diff --git a/extensions/copilot/src/platform/telemetry/common/telemetry.ts b/extensions/copilot/src/platform/telemetry/common/telemetry.ts
index 07e43a833ff..a19a044f29c 100644
--- a/extensions/copilot/src/platform/telemetry/common/telemetry.ts
+++ b/extensions/copilot/src/platform/telemetry/common/telemetry.ts
@@ -219,6 +219,11 @@ const COMPRESSED_CHUNK_SUFFIX = 'Chunk';
 // value from instead of having to branch on whether the value happened to be chunked.
 const ALWAYS_COMPRESSED_CHUNK_KEYS = new Set<string>(['messagesJson', 'diffsJSON']);
 
+// Overrides for the base name of the compressed chunk family. By default the chunk family is named
+// `<key>Chunk`, but some backend mappings expect a different casing. For `messagesJson` the backend
+// expects `messagesJSONChunk` (uppercased JSON), while the original `<key>` column keeps its casing.
+const COMPRESSED_CHUNK_KEY_OVERRIDES: { readonly [key: string]: string } = { messagesJson: 'messagesJSON' };
+
 // Compressor used by multiplexProperties to gzip + base64 encode oversized property values. It is
 // registered once by the Node layer (via setTelemetryPropertyCompressor) because Node's `zlib` is
 // unavailable in the common layer; until then multiplexProperties falls back to plain chunking. It
@@ -273,8 +278,9 @@ export async function multiplexProperties(
 			// padding). No redundant plain continuation family is produced.
 			newProperties[key] = value!.slice(0, MAX_PROPERTY_LENGTH);
 			const compressed = await compress(value!);
+			const chunkKey = COMPRESSED_CHUNK_KEY_OVERRIDES[key] ?? key;
 			for (let offset = 0, index = 1; offset < compressed.length && index <= MAX_CONCATENATED_PROPERTIES; offset += MAX_PROPERTY_LENGTH, index++) {
-				const columnName = index === 1 ? `${key}${COMPRESSED_CHUNK_SUFFIX}` : `${key}${COMPRESSED_CHUNK_SUFFIX}_${index}`;
+				const columnName = index === 1 ? `${chunkKey}${COMPRESSED_CHUNK_SUFFIX}` : `${chunkKey}${COMPRESSED_CHUNK_SUFFIX}_${index}`;
 				newProperties[columnName] = compressed.slice(offset, offset + MAX_PROPERTY_LENGTH);
 			}
 			continue;
diff --git a/extensions/copilot/src/platform/telemetry/test/node/telemetry.spec.ts b/extensions/copilot/src/platform/telemetry/test/node/telemetry.spec.ts
index 1c3edbda270..430bace261d 100644
--- a/extensions/copilot/src/platform/telemetry/test/node/telemetry.spec.ts
+++ b/extensions/copilot/src/platform/telemetry/test/node/telemetry.spec.ts
@@ -384,9 +384,11 @@ suite('multiplexProperties compression', function () {
 
 		// Known-large fields are always chunked in compressed form for backend uniformity.
 		expect(result.diffsJSONChunk).toBeDefined();
-		expect(result.messagesJsonChunk).toBeDefined();
+		// messagesJson uses the uppercase-JSON chunk family name expected by the backend.
+		expect(result.messagesJSONChunk).toBeDefined();
+		expect(result.messagesJsonChunk).toBeUndefined();
 		expect(gunzipFromBase64(joinCompressedChunks(result, 'diffsJSONChunk'))).toBe('small');
-		expect(gunzipFromBase64(joinCompressedChunks(result, 'messagesJsonChunk'))).toBe('tiny');
+		expect(gunzipFromBase64(joinCompressedChunks(result, 'messagesJSONChunk'))).toBe('tiny');
 		// The original columns still carry the (short) uncompressed value.
 		expect(result.diffsJSON).toBe('small');
 		expect(result.messagesJson).toBe('tiny');
@@ -395,6 +397,21 @@ suite('multiplexProperties compression', function () {
 		expect(result.otherChunk).toBeUndefined();
 	});
 
+	test('emits the messagesJSONChunk family (with numbered suffixes) for large messagesJson', async () => {
+		const original = pseudoRandomString(60000); // Poorly compressible -> compressed base64 > 8192.
+		const result = await multiplexProperties({ messagesJson: original }, gzipBase64) as { [key: string]: string };
+
+		// The original column carries just the first uncompressed chunk; no plain continuation family.
+		expect(result.messagesJson).toBe(original.slice(0, 8192));
+		expect(result.messagesJson_02).toBeUndefined();
+		// Compressed family uses the uppercase-JSON name, including numbered suffixes.
+		expect(result.messagesJSONChunk).toBeDefined();
+		expect(result.messagesJSONChunk_2).toBeDefined();
+		expect(result.messagesJsonChunk).toBeUndefined();
+		expect(result.messagesJsonChunk_2).toBeUndefined();
+		expect(gunzipFromBase64(joinCompressedChunks(result, 'messagesJSONChunk'))).toBe(original);
+	});
+
 	test('falls back to the plain continuation family when no compressor is provided', async () => {
 		const original = 'x'.repeat(20000);
 		const result = await multiplexProperties({ diffsJSON: original });