jsonl.co is a free online tool for viewing, editing, and converting JSONL (JSON Lines) files. It operates entirely within the user's browser, processing data locally without requiring file uploads to a server. The tool supports JSONL, NDJSON, JSON, and TXT file formats. JSONL is a text format where each line is a standalone JSON object separated by a newline, commonly used for streaming large datasets, application logs, and machine learning pipelines. The tool provides a viewer with streaming capabilities and syntax highlighting, along with real-time validation and editing functions. It includes a suite of conversion tools, such as JSONL to JSON, JSON to JSONL, JSONL to CSV, CSV to JSONL, JSONL to YAML, and JSONL to Excel. Additional utilities include a JSONL formatter, a JSONL validator, and a flatten JSON tool. The application can handle files up to 1 gigabyte in size. Typical use cases for jsonl.co include reviewing OpenAI fine-tuning datasets, inspecting Hugging Face training data, and debugging application logs. Users can interact with the tool by dropping a file, pasting text, or typing directly. Because the application processes data locally, it offers a private environment for working with sensitive data without requiring an account or sign-up.