diff --git a/src/python/jw/pkg/App.py b/src/python/jw/pkg/App.py
index fe93c585..af9b9311 100644
--- a/src/python/jw/pkg/App.py
+++ b/src/python/jw/pkg/App.py
@@ -21,7 +21,7 @@ if TYPE_CHECKING:
     import argparse
 
     from argparse import ArgumentParser
-    from typing import TypeAlias
+    from typing import Iterable, TypeAlias
 
     from .lib.PackageFilter import PackageFilter
 
@@ -77,12 +77,17 @@ class App(Base):
             raise Exception('Tried to access undefined pretty top directory')
         return self.___pretty_topdir
 
-    def __proj_dir(self, name: str, pretty: bool) -> str | None:
+    def __proj_dir(
+        self,
+        name: str,
+        pretty: bool,
+        projs_roots: Iterable[str | None] | None = None,
+    ) -> str | None:
         if name == self.__top_name:
             if pretty:
                 return self.__pretty_topdir
             return self.__topdir
-        for d in [self.__projs_root, self.___opt_root]:
+        for d in projs_roots or [self.__projs_root, self.___opt_root]:
             if d is None:
                 continue
             ret = d + '/' + name
@@ -93,12 +98,36 @@ class App(Base):
             return None
         raise Exception('No project path found for module "{}"'.format(name))
 
+    @cache
+    def __is_installed(self, name: str, devel: bool) -> bool:
+        # devel: the project is in the dev tree or the -devel
+        # package is installed: make/project.conf is present.
+        # run: the -run package is installed: a VERSION file is
+        # present in the project directory or
+        # /usr/share/doc/packages.
+        if devel and name == self.__top_name:
+            # the topdir is the project's own buildable checkout
+            if os.path.exists(f'{self.__topdir}/make/project.conf'):
+                return True
+        search, file = (
+            (self.__projs_root, self.___opt_root),
+            '/make/project.conf'
+        ) if devel else (
+            (self.__projs_root, '/usr/share/doc/packages'),
+            '/VERSION'
+        )
+        for root in search:
+            if root is not None and os.path.exists(f'{root}/{name}{file}'):
+                return True
+        return False
+
     def __find_dir(
         self,
         name: str,
         search_subdirs: list[str] | None = None,
         search_absdirs: list[str] | None = None,
         pretty: bool = True,
+        projs_roots: Iterable[str | None] | None = None,
     ) -> str | None:
         if search_subdirs is None:
             search_subdirs = []
@@ -134,7 +163,7 @@ class App(Base):
                 f'Tried to pretty-format directory {pd}, not implemented'
             )
 
-        pd = self.__proj_dir(name, False)
+        pd = self.__proj_dir(name, pretty = False, projs_roots = projs_roots)
         if pd is None:
             return None
         if not search_subdirs and not search_absdirs:
@@ -181,6 +210,7 @@ class App(Base):
         names_only: bool,
     ) -> None:
         name = self.strip_module_from_spec(spec)
+        mod = re.split('([=><]+)', spec)[0].strip()
         if names_only:
             spec = name
         if spec in buf:
@@ -190,6 +220,15 @@ class App(Base):
                 buf.append(spec)
             return
         visited.add(spec)
+        needed_subpackage = 'devel' if mod.endswith(('-dev', '-devel')) else 'run'
+        if not self.is_installed(name, devel = True):
+            if not mod.endswith(('-dev', '-devel')):
+                if self.is_installed(name, devel = False):
+                    return
+            raise Exception(
+                f'Unmet dependency on {mod}: the -{needed_subpackage} package '
+                f'of project {name} is not installed'
+            )
         vals = self.get_value(name, section, key)
         log(
             DEBUG,
@@ -398,6 +437,18 @@ class App(Base):
             raise Exception('No distro object')
         return self.__distro
 
+    def is_installed(
+        self,
+        name: str,
+        devel: bool = False,
+    ) -> bool:
+        """True if the project is installed: for devel,
+        make/project.conf is present in the dev tree or /opt; for
+        run, a VERSION file is present in the project directory
+        or /usr/share/doc/packages.
+        """
+        return self.__is_installed(name, devel)
+
     def find_dir(
         self,
         name: str,
@@ -405,8 +456,9 @@ class App(Base):
         search_absdirs: list[str] | None = None,
         pretty: bool = True,
         throw: bool = False,
+        projs_roots: Iterable[str | None] | None = None,
     ) -> str | None:
-        ret = self.__find_dir(name, search_subdirs, search_absdirs, pretty)
+        ret = self.__find_dir(name, search_subdirs, search_absdirs, pretty, projs_roots)
         if ret is not None:
             return ret
         if not throw:
diff --git a/src/python/jw/pkg/cmds/projects/CmdBuild.py b/src/python/jw/pkg/cmds/projects/CmdBuild.py
index 3972bda5..6677e71b 100644
--- a/src/python/jw/pkg/cmds/projects/CmdBuild.py
+++ b/src/python/jw/pkg/cmds/projects/CmdBuild.py
@@ -94,6 +94,14 @@ class CmdBuild(Cmd):  # export
     @override
     async def _run(self, args: Namespace) -> None:
 
+        @lru_cache(maxsize = None)
+        def proj_dir(module: str) -> str | None:
+            return self.app.find_dir(
+                module,
+                pretty = False,
+                projs_roots = [self.app.projs_root],
+            )
+
         @lru_cache(maxsize = None)
         def read_deps(cur: str, dep_flavour: str) -> list[str]:
             # dep cache doesn't make a difference at all
@@ -172,20 +180,27 @@ class CmdBuild(Cmd):  # export
                         dep_tree[k].remove(d)
             return 1
 
+        def log_skip(module: str, msg: str) -> None:
+            title = f'---- {module}'
+            log(NOTICE, f',{title} >')
+            log(NOTICE, f'| {msg}<')
+            log(NOTICE, f'`{title} <')
+
         async def run_make(
             module: str, target: str, cur_project: int, num_projects: int
         ) -> None:
 
             patt = self.app.is_excluded_from_build(module)
             if patt is not None:
-                title = f'---- {module}'
-                log(NOTICE, f',{title} >')
-                log(NOTICE, f'| Configured to skip build on platform >{patt}<')
-                log(NOTICE, f'`{title} <')
+                log_skip(module, f'Configured to skip build on platform >{patt}<')
+                return
+
+            wd = proj_dir(module)
+            if wd is None:
+                log_skip(module, 'Skipping: No buildable project directory')
                 return
 
             make_cmd = ['make', target]
-            wd = self.app.find_dir(module, pretty = False)
             title = '---- [%d/%d]: Running "%s" in %s -' % (
                 cur_project,
                 num_projects,