diff --git a/lib/src/repo.rs b/lib/src/repo.rs
index 91778446f..b63e6db7c 100644
--- a/lib/src/repo.rs
+++ b/lib/src/repo.rs
@@ -270,16 +270,31 @@ impl ReadonlyRepo {
     }
 
     pub fn shortest_unique_prefix_length(&self, target_id_bytes: &[u8]) -> usize {
-        // For `len = index.shortest(id)`, a prefix of length `len` will disambiguate
-        // `id` from all other ids in the index. This will be just as true for
-        // `max(len, anything_else)`, so a max of such lengths will disambiguate in all
-        // indices.
-        cmp::max(
-            self.commit_id_index()
-                .shortest_unique_prefix_len(target_id_bytes),
-            self.change_id_index()
-                .shortest_unique_prefix_len(target_id_bytes),
-        )
+        let root_commit_id = self.store().root_commit_id();
+        let root_change_id = backend::root_change_id();
+        if target_id_bytes == root_commit_id.as_bytes()
+            || target_id_bytes == root_change_id.as_bytes()
+        {
+            // The root change/commit ids share the same prefix, and they are found in both
+            // indices with different lengths. So we have to feed bytes of valid lengths.
+            cmp::max(
+                self.commit_id_index()
+                    .shortest_unique_prefix_len(root_commit_id.as_bytes()),
+                self.change_id_index()
+                    .shortest_unique_prefix_len(root_change_id.as_bytes()),
+            )
+        } else {
+            // For `len = index.shortest(id)`, a prefix of length `len` will disambiguate
+            // `id` from all other ids in the index. This will be just as true for
+            // `max(len, anything_else)`, so a max of such lengths will disambiguate in all
+            // indices.
+            cmp::max(
+                self.commit_id_index()
+                    .shortest_unique_prefix_len(target_id_bytes),
+                self.change_id_index()
+                    .shortest_unique_prefix_len(target_id_bytes),
+            )
+        }
     }
 
     pub fn store(&self) -> &Arc<Store> {
diff --git a/tests/test_alias.rs b/tests/test_alias.rs
index 4f1e437e3..3b75a1cb3 100644
--- a/tests/test_alias.rs
+++ b/tests/test_alias.rs
@@ -131,7 +131,7 @@ fn test_alias_cannot_override_builtin() {
     // Alias should be ignored
     let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-r", "root"]);
     insta::assert_snapshot!(stdout, @r###"
-    o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+    o 0[000000000]  1970-01-01 00:00:00.000 +00:00 0[000000000]
       (empty) (no description set)
     "###);
 }
diff --git a/tests/test_commit_template.rs b/tests/test_commit_template.rs
index b1e802c08..bc0fc7b24 100644
--- a/tests/test_commit_template.rs
+++ b/tests/test_commit_template.rs
@@ -68,7 +68,7 @@ fn test_log_default() {
     | (empty) description 1
     o 9a[45c67d3e] test.user@example.com 2001-02-03 04:05:08.000 +07:00 4[291e264ae]
     | add a file
-    o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+    o 0[000000000]  1970-01-01 00:00:00.000 +00:00 0[000000000]
       (empty) (no description set)
     "###);
 
@@ -78,7 +78,7 @@ fn test_log_default() {
     | (empty) description 1
     o 9a[45c67d3e] test.user@example.com 2001-02-03 04:05:08.000 +07:00 4[291e264ae]
     | add a file
-    o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+    o 0[000000000]  1970-01-01 00:00:00.000 +00:00 0[000000000]
       (empty) (no description set)
     "###);
 
@@ -99,7 +99,7 @@ fn test_log_default() {
     | (empty) description 1
     o 9a[45c67d3e] test.user@example.com 2001-02-03 04:05:08.000 +07:00 4[291e264ae]
     | add a file
-    o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+    o 0[000000000]  1970-01-01 00:00:00.000 +00:00 0[000000000]
       (empty) (no description set)
     "###);
 
@@ -110,7 +110,7 @@ fn test_log_default() {
     (empty) description 1
     9a[45c67d3e] test.user@example.com 2001-02-03 04:05:08.000 +07:00 4[291e264ae]
     add a file
-    000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+    0[000000000]  1970-01-01 00:00:00.000 +00:00 0[000000000]
     (empty) (no description set)
     "###);
 }
@@ -128,7 +128,7 @@ fn test_log_default_divergence() {
     insta::assert_snapshot!(stdout, @r###"
     @ 9[a45c67d3e] test.user@example.com 2001-02-03 04:05:08.000 +07:00 7[a17d52e63]
     | description 1
-    o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+    o 0[000000000]  1970-01-01 00:00:00.000 +00:00 0[000000000]
       (empty) (no description set)
     "###);
 
@@ -144,7 +144,7 @@ fn test_log_default_divergence() {
     | description 2
     | @ 9[a45c67d3e]?? test.user@example.com 2001-02-03 04:05:08.000 +07:00 7[a17d52e63]
     |/  description 1
-    o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+    o 0[000000000]  1970-01-01 00:00:00.000 +00:00 0[000000000]
       (empty) (no description set)
     "###);
 
@@ -155,7 +155,7 @@ fn test_log_default_divergence() {
     | description 2
     | @ 9[a45c67d3e]?? test.user@example.com 2001-02-03 04:05:08.000 +07:00 7[a17d52e63]
     |/  description 1
-    o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+    o 0[000000000]  1970-01-01 00:00:00.000 +00:00 0[000000000]
       (empty) (no description set)
     "###);
 }
diff --git a/tests/test_log_command.rs b/tests/test_log_command.rs
index c45b7a608..b8b11ab51 100644
--- a/tests/test_log_command.rs
+++ b/tests/test_log_command.rs
@@ -356,7 +356,7 @@ fn test_log_prefix_highlight_counts_hidden_commits() {
         test_env.jj_cmd_success(&repo_path, &["log", "-r", "all()", "-T", prefix_format]),
         @r###"
     @ Change 9[a45c67d3e] initial b[a1a30916d] original
-    o Change 000000000000 (no description set) 000000000000 
+    o Change 0[000000000] (no description set) 0[000000000] 
     "###
     );
     for i in 1..100 {
@@ -376,7 +376,7 @@ fn test_log_prefix_highlight_counts_hidden_commits() {
         test_env.jj_cmd_success(&repo_path, &["log", "-r", "all()", "-T", prefix_format]),
         @r###"
     @ Change 9a4[5c67d3e] commit99 de[3177d2ac] original
-    o Change 000000000000 (no description set) 000000000000 
+    o Change 000[0000000] (no description set) 000[0000000] 
     "###
     );
     insta::assert_snapshot!(
diff --git a/tests/test_revset_output.rs b/tests/test_revset_output.rs
index 1f5407f4d..c2834c312 100644
--- a/tests/test_revset_output.rs
+++ b/tests/test_revset_output.rs
@@ -161,13 +161,13 @@ fn test_alias() {
 
     let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-r", "my-root"]);
     insta::assert_snapshot!(stdout, @r###"
-    o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+    o 0[000000000]  1970-01-01 00:00:00.000 +00:00 0[000000000]
       (empty) (no description set)
     "###);
 
     let stdout = test_env.jj_cmd_success(&repo_path, &["log", "-r", "identity(my-root)"]);
     insta::assert_snapshot!(stdout, @r###"
-    o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+    o 0[000000000]  1970-01-01 00:00:00.000 +00:00 0[000000000]
       (empty) (no description set)
     "###);
 
@@ -263,7 +263,7 @@ fn test_bad_alias_decl() {
         .assert()
         .success();
     insta::assert_snapshot!(get_stdout_string(&assert), @r###"
-    o 000000000000  1970-01-01 00:00:00.000 +00:00 000000000000
+    o 0[000000000]  1970-01-01 00:00:00.000 +00:00 0[000000000]
       (empty) (no description set)
     "###);
     insta::assert_snapshot!(get_stderr_string(&assert), @r###"