langflow-ai/langflow · error · HTTPException

Failed to start MCP Composer. See logs for details.

Error message

Failed to start MCP Composer. See logs for details.

What it means

Error "Failed to start MCP Composer. See logs for details." thrown in langflow-ai/langflow.

Source

Thrown at src/backend/base/langflow/api/v1/mcp_projects.py:894

            raise HTTPException(status_code=400, detail="Invalid transport. Use 'sse' or 'streamablehttp'.")

        if use_mcp_composer:
            try:
                auth_config = await _get_mcp_composer_auth_config(project)
                await get_or_start_mcp_composer(auth_config, project.name, project_id)
                composer_streamable_http_url = await get_composer_streamable_http_url(project)
                sse_url = await get_composer_sse_url(project)
                connection_urls = [composer_streamable_http_url, sse_url]
            except MCPComposerError as e:
                await logger.aerror(
                    f"Failed to start MCP Composer for project '{project.name}' ({project_id}): {e.message}"
                )
                raise HTTPException(status_code=500, detail=e.message) from e
            except Exception as e:
                error_msg = f"Failed to start MCP Composer for project '{project.name}' ({project_id}): {e!s}"
                await logger.aerror(error_msg)
                error_detail = "Failed to start MCP Composer. See logs for details."
                raise HTTPException(status_code=500, detail=error_detail) from e

            # For OAuth/MCP Composer, use the special format
            settings = get_settings_service().settings
            command = "uvx"
            args = [
                *mcp_sdk_constraint_args(),
                f"mcp-composer{settings.mcp_composer_version}",
                "--mode",
                "http",
                "--endpoint",
                composer_streamable_http_url,
                "--sse-url",
                sse_url,
                "--client_auth_type",
                "oauth",
                "--disable-composer-tools",
            ]
        else:

View on GitHub (pinned to 976ec789d2)

Solutions

1. Check the Langflow logs for the MCP Composer startup failure; verify the required binaries are installed.

When it happens

Trigger: Occurs when the MCP Composer process fails to start; details are only available in server logs.

Common situations: See trigger scenarios.

AI-assisted analysis of langflow-ai/langflow@976ec789d2 (2026-08-14). Data as JSON: /api/errors/c8cbc0865b1eff45. Report an issue: GitHub.