diff --git a/README.md b/README.md
index c93b0e9..2006c79 100644
--- a/README.md
+++ b/README.md
@@ -47,6 +47,12 @@ Code](https://code.claude.com/docs/en/skills) and
   TADA, then how, then the challenges, tagged for an emoji retro.
 - [ibis-design](ibis-design/SKILL.md) — search a design space:
   issues, positions, and arguments.
+- [wiring-diagram](wiring-diagram/SKILL.md) — who holds what, drawn
+  like MarkM's ERTP slides: every arrow cites the code, and Mallory
+  shows what a leak doesn't reach.
+  TODO: consider copying or adapting its companion for message order,
+  sequence-diagram-actor-sim (agoric-sdk `.agents/skills/` on branch
+  `connolly/ago-1299-agentic-planning-tdd`).
 - [obsidian-markdown](obsidian-markdown/SKILL.md) — the workspace is
   an Obsidian vault: one line per paragraph, relative links, no file
   symlinks you expect Obsidian to show.
diff --git a/wiring-diagram/SKILL.md b/wiring-diagram/SKILL.md
new file mode 100644
index 0000000..bcd13ee
--- /dev/null
+++ b/wiring-diagram/SKILL.md
@@ -0,0 +1,113 @@
+---
+name: wiring-diagram
+description: >-
+  Capability wiring diagram, "who holds what" — use when asked for a wiring diagram, a picture of who holds what, a capability diagram, or "a diagram like the cookie-rotation one"; when reviewing a design for POLA and wanting to see the reference graph. MarkM ERTP-slide style: actors are balls, makers pale regions, objects rounded squares, state ledger tables, references dot-to-arrowhead; HTML/SVG plus a headless-Chromium PNG. Not for message order over time: that's a sequence diagram (sequence-diagram-actor-sim in agoric-sdk).
+---
+
+# Capability wiring diagram
+
+One picture that answers, for a design: **who holds a reference to what,
+and therefore who can do what.** It complements a sequence diagram,
+which shows *when* messages flow. This one shows the standing graph of
+authority that makes those messages possible. For sequences, see
+`~/projects/ymax-workspace/agoric-sdk/.agents/skills/sequence-diagram-actor-sim/SKILL.md`
+(on agoric-sdk branches `connolly/ago-1264-agentic-planning` and
+`connolly/ago-1299-agentic-planning-tdd`, not yet on master).
+
+Exemplars:
+- `~/oros-bots/capsite-cookie-rotation/docs/cookie-rotation-wiring.html`
+  (+ `.png`): the original.
+- `~/qtrx/packages/umb-download/docs/wiring.html` (+ `.png`): three
+  makers, two outside parties, and an adversary holding a webkey.
+
+Start from [`template.html`](template.html), next to this file. It has
+the drawing helpers and a small example scene to replace.
+
+## Read the graph from the code, not the story
+
+Every arrow must correspond to something real: a constructor argument, a
+stored id, a binding, a held token, or a file someone keeps. Before
+drawing, list the references as `holder → target: where in the code`.
+Read the factories (`make…`, `newX`, `configure`) and anything that
+stores an id: a DO id held as a string *is* a reference. If you can't
+cite where a reference comes from, it doesn't get an arrow.
+
+Watch for references that are dropped on purpose (e.g. a setup route
+that mints an instance, hands its id to one holder and forgets it).
+That's often the point of the design. Say so in a caption, or just
+leave the arrow out.
+
+## Vocabulary
+
+| element | helper | means |
+|---|---|---|
+| blue ball | `actor(cx, cy, label)` | a person or a program with its own agency |
+| grey ball | `actor(cx, cy, label, "url(#ext)")` | outside the system: a browser extension, a third-party service |
+| pale region | `maker(x, y, w, h, "makeFoo")` | a maker (factory, DO class) and the instances it made; the stacked inner panes say "there are many" |
+| rounded square | `obj(x, y, w, h, label, stacked)` | an object or facet; `stacked` when one is made per instance |
+| ledger table | `table(x, y, title, rows, [colW0, colW1])` | an object's state; rows are key/value; `rowY(i)` and `colX` give arrow anchors |
+| dot → arrowhead | `ref(x1, y1, x2, y2, bend)` | holder references target; `bend` (±0.05–0.3) curves around things |
+| speech bubble | the `path` + `code` block | the one call the picture is about, in `E(target).method(args)` form |
+| caption | `cap(x, y, text, attrs)` | what an actor holds, and what it can't reach |
+
+Facets are separate objects (`store` vs `readOnly`): drawing them apart
+is how the picture shows attenuation.
+
+## Include an adversary
+
+Add a Mallory who holds whatever is most likely to leak (an instance
+id, a webkey, a URL), with an arrow to exactly what it reaches. The
+caption says what she *can't* reach. Absent arrows are the security
+argument, and Mallory makes them visible.
+
+## Layout
+
+- Actors in a row along the top (y≈200), makers below (y≈320 onward),
+  captions along the bottom (y≈ height − 14).
+- Put each maker under the actors that mostly use it, so arrows run
+  short and downward.
+- Speech bubble at the top, its tail pointing at the speaker. Move the
+  bubble's path (`M x,y`) and the tail segment (`h… l-38,48 l6,-48`)
+  together.
+- Captions: two or three short lines, never on top of an arrow or a
+  ball. Use `"text-anchor": "start"` to set one beside a ball.
+- Long arrows that cross a maker: increase `|bend|` until the curve
+  clears tables and objects. The control point is the midpoint offset
+  by `bend × (−dy, dx)`.
+
+## Accessibility
+
+Keep `<title>` short. Write `<desc>` as prose that names every actor and
+maker, every reference, the bubble's call, and what the adversary
+can't reach. Someone who can't see the picture should get the same
+argument from it.
+
+## Render and check
+
+The page is self-contained (no network). Render a PNG with headless
+Chromium. The viewport comes out shorter than `--window-size`, so render
+taller and crop to the SVG size:
+
+```sh
+chromium --headless=new --disable-gpu --no-sandbox --hide-scrollbars \
+  --window-size=W,$((H+100)) --screenshot=full.png "file://$PWD/NAME.html"
+python3 -c "from PIL import Image; Image.open('full.png').crop((0,0,W,H)).save('NAME.png')" && rm full.png
+```
+
+Then **look at the PNG** (Read it) and fix overlaps: captions on arrows,
+arrows through tables, text clipped at the bottom. Expect two or three
+rounds. Commit the HTML (source) and the PNG separately, so the source
+diff stays readable.
+
+## Review checklist
+
+- Does every arrow cite something in the code? Is any real reference
+  missing?
+- Does each actor hold only what its job needs (POLA)? If the picture
+  shows an actor holding more, that's a finding about the design, not a
+  drawing problem.
+- Are facets drawn apart where they differ in authority?
+- Does the adversary's caption match what the arrows show?
+- Does `<desc>` tell the same story in prose?
+- Is any secret value drawn? Elide values (`session=…`, `{…}`): the
+  picture shows *that* a row exists, not its contents.
diff --git a/wiring-diagram/template.html b/wiring-diagram/template.html
new file mode 100644
index 0000000..bb8c73d
--- /dev/null
+++ b/wiring-diagram/template.html
@@ -0,0 +1,138 @@
+<!doctype html>
+<html><head><meta charset="utf-8">
+<title>TITLE: who holds what</title>
+<!-- drawn after the style of MarkM's ERTP slides; NAME.png is a headless-Chromium screenshot of this page -->
+<style>
+  html, body { margin: 0; background: #fff; }
+  .sans { font-family: "Helvetica Neue", "DejaVu Sans", sans-serif; }
+  .obj { font-family: "DejaVu Serif", serif; font-style: italic; }
+  .mono { font-family: "DejaVu Sans Mono", monospace; }
+</style></head>
+<body>
+<svg id="c" xmlns="http://www.w3.org/2000/svg" width="1120" height="820" viewBox="0 0 1120 820"
+  role="img" aria-labelledby="title desc">
+  <title id="title">TITLE: who holds what</title>
+  <desc id="desc">Prose for screen readers: name every actor and maker, every
+reference (who holds what), the speech bubble's call, and what the
+adversary can and cannot reach.</desc>
+  <defs>
+    <radialGradient id="ball" cx="40%" cy="35%" r="70%">
+      <stop offset="0%" stop-color="#7cc8f8"/>
+      <stop offset="100%" stop-color="#3aa6ee"/>
+    </radialGradient>
+    <radialGradient id="ext" cx="40%" cy="35%" r="70%">
+      <stop offset="0%" stop-color="#e3e3e3"/>
+      <stop offset="100%" stop-color="#b9b9b9"/>
+    </radialGradient>
+    <linearGradient id="objfill" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0%" stop-color="#6cbcf4"/>
+      <stop offset="100%" stop-color="#4aa8ec"/>
+    </linearGradient>
+    <marker id="head" viewBox="0 0 10 10" refX="9" refY="5" markerWidth="9" markerHeight="9" orient="auto-start-reverse">
+      <path d="M0,0 L10,5 L0,10 z" fill="#1d4f7a"/>
+    </marker>
+  </defs>
+</svg>
+<script>
+const svg = document.getElementById("c");
+const NS = "http://www.w3.org/2000/svg";
+const el = (name, attrs, parent = svg) => {
+  const e = document.createElementNS(NS, name);
+  for (const [k, v] of Object.entries(attrs)) e.setAttribute(k, v);
+  parent.appendChild(e);
+  return e;
+};
+const text = (x, y, s, cls, attrs = {}) => {
+  const t = el("text", { x, y, class: cls, "text-anchor": "middle", fill: "#1d3550", ...attrs });
+  t.textContent = s;
+  return t;
+};
+
+const INK = "#1d4f7a";
+const dot = (x, y) => el("circle", { cx: x, cy: y, r: 4.5, fill: INK });
+/** a reference: dot at the holder, arrowhead at the target */
+const ref = (x1, y1, x2, y2, bend = 0, extra = {}) => {
+  dot(x1, y1);
+  const mx = (x1 + x2) / 2 - (y2 - y1) * bend, my = (y1 + y2) / 2 + (x2 - x1) * bend;
+  el("path", { d: `M${x1},${y1} Q${mx},${my} ${x2},${y2}`, fill: "none", stroke: INK, "stroke-width": 2, "marker-end": "url(#head)", ...extra });
+};
+
+// maker box: a pale rounded region, with stacked instances inside
+function maker(x, y, w, h, label) {
+  el("rect", { x, y, width: w, height: h, rx: 26, fill: "#d6ecfb", stroke: "#b5dbf5", "stroke-width": 1.5 });
+  text(x + 18, y + 30, label, "obj", { "text-anchor": "start", "font-size": 20 });
+  for (let i = 2; i >= 0; i -= 1) {
+    el("rect", {
+      x: x + 18 + i * 9, y: y + 48 + i * 9, width: w - 52, height: h - 82, rx: 22,
+      fill: "#c8e5fa", stroke: "#8ec9f2", "stroke-width": 1.5, "fill-opacity": 0.9,
+    });
+  }
+}
+// an object: rounded square, optionally stacked (several made)
+function obj(x, y, w, h, label, stacked = false) {
+  const n = stacked ? 3 : 1;
+  for (let i = n - 1; i >= 0; i -= 1) {
+    el("rect", { x: x + i * 7, y: y - i * 7, width: w, height: h, rx: 12, fill: "url(#objfill)", stroke: "#2f8fd6", "stroke-width": 1.5 });
+  }
+  text(x + w / 2, y + h / 2 + 7, label, "obj", { "font-size": 19 });
+}
+function actor(cx, cy, label, fill = "url(#ball)") {
+  el("circle", { cx, cy, r: 52, fill, stroke: "#2f8fd6", "stroke-width": 1.5 });
+  text(cx, cy + 6, label, "sans", { "font-size": label.length > 8 ? 15 : 17 });
+}
+// a table like the ERTP ledger
+function table(x, y, title, rows, colW) {
+  const rowH = 28, w = colW[0] + colW[1];
+  el("rect", { x, y, width: w, height: 36 + rowH * rows.length, fill: "#5fb5f2", stroke: "#2f8fd6", "stroke-width": 1.5 });
+  text(x + w / 2, y + 25, title, "obj", { "font-size": 18 });
+  rows.forEach(([k, v], i) => {
+    const ry = y + 36 + i * rowH;
+    el("line", { x1: x, y1: ry, x2: x + w, y2: ry, stroke: "#2f8fd6", "stroke-width": 1.2 });
+    text(x + 10, ry + 19, k, "mono", { "text-anchor": "start", "font-size": 14 });
+    if (v) text(x + colW[0] + 10, ry + 19, v, "mono", { "text-anchor": "start", "font-size": 14 });
+  });
+  el("line", { x1: x + colW[0], y1: y + 36, x2: x + colW[0], y2: y + 36 + rowH * rows.length, stroke: "#1d4f7a", "stroke-width": 1.5 });
+  return { rowY: (i) => y + 36 + i * rowH + rowH / 2, x, w, colX: x + colW[0] };
+}
+
+// ---- scene: replace from here down ----
+
+// speech bubble: the one invocation the picture is about; tail toward the speaker
+el("path", {
+  d: "M30,22 h470 a16,16 0 0 1 16,16 v34 a16,16 0 0 1 -16,16 h-330 l-38,48 l6,-48 h-102 a16,16 0 0 1 -16,-16 v-34 a16,16 0 0 1 16,-16 z",
+  fill: "#d9d9d9", stroke: "#555", "stroke-width": 2,
+});
+const code = el("text", { x: 46, y: 62, class: "mono", "font-size": 17, fill: "#222" });
+code.innerHTML = '<tspan fill="#b0177b">await</tspan> E(store).set(<tspan fill="#1a7f37">"cookie"</tspan>, freshCookie);';
+
+// actors: people, programs, outside services (grey: "url(#ext)")
+actor(150, 200, "Alice");
+actor(560, 200, "client");
+actor(930, 200, "Mallory");
+
+// makers, with what they made inside
+maker(30, 320, 470, 470, "makeMapStore");
+maker(560, 320, 530, 470, "makeWebApp");
+obj(80, 420, 120, 80, "store");
+obj(80, 600, 120, 80, "readOnly", true);
+const kv = table(260, 430, "kv", [["cookie", "session=…"], ["", ""], ["", ""]], [90, 110]);
+obj(620, 560, 140, 90, "access", true);
+const init = table(830, 420, "init", [["url", "…"], ["headerStore", ""]], [130, 85]);
+
+// references: holder first; one comment per arrow, in the form "holder → target"
+ref(140, 228, 125, 418, 0.05); // Alice → store
+ref(172, 224, 508, 210, 0.04); // Alice → client (she wired it)
+ref(554, 218, 660, 553, 0.12); // client → access
+ref(916, 220, 720, 553, -0.08); // Mallory → access
+ref(160, 470, kv.x - 2, kv.rowY(0), 0.05); // store → kv
+ref(150, 666, kv.x - 2, kv.rowY(0) + 6, -0.25); // readOnly → kv
+ref(760, 600, init.x - 2, init.rowY(1), -0.1); // access → init
+ref(init.colX + 40, init.rowY(1), 208, 640, -0.28); // init.headerStore → readOnly
+
+// captions: what each actor holds, what the adversary can't reach
+const cap = (x, y, s, attrs = {}) => text(x, y, s, "sans", { "font-size": 14, fill: "#33536f", ...attrs });
+cap(930, 118, "holds the instance id:");
+cap(930, 136, "uses the cookie, can't reach store");
+cap(265, 806, "shapes: keyShape \"cookie\", valueShape M.string()");
+</script>
+</body></html>