diff --git a/alibuild_helpers/build.py b/alibuild_helpers/build.py
index 576447ec..ee845989 100644
--- a/alibuild_helpers/build.py
+++ b/alibuild_helpers/build.py
@@ -12,7 +12,7 @@
 from alibuild_helpers.utilities import validateDefaults
 from alibuild_helpers.utilities import Hasher
 from alibuild_helpers.utilities import yamlDump
-from alibuild_helpers.utilities import resolve_tag, resolve_version
+from alibuild_helpers.utilities import resolve_tag, resolve_version, short_commit_hash
 from alibuild_helpers.git import Git, git
 from alibuild_helpers.sl import Sapling
 from alibuild_helpers.scm import SCMError
@@ -1009,8 +1009,7 @@ def doBuild(args, parser):
       ("BUILD_REQUIRES", " ".join(spec["build_requires"])),
       ("CACHED_TARBALL", cachedTarball),
       ("CAN_DELETE", args.aggressiveCleanup and "1" or ""),
-      # Shorten the commit hash if it's a real commit hash and not simply the tag.
-      ("COMMIT_HASH", spec["tag"] if spec["tag"] == spec["commit_hash"] else spec["commit_hash"][:10]),
+      ("COMMIT_HASH", short_commit_hash(spec)),
       ("DEPS_HASH", spec.get("deps_hash", "")),
       ("DEVEL_HASH", spec.get("devel_hash", "")),
       ("DEVEL_PREFIX", develPrefix),
diff --git a/alibuild_helpers/utilities.py b/alibuild_helpers/utilities.py
index 747640cb..5f5318eb 100644
--- a/alibuild_helpers/utilities.py
+++ b/alibuild_helpers/utilities.py
@@ -90,6 +90,18 @@ def resolve_links_path(architecture, package):
   return "/".join(("TARS", architecture, package))
 
 
+def short_commit_hash(spec):
+  """Shorten the spec's commit hash to make it more human-readable.
+
+  This is complicated by the fact that the commit_hash property is not
+  necessarily a commit hash, but might be a tag name. If it is a tag name,
+  return it as-is, else assume it is actually a commit hash and shorten it.
+  """
+  if spec["tag"] == spec["commit_hash"]:
+    return spec["commit_hash"]
+  return spec["commit_hash"][:10]
+
+
 # Date fields to substitute: they are zero-padded
 now = datetime.now()
 nowKwds = { "year": str(now.year),
diff --git a/alibuild_helpers/workarea.py b/alibuild_helpers/workarea.py
index bfb6cde8..8be3f985 100644
--- a/alibuild_helpers/workarea.py
+++ b/alibuild_helpers/workarea.py
@@ -10,7 +10,7 @@
   from ordereddict import OrderedDict
 
 from alibuild_helpers.log import dieOnError, debug, error
-from alibuild_helpers.utilities import call_ignoring_oserrors, symlink
+from alibuild_helpers.utilities import call_ignoring_oserrors, symlink, short_commit_hash
 
 FETCH_LOG_NAME = "fetch-log.txt"
 
@@ -145,7 +145,9 @@ def scm_exec(command, directory=".", check=True):
     return 0
 
   source_parent_dir = os.path.join(work_dir, "SOURCES", spec["package"], spec["version"])
-  source_dir = os.path.join(source_parent_dir, spec["commit_hash"])
+  # The build script expects SOURCEDIR to be named after the shortened commit
+  # hash, not the full one.
+  source_dir = os.path.join(source_parent_dir, short_commit_hash(spec))
   os.makedirs(source_parent_dir, exist_ok=True)
 
   if spec["commit_hash"] != spec["tag"]:
diff --git a/tests/test_build.py b/tests/test_build.py
index e0184355..584476f1 100644
--- a/tests/test_build.py
+++ b/tests/test_build.py
@@ -104,22 +104,22 @@
 GIT_CLONE_REF_ZLIB_ARGS = ("clone", "--bare", "https://github.com/star-externals/zlib",
                            "/sw/MIRROR/zlib", "--filter=blob:none"), ".", False
 GIT_CLONE_SRC_ZLIB_ARGS = ("clone", "-n", "https://github.com/star-externals/zlib",
-                           "/sw/SOURCES/zlib/v1.2.3/8822efa61f2a385e0bc83ca5819d608111b2168a",
+                           "/sw/SOURCES/zlib/v1.2.3/8822efa61f",
                            "--reference", "/sw/MIRROR/zlib", "--filter=blob:none"), ".", False
 GIT_SET_URL_ZLIB_ARGS = ("remote", "set-url", "--push", "origin", "https://github.com/star-externals/zlib"), \
-    "/sw/SOURCES/zlib/v1.2.3/8822efa61f2a385e0bc83ca5819d608111b2168a", False
+    "/sw/SOURCES/zlib/v1.2.3/8822efa61f", False
 GIT_CHECKOUT_ZLIB_ARGS = ("checkout", "-f", "master"), \
-    "/sw/SOURCES/zlib/v1.2.3/8822efa61f2a385e0bc83ca5819d608111b2168a", False
+    "/sw/SOURCES/zlib/v1.2.3/8822efa61f", False
 
 GIT_FETCH_REF_ROOT_ARGS = ("fetch", "-f", "https://github.com/root-mirror/root", "+refs/tags/*:refs/tags/*",
                            "+refs/heads/*:refs/heads/*"), "/sw/MIRROR/root", False
 GIT_CLONE_SRC_ROOT_ARGS = ("clone", "-n", "https://github.com/root-mirror/root",
-                           "/sw/SOURCES/ROOT/v6-08-30/f7b336611753f1f4aaa94222b0d620748ae230c0",
+                           "/sw/SOURCES/ROOT/v6-08-30/f7b3366117",
                            "--reference", "/sw/MIRROR/root", "--filter=blob:none"), ".", False
 GIT_SET_URL_ROOT_ARGS = ("remote", "set-url", "--push", "origin", "https://github.com/root-mirror/root"), \
-    "/sw/SOURCES/ROOT/v6-08-30/f7b336611753f1f4aaa94222b0d620748ae230c0", False
+    "/sw/SOURCES/ROOT/v6-08-30/f7b3366117", False
 GIT_CHECKOUT_ROOT_ARGS = ("checkout", "-f", "v6-08-00-patches"), \
-    "/sw/SOURCES/ROOT/v6-08-30/f7b336611753f1f4aaa94222b0d620748ae230c0", False
+    "/sw/SOURCES/ROOT/v6-08-30/f7b3366117", False
 
 
 def dummy_git(args, directory=".", check=True, prompt=True):