diff --git a/src/commands.zig b/src/commands.zig
index 47f0822..2bdd392 100644
--- a/src/commands.zig
+++ b/src/commands.zig
@@ -559,10 +559,108 @@ fn reject_bad_session_fields(msg: *wire.Message, reply: *wire.Reply, cmd: []cons
     return reject_txn_fields(msg, reply, cmd);
 }
 
-fn cmd_end_sessions(_: *Context, _: *wire.Message, reply: *wire.Reply) !void {
+/// Still a no-op -- there is nothing to end -- but no longer a blind `ok`.
+///
+/// A driver sends this on close for every session it handed out, so it is the
+/// one session command that arrives in normal operation. Validating an array
+/// we then discard looks like ceremony; it is not. `ok: 1` to a malformed
+/// `endSessions` is the same class of answer as `ok: 1` to a transactional
+/// write: the client is told the server understood, and it did not. mongod's
+/// field path is `endSessions.endSessionsFromClient`, an IDL artefact -- the
+/// command's own field is `endSessions` and the parsed argument has a
+/// different name -- and it is reproduced rather than tidied, because a name
+/// that differs from the real server's is worse than an odd one.
+fn cmd_end_sessions(_: *Context, msg: *wire.Message, reply: *wire.Reply) !void {
+    const arena = reply.arena_alloc();
+    const sessions = switch (msg.body.get("endSessions").?) {
+        .array => |a| a,
+        else => |v| {
+            const text = try std.fmt.allocPrint(
+                arena,
+                "BSON field 'endSessions.endSessions' is the wrong type '{s}', expected type 'array'",
+                .{v.type_name()},
+            );
+            return reply.put_error(@intFromEnum(ErrorCode.type_mismatch), "TypeMismatch", text);
+        },
+    };
+
+    for (sessions, 0..) |entry, i| {
+        if (try reject_bad_session_entry(entry, i, reply)) return;
+    }
     try reply.put_ok();
 }
 
+/// One element of `endSessions`. Answers whether an error reply was written.
+fn reject_bad_session_entry(entry: bson.Value, i: usize, reply: *wire.Reply) !bool {
+    const arena = reply.arena_alloc();
+    const prefix = "BSON field 'endSessions.endSessionsFromClient";
+    const doc = switch (entry) {
+        .doc => |d| d,
+        else => {
+            const text = try std.fmt.allocPrint(
+                arena,
+                "{s}.{d}' is the wrong type '{s}', expected type 'object'",
+                .{ prefix, i, entry.type_name() },
+            );
+            try reply.put_error(@intFromEnum(ErrorCode.type_mismatch), "TypeMismatch", text);
+            return true;
+        },
+    };
+
+    var seen_id = false;
+    for (doc) |pair| {
+        if (std.mem.eql(u8, pair.key, "uid")) continue;
+        if (!std.mem.eql(u8, pair.key, "id")) {
+            const text = try std.fmt.allocPrint(
+                arena,
+                "{s}.{s}' is an unknown field.",
+                .{ prefix, pair.key },
+            );
+            try reply.put_error(@intFromEnum(ErrorCode.idl_unknown_field), "IDLUnknownField", text);
+            return true;
+        }
+        seen_id = true;
+        const bin = switch (pair.value) {
+            .binary => |b| b,
+            else => {
+                const text = try std.fmt.allocPrint(
+                    arena,
+                    "{s}.id' is the wrong type '{s}', expected type 'binData'",
+                    .{ prefix, pair.value.type_name() },
+                );
+                try reply.put_error(@intFromEnum(ErrorCode.type_mismatch), "TypeMismatch", text);
+                return true;
+            },
+        };
+        if (bin.subtype != 4) {
+            const text = try std.fmt.allocPrint(
+                arena,
+                "{s}.id' is the wrong binData type '{s}', expected type 'UUID'",
+                .{ prefix, subtype_name(bin.subtype) },
+            );
+            try reply.put_error(@intFromEnum(ErrorCode.type_mismatch), "TypeMismatch", text);
+            return true;
+        }
+        if (bin.data.len != 16) {
+            try reply.put_error(
+                @intFromEnum(ErrorCode.invalid_uuid),
+                "InvalidUUID",
+                "uuid must be a 16-byte binary field with UUID (4) subtype",
+            );
+            return true;
+        }
+    }
+    if (!seen_id) {
+        try reply.put_error(
+            @intFromEnum(ErrorCode.idl_failed_to_parse),
+            "IDLFailedToParse",
+            "BSON field 'endSessions.endSessionsFromClient.id' is missing but a required field",
+        );
+        return true;
+    }
+    return false;
+}
+
 fn cmd_connection_status(_: *Context, _: *wire.Message, reply: *wire.Reply) !void {
     const auth_info = try reply.arena_alloc().alloc(bson.Pair, 2);
     auth_info[0] = .{ .key = "authenticatedUsers", .value = .{ .array = &.{} } };
@@ -3194,6 +3292,47 @@ test "a transactional write is refused rather than applied" {
     try testing.expectEqual(@as(i64, 0), try doc_count(&ctx, "txn"));
 }
 
+test "endSessions judges the array it is handed" {
+    // Still a no-op, and that is not what is being tested. The command arrives
+    // in normal operation -- a driver sends it on close for every session it
+    // handed out -- so a blind `ok` here tells a client the server understood
+    // something it never looked at. Codes measured against mongod 8.3.7.
+    var threaded: std.Io.Threaded = .init_single_threaded;
+    defer threaded.deinit();
+    const io = threaded.io();
+    var tdb = try TestDb.init(io);
+    defer tdb.deinit();
+    var ctx = tdb.ctx(io);
+
+    const uuid = [_]u8{0x6B} ** 16;
+    const good = bson.Value{ .binary = .{ .subtype = 4, .data = &uuid } };
+    const one = bson.Value{ .doc = &.{.{ .key = "id", .value = good }} };
+
+    try testing.expectEqual(@as(?i32, null), try run_for_code(&ctx, "endSessions", .{ .array = &.{} }, &.{}));
+    try testing.expectEqual(@as(?i32, null), try run_for_code(&ctx, "endSessions", .{ .array = &.{one} }, &.{}));
+    // `uid` rides along once authentication is on, exactly as it does in lsid.
+    try testing.expectEqual(@as(?i32, null), try run_for_code(&ctx, "endSessions", .{ .array = &.{.{ .doc = &.{
+        .{ .key = "id", .value = good },
+        .{ .key = "uid", .value = .{ .binary = .{ .subtype = 0, .data = &[_]u8{0} ** 32 } } },
+    } }} }, &.{}));
+
+    try testing.expectEqual(@as(i32, 14), (try run_for_code(&ctx, "endSessions", .{ .string = "nope" }, &.{})).?);
+    try testing.expectEqual(@as(i32, 14), (try run_for_code(&ctx, "endSessions", .{ .array = &.{.{ .int32 = 5 }} }, &.{})).?);
+    try testing.expectEqual(@as(i32, 14), (try run_for_code(&ctx, "endSessions", .{ .array = &.{.{ .doc = &.{
+        .{ .key = "id", .value = .{ .string = "nope" } },
+    } }} }, &.{})).?);
+    try testing.expectEqual(@as(i32, 207), (try run_for_code(&ctx, "endSessions", .{ .array = &.{.{ .doc = &.{
+        .{ .key = "id", .value = .{ .binary = .{ .subtype = 4, .data = uuid[0..15] } } },
+    } }} }, &.{})).?);
+    try testing.expectEqual(@as(i32, 40414), (try run_for_code(&ctx, "endSessions", .{ .array = &.{.{ .doc = &.{} }} }, &.{})).?);
+    try testing.expectEqual(@as(i32, 40415), (try run_for_code(&ctx, "endSessions", .{ .array = &.{.{ .doc = &.{
+        .{ .key = "id", .value = good },
+        .{ .key = "bogus", .value = .{ .int32 = 1 } },
+    } }} }, &.{})).?);
+    // A bad entry after a good one is still a bad entry.
+    try testing.expectEqual(@as(i32, 14), (try run_for_code(&ctx, "endSessions", .{ .array = &.{ one, .{ .int32 = 5 } } }, &.{})).?);
+}
+
 test "an unknown command is reported before its session id is judged" {
     // Measured: mongod answers CommandNotFound to `{nosuchcmd: 1, lsid: 5}`,
     // so the lookup comes first and this is where the check belongs.