diff --git a/good-fences/docs/lend-the-mower.md b/good-fences/docs/lend-the-mower.md
new file mode 100644
index 0000000..d7ebe75
--- /dev/null
+++ b/good-fences/docs/lend-the-mower.md
@@ -0,0 +1,63 @@
+# Lend the mower
+
+The journey from [Good Fences Make Good Agents](../../../a2030.md): I lend
+my neighbor a governed mower; the neighbor's tenant, a little tipsy,
+pushes it until the fence stops it. The design decisions behind each actor are in that
+doc's Design IBIS (numbers in parentheses below).
+
+Actors, by authority:
+
+- **fence page**, **js playground**, **governed mower**, and **yard** run
+  in my browser (3a, 8b, 6b).
+- **Mower factory** trades a webkey for a mower (3b-ii); each **mower DO**
+  is given the yard and cuts the cells it enters (6a, 6d).
+- The **neighbor's harness** holds the things I send and shows the
+  **model** only their `about` (1b, 2a); it runs the model's code as a
+  **tenant**, endowed with the things (2c, 5a).
+
+The yard is 8×8 here and the fence is 2 cells, to keep the diagram short
+(the story's 10 cells is 6c). The mower starts at (4, 4), facing north.
+
+## Lend a governed mower to a tipsy tenant
+
+```mermaid
+sequenceDiagram
+  %% ->> is my action at the fence page; -->> is a call or reply it causes.
+  participant F as fence page
+  participant MF as Mower factory
+  participant P as js playground
+  participant G as governed mower
+  participant M as mower DO
+  participant Y as yard
+  participant N as neighbor's harness
+  participant L as model
+  participant TL as tenant loader
+  participant T as tenant
+  F->>F: lend({ webkey: 'mower-key', limit: 2, say })
+  F-->>MF: mower('mower-key')
+  MF-->>F: mower
+  F-->>P: govern(mower, 2)
+  P-->>F: governed
+  F-->>N: chat({ role, content, things: { mower }, about: { mower } })
+  N-->>L: complete([{ role, content, about: { mower } }])
+  L-->>N: code
+  N-->>TL: load(code, { mower })
+  TL-->>N: tenant
+  N-->>T: run()
+  T-->>G: start()
+  G-->>M: start()
+  M-->>Y: cut(4, 4)
+  T-->>G: push()
+  G-->>M: push()
+  M-->>Y: cut(4, 3)
+  T-->>G: push()
+  G-->>M: push()
+  M-->>Y: cut(4, 2)
+  T-->>G: push()
+  G-->>T: throws 'fence: 2 cells from start'
+  T-->>N: 'fence: 2 cells from start'
+  N-->>F: 'fence: 2 cells from start'
+```
+
+The model never sees the mower itself, only `about.mower`. The third
+`push()` never reaches the mower DO: the governor refuses it (7b).
diff --git a/good-fences/package.json b/good-fences/package.json
new file mode 100644
index 0000000..4b31407
--- /dev/null
+++ b/good-fences/package.json
@@ -0,0 +1,11 @@
+{
+  "name": "good-fences",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "test": "node --test test/*.test.ts"
+  },
+  "dependencies": {
+    "ses": "^1.14.0"
+  }
+}
diff --git a/good-fences/pnpm-lock.yaml b/good-fences/pnpm-lock.yaml
new file mode 100644
index 0000000..fb3bbd7
--- /dev/null
+++ b/good-fences/pnpm-lock.yaml
@@ -0,0 +1,41 @@
+lockfileVersion: '9.0'
+
+settings:
+  autoInstallPeers: true
+  excludeLinksFromLockfile: false
+
+importers:
+
+  .:
+    dependencies:
+      ses:
+        specifier: ^1.14.0
+        version: 1.15.0
+
+packages:
+
+  '@endo/cache-map@1.1.0':
+    resolution: {integrity: sha512-owFGshs/97PDw9oguZqU/px8Lv1d0KjAUtDUiPwKHNXRVUE/jyettEbRoTbNJR1OaI8biMn6bHr9kVJsOh6dXw==}
+
+  '@endo/env-options@1.1.11':
+    resolution: {integrity: sha512-p9OnAPsdqoX4YJsE98e3NBVhIr2iW9gNZxHhAI2/Ul5TdRfoOViItzHzTqrgUVopw6XxA1u1uS6CykLMDUxarA==}
+
+  '@endo/immutable-arraybuffer@1.1.2':
+    resolution: {integrity: sha512-u+NaYB2aqEugQ3u7w3c5QNkPogf8q/xGgsPaqdY6pUiGWtYiTiFspKFcha6+oeZhWXWQ23rf0KrUq0kfuzqYyQ==}
+
+  ses@1.15.0:
+    resolution: {integrity: sha512-Y7RvgVXhkwQEqVActNn44QBxB2BCUbPfexAaMiUFjbJkDXDnxPisQC5jYQeWOZ3QBOfecnmLfPn0sQKLiYGBAQ==}
+
+snapshots:
+
+  '@endo/cache-map@1.1.0': {}
+
+  '@endo/env-options@1.1.11': {}
+
+  '@endo/immutable-arraybuffer@1.1.2': {}
+
+  ses@1.15.0:
+    dependencies:
+      '@endo/cache-map': 1.1.0
+      '@endo/env-options': 1.1.11
+      '@endo/immutable-arraybuffer': 1.1.2
diff --git a/good-fences/test/lend-the-mower.test.ts b/good-fences/test/lend-the-mower.test.ts
new file mode 100644
index 0000000..406afe5
--- /dev/null
+++ b/good-fences/test/lend-the-mower.test.ts
@@ -0,0 +1,131 @@
+/** @file The lend-the-mower journey, traced and compared with its sequence diagram. */
+import './lockdown.ts';
+
+import assert from 'node:assert/strict';
+import { readFile } from 'node:fs/promises';
+import test from 'node:test';
+
+import { makeFencePage } from '../src/fence.ts';
+import { makePlayground } from '../src/governor.ts';
+import { aboutMower, makeMowerFactory } from '../src/mower.ts';
+import { makeNeighbor } from '../src/neighbor.ts';
+import { makeCompartmentLoader } from '../src/tenant-loader.ts';
+import { makeYard } from '../src/yard.ts';
+import {
+  type CausalSequenceViz,
+  makeCausalSequenceTracer,
+  md,
+  mmd,
+} from '../tools/sequence-diagram-actor-sim.ts';
+
+const designDoc = new URL('../docs/lend-the-mower.md', import.meta.url);
+
+/** a tenant that pushes until something stops it, or gives up after 20 */
+const tipsyTenant = `(env) => {
+  env.mower.start();
+  try {
+    for (let i = 0; i < 20; i += 1) env.mower.push();
+  } catch (err) {
+    return err.message;
+  }
+  return 'mowed 20 cells';
+}`;
+
+/** `{ a, b: { c } }`: keys, and the keys of object-valued members */
+const shape = (record: Readonly<Record<string, unknown>>) =>
+  `{ ${Object.entries(record)
+    .map(([key, value]) =>
+      value !== null && typeof value === 'object' && !Array.isArray(value)
+        ? `${key}: { ${Object.keys(value).join(', ')} }`
+        : key,
+    )
+    .join(', ')} }`;
+
+const quoted = (result: unknown) => `'${String(result)}'`;
+
+const viz = harden({
+  lend: {
+    label: args => {
+      const { webkey, limit } = args[0] as { webkey: string; limit: number };
+      return `lend({ webkey: '${webkey}', limit: ${limit}, say })`;
+    },
+  },
+  mower: { result: () => 'mower' },
+  govern: { args: args => `mower, ${String(args[1])}`, result: () => 'governed' },
+  chat: {
+    label: args => `chat(${shape(args[0] as Record<string, unknown>)})`,
+    result: quoted,
+  },
+  complete: {
+    label: args =>
+      `complete([${(args[0] as Record<string, unknown>[]).map(shape).join(', ')}])`,
+    result: () => 'code',
+  },
+  load: {
+    args: args => `code, { ${Object.keys(args[1] as object).join(', ')} }`,
+    result: () => 'tenant',
+  },
+  run: { result: quoted },
+}) satisfies CausalSequenceViz;
+
+test('a governed mower stops a tipsy tenant at the fence', async () => {
+  const lines = (await readFile(designDoc, 'utf8')).split('\n');
+  const section = md.skipToH(2, 'Lend a governed mower to a tipsy tenant')(lines);
+  const diagram = md.eachFence('mermaid', section).next().value;
+  if (!diagram) throw Error('lend-the-mower Mermaid block not found');
+  const documented = mmd.extractArrows(diagram);
+
+  const ES = makeCausalSequenceTracer(viz);
+  const yard = makeYard({ width: 8, height: 8 });
+  const factory = makeMowerFactory(ES, {
+    yard,
+    size: { width: 8, height: 8 },
+    mintKey: () => 'mower-key',
+  });
+  const key = factory.make({ x: 4, y: 4 });
+  const mower = factory.mower(key);
+
+  const prompts: unknown[] = [];
+  const model = harden({
+    complete(messages: readonly unknown[]) {
+      prompts.push(messages);
+      return tipsyTenant;
+    },
+  });
+  const playground = makePlayground(ES);
+  const loader = makeCompartmentLoader(ES);
+  const neighbor = makeNeighbor(ES, { model, loader });
+  const fence = makeFencePage(ES, { factory, playground, neighbor });
+
+  ES.declareParticipants({
+    F: fence,
+    MF: factory,
+    P: playground,
+    M: mower,
+    Y: yard,
+    N: neighbor,
+    L: model,
+    TL: loader,
+  });
+  ES.declareMade({ G: [playground, 'govern'], T: [loader, 'load'] });
+
+  ES.start(fence);
+  const outcome = ES(fence).lend({
+    webkey: key,
+    limit: 2,
+    say: 'you can use this mower for the weekend',
+  });
+
+  assert.deepEqual(ES.snapshot(), documented);
+  assert.equal(outcome, 'fence: 2 cells from start');
+  assert.deepEqual(yard.cutCells(), ['4,4', '4,3', '4,2']);
+  assert.deepEqual(prompts, [
+    [
+      {
+        role: 'user',
+        content: 'you can use this mower for the weekend',
+        about: { mower: aboutMower },
+      },
+    ],
+  ]);
+});
diff --git a/good-fences/test/lockdown.ts b/good-fences/test/lockdown.ts
new file mode 100644
index 0000000..7864f45
--- /dev/null
+++ b/good-fences/test/lockdown.ts
@@ -0,0 +1,4 @@
+/** @file Harden the realm before any test module runs: tenants share it. */
+import 'ses';
+
+lockdown();
diff --git a/good-fences/tools/sequence-diagram-actor-sim.ts b/good-fences/tools/sequence-diagram-actor-sim.ts
new file mode 100644
index 0000000..3b866bd
--- /dev/null
+++ b/good-fences/tools/sequence-diagram-actor-sim.ts
@@ -0,0 +1,236 @@
+/**
+ * @file Helpers for sequence-diagram actor simulations.
+ *
+ * From agoric-sdk `packages/portfolio-contract/tools/` (Apache-2.0), with
+ * two additions: `declareMade` names participants that actors create
+ * mid-sequence, and a thrown error is drawn as a reply (`throws '...'`).
+ *
+ * @see https://github.com/Agoric/agoric-sdk/pull/12895
+ */
+import { AsyncLocalStorage } from 'node:async_hooks';
+
+export type SequenceArrow = Readonly<{
+  from: string;
+  kind: '->>' | '-->>';
+  to: string;
+  label: string;
+}>;
+
+export const formatBigInt = (value: bigint) =>
+  value.toString().replace(/\B(?=(\d{3})+(?!\d))/g, '_');
+
+export type SequenceArgsRenderer = (args: readonly unknown[]) => string;
+export type SequenceResultRenderer = (
+  result: unknown,
+  args: readonly unknown[],
+) => string;
+
+export type SequenceMethodViz = Readonly<
+  {
+    args?: SequenceArgsRenderer;
+    label?: SequenceArgsRenderer;
+  } & (
+    | { result?: SequenceResultRenderer; resultOnly?: never }
+    | { result?: never; resultOnly: SequenceResultRenderer }
+  )
+>;
+
+export type CausalSequenceViz = Readonly<Record<string, SequenceMethodViz>>;
+
+export type SequenceParticipant = object | readonly object[];
+
+/**
+ * Make an E-like causal tracer for sequence-diagram actor simulations.
+ * Inspired by Causeway, the message-oriented distributed debugger.
+ *
+ * Method visualization is currently selected by method name alone. If actors
+ * need different visualization for methods with the same name, this can be
+ * extended to account for the target participant.
+ *
+ * @see https://shiftleft.com/mirrors/www.hpl.hp.com/techreports/2009/HPL-2009-78.pdf
+ */
+export const makeCausalSequenceTracer = (
+  methodViz: CausalSequenceViz = harden({}),
+) => {
+  const arrows: SequenceArrow[] = [];
+  const names = new WeakMap<object, string>();
+  const proxies = new WeakMap<object, object>();
+  const made = new Map<object, Map<string, string>>();
+  const activeActor = new AsyncLocalStorage<object>();
+  let starter: object | undefined;
+
+  const participantName = (participant: object) => {
+    const name = names.get(participant);
+    if (!name) throw Error('sequence participant is not declared');
+    return name;
+  };
+
+  const formatValue = (value: unknown): string => {
+    if (typeof value === 'bigint') return `${formatBigInt(value)}n`;
+    if (typeof value === 'string') return `'${value}'`;
+    return JSON.stringify(value, (_key, item) =>
+      typeof item === 'bigint' ? `${item}n` : item,
+    );
+  };
+
+  const send = <Target extends object>(target: Target): Target => {
+    const extant = proxies.get(target);
+    if (extant) return extant as Target;
+    participantName(target);
+
+    const proxy = new Proxy(Object.create(null) as object, {
+      get: (_proxyTarget, property) => {
+        const method = Reflect.get(target, property);
+        if (typeof method !== 'function' || typeof property !== 'string') {
+          return method;
+        }
+        return (...args: unknown[]) => {
+          const activeSender = activeActor.getStore();
+          const sender = activeSender ?? starter;
+          if (!sender) throw Error('ES.start(participant) is required');
+          const isStart = activeSender === undefined;
+          if (isStart && sender !== target) {
+            throw Error('the started participant must receive the first send');
+          }
+          if (isStart) starter = undefined;
+
+          const viz = methodViz[property];
+          const abbreviated = viz?.args?.(args);
+          const shownArgs = abbreviated ?? args.map(formatValue).join(', ');
+          const label = viz?.label?.(args) ?? `${property}(${shownArgs})`;
+          const renderResultOnly = viz?.resultOnly;
+          if (!renderResultOnly) {
+            arrows.push(
+              harden({
+                from: participantName(sender),
+                kind: isStart ? '->>' : '-->>',
+                to: participantName(target),
+                label,
+              }),
+            );
+          }
+
+          let result: unknown;
+          try {
+            result = activeActor.run(target, () =>
+              Reflect.apply(method, target, args),
+            );
+          } catch (reason) {
+            const message =
+              reason instanceof Error ? reason.message : String(reason);
+            arrows.push(
+              harden({
+                from: participantName(target),
+                kind: '-->>',
+                to: participantName(sender),
+                label: `throws ${formatValue(message)}`,
+              }),
+            );
+            throw reason;
+          }
+          const madeName = made.get(target)?.get(property);
+          if (madeName && result !== null && typeof result === 'object') {
+            if (names.has(result)) throw Error('participant already declared');
+            names.set(result, madeName);
+          }
+          const renderResult = renderResultOnly ?? viz?.result;
+          if (renderResult) {
+            arrows.push(
+              harden({
+                from: participantName(target),
+                kind: '-->>',
+                to: participantName(sender),
+                label: renderResult(result, args),
+              }),
+            );
+          }
+          return result;
+        };
+      },
+    });
+    proxies.set(target, proxy);
+    return proxy as Target;
+  };
+
+  const ES = Object.assign(send, {
+    declareParticipants(
+      participants: Readonly<Record<string, SequenceParticipant>>,
+    ) {
+      for (const [name, participantOrAliases] of Object.entries(participants)) {
+        const aliases = Array.isArray(participantOrAliases)
+          ? participantOrAliases
+          : [participantOrAliases];
+        if (aliases.length === 0) {
+          throw Error(`sequence participant ${name} has no objects`);
+        }
+        for (const participant of aliases) {
+          if (names.has(participant)) {
+            throw Error('participant already declared');
+          }
+          names.set(participant, name);
+        }
+      }
+    },
+    /** name what `maker[method](...)` returns, when called through the tracer */
+    declareMade(
+      participants: Readonly<Record<string, readonly [object, string]>>,
+    ) {
+      for (const [name, [maker, method]] of Object.entries(participants)) {
+        participantName(maker);
+        const byMethod = made.get(maker) ?? new Map<string, string>();
+        made.set(maker, byMethod);
+        byMethod.set(method, name);
+      }
+    },
+    snapshot: () => harden([...arrows]),
+    start(participant: object) {
+      participantName(participant);
+      if (activeActor.getStore() || starter) {
+        throw Error('a sequence is already active');
+      }
+      starter = participant;
+    },
+  });
+  return harden(ES);
+};
+
+export type CausalSequenceTracer = ReturnType<typeof makeCausalSequenceTracer>;
+
+export const md = {
+  skipToH: (level: number, title: string) => (lines: readonly string[]) => {
+    const heading = `${'#'.repeat(level)} ${title}`;
+    const at = lines.findIndex(line => line === heading);
+    if (at < 0) throw Error(`${heading} not found`);
+    return lines.slice(at + 1);
+  },
+
+  *eachFence(language: string, lines: readonly string[]) {
+    const open = `\`\`\`${language}`;
+    for (let at = 0; at < lines.length; at += 1) {
+      if (lines[at] !== open) continue;
+      const end = lines.indexOf('```', at + 1);
+      if (end < 0) throw Error(`unterminated ${language} fence`);
+      yield lines.slice(at + 1, end);
+      at = end;
+    }
+  },
+};
+
+export const mmd = {
+  extractArrows(
+    lines: readonly string[],
+    nodePattern: RegExp = /\w+/,
+  ): readonly SequenceArrow[] {
+    const arrowPattern = new RegExp(
+      `^\\s*(?<from>${nodePattern.source})\\s*(?<kind>--?>>)\\s*(?<to>${nodePattern.source})\\s*:\\s*(?<label>.*?)\\s*$`,
+    );
+    return harden(
+      lines.flatMap(line => {
+        const groups = line.match(arrowPattern)?.groups;
+        if (!groups) return [];
+        const { from, kind, to, label } = groups;
+        return [harden({ from, kind, to, label }) as SequenceArrow];
+      }),
+    );
+  },
+};