diff --git a/adr/004-m2m-audit-logging-join-table-query.md b/adr/004-m2m-audit-logging-join-table-query.md
index 426be6229..c46c607f8 100644
--- a/adr/004-m2m-audit-logging-join-table-query.md
+++ b/adr/004-m2m-audit-logging-join-table-query.md
@@ -414,26 +414,14 @@ case IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE:
 
 To add M2M audit support for a new entity:
 
-1. Add a combined `EVENT_COLLECTION_MANYTOMANY_UPDATE`/`DELETE` case to the entity's formatter's `switch`:
+1. Add a combined `EVENT_COLLECTION_MANYTOMANY_UPDATE`/`DELETE` case to the entity's formatter's `switch`, routing both events to the shared base method with the entity wording as the label:
    ```php
    case IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE:
    case IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE:
-       return $this->handleEntityManyToManyCollection($change_set, $id, $name);
+       return $this->formatManyToManyCollectionChange($change_set, sprintf("Entity '%s' (%s)", $name, $id));
    ```
-2. Implement the private handler following the two-step pattern:
-   ```php
-   private function handleEntityManyToManyCollection(array $change_set, $id, $name): ?string
-   {
-       $metadata = $this->handleManyToManyCollection($change_set);
-       if (!$metadata) return null;
-       $collectionData = $this->processCollection($metadata);
-       if (!$collectionData) return null;
-       return $this->event_type === IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE
-           ? $this->formatManyToManyDelete($collectionData, $id, $name)
-           : $this->formatManyToManyUpdate($collectionData, $id, $name);
-   }
-   ```
-3. Implement private `formatManyToManyUpdate()` and `formatManyToManyDelete()` with entity-specific message formatting.
+2. Nothing else is needed in the formatter. `AbstractAuditLogFormatter::formatManyToManyCollectionChange(array $change_set, string $label): ?string` runs the two-step pipeline (`handleManyToManyCollection()` then `processCollection()`), picks update or delete from `$this->event_type`, and builds the message `<label> <field> (<TargetEntity>) updated: Added IDs: [..], Removed IDs: [..] by user X` or `<label> <field> (<TargetEntity>) deleted: Removed IDs: [..] by user X`. It returns `null` when there is nothing to report. Do not copy private `formatManyToManyUpdate()` / `formatManyToManyDelete()` methods into a new formatter.
+3. If an entity needs a message that does not fit the shared shape, build it from `handleManyToManyCollection()` + `processCollection()` directly instead of overriding the base method.
 4. Register the entity in `config/audit_log.php` if not already present.
 
 When one entity is formatted by several classes — `Presentation` has a default strategy plus route-scoped strategies in `config/audit_log.php` — keep the handler in a single per-entity trait (`FormatsPresentationManyToManyCollections`) so every formatter of that entity shares it. An update without diff, or a delete with nothing to remove, returns `null` so no empty entry is emitted.
@@ -443,8 +431,25 @@ Current entities with M2M audit support:
 - `PresentationCategoryGroup` — `$categories` collection (`PresentationCategoryGroup_Categories`)
 - `Presentation` — `$sponsors`, `$tags`, `$allowed_ticket_types` (`BasePresentationAuditLogFormatter` subclasses and `PresentationUserSubmissionAuditLogFormatter`, via the `FormatsPresentationManyToManyCollections` trait)
 - `SummitEvent` — `$sponsors`, `$tags`, `$allowed_ticket_types` (`SummitEventAuditLogFormatter`)
-
-Every other entity registered in `config/audit_log.php` that owns a many-to-many collection still drops these events, because the factory resolves the configured formatter and only falls back to the `Default*` formatters when an entity has no configuration. Extending them follows the steps above, one entity at a time.
+- `Summit` — `$excluded_categories_for_accepted_presentations`, `$excluded_categories_for_alternate_presentations`, `$excluded_categories_for_rejected_presentations`, `$excluded_categories_for_upload_slide_decks`, `$registration_companies` (`SummitAuditLogFormatter`)
+- `SelectionPlan` — `$category_groups`, `$event_types` (`SelectionPlanAuditLogFormatter`)
+- `SummitTrackChair` — `$categories` (`SummitTrackChairAuditLogFormatter`)
+- `SummitSubmissionInvitation` — `$tags` (`SubmissionInvitationAuditLogFormatter`)
+- `PresentationSpeaker` — `$languages`, `$organizational_roles`, `$active_involvements` (`PresentationSpeakerAuditLogFormatter`)
+- `PresentationCategory` — `$allowed_access_levels`, `$allowed_tags`, `$extra_questions` (`PresentationCategoryAuditLogFormatter`)
+- `PresentationType` — `$allowed_ticket_types` (`PresentationTypeAuditLogFormatter`)
+- `SummitEventType` — `$allowed_ticket_types` (`SummitEventTypeAuditLogFormatter`)
+- `SummitMediaUploadType` — `$presentation_types` (`SummitMediaUploadTypeAuditLogFormatter`)
+- `SummitDocument` — `$event_types` (`SummitDocumentAuditLogFormatter`)
+- `SummitBadgeType` — `$access_levels`, `$badge_features`, `$allowed_view_types` (`SummitBadgeTypeAuditLogFormatter`)
+- `SummitAttendeeBadge` — `$features` (`SummitAttendeeBadgeAuditLogFormatter`)
+- `SummitOrderExtraQuestionType` — `$allowed_ticket_types`, `$allowed_badge_features_types` (`SummitOrderExtraQuestionTypeAuditLogFormatter`)
+- `SummitTaxType` — `$ticket_types` (`SummitTaxTypeAuditLogFormatter`)
+- `PrePaidSummitRegistrationDiscountCode`, `SponsorSummitRegistrationDiscountCode`, `SponsorSummitRegistrationPromoCode` — `$badge_features`, `$allowed_ticket_types`, `$tags`, inherited from `SummitRegistrationPromoCode` (their three formatters)
+- `Sponsor` — `$members` (`SponsorAuditLogFormatter`)
+- `SummitBookableVenueRoom` — `$attributes` (`SummitBookableVenueRoomAuditLogFormatter`)
+
+The factory resolves the configured formatter and only falls back to the `Default*` formatters when an entity has no configuration, so a configured entity whose formatter has no M2M case would drop these events. `tests/Unit/Audit/ManyToManyAuditCoverageTest.php` prevents that: for every entity in `config/audit_log.php` that owns a many-to-many collection (Doctrine metadata, owning side) it asserts that each configured formatter, including route-scoped `strategies[]` entries, handles `EVENT_COLLECTION_MANYTOMANY_UPDATE` and `DELETE`. There is no allowlist; configuring a new entity with an unhandled collection fails the test and names the entity, its collections and the formatter to fix.
 
 ### Test infrastructure
 
diff --git a/app/Audit/ConcreteFormatters/SponsorAuditLogFormatter.php b/app/Audit/ConcreteFormatters/SponsorAuditLogFormatter.php
index c58859947..90d48bbfd 100644
--- a/app/Audit/ConcreteFormatters/SponsorAuditLogFormatter.php
+++ b/app/Audit/ConcreteFormatters/SponsorAuditLogFormatter.php
@@ -65,6 +65,10 @@ public function format($subject, array $change_set): ?string
                         $summit_name,
                         $this->getUserInfo()
                     );
+
+                case IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE:
+                case IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE:
+                    return $this->formatManyToManyCollectionChange($change_set, sprintf("Sponsor (%s) for Company '%s' in Summit '%s'", $id, $company_name, $summit_name));
             }
         } catch (\Exception $ex) {
             Log::warning("SponsorAuditLogFormatter error: " . $ex->getMessage());
diff --git a/app/Audit/ConcreteFormatters/SummitAuditLogFormatter.php b/app/Audit/ConcreteFormatters/SummitAuditLogFormatter.php
index 6531556ee..ed3daa078 100644
--- a/app/Audit/ConcreteFormatters/SummitAuditLogFormatter.php
+++ b/app/Audit/ConcreteFormatters/SummitAuditLogFormatter.php
@@ -57,6 +57,10 @@ public function format($subject, array $change_set): ?string
                         $id,
                         $this->getUserInfo()
                     );
+
+                case IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE:
+                case IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE:
+                    return $this->formatManyToManyCollectionChange($change_set, sprintf("Summit '%s' (%s)", $name, $id));
             }
         } catch (\Exception $ex) {
             Log::warning("SummitAuditLogFormatter error: " . $ex->getMessage());
diff --git a/app/Audit/ConcreteFormatters/SummitBookableVenueRoomAuditLogFormatter.php b/app/Audit/ConcreteFormatters/SummitBookableVenueRoomAuditLogFormatter.php
index a81aa2608..4a45f9054 100644
--- a/app/Audit/ConcreteFormatters/SummitBookableVenueRoomAuditLogFormatter.php
+++ b/app/Audit/ConcreteFormatters/SummitBookableVenueRoomAuditLogFormatter.php
@@ -69,6 +69,10 @@ public function format($subject, array $change_set): ?string
                         $summit_name,
                         $this->getUserInfo()
                     );
+
+                case IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE:
+                case IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE:
+                    return $this->formatManyToManyCollectionChange($change_set, sprintf("Bookable Venue Room '%s' (%s) in Venue '%s' for Summit '%s'", $name, $id, $venue_name, $summit_name));
             }
         } catch (\Exception $ex) {
             Log::warning("SummitBookableVenueRoomAuditLogFormatter error: " . $ex->getMessage());
diff --git a/tests/OpenTelemetry/Formatters/SponsorManyToManyAuditLogFormatterTest.php b/tests/OpenTelemetry/Formatters/SponsorManyToManyAuditLogFormatterTest.php
new file mode 100644
index 000000000..5716cc490
--- /dev/null
+++ b/tests/OpenTelemetry/Formatters/SponsorManyToManyAuditLogFormatterTest.php
@@ -0,0 +1,158 @@
+<?php
+
+namespace Tests\OpenTelemetry\Formatters;
+
+/**
+ * Copyright 2026 OpenStack Foundation
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ **/
+
+use App\Audit\ConcreteFormatters\SponsorAuditLogFormatter;
+use App\Audit\Interfaces\IAuditStrategy;
+use Mockery;
+use models\main\Company;
+use models\main\Member;
+use models\summit\Sponsor;
+use models\summit\Summit;
+use Tests\OpenTelemetry\Formatters\Support\AuditContextBuilder;
+use Tests\OpenTelemetry\Formatters\Support\PersistentCollectionTestHelper;
+use Tests\TestCase;
+
+/**
+ * Sponsor owns one many-to-many collection: members.
+ */
+class SponsorManyToManyAuditLogFormatterTest extends TestCase
+{
+    private const MESSAGE_PREFIX = "Sponsor (17) for Company 'Acme' in Summit 'Test Summit'";
+
+    protected function tearDown(): void
+    {
+        Mockery::close();
+        parent::tearDown();
+    }
+
+    public function testUpdateEventReportsAddedAndRemovedIdsFromDiff(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection' => $this->makeCollection('members', Member::class, [10, 11], [11, 12]),
+        ]);
+
+        $this->assertStringStartsWith(
+            self::MESSAGE_PREFIX . " members (Member) updated: Added IDs: [12], Removed IDs: [10] by user ",
+            $result
+        );
+    }
+
+    public function testUpdateEventReportsAddedIds(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection' => $this->makeCollection('members', Member::class, [], [5, 6]),
+        ]);
+
+        $this->assertStringStartsWith(
+            self::MESSAGE_PREFIX . " members (Member) updated: Added IDs: [5,6] by user ",
+            $result
+        );
+    }
+
+    public function testDeleteEventReportsRemovedIdsFromPayload(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection'  => $this->makeCollection('members', Member::class, [], []),
+            'deleted_ids' => [3, 4],
+        ]);
+
+        $this->assertStringStartsWith(
+            self::MESSAGE_PREFIX . " members (Member) deleted: Removed IDs: [3,4] by user ",
+            $result
+        );
+    }
+
+    public function testUpdateEventWithoutDiffIsSuppressed(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection' => $this->makeCollection('members', Member::class, [3], [3]),
+        ]);
+
+        $this->assertNull($result);
+    }
+
+    public function testDeleteEventWithNothingToRemoveIsSuppressed(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection'  => $this->makeCollection('members', Member::class, [], []),
+            'deleted_ids' => [],
+        ]);
+
+        $this->assertNull($result);
+    }
+
+    public function testManyToManyEventWithoutCollectionReturnsNull(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $this->assertNull($formatter->format($this->makeSubject(), []));
+    }
+
+    public function testManyToManyEventWithInvalidSubjectReturnsNull(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $this->assertNull($formatter->format(new \stdClass(), [
+            'collection' => $this->makeCollection('members', Member::class, [], [7]),
+        ]));
+    }
+
+    private function makeFormatter(string $eventType): SponsorAuditLogFormatter
+    {
+        $formatter = new SponsorAuditLogFormatter($eventType);
+        $formatter->setContext(AuditContextBuilder::default()->build());
+        return $formatter;
+    }
+
+    private function makeCollection(string $field, string $targetEntity, array $snapshotIds, array $currentIds)
+    {
+        return PersistentCollectionTestHelper::buildManyToManyCollection(
+            Sponsor::class,
+            $field,
+            $targetEntity,
+            $snapshotIds,
+            $currentIds
+        );
+    }
+
+    private function makeSubject(): Sponsor
+    {
+        $company = Mockery::mock(Company::class);
+        $company->shouldReceive('getName')->andReturn('Acme');
+
+        $summit = Mockery::mock(Summit::class);
+        $summit->shouldReceive('getName')->andReturn('Test Summit');
+
+        $sponsor = Mockery::mock(Sponsor::class);
+        $sponsor->shouldReceive('getId')->andReturn(17);
+        $sponsor->shouldReceive('getCompany')->andReturn($company);
+        $sponsor->shouldReceive('getSummit')->andReturn($summit);
+        $sponsor->shouldReceive('getOrder')->andReturn(1);
+
+        return $sponsor;
+    }
+}
diff --git a/tests/OpenTelemetry/Formatters/SummitBookableVenueRoomManyToManyAuditLogFormatterTest.php b/tests/OpenTelemetry/Formatters/SummitBookableVenueRoomManyToManyAuditLogFormatterTest.php
new file mode 100644
index 000000000..9bd88e019
--- /dev/null
+++ b/tests/OpenTelemetry/Formatters/SummitBookableVenueRoomManyToManyAuditLogFormatterTest.php
@@ -0,0 +1,158 @@
+<?php
+
+namespace Tests\OpenTelemetry\Formatters;
+
+/**
+ * Copyright 2026 OpenStack Foundation
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ **/
+
+use App\Audit\ConcreteFormatters\SummitBookableVenueRoomAuditLogFormatter;
+use App\Audit\Interfaces\IAuditStrategy;
+use Mockery;
+use models\summit\Summit;
+use models\summit\SummitBookableVenueRoom;
+use models\summit\SummitBookableVenueRoomAttributeValue;
+use models\summit\SummitVenue;
+use Tests\OpenTelemetry\Formatters\Support\AuditContextBuilder;
+use Tests\OpenTelemetry\Formatters\Support\PersistentCollectionTestHelper;
+use Tests\TestCase;
+
+/**
+ * SummitBookableVenueRoom owns one many-to-many collection: attributes.
+ */
+class SummitBookableVenueRoomManyToManyAuditLogFormatterTest extends TestCase
+{
+    private const MESSAGE_PREFIX = "Bookable Venue Room 'Room A' (9) in Venue 'Main Venue' for Summit 'Test Summit'";
+
+    protected function tearDown(): void
+    {
+        Mockery::close();
+        parent::tearDown();
+    }
+
+    public function testUpdateEventReportsAddedAndRemovedIdsFromDiff(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection' => $this->makeCollection('attributes', SummitBookableVenueRoomAttributeValue::class, [10, 11], [11, 12]),
+        ]);
+
+        $this->assertStringStartsWith(
+            self::MESSAGE_PREFIX . " attributes (SummitBookableVenueRoomAttributeValue) updated: Added IDs: [12], Removed IDs: [10] by user ",
+            $result
+        );
+    }
+
+    public function testUpdateEventReportsAddedIds(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection' => $this->makeCollection('attributes', SummitBookableVenueRoomAttributeValue::class, [], [5, 6]),
+        ]);
+
+        $this->assertStringStartsWith(
+            self::MESSAGE_PREFIX . " attributes (SummitBookableVenueRoomAttributeValue) updated: Added IDs: [5,6] by user ",
+            $result
+        );
+    }
+
+    public function testDeleteEventReportsRemovedIdsFromPayload(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection'  => $this->makeCollection('attributes', SummitBookableVenueRoomAttributeValue::class, [], []),
+            'deleted_ids' => [3, 4],
+        ]);
+
+        $this->assertStringStartsWith(
+            self::MESSAGE_PREFIX . " attributes (SummitBookableVenueRoomAttributeValue) deleted: Removed IDs: [3,4] by user ",
+            $result
+        );
+    }
+
+    public function testUpdateEventWithoutDiffIsSuppressed(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection' => $this->makeCollection('attributes', SummitBookableVenueRoomAttributeValue::class, [3], [3]),
+        ]);
+
+        $this->assertNull($result);
+    }
+
+    public function testDeleteEventWithNothingToRemoveIsSuppressed(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection'  => $this->makeCollection('attributes', SummitBookableVenueRoomAttributeValue::class, [], []),
+            'deleted_ids' => [],
+        ]);
+
+        $this->assertNull($result);
+    }
+
+    public function testManyToManyEventWithoutCollectionReturnsNull(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $this->assertNull($formatter->format($this->makeSubject(), []));
+    }
+
+    public function testManyToManyEventWithInvalidSubjectReturnsNull(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $this->assertNull($formatter->format(new \stdClass(), [
+            'collection' => $this->makeCollection('attributes', SummitBookableVenueRoomAttributeValue::class, [], [7]),
+        ]));
+    }
+
+    private function makeFormatter(string $eventType): SummitBookableVenueRoomAuditLogFormatter
+    {
+        $formatter = new SummitBookableVenueRoomAuditLogFormatter($eventType);
+        $formatter->setContext(AuditContextBuilder::default()->build());
+        return $formatter;
+    }
+
+    private function makeCollection(string $field, string $targetEntity, array $snapshotIds, array $currentIds)
+    {
+        return PersistentCollectionTestHelper::buildManyToManyCollection(
+            SummitBookableVenueRoom::class,
+            $field,
+            $targetEntity,
+            $snapshotIds,
+            $currentIds
+        );
+    }
+
+    private function makeSubject(): SummitBookableVenueRoom
+    {
+        $venue = Mockery::mock(SummitVenue::class);
+        $venue->shouldReceive('getName')->andReturn('Main Venue');
+
+        $summit = Mockery::mock(Summit::class);
+        $summit->shouldReceive('getName')->andReturn('Test Summit');
+
+        $room = Mockery::mock(SummitBookableVenueRoom::class);
+        $room->shouldReceive('getId')->andReturn(9);
+        $room->shouldReceive('getName')->andReturn('Room A');
+        $room->shouldReceive('getVenue')->andReturn($venue);
+        $room->shouldReceive('getSummit')->andReturn($summit);
+
+        return $room;
+    }
+}
diff --git a/tests/OpenTelemetry/Formatters/SummitManyToManyAuditLogFormatterTest.php b/tests/OpenTelemetry/Formatters/SummitManyToManyAuditLogFormatterTest.php
new file mode 100644
index 000000000..29c0fda22
--- /dev/null
+++ b/tests/OpenTelemetry/Formatters/SummitManyToManyAuditLogFormatterTest.php
@@ -0,0 +1,149 @@
+<?php
+
+namespace Tests\OpenTelemetry\Formatters;
+
+/**
+ * Copyright 2026 OpenStack Foundation
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ **/
+
+use App\Audit\ConcreteFormatters\SummitAuditLogFormatter;
+use App\Audit\Interfaces\IAuditStrategy;
+use Mockery;
+use models\main\Company;
+use models\summit\PresentationCategory;
+use models\summit\Summit;
+use Tests\OpenTelemetry\Formatters\Support\AuditContextBuilder;
+use Tests\OpenTelemetry\Formatters\Support\PersistentCollectionTestHelper;
+use Tests\TestCase;
+
+/**
+ * Summit owns five many-to-many collections: the four excluded_categories_for_* collections (PresentationCategory) and registration_companies (Company).
+ */
+class SummitManyToManyAuditLogFormatterTest extends TestCase
+{
+    private const MESSAGE_PREFIX = "Summit 'Test Summit' (3)";
+
+    protected function tearDown(): void
+    {
+        Mockery::close();
+        parent::tearDown();
+    }
+
+    public function testUpdateEventReportsAddedAndRemovedIdsFromDiff(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection' => $this->makeCollection('excluded_categories_for_accepted_presentations', PresentationCategory::class, [10, 11], [11, 12]),
+        ]);
+
+        $this->assertStringStartsWith(
+            self::MESSAGE_PREFIX . " excluded_categories_for_accepted_presentations (PresentationCategory) updated: Added IDs: [12], Removed IDs: [10] by user ",
+            $result
+        );
+    }
+
+    public function testUpdateEventReportsAddedIds(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection' => $this->makeCollection('registration_companies', Company::class, [], [5, 6]),
+        ]);
+
+        $this->assertStringStartsWith(
+            self::MESSAGE_PREFIX . " registration_companies (Company) updated: Added IDs: [5,6] by user ",
+            $result
+        );
+    }
+
+    public function testDeleteEventReportsRemovedIdsFromPayload(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection'  => $this->makeCollection('excluded_categories_for_accepted_presentations', PresentationCategory::class, [], []),
+            'deleted_ids' => [3, 4],
+        ]);
+
+        $this->assertStringStartsWith(
+            self::MESSAGE_PREFIX . " excluded_categories_for_accepted_presentations (PresentationCategory) deleted: Removed IDs: [3,4] by user ",
+            $result
+        );
+    }
+
+    public function testUpdateEventWithoutDiffIsSuppressed(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection' => $this->makeCollection('registration_companies', Company::class, [3], [3]),
+        ]);
+
+        $this->assertNull($result);
+    }
+
+    public function testDeleteEventWithNothingToRemoveIsSuppressed(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_DELETE);
+
+        $result = $formatter->format($this->makeSubject(), [
+            'collection'  => $this->makeCollection('registration_companies', Company::class, [], []),
+            'deleted_ids' => [],
+        ]);
+
+        $this->assertNull($result);
+    }
+
+    public function testManyToManyEventWithoutCollectionReturnsNull(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $this->assertNull($formatter->format($this->makeSubject(), []));
+    }
+
+    public function testManyToManyEventWithInvalidSubjectReturnsNull(): void
+    {
+        $formatter = $this->makeFormatter(IAuditStrategy::EVENT_COLLECTION_MANYTOMANY_UPDATE);
+
+        $this->assertNull($formatter->format(new \stdClass(), [
+            'collection' => $this->makeCollection('excluded_categories_for_accepted_presentations', PresentationCategory::class, [], [7]),
+        ]));
+    }
+
+    private function makeFormatter(string $eventType): SummitAuditLogFormatter
+    {
+        $formatter = new SummitAuditLogFormatter($eventType);
+        $formatter->setContext(AuditContextBuilder::default()->build());
+        return $formatter;
+    }
+
+    private function makeCollection(string $field, string $targetEntity, array $snapshotIds, array $currentIds)
+    {
+        return PersistentCollectionTestHelper::buildManyToManyCollection(
+            Summit::class,
+            $field,
+            $targetEntity,
+            $snapshotIds,
+            $currentIds
+        );
+    }
+
+    private function makeSubject(): Summit
+    {
+        $summit = Mockery::mock(Summit::class);
+        $summit->shouldReceive('getId')->andReturn(3);
+        $summit->shouldReceive('getName')->andReturn('Test Summit');
+
+        return $summit;
+    }
+}
diff --git a/tests/Unit/Audit/ManyToManyAuditCoverageTest.php b/tests/Unit/Audit/ManyToManyAuditCoverageTest.php
new file mode 100644
index 000000000..49a46d5c3
--- /dev/null
+++ b/tests/Unit/Audit/ManyToManyAuditCoverageTest.php
@@ -0,0 +1,136 @@
+<?php namespace Tests\Unit\Audit;
+/**
+ * Copyright 2026 OpenStack Foundation
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ **/
+
+use App\Audit\AbstractAuditLogFormatter;
+use Doctrine\ORM\Mapping\ClassMetadata;
+use Doctrine\Persistence\ManagerRegistry;
+use ReflectionClass;
+use Tests\TestCase;
+
+/**
+ * AuditLogFormatterFactory hands EVENT_COLLECTION_MANYTOMANY_UPDATE / DELETE to the formatter
+ * configured for the owner entity in config/audit_log.php and only falls back to the
+ * Default* formatters when the entity has no configuration (ADR-004). A configured formatter
+ * without a case for those events returns null and the entry is dropped, so every configured
+ * entity that owns a many-to-many collection must have all its formatters handle them.
+ *
+ * Handling is detected the way AuditLogFormatterUpdateGatewayTest does it: a source scan of the
+ * formatter class, its parents below AbstractAuditLogFormatter and the traits they use.
+ */
+class ManyToManyAuditCoverageTest extends TestCase
+{
+    private const REQUIRED_EVENTS = [
+        'EVENT_COLLECTION_MANYTOMANY_UPDATE',
+        'EVENT_COLLECTION_MANYTOMANY_DELETE',
+    ];
+
+    public function testEveryConfiguredFormatterHandlesManyToManyEventsOfItsEntity(): void
+    {
+        $em = app(ManagerRegistry::class)->getManager();
+        $offenders = [];
+
+        foreach (config('audit_log.entities') as $entity => $config) {
+            if (($config['enabled'] ?? true) === false) {
+                continue;
+            }
+
+            $fields = $this->getOwningManyToManyFields($em->getClassMetadata($entity));
+            if (empty($fields)) {
+                continue;
+            }
+
+            foreach ($this->getConfiguredFormatters($config) as $formatter) {
+                $missing = array_diff(self::REQUIRED_EVENTS, $this->getHandledEvents($formatter));
+                if (!empty($missing)) {
+                    $offenders[] = sprintf(
+                        "%s (%s) is formatted by %s, which does not handle %s",
+                        $entity,
+                        implode(', ', $fields),
+                        $formatter,
+                        implode(' / ', $missing)
+                    );
+                }
+            }
+        }
+
+        $this->assertEmpty(
+            $offenders,
+            "Configured entities that own many-to-many collections must have their formatter handle " .
+            "EVENT_COLLECTION_MANYTOMANY_UPDATE / DELETE, otherwise those changes are dropped from the audit log " .
+            "(see adr/004-m2m-audit-logging-join-table-query.md, 'Adding M2M audit to a new entity'). Offenders:\n" .
+            implode("\n", $offenders)
+        );
+    }
+
+    /**
+     * @return string[] owning-side many-to-many association field names
+     */
+    private function getOwningManyToManyFields(ClassMetadata $metadata): array
+    {
+        $fields = [];
+        foreach ($metadata->associationMappings as $field => $mapping) {
+            $type = is_array($mapping) ? $mapping['type'] : $mapping->type();
+            $owning = is_array($mapping) ? $mapping['isOwningSide'] : $mapping->isOwningSide();
+            if ($type === ClassMetadata::MANY_TO_MANY && $owning) {
+                $fields[] = $field;
+            }
+        }
+        return $fields;
+    }
+
+    /**
+     * @return string[] the default strategy plus every route-scoped strategies[] formatter
+     */
+    private function getConfiguredFormatters(array $config): array
+    {
+        $formatters = [];
+        if (!empty($config['strategy'])) {
+            $formatters[] = $config['strategy'];
+        }
+        foreach ($config['strategies'] ?? [] as $strategy) {
+            if (!empty($strategy['formatter'])) {
+                $formatters[] = $strategy['formatter'];
+            }
+        }
+        return array_values(array_unique($formatters));
+    }
+
+    /**
+     * @return string[] required event constants found in the formatter class, its parents
+     *                  below AbstractAuditLogFormatter and the traits they use
+     */
+    private function getHandledEvents(string $formatter): array
+    {
+        $source = '';
+        $class = new ReflectionClass($formatter);
+        while ($class && $class->getName() !== AbstractAuditLogFormatter::class) {
+            $source .= $this->getSourceWithTraits($class);
+            $class = $class->getParentClass() ?: null;
+        }
+
+        return array_values(array_filter(
+            self::REQUIRED_EVENTS,
+            fn(string $event) => str_contains($source, $event)
+        ));
+    }
+
+    private function getSourceWithTraits(ReflectionClass $class): string
+    {
+        $source = file_get_contents($class->getFileName());
+        foreach ($class->getTraits() as $trait) {
+            $source .= $this->getSourceWithTraits($trait);
+        }
+        return $source;
+    }
+}