diff --git a/nanobot/utils/gitstore.py b/nanobot/utils/gitstore.py
index 01ab1fb9a..f09325077 100644
--- a/nanobot/utils/gitstore.py
+++ b/nanobot/utils/gitstore.py
@@ -176,7 +176,10 @@ class GitStore:
             )
             if cast(object, sha_bytes) is None:
                 return None
-            sha = sha_bytes.hex()[:8]
+            # porcelain.commit returns the id as a 40-char hex string that is
+            # already encoded to bytes; .hex() would encode those ASCII bytes
+            # again and produce an id no git command can resolve.
+            sha = sha_bytes.decode()[:8]
             logger.debug("Git auto-commit: {} ({})", sha, message)
             return sha
         except Exception as exc:
@@ -200,7 +203,7 @@ class GitStore:
                     return None
 
                 while sha:
-                    if sha.hex().startswith(short_sha):
+                    if sha.decode().startswith(short_sha):
                         return sha
                     commit_obj = repo[sha]
                     if commit_obj.type_name != b"commit":
@@ -280,7 +283,7 @@ class GitStore:
                     msg = commit.message.decode("utf-8", errors="replace").strip()
                     if message_prefix is None or msg.startswith(message_prefix):
                         entries.append(CommitInfo(
-                            sha=sha.hex()[:8],
+                            sha=sha.decode()[:8],
                             message=msg,
                             timestamp=ts,
                         ))
@@ -484,7 +487,7 @@ class GitStore:
                     with Repo(str(self._workspace)) as repo:
                         commit = cast("Commit", repo[full_sha])
                         parent = commit.parents[0] if commit.parents else None
-                    diff = self.diff_commits(parent.hex()[:8], c.sha) if parent else ""
+                    diff = self.diff_commits(parent.decode()[:8], c.sha) if parent else ""
                     return c, diff
             return None
         except Exception as exc:
diff --git a/tests/utils/test_gitstore.py b/tests/utils/test_gitstore.py
index c5b7c16ff..14a2d7fd6 100644
--- a/tests/utils/test_gitstore.py
+++ b/tests/utils/test_gitstore.py
@@ -310,3 +310,25 @@ class TestNestedRepoProtection:
 
         assert result is False
         assert not (workspace / ".git").exists()
+
+
+class TestCommitIdEncoding:
+    """Commit ids must be usable with git, not hex-of-hex."""
+
+    def test_auto_commit_returns_the_real_short_sha(self, git, tmp_path):
+        (tmp_path / "MEMORY.md").write_text("- a fact\n", encoding="utf-8")
+        sha = git.auto_commit("memory update")
+        expected = subprocess.run(
+            ["git", "-C", str(tmp_path), "log", "-1", "--format=%h", "--abbrev=8"],
+            capture_output=True, text=True, check=True,
+        ).stdout.strip()
+        assert sha == expected
+
+    def test_a_real_git_sha_resolves(self, git, tmp_path):
+        (tmp_path / "MEMORY.md").write_text("- a fact\n", encoding="utf-8")
+        git.auto_commit("memory update")
+        real = subprocess.run(
+            ["git", "-C", str(tmp_path), "log", "-1", "--format=%h", "--abbrev=8"],
+            capture_output=True, text=True, check=True,
+        ).stdout.strip()
+        assert git._resolve_sha(real) is not None