diff --git a/tests/ClaudeDo.Worker.Tests/External/ExternalMcpToolSchemaTests.cs b/tests/ClaudeDo.Worker.Tests/External/ExternalMcpToolSchemaTests.cs
index fd95dc2a..9f6ba4ee 100644
--- a/tests/ClaudeDo.Worker.Tests/External/ExternalMcpToolSchemaTests.cs
+++ b/tests/ClaudeDo.Worker.Tests/External/ExternalMcpToolSchemaTests.cs
@@ -84,4 +84,57 @@ public sealed class ExternalMcpToolSchemaTests
 
         Assert.True(hasHint, "No external tool description carries McpToolDocs.TaskNumberHint.");
     }
+
+    // Full inventory of registered external tool names, computed via the SDK's own
+    // McpServerTool.Create (the same call Program.cs's WithTools<T>() makes internally) rather
+    // than reimplementing the PascalCase-to-snake_case conversion — so this only breaks on an
+    // actual rename/add/remove, not on a divergent reimplementation of the naming rule. Used to
+    // prove that splitting ExternalMcpService.cs into topic files (MergeMcpTools, WorktreeMcpTools,
+    // ...) is a byte-identical rename-preserving move: the set below must not change across that split.
+    private static readonly string[] ExpectedToolNames =
+    [
+        "list_task_lists", "list_tasks", "get_task", "add_task", "update_task", "add_subtask",
+        "update_task_status", "review_task", "run_task_now", "cancel_task", "delete_task",
+        "get_task_worktree", "get_task_diff", "merge_task", "continue_merge", "verify_merges",
+        "abort_merge", "get_merge_conflicts", "resolve_conflict_hunk", "preview_merge",
+        "preview_merge_set", "revert_merge", "list_worktrees", "cleanup_task_worktree",
+        "continue_task", "get_daily_prep_candidates", "set_my_day",
+        "batch_get_tasks", "batch_add_tasks", "batch_update_task_status", "batch_cancel_tasks",
+        "batch_delete_tasks", "batch_set_my_day", "batch_cleanup_task_worktrees",
+        "create_list", "update_list", "delete_list",
+        "get_list_config", "set_list_config", "set_task_config", "get_task_config", "get_effective_run_config",
+        "list_runs", "get_run", "get_task_log",
+        "list_agents",
+        "reset_failed_task", "submit_task_for_review",
+        "handoff_list_handler",
+        "get_app_settings",
+        "wait_for_task_change",
+        "get_queue_state",
+        "add_task_attachment", "list_task_attachments", "remove_task_attachment",
+        "save_finding",
+        "add_note",
+    ];
+
+    [Fact]
+    public void ExternalToolNames_MatchExpectedInventory()
+    {
+        // A throwaway, never-constructed instance per declaring type — McpServerTool.Create needs a
+        // non-null target reference to bind the method to, but naming/schema derivation never
+        // touches instance state, so skipping the real constructor (and its DI dependencies) is safe.
+        var targets = new Dictionary<Type, object>();
+        var actual = ExternalToolMethods()
+            .Select(m =>
+            {
+                var target = targets.TryGetValue(m.DeclaringType!, out var t)
+                    ? t
+                    : targets[m.DeclaringType!] = System.Runtime.CompilerServices.RuntimeHelpers.GetUninitializedObject(m.DeclaringType!);
+                return McpServerTool.Create(m, target).ProtocolTool.Name;
+            })
+            .OrderBy(n => n, StringComparer.Ordinal)
+            .ToList();
+
+        var expected = ExpectedToolNames.OrderBy(n => n, StringComparer.Ordinal).ToList();
+
+        Assert.Equal(expected, actual);
+    }
 }