diff --git a/docker/zammad/Dockerfile b/docker/zammad/Dockerfile
index d16b21b..cbfbdbe 100644
--- a/docker/zammad/Dockerfile
+++ b/docker/zammad/Dockerfile
@@ -42,6 +42,15 @@ RUN pnpm install --frozen-lockfile
 # This extracts addon files including CoffeeScript, Vue components, TypeScript, and CSS
 RUN ruby contrib/link/install.rb
 
+# Fix OpenSearch compatibility: Replace 'flattened' with 'flat_object'
+# Elasticsearch uses 'flattened' but OpenSearch uses 'flat_object'
+# Without this fix, search index creation fails with:
+#   [o.o.c.m.MetadataCreateIndexService] failed on parsing mappings on index creation
+#   org.opensearch.index.mapper.MapperParsingException: Failed to parse mapping [_doc]:
+#   No handler for type [flattened] declared on field [preferences]
+# See: https://github.com/zammad/zammad/blob/bfd2f5befc3aec3fe607a5b6146788ec9af461e4/lib/search_index_backend.rb#L896
+RUN sed -i "s/'flattened'/'flat_object'/g" /opt/zammad/lib/search_index_backend.rb
+
 # Precompile assets with addon CoffeeScript files included
 # Use ZAMMAD_SAFE_MODE=1 and dummy DATABASE_URL to avoid needing real database
 RUN touch db/schema.rb && \
@@ -93,6 +102,8 @@ COPY --from=builder --chown=zammad:zammad /opt/zammad/app/policies/controllers/*
 COPY --from=builder --chown=zammad:zammad /opt/zammad/config/initializers/*cdr* /opt/zammad/config/initializers/
 COPY --from=builder --chown=zammad:zammad /opt/zammad/config/routes/*cdr* /opt/zammad/config/routes/
 COPY --from=builder --chown=zammad:zammad /opt/zammad/lib/cdr* /opt/zammad/lib/
+# CRITICAL: Copy modified search_index_backend.rb with OpenSearch fix
+COPY --from=builder --chown=zammad:zammad /opt/zammad/lib/search_index_backend.rb /opt/zammad/lib/search_index_backend.rb
 COPY --from=builder --chown=zammad:zammad /opt/zammad/public/assets/images/icons/*cdr* /opt/zammad/public/assets/images/icons/
 COPY --from=builder --chown=zammad:zammad /opt/zammad/app/views/mailer/ticket_create/ /opt/zammad/app/views/mailer/ticket_create/
 COPY --from=builder --chown=zammad:zammad /opt/zammad/public/assets/images/logo* /opt/zammad/public/assets/images/