diff --git a/src/commands.zig b/src/commands.zig
index e6d9538..292aab8 100644
--- a/src/commands.zig
+++ b/src/commands.zig
@@ -31,9 +31,44 @@ pub const ErrorCode = enum(i32) {
     invalid_pipeline_operator = 40324,
 };
 
+/// Which lock (if any) a command needs on the engine. Contract: only
+/// `.write` commands may call engine mutation functions (insert, replace,
+/// remove, drop*, get_or_create_collection, compact); `.read` commands may
+/// only read (`get_collection`, `database_names`, `collection_names`);
+/// `.none` commands must not touch the engine at all.
+const CommandKind = enum { none, read, write };
+
+fn command_kind(name: []const u8) CommandKind {
+    // Read-only: scan the engine without mutating it.
+    if (std.mem.eql(u8, name, "find") or
+        std.mem.eql(u8, name, "count") or
+        std.mem.eql(u8, name, "aggregate") or
+        std.mem.eql(u8, name, "listDatabases") or
+        std.mem.eql(u8, name, "listCollections")) return .read;
+    // Writes: exclusive, totally ordered.
+    if (std.mem.eql(u8, name, "create") or
+        std.mem.eql(u8, name, "drop") or
+        std.mem.eql(u8, name, "dropDatabase") or
+        std.mem.eql(u8, name, "insert") or
+        std.mem.eql(u8, name, "update") or
+        std.mem.eql(u8, name, "delete") or
+        std.mem.eql(u8, name, "findAndModify")) return .write;
+    // Everything else (handshake, admin info, no-ops) needs no lock.
+    return .none;
+}
+
 pub fn dispatch(ctx: *Context, msg: *wire.Message, reply: *wire.Reply) !void {
-    try ctx.engine.lock();
-    defer ctx.engine.unlock();
+    switch (command_kind(msg.command_name())) {
+        .none => {},
+        .read => {
+            try ctx.engine.lock_read();
+            defer ctx.engine.unlock_read();
+        },
+        .write => {
+            try ctx.engine.lock();
+            defer ctx.engine.unlock();
+        },
+    }
 
     const name = msg.command_name();
     if (std.mem.eql(u8, name, "hello")) return cmd_hello(ctx, reply);