src/broker-nats.ts

@@ -46,6 +46,24 @@ export interface Credential {
   claims?: { seat: string; scope?: string; serves?: string[] }[];
 }
 
+/**
+ * What the mesh issued this assignment (novox/hq ADR 0160): where its tools are served, in which
+ * queue, the verbs of the seats it holds, where its events land, what it may reach. Read from the
+ * ASSIGNMENTS stream at `mesh.assignment.<node>.<module>` — the one subject a runtime derives for
+ * itself — and followed live. Absent for a mesh older than the membership, and then the runtime
+ * serves the shape it always derived, and says so.
+ */
+export interface Membership {
+  node: string;
+  module: string;
+  /** Addresses a tool is answered on; `{tool}` stands for the tool's name. */
+  serves: { subject: string; queue?: string }[];
+  seats?: { seat: string; verb: string; subject: string }[];
+  emits: string;
+  reaches?: Record<string, string[]>;
+  tools: string;
+}
+
 /** What a tool call answers: the module's own result, and which machine answered it
  *  (novox/hq ADR 0159) — a module on several machines is otherwise an answer from nowhere. */
 export interface Answered<Res> {
@@ -57,8 +75,20 @@ export interface Answered<Res> {
  *  an answer that says which machine gave it, and serving a subject that is not a module's own tool
  *  (a seat's verb). */
 export interface RuntimeBroker extends Broker {
-  ask<Req, Res>(key: string, body: Req): Promise<Answered<Res>>;
+  /** Call a tool by key, or — when `on` names a subject the mesh listed for it (ADR 0160) — there. */
+  ask<Req, Res>(key: string, body: Req, on?: string): Promise<Answered<Res>>;
   handleSubject<Req, Res>(subject: string, handler: (body: Req) => Promise<Res>): Promise<() => void>;
+  /** What the mesh issued this assignment, or undefined when nothing has been issued yet. */
+  membership(): Membership | undefined;
+  /** Called when the mesh issues a new membership; the runtime re-serves on it. */
+  onMembership(handler: (m: Membership) => void): void;
+}
+
+const ASSIGNMENTS_STREAM = "ASSIGNMENTS";
+
+/** The one address a runtime derives for itself (ADR 0160). */
+export function membershipSubject(node: string, module: string): string {
+  return `mesh.assignment.${node}.${module}`;
 }
 
 /** Whether a connection failure is worth retrying, or is a fact about this configuration that
@@ -123,6 +153,80 @@ export async function connectNats(
 
   const node = cred.node;
 
+  // The membership, read once at connect and followed. A direct get is one request on the
+  // stream's API, which is the whole of what this account may ask JetStream for its own subject;
+  // a 404 is a mesh that has not issued one, which is a fact to say and not an error to retry.
+  let issued: Membership | undefined;
+  const issuedHandlers: ((m: Membership) => void)[] = [];
+  const subjectOfMine = node ? membershipSubject(node, self) : "";
+  if (subjectOfMine) {
+    try {
+      const got = await conn.request(`$JS.API.DIRECT.GET.${ASSIGNMENTS_STREAM}`,
+        sc.encode(JSON.stringify({ last_by_subj: subjectOfMine })), { timeout: 5_000 });
+      const status = got.headers?.code ?? 0;
+      if (status === 0 && got.data.length > 0) {
+        issued = JSON.parse(sc.decode(got.data)) as Membership;
+      }
+    } catch {
+      // Not readable here: an older mesh, a stream not yet asserted, or no grant. Said below.
+    }
+    if (!issued) {
+      console.log(`[mesh-tools] no membership issued for ${self} on ${node} yet; serving the derived shape until one arrives`);
+    }
+    try {
+      const live = conn.subscribe(subjectOfMine);
+      subs.push(live);
+      void (async () => {
+        for await (const msg of live) {
+          try {
+            issued = JSON.parse(sc.decode(msg.data)) as Membership;
+            console.log(`[mesh-tools] ${self} on ${node} was issued a new membership; re-serving on it`);
+            for (const h of issuedHandlers) h(issued);
+          } catch (err) {
+            console.log(`[mesh-tools] a membership arrived that is not one: ${err}`);
+          }
+        }
+      })();
+    } catch {
+      // A subscription this account may not make is a mesh older than the membership.
+    }
+  }
+
+  /** The subjects a tool of this module is served on: from the membership when issued, derived
+   *  otherwise (the shape the mesh issues on day one, so the two agree). */
+  const servedOn = (tool: string): { subject: string; queue?: string }[] => {
+    if (issued) {
+      const m = issued;
+      const out = m.serves.map((s) => ({ subject: s.subject.replace("{tool}", tool), queue: s.queue }));
+      // The verb that lists what this module serves is answered on the mesh's plain address for it
+      // whatever the placement — one answer suffices, so a queue — and on this machine's beside it.
+      if (tool === "tools" && m.tools && !out.some((s) => s.subject === m.tools)) {
+        out.unshift({ subject: m.tools, queue: `serve.${self}` });
+      }
+      return out;
+    }
+    const base = `mesh.mod.${self}.tool.${tool}`;
+    const out: { subject: string; queue?: string }[] = [{ subject: base, queue: `serve.${self}` }];
+    if (node) out.push({ subject: `${base}.${node}` });
+    return out;
+  };
+
+  /** Where a call by key goes: a subject the membership says this module reaches, when it says
+   *  one — the machine's when named — else the derived shape. */
+  const reachedAt = (key: string): string => {
+    const [name, wanted] = key.split("@", 2);
+    const reach = issued?.reaches?.[name];
+    if (reach && reach.length > 0) {
+      if (wanted) {
+        const at = reach.find((s) => s.endsWith(`.${wanted}`));
+        if (at) return at;
+      } else {
+        return reach[0];
+      }
+    }
+    return toolSubject(key, self);
+  };
+
   /** Answer one subject with one handler, and say which machine answered (novox/hq ADR 0159). */
   const answerOn = <Req, Res>(
     subject: string,
@@ -156,8 +260,8 @@ export async function connectNats(
    * request carries, which the responder may answer because its account has `allow_responses`
    * — one reply to a message it actually received, and nothing wider.
    */
-  const ask = async <Req, Res>(key: string, body: Req): Promise<Answered<Res>> => {
+  const ask = async <Req, Res>(key: string, body: Req, on?: string): Promise<Answered<Res>> => {
-    const msg = await conn.request(toolSubject(key, self), sc.encode(JSON.stringify(body)), {
+    const msg = await conn.request(on ?? reachedAt(key), sc.encode(JSON.stringify(body)), {
       timeout: REQUEST_TIMEOUT_MS,
     });
     const reply = JSON.parse(sc.decode(msg.data)) as { result?: Res; error?: string; node?: string };
@@ -180,11 +284,32 @@ export async function connectNats(
      * which is how it always behaved.
      */
     async handle<Req, Res>(key: string, handler: (body: Req) => Promise<Res>): Promise<() => void> {
-      const stops = [answerOn(toolSubject(key, self), `serve.${self}`, handler)];
+      // A seat's verb named outright is served on the seat's subject as given, for a holder that
-      if (node) stops.push(answerOn(`${toolSubject(key, self)}.${node}`, undefined, handler));
+      // knows its role without a membership; everything else is this module's own tool, served
+      // where the mesh issued it (ADR 0160). A key naming another module is not served here at all.
+      if (key.startsWith("seat:")) {
+        const stop = answerOn(toolSubject(key, self), undefined, handler);
+        return () => stop();
+      }
+      const dot = key.indexOf(".");
+      if (dot >= 0 && key.slice(0, dot) !== self) {
+        throw new Error(`${self} cannot serve ${key}: a module serves its own tools`);
+      }
+      const tool = dot < 0 ? key : key.slice(dot + 1);
+      let stops = servedOn(tool).map((s) => answerOn(s.subject, s.queue, handler));
+      // When a new membership arrives, serve where it now says and stop serving where it no longer does.
+      issuedHandlers.push(() => {
+        stops.forEach((stop) => stop());
+        stops = servedOn(tool).map((s) => answerOn(s.subject, s.queue, handler));
+      });
       return () => stops.forEach((stop) => stop());
     },
 
+    membership: () => issued,
+    onMembership: (handler: (m: Membership) => void) => {
+      issuedHandlers.push(handler);
+    },
+
     async handleSubject<Req, Res>(subject: string, handler: (body: Req) => Promise<Res>): Promise<() => void> {
       return answerOn(subject, undefined, handler);
     },

src/client.ts

@@ -40,6 +40,9 @@ export interface Tool {
   input?: unknown;
   /** True for a role's tool: addressed to the seat, answered by whoever holds it (ADR 0132). */
   seat?: boolean;
+  /** Where the tool is answered, as the mesh issued it (ADR 0160): the plain subject first when the
+   *  module answers for itself anywhere, then one per machine. Absent for a runtime older than this. */
+  subjects?: string[];
 }
 
 /**
@@ -188,7 +191,7 @@ export async function toolsOn(bus: Broker): Promise<Listing> {
     const module = names[i]!;
     if (outcome.status === "fulfilled" && Array.isArray(outcome.value?.tools)) {
       for (const t of outcome.value.tools) {
-        tools.push({ module, name: t.name, description: t.description, input: t.input });
+        tools.push({ module, name: t.name, description: t.description, input: t.input, subjects: t.subjects });
       }
     } else {
       notAnswering.push(module);
@@ -204,7 +207,13 @@ export async function toolsOn(bus: Broker): Promise<Listing> {
 /** Call a tool and learn which machine answered (novox/hq ADR 0159). `<module>.<tool>@<node>` asks
  *  the instance on one machine; without it, whichever instance answers first does, and the answer
  *  says which. */
-export async function callTool(bus: Broker, key: string, args: unknown, seats?: Seats): Promise<Answered<unknown>> {
+export async function callTool(
+  bus: Broker,
+  key: string,
+  args: unknown,
+  seats?: Seats,
+  listing?: Listing,
+): Promise<Answered<unknown>> {
   const at = key.indexOf("@");
   const name = at < 0 ? key : key.slice(0, at);
   const node = at < 0 ? "" : key.slice(at + 1);
@@ -215,11 +224,28 @@ export async function callTool(bus: Broker, key: string, args: unknown, seats?:
     );
   }
   const resolved = toolKey(name, seats) + (node ? `@${node}` : "");
-  const asking = bus as Broker & { ask?: <Req, Res>(k: string, b: Req) => Promise<Answered<Res>> };
+  // Where the tool is answered is the module's to say and the mesh's to issue (ADR 0160): when the
-  if (typeof asking.ask === "function") return asking.ask<unknown, unknown>(resolved, args ?? {});
+  // listing carried subjects for it, the call goes to one of those and composes nothing.
+  const on = subjectListed(name, node, listing);
+  const asking = bus as Broker & { ask?: <Req, Res>(k: string, b: Req, on?: string) => Promise<Answered<Res>> };
+  if (typeof asking.ask === "function") return asking.ask<unknown, unknown>(resolved, args ?? {}, on);
   return { result: await bus.request<unknown, unknown>(resolved, args ?? {}) };
 }
 
+/** The subject the listing says answers `<module>.<tool>` — the machine's when one is named, else
+ *  the plain one — or undefined when the listing said none, and the key is composed as before. */
+export function subjectListed(name: string, node: string, listing?: Listing): string | undefined {
+  if (!listing) return undefined;
+  const dot = name.indexOf(".");
+  const module = name.slice(0, dot);
+  const tool = name.slice(dot + 1);
+  const found = listing.tools.find((t) => t.module === module && t.name === tool && !t.seat);
+  const subjects = found?.subjects ?? [];
+  if (subjects.length === 0) return undefined;
+  if (node) return subjects.find((s) => s.endsWith(`.${node}`));
+  return subjects[0];
+}
+
 /**
  * Why a call failed, said so that the remedy is in the words.
  *

src/mcp.ts

@@ -139,7 +139,8 @@ export function mcpSurface(bus: Broker, who: string): Surface {
           delete args.node;
           const name = node && !given.includes("@") ? `${given}@${node}` : given;
           try {
-            const { result, node: answeredBy } = await callTool(bus, name, args, await roles());
+            const have = await listing().catch(() => undefined);
+            const { result, node: answeredBy } = await callTool(bus, name, args, have && seatsIn(have), have);
             // Text, because that is what every host renders. The content is the module's answer
             // as JSON, unshaped: an adapter that flattened it would be deciding what matters in
             // somebody else's answer. Which machine answered follows it as its own line.

src/mesh.ts

@@ -173,8 +173,8 @@ async function calling(bus: Broker, args: string[]): Promise<number> {
   try {
     // `<seat>.<verb>` reaches the role when the mesh lists that verb for the seat; `seat:` says so
     // outright and asks nothing first.
-    const roles = key.startsWith("seat:") ? undefined : seatsIn(await toolsOn(bus).catch(() => ({ tools: [], notAnswering: [] })));
+    const have = key.startsWith("seat:") ? undefined : await toolsOn(bus).catch(() => undefined);
-    const { result, node } = await callTool(bus, key, parsed, roles);
+    const { result, node } = await callTool(bus, key, parsed, have && seatsIn(have), have);
     console.log(JSON.stringify(result, null, 2));
     if (node) console.error(`answered by ${node}`);
     return 0;

src/runtime.ts

@@ -6,7 +6,7 @@
 import { pathToFileURL } from "node:url";
 import { resolve } from "node:path";
 import { useBroker } from "@novox/mesh-sdk/messaging";
-import { collectTools, serveTools, listTools, toolKey } from "@novox/mesh-sdk/tools";
+import { collectTools, toolKey } from "@novox/mesh-sdk/tools";
 import type { Broker } from "@novox/mesh-sdk/messaging";
 import { seatToolSubject, type Credential, type RuntimeBroker } from "./broker-nats.js";
 
@@ -20,7 +20,14 @@ export const TOOLS_VERB = "tools";
 /** What `tools` answers for one module. */
 export interface ToolsAnswer {
   module: string;
-  tools: { name: string; description: string; input: Readonly<Record<string, unknown>> }[];
+  tools: {
+    name: string;
+    description: string;
+    input: Readonly<Record<string, unknown>>;
+    /** Where this tool is answered, as the mesh issued it (ADR 0160): the module's plain subject
+     *  first when there is one, then this machine's. A caller composes nothing. */
+    subjects?: string[];
+  }[];
 }
 
 export interface RuntimeOptions {
@@ -45,14 +52,36 @@ export async function runTools(opts: RuntimeOptions): Promise<() => void> {
   // Serve the RPC endpoint only if a module actually registered a tool. A pure-events module (the
   // audit logger) registers none, and its scoped account may not declare the serve queue — so a
   // runtime that always served would fail for exactly the modules that never needed it.
-  const tools = listTools();
+  // A registration under a seat's name is the module's implementation of that seat's verbs
-  const stop = tools.length > 0 ? await serveTools(opts.broker) : () => {};
+  // (ADR 0159, 0160): served on the seat's subjects by serveClaimedSeats, never as a module's
+  // tools and never listed among them. Everything else is the module's own.
+  // A module named like its seat (the catalogue is the mesh-catalog seat) registers once and is
+  // both: its tools are the module's and the seat's verbs alike.
+  const self = opts.credential?.module;
+  const seatNames = new Set((opts.credential?.claims ?? []).map((c) => c.seat));
+  const ownRegistrations = collectTools().filter(({ module }) => module === self || !seatNames.has(module));
+  const tools = ownRegistrations.flatMap(({ module, tools: own }) => own.map((t) => ({ module, name: t.name })));
+  const stops: Array<() => void> = [];
+  const stop = (): void => stops.splice(0).forEach((s) => s());
+  // Each tool on its own key, namespaced by its module (ADR 0047); where that key is answered is
+  // the broker's to know from the membership (ADR 0160).
+  for (const { module, tools: own } of ownRegistrations) {
+    const seen = new Set<string>();
+    for (const t of own) {
+      if (seen.has(t.name)) {
+        stop();
+        throw new Error(`${module} exposes two tools named ${t.name} — refused`);
+      }
+      seen.add(t.name);
+      stops.push(await opts.broker.handle(toolKey(module, t.name), (args: Record<string, unknown> | undefined) => t.run(args ?? {})));
+    }
+  }
 
   // And, for every module that serves any, the verb that says what it serves. Refused before
   // anything is bound if a module named a tool of its own `tools`: one name answering two things
   // is the fault nobody can diagnose afterwards, and the runtime is the only place that sees both.
-  const stops: Array<() => void> = [stop];
+  const runtime = opts.broker as RuntimeBroker;
-  for (const { module, tools: own } of collectTools()) {
+  for (const { module, tools: own } of ownRegistrations) {
     if (own.length === 0) continue;
     if (own.some((t) => t.name === TOOLS_VERB)) {
       stop();
@@ -61,9 +90,16 @@ export async function runTools(opts: RuntimeOptions): Promise<() => void> {
           "module with what it serves (novox/hq ADR 0152) — refused, rename it",
       );
     }
+    const subjectsOf = (tool: string): string[] | undefined => {
+      const issued = typeof runtime.membership === "function" ? runtime.membership() : undefined;
+      if (!issued) return undefined;
+      const plain = issued.serves.filter((s) => s.queue).map((s) => s.subject.replace("{tool}", tool));
+      const mine = issued.serves.filter((s) => !s.queue).map((s) => s.subject.replace("{tool}", tool));
+      return [...plain, ...mine];
+    };
     const answer: ToolsAnswer = {
       module,
-      tools: own.map((t) => ({ name: t.name, description: t.description, input: t.input })),
+      tools: own.map((t) => ({ name: t.name, description: t.description, input: t.input, subjects: subjectsOf(t.name) })),
     };
     stops.push(await opts.broker.handle(toolKey(module, TOOLS_VERB), async () => answer));
   }
@@ -85,22 +121,37 @@ export async function runTools(opts: RuntimeOptions): Promise<() => void> {
 async function serveClaimedSeats(broker: RuntimeBroker, credential?: Credential): Promise<() => void> {
   const claims = credential?.claims ?? [];
   if (claims.length === 0 || typeof broker.handleSubject !== "function") return () => {};
-  const byName = new Map<string, (args: Record<string, unknown>) => Promise<unknown>>();
+  // A seat's verbs are the role's, not the software's (ADR 0159): implemented under the seat's
-  for (const { tools } of collectTools()) {
+  // name — `registerModuleTools("mesh-store", …)` — and never confused with the module's own tools.
-    for (const t of tools) byName.set(t.name, (args) => t.run(args));
+  const implementations = new Map<string, Map<string, (args: Record<string, unknown>) => Promise<unknown>>>();
+  for (const { module, tools } of collectTools()) {
+    if (!claims.some((c) => c.seat === module)) continue;
+    const verbs = new Map<string, (args: Record<string, unknown>) => Promise<unknown>>();
+    for (const t of tools) verbs.set(t.name, (args) => t.run(args));
+    implementations.set(module, verbs);
   }
-  const stops: (() => void)[] = [];
+  let stops: (() => void)[] = [];
+  const serve = async (): Promise<void> => {
+    stops.forEach((s) => s());
+    stops = [];
+    const issued = typeof broker.membership === "function" ? broker.membership() : undefined;
     for (const claim of claims) {
+      const verbs = implementations.get(claim.seat);
       for (const verb of claim.serves ?? []) {
-      const run = byName.get(verb);
+        const run = verbs?.get(verb);
         if (!run) {
-        console.log(`[mesh-tools] claims ${claim.seat} and has no tool named ${verb}, which that seat promises; not served`);
+          console.log(`[mesh-tools] claims ${claim.seat} and implements no ${verb}, which that seat promises; not served`);
           continue;
         }
-      const subject = seatToolSubject(claim.seat, verb, claim.scope, credential?.node);
+        // Where the mesh issued the verb when it has; the derived shape until then.
+        const subject = issued?.seats?.find((s) => s.seat === claim.seat && s.verb === verb)?.subject
+          ?? seatToolSubject(claim.seat, verb, claim.scope, credential?.node);
         stops.push(await broker.handleSubject(subject, run));
         console.log(`[mesh-tools] serving ${claim.seat}'s ${verb} on ${subject}, admitted where this module holds the seat`);
       }
     }
+  };
+  await serve();
+  if (typeof broker.onMembership === "function") broker.onMembership(() => void serve());
   return () => stops.forEach((s) => s());
 }

test/fixtures/shop-seat.mjs

@@ -0,0 +1,12 @@
+// The shop again, in a file of its own: a module imported once stays imported, so a second test needs a second entrypoint.
+import { registerModuleTools } from "@novox/mesh-sdk/tools";
+
+registerModuleTools("shop", () => [
+  {
+    name: "price",
+    description: "what something costs",
+    input: { of: { type: "string", description: "the thing" } },
+    run: async (args) => ({ of: args.of ?? "nothing", cost: 12 }),
+  },
+  { name: "refund", description: "give it back", input: {}, run: async () => ({ done: true }) },
+]);

test/fixtures/store-seat.mjs

@@ -0,0 +1,12 @@
+// A module that is both software and a role: postgres's own tools under its name, and its
+// implementation of the mesh-store seat's verbs under the seat's (novox/hq ADR 0159, 0160).
+import { registerModuleTools } from "@novox/mesh-sdk/tools";
+
+registerModuleTools("postgres", () => [
+  { name: "postgres_create_database", description: "make one", input: {}, run: async () => ({ made: true }) },
+  { name: "databases", description: "postgres's own listing", input: {}, run: async () => ({ software: "postgres" }) },
+]);
+
+registerModuleTools("mesh-store", () => [
+  { name: "databases", description: "what the store holds", input: {}, run: async () => ({ seat: "mesh-store" }) },
+]);

test/membership.test.ts

@@ -0,0 +1,197 @@
+/**
+ * The mesh issues an assignment's subjects, and a runtime serves what it is issued (novox/hq ADR
+ * 0160). A runtime derives one address for itself — `mesh.assignment.<node>.<module>` — reads the
+ * membership there, serves exactly what it says, and re-serves when a new one arrives. Against a
+ * real bus with JetStream, because the membership is a direct get on a stream.
+ *
+ *   docker run -d --rm --name t -p 14232:4222 nats:2.10-alpine -js
+ *   MESH_TEST_NATS=nats://127.0.0.1:14232 node --test --experimental-strip-types test/membership.test.ts
+ */
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { fileURLToPath } from "node:url";
+import { connect, StringCodec } from "nats";
+
+import { resetTools } from "@novox/mesh-sdk/tools";
+
+import { connectNats, membershipSubject } from "../dist/broker-nats.js";
+import { callTool, subjectListed } from "../dist/client.js";
+import { runTools } from "../dist/runtime.js";
+
+const url = process.env.MESH_TEST_NATS;
+const fixture = (name: string) => fileURLToPath(new URL(`./fixtures/${name}`, import.meta.url));
+const sc = StringCodec();
+
+/** The ASSIGNMENTS stream as the controller asserts it: last-per-subject, readable by direct get. */
+async function anAssignmentsStream() {
+  const nc = await connect({ servers: url! });
+  const jsm = await nc.jetstreamManager();
+  try {
+    await jsm.streams.delete("ASSIGNMENTS");
+  } catch {
+    // none yet
+  }
+  await jsm.streams.add({
+    name: "ASSIGNMENTS",
+    subjects: ["mesh.assignment.>"],
+    max_msgs_per_subject: 1,
+    allow_direct: true,
+  } as never);
+  return {
+    async issue(m: object & { node: string; module: string }) {
+      await nc.jetstream().publish(membershipSubject(m.node, m.module), sc.encode(JSON.stringify(m)));
+    },
+    async close() {
+      await nc.close();
+    },
+  };
+}
+
+test("a runtime serves exactly the subjects it is issued, and the listing carries them", async (t) => {
+  if (!url) return t.skip("MESH_TEST_NATS unset");
+  resetTools();
+  const stream = await anAssignmentsStream();
+  // The mesh placed shop on two machines that are not interchangeable: each is served by name only.
+  await stream.issue({
+    node: "anchor",
+    module: "shop",
+    serves: [{ subject: "mesh.mod.shop.tool.{tool}.anchor" }],
+    emits: "mesh.mod.shop.event.{event}",
+    tools: "mesh.mod.shop.tool.tools",
+  });
+  const shop = await connectNats({ url, node: "anchor", module: "shop" });
+  const asker = await connectNats({ url, module: "console", node: "workstation" });
+  let stop = () => {};
+  try {
+    stop = await runTools({ broker: shop, moduleEntrypoints: [fixture("shop-tools.mjs")] });
+    assert.equal(shop.membership()?.module, "shop", "the runtime read what the mesh issued");
+    // By name it answers; the plain subject was not issued, so nothing serves it.
+    const named = await callTool(asker, "shop.price@anchor", { of: "a hat" });
+    assert.deepEqual(named.result, { of: "a hat", cost: 12 });
+    assert.equal(named.node, "anchor");
+    await assert.rejects(callTool(asker, "shop.price", {}), /no responders|503/i, "a subject not issued is not served");
+    // The tools answer says where each is served, and the client composes nothing.
+    const tools = await asker.request<Record<string, never>, { tools: { name: string; subjects?: string[] }[] }>(
+      "shop.tools",
+      {},
+    );
+    assert.deepEqual(tools.tools.find((x) => x.name === "price")?.subjects, ["mesh.mod.shop.tool.price.anchor"]);
+    const listing = { tools: [{ module: "shop", name: "price", subjects: ["mesh.mod.shop.tool.price.anchor"] }], notAnswering: [] };
+    assert.equal(subjectListed("shop.price", "anchor", listing as never), "mesh.mod.shop.tool.price.anchor");
+    assert.equal(subjectListed("shop.price", "elsewhere", listing as never), undefined);
+
+    // The mesh re-issues the membership with the plain subject too (the modules became
+    // interchangeable); the runtime re-serves on it without a restart.
+    await stream.issue({
+      node: "anchor",
+      module: "shop",
+      serves: [{ subject: "mesh.mod.shop.tool.{tool}", queue: "serve.shop" }, { subject: "mesh.mod.shop.tool.{tool}.anchor" }],
+      emits: "mesh.mod.shop.event.{event}",
+      tools: "mesh.mod.shop.tool.tools",
+    });
+    await new Promise((r) => setTimeout(r, 300));
+    const plain = await callTool(asker, "shop.price", { of: "a coat" });
+    assert.deepEqual(plain.result, { of: "a coat", cost: 12 });
+    assert.equal(plain.node, "anchor");
+  } finally {
+    stop();
+    await asker.close();
+    await shop.close();
+    await stream.close();
+  }
+});
+
+test("a seat's verbs are implemented under the seat's name, served where issued, never listed as the module's", async (t) => {
+  if (!url) return t.skip("MESH_TEST_NATS unset");
+  resetTools();
+  const stream = await anAssignmentsStream();
+  await stream.issue({
+    node: "anchor",
+    module: "postgres",
+    serves: [{ subject: "mesh.mod.postgres.tool.{tool}", queue: "serve.postgres" }, { subject: "mesh.mod.postgres.tool.{tool}.anchor" }],
+    seats: [{ seat: "mesh-store", verb: "databases", subject: "mesh.seat.mesh-store.tool.databases" }],
+    emits: "mesh.mod.postgres.event.{event}",
+    tools: "mesh.mod.postgres.tool.tools",
+  });
+  const credential = { url, node: "anchor", module: "postgres", claims: [{ seat: "mesh-store", scope: "mesh", serves: ["databases", "query"] }] };
+  const pg = await connectNats(credential);
+  const asker = await connectNats({ url, module: "console", node: "workstation" });
+  let stop = () => {};
+  try {
+    stop = await runTools({ broker: pg, credential, moduleEntrypoints: [fixture("store-seat.mjs")] });
+    // The module's own `databases` and the seat's are two tools: postgres's on its subject, the
+    // store's on the seat's, each answering as itself.
+    const own = await callTool(asker, "postgres.databases", {});
+    assert.deepEqual(own.result, { software: "postgres" });
+    const seat = await callTool(asker, "seat:mesh-store.databases", {});
+    assert.deepEqual(seat.result, { seat: "mesh-store" });
+    assert.equal(seat.node, "anchor");
+    // What the seat does not promise — creating a database — is postgres's tool and not the store's.
+    await assert.rejects(callTool(asker, "seat:mesh-store.postgres_create_database", {}), /no responders|503/i);
+    // And the module's `tools` lists only postgres's own, never the seat's implementation.
+    const tools = await asker.request<Record<string, never>, { module: string; tools: { name: string }[] }>("postgres.tools", {});
+    assert.deepEqual(tools.tools.map((x) => x.name).sort(), ["databases", "postgres_create_database"]);
+    await assert.rejects(asker.request("mesh-store.tools", {}), /no responders|503/i, "a seat is not a module with a tools verb");
+  } finally {
+    stop();
+    await asker.close();
+    await pg.close();
+    await stream.close();
+  }
+});
+
+test("a module named like its seat registers once, and answers as the module and as the seat", async (t) => {
+  if (!url) return t.skip("MESH_TEST_NATS unset");
+  resetTools();
+  const stream = await anAssignmentsStream();
+  await stream.issue({
+    node: "anchor",
+    module: "shop",
+    serves: [{ subject: "mesh.mod.shop.tool.{tool}", queue: "serve.shop" }, { subject: "mesh.mod.shop.tool.{tool}.anchor" }],
+    seats: [{ seat: "shop", verb: "price", subject: "mesh.seat.shop.tool.price" }],
+    emits: "mesh.mod.shop.event.{event}",
+    tools: "mesh.mod.shop.tool.tools",
+  });
+  const credential = { url, node: "anchor", module: "shop", claims: [{ seat: "shop", scope: "mesh", serves: ["price"] }] };
+  const shop = await connectNats(credential);
+  const asker = await connectNats({ url, module: "console", node: "workstation" });
+  let stop = () => {};
+  try {
+    stop = await runTools({ broker: shop, credential, moduleEntrypoints: [fixture("shop-seat.mjs")] });
+    assert.deepEqual((await callTool(asker, "shop.price", { of: "a hat" })).result, { of: "a hat", cost: 12 });
+    assert.deepEqual((await callTool(asker, "seat:shop.price", { of: "a hat" })).result, { of: "a hat", cost: 12 });
+    const tools = await asker.request<Record<string, never>, { tools: { name: string }[] }>("shop.tools", {});
+    assert.deepEqual(tools.tools.map((x) => x.name), ["price", "refund"]);
+  } finally {
+    stop();
+    await asker.close();
+    await shop.close();
+    await stream.close();
+  }
+});
+
+test("a mesh that has issued nothing yet gets the derived shape, and says so", async (t) => {
+  if (!url) return t.skip("MESH_TEST_NATS unset");
+  const stream = await anAssignmentsStream();
+  const said: string[] = [];
+  const log = console.log;
+  console.log = (...a: unknown[]) => said.push(a.join(" "));
+  let lone;
+  try {
+    lone = await connectNats({ url, node: "home-server", module: "lone" });
+  } finally {
+    console.log = log;
+  }
+  const asker = await connectNats({ url, module: "console", node: "workstation" });
+  try {
+    assert.equal(lone.membership(), undefined);
+    assert.ok(said.some((s) => /no membership issued for lone on home-server/.test(s)), said.join("\n"));
+    await lone.handle("ping", async () => ({ pong: true }));
+    assert.deepEqual((await callTool(asker, "lone.ping", {})).result, { pong: true });
+    assert.deepEqual((await callTool(asker, "lone.ping@home-server", {})).result, { pong: true });
+  } finally {
+    await asker.close();
+    await lone.close();
+    await stream.close();
+  }
+});