diff --git a/src/modules/atproto/application/useCases/ProcessMarginCollectionItemFirehoseEventUseCase.ts b/src/modules/atproto/application/useCases/ProcessMarginCollectionItemFirehoseEventUseCase.ts
index b7bea7de..d9413ee8 100644
--- a/src/modules/atproto/application/useCases/ProcessMarginCollectionItemFirehoseEventUseCase.ts
+++ b/src/modules/atproto/application/useCases/ProcessMarginCollectionItemFirehoseEventUseCase.ts
@@ -9,6 +9,7 @@ import {
   UpdateUrlCardAssociationsUseCase,
   OperationContext,
 } from '../../../cards/application/useCases/commands/UpdateUrlCardAssociationsUseCase';
+import { ATPROTO_NSID } from 'src/shared/constants/atproto';
 
 export interface ProcessMarginCollectionItemFirehoseEventDTO {
   atUri: string;
@@ -96,7 +97,7 @@ export class ProcessMarginCollectionItemFirehoseEventUseCase
       }
 
       const annotationCollection = annotationUriResult.value.collection;
-      if (annotationCollection !== 'at.margin.bookmark') {
+      if (annotationCollection !== ATPROTO_NSID.MARGIN.BOOKMARK) {
         if (ENABLE_FIREHOSE_LOGGING) {
           console.log(
             `[FirehoseWorker] Ignoring Margin collection item for non-bookmark annotation type: ${annotationCollection}, itemUri: ${request.atUri}`,
diff --git a/src/modules/feeds/application/useCases/commands/AddActivityToFeedUseCase.ts b/src/modules/feeds/application/useCases/commands/AddActivityToFeedUseCase.ts
index 3a663bf8..3655cbab 100644
--- a/src/modules/feeds/application/useCases/commands/AddActivityToFeedUseCase.ts
+++ b/src/modules/feeds/application/useCases/commands/AddActivityToFeedUseCase.ts
@@ -8,6 +8,8 @@ import { CollectionId } from '../../../../cards/domain/value-objects/CollectionI
 import { ActivityTypeEnum } from '../../../domain/value-objects/ActivityType';
 import { FeedService } from 'src/modules/feeds/domain/services/FeedService';
 import { ICardRepository } from '../../../../cards/domain/ICardRepository';
+import { SourceTypeEnum } from '../../../domain/value-objects/SourceType';
+import { ATPROTO_NSID } from '../../../../../shared/constants/atproto';
 
 export interface AddCardCollectedActivityDTO {
   type: ActivityTypeEnum.CARD_COLLECTED;
@@ -117,8 +119,8 @@ export class AddActivityToFeedUseCase
         const libraryInfo = cardResult.value.getLibraryInfo(actorId);
         if (libraryInfo?.publishedRecordId) {
           const uri = libraryInfo.publishedRecordId.uri;
-          if (uri.includes('/at.margin.')) {
-            source = 'margin';
+          if (uri.includes(`/${ATPROTO_NSID.MARGIN.NAMESPACE}.`)) {
+            source = SourceTypeEnum.MARGIN;
           }
           // Cosmik activities remain undefined (default)
         }
diff --git a/src/modules/feeds/domain/value-objects/SourceType.ts b/src/modules/feeds/domain/value-objects/SourceType.ts
new file mode 100644
index 00000000..0990a8d8
--- /dev/null
+++ b/src/modules/feeds/domain/value-objects/SourceType.ts
@@ -0,0 +1,43 @@
+import { ValueObject } from '../../../../shared/domain/ValueObject';
+import { Result, ok, err } from '../../../../shared/core/Result';
+
+export enum SourceTypeEnum {
+  MARGIN = 'margin',
+}
+
+export class SourceTypeError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = 'SourceTypeError';
+  }
+}
+
+interface SourceTypeProps {
+  value: SourceTypeEnum;
+}
+
+export class SourceType extends ValueObject<SourceTypeProps> {
+  get value(): SourceTypeEnum {
+    return this.props.value;
+  }
+
+  private constructor(props: SourceTypeProps) {
+    super(props);
+  }
+
+  public static create(value: string): Result<SourceType, SourceTypeError> {
+    if (!Object.values(SourceTypeEnum).includes(value as SourceTypeEnum)) {
+      return err(new SourceTypeError(`Invalid source type: ${value}`));
+    }
+
+    return ok(new SourceType({ value: value as SourceTypeEnum }));
+  }
+
+  public static margin(): Result<SourceType, SourceTypeError> {
+    return ok(new SourceType({ value: SourceTypeEnum.MARGIN }));
+  }
+
+  public toString(): string {
+    return this.props.value;
+  }
+}
diff --git a/src/shared/constants/atproto.ts b/src/shared/constants/atproto.ts
new file mode 100644
index 00000000..9ce1c725
--- /dev/null
+++ b/src/shared/constants/atproto.ts
@@ -0,0 +1,18 @@
+/**
+ * AT Protocol NSID constants
+ * Namespaced identifiers for AT Protocol collections
+ */
+export const ATPROTO_NSID = {
+  MARGIN: {
+    NAMESPACE: 'at.margin',
+    BOOKMARK: 'at.margin.bookmark',
+    COLLECTION: 'at.margin.collection',
+    COLLECTION_ITEM: 'at.margin.collectionItem',
+  },
+  COSMIK: {
+    NAMESPACE: 'network.cosmik',
+    CARD: 'network.cosmik.card',
+    COLLECTION: 'network.cosmik.collection',
+    COLLECTION_LINK: 'network.cosmik.collectionLink',
+  },
+} as const;
diff --git a/src/shared/infrastructure/config/EnvironmentConfigService.ts b/src/shared/infrastructure/config/EnvironmentConfigService.ts
index 92fa5bb9..9d82e0e1 100644
--- a/src/shared/infrastructure/config/EnvironmentConfigService.ts
+++ b/src/shared/infrastructure/config/EnvironmentConfigService.ts
@@ -1,3 +1,5 @@
+import { ATPROTO_NSID } from 'src/shared/constants/atproto';
+
 export enum Environment {
   LOCAL = 'local',
   DEV = 'dev',
@@ -106,25 +108,25 @@ export class EnvironmentConfigService {
         baseUrl: process.env.BASE_URL || 'http://127.0.0.1:3000',
         baseCollection:
           environment === Environment.PROD
-            ? 'network.cosmik'
-            : `network.cosmik.${environment}`,
+            ? ATPROTO_NSID.COSMIK.NAMESPACE
+            : `${ATPROTO_NSID.COSMIK.NAMESPACE}.${environment}`,
         collections: {
           card:
             environment === Environment.PROD
-              ? 'network.cosmik.card'
-              : `network.cosmik.${environment}.card`,
+              ? ATPROTO_NSID.COSMIK.CARD
+              : `${ATPROTO_NSID.COSMIK.NAMESPACE}.${environment}.card`,
           collection:
             environment === Environment.PROD
-              ? 'network.cosmik.collection'
-              : `network.cosmik.${environment}.collection`,
+              ? ATPROTO_NSID.COSMIK.COLLECTION
+              : `${ATPROTO_NSID.COSMIK.NAMESPACE}.${environment}.collection`,
           collectionLink:
             environment === Environment.PROD
-              ? 'network.cosmik.collectionLink'
-              : `network.cosmik.${environment}.collectionLink`,
+              ? ATPROTO_NSID.COSMIK.COLLECTION_LINK
+              : `${ATPROTO_NSID.COSMIK.NAMESPACE}.${environment}.collectionLink`,
           // Margin collections - no environment suffix
-          marginBookmark: 'at.margin.bookmark',
-          marginCollection: 'at.margin.collection',
-          marginCollectionItem: 'at.margin.collectionItem',
+          marginBookmark: ATPROTO_NSID.MARGIN.BOOKMARK,
+          marginCollection: ATPROTO_NSID.MARGIN.COLLECTION,
+          marginCollectionItem: ATPROTO_NSID.MARGIN.COLLECTION_ITEM,
         },
         serviceAccount: {
           identifier: process.env.BSKY_SERVICE_ACCOUNT_IDENTIFIER || '',