diff --git a/tools/content-node/interactors-endpoints.ts b/tools/content-node/interactors-endpoints.ts
index a29e6adb..b385e5ba 100644
--- a/tools/content-node/interactors-endpoints.ts
+++ b/tools/content-node/interactors-endpoints.ts
@@ -15,6 +15,7 @@ import {
 } from './spring.ts';
 import {
   RawJson,
+  deepShare,
   javaError,
   neo4jFailureMessage,
   nonEmptyJson,
@@ -315,6 +316,9 @@ export async function refreshPsicquicResources({ timeout = 60_000 } = {}) {
   }
 }
 
+/** Batches of molecule lookups running at once (POST /interactors/static/molecules/details). */
+const MOLECULE_BATCHES = deepShare(4);
+
 export const interactorsEndpoints: Endpoint[] = [
   {
     path: '/ContentService/interactors/static/molecule/{id}/details',
@@ -464,12 +468,36 @@ export const interactorsEndpoints: Endpoint[] = [
       const proteins = plainText(request.body, contentType);
       const accs = new JavaHashMap<string, boolean>();
       for (const acc of plainIds(proteins)) accs.set(acc, true);
+      // One query per accession, one at a time as Java asks: a diagram's
+      // proteins (273 for R-HSA-69620) take a second, and a 10MB body holds
+      // hundreds of thousands. Batches have a share of their own, and stop
+      // when nobody is left to read the answer (as the xrefs batch does).
+      const gone = new AbortController();
+      request.res?.on('close', () => {
+        if (!request.res?.writableFinished) gone.abort();
+      });
+      if (!MOLECULE_BATCHES.enter())
+        return {
+          ...javaError(
+            request,
+            503,
+            'SERVICE_UNAVAILABLE',
+            'The service is busy; try again shortly'
+          ),
+          headers: { 'Retry-After': '5' },
+        };
       const entities: [string, InteractionRow[]][] = [];
-      for (const acc of accs.keys()) {
-        const rows = await interactionsOf(request, acc, window);
-        if ('answer' in rows) return rows.answer;
-        if (rows.some((row) => row.accession === null)) return unexpected(request);
-        entities.push([acc, rows]);
+      try {
+        for (const acc of accs.keys()) {
+          // Nobody to answer: whatever is written goes nowhere.
+          if (gone.signal.aborted) return { status: 500 };
+          const rows = await interactionsOf(request, acc, window);
+          if ('answer' in rows) return rows.answer;
+          if (rows.some((row) => row.accession === null)) return unexpected(request);
+          entities.push([acc, rows]);
+        }
+      } finally {
+        MOLECULE_BATCHES.leave();
       }
       if (entities.every(([, rows]) => rows.length === 0))
         return notFound(request, `No interactors found for accession: ${proteins}`);
diff --git a/tools/content-node/molecule-details-bound.spec.mjs b/tools/content-node/molecule-details-bound.spec.mjs
new file mode 100644
index 00000000..4fbe3da6
--- /dev/null
+++ b/tools/content-node/molecule-details-bound.spec.mjs
@@ -0,0 +1,109 @@
+// @vitest-environment node
+//
+// POST /interactors/static/molecules/details runs one query per accession, one
+// at a time, and a 10MB body holds hundreds of thousands: its batches have a
+// share of their own, and stop when the client is gone. A stand-in graph
+// holds every query open until the test lets it go.
+import { afterAll, beforeAll, describe, expect, it, vi } from 'vitest';
+import { request as httpRequest } from 'node:http';
+
+const held = [];
+let asked = 0;
+vi.mock('./graph.ts', () => ({
+  read: () => {
+    asked += 1;
+    return new Promise((resolve) => held.push(() => resolve([])));
+  },
+  configured: () => true,
+  close: async () => {},
+  envSetting: () => '',
+}));
+
+const { app } = await import('./server.ts');
+
+let server;
+let port;
+beforeAll(async () => {
+  await new Promise((resolve) => {
+    server = app().listen(0, '127.0.0.1', resolve);
+  });
+  port = server.address().port;
+});
+afterAll(() => new Promise((resolve) => server.close(resolve)));
+
+/** A POST of these accessions: its response, and a way to drop the connection. */
+function post(accessions) {
+  const sent = httpRequest({
+    host: '127.0.0.1',
+    port,
+    // A connection of its own, closed with its answer: the server's count of
+    // open connections then says when a dropped one has gone.
+    agent: false,
+    method: 'POST',
+    path: '/ContentService/interactors/static/molecules/details',
+    headers: { 'Content-Type': 'text/plain' },
+  });
+  const answered = new Promise((resolve, reject) => {
+    sent.on('response', (response) => {
+      let body = '';
+      response.on('data', (chunk) => (body += chunk));
+      response.on('end', () => resolve({ status: response.statusCode, response, body }));
+    });
+    sent.on('error', reject);
+  });
+  sent.end(accessions.join(','));
+  return { answered, drop: () => sent.destroy() };
+}
+
+const until = async (condition) => {
+  for (let i = 0; i < 200 && !condition(); i += 1) await new Promise((r) => setTimeout(r, 10));
+  expect(condition()).toBe(true);
+};
+const release = () => {
+  for (const go of held.splice(0)) go();
+};
+
+describe('the molecule details batch', () => {
+  it('runs at most four at once; a fifth is the 503 to try again shortly', async () => {
+    const running = [1, 2, 3, 4].map((n) => post([`P0000${n}`]));
+    await until(() => held.length === 4);
+    const fifth = await post(['P00005']).answered;
+    expect(fifth.status).toBe(503);
+    expect(fifth.response.headers['retry-after']).toBe('5');
+    expect(JSON.parse(fifth.body).messages).toEqual(['The service is busy; try again shortly']);
+    release();
+    // None found for any is Java's 404, and each batch gives its place back.
+    for (const { answered } of running) expect((await answered).status).toBe(404);
+    const after = post(['P00006']);
+    await until(() => held.length === 1);
+    release();
+    expect((await after.answered).status).toBe(404);
+  });
+
+  it('stops asking once the client is gone, and gives its place back', async () => {
+    asked = 0;
+    const { answered, drop } = post(['P1', 'P2', 'P3', 'P4', 'P5']);
+    answered.catch(() => {});
+    await until(() => held.length === 1);
+    drop();
+    // The server has seen the close before the first lookup returns.
+    const open = () =>
+      new Promise((resolve) => server.getConnections((_error, count) => resolve(count)));
+    let connections = 1;
+    for (let i = 0; i < 200 && connections > 0; i += 1) {
+      connections = await open();
+      if (connections > 0) await new Promise((r) => setTimeout(r, 10));
+    }
+    expect(connections).toBe(0);
+    release();
+    await new Promise((r) => setTimeout(r, 20));
+    expect(asked).toBe(1);
+    // All four places are free again: four run, and a fifth is refused. A
+    // dropped batch that kept its place would leave three.
+    const four = [1, 2, 3, 4].map((n) => post([`Q0000${n}`]));
+    await until(() => held.length === 4);
+    expect((await post(['Q00005']).answered).status).toBe(503);
+    release();
+    for (const { answered: each } of four) expect((await each).status).toBe(404);
+  });
+});