diff --git a/.gitmodules b/.gitmodules
new file mode 100644
index 0000000..3fd4a62
--- /dev/null
+++ b/.gitmodules
@@ -0,0 +1,4 @@
+[submodule "scache"]
+	path = scache
+	url = https://github.com/moyodiallo/scache.git
+	branch = master
diff --git a/Dockerfile b/Dockerfile
index 8b793ce..8c7938d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,9 +1,11 @@
 FROM ocaml/opam:debian-12-ocaml-5.1 AS build
 RUN sudo apt-get update && sudo apt-get install libev-dev capnproto m4 pkg-config libsqlite3-dev libgmp-dev libzstd-dev -y --no-install-recommends
-RUN cd ~/opam-repository && git fetch -q origin master && git reset --hard b61304c6db353e679a36720d8b914b029d6fbc0c && opam update
+RUN cd ~/opam-repository && git fetch -q origin master && git reset --hard bc52affc41b55ff00c0d3ac9a376538d79695aaf && opam update
 RUN sudo ln -sf /usr/bin/opam-2.1 /usr/bin/opam
+COPY --chown=opam scache/scache.opam /src/scache/
 COPY --chown=opam solver-service.opam solver-service-api.opam /src/
 WORKDIR /src
+RUN opam pin add -yn scache.dev "./scache"
 RUN opam install -y --deps-only .
 ADD --chown=opam . .
 RUN opam exec -- dune build @install
diff --git a/bin/main.ml b/bin/main.ml
index 2d19373..9d759cc 100644
--- a/bin/main.ml
+++ b/bin/main.ml
@@ -75,24 +75,25 @@ let start_server ~service vat_config =
   let+ vat = Capnp_rpc_unix.serve vat_config ~restore in
   Capnp_rpc_unix.Vat.sturdy_uri vat service_id
 
-let main_service () solver cap_file vat_config =
-  let uri = start_server ~service:(Solver_service.Service.v solver) vat_config in
+let main_service () solver cacheable cap_file vat_config =
+  let uri = start_server ~service:(Solver_service.Service.v ~cacheable solver) vat_config in
   Capnp_rpc_unix.Cap_file.save_uri uri cap_file |> or_die;
   Fmt.pr "Wrote solver service's address to %S@." cap_file;
   Fiber.await_cancel ()
 
-let main_service_pipe () solver =
+let main_service_pipe () solver cacheable =
   let socket = Lwt_unix.stdin in
   (* Run locally reading from socket *)
-  export (Solver_service.Service.v solver) ~on:socket
+  export (Solver_service.Service.v ~cacheable solver) ~on:socket
 
-let main_cluster () solver name capacity register_addr =
+let main_cluster () solver cacheable name capacity register_addr =
   let vat = Capnp_rpc_unix.client_only_vat () in
   let sr = Capnp_rpc_unix.Vat.import_exn vat register_addr in
   let `Cancelled =
     Solver_worker.run solver sr
       ~name
       ~capacity
+      ~cacheable
   in
   ()
 
@@ -128,6 +129,11 @@ let capacity =
   @@ Arg.info ~doc:"The number of cluster jobs that can run in parallel" ~docv:"N"
     [ "capacity" ]
 
+let cacheable =
+  Arg.value
+  @@ Arg.flag
+  @@ Arg.info ~doc:"Activate the cache system" [ "activate-cache"; "cache" ]
+
 let cap_file =
   Arg.required
   @@ Arg.opt Arg.(some string) None
@@ -168,6 +174,7 @@ let () =
         const main_service
         $ setup_log
         $ solver
+        $ cacheable
         $ cap_file
         $ Capnp_rpc_unix.Vat_config.cmd
       )
@@ -179,6 +186,7 @@ let () =
         const main_service_pipe
         $ setup_log
         $ solver
+        $ cacheable
       )
   in
   let run_agent =
@@ -188,6 +196,7 @@ let () =
         const main_cluster
         $ setup_log
         $ solver
+        $ cacheable
         $ worker_name
         $ capacity
         $ register_addr
diff --git a/dune b/dune
index 1e8abdc..ca70495 100644
--- a/dune
+++ b/dune
@@ -1 +1,3 @@
-(dirs :standard \ var cache)
+(dirs :standard \ var)
+
+(vendored_dirs scache)
diff --git a/dune-project b/dune-project
index 2711d3d..3bbfbff 100644
--- a/dune-project
+++ b/dune-project
@@ -1,48 +1,97 @@
 (lang dune 3.7)
+
 (name solver-service)
 
-(formatting (enabled_for dune))
+(formatting
+ (enabled_for dune))
+
 (generate_opam_files true)
-(source (github ocurrent/solver-service))
-(authors "Thomas Leonard <talex5@gmail.com>" "Patrick Ferris <patrick@sirref.org>")
-(maintainers "alpha@tarides.com" "Tim McGilchrist <timmcgil@gmail.com>")
+
+(source
+ (github ocurrent/solver-service))
+
+(authors
+ "Thomas Leonard <talex5@gmail.com>"
+ "Patrick Ferris <patrick@sirref.org>")
+
+(maintainers alpha@tarides.com "Tim McGilchrist <timmcgil@gmail.com>")
 
 (package
  (name solver-service)
  (synopsis "Choose package versions to test")
  (depends
-  (ocaml (>= 5.0.0))
-  ; Examples dependencies
-  (current_web (and (>= 0.6.4) :with-test))
-  (current_git (and (>= 0.6.4) :with-test))
-  (current_github (and (>= 0.6.4) :with-test))
-  (current_ocluster (and (>= 0.2.1) :with-test))
-  (ppx_deriving_yojson (>= 3.6.1))
-  (ppx_deriving (>= 5.1))
-  (yojson (>= 2.1.0))
-  (lwt (>= 5.6.1))
-  (eio (>= 0.12))
-  (eio_main (>= 0.12))
-  (lwt_eio (>= 0.5))
-  (logs (>= 0.7.0))
-  (fmt (>= 0.9.0))
-  (ocaml-version (>= 3.6.1))
-  (solver-service-api (= :version))
-  (dune-build-info (>= 3.8.0))
-  (opam-0install (>= 0.4.3))
-  (git-unix (>= 3.12.0))
-  (ocluster-api (>= 0.2.1))
-  (prometheus-app (>= 1.2))
-  (capnp-rpc-net (>= 1.2.3))
-  (capnp-rpc-unix (>= 1.2.3)))
- (conflicts (carton (< 0.4.2))))
+  (opam-file-format
+   (>= 2.1.6))
+  (ocaml
+   (>= 5.0.0))
+  (current_web
+   (and
+    (>= 0.6.4)
+    :with-test))
+  (current_git
+   (and
+    (>= 0.6.4)
+    :with-test))
+  (current_github
+   (and
+    (>= 0.6.4)
+    :with-test))
+  (current_ocluster
+   (and
+    (>= 0.2.1)
+    :with-test))
+  (ppx_deriving_yojson
+   (>= 3.6.1))
+  (ppx_deriving
+   (>= 5.1))
+  (yojson
+   (>= 2.1.0))
+  (lwt
+   (>= 5.6.1))
+  (eio
+   (>= 0.12))
+  (eio_main
+   (>= 0.12))
+  (lwt_eio
+   (>= 0.5))
+  (logs
+   (>= 0.7.0))
+  (fmt
+   (>= 0.9.0))
+  sqlite3
+  (ocaml-version
+   (>= 3.6.1))
+  (solver-service-api
+   (= :version))
+  (dune-build-info
+   (>= 3.8.0))
+  (opam-0install
+   (>= 0.4.3))
+  (git-unix
+   (>= 3.12.0))
+  (ocluster-api
+   (>= 0.2.1))
+  (prometheus-app
+   (>= 1.2))
+  (capnp-rpc-net
+   (>= 1.2.3))
+  (capnp-rpc-unix
+   (>= 1.2.3)))
+ (conflicts
+  (carton
+   (< 0.4.2))))
 
 (package
  (name solver-service-api)
  (synopsis "Cap'n Proto API for the solver service")
  (depends
-  (ocaml (>= 4.14.1))
-  (capnp (>= 3.5.0))
-  (capnp-rpc-lwt (>= 1.2.3))
-  (ppx_deriving_yojson (>= 3.6.1))
-  (ppx_deriving (>= 5.1))))
+  (ocaml
+   (>= 4.14.1))
+  (capnp
+   (>= 3.5.0))
+  (capnp-rpc-lwt
+   (>= 1.2.3))
+  (ppx_deriving_yojson
+   (>= 3.6.1))
+  (ppx_deriving
+   (>= 5.1))))
\ No newline at end of file
diff --git a/scache b/scache
new file mode 160000
index 0000000..265c821
--- /dev/null
+++ b/scache
@@ -0,0 +1 @@
+Subproject commit 265c8217f251edd59ed0a8074adcb4c42c5e57cf
diff --git a/service/dune b/service/dune
index bd4a7e8..82e85b6 100644
--- a/service/dune
+++ b/service/dune
@@ -7,5 +7,9 @@
   solver-service-api
   opam-0install
   capnp-rpc-net
+  opam-file-format
   git-unix
-  ocaml-version))
+  scache
+  ocaml-version)
+ (preprocess
+  (pps ppx_deriving_yojson)))
diff --git a/service/git_clone.ml b/service/git_clone.ml
new file mode 100644
index 0000000..ddb32fe
--- /dev/null
+++ b/service/git_clone.ml
@@ -0,0 +1,181 @@
+open Eio.Std
+
+module Store = Git_unix.Store
+
+module type CacheType = sig
+  type cache
+  val dir: cache -> string
+  val process_mgr : cache -> [`Generic] Eio.Process.mgr_ty r
+end
+
+module Make (Cache : CacheType) = struct
+
+  type t = Cache.cache
+
+  let git_command ?cwd args =
+    "git" ::
+    match cwd with
+    | Some dir -> "-C" :: dir :: args
+    | None -> args
+
+  let [@warning "-27"] grep_command ?cwd args = "grep" :: args
+
+  let [@warning "-27"] rm_command ?cwd args = "rm" :: args
+
+  let run_git ?cwd t args =
+    Eio.Process.run (Cache.process_mgr t) (git_command ?cwd args)
+
+  let run_rm ?cwd t args =
+    Eio.Process.run (Cache.process_mgr t) (rm_command ?cwd args)
+
+  let line_opt r =
+    if Eio.Buf_read.at_end_of_input r then None
+    else Some (Eio.Buf_read.line r)
+
+  let run_git_line ?cwd t args =
+    Eio.Process.parse_out (Cache.process_mgr t) line_opt (git_command ?cwd args)
+
+  let take_all_opt r =
+    if Eio.Buf_read.at_end_of_input r then None
+    else Some (Eio.Buf_read.take_all r)
+
+  let run_take_all ?cwd ?stdin t args command =
+    Eio.Process.parse_out ?stdin (Cache.process_mgr t) take_all_opt (command ?cwd args)
+
+  let lines_opt r =
+    if Eio.Buf_read.at_end_of_input r then None
+    else (Eio.Buf_read.map List.of_seq Eio.Buf_read.lines) r |> Option.some
+
+  let run_lines ?cwd ?stdin t args command =
+    Eio.Process.parse_out ?stdin (Cache.process_mgr t) lines_opt (command ?cwd args)
+
+  let run_git_lines ?cwd ?stdin t args = run_lines ?cwd ?stdin t args git_command
+
+  let run_git_take_all ?cwd ?stdin t args = run_take_all ?cwd ?stdin t args git_command 
+
+  let run_grep_lines ?cwd ?stdin t args = run_lines ?cwd ?stdin t args grep_command
+
+  let replace_special =
+    String.map @@ function
+    | 'A'..'Z'
+      | 'a'..'z'
+      | '0'..'9'
+      | '-' as c -> c
+    | _ -> '_'
+  let rec mkdir_p path =
+    try Unix.mkdir path 0o700 with
+      | Unix.Unix_error (EEXIST, _, _) -> ()
+      | Unix.Unix_error (ENOENT, _, _) ->
+      let parent = Filename.dirname path in
+      mkdir_p parent;
+      Unix.mkdir path 0o700
+
+  let repo_url_to_clone_path t repo_url =
+    let cache_dir = Cache.dir t in
+    let uri = Uri.of_string repo_url in
+    let sane_host =
+      match Uri.host uri with
+      | Some host -> replace_special host
+      | None -> "no_host"
+    in
+    let sane_path =
+      Uri.(
+      path uri
+      |> pct_decode
+      |> Filename.chop_extension
+      |> replace_special)
+    in
+    Fpath.(v cache_dir / sane_host / sane_path)
+
+  let remove t repo_url =
+    let clone_path = repo_url_to_clone_path t repo_url in
+    let clone_path_str = Fpath.to_string clone_path in
+    match Unix.lstat clone_path_str with
+    | Unix.{ st_kind = S_DIR; _ } -> (
+      try
+        run_rm t ["-fr"; clone_path_str]
+      with Eio.Io _ as ex ->
+        let bt = Printexc.get_raw_backtrace () in
+        Eio.Exn.reraise_with_context ex bt "removing %S" clone_path_str)
+    | _ -> ()
+
+  let clone ~bare t repo_url =
+    let clone_path = repo_url_to_clone_path t repo_url in
+    let clone_parent = Fpath.parent clone_path |> Fpath.to_string in
+    let clone_path_str = Fpath.to_string clone_path in
+    match Unix.lstat clone_path_str with
+    | Unix.{ st_kind = S_DIR; _ } -> ()
+    | _ -> Fmt.failwith "%S is not a directory!" clone_path_str
+    | exception Unix.Unix_error (Unix.ENOENT, _, _) ->
+      mkdir_p clone_parent;
+      try
+        if bare then
+          run_git t ["clone"; "--bare"; repo_url; clone_path_str]
+        else
+          run_git t ["clone"; repo_url; clone_path_str]
+      with Eio.Io _ as ex ->
+        let bt = Printexc.get_raw_backtrace () in
+        Eio.Exn.reraise_with_context ex bt "cloning %S" repo_url
+
+  let clone_bare t repo_url = clone ~bare:true t repo_url
+
+  let clone t repo_url = clone ~bare:false t repo_url
+
+  let fetch t repo_url =
+    try
+      let clone_path = repo_url_to_clone_path t repo_url |> Fpath.to_string in
+      run_git t ~cwd:clone_path ["fetch"; "origin"]
+    with Eio.Io _ as ex ->
+      let bt = Printexc.get_raw_backtrace () in
+      Eio.Exn.reraise_with_context ex bt "fetching %S" repo_url
+
+  let pull t repo_url =
+    try
+      let clone_path = repo_url_to_clone_path t repo_url |> Fpath.to_string in
+      run_git t ~cwd:clone_path ["pull"; "origin"]
+    with Eio.Io _ as ex ->
+      let bt = Printexc.get_raw_backtrace () in
+      Eio.Exn.reraise_with_context ex bt "pulling %S" repo_url
+
+  let all_commits_rev t repo_url =
+    let clone_path = repo_url_to_clone_path t repo_url |> Fpath.to_string in
+    run_git_lines t ~cwd:clone_path
+    @@ "log"
+       :: "--reverse"
+       :: [ "--format=format:%H" ]
+    |> Option.value ~default:[]
+
+  let diff_pkgs t ~repo_url ~new_commit ~old_commit =
+    let clone_path = repo_url_to_clone_path t repo_url |> Fpath.to_string in
+    run_git_take_all t ~cwd:clone_path
+    @@ "diff"
+       :: old_commit
+       :: new_commit
+       :: "--"
+       :: [ "packages" ]
+    |> function
+    | None -> []
+    | Some diff ->
+      try
+        run_grep_lines ~stdin:(Eio.Flow.string_source diff) t ["^... ./packages/.*/opam"]
+        |> Option.value ~default:[]
+        |> List.filter_map (fun path -> Astring.String.cut ~sep:"/" path |> Option.map snd)
+      with _ -> [] (* grep could exits with status 1 if there's no match *)
+
+  let oldest_commit_with t ~repo_url ~from paths =
+    let clone_path = repo_url_to_clone_path t repo_url |> Fpath.to_string in
+    run_git_line t ~cwd:clone_path
+    @@ "log"
+       :: "-n" :: "1"
+       :: "--format=format:%H"
+       :: from
+       :: "--"
+       :: paths
+
+  let open_store t repo_url =
+    let path = repo_url_to_clone_path t repo_url in
+    match Lwt_eio.run_lwt (fun () -> Git_unix.Store.v ~dotgit:path path) with
+    | Ok x -> x
+    | Error e ->
+      Fmt.failwith "Failed to open %a: %a" Fpath.pp path Store.pp_error e
+end
diff --git a/service/service.ml b/service/service.ml
index 2732d8c..85393a7 100644
--- a/service/service.ml
+++ b/service/service.ml
@@ -1,6 +1,6 @@
 module Worker = Solver_service_api.Worker
 
-let v t =
+let v ?cacheable t =
   let open Capnp_rpc_lwt in
   let module X = Solver_service_api.Raw.Service.Solver in
   X.local
@@ -27,7 +27,7 @@ let v t =
                (Capnp_rpc.Error.exn "Bad JSON in request: %s" msg))
         | Ok request ->
           Lwt_eio.run_eio @@ fun () ->
-          let selections = Solver.solve t ~log request in
+          let selections = Solver.solve ?cacheable t ~log request in
           let json =
             Yojson.Safe.to_string
               (Worker.Solve_response.to_yojson selections)
diff --git a/service/service.mli b/service/service.mli
index bb2d4c7..7cc6f87 100644
--- a/service/service.mli
+++ b/service/service.mli
@@ -1,2 +1,2 @@
-val v : Solver.t -> Solver_service_api.Solver.t
+val v : ?cacheable:bool -> Solver.t -> Solver_service_api.Solver.t
 (** [capnp_service t] is a Cap'n Proto service that handles requests using [t]. *)
diff --git a/service/solve_cache.ml b/service/solve_cache.ml
new file mode 100644
index 0000000..526fbd3
--- /dev/null
+++ b/service/solve_cache.ml
@@ -0,0 +1,254 @@
+open Eio.Std
+
+module Worker = Solver_service_api.Worker
+module Log_data = Solver_service_api.Solver.Log
+module Cache = Scache.Cache
+module Selections = Set.Make(Worker.Selection)
+
+exception Invalidated
+
+type t = {
+  cache_dir : string;
+  process_mgr : [`Generic] Eio.Process.mgr_ty r;
+}
+
+let create ~cache_dir ~proc_mgr =
+  {
+    cache_dir = Fpath.(v cache_dir / "solve") |> Fpath.to_string;
+    process_mgr = proc_mgr
+  }
+
+module Git_clone = Git_clone.Make ( struct
+    type cache = t
+    let dir t = t.cache_dir
+    let process_mgr t = t.process_mgr
+end)
+
+module Solve_cache = struct
+
+  type t = {
+    request: Worker.Solve_request.t;
+    (** The request *)
+    solve_response : Worker.Solve_response.t;
+    (** Response of the solved request *)
+    last_opam_repository_commits: (string * string) list;
+    (** Pair of repo URL and commit hash, for each last opam-repository used during the request *)
+  }[@@deriving yojson]
+
+  let marshal t = t |> to_yojson |> Yojson.Safe.to_string
+  let unmarshal t = t |> Yojson.Safe.from_string |> of_yojson |> Result.get_ok
+
+end
+
+let mutex = Lazy.from_fun (fun () -> Eio.Mutex.create ())
+
+let cache = Cache.start ~name:"solve"
+
+let digest_request request =
+  request
+  |> Worker.Solve_request.to_yojson 
+  |> Yojson.Safe.to_string
+  |> Digest.string
+  |> Digest.to_hex
+
+let get_solve ~cache ~digest : Solve_cache.t option =
+  match Cache.get cache ~key:digest with
+  | None -> None
+  | Some r -> Solve_cache.unmarshal r |> Option.some
+
+let set_solve ~cache ~solve_cache ~digest =
+  solve_cache
+  |> Solve_cache.marshal
+  |> fun response -> Cache.set cache ~key:digest ~value:response
+
+let remove_commits opam_repository_commits =
+  opam_repository_commits |> List.map (fun (url,_) -> (url,""))
+
+(* important because of the digest_request *)
+let sort_by_url opam_repository_commits = 
+  opam_repository_commits
+  |> List.sort (fun (url1,_) (url2,_) -> String.compare url1 url2)
+
+let is_same_solution ~solve_response_cache ~solve_response =
+  match solve_response_cache, solve_response with
+  | Error _, _ -> false
+  | _, Error _ -> false
+  | Ok selections_cache, Ok selections ->
+    let selections_cache =
+      List.map (fun sel -> { sel with Worker.Selection.commits = []}) selections_cache
+    in
+    let selections =
+      List.map (fun sel -> { sel with Worker.Selection.commits = []}) selections
+    in
+    Selections.equal (Selections.of_list selections_cache) (Selections.of_list selections)
+
+let yojson_of_list l = l |> [%to_yojson: string list]
+let yojosn_to_list l = l |> [%of_yojson: string list]
+  
+(* opam-repo comits with their rank *)
+let opam_commits = Lazy.from_fun (fun () -> Hashtbl.create 10)
+
+let update_commit t repo_url commit =
+  let opam_commits = Lazy.force opam_commits in
+  let mutex = Lazy.force mutex in
+  let get_repo t repo_url =
+    Git_clone.clone t repo_url;
+    Git_clone.pull t repo_url; 
+    Git_clone.all_commits_rev t repo_url
+  in
+  match Hashtbl.find_opt opam_commits commit with
+  | Some _ -> ()
+  | None -> 
+    Eio.Mutex.use_rw mutex ~protect:true (fun () ->
+      try
+        get_repo t repo_url;
+      with _ -> (
+        (* could be a conflict between commits when pulling *)
+        Git_clone.remove t repo_url;
+        get_repo t repo_url))
+    |> List.iteri (fun rank commit -> Hashtbl.replace opam_commits commit rank)
+
+let changed_packages t ~new_opam_repo ~old_opam_repo =
+  if new_opam_repo = old_opam_repo then
+    Some []
+  else
+    let opam_commits = Lazy.force opam_commits in
+    try
+      (* new_opam_repo and old_opam_repo nead to be sorted by url *)
+      List.combine new_opam_repo old_opam_repo
+      |> List.map (fun ((repo_url,new_commit), (_,old_commit)) ->
+        let key = ("diff"^new_commit^"-"^old_commit) in
+        match Cache.get cache ~key with
+        | Some pkgs -> Yojson.Safe.from_string pkgs |> yojosn_to_list |> Result.get_ok
+        | None -> (
+          update_commit t repo_url new_commit;
+          update_commit t repo_url old_commit;
+          match Hashtbl.find_opt opam_commits new_commit, Hashtbl.find_opt opam_commits old_commit with
+          | Some new_rank, Some old_rank when new_rank < old_rank ->
+            (* This new commit is supposed to be newer in the commit history,
+             this could be a specific request on opam commits, like fixed demand
+             so it invalidated *)
+            raise Invalidated
+          | Some _, Some _ ->
+            let pkgs_filename = Git_clone.diff_pkgs t ~repo_url ~new_commit ~old_commit in
+            Cache.set cache ~key ~value:(Yojson.Safe.to_string (yojson_of_list pkgs_filename));
+            pkgs_filename
+          | None, _ ->
+            Fmt.epr "The repo %s has not the commit %s@." repo_url new_commit; raise Invalidated
+          | _, None ->
+            Fmt.epr "The repo %s has not the commit %s@." repo_url old_commit; raise Invalidated))
+      |> List.flatten
+      |> Option.some
+    with Invalidated -> None
+
+let get_names = OpamFormula.fold_left (fun a (name, _) -> name :: a) []
+
+let deps_of_opam_file opam_pkgs =
+  opam_pkgs
+  |> List.map (fun (_, content) ->
+    OpamFile.OPAM.read_from_string content |> OpamFile.OPAM.depends |> get_names)
+
+let is_invalidated t ~request ~solve_cache =
+  let {
+    Worker.Solve_request.opam_repository_commits;
+    root_pkgs;
+    pinned_pkgs; _ } = request 
+  in
+  let request_pkgs () =
+    List.concat_map (fun pkgs_name -> deps_of_opam_file pkgs_name) [root_pkgs; pinned_pkgs]
+    |> List.flatten
+    |> OpamPackage.Name.Set.of_list
+  in
+  let response_pkgs () =
+    solve_cache.Solve_cache.solve_response
+    |> Result.get_ok
+    |> List.map (fun selection -> selection.Worker.Selection.packages)
+    |> List.concat
+    |> List.map (fun pkg_version ->
+      pkg_version
+      |> Astring.String.cut ~sep:"."
+      |> Option.get
+      |> fun (name,version) ->
+        OpamPackage.create  (OpamPackage.Name.of_string name) (OpamPackage.Version.of_string version))
+    |> OpamPackage.Set.of_list
+  in
+  let old_opam_repo =
+    solve_cache.Solve_cache.last_opam_repository_commits
+    |> List.sort (fun (url1,_) (url2,_) -> String.compare url1 url2)
+  in
+  let new_opam_repo =
+    opam_repository_commits
+    |> List.sort (fun (url1,_) (url2,_) -> String.compare url1 url2)
+  in
+  match changed_packages t ~old_opam_repo ~new_opam_repo with
+  | None -> true (* Invalidate when a commit does not exist *)
+  | Some pkgs ->
+    pkgs
+    |> List.find_opt (fun pkg ->
+      let request_pkgs = request_pkgs () in
+      let response_pkgs = response_pkgs () in
+      OpamFilename.raw pkg
+      |> OpamPackage.of_filename
+      |> Option.get
+      |> fun opam_pkg ->
+        OpamPackage.Name.Set.mem (OpamPackage.name opam_pkg) request_pkgs || OpamPackage.Set.mem opam_pkg response_pkgs)
+    |> Option.is_some
+
+(**
+    There is 2 stage of looking for the cache:
+      * With opam repository URL and their commit: (url,commit) list
+      * Only the opam repository URL: (url,_) list
+
+    When the cache is hited with only opam URLs, it try to invalidate it
+    because of the opam repository commit could be updated with new commit.
+
+    The invalidation is about looking if the request packages is involve in
+    the 2 different commit, the commit of the cached response and the commit of
+    the request. Also the cache response contain the transitive dependencies, we
+    make sure those ones are not also involve in the commit changes.
+
+    The oldest commit used during the solve is kept when the response is the same
+    as previous solve.
+*)
+let solve t ~solve log (request: Worker.Solve_request.t) =
+  let request =
+    { request with opam_repository_commits = sort_by_url request.opam_repository_commits }
+  in
+  let solve () = solve ~log request in
+  match get_solve ~cache ~digest:(digest_request request) with
+  | Some solve_cache when Result.is_ok solve_cache.solve_response -> (
+    Log_data.info log "From cache@.";
+    solve_cache.solve_response)
+  | _ -> (
+    let req =
+      { request with opam_repository_commits = remove_commits request.opam_repository_commits }
+    in
+    match get_solve ~cache ~digest:(digest_request req) with
+    | Some solve_cache when not (Result.is_error solve_cache.solve_response || is_invalidated t ~request ~solve_cache) ->
+      Log_data.info log "From cache@.";
+      let solve_cache =
+        { solve_cache with last_opam_repository_commits = request.opam_repository_commits }
+      in
+      set_solve ~cache ~solve_cache ~digest:(digest_request req);
+      set_solve ~cache ~solve_cache ~digest:(digest_request request);
+      solve_cache.solve_response
+    | Some solve_cache when Result.is_ok solve_cache.solve_response ->
+      let solve_response = solve () in
+      let solve_cache =
+        if is_same_solution ~solve_response_cache:solve_cache.solve_response ~solve_response then
+          { solve_cache with last_opam_repository_commits = request.opam_repository_commits }
+        else
+          {request; solve_response; last_opam_repository_commits = request.opam_repository_commits }
+      in
+      set_solve ~cache ~solve_cache ~digest:(digest_request req);
+      set_solve ~cache ~solve_cache ~digest:(digest_request request);
+      solve_cache.solve_response
+    | _ ->
+      let solve_response = solve () in
+      let solve_cache =
+        { Solve_cache.request; solve_response; last_opam_repository_commits = request.opam_repository_commits }
+      in
+      set_solve ~cache ~digest:(digest_request req) ~solve_cache;
+      set_solve ~cache ~digest:(digest_request request) ~solve_cache;
+      solve_response
+  )
diff --git a/service/solve_cache.mli b/service/solve_cache.mli
new file mode 100644
index 0000000..1794d0d
--- /dev/null
+++ b/service/solve_cache.mli
@@ -0,0 +1,18 @@
+open Eio.Std
+module Worker = Solver_service_api.Worker
+
+(** Cache system where try to hit in a Sqlite3 database *)
+
+type t
+
+val create : cache_dir: string -> proc_mgr: [`Generic] Eio.Process.mgr_ty r -> t
+
+val solve :
+  t ->
+  solve:(log:Solver_service_api.Solver.Log.t -> Worker.Solve_request.t -> Worker.Solve_response.t) ->
+  Solver_service_api.Solver.Log.t ->
+  Worker.Solve_request.t ->
+  Worker.Solve_response.t
+(** [solve t ~solve log request] try to hit the cache, if missed it uses [t] and [solve] to solve the [request]
+
+    @param log Diagnostics about failed solves (and other logging) goes here. *)
diff --git a/service/solver.ml b/service/solver.ml
index 160d9da..db05ff5 100644
--- a/service/solver.ml
+++ b/service/solver.ml
@@ -8,6 +8,8 @@ let (let*!) = Result.bind
 type t = {
   pool : (Domain_worker.request, Domain_worker.reply) Pool.t;
   stores : Stores.t;
+  cache_dir : string;
+  process_mgr : [`Generic] Eio.Process.mgr_ty r;
 }
 
 let ocaml = OpamPackage.Name.of_string "ocaml"
@@ -30,7 +32,7 @@ let env vars v =
   if List.mem v OpamPackageVar.predefined_depends_variables then None
   else Domain_worker.env vars (OpamVariable.Full.to_string v)
 
-let solve_for_platform ?cancelled t ~log ~opam_repository_commits ~packages ~root_pkgs ~pinned_pkgs ~pins ~vars id =
+let solve_for_platform ?cancelled t ~cacheable ~log ~opam_repository_commits ~packages ~root_pkgs ~pinned_pkgs ~pins ~vars id =
   let ocaml_version = OpamPackage.Version.of_string vars.Worker.Vars.ocaml_version in
   let root_pkgs =
     root_pkgs
@@ -55,18 +57,25 @@ let solve_for_platform ?cancelled t ~log ~opam_repository_commits ~packages ~roo
         Error (`No_solution e)
       | Ok packages ->
         Log.info log "%s: found solution in %.2f s" id time;
-        let repo_packages =
-          packages
-          |> List.filter_map (fun (pkg : OpamPackage.t) ->
-              if OpamPackage.Name.Set.mem pkg.name pins then None
-              else Some pkg)
-        in
-        (* Hack: ocaml-ci sometimes also installs odoc, but doesn't tell us about it.
+        let commits =
+          if cacheable then
+            (* The cache system handle a sort of oldest_commit*)
+            opam_repository_commits
+          else
+            let repo_packages =
+              packages
+              |> List.filter_map (fun (pkg : OpamPackage.t) ->
+                if OpamPackage.Name.Set.mem pkg.name pins then None
+                else Some pkg)
+            in
+            (* Hack: ocaml-ci sometimes also installs odoc, but doesn't tell us about it.
            Make sure we have at least odoc 2.1.1 available, otherwise it won't work on OCaml 5.0. *)
-        let repo_packages =
-          OpamPackage.of_string "odoc.2.1.1" :: repo_packages
+            let repo_packages =
+              OpamPackage.of_string "odoc.2.1.1" :: repo_packages
+            in
+            let commits = Stores.oldest_commits_with t.stores repo_packages ~from:opam_repository_commits in
+            commits
         in
-        let commits = Stores.oldest_commits_with t.stores repo_packages ~from:opam_repository_commits in
         let compat_pkgs =
           let to_string (name, (version,_)) = OpamPackage.to_string (OpamPackage.create name version) in
           List.map to_string root_pkgs
@@ -96,7 +105,7 @@ let rec parse_opams = function
     Ok (x :: xs)
 
 (* Handle a request by distributing it among the worker processes and then aggregating their responses. *)
-let solve ?cancelled t ~log request =
+let solve ?cancelled ~cacheable t ~log request =
   let {
     Worker.Solve_request.opam_repository_commits;
     platforms;
@@ -124,6 +133,7 @@ let solve ?cancelled t ~log request =
         let result =
           solve_for_platform t id
             ?cancelled
+            ~cacheable
             ~log
             ~opam_repository_commits
             ~packages
@@ -161,10 +171,20 @@ let solve ?cancelled t ~log request =
     | [] -> Ok results
     | errors -> Fmt.error_msg "@[<v>%a@]" Fmt.(list ~sep:cut string) errors
 
+let solve ?cacheable ?cancelled t ~log request =
+  match cacheable with
+  | Some true ->
+    let solve = solve t ?cancelled ~cacheable:true in
+    let cache = Solve_cache.create ~cache_dir:t.cache_dir ~proc_mgr:t.process_mgr in 
+    Solve_cache.solve cache ~solve log request
+  | _ -> solve ?cancelled ~cacheable:false t ~log request
+
 let create ~sw ~domain_mgr ~process_mgr ~cache_dir ~n_workers =
   let stores = Stores.create ~process_mgr ~cache_dir in
   let pool = Pool.create ~sw ~domain_mgr ~n_workers Domain_worker.solve in
   {
     stores;
     pool;
+    cache_dir;
+    process_mgr = (process_mgr :> [`Generic] Eio.Process.mgr_ty r);
   }
diff --git a/service/solver.mli b/service/solver.mli
index 7e3c19a..973356f 100644
--- a/service/solver.mli
+++ b/service/solver.mli
@@ -17,6 +17,7 @@ val create :
     @param n_workers Maximum number of worker domains. *)
 
 val solve :
+  ?cacheable:bool ->
   ?cancelled:unit Eio.Promise.t ->
   t ->
   log:Solver_service_api.Solver.Log.t ->
diff --git a/service/stores.ml b/service/stores.ml
index f546db5..6ace9b2 100644
--- a/service/stores.ml
+++ b/service/stores.ml
@@ -16,110 +16,31 @@ type t = {
   mutable index_cache : (commit list * Packages.t Promise.or_exn) option;
 }
 
-let git_command ?cwd args =
-  "git" ::
-  match cwd with
-  | Some dir -> "-C" :: dir :: args
-  | None -> args
-
-let run_git ?cwd t args =
-  Eio.Process.run t.process_mgr (git_command ?cwd args)
-
-let line_opt r =
-  if Eio.Buf_read.at_end_of_input r then None
-  else Some (Eio.Buf_read.line r)
-
-let run_git_line ?cwd t args =
-  Eio.Process.parse_out t.process_mgr line_opt (git_command ?cwd args)
-
 module Git_clone = struct
-  let replace_special =
-    String.map @@ function
-    | 'A'..'Z'
-    | 'a'..'z'
-    | '0'..'9'
-    | '-' as c -> c
-    | _ -> '_'
-
-  let rec mkdir_p path =
-    try Unix.mkdir path 0o700 with
-    | Unix.Unix_error (EEXIST, _, _) -> ()
-    | Unix.Unix_error (ENOENT, _, _) ->
-      let parent = Filename.dirname path in
-      mkdir_p parent;
-      Unix.mkdir path 0o700
-
-  let repo_url_to_clone_path t repo_url =
-    let uri = Uri.of_string repo_url in
-    let sane_host =
-      match Uri.host uri with
-      | Some host -> replace_special host
-      | None -> "no_host"
-    in
-    let sane_path =
-      Uri.(
-        path uri
-        |> pct_decode
-        |> Filename.chop_extension
-        |> replace_special)
-    in
-    Fpath.(v t.cache_dir / sane_host / sane_path)
-
-  let clone t repo_url =
-    let clone_path = repo_url_to_clone_path t repo_url in
-    let clone_parent = Fpath.parent clone_path |> Fpath.to_string in
-    let clone_path_str = Fpath.to_string clone_path in
-    match Unix.lstat clone_path_str with
-    | Unix.{ st_kind = S_DIR; _ } -> ()
-    | _ -> Fmt.failwith "%S is not a directory!" clone_path_str
-    | exception Unix.Unix_error (Unix.ENOENT, _, _) ->
-      mkdir_p clone_parent;
-      try
-        run_git t ["clone"; "--bare"; repo_url; clone_path_str]
-      with Eio.Io _ as ex ->
-        let bt = Printexc.get_raw_backtrace () in
-        Eio.Exn.reraise_with_context ex bt "cloning %S" repo_url
-
-  let open_store t repo_url =
-    let path = repo_url_to_clone_path t repo_url in
-    match Lwt_eio.run_lwt (fun () -> Git_unix.Store.v ~dotgit:path path) with
-    | Ok x -> x
-    | Error e ->
-      Fmt.failwith "Failed to open %a: %a" Fpath.pp path Store.pp_error e
-
-  let oldest_commit_with t ~repo_url ~from paths =
-    let clone_path = repo_url_to_clone_path t repo_url |> Fpath.to_string in
-    run_git_line t ~cwd:clone_path
-    @@ "log"
-       :: "-n" :: "1"
-       :: "--format=format:%H"
-       :: from
-       :: "--"
-       :: paths
-
-  let oldest_commits_with t ~from pkgs =
-    let paths =
-      pkgs
-      |> List.map (fun pkg ->
-          let name = OpamPackage.name_to_string pkg in
-          let version = OpamPackage.version_to_string pkg in
-          Printf.sprintf "packages/%s/%s.%s" name name version)
-    in
-    from
-    |> Fiber.List.filter_map (fun (repo_url, hash) ->
-        oldest_commit_with t ~repo_url ~from:hash paths
-        |> Option.map (fun commit -> (repo_url, commit))
-      )
 
-  let fetch t repo_url =
-    try
-      let clone_path = repo_url_to_clone_path t repo_url |> Fpath.to_string in
-      run_git t ~cwd:clone_path ["fetch"; "origin"]
-    with Eio.Io _ as ex ->
-      let bt = Printexc.get_raw_backtrace () in
-      Eio.Exn.reraise_with_context ex bt "fetching %S" repo_url
+  include ( Git_clone.Make ( struct
+    type cache = t
+    let dir t = t.cache_dir
+    let process_mgr t = t.process_mgr
+  end))
+
+  let clone = clone_bare
 end
 
+let oldest_commits_with t ~from pkgs =
+  let paths =
+    pkgs
+    |> List.map (fun pkg ->
+      let name = OpamPackage.name_to_string pkg in
+      let version = OpamPackage.version_to_string pkg in
+      Printf.sprintf "packages/%s/%s.%s" name name version)
+  in
+  from
+  |> Fiber.List.filter_map (fun (repo_url, hash) ->
+    Git_clone.oldest_commit_with t ~repo_url ~from:hash paths
+    |> Option.map (fun commit -> (repo_url, commit))
+  )
+
 let oldest_commit = Eio.Semaphore.make 180
 (* we are using at most 360 pipes at the same time and that's enough to keep the current
  * performance and prevent some jobs to fail because of file descriptors exceed the limit.*)
@@ -191,7 +112,7 @@ let create ~process_mgr ~cache_dir =
 let oldest_commits_with t ~from repo_packages =
   Eio.Semaphore.acquire oldest_commit;
   Fun.protect ~finally:(fun () -> Eio.Semaphore.release oldest_commit) @@ fun () ->
-  Git_clone.oldest_commits_with t repo_packages ~from
+  oldest_commits_with t repo_packages ~from
 
 (* We could do this in parallel, except that there might be duplicate repos in the list. *)
 let rec fetch_commits t = function
diff --git a/solver-service.opam b/solver-service.opam
index a0feb66..a5cc92f 100644
--- a/solver-service.opam
+++ b/solver-service.opam
@@ -9,6 +9,7 @@ homepage: "https://github.com/ocurrent/solver-service"
 bug-reports: "https://github.com/ocurrent/solver-service/issues"
 depends: [
   "dune" {>= "3.7"}
+  "opam-file-format" {>= "2.1.6"}
   "ocaml" {>= "5.0.0"}
   "current_web" {>= "0.6.4" & with-test}
   "current_git" {>= "0.6.4" & with-test}
@@ -23,6 +24,7 @@ depends: [
   "lwt_eio" {>= "0.5"}
   "logs" {>= "0.7.0"}
   "fmt" {>= "0.9.0"}
+  "sqlite3"
   "ocaml-version" {>= "3.6.1"}
   "solver-service-api" {= version}
   "dune-build-info" {>= "3.8.0"}
diff --git a/test/dune b/test/dune
index f6453d5..658aa25 100644
--- a/test/dune
+++ b/test/dune
@@ -1,4 +1,4 @@
 (tests
  (names test)
  (package solver-service)
- (libraries solver-service eio_main lwt_eio))
+ (libraries solver-service scache eio_main lwt_eio))
diff --git a/test/test.expected b/test/test.expected
index d0e8843..dec48b2 100644
--- a/test/test.expected
+++ b/test/test.expected
@@ -246,3 +246,114 @@ results:
      packages: [foo.dev; ocaml-base-compiler.5.0]
      commits: [(opam-repo.git, 1bc28b8e8d98db6e524822c6f28bddebbc3504a3)]
      lower_bound: false]
+
+# Solve_cache
+
+## Select foo.1.0 ##
+
+commits: [(opam-repo.git,
+           [ocaml-base-compiler.5.0; bar.1.0; baz.1.0; foo.1.0; foobar.0.1])]
+root_pkgs: [app.dev]
+platforms: [debian-12-ocaml-5]
+Need to update opam-repo.git to get new commit 4733cec979c5946f667f86e0df0ac20741277d68
+results:
+  [debian-12-ocaml-5:
+     compat_pkgs: [app.dev]
+     packages: [app.dev; bar.1.0; baz.1.0; foo.1.0; ocaml-base-compiler.5.0]
+     commits: [(opam-repo.git, 4733cec979c5946f667f86e0df0ac20741277d68)]
+     lower_bound: false]
+
+## Foo 1.1 now available (A direct dependency, the result will contain the new commit) ##
+
+commits: [(opam-repo.git,
+           [foo.1.1; ocaml-base-compiler.5.0; bar.1.0; baz.1.0; foo.1.0;
+            foobar.0.1])]
+root_pkgs: [app.dev]
+platforms: [debian-12-ocaml-5]
+Already up to date.
+Need to update opam-repo.git to get new commit e82813b4ed60505d8511d132e77ca062776b0b10
+results:
+  [debian-12-ocaml-5:
+     compat_pkgs: [app.dev]
+     packages: [app.dev; bar.1.0; baz.1.0; foo.1.1; ocaml-base-compiler.5.0]
+     commits: [(opam-repo.git, e82813b4ed60505d8511d132e77ca062776b0b10)]
+     lower_bound: false]
+
+## Foo 1.1 again (hit the cache, the commit won't change) ##
+
+commits: [(opam-repo.git,
+           [foo.1.1; foo.1.1; ocaml-base-compiler.5.0; bar.1.0; baz.1.0;
+            foo.1.0; foobar.0.1])]
+root_pkgs: [app.dev]
+platforms: [debian-12-ocaml-5]
+Updating e82813b..07bc05a
+Fast-forward
+results:
+  [debian-12-ocaml-5:
+     compat_pkgs: [app.dev]
+     packages: [app.dev; bar.1.0; baz.1.0; foo.1.1; ocaml-base-compiler.5.0]
+     commits: [(opam-repo.git, e82813b4ed60505d8511d132e77ca062776b0b10)]
+     lower_bound: false]
+
+## Baz 1.0 is a transitive dep of Foo, the cache will be invalidated(the result will contain the new commit) ##
+
+commits: [(opam-repo.git,
+           [baz.1.0; foo.1.1; foo.1.1; ocaml-base-compiler.5.0; bar.1.0;
+            foo.1.0; foobar.0.1])]
+root_pkgs: [app.dev]
+platforms: [debian-12-ocaml-5]
+Updating 07bc05a..86fca69
+Fast-forward
+ packages/baz/baz.1.0/opam | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+Need to update opam-repo.git to get new commit 86fca698addda8dd554cffc2ea09bf616c351332
+results:
+  [debian-12-ocaml-5:
+     compat_pkgs: [app.dev]
+     packages: [app.dev; bar.1.0; baz.1.0; foo.1.1; foobar.0.1;
+                ocaml-base-compiler.5.0]
+     commits: [(opam-repo.git, 86fca698addda8dd554cffc2ea09bf616c351332)]
+     lower_bound: false]
+
+## Oof 1.0 now available (hit the cache, the commit won't change in the result) ##
+
+commits: [(opam-repo.git,
+           [oof.1.0; baz.1.0; foo.1.1; foo.1.1; ocaml-base-compiler.5.0;
+            bar.1.0; foo.1.0; foobar.0.1])]
+root_pkgs: [app.dev]
+platforms: [debian-12-ocaml-5]
+Updating 86fca69..0eb9328
+Fast-forward
+ packages/oof/oof.1.0/opam | 4 ++++
+ 1 file changed, 4 insertions(+)
+ create mode 100644 packages/oof/oof.1.0/opam
+results:
+  [debian-12-ocaml-5:
+     compat_pkgs: [app.dev]
+     packages: [app.dev; bar.1.0; baz.1.0; foo.1.1; foobar.0.1;
+                ocaml-base-compiler.5.0]
+     commits: [(opam-repo.git, 86fca698addda8dd554cffc2ea09bf616c351332)]
+     lower_bound: false]
+
+## Oof 1.1 now available (will invalidate the cache because foo 1.1 will be removed, the result will contain the new commit) ##
+
+commits: [(opam-repo.git,
+           [oof.1.1; ocaml-base-compiler.5.0; bar.1.0; baz.1.0; foo.1.0;
+            foobar.0.1])]
+root_pkgs: [app.dev]
+platforms: [debian-12-ocaml-5]
+Updating 0eb9328..44d1c6f
+Fast-forward
+ packages/baz/baz.1.0/opam              | 2 +-
+ packages/foo/foo.1.1/opam              | 4 ----
+ packages/oof/{oof.1.0 => oof.1.1}/opam | 0
+ 3 files changed, 1 insertion(+), 5 deletions(-)
+ delete mode 100644 packages/foo/foo.1.1/opam
+ rename packages/oof/{oof.1.0 => oof.1.1}/opam (100%)
+Need to update opam-repo.git to get new commit 44d1c6f9bf942a6ea6719e70e0f1340cfb6b3c9e
+results:
+  [debian-12-ocaml-5:
+     compat_pkgs: [app.dev]
+     packages: [app.dev; bar.1.0; baz.1.0; foo.1.0; ocaml-base-compiler.5.0]
+     commits: [(opam-repo.git, 44d1c6f9bf942a6ea6719e70e0f1340cfb6b3c9e)]
+     lower_bound: false]
diff --git a/test/test.ml b/test/test.ml
index cb37273..0498b56 100644
--- a/test/test.ml
+++ b/test/test.ml
@@ -231,6 +231,59 @@ let test_available t =
       "mac", { debian_12_ocaml_5 with os = "macos" };
     ]
 
+let test_solve_cache t =
+  let solve = solve_cache in
+  let opam_repo = Opam_repo.create "opam-repo.git" in
+  let root_pkgs = ["app.dev", {| depends: [ "foo" ] |}] in
+  let depends = {| depends: [ "ocaml-base-compiler" "bar" ] |} in
+  let opam_packages = [
+    "ocaml-base-compiler.5.0", "";
+    "bar.1.0", {| depends: [ "baz" ] |};
+    "baz.1.0", "";
+    "foo.1.0",depends;
+    "foobar.0.1", "";
+  ]
+  in
+  let first_opam_packages = opam_packages in
+  let recent_commits =
+    solve t "Select foo.1.0" ~platforms ~root_pkgs ~previous_commits:[opam_repo,[]]
+      ~commits:[opam_repo, opam_packages]
+  in
+  let opam_packages = ("foo.1.1",depends) :: opam_packages in
+  let recent_commits =
+  solve t "Foo 1.1 now available (A direct dependency, the result will contain the new commit)" ~previous_commits:recent_commits ~platforms ~root_pkgs ~commits:[
+    opam_repo, opam_packages
+  ]
+  in
+  let opam_packages = ("foo.1.1",depends)::opam_packages in
+  let recent_commits =
+    solve t "Foo 1.1 again (hit the cache, the commit won't change)" ~previous_commits:recent_commits ~platforms ~root_pkgs ~commits:[
+      opam_repo, opam_packages
+    ]
+  in
+  let opam_packages =
+   ("baz.1.0",{|depends: [ "foobar" ]|})
+    ::(List.remove_assoc "baz.1.0" opam_packages)
+  in
+  let recent_commits =
+    solve t "Baz 1.0 is a transitive dep of Foo, the cache will be invalidated(the result will contain the new commit)" ~previous_commits:recent_commits ~platforms ~root_pkgs ~commits:[
+      opam_repo, opam_packages
+    ]
+  in
+
+  let opam_packages = ("oof.1.0","") :: opam_packages in
+  let recent_commits =
+  solve t "Oof 1.0 now available (hit the cache, the commit won't change in the result)" ~previous_commits:recent_commits ~platforms ~root_pkgs ~commits:[
+    opam_repo, opam_packages
+  ]
+  in
+  solve t
+    "Oof 1.1 now available (will invalidate the cache because foo 1.1 will be removed, the result will contain the new commit)"
+    ~previous_commits:recent_commits ~platforms ~root_pkgs ~commits:[
+    opam_repo, ("oof.1.1","") :: first_opam_packages
+  ] |> ignore;
+  ()
+
 let () =
   Eio_main.run @@ fun env ->
   let domain_mgr = env#domain_mgr in
@@ -250,6 +303,7 @@ let () =
     "Pinned", test_pinned;
     "Cancel", test_cancel;
     "Available", test_available;
+    "Solve_cache", test_solve_cache;
   ]
   |> List.iter (fun (name, fn) ->
       Fmt.pr "@.# %s@." name;
diff --git a/test/utils.ml b/test/utils.ml
index 557879b..a206883 100644
--- a/test/utils.ml
+++ b/test/utils.ml
@@ -6,13 +6,18 @@ let add_opam_header s =
     synopsis: "Test package"
   |} ^ s
 
+module S = Git_unix.Store
+
 module Opam_repo : sig
-  type t
+  type t = {
+    name : string;
+    store : S.t;
+  }
 
   val create : string -> t
   (** [create path] opens a Git repository at [path], creating it if needed. *)
 
-  val commit : t -> (string * string) list -> string * string
+  val commit : ?parents:S.Value.Commit.hash list -> t -> (string * string) list -> string * string
   (** [commit t files] creates a commit with the given opam package files and returns
       the [(repo_url, hash]) pair for it. *)
 
@@ -20,7 +25,6 @@ module Opam_repo : sig
 end = struct
   (* A fake opam-repository upstream for testing. *)
 
-  module S = Git_unix.Store
 
   type t = {
     name : string;
@@ -85,7 +89,7 @@ end = struct
     in
     write_tree t (List.map write_version versions)
 
-  let commit t pkgs =
+  let commit ?parents t pkgs =
     let pkgs =
       group_packages pkgs
       |> OpamPackage.Name.Map.to_seq
@@ -95,10 +99,11 @@ end = struct
         )
     in
     let tree = write_tree t ["packages", `Dir (write_tree t pkgs)] in
-    let commit = S.Value.Commit.make ~tree ~author:test_user ~committer:test_user (Some "Commit") in
+    let commit = S.Value.Commit.make ?parents ~tree ~author:test_user ~committer:test_user (Some "Commit") in
     let commit = write t (S.Value.commit commit) in
     set_branch t Git.Reference.master commit;
     (t.name, S.Hash.to_hex commit)
+
 end
 
 let stderr_log =
@@ -153,3 +158,39 @@ let solve ?cancelled ?(pinned_pkgs=[]) t label ~commits ~root_pkgs ~platforms =
   in
   let response = Solver_service.Solver.solve ?cancelled t ~log:stderr_log req in
   Fmt.pr "@[<v2>results:@,%a@]@." pp_response response
+
+let solve_cache ?cancelled ?(pinned_pkgs=[])
+  t label ~previous_commits ~commits ~root_pkgs ~platforms =
+  Fmt.pr "@.## %s ##@.@.commits: %a@.root_pkgs: %a@.platforms: %a@."
+    label
+    pp_commits commits
+    pp_packages root_pkgs
+    pp_platforms platforms;
+  if pinned_pkgs <> [] then Fmt.pr "pinned: %a@." pp_packages pinned_pkgs;
+  let root_pkgs = List.map (fun (pkg, opam) -> pkg, add_opam_header opam) root_pkgs in
+  let pinned_pkgs = List.map (fun (pkg, opam) -> pkg, add_opam_header opam) pinned_pkgs in
+  let opam_repository_commits =
+    commits
+    |> List.map (fun (repo, packages) ->
+      let (_, parents) =
+        List.find (fun (repo_prev,_) -> String.equal repo_prev.Opam_repo.name repo.Opam_repo.name) previous_commits
+      in
+      let parents = List.map S.Hash.of_hex parents in
+      Opam_repo.commit ~parents repo packages)
+  in
+  let req = { Solver_service_api.Worker.Solve_request.
+              opam_repository_commits;
+              root_pkgs;
+              pinned_pkgs;
+              platforms;
+            }
+  in
+  let response = Solver_service.Solver.solve ~cacheable:true ?cancelled t ~log:stderr_log req in
+  Fmt.pr "@[<v2>results:@,%a@]@." pp_response response;
+  let recent_commits =
+    List.map (fun (repo_prev, repo_previous_commits) ->
+      let (_, commit) =
+        List.find (fun (repo,_) -> String.equal repo_prev.Opam_repo.name  repo) opam_repository_commits
+      in repo_prev, commit::repo_previous_commits) previous_commits
+  in
+  recent_commits
diff --git a/worker/custom.ml b/worker/custom.ml
index 3b3cbb8..c93399b 100644
--- a/worker/custom.ml
+++ b/worker/custom.ml
@@ -28,14 +28,14 @@ let cluster_worker_log log =
       Capnp_rpc_lwt.Service.return_empty ()
   end
 
-let solve ~cancelled ~solver ~log c =
+let solve ~cacheable ~cancelled ~solver ~log c =
   let selections =
     let*! request = solve_of_custom c in
     let log = cluster_worker_log log in
     Lwt_eio.run_lwt @@ fun () ->
     Capnp_rpc_lwt.Capability.with_ref log @@ fun log ->
     Lwt_eio.run_eio @@ fun () ->
-    Solver_service.Solver.solve ~cancelled solver ~log request
+    Solver_service.Solver.solve ~cacheable ~cancelled solver ~log request
   in
   begin match selections with
     | Ok sels ->
diff --git a/worker/custom.mli b/worker/custom.mli
index 3082d65..9a5abc6 100644
--- a/worker/custom.mli
+++ b/worker/custom.mli
@@ -2,6 +2,7 @@
     custom job specification. *)
 
 val solve :
+  cacheable:bool ->
   cancelled:unit Eio.Promise.t ->
   solver:Solver_service.Solver.t ->
   log:Log_data.t ->
diff --git a/worker/solver_worker.ml b/worker/solver_worker.ml
index bd594f9..d3b0403 100644
--- a/worker/solver_worker.ml
+++ b/worker/solver_worker.ml
@@ -34,6 +34,7 @@ type t = {
   name : string;
   registration_service : Cluster_api.Raw.Client.Registration.t Sturdy_ref.t;
   capacity : int;
+  cacheable : bool;
   mutable in_use : int; (* Number of active builds *)
   cond : unit Lwt_condition.t;
   (* Fires when a build finishes (or switch turned off) *)
@@ -50,7 +51,7 @@ let metrics = function
   | `Host ->
     failwith "No host metrics from solver service"
 
-let build ~cancelled ~log t descr =
+let build ~cacheable ~cancelled ~log t descr =
   let module R = Cluster_api.Raw.Reader.JobDescr in
   match Cluster_api.Submission.get_action descr with
   | Custom_build c ->
@@ -58,7 +59,7 @@ let build ~cancelled ~log t descr =
         f "Got request to build a job of kind \"%s\""
           (Cluster_api.Custom.kind c));
     (* Oddly, the protocol has us report cancellation and errors as "successful" jobs with the error inside! *)
-    let output = Custom.solve ~cancelled ~solver:t.solver ~log c in
+    let output = Custom.solve ~cacheable ~cancelled ~solver:t.solver ~log c in
     Log_data.write log "Job succeeded\n";
     (Ok output, "ok")
   | Obuilder_build _ | Docker_build _ ->
@@ -95,7 +96,7 @@ let loop t queue =
                Lwt_eio.run_eio @@ fun () ->
                Log_data.info log "Building on %s" t.name;
                let t0 = Unix.gettimeofday () in
-               match build ~cancelled ~log t request with
+               match build ~cacheable:t.cacheable ~cancelled ~log t request with
                | (outcome, metric_label) ->
                  let t1 = Unix.gettimeofday () in
                  Prometheus.Summary.observe
@@ -127,7 +128,7 @@ let loop t queue =
 
 let self_update () = failwith "TODO: Self-update"
 
-let run ~name ~capacity solver registration_service =
+let run ~cacheable ~name ~capacity solver registration_service =
   Lwt_eio.run_lwt @@ fun () ->
   let t = {
     solver;
@@ -136,6 +137,7 @@ let run ~name ~capacity solver registration_service =
     cond = Lwt_condition.create ();
     capacity;
     in_use = 0;
+    cacheable;
   } in
   let rec reconnect () =
     let connect_time = Unix.gettimeofday () in
diff --git a/worker/solver_worker.mli b/worker/solver_worker.mli
index 564ff13..3bf159c 100644
--- a/worker/solver_worker.mli
+++ b/worker/solver_worker.mli
@@ -1,6 +1,7 @@
 open Capnp_rpc_lwt
 
 val run :
+  cacheable: bool ->
   name:string ->
   capacity:int ->
   Solver_service.Solver.t ->