What causes "Failed to reload bundle" in langflow-ai/langflow?

Check the aerror log line 'Unexpected error starting MCP Composer ... rolling back auth settings' for the underlying exception.

langflow-ai/langflow · error · Error

Failed to reload bundle

Error message

Failed to reload bundle

What it means

500 raised inside the PATCH handler when starting MCP Composer for OAuth throws an unexpected (non-MCPComposerError) exception. The handler deliberately rolls back project.auth_settings to the pre-request value before failing, so the DB stays consistent, and echoes the raw exception text.

Solutions

Check the aerror log line 'Unexpected error starting MCP Composer ... rolling back auth settings' for the underlying exception.
Verify 'uvx --version' works in the Langflow runtime environment and that the mcp-composer version pin (settings.mcp_composer_version) resolves.
Ensure the composer host/port in auth settings is free and reachable.
Retry the PATCH after fixing the environment — the rollback means no state cleanup is needed.

Defensive patterns

Strategy: try-catch

Validate before calling

import shutil
if shutil.which("uvx") is None:
    raise EnvironmentError("uvx not on PATH; MCP Composer cannot start")

Try / catch

try:
    await client.patch(project_url, json={"auth_type": "oauth", ...})
except httpx.HTTPStatusError as e:
    if e.response.status_code == 500:
        detail = e.response.json().get("detail", "")
        # auth settings were rolled back server-side; safe to fix env and retry

Prevention

Verify uvx and the pinned mcp-composer version work before enabling OAuth on projects.
Reserve a known-free host/port for the composer.
Remember the rollback: after this 500 the previous auth settings are still active, so retrying is idempotent.

When it happens

Trigger: PATCH /api/v1/mcp/projects/{project_id} with auth_type='oauth' while mcp_composer_enabled=True, and get_or_start_mcp_composer raises e.g. a subprocess spawn error (uvx missing), port allocation failure, or an httpx connection error.

Common situations: uv not installed / uvx not on PATH in the container; composer port already in use by a zombie process; first-run image missing the mcp-composer package; OAuth host/port misconfigured.

Related errors

AI-assisted analysis of langflow-ai/langflow@976ec789d2 (2026-08-14). Data as JSON: /api/errors/01968c7718a8780d. Report an issue: GitHub.

Appendix: source

Thrown at src/frontend/src/controllers/API/queries/extensions/use-reload-bundle.ts:85

        typeof detail === "object" &&
        (detail as ReloadInProgressDetail).code === "reload-in-progress"
      ) {
        const inProgress = detail as ReloadInProgressDetail;
        throw new Error(`reload-in-progress: ${inProgress.message}`);
      }

      // 422 carries the full ReloadResult in detail.result so structural
      // failures keep the same body shape as a 200 -- the caller's
      // onSuccess handler renders the typed errors inline via the
      // existing ok=false branch.
      if (status === 422 && detail && typeof detail === "object") {
        const result = (detail as { result?: ReloadBundleResponse }).result;
        if (result && typeof result === "object") {
          return result;
        }
      }

      throw new Error(
        extractApiErrorMessage(
          error as Parameters<typeof extractApiErrorMessage>[0],
          "Failed to reload bundle",
        ),
      );
    }
  }

  const mutation: UseMutationResult<
    ReloadBundleResponse,
    Error,
    ReloadBundleVariables
  > = mutate(["useReloadBundle"], reloadBundle, options);

  return mutation;
};

View on GitHub (pinned to 976ec789d2)