diff --git a/CHANGELOG.md b/CHANGELOG.md
index 429b650f3..d96758219 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- Fixed GitLab connection topic filters (`topics` / `exclude.topics`) being case-sensitive on the project side, so mixed-case GitLab project topics (e.g. `Backend`) now match lowercase config topics. [#1388](https://github.com/sourcebot-dev/sourcebot/issues/1388)
+
 ## [5.1.0] - 2026-07-10
 
 ### Changed
diff --git a/packages/backend/src/gitlab.test.ts b/packages/backend/src/gitlab.test.ts
index a502488df..956374a87 100644
--- a/packages/backend/src/gitlab.test.ts
+++ b/packages/backend/src/gitlab.test.ts
@@ -141,16 +141,29 @@ test('shouldExcludeProject returns false when exclude.topics does not match any
     })).toBe(false);
 });
 
-test('shouldExcludeProject include.topics matching is case-sensitive on the project side.', () => {
+test('shouldExcludeProject include.topics matching is case-insensitive on the project side.', () => {
     const project = {
         path_with_namespace: 'test/project',
         topics: ['Backend'],
     } as unknown as ProjectSchema;
 
-    // The function lowercases config topics but not project topics,
-    // so 'Backend' does not match the lowercased pattern 'backend'.
+    // Both config and project topics are lowercased before matching,
+    // so the mixed-case project topic 'Backend' matches the pattern 'backend'.
     expect(shouldExcludeProject({
         project,
         include: { topics: ['backend'] },
+    })).toBe(false);
+});
+
+test('shouldExcludeProject exclude.topics matching is case-insensitive on the project side.', () => {
+    const project = {
+        path_with_namespace: 'test/project',
+        topics: ['Deprecated'],
+    } as unknown as ProjectSchema;
+
+    // The mixed-case project topic 'Deprecated' matches the exclude pattern 'deprecated'.
+    expect(shouldExcludeProject({
+        project,
+        exclude: { topics: ['deprecated'] },
     })).toBe(true);
 });
diff --git a/packages/backend/src/gitlab.ts b/packages/backend/src/gitlab.ts
index 94a6e0710..6cdadb43a 100644
--- a/packages/backend/src/gitlab.ts
+++ b/packages/backend/src/gitlab.ts
@@ -249,7 +249,7 @@ export const shouldExcludeProject = ({
 
         if (include?.topics) {
             const configTopics = include.topics.map(topic => topic.toLowerCase());
-            const projectTopics = project.topics ?? [];
+            const projectTopics = (project.topics ?? []).map(topic => topic.toLowerCase());
 
             const matchingTopics = projectTopics.filter((topic) => micromatch.isMatch(topic, configTopics));
             if (matchingTopics.length === 0) {
@@ -260,7 +260,7 @@ export const shouldExcludeProject = ({
 
         if (exclude?.topics) {
             const configTopics = exclude.topics.map(topic => topic.toLowerCase());
-            const projectTopics = project.topics ?? [];
+            const projectTopics = (project.topics ?? []).map(topic => topic.toLowerCase());
 
             const matchingTopics = projectTopics.filter((topic) => micromatch.isMatch(topic, configTopics));
             if (matchingTopics.length > 0) {