diff --git a/docker-compose.yml b/docker-compose.yml
index 35458aee..c2c3dfcc 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -8,6 +8,7 @@ services:
       - .env
     environment:
       CLINICAL_ASSISTANT_MCP_URL: http://mcp:8000/mcp
+      CLINICAL_ASSISTANT_SOURCE_FILE_PATH: Medical Library/Pediatrics/Kliegman%20R.%20Nelson%20Textbook%20of%20Pediatrics%20%202-Volume%20Set%2022ed%202024.pdf
       REDIS_URL: redis://ped-ai-redis:6379
       LOKI_URL: http://monitoring-loki:3100
       LITELLM_API_BASE: http://litellm:4000
diff --git a/src/routes/clinicalAssistant.js b/src/routes/clinicalAssistant.js
index 9e45e658..602fe33b 100644
--- a/src/routes/clinicalAssistant.js
+++ b/src/routes/clinicalAssistant.js
@@ -28,6 +28,7 @@ var {
 var {
   normalizeMcpSearchResponse,
   normalizeMcpMultimodalResponse,
+  filterSourcesByFilePath,
   dedupeSources,
   isVisualSourceQuery,
   buildMultimodalSearchQuery,
@@ -48,6 +49,7 @@ var MAX_SAVED_CHATS_PER_USER = 100;
 var MAX_SAVED_CHAT_PAYLOAD = 250000;
 var MAX_SAVED_CHAT_TITLE = 160;
 var IMAGE_JOB_TTL_SECONDS = 15 * 60;
+var SOURCE_LOCK_SEARCH_LIMIT = 50;
 var imageJobs = new Map();
 var promptPool = createClinicalPromptPool({
   redisCache: redisCache,
@@ -375,6 +377,7 @@ async function prepareAssistantChat(body) {
   var contextChars = clampInt(await getSetting('clinical_assistant.context_chars', '1400'), 300, 4000, 1400);
   var behavior = await getSetting('clinical_assistant.system_behavior', DEFAULT_BEHAVIOR) || DEFAULT_BEHAVIOR;
   var includeContext = body.includeContext !== false;
+  var sourceFilePath = String(process.env.CLINICAL_ASSISTANT_SOURCE_FILE_PATH || '').trim();
 
   if (GREETING_RE.test(message)) {
     return { direct: {
@@ -393,7 +396,8 @@ async function prepareAssistantChat(body) {
     return message;
   });
   var searchResponse = await semanticSearch(searchQuery, {
-    limit: searchLimit,
+    // MCP has no source-path filter. Retrieve a larger candidate set, then enforce the lock locally.
+    limit: sourceFilePath ? SOURCE_LOCK_SEARCH_LIMIT : searchLimit,
     includeContext: includeContext,
     contextChars: contextChars
   });
@@ -402,10 +406,10 @@ async function prepareAssistantChat(body) {
     console.warn('[clinical-assistant] multimodal search skipped:', e.message);
     return null;
   }) : null;
-  var rawTextResults = normalizeMcpSearchResponse(searchResponse);
+  var rawTextResults = filterSourcesByFilePath(normalizeMcpSearchResponse(searchResponse), sourceFilePath);
   var rawMultimodalResults = await classifyAndRerankMultimodalResults(
     message + ' ' + searchQuery,
-    normalizeMcpMultimodalResponse(multimodalResponse)
+    filterSourcesByFilePath(normalizeMcpMultimodalResponse(multimodalResponse), sourceFilePath)
   );
   var rawResults = rawTextResults.concat(rawMultimodalResults);
   console.info('[clinical-assistant] retrieval counts:', {
@@ -446,7 +450,8 @@ async function prepareAssistantChat(body) {
       query: searchQuery,
       rewritten: searchQuery !== message,
       verifiedChunkCount: searchResponse.verified_chunk_count || searchResponse.verifiedChunkCount || 0,
-      droppedDocumentCount: searchResponse.dropped_document_count || searchResponse.droppedDocumentCount || 0
+      droppedDocumentCount: searchResponse.dropped_document_count || searchResponse.droppedDocumentCount || 0,
+      sourceLocked: Boolean(sourceFilePath)
     }
   };
 }
diff --git a/src/utils/clinicalRetrieval.js b/src/utils/clinicalRetrieval.js
index 6eedf1a4..eea2e521 100644
--- a/src/utils/clinicalRetrieval.js
+++ b/src/utils/clinicalRetrieval.js
@@ -89,6 +89,18 @@ function normalizeMcpMultimodalResponse(result) {
   }).filter(function(r) { return r.page; });
 }
 
+function filterSourcesByFilePath(results, filePath) {
+  if (!filePath) return results;
+  var wanted = normalizeFilePath(filePath);
+  return results.filter(function(result) {
+    return normalizeFilePath(result && result.file_path) === wanted;
+  });
+}
+
+function normalizeFilePath(filePath) {
+  try { return decodeURIComponent(String(filePath || '')).replace(/^\/+/, ''); } catch (e) { return String(filePath || '').replace(/^\/+/, ''); }
+}
+
 function dedupeSources(results) {
   var seen = new Map();
   var out = [];
@@ -271,6 +283,7 @@ function cleanSourceExcerpt(text) {
 module.exports = {
   normalizeMcpSearchResponse: normalizeMcpSearchResponse,
   normalizeMcpMultimodalResponse: normalizeMcpMultimodalResponse,
+  filterSourcesByFilePath: filterSourcesByFilePath,
   dedupeSources: dedupeSources,
   isVisualSourceQuery: isVisualSourceQuery,
   buildMultimodalSearchQuery: buildMultimodalSearchQuery,
diff --git a/test/clinical-retrieval-source-lock.test.js b/test/clinical-retrieval-source-lock.test.js
new file mode 100644
index 00000000..342fffe7
--- /dev/null
+++ b/test/clinical-retrieval-source-lock.test.js
@@ -0,0 +1,13 @@
+const test = require('node:test');
+const assert = require('node:assert/strict');
+
+const { filterSourcesByFilePath } = require('../src/utils/clinicalRetrieval');
+
+test('source lock keeps only the selected book regardless of URL encoding', () => {
+  const path = 'Medical Library/Pediatrics/Kliegman R. Nelson Textbook of Pediatrics  2-Volume Set 22ed 2024.pdf';
+  const sources = [
+    { title: 'Nelson', file_path: 'Medical Library/Pediatrics/Kliegman%20R.%20Nelson%20Textbook%20of%20Pediatrics%20%202-Volume%20Set%2022ed%202024.pdf' },
+    { title: 'Other book', file_path: 'Medical Library/Pediatrics/Other.pdf' }
+  ];
+  assert.deepEqual(filterSourcesByFilePath(sources, path), [sources[0]]);
+});