DRIVERS-3684 Mandate reporting write concern errors

source/read-write-concern/read-write-concern.md

@@ -399,9 +399,8 @@ rs0:PRIMARY> db.runCommand({insert: "foo", documents: [{x:1}], writeConcern: { w
 }
 ```
 
-Drivers SHOULD parse server replies for a "writeConcernError" field and report the error only in the command-specific
-helper methods for commands that write, from the list above. For example, helper methods for "findAndModify" or
-"aggregate" SHOULD parse the server reply for "writeConcernError".
+Drivers MUST parse server replies for a "writeConcernError" field and report the error from all driver methods that
+accept a `WriteConcern`.
 
 Drivers SHOULD report writeConcernErrors however they report other server errors: by raising an exception, returning
 "false", or another idiom that is consistent with other server errors. Drivers SHOULD report writeConcernErrors with a
@@ -504,6 +503,8 @@ Below are English descriptions of other items that should be tested:
 
 1. Commands supporting a write concern MUST NOT send the default write concern to the server.
 2. Commands supporting a write concern MUST send any non-default write concern to the server in the command.
+3. Commands supporting a write concern MUST report a `writeConcernError` returned in the server reply from
+    command-specific helper methods.
 
 ## Reference Implementation
 
@@ -521,6 +522,8 @@ instance, we send it.
 
 ## Changelog
 
+- 2026-10-23: Require drivers to report `writeConcernError`s.
+
 - 2026-06-17: Remove pre-4.2 version references.
 
 - 2026-05-04: Describe how `readConcern.afterClusterTime` is used in causally-consistent sessions on reads and writes.

source/read-write-concern/tests/README.md

@@ -42,6 +42,13 @@ have the following keys:
 - `isServerDefault:` Indicates whether the read or write concern is considered the server's default.
 - `isAcknowledged:` Indicates if the write concern should be considered acknowledged.
 
+### Write Concern Errors
+
+These tests check that drivers report a `writeConcernError` returned in an `ok: 1` server reply for each driver method
+that supports setting a write concern.
+
+The tests utilize the [Unified Test Format](../../unified-test-format/unified-test-format.md).
+
 ### Operation
 
 These tests check that the default write concern is omitted in operations.

source/read-write-concern/tests/write-concern-errors/aggregate.yml

@@ -0,0 +1,67 @@
+description: write-concern-errors-aggregate
+
+schemaVersion: "1.0"
+
+createEntities:
+  - client:
+      id: &client0 client0
+      useMultipleMongoses: false
+  - database:
+      id: &database0 database0
+      client: *client0
+      databaseName: &database_name write-concern-errors-tests
+  - collection:
+      id: &collection0 collection0
+      database: *database0
+      collectionName: &collection_name coll
+
+initialData:
+  - collectionName: *collection_name
+    databaseName: *database_name
+    documents:
+      - { _id: 1, x: 11 }
+      - { _id: 2, x: 22 }
+
+tests:
+  - description: aggregate with $out reports a writeConcernError
+    operations:
+      - name: failPoint
+        object: testRunner
+        arguments:
+          client: *client0
+          failPoint:
+            configureFailPoint: failCommand
+            mode: { times: 1 }
+            data:
+              failCommands: [ aggregate ]
+              writeConcernError: &write_concern_error
+                code: 79
+                codeName: UnknownReplWriteConcern
+      - name: aggregate
+        object: *collection0
+        arguments:
+          pipeline:
+            - $match: { _id: { $gt: 1 } }
+            - $out: "aggregate_out"
+        expectError: &expect_write_concern_error
+          errorCode: 79
+
+  - description: aggregate with $merge reports a writeConcernError
+    operations:
+      - name: failPoint
+        object: testRunner
+        arguments:
+          client: *client0
+          failPoint:
+            configureFailPoint: failCommand
+            mode: { times: 1 }
+            data:
+              failCommands: [ aggregate ]
+              writeConcernError: *write_concern_error
+      - name: aggregate
+        object: *collection0
+        arguments:
+          pipeline:
+            - $match: { _id: { $gt: 1 } }
+            - $merge: { into: "aggregate_merge" }
+        expectError: *expect_write_concern_error

source/read-write-concern/tests/write-concern-errors/bulkWrite.yml

@@ -0,0 +1,45 @@
+description: write-concern-errors-bulkWrite
+
+schemaVersion: "1.0"
+
+createEntities:
+  - client:
+      id: &client0 client0
+      useMultipleMongoses: false
+  - database:
+      id: &database0 database0
+      client: *client0
+      databaseName: write-concern-errors-tests
+  - collection:
+      id: &collection0 collection0
+      database: *database0
+      collectionName: coll
+
+initialData:
+  - databaseName: write-concern-errors-tests
+    collectionName: coll
+    documents: []
+
+tests:
+  - description: bulkWrite reports a writeConcernError
+    operations:
+      - name: failPoint
+        object: testRunner
+        arguments:
+          client: *client0
+          failPoint:
+            configureFailPoint: failCommand
+            mode: { times: 1 }
+            data:
+              failCommands: [ insert ]
+              writeConcernError:
+                code: 79
+                codeName: UnknownReplWriteConcern
+      - name: bulkWrite
+        object: *collection0
+        arguments:
+          requests:
+            - insertOne:
+                document: { _id: 3, x: 33 }
+        expectError:
+          isError: true