diff --git a/src/commands.zig b/src/commands.zig
index a19d7fe..2cfdda0 100644
--- a/src/commands.zig
+++ b/src/commands.zig
@@ -62,6 +62,21 @@ pub const ErrorCode = enum(i32) {
     invalid_uuid = 207,
     idl_failed_to_parse = 40414,
     idl_unknown_field = 40415,
+    // Aggregation codes, measured against mongod 8.3.7 rather than recalled --
+    // the four-and-five-digit ones are `Location` codes, which mongod names
+    // after the number rather than after a symbol.
+    //
+    // These are what an unimplemented construct answers with, which is the same
+    // choice `location_unrecognized_stage` already made for `$addFields`: this
+    // server reports what it does not implement using MongoDB's own code for
+    // "no such thing", because a code MongoDB never emits would break the
+    // error-code parity every milestone is held to. The message names the
+    // construct, so the answer is a bug report rather than a wrong number.
+    invalid_pipeline_operator = 168,
+    location_unknown_group_operator = 15952,
+    location_group_needs_id = 15955,
+    location_accumulator_not_object = 40234,
+    location_one_accumulator = 40238,
 };
 
 /// Which lock (if any) a command needs on the engine. Contract: only
@@ -2331,8 +2346,64 @@ fn count_only_pipeline(reply: *wire.Reply, stages: []const bson.Value) !?CountSh
     return .{ .filter = filter, .id_value = id_value, .accs = accs.items };
 }
 
-/// Minimal $group: supports `_id` of null/literal/"$field" and `$sum`
-/// accumulators (constant or "$field").
+/// The whole vocabulary this engine can evaluate: a field path, or a constant.
+///
+/// There is no expression evaluator (PLAN amendment A6 puts one in M2.5), and
+/// until there is, anything else has to be *refused*. It used to fall through
+/// to a zero: `{$avg: "$x"}` answered `0` with `ok: 1`, and so did `$max` and
+/// `$push`, and a compound `_id` collapsed every document into one group keyed
+/// by the unevaluated expression. A wrong number that reports success is worse
+/// than an error, because nobody files it.
+const GroupExpr = union(enum) {
+    path: []const u8,
+    constant: bson.Value,
+};
+
+/// Classify `v`, or answer the client and return null.
+///
+/// `what` names the position for the message -- mongod's own messages name the
+/// field, and a refusal that does not say what it refused is only half an
+/// improvement on a silent zero.
+fn classify_expr(reply: *wire.Reply, v: bson.Value, what: []const u8) !?GroupExpr {
+    const detail = switch (v) {
+        .string => |str| {
+            // "$x" is a path; "x" is the string itself. This is the only place
+            // that distinction is made now, where it used to be open-coded at
+            // each use and disagree between them.
+            if (str.len > 0 and str[0] == '$') return GroupExpr{ .path = str[1..] };
+            return GroupExpr{ .constant = v };
+        },
+        // An operator document is the one shape mongod also refuses, and 168 is
+        // the code it uses, naming the operator. A compound expression
+        // (`{a: "$x"}`) mongod would happily evaluate -- so the code is the
+        // same and the message says what is actually true here instead.
+        .doc => |d| if (d.len > 0 and d[0].key.len > 0 and d[0].key[0] == '$')
+            try std.fmt.allocPrint(reply.arena_alloc(), "Unrecognized expression '{s}'", .{d[0].key})
+        else
+            try std.fmt.allocPrint(
+                reply.arena_alloc(),
+                "{s} must be a field path or a constant: this server evaluates no expressions",
+                .{what},
+            ),
+        .array => try std.fmt.allocPrint(
+            reply.arena_alloc(),
+            "{s} must be a field path or a constant: this server evaluates no expressions",
+            .{what},
+        ),
+        else => return GroupExpr{ .constant = v },
+    };
+    try reply.put_error(@intFromEnum(ErrorCode.invalid_pipeline_operator), "InvalidPipelineOperator", detail);
+    return null;
+}
+
+/// One output field of a `$group`, with its argument already classified.
+const Accumulator = struct {
+    key: []const u8,
+    arg: GroupExpr,
+};
+
+/// Minimal $group: `_id` of a constant or "$field", and `$sum` accumulators
+/// over the same. Everything else is refused rather than answered.
 fn run_group(
     ctx: *Context,
     reply: *wire.Reply,
@@ -2342,15 +2413,58 @@ fn run_group(
 ) !?std.ArrayListUnmanaged(*const bson.Document) {
     const arena = reply.arena_alloc();
     const id_expr = bson.get_pair(group_pairs, "_id") orelse {
-        try bad_value(reply, "$group requires _id");
+        try reply.put_error(
+            @intFromEnum(ErrorCode.location_group_needs_id),
+            "Location15955",
+            "a group specification must include an _id",
+        );
         return null;
     };
+    const id_class = (try classify_expr(reply, id_expr, "the _id of a $group")) orelse return null;
 
-    var accs: std.ArrayListUnmanaged(bson.Pair) = .empty;
+    // Every accumulator is validated before a single document is read, so a
+    // pipeline that cannot be answered is refused rather than half-answered.
+    var accs: std.ArrayListUnmanaged(Accumulator) = .empty;
     defer accs.deinit(arena);
     for (group_pairs) |p| {
         if (std.mem.eql(u8, p.key, "_id")) continue;
-        try accs.append(arena, p);
+        const spec = switch (p.value) {
+            .doc => |d| d,
+            else => {
+                const detail = try std.fmt.allocPrint(
+                    arena,
+                    "The field '{s}' must be an accumulator object",
+                    .{p.key},
+                );
+                try reply.put_error(@intFromEnum(ErrorCode.location_accumulator_not_object), "Location40234", detail);
+                return null;
+            },
+        };
+        if (spec.len != 1) {
+            const detail = try std.fmt.allocPrint(
+                arena,
+                "The field '{s}' must specify one accumulator",
+                .{p.key},
+            );
+            try reply.put_error(@intFromEnum(ErrorCode.location_one_accumulator), "Location40238", detail);
+            return null;
+        }
+        if (!std.mem.eql(u8, spec[0].key, "$sum")) {
+            // `$avg`, `$max`, `$push` and the rest are real MongoDB operators
+            // that this server does not implement; see the note on the error
+            // codes for why they are reported as unknown rather than with an
+            // invented code. They used to answer `0`.
+            const detail = try std.fmt.allocPrint(
+                arena,
+                "unknown group operator '{s}'",
+                .{spec[0].key},
+            );
+            try reply.put_error(@intFromEnum(ErrorCode.location_unknown_group_operator), "Location15952", detail);
+            return null;
+        }
+        const what = try std.fmt.allocPrint(arena, "the argument of '{s}'", .{p.key});
+        const arg = (try classify_expr(reply, spec[0].value, what)) orelse return null;
+        try accs.append(arena, .{ .key = p.key, .arg = arg });
     }
 
     const Group = struct {
@@ -2373,9 +2487,9 @@ fn run_group(
     defer walk_arena.deinit();
     for (docs) |off| {
         const doc = coll.doc_bytes(off);
-        const id_value: bson.Value = switch (id_expr) {
-            .string => |s| if (s.len > 0 and s[0] == '$') (try query_path_value_bytes(walk_arena.allocator(), doc, s[1..])) orelse .null else id_expr,
-            else => id_expr,
+        const id_value: bson.Value = switch (id_class) {
+            .path => |path| (try query_path_value_bytes(walk_arena.allocator(), doc, path)) orelse .null,
+            .constant => |v| v,
         };
         id_key_buf.clearRetainingCapacity();
         try bson.write_value(id_value, ctx.gpa, &id_key_buf);
@@ -2392,29 +2506,19 @@ fn run_group(
             gop.value_ptr.* = .{ .id_value = id_value, .sums = sums };
         }
         for (accs.items, 0..) |acc, i| {
-            var expr = acc.value;
-            // Unwrap {$sum: <expr>} accumulator documents.
-            if (expr == .doc) {
-                if (bson.get_pair(expr.doc, "$sum")) |inner| {
-                    expr = inner;
-                } else continue;
-            }
-            const term: f64 = switch (expr) {
+            // A `$sum` over something that is not a number contributes nothing,
+            // which is MongoDB's rule and not a stand-in for an unimplemented
+            // one: `{$sum: "$name"}` over strings really is zero.
+            const v: bson.Value = switch (acc.arg) {
+                .path => |path| (try query_path_value_bytes(walk_arena.allocator(), doc, path)) orelse .null,
+                .constant => |c| c,
+            };
+            gop.value_ptr.sums[i] += switch (v) {
                 .int32 => |n| @floatFromInt(n),
                 .int64 => |n| @floatFromInt(n),
                 .double => |n| n,
-                .string => |s| if (s.len > 0 and s[0] == '$')
-                    switch ((try query_path_value_bytes(walk_arena.allocator(), doc, s[1..])) orelse .null) {
-                        .int32 => |n| @floatFromInt(n),
-                        .int64 => |n| @floatFromInt(n),
-                        .double => |n| n,
-                        else => 0,
-                    }
-                else
-                    0,
                 else => 0,
             };
-            gop.value_ptr.sums[i] += term;
         }
     }
 
@@ -3872,6 +3976,147 @@ test "aggregate $sort without a preceding $group sorts and frees correctly" {
     }
 }
 
+test "$group refuses what it cannot compute instead of answering zero" {
+    // The failure this closes was not a missing feature, it was a wrong number
+    // reported as success. `{$avg: "$x"}` answered `0`, and so did `$max` and
+    // `$push`; a compound `_id` collapsed every document into one group keyed
+    // by the unevaluated expression; `{$literal: 1}` came back echoed. Six of
+    // eight probed pipelines answered `ok: 1` with a wrong result. An
+    // unrecognised stage is a bug report; an `$avg` that returns `0` is a
+    // corrupted report nobody files.
+    //
+    // Every code and codeName below was read off mongod 8.3.7, not recalled,
+    // and this test is where they are pinned.
+    //
+    // Mutation check: drop any one arm of the validation in `run_group` and
+    // the matching row answers `ok: 1` again.
+    var threaded = std.Io.Threaded.init(testing.allocator, .{});
+    defer threaded.deinit();
+    const io = threaded.io();
+
+    var tdb = try TestDb.init(io);
+    defer tdb.deinit();
+    try dispatch_insert(&tdb, io, "g", &.{
+        .{ .doc = &.{ .{ .key = "_id", .value = .{ .int32 = 1 } }, .{ .key = "k", .value = .{ .string = "a" } }, .{ .key = "x", .value = .{ .int32 = 10 } } } },
+        .{ .doc = &.{ .{ .key = "_id", .value = .{ .int32 = 2 } }, .{ .key = "k", .value = .{ .string = "a" } }, .{ .key = "x", .value = .{ .int32 = 20 } } } },
+    });
+
+    const Case = struct { name: []const u8, group: []const bson.Pair, code: i32 };
+    const path_x = bson.Value{ .string = "$x" };
+    const sum_one = bson.Value{ .doc = &.{.{ .key = "$sum", .value = .{ .int32 = 1 } }} };
+    const cases = [_]Case{
+        .{
+            .name = "$avg",
+            .group = &.{
+                .{ .key = "_id", .value = .{ .string = "$k" } },
+                .{ .key = "v", .value = .{ .doc = &.{.{ .key = "$avg", .value = path_x }} } },
+            },
+            .code = 15952,
+        },
+        .{
+            .name = "$push",
+            .group = &.{
+                .{ .key = "_id", .value = .{ .string = "$k" } },
+                .{ .key = "v", .value = .{ .doc = &.{.{ .key = "$push", .value = path_x }} } },
+            },
+            .code = 15952,
+        },
+        .{
+            .name = "an accumulator that is not a document",
+            .group = &.{
+                .{ .key = "_id", .value = .{ .string = "$k" } },
+                .{ .key = "v", .value = path_x },
+            },
+            .code = 40234,
+        },
+        .{
+            .name = "two operators in one accumulator",
+            .group = &.{
+                .{ .key = "_id", .value = .{ .string = "$k" } },
+                .{ .key = "v", .value = .{ .doc = &.{
+                    .{ .key = "$sum", .value = path_x },
+                    .{ .key = "$max", .value = path_x },
+                } } },
+            },
+            .code = 40238,
+        },
+        .{
+            .name = "no _id",
+            .group = &.{.{ .key = "v", .value = sum_one }},
+            .code = 15955,
+        },
+        .{
+            .name = "a compound _id",
+            .group = &.{
+                .{ .key = "_id", .value = .{ .doc = &.{.{ .key = "k", .value = .{ .string = "$k" } }} } },
+                .{ .key = "v", .value = sum_one },
+            },
+            .code = 168,
+        },
+        .{
+            .name = "an operator expression in _id",
+            .group = &.{
+                .{ .key = "_id", .value = .{ .doc = &.{.{ .key = "$literal", .value = .{ .int32 = 1 } }} } },
+                .{ .key = "v", .value = sum_one },
+            },
+            .code = 168,
+        },
+        .{
+            .name = "an expression argument to $sum",
+            .group = &.{
+                .{ .key = "_id", .value = .null },
+                .{ .key = "v", .value = .{ .doc = &.{.{ .key = "$sum", .value = .{ .doc = &.{
+                    .{ .key = "$multiply", .value = .{ .array = &.{ path_x, .{ .int32 = 2 } } } },
+                } } }} } },
+            },
+            .code = 168,
+        },
+    };
+
+    var ctx = tdb.ctx(io);
+    for (cases) |c| {
+        const stages = [_]bson.Value{.{ .doc = &.{.{ .key = "$group", .value = .{ .doc = c.group } }} }};
+        var msg = try parse_fake_msg("aggregate", .{ .string = "g" }, &.{
+            .{ .key = "pipeline", .value = .{ .array = &stages } },
+            .{ .key = "cursor", .value = .{ .doc = &.{} } },
+        });
+        defer msg.deinit();
+        var reply = wire.Reply.init(testing.allocator);
+        defer reply.deinit();
+        try dispatch(&ctx, &msg, &reply);
+        const ok = bson.get_pair(reply.pairs.items, "ok").?.double;
+        testing.expectEqual(@as(f64, 0.0), ok) catch |err| {
+            std.debug.print("  {s}: answered ok:1\n", .{c.name});
+            return err;
+        };
+        const code = bson.get_pair(reply.pairs.items, "code").?.int32;
+        testing.expectEqual(c.code, code) catch |err| {
+            std.debug.print("  {s}: code {d}, wanted {d}\n", .{ c.name, code, c.code });
+            return err;
+        };
+    }
+
+    // And the one shape that *is* implemented still answers, so the refusals
+    // above are a fence and not a wall.
+    const good = [_]bson.Value{.{ .doc = &.{.{ .key = "$group", .value = .{ .doc = &.{
+        .{ .key = "_id", .value = .{ .string = "$k" } },
+        .{ .key = "v", .value = .{ .doc = &.{.{ .key = "$sum", .value = path_x }} } },
+    } } }} }};
+    var msg = try parse_fake_msg("aggregate", .{ .string = "g" }, &.{
+        .{ .key = "pipeline", .value = .{ .array = &good } },
+        .{ .key = "cursor", .value = .{ .doc = &.{} } },
+    });
+    defer msg.deinit();
+    var reply = wire.Reply.init(testing.allocator);
+    defer reply.deinit();
+    try dispatch(&ctx, &msg, &reply);
+    try testing.expectEqual(@as(f64, 1.0), bson.get_pair(reply.pairs.items, "ok").?.double);
+    const cur = bson.get_pair(reply.pairs.items, "cursor").?;
+    const batch = bson.get_pair(cur.doc, "firstBatch").?.array;
+    try testing.expectEqual(@as(usize, 1), batch.len);
+    try testing.expectEqual(@as(i32, 30), bson.get_pair(batch[0].doc, "v").?.int32);
+}
+
 test "count_only_pipeline accepts only shapes a count can answer" {
     // The fast path skips materializing documents, so mis-accepting a
     // pipeline would silently return a wrong aggregate rather than a slow