diff --git a/Makefile.config.in b/Makefile.config.in
index 6b23ad3e174..9171697571a 100644
--- a/Makefile.config.in
+++ b/Makefile.config.in
@@ -9,6 +9,7 @@ PACKS = $(filter-out no,@OCAML_PKG_unix@ @OCAML_PKG_bigarray@ @OCAML_PKG_extlib@
 CONF_OCAMLFLAGS = @CONF_OCAMLFLAGS@
 
 MCCS_ENABLED = @MCCS_ENABLED@
+OPAM_0INSTALL_SOLVER_ENABLED = @OPAM_0INSTALL_SOLVER_ENABLED@
 
 OCAMLLIB = @OCAMLLIB@
 
diff --git a/configure b/configure
index 5aa7d8f4dca..4c36ec72777 100755
--- a/configure
+++ b/configure
@@ -627,6 +627,7 @@ hasalldeps
 ac_ct_CXX
 CXXFLAGS
 CXX
+OCAML_PKG_opam_0install_cudf
 OCAML_PKG_mccs
 OCAML_PKG_opam_file_format
 OCAML_PKG_dose3_algo
@@ -662,6 +663,7 @@ CONF_LIBACL_LINK
 EGREP
 GREP
 CPP
+OPAM_0INSTALL_SOLVER_ENABLED
 MCCS_ENABLED
 DEVELOPER
 OBJEXT
@@ -740,6 +742,7 @@ enable_version_check
 enable_checks
 enable_developer_mode
 with_mccs
+with_0install_solver
 with_libacl
 with_private_runtime
 enable_cold_check
@@ -1393,6 +1396,10 @@ Optional Packages:
   --without-PACKAGE       do not use PACKAGE (same as --with-PACKAGE=no)
   --without-mccs          Compile without a built-in Cudf solver (only works
                           if 'mccs' is not otherwise installed)
+  --without-0install-solver
+                          Compile without the built-in 0install solver (only
+                          works if 'opam-0install-cudf' is not otherwise
+                          installed)
   --with-libacl           Compile opam with libacl support
   --with-private-runtime  For a mingw-w64 build, manifest the runtime DLLs
                           locally in Opam.Runtime.arch
@@ -3451,6 +3458,16 @@ else
 fi
 
 
+# Check whether --with-0install-solver was given.
+if test "${with_0install_solver+set}" = set; then :
+  withval=$with_0install_solver;
+else
+  OPAM_0INSTALL_SOLVER_DEFAULT=yes
+
+fi
+
+
+
 
 # Check whether --with-libacl was given.
 if test "${with_libacl+set}" = set; then :
@@ -4276,6 +4293,13 @@ else
   MCCS_ENABLED=true
 
 fi
+if test "x${with_0install_solver}" = "xno"; then :
+  OPAM_0INSTALL_SOLVER_ENABLED=false
+
+else
+  OPAM_0INSTALL_SOLVER_ENABLED=true
+
+fi
 
 
 ac_ext=c
@@ -6020,9 +6044,36 @@ $as_echo "not found" >&6; }
 
 
 
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking for OCaml findlib package opam-0install-cudf" >&5
+$as_echo_n "checking for OCaml findlib package opam-0install-cudf... " >&6; }
+
+  unset found
+  unset pkg
+  found=no
+  for pkg in opam-0install-cudf  ; do
+    if $OCAMLFIND query $pkg >/dev/null 2>/dev/null; then
+      { $as_echo "$as_me:${as_lineno-$LINENO}: result: found" >&5
+$as_echo "found" >&6; }
+      OCAML_PKG_opam_0install_cudf=$pkg
+      found=yes
+      break
+    fi
+  done
+  if test "$found" = "no" ; then
+    { $as_echo "$as_me:${as_lineno-$LINENO}: result: not found" >&5
+$as_echo "not found" >&6; }
+    OCAML_PKG_opam_0install_cudf=no
+  fi
+
+
+
+
 if test "x${with_mccs}" = "xno" && test "x$OCAML_PKG_mccs" != "xno"; then :
   as_fn_error $? "Option --without-mccs is not available without uninstalling the 'mccs' package" "$LINENO" 5
 fi
+if test "x${with_0install_solver}" = "xno" && test "x$OCAML_PKG_opam_0install_cudf" != "xno"; then :
+  as_fn_error $? "Option --without-0install-solver is not available without uninstalling the 'opam-0install-cudf' package" "$LINENO" 5
+fi
 
 
 if test "x$MCCS_ENABLED" = "xtrue"; then :
@@ -6311,6 +6362,57 @@ fi
 
 fi
 
+if test "x$MCCS_ENABLED" = "xtrue"; then :
+
+
+
+
+  # Used to indicate true or false condition
+  ax_compare_version=false
+
+  # Convert the two version strings to be compared into a format that
+  # allows a simple string comparison.  The end result is that a version
+  # string of the form 1.12.5-r617 will be converted to the form
+  # 0001001200050617.  In other words, each number is zero padded to four
+  # digits, and non digits are removed.
+
+  ax_compare_version_A=`echo "$OCAMLVERSION" | sed -e 's/\([0-9]*\)/Z\1Z/g' \
+                     -e 's/Z\([0-9]\)Z/Z0\1Z/g' \
+                     -e 's/Z\([0-9][0-9]\)Z/Z0\1Z/g' \
+                     -e 's/Z\([0-9][0-9][0-9]\)Z/Z0\1Z/g' \
+                     -e 's/[^0-9]//g'`
+
+
+  ax_compare_version_B=`echo "4.08" | sed -e 's/\([0-9]*\)/Z\1Z/g' \
+                     -e 's/Z\([0-9]\)Z/Z0\1Z/g' \
+                     -e 's/Z\([0-9][0-9]\)Z/Z0\1Z/g' \
+                     -e 's/Z\([0-9][0-9][0-9]\)Z/Z0\1Z/g' \
+                     -e 's/[^0-9]//g'`
+
+
+    ax_compare_version=`echo "x$ax_compare_version_A
+x$ax_compare_version_B" | sed 's/^ *//' | sort -r | sed "s/x${ax_compare_version_A}/false/;s/x${ax_compare_version_B}/true/;1q"`
+
+
+
+    if test "$ax_compare_version" = "true" ; then
+
+    if test "x$OPAM_0INSTALL_SOLVER_DEFAULT" = "xyes"; then :
+
+      OPAM_0INSTALL_SOLVER_ENABLED=false
+
+
+else
+
+      as_fn_error $? "Your version of OCaml: $OCAMLVERSION does not support the requested 0install-solver. \
+                    You can either re-run the configure script with --without-0install-solver or use make cold" "$LINENO" 5
+
+fi
+
+      fi
+
+
+fi
 
 echo
 
@@ -6323,7 +6425,8 @@ if test "x${enable_checks}" != "xno" && {
        test "x$OCAML_PKG_dose3_common" = "xno" ||
        test "x$OCAML_PKG_opam_file_format" = "xno" ||
        test "x$CPPO" = "x" ||
-       test "x$OCAML_PKG_mccs$MCCS_ENABLED" = "xnotrue";}; then :
+       test "x$OCAML_PKG_mccs$MCCS_ENABLED" = "xnotrue" ||
+       test "x$OCAML_PKG_opam_0install_cudf$OPAM_0INSTALL_SOLVER_ENABLED" = "xnotrue";}; then :
 
   echo "============================================================================"
   echo "Some dependencies are missing. If you are just interested in the stand-alone"
@@ -7549,7 +7652,7 @@ bindir="`eval echo ${bindir}`"
 mandir="`eval echo ${mandir}`"
 mandir="`eval echo ${mandir}`"
 
-if test "x$MCCS_ENABLED" = "xfalse"; then :
+if test "x$MCCS_ENABLED" = "xfalse" -a "x$OPAM_0INSTALL_SOLVER_ENABLED" = "xfalse"; then :
 
   echo "Opam will be built WITHOUT a built-in solver"
 
diff --git a/configure.ac b/configure.ac
index ff5c7ded63f..332b66b3717 100644
--- a/configure.ac
+++ b/configure.ac
@@ -57,6 +57,13 @@ AC_ARG_WITH([mccs],
   AC_HELP_STRING([--without-mccs],
                  [Compile without a built-in Cudf solver (only works if 'mccs' is not otherwise installed)]),[],[MCCS_DEFAULT=yes]
 )
+AC_ARG_WITH([0install-solver],
+  AC_HELP_STRING([--without-0install-solver],
+                 [Compile without the built-in 0install solver (only works if 'opam-0install-cudf' is not otherwise installed)]),
+                 [],
+                 [OPAM_0INSTALL_SOLVER_DEFAULT=yes]
+)
+
 
 AC_ARG_WITH([libacl],
   AC_HELP_STRING([--with-libacl],
@@ -144,6 +151,7 @@ AS_IF([test "x${enable_developer_mode}" = "xyes"],[
 ])
 
 AS_IF([test "x${with_mccs}" = "xno"], [AC_SUBST(MCCS_ENABLED,false)], [AC_SUBST(MCCS_ENABLED,true)])
+AS_IF([test "x${with_0install_solver}" = "xno"], [AC_SUBST(OPAM_0INSTALL_SOLVER_ENABLED,false)], [AC_SUBST(OPAM_0INSTALL_SOLVER_ENABLED,true)])
 
 AS_IF([test "x${with_libacl}" != "xno"],[
   have_libacl=yes
@@ -321,9 +329,12 @@ AC_CHECK_OCAML_PKG(dose3.common,dose.common)
 AC_CHECK_OCAML_PKG(dose3.algo,dose.algo)
 AC_CHECK_OCAML_PKG([opam-file-format])
 AC_CHECK_OCAML_PKG([mccs])
+AC_CHECK_OCAML_PKG([opam-0install-cudf])
 
 AS_IF([test "x${with_mccs}" = "xno" && test "x$OCAML_PKG_mccs" != "xno"],
       [AC_MSG_ERROR([Option --without-mccs is not available without uninstalling the 'mccs' package])])
+AS_IF([test "x${with_0install_solver}" = "xno" && test "x$OCAML_PKG_opam_0install_cudf" != "xno"],
+      [AC_MSG_ERROR([Option --without-0install-solver is not available without uninstalling the 'opam-0install-cudf' package])])
 
 dnl -- that's what we would like to do, but no way to disable mccs in jbuilder
 dnl -- if it's installed, at the moment
@@ -349,6 +360,16 @@ AS_IF([test "x$MCCS_ENABLED" = "xtrue"],[
   ])
 ])
 
+AS_IF([test "x$MCCS_ENABLED" = "xtrue"],[
+  AX_COMPARE_VERSION([$OCAMLVERSION], [lt], [4.08],[
+    AS_IF([test "x$OPAM_0INSTALL_SOLVER_DEFAULT" = "xyes"],[
+      AC_SUBST(OPAM_0INSTALL_SOLVER_ENABLED,false)
+    ],[
+      AC_MSG_ERROR([Your version of OCaml: $OCAMLVERSION does not support the requested 0install-solver. \
+                    You can either re-run the configure script with --without-0install-solver or use make cold])
+    ])
+  ])
+])
 
 dnl echo
 dnl echo "extlib........................ ${OCAML_PKG_extlib}"
@@ -368,7 +389,8 @@ AS_IF([test "x${enable_checks}" != "xno" && {
        test "x$OCAML_PKG_dose3_common" = "xno" ||
        test "x$OCAML_PKG_opam_file_format" = "xno" ||
        test "x$CPPO" = "x" ||
-       test "x$OCAML_PKG_mccs$MCCS_ENABLED" = "xnotrue";}],[
+       test "x$OCAML_PKG_mccs$MCCS_ENABLED" = "xnotrue" ||
+       test "x$OCAML_PKG_opam_0install_cudf$OPAM_0INSTALL_SOLVER_ENABLED" = "xnotrue";}],[
   echo "============================================================================"
   echo "Some dependencies are missing. If you are just interested in the stand-alone"
   echo "'opam' binary, run 'make lib-ext' to download and include them."
@@ -423,7 +445,7 @@ bindir="`eval echo ${bindir}`"
 mandir="`eval echo ${mandir}`"
 mandir="`eval echo ${mandir}`"
 
-AS_IF([test "x$MCCS_ENABLED" = "xfalse"],[
+AS_IF([test "x$MCCS_ENABLED" = "xfalse" -a "x$OPAM_0INSTALL_SOLVER_ENABLED" = "xfalse"],[
   echo "Opam will be built WITHOUT a built-in solver"
 ],[
   echo "Opam will be built WITH a built-in solver"
diff --git a/master_changes.md b/master_changes.md
index 828d0556e47..bb8357ff2c0 100644
--- a/master_changes.md
+++ b/master_changes.md
@@ -120,6 +120,7 @@ New option/command/subcommand are prefixed with ◈.
   * Fix Cudf preprocessing [#4534 #4627 @AltGr - fix #4624]
   * Allow to upgrade to a hidden-version package if a hidden-version package is already installed [#4525 @kit-ty-kate]
   * Add support for a few select criteria useful to CI to the 0install solver: `+count[version-lag,solution]` to always choose the oldest version available, `+removed` to not try to keep installed packages [#4631 @kit-ty-kate]
+  * Build opam with the builtin 0install solver whenever possible [#4643 @kit-ty-kate]
 
 ## Client
   * ✘ Environment variables initialised only at opam client launch, no more via libraries [#4606 @rjbou]
diff --git a/src_ext/Makefile b/src_ext/Makefile
index 7a799035839..d577435e484 100644
--- a/src_ext/Makefile
+++ b/src_ext/Makefile
@@ -33,12 +33,16 @@ MD5_PKG_$(1) = $(MD5_$(1))
 endef
 
 SRC_EXTS = cppo extlib re cmdliner ocamlgraph cudf dose3 opam-file-format result seq stdlib-shims
-PKG_EXTS = $(SRC_EXTS) dune-local findlib ocamlbuild topkg mccs
+PKG_EXTS = $(SRC_EXTS) dune-local findlib ocamlbuild topkg mccs opam-0install-cudf 0install-solver
 
 ifeq ($(MCCS_ENABLED),true)
 SRC_EXTS := $(SRC_EXTS) mccs
 endif
 
+ifeq ($(0INSTALL_SOLVER_ENABLED),true)
+SRC_EXTS := $(SRC_EXTS) opam-0install-cudf 0install-solver
+endif
+
 include Makefile.sources
 ifneq ($(shell PATH="$(PATH)" command -v ocamlc 2>/dev/null),)
 include Makefile.packages
diff --git a/src_ext/Makefile.packages b/src_ext/Makefile.packages
index 76cf8b0482d..bd2c8900151 100644
--- a/src_ext/Makefile.packages
+++ b/src_ext/Makefile.packages
@@ -41,6 +41,10 @@ dose3.pkgbuild: findlib.pkgbuild ocamlbuild.pkgbuild cppo.pkgbuild extlib.pkgbui
 # mccs.pkgbuild depends on findlib because the files are explicitly installed using it
 mccs.pkgbuild: dune-local.pkgbuild cudf.pkgbuild findlib.pkgbuild
 
+opam-0install-cudf.pkgbuild: dune-local.pkgbuild cudf.pkgbuild 0install-solver
+
+0install-solver.pkgbuild: dune-local.pkgbuild
+
 stdlib-shims.pkgbuild: dune-local.pkgbuild
 
 dune-local.pkgbuild: findlib.pkgbuild
@@ -107,6 +111,14 @@ mccs-pkg-build:
 	ocamlfind install mccs $(addprefix _build/install/default/lib/,$(addprefix mccs/,META libmccs_stubs.$(EXT_LIB) mccs.*) stublibs/*.$(EXT_DLL))
 	cp -RL _build/install/default/lib/mccs/glpk $(SITELIB)/mccs/
 
+opam-0install-cudf-pkg-build:
+	dune build @install -p opam-0install-cudf
+	dune install -p opam-0install-cudf opam-0install-cudf
+
+0install-solver-pkg-build:
+	dune build @install -p 0install-solver
+	dune install -p 0install-solver 0install-solver
+
 seq-pkg-build:
 	dune build @install -p seq
 	dune install -p seq seq
diff --git a/src_ext/Makefile.sources b/src_ext/Makefile.sources
index c495835177c..2d58a2a982a 100644
--- a/src_ext/Makefile.sources
+++ b/src_ext/Makefile.sources
@@ -38,6 +38,16 @@ MD5_mccs = 7086eefb41e631a63e26b23577cef582
 
 $(call PKG_SAME,mccs)
 
+URL_opam-0install-cudf = /~https://github.com/ocaml-opam/opam-0install-solver/releases/download/v0.4.1/opam-0install-cudf-v0.4.1.tbz
+MD5_opam-0install-cudf = 44e6d5a7fcdbc130db5dc86a6b9aeca6
+
+$(call PKG_SAME,opam-0install-cudf)
+
+URL_0install-solver = /~https://github.com/0install/0install/releases/download/v2.17/0install-v2.17.tbz
+MD5_0install-solver = 50daf035b04b29399a3c6e6f965ac447
+
+$(call PKG_SAME,0install-solver)
+
 URL_opam-file-format = /~https://github.com/ocaml/opam-file-format/archive/2.1.2.tar.gz
 MD5_opam-file-format = f16ed774167fc1881876b8185087e0ab