diff --git a/src/ci_orchestrator/commands/build.py b/src/ci_orchestrator/commands/build.py
index 28581b1..26f6aca 100644
--- a/src/ci_orchestrator/commands/build.py
+++ b/src/ci_orchestrator/commands/build.py
@@ -151,8 +151,15 @@ def _linux_build(
             )
             t.run(cmd)
             if clone_commit:
+                # The shallow --depth 1 --branch clone only has the branch
+                # tip. If the branch advanced between workflow trigger and
+                # runner pickup, the pinned commit is absent ("reference is
+                # not a tree"). Fetch exactly that commit, then check out.
                 t.run(
-                    f"git -C {_sh_quote(workdir)} checkout {_sh_quote(clone_commit)}"
+                    f"git -C {_sh_quote(workdir)} fetch --depth 1 origin "
+                    f"{_sh_quote(clone_commit)} && "
+                    f"git -C {_sh_quote(workdir)} checkout "
+                    f"{_sh_quote(clone_commit)}"
                 )
         else:
             raise click.ClickException(
@@ -275,7 +282,12 @@ def _windows_build(
                 f"{_ps_quote(clone_url)} (Split-Path {_ps_quote(workdir)} -Leaf)"
             )
             if clone_commit:
+                # Shallow clone has only the branch tip; fetch the pinned
+                # commit explicitly so checkout works even if the branch
+                # advanced between trigger and runner pickup.
                 t.run(
+                    f"git -C {_ps_quote(workdir)} fetch --depth 1 origin "
+                    f"{_ps_quote(clone_commit)}; "
                     f"git -C {_ps_quote(workdir)} checkout {_ps_quote(clone_commit)}"
                 )
         else: