add second layer of result validation

backend/package-lock.json

@@ -11,7 +11,8 @@
                 "@anthropic-ai/sdk": "^0.74.0",
                 "cors": "^2.8.5",
                 "dotenv": "^16.4.7",
-                "express": "^4.21.2"
+                "express": "^4.21.2",
+                "voyageai": "^0.1.0"
             },
             "devDependencies": {
                 "nodemon": "^3.1.9",
@@ -1013,6 +1014,18 @@
                 "url": "https://opencollective.com/vitest"
             }
         },
+        "node_modules/abort-controller": {
+            "version": "3.0.0",
+            "resolved": "https://registry.npmjs.org/abort-controller/-/abort-controller-3.0.0.tgz",
+            "integrity": "sha512-h8lQ8tacZYnR3vNQTgibj+tODHI5/+l06Au2Pcriv/Gmet0eaj4TwWH41sO9wnHDiQsEj19q0drzdWdeAHtweg==",
+            "license": "MIT",
+            "dependencies": {
+                "event-target-shim": "^5.0.0"
+            },
+            "engines": {
+                "node": ">=6.5"
+            }
+        },
         "node_modules/accepts": {
             "version": "1.3.8",
             "resolved": "https://registry.npmjs.org/accepts/-/accepts-1.3.8.tgz",
@@ -1067,7 +1080,6 @@
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-            "dev": true,
             "license": "MIT"
         },
         "node_modules/balanced-match": {
@@ -1077,6 +1089,26 @@
             "dev": true,
             "license": "MIT"
         },
+        "node_modules/base64-js": {
+            "version": "1.5.1",
+            "resolved": "https://registry.npmjs.org/base64-js/-/base64-js-1.5.1.tgz",
+            "integrity": "sha512-AKpaYlHn8t4SVbOHCy+b5+KKgvR4vrsD8vbvrbiQJps7fKDTkjkDry6ji0rUJjC0kzbNePLwzxq8iypo41qeWA==",
+            "funding": [
+                {
+                    "type": "github",
+                    "url": "https://github.com/sponsors/feross"
+                },
+                {
+                    "type": "patreon",
+                    "url": "https://www.patreon.com/feross"
+                },
+                {
+                    "type": "consulting",
+                    "url": "https://feross.org/support"
+                }
+            ],
+            "license": "MIT"
+        },
         "node_modules/binary-extensions": {
             "version": "2.3.0",
             "resolved": "https://registry.npmjs.org/binary-extensions/-/binary-extensions-2.3.0.tgz",
@@ -1138,6 +1170,30 @@
                 "node": ">=8"
             }
         },
+        "node_modules/buffer": {
+            "version": "6.0.3",
+            "resolved": "https://registry.npmjs.org/buffer/-/buffer-6.0.3.tgz",
+            "integrity": "sha512-FTiCpNxtwiZZHEZbcbTIcZjERVICn9yq/pDFkTl95/AxzD1naBctN7YO68riM/gLSDY7sdrMby8hofADYuuqOA==",
+            "funding": [
+                {
+                    "type": "github",
+                    "url": "https://github.com/sponsors/feross"
+                },
+                {
+                    "type": "patreon",
+                    "url": "https://www.patreon.com/feross"
+                },
+                {
+                    "type": "consulting",
+                    "url": "https://feross.org/support"
+                }
+            ],
+            "license": "MIT",
+            "dependencies": {
+                "base64-js": "^1.3.1",
+                "ieee754": "^1.2.1"
+            }
+        },
         "node_modules/bytes": {
             "version": "3.1.2",
             "resolved": "https://registry.npmjs.org/bytes/-/bytes-3.1.2.tgz",
@@ -1215,7 +1271,6 @@
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-            "dev": true,
             "license": "MIT",
             "dependencies": {
                 "delayed-stream": "~1.0.0"
@@ -1314,7 +1369,6 @@
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             "resolved": "https://registry.npmjs.org/delayed-stream/-/delayed-stream-1.0.0.tgz",
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-            "dev": true,
             "license": "MIT",
             "engines": {
                 "node": ">=0.4.0"
@@ -1432,7 +1486,6 @@
             "version": "2.1.0",
             "resolved": "https://registry.npmjs.org/es-set-tostringtag/-/es-set-tostringtag-2.1.0.tgz",
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-            "dev": true,
             "license": "MIT",
             "dependencies": {
                 "es-errors": "^1.3.0",
@@ -1511,6 +1564,24 @@
                 "node": ">= 0.6"
             }
         },
+        "node_modules/event-target-shim": {
+            "version": "5.0.1",
+            "resolved": "https://registry.npmjs.org/event-target-shim/-/event-target-shim-5.0.1.tgz",
+            "integrity": "sha512-i/2XbnSz/uxRCU6+NdVJgKWDTM427+MqYbkQzD321DuCQJUqOuJKIA0IM2+W2xtYHdKOmZ4dR6fExsd4SXL+WQ==",
+            "license": "MIT",
+            "engines": {
+                "node": ">=6"
+            }
+        },
+        "node_modules/events": {
+            "version": "3.3.0",
+            "resolved": "https://registry.npmjs.org/events/-/events-3.3.0.tgz",
+            "integrity": "sha512-mQw+2fkQbALzQ7V0MY0IqdnXNOeTtP4r0lN9z7AAawCXgqea7bDii20AYrIBrFd/Hx0M2Ocz6S111CaFkUcb0Q==",
+            "license": "MIT",
+            "engines": {
+                "node": ">=0.8.x"
+            }
+        },
         "node_modules/expect-type": {
             "version": "1.3.0",
             "resolved": "https://registry.npmjs.org/expect-type/-/expect-type-1.3.0.tgz",
@@ -1609,7 +1680,6 @@
             "version": "4.0.5",
             "resolved": "https://registry.npmjs.org/form-data/-/form-data-4.0.5.tgz",
             "integrity": "sha512-8RipRLol37bNs2bhoV67fiTEvdTrbMUYcFTiy3+wuuOnUog2QBHCZWXDRijWQfAkhBj2Uf5UnVaiWwA5vdd82w==",
-            "dev": true,
             "license": "MIT",
             "dependencies": {
                 "asynckit": "^0.4.0",
@@ -1622,6 +1692,15 @@
                 "node": ">= 6"
             }
         },
+        "node_modules/formdata-node": {
+            "version": "6.0.3",
+            "resolved": "https://registry.npmjs.org/formdata-node/-/formdata-node-6.0.3.tgz",
+            "integrity": "sha512-8e1++BCiTzUno9v5IZ2J6bv4RU+3UKDmqWUQD0MIMVCd9AdhWkO1gw57oo1mNEX1dMq2EGI+FbWz4B92pscSQg==",
+            "license": "MIT",
+            "engines": {
+                "node": ">= 18"
+            }
+        },
         "node_modules/formidable": {
             "version": "3.5.4",
             "resolved": "https://registry.npmjs.org/formidable/-/formidable-3.5.4.tgz",
@@ -1770,7 +1849,6 @@
             "version": "1.0.2",
             "resolved": "https://registry.npmjs.org/has-tostringtag/-/has-tostringtag-1.0.2.tgz",
             "integrity": "sha512-NqADB8VjPFLM2V0VvHUewwwsw0ZWBaIdgo+ieHtK3hasLz4qeCRjYcqfB6AQrBggRKppKF8L52/VqdVsO47Dlw==",
-            "dev": true,
             "license": "MIT",
             "dependencies": {
                 "has-symbols": "^1.0.3"
@@ -1826,6 +1904,26 @@
                 "node": ">=0.10.0"
             }
         },
+        "node_modules/ieee754": {
+            "version": "1.2.1",
+            "resolved": "https://registry.npmjs.org/ieee754/-/ieee754-1.2.1.tgz",
+            "integrity": "sha512-dcyqhDvX1C46lXZcVqCpK+FtMRQVdIMN6/Df5js2zouUsqG7I6sFxitIC+7KYK29KdXOLHdu9zL4sFnoVQnqaA==",
+            "funding": [
+                {
+                    "type": "github",
+                    "url": "https://github.com/sponsors/feross"
+                },
+                {
+                    "type": "patreon",
+                    "url": "https://www.patreon.com/feross"
+                },
+                {
+                    "type": "consulting",
+                    "url": "https://feross.org/support"
+                }
+            ],
+            "license": "BSD-3-Clause"
+        },
         "node_modules/ignore-by-default": {
             "version": "1.0.1",
             "resolved": "https://registry.npmjs.org/ignore-by-default/-/ignore-by-default-1.0.1.tgz",
@@ -1894,6 +1992,12 @@
                 "node": ">=0.12.0"
             }
         },
+        "node_modules/js-base64": {
+            "version": "3.7.2",
+            "resolved": "https://registry.npmjs.org/js-base64/-/js-base64-3.7.2.tgz",
+            "integrity": "sha512-NnRs6dsyqUXejqk/yv2aiXlAvOs56sLkX6nUdeaNezI5LFFLlsZjOThmwnrcwh5ZZRwZlCMnVAY3CvhIhoVEKQ==",
+            "license": "BSD-3-Clause"
+        },
         "node_modules/json-schema-to-ts": {
             "version": "3.1.1",
             "resolved": "https://registry.npmjs.org/json-schema-to-ts/-/json-schema-to-ts-3.1.1.tgz",
@@ -2033,6 +2137,26 @@
                 "node": ">= 0.6"
             }
         },
+        "node_modules/node-fetch": {
+            "version": "2.7.0",
+            "resolved": "https://registry.npmjs.org/node-fetch/-/node-fetch-2.7.0.tgz",
+            "integrity": "sha512-c4FRfUm/dbcWZ7U+1Wq0AwCyFL+3nt2bEw05wfxSz+DWpWsitgmSgYmy2dQdWyKC1694ELPqMs/YzUSNozLt8A==",
+            "license": "MIT",
+            "dependencies": {
+                "whatwg-url": "^5.0.0"
+            },
+            "engines": {
+                "node": "4.x || >=6.0.0"
+            },
+            "peerDependencies": {
+                "encoding": "^0.1.0"
+            },
+            "peerDependenciesMeta": {
+                "encoding": {
+                    "optional": true
+                }
+            }
+        },
         "node_modules/nodemon": {
             "version": "3.1.11",
             "resolved": "https://registry.npmjs.org/nodemon/-/nodemon-3.1.11.tgz",
@@ -2222,6 +2346,15 @@
                 "node": "^10 || ^12 || >=14"
             }
         },
+        "node_modules/process": {
+            "version": "0.11.10",
+            "resolved": "https://registry.npmjs.org/process/-/process-0.11.10.tgz",
+            "integrity": "sha512-cdGef/drWFoydD1JsMzuFf8100nZl+GT+yacc2bEced5f9Rjk4z+WtFUTBu9PhOi9j/jfmBPu0mMEY4wIdAF8A==",
+            "license": "MIT",
+            "engines": {
+                "node": ">= 0.6.0"
+            }
+        },
         "node_modules/proxy-addr": {
             "version": "2.0.7",
             "resolved": "https://registry.npmjs.org/proxy-addr/-/proxy-addr-2.0.7.tgz",
@@ -2281,6 +2414,22 @@
                 "node": ">= 0.8"
             }
         },
+        "node_modules/readable-stream": {
+            "version": "4.7.0",
+            "resolved": "https://registry.npmjs.org/readable-stream/-/readable-stream-4.7.0.tgz",
+            "integrity": "sha512-oIGGmcpTLwPga8Bn6/Z75SVaH1z5dUut2ibSyAMVhmUggWpmDn2dapB0n7f8nwaSiRtepAsfJyfXIO5DCVAODg==",
+            "license": "MIT",
+            "dependencies": {
+                "abort-controller": "^3.0.0",
+                "buffer": "^6.0.3",
+                "events": "^3.3.0",
+                "process": "^0.11.10",
+                "string_decoder": "^1.3.0"
+            },
+            "engines": {
+                "node": "^12.22.0 || ^14.17.0 || >=16.0.0"
+            }
+        },
         "node_modules/readdirp": {
             "version": "3.6.0",
             "resolved": "https://registry.npmjs.org/readdirp/-/readdirp-3.6.0.tgz",
@@ -2554,6 +2703,15 @@
             "dev": true,
             "license": "MIT"
         },
+        "node_modules/string_decoder": {
+            "version": "1.3.0",
+            "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.3.0.tgz",
+            "integrity": "sha512-hkRX8U1WjJFd8LsDJ2yQ/wWWxaopEsABU1XfkM8A+j0+85JAGppt16cr1Whg6KIbb4okU6Mql6BOj+uup/wKeA==",
+            "license": "MIT",
+            "dependencies": {
+                "safe-buffer": "~5.2.0"
+            }
+        },
         "node_modules/superagent": {
             "version": "10.3.0",
             "resolved": "https://registry.npmjs.org/superagent/-/superagent-10.3.0.tgz",
@@ -2759,6 +2917,12 @@
                 "nodetouch": "bin/nodetouch.js"
             }
         },
+        "node_modules/tr46": {
+            "version": "0.0.3",
+            "resolved": "https://registry.npmjs.org/tr46/-/tr46-0.0.3.tgz",
+            "integrity": "sha512-N3WMsuqV66lT30CrXNbEjx4GEwlow3v6rr4mCcv6prnfwhS01rkgyFdjPNBYd9br7LpXV1+Emh01fHnq2Gdgrw==",
+            "license": "MIT"
+        },
         "node_modules/ts-algebra": {
             "version": "2.0.0",
             "resolved": "https://registry.npmjs.org/ts-algebra/-/ts-algebra-2.0.0.tgz",
@@ -2794,6 +2958,12 @@
                 "node": ">= 0.8"
             }
         },
+        "node_modules/url-join": {
+            "version": "4.0.1",
+            "resolved": "https://registry.npmjs.org/url-join/-/url-join-4.0.1.tgz",
+            "integrity": "sha512-jk1+QP6ZJqyOiuEI9AEWQfju/nB2Pw466kbA0LEZljHwKeMgd9WrAEgEGxjPDD2+TNbbb37rTyhEfrCXfuKXnA==",
+            "license": "MIT"
+        },
         "node_modules/utils-merge": {
             "version": "1.0.1",
             "resolved": "https://registry.npmjs.org/utils-merge/-/utils-merge-1.0.1.tgz",
@@ -3011,6 +3181,52 @@
                 "url": "https://github.com/sponsors/jonschlinkert"
             }
         },
+        "node_modules/voyageai": {
+            "version": "0.1.0",
+            "resolved": "https://registry.npmjs.org/voyageai/-/voyageai-0.1.0.tgz",
+            "integrity": "sha512-QnalSSHRigZqzKJpXLsgp3tKaKkSv2kGO4rcXQ6mr6PKHa0vRirs0jZ72olvFJrGJkbu1hHijcyr6/eI4L5aRw==",
+            "license": "MIT",
+            "dependencies": {
+                "form-data": "^4.0.0",
+                "formdata-node": "^6.0.3",
+                "js-base64": "3.7.2",
+                "node-fetch": "2.7.0",
+                "qs": "6.11.2",
+                "readable-stream": "^4.5.2",
+                "url-join": "4.0.1"
+            }
+        },
+        "node_modules/voyageai/node_modules/qs": {
+            "version": "6.11.2",
+            "resolved": "https://registry.npmjs.org/qs/-/qs-6.11.2.tgz",
+            "integrity": "sha512-tDNIz22aBzCDxLtVH++VnTfzxlfeK5CbqohpSqpJgj1Wg/cQbStNAz3NuqCs5vV+pjBsK4x4pN9HlVh7rcYRiA==",
+            "license": "BSD-3-Clause",
+            "dependencies": {
+                "side-channel": "^1.0.4"
+            },
+            "engines": {
+                "node": ">=0.6"
+            },
+            "funding": {
+                "url": "https://github.com/sponsors/ljharb"
+            }
+        },
+        "node_modules/webidl-conversions": {
+            "version": "3.0.1",
+            "resolved": "https://registry.npmjs.org/webidl-conversions/-/webidl-conversions-3.0.1.tgz",
+            "integrity": "sha512-2JAn3z8AR6rjK8Sm8orRC0h/bcl/DqL7tRPdGZ4I1CjdF+EaMLmYxBHyXuKL849eucPFhvBoxMsflfOb8kxaeQ==",
+            "license": "BSD-2-Clause"
+        },
+        "node_modules/whatwg-url": {
+            "version": "5.0.0",
+            "resolved": "https://registry.npmjs.org/whatwg-url/-/whatwg-url-5.0.0.tgz",
+            "integrity": "sha512-saE57nupxk6v3HY35+jzBwYa0rKSy0XR8JSxZPwgLr7ys0IBzhGviA1/TUGJLmSVqs8pb9AnvICXEuOHLprYTw==",
+            "license": "MIT",
+            "dependencies": {
+                "tr46": "~0.0.3",
+                "webidl-conversions": "^3.0.0"
+            }
+        },
         "node_modules/why-is-node-running": {
             "version": "2.3.0",
             "resolved": "https://registry.npmjs.org/why-is-node-running/-/why-is-node-running-2.3.0.tgz",

backend/package.json

@@ -13,7 +13,8 @@
         "@anthropic-ai/sdk": "^0.74.0",
         "cors": "^2.8.5",
         "dotenv": "^16.4.7",
-        "express": "^4.21.2"
+        "express": "^4.21.2",
+        "voyageai": "^0.1.0"
     },
     "devDependencies": {
         "nodemon": "^3.1.9",

backend/routes/notes.routes.js

@@ -23,8 +23,8 @@ router.get('/', async (req, res) => {
 router.post('/cluster', async (req, res) => {
   try {
     const notes = await loadNotes();
-    const clusters = await clusterNotes(notes);
+    const result = await clusterNotes(notes);
-    res.json(clusters);
+    res.json(result);
   } catch (err) {
     console.error(`Clustering failed: ${err}`);
     res.status(500).json({ error: 'Clustering failed' });

backend/services/clustering.service.js

@@ -1,4 +1,6 @@
 import Anthropic from "@anthropic-ai/sdk";
+import { embedNotes } from "./embedding.service.js";
+import { validateStructure, computeCohesionScore } from "./validation.service.js";
 
 const client = new Anthropic({
   apiKey: process.env.ANTHROPIC_API_KEY,
@@ -31,7 +33,7 @@ Here are the notes:
 ${notesJson}`;
 };
 
-export const clusterNotes = async (notes) => {
+const requestClusters = async (notes) => {
   const response = await client.messages.create({
     model: "claude-sonnet-4-20250514",
     max_tokens: 4096,
@@ -52,3 +54,20 @@ export const clusterNotes = async (notes) => {
     throw new Error('LLM API returned non-JSON response');
   }
 };
+
+export const clusterNotes = async (notes) => {
+  const [clusters, embeddingMap] = await Promise.all([
+    requestClusters(notes),
+    embedNotes(notes),
+  ]);
+
+  const noteIds = notes.map((n) => n.id);
+  const { valid, reasons } = validateStructure(clusters, noteIds);
+  if (!valid) {
+    throw new Error(`Cluster validation failed: ${reasons.join('; ')}`);
+  }
+
+  const score = computeCohesionScore(clusters, embeddingMap);
+
+  return { clusters, score: Math.round(score * 100) / 100 };
+};

backend/services/embedding.service.js

@@ -0,0 +1,25 @@
+import { VoyageAIClient } from "voyageai";
+
+const client = new VoyageAIClient({
+  apiKey: process.env.VOYAGEAI_API_KEY,
+});
+
+/**
+ * @param {Array<{id: string, text: string}>} notes
+ * @returns {Promise<Map<string, number[]>>} noteId → embedding vector
+ */
+export const embedNotes = async (notes) => {
+  const texts = notes.map((n) => n.text);
+
+  const response = await client.embed({
+    input: texts,
+    model: "voyage-3-lite",
+  });
+
+  const embeddingMap = new Map();
+  response.data.forEach((item, i) => {
+    embeddingMap.set(notes[i].id, item.embedding);
+  });
+
+  return embeddingMap;
+};

backend/services/validation.service.js

@@ -0,0 +1,129 @@
+/**
+ * Structural validation: confirms the LLM output is well-formed
+ * before it reaches the frontend.
+ *
+ * @param {Array<{label: string, noteIds: string[]}>} clusters
+ * @param {string[]} inputNoteIds - the original note IDs that were sent to the LLM
+ * @returns {{valid: boolean, reasons: string[]}}
+ */
+export const validateStructure = (clusters, inputNoteIds) => {
+  const reasons = [];
+
+  if (!Array.isArray(clusters) || clusters.length === 0) {
+    return { valid: false, reasons: ['Response is not a non-empty array'] };
+  }
+
+  const assignedIds = [];
+  for (const cluster of clusters) {
+    if (!cluster.label || typeof cluster.label !== 'string') {
+      reasons.push(`Cluster missing a valid label`);
+    }
+    if (!Array.isArray(cluster.noteIds) || cluster.noteIds.length === 0) {
+      reasons.push(`Cluster "${cluster.label ?? '(unlabeled)'}" has no noteIds`);
+    }
+    assignedIds.push(...(cluster.noteIds ?? []));
+  }
+
+  const inputSet = new Set(inputNoteIds);
+  const assignedSet = new Set(assignedIds);
+
+  if (assignedIds.length !== assignedSet.size) {
+    reasons.push('One or more notes appear in multiple clusters');
+  }
+
+  const missing = inputNoteIds.filter((id) => !assignedSet.has(id));
+  if (missing.length > 0) {
+    reasons.push(`Notes missing from clusters: ${missing.join(', ')}`);
+  }
+
+  const extra = assignedIds.filter((id) => !inputSet.has(id));
+  if (extra.length > 0) {
+    reasons.push(`Unknown noteIds in clusters: ${[...new Set(extra)].join(', ')}`);
+  }
+
+  if (clusters.length > inputNoteIds.length) {
+    reasons.push(`More clusters (${clusters.length}) than notes (${inputNoteIds.length})`);
+  }
+
+  return { valid: reasons.length === 0, reasons };
+};
+
+const cosineSimilarity = (a, b) => {
+  let dot = 0;
+  let magA = 0;
+  let magB = 0;
+  for (let i = 0; i < a.length; i++) {
+    dot += a[i] * b[i];
+    magA += a[i] * a[i];
+    magB += b[i] * b[i];
+  }
+  const denom = Math.sqrt(magA) * Math.sqrt(magB);
+  return denom === 0 ? 0 : dot / denom;
+};
+
+/**
+ * Computes a silhouette-style cohesion score for the clustering.
+ *
+ * For each note, measures how much more similar it is to its own cluster
+ * versus the nearest neighboring cluster. Returns a score in [-1, 1]
+ * where higher is better.
+ *
+ * @param {Array<{label: string, noteIds: string[]}>} clusters
+ * @param {Map<string, number[]>} embeddingMap - noteId → vector
+ * @returns {number} average silhouette score
+ */
+export const computeCohesionScore = (clusters, embeddingMap) => {
+  if (clusters.length <= 1) return 1.0;
+
+  const scores = [];
+
+  for (let ci = 0; ci < clusters.length; ci++) {
+    const clusterIds = clusters[ci].noteIds;
+    if (clusterIds.length <= 1) {
+      scores.push(0);
+      continue;
+    }
+
+    for (const noteId of clusterIds) {
+      const vec = embeddingMap.get(noteId);
+      if (!vec) continue;
+
+      // a(i): avg distance to other notes in same cluster
+      let intraSum = 0;
+      let intraCount = 0;
+      for (const otherId of clusterIds) {
+        if (otherId === noteId) continue;
+        const otherVec = embeddingMap.get(otherId);
+        if (!otherVec) continue;
+        intraSum += 1 - cosineSimilarity(vec, otherVec);
+        intraCount++;
+      }
+      const a = intraCount > 0 ? intraSum / intraCount : 0;
+
+      // b(i): min avg distance to notes in any other cluster
+      let b = Infinity;
+      for (let oi = 0; oi < clusters.length; oi++) {
+        if (oi === ci) continue;
+        const otherClusterIds = clusters[oi].noteIds;
+        let interSum = 0;
+        let interCount = 0;
+        for (const otherId of otherClusterIds) {
+          const otherVec = embeddingMap.get(otherId);
+          if (!otherVec) continue;
+          interSum += 1 - cosineSimilarity(vec, otherVec);
+          interCount++;
+        }
+        if (interCount > 0) {
+          b = Math.min(b, interSum / interCount);
+        }
+      }
+      if (b === Infinity) b = 0;
+
+      const max = Math.max(a, b);
+      scores.push(max === 0 ? 0 : (b - a) / max);
+    }
+  }
+
+  if (scores.length === 0) return 0;
+  return scores.reduce((sum, s) => sum + s, 0) / scores.length;
+};

backend/tests/Clustering.service.test.js

@@ -1,7 +1,11 @@
 import { describe, it, expect, vi, beforeEach } from 'vitest';
 
-const { createMock } = vi.hoisted(() => {
+const { createMock, mockEmbeddings } = vi.hoisted(() => {
-  return { createMock: vi.fn() };
+  const embeddings = new Map([
+    ['note_001', [1.0, 0.0, 0.0]],
+    ['note_002', [0.0, 1.0, 0.0]],
+  ]);
+  return { createMock: vi.fn(), mockEmbeddings: embeddings };
 });
 
 vi.mock('@anthropic-ai/sdk', () => {
@@ -14,6 +18,10 @@ vi.mock('@anthropic-ai/sdk', () => {
   };
 });
 
+vi.mock('../services/embedding.service.js', () => ({
+  embedNotes: vi.fn().mockResolvedValue(mockEmbeddings),
+}));
+
 import { clusterNotes } from '../services/clustering.service.js';
 
 const MOCK_NOTES = [
@@ -60,14 +68,17 @@ describe('clusterNotes service', () => {
     expect(prompt).toContain('Export fails');
   });
 
-  it('should parse and return the clustered JSON from the API response', async () => {
+  it('should return clusters and a cohesion score', async () => {
     createMock.mockResolvedValue({
       content: [{ type: 'text', text: JSON.stringify(MOCK_CLUSTERS) }],
     });
 
     const result = await clusterNotes(MOCK_NOTES);
 
-    expect(result).toEqual(MOCK_CLUSTERS);
+    expect(result.clusters).toEqual(MOCK_CLUSTERS);
+    expect(typeof result.score).toBe('number');
+    expect(result.score).toBeGreaterThanOrEqual(-1);
+    expect(result.score).toBeLessThanOrEqual(1);
   });
 
   it('should throw error when the API returns non-JSON', async () => {
@@ -89,4 +100,15 @@ describe('clusterNotes service', () => {
 
     await expect(clusterNotes(MOCK_NOTES)).rejects.toThrow('401 Unauthorized');
   });
+
+  it('should throw a validation error when a note is missing from clusters', async () => {
+    const incompleteClusters = [
+      { label: 'Auth Issues', noteIds: ['note_001'] },
+    ];
+    createMock.mockResolvedValue({
+      content: [{ type: 'text', text: JSON.stringify(incompleteClusters) }],
+    });
+
+    await expect(clusterNotes(MOCK_NOTES)).rejects.toThrow('Cluster validation failed');
+  });
 });

frontend/src/api/api.ts

@@ -1,5 +1,5 @@
 import { useQuery, useMutation } from '@tanstack/react-query';
-import type { Sticky, Cluster } from '../types/types';
+import type { Sticky, ClusterResponse } from '../types/types';
 
 const API_BASE = `${import.meta.env.VITE_API_BASE}/v1/notes`;
 
@@ -11,7 +11,7 @@ const fetchStickies = async (): Promise<Sticky[]> => {
   return response.json();
 };
 
-const fetchClusters = async (): Promise<Cluster[]> => {
+const fetchClusters = async (): Promise<ClusterResponse> => {
   const response = await fetch(`${API_BASE}/cluster`, {
     method: 'POST',
   });
@@ -29,7 +29,7 @@ export const useGetStickies = () => {
 };
 
 export const useClusterStickies = () => {
-  return useMutation<Cluster[]>({
+  return useMutation<ClusterResponse>({
     mutationFn: fetchClusters,
   });
 };

frontend/src/components/stickies.tsx

@@ -4,12 +4,22 @@ import Sticky from './sticky';
 import Button from './button';
 import '../styles/stickies.css';
 
+const scoreLabel = (score: number): string => {
+  if (score >= 0.7) return 'Strong';
+  if (score >= 0.4) return 'Moderate';
+  if (score >= 0.1) return 'Weak';
+  return 'Poor';
+};
+
 const Stickies = () => {
   const { data: stickies, isLoading, error } = useGetStickies();
-  const { mutate: cluster, data: clusters, isPending } = useClusterStickies();
+  const { mutate: cluster, data: clusterResponse, isPending } = useClusterStickies();
   if (isLoading) return <div>Loading...</div>;
   if (error) return <div>Error: {error.message}</div>;
 
+  const clusters = clusterResponse?.clusters;
+  const score = clusterResponse?.score;
+
   const handleCluster = () => {
     cluster();
   };
@@ -30,6 +40,11 @@ const Stickies = () => {
       <Button onClick={handleCluster} isLoading={isPending} label="Group Stickies By Topic" />
       {clusters ? (
         <div className="clusters-container">
+          {score != null && (
+            <div className="cohesion-score">
+              Cluster cohesion: <strong>{score.toFixed(2)}</strong> — {scoreLabel(score)}
+            </div>
+          )}
           {clusters.map((group) => (
             <div key={group.label} className="cluster-group">
               <h3 className="cluster-label">{group.label}</h3>

frontend/src/styles/stickies.css

@@ -29,3 +29,14 @@
   font-size: 1.2em;
   font-weight: 600;
 }
+
+.cohesion-score {
+  text-align: center;
+  font-size: 0.95em;
+  color: #e0e0e0;
+  padding: 8px 16px;
+  background: rgba(109, 214, 244, 0.1);
+  border-radius: 6px;
+  width: fit-content;
+  margin: 0 auto;
+}

frontend/src/tests/stickies.test.tsx

@@ -9,10 +9,13 @@ const MOCK_STICKIES = [
   { id: 'note_002', text: 'Export takes too long', x: 798, y: 211, author: 'user_2', color: 'green' },
 ];
 
-const MOCK_CLUSTERS = [
+const MOCK_CLUSTER_RESPONSE = {
-  { label: 'Auth Issues', noteIds: ['note_001'] },
+  clusters: [
-  { label: 'Export Issues', noteIds: ['note_002'] },
+    { label: 'Auth Issues', noteIds: ['note_001'] },
-];
+    { label: 'Export Issues', noteIds: ['note_002'] },
+  ],
+  score: 0.74,
+};
 
 let fetchMock: ReturnType<typeof vi.fn>;
 
@@ -73,7 +76,7 @@ describe('Stickies', () => {
     // First call = GET notes, second call = POST cluster
     fetchMock
       .mockReturnValueOnce(mockFetchOk(MOCK_STICKIES))
-      .mockReturnValueOnce(mockFetchOk(MOCK_CLUSTERS));
+      .mockReturnValueOnce(mockFetchOk(MOCK_CLUSTER_RESPONSE));
 
     const { Wrapper } = createTestWrapper();
     render(<Stickies />, { wrapper: Wrapper });
@@ -90,7 +93,7 @@ describe('Stickies', () => {
   it('should still render sticky note text inside clusters', async () => {
     fetchMock
       .mockReturnValueOnce(mockFetchOk(MOCK_STICKIES))
-      .mockReturnValueOnce(mockFetchOk(MOCK_CLUSTERS));
+      .mockReturnValueOnce(mockFetchOk(MOCK_CLUSTER_RESPONSE));
 
     const { Wrapper } = createTestWrapper();
     render(<Stickies />, { wrapper: Wrapper });

frontend/src/types/types.d.ts

@@ -11,3 +11,8 @@ export type Cluster = {
   label: string;
   noteIds: string[];
 };
+
+export type ClusterResponse = {
+  clusters: Cluster[];
+  score: number;
+};