diff --git a/bin/arg.ml b/bin/arg.ml
index 7cffc60593a..b00a7e14c6a 100644
--- a/bin/arg.ml
+++ b/bin/arg.ml
@@ -41,6 +41,7 @@ module Dep = struct
 
   type t = Dep_conf.t
 
+  let equal = Dep_conf.equal
   let file s = Dep_conf.File (String_with_vars.make_text Loc.none s)
 
   let make_alias_sw ~dir s =
diff --git a/bin/arg.mli b/bin/arg.mli
index c1f7b3f5a23..ca272582701 100644
--- a/bin/arg.mli
+++ b/bin/arg.mli
@@ -21,6 +21,7 @@ end
 module Dep : sig
   type t = Dune_lang.Dep_conf.t
 
+  val equal : t -> t -> bool
   val file : string -> t
   val alias : dir:Stdune.Path.Local.t -> Dune_engine.Alias.Name.t -> t
   val alias_rec : dir:Stdune.Path.Local.t -> Dune_engine.Alias.Name.t -> t
diff --git a/bin/build_cmd.ml b/bin/build_cmd.ml
index f90d2c17806..5ba33fa6964 100644
--- a/bin/build_cmd.ml
+++ b/bin/build_cmd.ml
@@ -205,7 +205,7 @@ let build =
        state of the lock could otherwise change between checking it and taking
        it. *)
     match Dune_util.Global_lock.lock ~timeout:None with
-    | Error () ->
+    | Error lock_held_by ->
       (* This case is reached if dune detects that another instance of dune
          is already running. Rather than performing the build itself, the
          current instance of dune will instruct the already-running instance to
@@ -215,6 +215,16 @@ let build =
          perform the RPC call.
       *)
       Scheduler.go_without_rpc_server ~common ~config (fun () ->
+        if not (Common.Builder.equal builder Common.Builder.default)
+        then
+          User_warning.emit
+            [ Pp.textf
+                "Your build request is being forwarded to a running Dune instance%s so \
+                 most command-line arguments will be ignored."
+                (match (lock_held_by : Dune_util.Global_lock.Lock_held_by.t) with
+                 | Unknown -> ""
+                 | Pid_from_lockfile pid -> sprintf " (pid: %d)" pid)
+            ];
         build_via_rpc_server ~print_on_success:true ~targets)
     | Ok () ->
       let request setup =
diff --git a/bin/common.ml b/bin/common.ml
index dd8cd606f12..2b09eb2dae2 100644
--- a/bin/common.ml
+++ b/bin/common.ml
@@ -22,7 +22,38 @@ module Workspace = Source.Workspace
 open struct
   open Cmdliner
   module Cmd = Cmd
-  module Term = Term
+
+  module Term = struct
+    include Term
+
+    (* Evaluate a parser, passing it no command-line arguments, in an
+       environment with no variables set. It's only valid to pass a parser
+       which can be evaluated with no arguments, otherwise a code error will
+       be raised. Returns the result of the parser. This is intended to be
+       used to extract the default value for a type implied by the behaviour
+       of its parser when no command-line arguments are passed to it. *)
+    let eval_no_args_empty_env t =
+      let raise_code_error data =
+        Code_error.raise "Unexpected result evaluating term with no args" data
+      in
+      (* Cmdliner doesn't allow argv to be empty. *)
+      let argv = [| "dune" |] in
+      let env _ = None in
+      match Cmd.eval_value ~argv ~env (Cmd.v (Cmd.info "dune") t) with
+      | Ok (`Ok x) -> x
+      | Ok `Help -> raise_code_error [ "ok", Dyn.string "help" ]
+      | Ok `Version -> raise_code_error [ "ok", Dyn.string "version" ]
+      | Error e ->
+        let error_string =
+          match e with
+          | `Parse -> "parse"
+          | `Term -> "term"
+          | `Exn -> "exn"
+        in
+        raise_code_error [ "error", Dyn.string error_string ]
+    ;;
+  end
+
   module Manpage = Manpage
 end
 
@@ -1025,6 +1056,94 @@ module Builder = struct
     ; log_file = Default
     }
   ;;
+
+  let default = Term.eval_no_args_empty_env term
+
+  let equal
+        t
+        { debug_dep_path
+        ; debug_backtraces
+        ; debug_artifact_substitution
+        ; debug_load_dir
+        ; debug_digests
+        ; debug_package_logs
+        ; wait_for_filesystem_clock
+        ; only_packages
+        ; capture_outputs
+        ; diff_command
+        ; promote
+        ; ignore_promoted_rules
+        ; force
+        ; no_print_directory
+        ; ignore_lock_dir
+        ; store_orig_src_dir
+        ; default_target
+        ; watch
+        ; print_metrics
+        ; dump_memo_graph_file
+        ; dump_memo_graph_format
+        ; dump_memo_graph_with_timing
+        ; dump_gc_stats
+        ; always_show_command_line
+        ; promote_install_files
+        ; file_watcher
+        ; workspace_config
+        ; cache_debug_flags
+        ; report_errors_config
+        ; separate_error_messages
+        ; stop_on_first_error
+        ; require_dune_project_file
+        ; watch_exclusions
+        ; build_dir
+        ; root
+        ; stats_trace_file
+        ; stats_trace_extended
+        ; allow_builds
+        ; default_root_is_cwd
+        ; log_file
+        }
+    =
+    Bool.equal t.debug_dep_path debug_dep_path
+    && Bool.equal t.debug_backtraces debug_backtraces
+    && Bool.equal t.debug_artifact_substitution debug_artifact_substitution
+    && Bool.equal t.debug_load_dir debug_load_dir
+    && Bool.equal t.debug_digests debug_digests
+    && Bool.equal t.debug_package_logs debug_package_logs
+    && Bool.equal t.wait_for_filesystem_clock wait_for_filesystem_clock
+    && Only_packages.Clflags.equal t.only_packages only_packages
+    && Bool.equal t.capture_outputs capture_outputs
+    && Option.equal String.equal t.diff_command diff_command
+    && Option.equal Dune_engine.Clflags.Promote.equal t.promote promote
+    && Bool.equal t.ignore_promoted_rules ignore_promoted_rules
+    && Bool.equal t.force force
+    && Bool.equal t.no_print_directory no_print_directory
+    && Bool.equal t.ignore_lock_dir ignore_lock_dir
+    && Bool.equal t.store_orig_src_dir store_orig_src_dir
+    && Arg.Dep.equal t.default_target default_target
+    && Dune_rpc_impl.Watch_mode_config.equal t.watch watch
+    && Bool.equal t.print_metrics print_metrics
+    && Option.equal Path.External.equal t.dump_memo_graph_file dump_memo_graph_file
+    && Graph.File_format.equal t.dump_memo_graph_format dump_memo_graph_format
+    && Bool.equal t.dump_memo_graph_with_timing dump_memo_graph_with_timing
+    && Option.equal Path.External.equal t.dump_gc_stats dump_gc_stats
+    && Bool.equal t.always_show_command_line always_show_command_line
+    && Bool.equal t.promote_install_files promote_install_files
+    && Dune_engine.Scheduler.Run.file_watcher_equal t.file_watcher file_watcher
+    && Source.Workspace.Clflags.equal t.workspace_config workspace_config
+    && Dune_engine.Cache_debug_flags.equal t.cache_debug_flags cache_debug_flags
+    && Dune_engine.Report_errors_config.equal t.report_errors_config report_errors_config
+    && Bool.equal t.separate_error_messages separate_error_messages
+    && Bool.equal t.stop_on_first_error stop_on_first_error
+    && Bool.equal t.require_dune_project_file require_dune_project_file
+    && List.equal String.equal t.watch_exclusions watch_exclusions
+    && String.equal t.build_dir build_dir
+    && Option.equal String.equal t.root root
+    && Option.equal String.equal t.stats_trace_file stats_trace_file
+    && Bool.equal t.stats_trace_extended stats_trace_extended
+    && Bool.equal t.allow_builds allow_builds
+    && Bool.equal t.default_root_is_cwd default_root_is_cwd
+    && Log.File.equal t.log_file log_file
+  ;;
 end
 
 type t =
diff --git a/bin/common.mli b/bin/common.mli
index 39b1cf52439..1ed5e618d6b 100644
--- a/bin/common.mli
+++ b/bin/common.mli
@@ -28,6 +28,7 @@ val prefix_target : t -> string -> string
 module Builder : sig
   type t
 
+  val equal : t -> t -> bool
   val root : t -> string option
   val set_root : t -> string -> t
   val forbid_builds : t -> t
@@ -38,6 +39,7 @@ module Builder : sig
   val set_promote : t -> Dune_engine.Clflags.Promote.t -> t
   val default_target : t -> Arg.Dep.t
   val term : t Cmdliner.Term.t
+  val default : t
 end
 
 (** [init] creates a [Common.t] by executing a sequence of side-effecting actions to
diff --git a/bin/tools/tools_common.ml b/bin/tools/tools_common.ml
index 0d722304a0e..924ba768581 100644
--- a/bin/tools/tools_common.ml
+++ b/bin/tools/tools_common.ml
@@ -29,7 +29,7 @@ let build_dev_tool_via_rpc dev_tool =
 let lock_and_build_dev_tool ~common ~config dev_tool =
   let open Fiber.O in
   match Dune_util.Global_lock.lock ~timeout:None with
-  | Error () ->
+  | Error _lock_held_by ->
     Scheduler.go_without_rpc_server ~common ~config (fun () ->
       let* () = Lock_dev_tool.lock_dev_tool dev_tool |> Memo.run in
       build_dev_tool_via_rpc dev_tool)
diff --git a/src/dune_cache/config.ml b/src/dune_cache/config.ml
index e14ab5f7ae4..df66c9825ae 100644
--- a/src/dune_cache/config.ml
+++ b/src/dune_cache/config.ml
@@ -8,6 +8,13 @@ module Reproducibility_check = struct
     | Check_with_probability of float
     | Check
 
+  let equal a b =
+    match a, b with
+    | Skip, Skip | Check, Check -> true
+    | Check_with_probability a, Check_with_probability b -> a = b
+    | _, _ -> false
+  ;;
+
   let sample = function
     | Skip -> false
     | Check_with_probability p -> Random.float 1. < p
diff --git a/src/dune_cache/config.mli b/src/dune_cache/config.mli
index e7461c5baa3..0168f00c9b3 100644
--- a/src/dune_cache/config.mli
+++ b/src/dune_cache/config.mli
@@ -16,6 +16,8 @@ module Reproducibility_check : sig
     | Check_with_probability of float (** [0 < p < 1] *)
     | Check
 
+  val equal : t -> t -> bool
+
   (** Should we check the current build rule for reproducibility?
 
       - If [t = Skip], return [false].
diff --git a/src/dune_cache_storage/mode.ml b/src/dune_cache_storage/mode.ml
index f75110f4c76..dbc1ed7555d 100644
--- a/src/dune_cache_storage/mode.ml
+++ b/src/dune_cache_storage/mode.ml
@@ -4,6 +4,12 @@ type t =
   | Hardlink
   | Copy
 
+let equal a b =
+  match a, b with
+  | Hardlink, Hardlink | Copy, Copy -> true
+  | _, _ -> false
+;;
+
 let all = [ "hardlink", Hardlink; "copy", Copy ]
 
 let of_string s =
diff --git a/src/dune_cache_storage/mode.mli b/src/dune_cache_storage/mode.mli
index 6af6d4217fd..811455f3e3e 100644
--- a/src/dune_cache_storage/mode.mli
+++ b/src/dune_cache_storage/mode.mli
@@ -24,6 +24,7 @@ type t =
   | Hardlink
   | Copy
 
+val equal : t -> t -> bool
 val all : (string * t) list
 val to_string : t -> string
 val of_string : string -> (t, string) result
diff --git a/src/dune_config_file/display.ml b/src/dune_config_file/display.ml
index 9b89c81546a..bc284a53ae0 100644
--- a/src/dune_config_file/display.ml
+++ b/src/dune_config_file/display.ml
@@ -8,6 +8,15 @@ type t =
       }
   | Tui
 
+let equal a b =
+  match a, b with
+  | ( Simple { status_line = a_status_line; verbosity = a_verbosity }
+    , Simple { status_line = b_status_line; verbosity = b_verbosity } ) ->
+    Bool.equal a_status_line b_status_line && Display.equal a_verbosity b_verbosity
+  | Tui, Tui -> true
+  | _, _ -> false
+;;
+
 let progress = Simple { status_line = true; verbosity = Quiet }
 let verbose = Simple { status_line = true; verbosity = Verbose }
 let short = Simple { status_line = true; verbosity = Short }
diff --git a/src/dune_config_file/display.mli b/src/dune_config_file/display.mli
index 7dd164c79e9..2f788291329 100644
--- a/src/dune_config_file/display.mli
+++ b/src/dune_config_file/display.mli
@@ -9,6 +9,8 @@ type t =
       }
   | Tui
 
+val equal : t -> t -> bool
+
 (** All the supported display modes for setting from the command line. *)
 val all : (string * t) list
 
diff --git a/src/dune_config_file/dune_config_file.ml b/src/dune_config_file/dune_config_file.ml
index 46c094ce653..b45a09e132e 100644
--- a/src/dune_config_file/dune_config_file.ml
+++ b/src/dune_config_file/dune_config_file.ml
@@ -25,6 +25,13 @@ module Dune_config = struct
       ; license : string list option
       }
 
+    let equal t { authors; maintainers; maintenance_intent; license } =
+      Option.equal (List.equal String.equal) t.authors authors
+      && Option.equal (List.equal String.equal) t.authors maintainers
+      && Option.equal (List.equal String.equal) t.authors maintenance_intent
+      && Option.equal (List.equal String.equal) t.authors license
+    ;;
+
     let decode =
       fields
         (let+ authors = field_o "authors" (repeat string)
@@ -59,6 +66,14 @@ module Dune_config = struct
       ]
     ;;
 
+    let equal a b =
+      match a, b with
+      | Preserve, Preserve
+      | Clear_on_rebuild, Clear_on_rebuild
+      | Clear_on_rebuild_and_flush_history, Clear_on_rebuild_and_flush_history -> true
+      | _, _ -> false
+    ;;
+
     let to_dyn = function
       | Preserve -> Dyn.Variant ("Preserve", [])
       | Clear_on_rebuild -> Dyn.Variant ("Clear_on_rebuild", [])
@@ -74,6 +89,13 @@ module Dune_config = struct
       | Fixed of int
       | Auto
 
+    let equal a b =
+      match a, b with
+      | Fixed a, Fixed b -> Int.equal a b
+      | Auto, Auto -> true
+      | _, _ -> false
+    ;;
+
     let error = Error "invalid concurrency value, must be 'auto' or a positive number"
 
     let of_string = function
@@ -105,6 +127,8 @@ module Dune_config = struct
   module Sandboxing_preference = struct
     type t = Sandbox_mode.t list
 
+    let equal = List.equal Sandbox_mode.equal
+
     let decode : Sandbox_mode.t Dune_sexp.Decoder.t =
       let open Dune_sexp.Decoder in
       enum
@@ -125,6 +149,14 @@ module Dune_config = struct
         | Enabled_except_user_rules
         | Enabled
 
+      let equal a b =
+        match a, b with
+        | Disabled, Disabled
+        | Enabled_except_user_rules, Enabled_except_user_rules
+        | Enabled, Enabled -> true
+        | _, _ -> false
+      ;;
+
       let to_string = function
         | Disabled -> "disabled"
         | Enabled_except_user_rules -> "enabled-except-user-rules"
@@ -166,6 +198,8 @@ module Dune_config = struct
     module Storage_mode = struct
       type t = Dune_cache_storage.Mode.t option
 
+      let equal = Option.equal Dune_cache_storage.Mode.equal
+
       let all =
         ("auto", None)
         :: List.map ~f:(fun (name, mode) -> name, Some mode) Dune_cache_storage.Mode.all
@@ -206,6 +240,56 @@ module Dune_config = struct
       }
   end
 
+  module Make_equal
+      (M : S)
+      (Equal : sig
+         val field : ('a -> 'a -> bool) -> 'a M.field -> 'a M.field -> bool
+       end) =
+  struct
+    let field = Equal.field
+
+    let equal
+          (t : M.t)
+          { M.display
+          ; concurrency
+          ; terminal_persistence
+          ; sandboxing_preference
+          ; cache_enabled
+          ; cache_reproducibility_check
+          ; cache_storage_mode
+          ; action_stdout_on_success
+          ; action_stderr_on_success
+          ; project_defaults
+          ; experimental
+          }
+      =
+      field Display.equal t.display display
+      && field Concurrency.equal t.concurrency concurrency
+      && field Terminal_persistence.equal t.terminal_persistence terminal_persistence
+      && field Sandboxing_preference.equal t.sandboxing_preference sandboxing_preference
+      && field Cache.Toggle.equal t.cache_enabled cache_enabled
+      && field
+           Dune_cache.Config.Reproducibility_check.equal
+           t.cache_reproducibility_check
+           cache_reproducibility_check
+      && field Cache.Storage_mode.equal t.cache_storage_mode cache_storage_mode
+      && field
+           Action_output_on_success.equal
+           t.action_stdout_on_success
+           action_stdout_on_success
+      && field
+           Action_output_on_success.equal
+           t.action_stderr_on_success
+           action_stderr_on_success
+      && field Project_defaults.equal t.project_defaults project_defaults
+      && field
+           (List.equal
+              (Tuple.T2.equal String.equal (Tuple.T2.equal Loc.equal String.equal)))
+           t.experimental
+           experimental
+    ;;
+  end
+
   module Make_superpose
       (A : S)
       (B : S)
@@ -318,6 +402,13 @@ module Dune_config = struct
         (struct
           let field f = Dyn.option f
         end)
+
+    include
+      Make_equal
+        (M)
+        (struct
+          let field f = Option.equal f
+        end)
   end
 
   include
@@ -333,6 +424,13 @@ module Dune_config = struct
         let field f = f
       end)
 
+  include
+    Make_equal
+      (M)
+      (struct
+        let field f = f
+      end)
+
   let standard_watch_exclusions =
     [ {|^_opam|}
     ; {|/_opam|}
@@ -352,7 +450,6 @@ module Dune_config = struct
   ;;
 
   let hash = Poly.hash
-  let equal a b = Poly.equal a b
 
   let default =
     { display = Simple { verbosity = Quiet; status_line = not Execution_env.inside_dune }
diff --git a/src/dune_config_file/dune_config_file.mli b/src/dune_config_file/dune_config_file.mli
index 743ada5e59f..7c1638a8da6 100644
--- a/src/dune_config_file/dune_config_file.mli
+++ b/src/dune_config_file/dune_config_file.mli
@@ -20,6 +20,7 @@ module Dune_config : sig
       | Fixed of int
       | Auto
 
+    val equal : t -> t -> bool
     val of_string : string -> (t, string) result
     val to_string : t -> string
   end
@@ -94,6 +95,7 @@ module Dune_config : sig
     val empty : t
     val superpose : t -> t -> t
     val to_dyn : t -> Dyn.t
+    val equal : t -> t -> bool
   end
 
   (** A standard list of watch exclusions *)
diff --git a/src/dune_engine/cache_debug_flags.ml b/src/dune_engine/cache_debug_flags.ml
index fa536b2eabb..9caa7ba56c5 100644
--- a/src/dune_engine/cache_debug_flags.ml
+++ b/src/dune_engine/cache_debug_flags.ml
@@ -7,3 +7,9 @@ type t =
   ; workspace_local_cache : bool
   ; fs_cache : bool
   }
+
+let equal t { shared_cache; workspace_local_cache; fs_cache } =
+  Bool.equal t.shared_cache shared_cache
+  && Bool.equal t.workspace_local_cache workspace_local_cache
+  && Bool.equal t.fs_cache fs_cache
+;;
diff --git a/src/dune_engine/clflags.ml b/src/dune_engine/clflags.ml
index 7bb186f8190..de5346bb6ec 100644
--- a/src/dune_engine/clflags.ml
+++ b/src/dune_engine/clflags.ml
@@ -3,6 +3,12 @@ module Promote = struct
     | Automatically
     | Never
 
+  let equal a b =
+    match a, b with
+    | Automatically, Automatically | Never, Never -> true
+    | _, _ -> false
+  ;;
+
   let to_dyn = function
     | Automatically -> Dyn.variant "Automatically" []
     | Never -> Dyn.variant "Never" []
diff --git a/src/dune_engine/clflags.mli b/src/dune_engine/clflags.mli
index 3b7c2f2bcbf..c5f2bb87356 100644
--- a/src/dune_engine/clflags.mli
+++ b/src/dune_engine/clflags.mli
@@ -22,6 +22,7 @@ module Promote : sig
     | Automatically
     | Never
 
+  val equal : t -> t -> bool
   val to_dyn : t -> Dyn.t
 end
 
diff --git a/src/dune_engine/display.ml b/src/dune_engine/display.ml
index c2f91998cc0..3a79ca67507 100644
--- a/src/dune_engine/display.ml
+++ b/src/dune_engine/display.ml
@@ -3,6 +3,12 @@ type t =
   | Short
   | Verbose
 
+let equal a b =
+  match a, b with
+  | Quiet, Quiet | Short, Short | Verbose, Verbose -> true
+  | _, _ -> false
+;;
+
 let to_dyn : t -> Dyn.t = function
   | Quiet -> Variant ("Quiet", [])
   | Short -> Variant ("Short", [])
diff --git a/src/dune_engine/display.mli b/src/dune_engine/display.mli
index 64e1b8a1d0c..eb4ec024bb2 100644
--- a/src/dune_engine/display.mli
+++ b/src/dune_engine/display.mli
@@ -10,4 +10,5 @@ type t =
   | Short
   | Verbose
 
+val equal : t -> t -> bool
 val to_dyn : t -> Dyn.t
diff --git a/src/dune_engine/report_errors_config.ml b/src/dune_engine/report_errors_config.ml
index 4d7119f9fba..d8c845fca3a 100644
--- a/src/dune_engine/report_errors_config.ml
+++ b/src/dune_engine/report_errors_config.ml
@@ -15,6 +15,12 @@ type t =
      scheduling. *)
   | Twice
 
+let equal a b =
+  match a, b with
+  | Early, Early | Deterministic, Deterministic | Twice, Twice -> true
+  | _, _ -> false
+;;
+
 let default : t =
   match Dune_util.Execution_env.inside_dune with
   | true -> Deterministic
diff --git a/src/dune_engine/scheduler.ml b/src/dune_engine/scheduler.ml
index 09b2d37b516..5b64f909658 100644
--- a/src/dune_engine/scheduler.ml
+++ b/src/dune_engine/scheduler.ml
@@ -1109,6 +1109,12 @@ module Run = struct
     | Automatic
     | No_watcher
 
+  let file_watcher_equal a b =
+    match a, b with
+    | Automatic, Automatic | No_watcher, No_watcher -> true
+    | _, _ -> false
+  ;;
+
   module Build_outcome = Build_outcome
   module Event_queue = Event.Queue
   module Event = Handler.Event
diff --git a/src/dune_engine/scheduler.mli b/src/dune_engine/scheduler.mli
index 85c10d8bc4e..8d90199f6dd 100644
--- a/src/dune_engine/scheduler.mli
+++ b/src/dune_engine/scheduler.mli
@@ -30,6 +30,8 @@ module Run : sig
     | Automatic
     | No_watcher
 
+  val file_watcher_equal : file_watcher -> file_watcher -> bool
+
   module Shutdown : sig
     module Reason : sig
       type t =
diff --git a/src/dune_graph/graph.ml b/src/dune_graph/graph.ml
index d4484dcd6f1..b159661b33e 100644
--- a/src/dune_graph/graph.ml
+++ b/src/dune_graph/graph.ml
@@ -6,6 +6,12 @@ module File_format = struct
     | Dot
     | Summary
 
+  let equal a b =
+    match a, b with
+    | Gexf, Gexf | Dot, Dot | Summary, Summary -> true
+    | _, _ -> false
+  ;;
+
   let conv =
     ( (function
         | "gexf" -> Ok Gexf
diff --git a/src/dune_graph/graph.mli b/src/dune_graph/graph.mli
index cdc26504ad9..649b161e8f3 100644
--- a/src/dune_graph/graph.mli
+++ b/src/dune_graph/graph.mli
@@ -6,6 +6,7 @@ module File_format : sig
     | Dot
     | Summary
 
+  val equal : t -> t -> bool
   val conv : (string -> (t, [> `Msg of string ]) result) * (Format.formatter -> t -> unit)
 end
 
diff --git a/src/dune_lang/dep_conf.ml b/src/dune_lang/dep_conf.ml
index d8866d8c3ad..c417b8e1da0 100644
--- a/src/dune_lang/dep_conf.ml
+++ b/src/dune_lang/dep_conf.ml
@@ -7,6 +7,10 @@ module Glob_files = struct
     ; recursive : bool
     }
 
+  let equal t { glob; recursive } =
+    String_with_vars.equal t.glob glob && Bool.equal t.recursive recursive
+  ;;
+
   let to_dyn { glob; recursive } =
     Dyn.record [ "glob", String_with_vars.to_dyn glob; "recursive", Dyn.bool recursive ]
   ;;
@@ -15,6 +19,16 @@ end
 module Sandbox_config = struct
   type t = Loc.t * [ `None | `Always | `Preserve_file_kind ] list
 
+  let equal =
+    Tuple.T2.equal
+      Loc.equal
+      (List.equal (fun a b ->
+         match a, b with
+         | `None, `None | `Always, `Always | `Preserve_file_kind, `Preserve_file_kind ->
+           true
+         | _, _ -> false))
+  ;;
+
   let all =
     [ "none", `None; "always", `Always; "preserve_file_kind", `Preserve_file_kind ]
   ;;
@@ -50,6 +64,21 @@ type t =
   | Sandbox_config of Sandbox_config.t
   | Include of string
 
+let equal a b =
+  match a, b with
+  | File a, File b
+  | Alias a, Alias b
+  | Alias_rec a, Alias_rec b
+  | Source_tree a, Source_tree b
+  | Package a, Package b
+  | Env_var a, Env_var b -> String_with_vars.equal a b
+  | Glob_files a, Glob_files b -> Glob_files.equal a b
+  | Universe, Universe -> true
+  | Sandbox_config a, Sandbox_config b -> Sandbox_config.equal a b
+  | Include a, Include b -> String.equal a b
+  | _, _ -> false
+;;
+
 let remove_locs = function
   | File sw -> File (String_with_vars.remove_locs sw)
   | Alias sw -> Alias (String_with_vars.remove_locs sw)
diff --git a/src/dune_lang/dep_conf.mli b/src/dune_lang/dep_conf.mli
index a2798be623a..cee2a4db4d0 100644
--- a/src/dune_lang/dep_conf.mli
+++ b/src/dune_lang/dep_conf.mli
@@ -42,6 +42,7 @@ type t =
   | Sandbox_config of Sandbox_config.t
   | Include of string
 
+val equal : t -> t -> bool
 val remove_locs : t -> t
 
 include Conv.S with type t := t
diff --git a/src/dune_rpc_impl/watch_mode_config.ml b/src/dune_rpc_impl/watch_mode_config.ml
index 517659e1910..c4c8f1ac1b5 100644
--- a/src/dune_rpc_impl/watch_mode_config.ml
+++ b/src/dune_rpc_impl/watch_mode_config.ml
@@ -8,6 +8,12 @@ type t =
   | No
   | Yes of rebuild_trigger
 
+let equal a b =
+  match a, b with
+  | No, No | Yes Eager, Yes Eager | Yes Passive, Yes Passive -> true
+  | _, _ -> false
+;;
+
 let all = [ No; Yes Eager; Yes Passive ]
 
 let to_string = function
diff --git a/src/dune_rpc_impl/watch_mode_config.mli b/src/dune_rpc_impl/watch_mode_config.mli
index 38d553d3c0e..83aff53318f 100644
--- a/src/dune_rpc_impl/watch_mode_config.mli
+++ b/src/dune_rpc_impl/watch_mode_config.mli
@@ -6,5 +6,6 @@ type t =
   | No
   | Yes of rebuild_trigger
 
+val equal : t -> t -> bool
 val to_string : t -> string
 val of_string : string -> (t, string) Result.t
diff --git a/src/dune_util/global_lock.ml b/src/dune_util/global_lock.ml
index 0ae0a729b67..ab34631affb 100644
--- a/src/dune_util/global_lock.ml
+++ b/src/dune_util/global_lock.ml
@@ -66,6 +66,30 @@ end
 
 let locked = ref false
 
+module Lock_held_by = struct
+  type t =
+    | Pid_from_lockfile of int
+    | Unknown
+
+  let read_lock_file () =
+    match Io.read_file (Path.build lock_file) with
+    | exception _ -> Unknown
+    | pid ->
+      (match int_of_string_opt pid with
+       | Some pid -> Pid_from_lockfile pid
+       | None ->
+         User_error.raise
+           [ Pp.textf
+               "Unexpected contents of build directory global lock file (%s). Expected \
+                an integer PID. Found: %s"
+               (Path.Build.to_string_maybe_quoted lock_file)
+               pid
+           ]
+           ~hints:
+             [ Pp.textf "Try deleting %s" (Path.Build.to_string_maybe_quoted lock_file) ])
+  ;;
+end
+
 let lock ~timeout =
   match Config.(get global_lock) with
   | `Disabled -> Ok ()
@@ -90,20 +114,22 @@ let lock ~timeout =
       | `Success ->
         locked := true;
         Ok ()
-      | `Failure -> Error ())
+      | `Failure ->
+        let lock_held_by = Lock_held_by.read_lock_file () in
+        Error lock_held_by)
 ;;
 
 let lock_exn ~timeout =
   match lock ~timeout with
   | Ok () -> ()
-  | Error () ->
+  | Error lock_held_by ->
     User_error.raise
       [ Pp.textf
           "A running dune%s instance has locked the build directory. If this is not the \
-           case, please delete %s"
-          (match Io.read_file (Path.build lock_file) with
-           | exception _ -> ""
-           | pid -> sprintf " (pid: %s)" pid)
+           case, please delete %S."
+          (match lock_held_by with
+           | Unknown -> ""
+           | Pid_from_lockfile pid -> sprintf " (pid: %d)" pid)
           (Path.Build.to_string_maybe_quoted lock_file)
       ]
 ;;
diff --git a/src/dune_util/global_lock.mli b/src/dune_util/global_lock.mli
index da66837b4df..1b5eef0209b 100644
--- a/src/dune_util/global_lock.mli
+++ b/src/dune_util/global_lock.mli
@@ -2,10 +2,16 @@
 
     Before starting rpc, writing to the build dir, this lock should be locked. *)
 
-(** attempt to acquire a lock. once a lock is locked, subsequent locks always
+module Lock_held_by : sig
+  type t =
+    | Pid_from_lockfile of int
+    | Unknown
+end
+
+(** Attempt to acquire a lock. once a lock is locked, subsequent locks always
     succeed. Returns [Ok ()] if the lock is acquired within [timeout] seconds,
     and [Error ()] otherwise. *)
-val lock : timeout:float option -> (unit, unit) result
+val lock : timeout:float option -> (unit, Lock_held_by.t) result
 
 val lock_exn : timeout:float option -> unit
 
diff --git a/src/dune_util/log.ml b/src/dune_util/log.ml
index 15d66becbd1..3600067d483 100644
--- a/src/dune_util/log.ml
+++ b/src/dune_util/log.ml
@@ -6,7 +6,14 @@ module File = struct
     | Default
     | No_log_file
     | This of Path.t
-    | Out_channel of out_channel
+    | Stderr
+
+  let equal a b =
+    match a, b with
+    | Default, Default | No_log_file, No_log_file | Stderr, Stderr -> true
+    | This a, This b -> Path.equal a b
+    | _, _ -> false
+  ;;
 end
 
 type real = { oc : out_channel option }
@@ -18,7 +25,7 @@ let init ?(file = File.Default) () =
   let oc =
     match file with
     | No_log_file -> None
-    | Out_channel s -> Some s
+    | Stderr -> Some stderr
     | This path ->
       Path.mkdir_p (Path.parent_exn path);
       Some (Io.open_out path)
diff --git a/src/dune_util/log.mli b/src/dune_util/log.mli
index f4fc2468113..dc350ec8671 100644
--- a/src/dune_util/log.mli
+++ b/src/dune_util/log.mli
@@ -6,7 +6,9 @@ module File : sig
     | Default
     | No_log_file
     | This of Path.t
-    | Out_channel of out_channel
+    | Stderr
+
+  val equal : t -> t -> bool
 end
 
 (** Initialise the log file *)
diff --git a/src/source/only_packages.ml b/src/source/only_packages.ml
index cefb4654c03..fd438d3dcdd 100644
--- a/src/source/only_packages.ml
+++ b/src/source/only_packages.ml
@@ -8,6 +8,16 @@ module Clflags = struct
         ; command_line_option : string
         }
 
+  let equal a b =
+    match a, b with
+    | No_restriction, No_restriction -> true
+    | ( Restrict { names = a_names; command_line_option = a_command_line_options }
+      , Restrict { names = b_names; command_line_option = b_command_line_options } ) ->
+      Package.Name.Set.equal a_names b_names
+      && String.equal a_command_line_options b_command_line_options
+    | _, _ -> false
+  ;;
+
   let to_dyn = function
     | No_restriction -> Dyn.Variant ("No_restriction", [])
     | Restrict { names; command_line_option } ->
diff --git a/src/source/only_packages.mli b/src/source/only_packages.mli
index 01ee36ced6a..87b26521e80 100644
--- a/src/source/only_packages.mli
+++ b/src/source/only_packages.mli
@@ -11,6 +11,8 @@ module Clflags : sig
           (** Which of [-p], [--only-packages], ... was passed *)
         }
 
+  val equal : t -> t -> bool
+
   (** This must be called exactly once *)
   val set : t -> unit
 end
diff --git a/src/source/workspace.ml b/src/source/workspace.ml
index e079e07ba8b..00db72a6243 100644
--- a/src/source/workspace.ml
+++ b/src/source/workspace.ml
@@ -749,6 +749,24 @@ module Clflags = struct
     ; config_from_config_file : Dune_config.Partial.t
     }
 
+  let equal
+        t
+        { x
+        ; profile
+        ; instrument_with
+        ; workspace_file
+        ; config_from_command_line
+        ; config_from_config_file
+        }
+    =
+    Option.equal Context_name.equal t.x x
+    && Option.equal Profile.equal t.profile profile
+    && Option.equal (List.equal Lib_name.equal) t.instrument_with instrument_with
+    && Option.equal Path.Outside_build_dir.equal t.workspace_file workspace_file
+    && Dune_config.Partial.equal t.config_from_command_line config_from_command_line
+    && Dune_config.Partial.equal t.config_from_config_file config_from_config_file
+  ;;
+
   let to_dyn
         { x
         ; profile
diff --git a/src/source/workspace.mli b/src/source/workspace.mli
index e43020d958e..c6d5c201bd7 100644
--- a/src/source/workspace.mli
+++ b/src/source/workspace.mli
@@ -143,6 +143,8 @@ module Clflags : sig
     ; config_from_config_file : Dune_config.Partial.t
     }
 
+  val equal : t -> t -> bool
+
   (** This must be called exactly once *)
   val set : t -> unit
 end
diff --git a/test/blackbox-tests/test-cases/watching/watching-eager-concurrent-build-command.t b/test/blackbox-tests/test-cases/watching/watching-eager-concurrent-build-command.t
index f226d336979..0285d45db1f 100644
--- a/test/blackbox-tests/test-cases/watching/watching-eager-concurrent-build-command.t
+++ b/test/blackbox-tests/test-cases/watching/watching-eager-concurrent-build-command.t
@@ -9,6 +9,7 @@ Build the project once before starting the watch server so the watch server star
   $ dune build --watch &
   Success, waiting for filesystem changes...
   Success, waiting for filesystem changes...
+  Success, waiting for filesystem changes...
   File "foo.ml", line 1, characters 9-21:
   1 | let () = print_endlin "Hello, World!"
                ^^^^^^^^^^^^
@@ -26,6 +27,13 @@ Demonstrate that we can run "dune build" while the watch server is running.
   $ dune build
   Success
 
+Demonstrate that a warning is displayed when extra arguments are passed to
+"dune build", since those arguments will be ignored.
+  $ dune build --auto-promote 2>&1 | sed 's/pid: [0-9]*/pid: PID/g'
+  Warning: Your build request is being forwarded to a running Dune instance
+  (pid: PID) so most command-line arguments will be ignored.
+  Success
+
 Demonstrate that error messages are still printed by "dune build" when it's
 acting as an RPC client while running concurrently with an RPC server.
   $ echo 'let () = print_endlin "Hello, World!"' > foo.ml
diff --git a/test/expect-tests/dune_rpc_e2e/dune_rpc_e2e.ml b/test/expect-tests/dune_rpc_e2e/dune_rpc_e2e.ml
index 117f01eb352..8cda8362173 100644
--- a/test/expect-tests/dune_rpc_e2e/dune_rpc_e2e.ml
+++ b/test/expect-tests/dune_rpc_e2e/dune_rpc_e2e.ml
@@ -9,7 +9,7 @@ module Session = Csexp_rpc.Session
 
 (* enable to debug process stdout/stderr *)
 let debug = false
-let () = if debug then Dune_util.Log.init ~file:(Out_channel stderr) ()
+let () = if debug then Dune_util.Log.init ~file:Stderr ()
 
 let dune_prog =
   lazy