diff --git a/modules/module-mongodb/src/replication/ChangeStream.ts b/modules/module-mongodb/src/replication/ChangeStream.ts
index 0a516af82..cd0bf3bc5 100644
--- a/modules/module-mongodb/src/replication/ChangeStream.ts
+++ b/modules/module-mongodb/src/replication/ChangeStream.ts
@@ -226,7 +226,10 @@ export class ChangeStream {
   private async getSnapshotLsn(): Promise<string> {
     const hello = await this.defaultDb.command({ hello: 1 });
     // Basic sanity check
-    if (hello.msg == 'isdbgrid') {
+    if (hello.internal?.cosmos_versions != null) {
+      // Exmaple: internal: { cosmos_versions: [ '1.104-1', '1.105.0', '12.1-1' ] },
+      this.logger.info('CosmosDB detected. CosmosDB support is experimental and may not work as expected.');
+    } else if (hello.msg == 'isdbgrid') {
       throw new ServiceError(
         ErrorCode.PSYNC_S1341,
         'Sharded MongoDB Clusters are not supported yet (including MongoDB Serverless instances).'
@@ -280,10 +283,13 @@ export class ChangeStream {
         if (!this.checkpointStreamId.equals(checkpointId)) {
           continue;
         }
+
+        // CosmosDB workaround: use walltime
         const { comparable: lsn } = new MongoLSN({
-          timestamp: changeDocument.clusterTime!,
+          timestamp: mongo.Timestamp.fromBits(0, (changeDocument as any).wallTime!.getTime() / 1000),
           resume_token: changeDocument._id
         });
+
         return lsn;
       }
 
@@ -389,7 +395,8 @@ export class ChangeStream {
     const collection = await this.getCollectionInfo(this.defaultDb.databaseName, CHECKPOINTS_COLLECTION);
     if (collection == null) {
       await this.defaultDb.createCollection(CHECKPOINTS_COLLECTION, {
-        changeStreamPreAndPostImages: { enabled: true }
+        // Not supported by CosmosDB:
+        // changeStreamPreAndPostImages: { enabled: true }
       });
     } else if (this.usePostImages && collection.options?.changeStreamPreAndPostImages?.enabled != true) {
       // Drop + create requires less permissions than collMod,
@@ -718,8 +725,9 @@ export class ChangeStream {
     const pipeline: mongo.Document[] = [
       {
         $match: filters.$match
-      },
-      { $changeStreamSplitLargeEvent: {} }
+      }
+
+      // { $changeStreamSplitLargeEvent: {} } // not supported on CosmosDB
     ];
 
     let fullDocument: 'required' | 'updateLookup';
@@ -734,7 +742,7 @@ export class ChangeStream {
     }
 
     const streamOptions: mongo.ChangeStreamOptions = {
-      showExpandedEvents: true,
+      // showExpandedEvents: true, // not supported on CosmosDB
       maxAwaitTimeMS: options.maxAwaitTimeMs ?? this.maxAwaitTimeMS,
       fullDocument: fullDocument
     };
@@ -744,7 +752,7 @@ export class ChangeStream {
      */
     if (resumeAfter) {
       streamOptions.resumeAfter = resumeAfter;
-    } else {
+    } else if (streamOptions.startAtOperationTime != null) {
       // Legacy: We don't persist lsns without resumeTokens anymore, but we do still handle the
       // case if we have an old one.
       streamOptions.startAtOperationTime = startAfter;
@@ -756,7 +764,10 @@ export class ChangeStream {
       stream = this.client.watch(pipeline, streamOptions);
     } else {
       // Same general result, but requires less permissions than the above
-      stream = this.defaultDb.watch(pipeline, streamOptions);
+      // Core issue: Watching on an entire database is not supported on CosmosDB.
+      // stream = this.defaultDb.watch(pipeline, streamOptions);
+      // Temp workaround just to test other behavior
+      stream = this.defaultDb.collection('_powersync_checkpoints').watch(pipeline, streamOptions);
     }
 
     this.abort_signal.addEventListener('abort', () => {
@@ -835,6 +846,8 @@ export class ChangeStream {
             break;
           }
 
+          console.log('change doc', originalChangeDocument);
+
           if (originalChangeDocument == null) {
             // We get a new null document after `maxAwaitTimeMS` if there were no other events.
             // In this case, stream.resumeToken is the resume token associated with the last response.
@@ -961,10 +974,17 @@ export class ChangeStream {
             if (!(checkpointId == STANDALONE_CHECKPOINT_ID || this.checkpointStreamId.equals(checkpointId))) {
               continue;
             }
+
+            // CosmosDB workaround: use walltime
+            const wallTime = (changeDocument as any).wallTime;
             const { comparable: lsn } = new MongoLSN({
-              timestamp: changeDocument.clusterTime!,
+              timestamp: mongo.Timestamp.fromBits(0, wallTime!.getTime() / 1000),
               resume_token: changeDocument._id
             });
+            // const { comparable: lsn } = new MongoLSN({
+            //   timestamp: changeDocument.clusterTime!,
+            //   resume_token: changeDocument._id
+            // });
             if (batch.lastCheckpointLsn != null && lsn < batch.lastCheckpointLsn) {
               // Checkpoint out of order - should never happen with MongoDB.
               // If it does happen, we throw an error to stop the replication - restarting should recover.
diff --git a/modules/module-mongodb/src/replication/MongoRelation.ts b/modules/module-mongodb/src/replication/MongoRelation.ts
index 4ac11efb7..fcbe28776 100644
--- a/modules/module-mongodb/src/replication/MongoRelation.ts
+++ b/modules/module-mongodb/src/replication/MongoRelation.ts
@@ -3,7 +3,7 @@ import { storage } from '@powersync/service-core';
 import { JSONBig, JsonContainer } from '@powersync/service-jsonbig';
 import { SqliteRow, SqliteValue } from '@powersync/service-sync-rules';
 
-import { ErrorCode, ServiceError } from '@powersync/lib-services-framework';
+import { ErrorCode, ServiceAssertionError, ServiceError } from '@powersync/lib-services-framework';
 import { MongoLSN } from '../common/MongoLSN.js';
 import { CHECKPOINTS_COLLECTION } from './replication-utils.js';
 
@@ -181,9 +181,19 @@ export async function createCheckpoint(
         session
       }
     );
-    const time = session.operationTime!;
+
+    let time = session.operationTime;
+    if (time == null) {
+      // CosmosDB workaround
+      const hello = await db.command({ hello: 1 }, { session });
+      if (hello.operationTime == null) {
+        throw new ServiceAssertionError('Failed to create checkpoint: no operation time available');
+      }
+      time = hello.operationTime!;
+    }
+
     // TODO: Use the above when we support custom write checkpoints
-    return new MongoLSN({ timestamp: time }).comparable;
+    return new MongoLSN({ timestamp: time! }).comparable;
   } finally {
     await session.endSession();
   }