diff --git a/.env.example b/.env.example
index f95c57d0..541ae47a 100644
--- a/.env.example
+++ b/.env.example
@@ -260,6 +260,7 @@ DB_PASSWORD=pedscribe_secret_change_me
 # CLINICAL_ASSISTANT_MCP_CONCURRENCY=3   # library searches in flight at once; they used to run one at a time
 # CLINICAL_ASSISTANT_MCP_QUEUE_MAX=        # callers allowed to wait for a slot (default 4 x concurrency); past it: "busy, try again"
 # CLINICAL_ASSISTANT_MCP_QUEUE_WAIT_MS=8000 # how long a caller waits in that line before being told the library is busy
+# CLINICAL_ASSISTANT_COLLECTIONS_TTL_MS=600000 # how often the list of search collections is re-read (off the query path); with one collection nothing changes
 # CLINICAL_ASSISTANT_ASK_LIMIT_PER_MINUTE=30 # paid assistant questions per user per minute, counted in Redis (no Redis: no limit)
 # CLINICAL_ASSISTANT_RETRIEVAL_CACHE_TTL_S=60 # same library search within this window is answered from Redis; 0 disables
 # CLINICAL_ASSISTANT_MCP_WARMUP=            # open a session at boot
diff --git a/src/utils/clinicalMcpClient.js b/src/utils/clinicalMcpClient.js
index 087b7566..2f7992d0 100644
--- a/src/utils/clinicalMcpClient.js
+++ b/src/utils/clinicalMcpClient.js
@@ -1,5 +1,6 @@
 var axios = require('axios');
 var sleep = require('node:timers/promises').setTimeout;
+var fanout = require('./collectionFanout');
 
 var MCP_URLS = buildMcpUrls();
 var _lastGoodMcpUrl = MCP_URLS[0];
@@ -87,9 +88,8 @@ function closeMcpSession() {
   return _closePromise;
 }
 
-async function semanticSearch(query, opts) {
-  opts = opts || {};
-  return callMcpTool(SEARCH_TOOL_NAME, {
+function searchArgs(query, opts, collection) {
+  var args = {
     query: query,
     limit: opts.limit,
     doc_types: ['file'],
@@ -97,7 +97,65 @@ async function semanticSearch(query, opts) {
     fusion: 'rrf',
     include_context: opts.includeContext,
     context_chars: opts.contextChars
+  };
+  // The default collection is asked for by omission, exactly as before the
+  // service could hold more than one.
+  if (collection) args.collection = collection;
+  return args;
+}
+
+async function semanticSearch(query, opts) {
+  opts = opts || {};
+  var extra = knownExtraCollections();
+  if (extra.length === 0) return callMcpTool(SEARCH_TOOL_NAME, searchArgs(query, opts));
+  // Every collection at once, the default included. A collection whose call
+  // fails is left out of the fused list rather than failing the answer; the
+  // default failing is the same error it always was.
+  var calls = [{ collection: null, promise: callMcpTool(SEARCH_TOOL_NAME, searchArgs(query, opts)) }]
+    .concat(extra.map(function(name) {
+      return { collection: name, promise: callMcpTool(SEARCH_TOOL_NAME, searchArgs(query, opts, name)) };
+    }));
+  var settled = await Promise.allSettled(calls.map(function(c) { return c.promise; }));
+  if (settled[0].status === 'rejected') throw settled[0].reason;
+  var responses = [];
+  settled.forEach(function(s, i) {
+    if (s.status === 'fulfilled') responses.push({ collection: calls[i].collection, response: s.value });
+    else console.warn('[clinical-assistant] collection search skipped:', calls[i].collection, s.reason && s.reason.message);
   });
+  return fanout.mergeCollectionResults(responses, positiveInt(opts.limit, 0));
+}
+
+// The collections the service will search besides its default, learned off
+// the query path: at warm-up and on the session timer, never while someone
+// waits for an answer. Until the first listing arrives, or on a service that
+// has no listing tool, this is empty and a search is the single call it
+// always was.
+var COLLECTIONS_TTL_MS = positiveInt(process.env.CLINICAL_ASSISTANT_COLLECTIONS_TTL_MS, 10 * 60 * 1000);
+var _extraCollections = [];
+var _collectionsFetchedAt = 0;
+var _collectionsPromise = null;
+
+function knownExtraCollections() {
+  return _extraCollections;
+}
+
+function refreshCollections() {
+  if (_collectionsPromise) return _collectionsPromise;
+  _collectionsPromise = callMcpTool('clinical_list_collections', {}).then(function(listing) {
+    _extraCollections = fanout.extraCollectionNames(listing);
+    _collectionsFetchedAt = Date.now();
+    return _extraCollections;
+  }).catch(function(e) {
+    // No listing tool, or a transient error: keep whatever was known.
+    _collectionsFetchedAt = Date.now();
+    console.info('[clinical-assistant] collection listing unavailable:', e && e.message);
+    return _extraCollections;
+  }).finally(function() { _collectionsPromise = null; });
+  return _collectionsPromise;
+}
+
+function collectionsStale() {
+  return Date.now() - _collectionsFetchedAt > COLLECTIONS_TTL_MS;
 }
 
 async function indexedTopicSuggestions(limit) {
@@ -117,10 +175,14 @@ function warmMcpSession() {
         if (soon) { var old = _mcpSession; _mcpSession = null; endMcpSession(old); }
         getMcpSession().catch(function() {});
       }
+      if (collectionsStale()) refreshCollections();
     }, Math.max(30000, Math.floor(MCP_SESSION_TTL_MS / 3)));
     if (_warmTimer.unref) _warmTimer.unref();
   }
-  return getMcpSession();
+  return getMcpSession().then(function(session) {
+    if (collectionsStale()) refreshCollections();
+    return session;
+  });
 }
 
 function busyError() {
@@ -402,6 +464,8 @@ function parseMcpResponse(body) {
 
 module.exports = {
   semanticSearch: semanticSearch,
+  knownExtraCollections: knownExtraCollections,
+  refreshCollections: refreshCollections,
   queueDepth: queueDepth,
   indexedTopicSuggestions: indexedTopicSuggestions,
   getMcpHealth: getMcpHealth,
diff --git a/src/utils/collectionFanout.js b/src/utils/collectionFanout.js
new file mode 100644
index 00000000..7b0a623d
--- /dev/null
+++ b/src/utils/collectionFanout.js
@@ -0,0 +1,103 @@
+// ============================================================
+// COLLECTION FAN-OUT
+// Searching more than one Milvus collection without slowing down the one.
+//
+// The search service can hold several collections, each embedded with its
+// own model (MILVUS_COLLECTIONS on the service; clinical_list_collections
+// reports them). A search names one collection, or none for the default. So
+// a second collection is only useful to the assistant if the client asks
+// both — and the one-collection case, which is every deployment today, must
+// cost exactly what it did: one call, no listing on the query path.
+//
+// The list is therefore fetched off the query path (at session warm-up and
+// on a slow timer) and the fan-out is decided from that cache. With one
+// collection cached, or none yet, the search is the single call it always
+// was. With more, one call per collection runs in parallel and the ranked
+// lists are fused by reciprocal rank — the same fusion the service uses
+// within a collection — so a strong hit in a small collection is not buried
+// under a large one's scores, which are not comparable across embedders.
+// ============================================================
+
+var RRF_K = 60;
+
+// The service's result list, wherever the MCP envelope put it.
+function extractResults(response) {
+  var data = response && (response.structuredContent || response.data || response);
+  if ((!data || !Array.isArray(data.results)) && response && Array.isArray(response.content)) {
+    for (var i = 0; i < response.content.length; i++) {
+      var c = response.content[i];
+      if (c && c.type === 'text' && c.text) {
+        try {
+          var parsed = JSON.parse(c.text);
+          if (parsed && Array.isArray(parsed.results)) data = parsed;
+        } catch (e) {}
+      }
+    }
+  }
+  return data && Array.isArray(data.results) ? data : null;
+}
+
+function resultKey(r) {
+  if (r && r.id != null) return String(r.id);
+  return [r && r.file_path, r && r.page_number, r && r.chunk_index].join('|');
+}
+
+// Fuse per-collection ranked lists into one list of at most `limit`.
+// `responses` is [{ collection, response }]; a collection whose call failed
+// is simply absent. Each merged result carries the collection it came from.
+function mergeCollectionResults(responses, limit) {
+  var fused = new Map();
+  var verified = 0, dropped = 0, searched = [];
+  responses.forEach(function(entry) {
+    var data = extractResults(entry.response);
+    if (!data) return;
+    searched.push(entry.collection);
+    verified += Number(data.verified_chunk_count || data.verifiedChunkCount || 0);
+    dropped += Number(data.dropped_document_count || data.droppedDocumentCount || 0);
+    data.results.forEach(function(r, rank) {
+      var key = resultKey(r);
+      var slot = fused.get(key);
+      var contribution = 1 / (RRF_K + rank + 1);
+      if (slot) { slot.fused += contribution; return; }
+      fused.set(key, { fused: contribution, result: Object.assign({}, r, { collection: entry.collection }) });
+    });
+  });
+  var merged = Array.from(fused.values())
+    .sort(function(a, b) { return b.fused - a.fused; })
+    .map(function(s) { return s.result; });
+  if (limit > 0) merged = merged.slice(0, limit);
+  return {
+    results: merged,
+    verified_chunk_count: verified,
+    dropped_document_count: dropped,
+    collections: searched
+  };
+}
+
+// The names worth a separate call: everything the service lists beyond its
+// default. An empty or one-entry list means "search as before".
+function extraCollectionNames(listing) {
+  var data = listing && (listing.structuredContent || listing.data || listing);
+  if ((!data || !Array.isArray(data.collections)) && listing && Array.isArray(listing.content)) {
+    for (var i = 0; i < listing.content.length; i++) {
+      var c = listing.content[i];
+      if (c && c.type === 'text' && c.text) {
+        try {
+          var parsed = JSON.parse(c.text);
+          if (parsed && Array.isArray(parsed.collections)) data = parsed;
+        } catch (e) {}
+      }
+    }
+  }
+  if (!data || !Array.isArray(data.collections)) return [];
+  return data.collections
+    .filter(function(c) { return c && c.name && !c.default && !c.is_default; })
+    .map(function(c) { return String(c.name); });
+}
+
+module.exports = {
+  extractResults: extractResults,
+  mergeCollectionResults: mergeCollectionResults,
+  extraCollectionNames: extraCollectionNames,
+  RRF_K: RRF_K
+};
diff --git a/test/mcp-collection-fanout.test.js b/test/mcp-collection-fanout.test.js
new file mode 100644
index 00000000..14c5d3e2
--- /dev/null
+++ b/test/mcp-collection-fanout.test.js
@@ -0,0 +1,60 @@
+// A second search collection is searched alongside the first and fused by
+// rank; with one collection the search is the single call it always was.
+const test = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const fanout = require('../src/utils/collectionFanout');
+const src = fs.readFileSync(path.join(__dirname, '..', 'src/utils/clinicalMcpClient.js'), 'utf8');
+
+function listing(names, defaultName) {
+  return { structuredContent: { collections: names.map(n => ({ name: n, default: n === defaultName, model: 'm' })) } };
+}
+
+test('the default collection alone means nothing extra to search', () => {
+  assert.deepEqual(fanout.extraCollectionNames(listing(['mcp_bge_m3_1024'], 'mcp_bge_m3_1024')), []);
+  assert.deepEqual(fanout.extraCollectionNames({}), []);
+  assert.deepEqual(fanout.extraCollectionNames(null), []);
+});
+
+test('collections beyond the default are named, from either envelope', () => {
+  assert.deepEqual(fanout.extraCollectionNames(listing(['a', 'b', 'c'], 'a')), ['b', 'c']);
+  const textEnvelope = { content: [{ type: 'text', text: JSON.stringify({ collections: [{ name: 'a', default: true }, { name: 'peds' }] }) }] };
+  assert.deepEqual(fanout.extraCollectionNames(textEnvelope), ['peds']);
+});
+
+test('ranked lists from two collections fuse by reciprocal rank and keep their origin', () => {
+  const first = { structuredContent: { results: [{ id: 'x', score: 0.9 }, { id: 'y', score: 0.8 }], verified_chunk_count: 2 } };
+  const second = { structuredContent: { results: [{ id: 'z', score: 0.1 }, { id: 'y', score: 0.05 }], verified_chunk_count: 2, dropped_document_count: 1 } };
+  const merged = fanout.mergeCollectionResults([{ collection: null, response: first }, { collection: 'peds', response: second }], 10);
+  // y appears in both lists (ranks 2 and 2) and outranks either list's first.
+  assert.equal(merged.results[0].id, 'y');
+  assert.deepEqual(merged.results.map(r => r.id).sort(), ['x', 'y', 'z']);
+  assert.equal(merged.results.find(r => r.id === 'z').collection, 'peds');
+  assert.equal(merged.results.find(r => r.id === 'x').collection, null);
+  assert.equal(merged.verified_chunk_count, 4);
+  assert.equal(merged.dropped_document_count, 1);
+  assert.deepEqual(merged.collections, [null, 'peds']);
+});
+
+test('the fused list respects the limit and a failed collection is simply absent', () => {
+  const one = { structuredContent: { results: [{ id: 1 }, { id: 2 }, { id: 3 }] } };
+  const merged = fanout.mergeCollectionResults([{ collection: null, response: one }], 2);
+  assert.equal(merged.results.length, 2);
+  assert.deepEqual(merged.collections, [null]);
+});
+
+test('the search itself stays one call when no extra collection is known', () => {
+  // The single-call path is the first thing semanticSearch does, before any
+  // fan-out machinery, and it names no collection.
+  const body = src.slice(src.indexOf('async function semanticSearch'), src.indexOf('// The collections the service will search'));
+  assert.match(body, /if \(extra\.length === 0\) return callMcpTool\(SEARCH_TOOL_NAME, searchArgs\(query, opts\)\)/);
+  assert.match(src, /if \(collection\) args\.collection = collection;/);
+});
+
+test('the collection list is learned off the query path, never inside a search', () => {
+  const search = src.slice(src.indexOf('async function semanticSearch'), src.indexOf('// The collections the service will search'));
+  assert.doesNotMatch(search, /refreshCollections|clinical_list_collections/);
+  const warm = src.slice(src.indexOf('function warmMcpSession'), src.indexOf('function busyError'));
+  assert.match(warm, /if \(collectionsStale\(\)\) refreshCollections\(\);/);
+});