diff --git a/lib/analyse.ml b/lib/analyse.ml
index 43361c3..36e5cf3 100644
--- a/lib/analyse.ml
+++ b/lib/analyse.ml
@@ -53,6 +53,7 @@ let make_placeholder_selections ~platforms ~opam_repository_commits =
          {
            Selection.variant = fst platform;
            packages = [];
+           only_packages = [];
            commits =
              List.map
                (fun y ->
@@ -196,12 +197,11 @@ module Analysis = struct
       None
 
   (** If the opam file for the given root_pkgs includes a build command, use
-      that instead of the default (dune build @install @runtests).  This is
-      put into Selection.command to be picked up by the build phase.
+      that instead of the default (dune build \@install \@runtests). This is put
+      into Selection.command to be picked up by the build phase.
 
-      This is only implemented if root_pkgs is a singleton, otherwise we
-      fall back to the default build command.
-    *)
+      This is only implemented if root_pkgs is a singleton, otherwise we fall
+      back to the default build command. *)
   let maybe_add_build_command ~root_pkgs selection =
     match root_pkgs with
     | [ (pkg_name, opam_str) ] ->
@@ -211,6 +211,7 @@ module Analysis = struct
         {
           Selection.variant = selection.Selection.variant;
           packages = selection.packages;
+          only_packages = selection.only_packages;
           commits = selection.commits;
           command;
         }
@@ -283,7 +284,9 @@ module Analysis = struct
     Capnp_rpc_lwt.Capability.with_ref (job_log job) @@ fun log ->
     Backend_solver.solve solver job request ~log >|= function
     | Ok [] -> Fmt.error_msg "No solution found for any supported platform"
-    | Ok x -> Ok (List.map Selection.of_worker x)
+    | Ok x ->
+        let root_pkgs = List.map fst root_pkgs in
+        Ok (List.map (Selection.of_worker ~root_pkgs) x)
     | Error (`Msg msg) -> Fmt.error_msg "Error from solver: %s" msg
 
   let find_opam_files ~job ~dir =
diff --git a/lib/opam_build.ml b/lib/opam_build.ml
index f916c12..387b05a 100644
--- a/lib/opam_build.ml
+++ b/lib/opam_build.ml
@@ -218,10 +218,16 @@ let spec_script ~repo ~base ~opam_files ~compiler_commit ~selection ~cmds =
     ~cmds:(run_all_opam_exec cmds)
 
 let spec_dune ~repo ~base ~opam_files ~compiler_commit ~selection =
+  let only_packages =
+    let to_name x = OpamPackage.of_string x |> OpamPackage.name_to_string in
+    match selection.Selection.only_packages with
+    | [] -> ""
+    | pkgs -> " --only-packages=" ^ String.concat "," (List.map to_name pkgs)
+  in
   let cmd =
     match selection.Selection.command with
     | Some c -> c
-    | None -> "dune build @install @runtest"
+    | None -> Printf.sprintf "dune build%s @install @runtest" only_packages
   in
   let cmds = [ "opam install dune"; cmd ^ " && rm -rf _build" ] in
   spec_script ~repo ~base ~opam_files ~compiler_commit ~selection ~cmds
diff --git a/lib/selection.ml b/lib/selection.ml
index c7781bb..be7c0d9 100644
--- a/lib/selection.ml
+++ b/lib/selection.ml
@@ -1,21 +1,25 @@
 type t = {
   variant : Variant.t;  (** The variant image to build on. *)
   packages : string list;  (** The selected packages ("name.version"). *)
+  only_packages : string list; [@default []]
+      (** Local root packages to include (empty to include all). *)
   commits : (string * string) list;
       (** The list of (repo_url,commit) opam-repositories from the analysis, and
           they're all usefull during the build.*)
   command : string option;
-      (** The build command to use (will default to dune build @install @runtest if not specified) *)
+      (** The build command to use (will default to dune build \@install
+          \@runtest if not specified) *)
 }
 [@@deriving yojson, ord]
 (** A set of packages for a single build. *)
 
-let of_worker w =
+let of_worker ~root_pkgs w =
   let module W = Ocaml_multicore_ci_api.Worker.Selection in
-  let { W.id; packages; commits; _ } = w in
+  let { W.id; packages; commits; compat_pkgs } = w in
   let variant = Variant.of_string id in
+  let only_packages = if root_pkgs = compat_pkgs then [] else compat_pkgs in
   let commits = commits in
-  { variant; packages; commits; command = None }
+  { variant; packages; only_packages; commits; command = None }
 
 let remove_package t ~package =
   {
diff --git a/lib/selection.mli b/lib/selection.mli
index df1baa3..3ad031f 100644
--- a/lib/selection.mli
+++ b/lib/selection.mli
@@ -1,14 +1,19 @@
 type t = {
   variant : Variant.t;  (** The variant image to build on. *)
   packages : string list;  (** The selected packages ("name.version"). *)
+  only_packages : string list; [@default []]
+      (** Local root packages to include (empty to include all). *)
   commits : (string * string) list;
       (** The list of (repo_url,commit) opam-repositories from the analysis, and
           they're all usefull during the build.*)
   command : string option;
-      (** The build command to use (will default to dune build @install @runtest if not specified) *)
+      (** The build command to use (will default to dune build \@install
+          \@runtest if not specified) *)
 }
 [@@deriving yojson, ord]
 (** A set of packages for a single build. *)
 
-val of_worker : Ocaml_multicore_ci_api.Worker.Selection.t -> t
+val of_worker :
+  root_pkgs:string list -> Ocaml_multicore_ci_api.Worker.Selection.t -> t
+
 val remove_package : t -> package:string -> t