'stopped';
+					await tx
+						.update(BoxTable)
+						.set({
+							state: box.state,
+							stopReason: stopped ? box.reason : null,
+							stopClean: stopped ? box.clean : null
+						})
+						.where(and(eq(BoxTable.id, box.boxId), eq(BoxTable.machineId, input.machineId)));
+				}
+
+				// Everything placed here that the snapshot did not mention, and
+				// that we believed was running.
+				//
+				// **`running` and not "anything not stopped"**, which is narrower
+				// than it first looks it should be. A box is created here, by a
+				// person, before its host has ever been told about it — so between
+				// creation and the job that starts it there is a `created` box the
+				// host correctly does not mention, and stopping it on that basis
+				// would break the ordinary path rather than catch a divergence. A
+				// box we were told was running and that has since vanished from its
+				// host's own inventory is the real disagreement, and it is the one
+				// that leaves a person looking at a box nothing is running.
+				//
+				// `notInArray` on an empty list matches nothing in SQL rather than
+				// everything, so the empty snapshot — a host that has just come up
+				// holding no boxes — is spelt out instead of relying on that.
+				const missing = await tx
+					.update(BoxTable)
+					.set({ state: 'stopped', stopReason: OMITTED_FROM_REPORT, stopClean: false })
+					.where(
+						and(
+							eq(BoxTable.machineId, input.machineId),
+							isNull(BoxTable.timeDeleted),
+							eq(BoxTable.state, 'running'),
+							seen.length > 0 ? notInArray(BoxTable.id, seen) : undefined
+						)
+					)
+					.returning({ id: BoxTable.id })
+					.then((rows) => rows.map((row) => row.id));
+
+				return { recorded: seen.length, unknown, markedStopped: missing };
+			});
+		}
+	);
+
 	export function serialize(input: typeof BoxTable.$inferSelect): z.infer<typeof Info> {
 		return {
 			id: input.id,