Partner APIs are more prevalent than public-facing ones, yet often suffer from poor documentation, which significantly hampers integration efforts. According to the 2024 State of the API Report, nearly 40% of developers cite inconsistent documentation as a major challenge, leading to errors, miscommunications, and delays. A documentation-first approach, grounded in API specifications and managed through a git-based environment, can address these issues by providing a consistent source of truth, aligning with modern DevOps practices like CI/CD, and reducing integration friction. This method ensures that all API collaborators are on the same page, thereby enhancing partner relationships and improving the overall developer experience, which is crucial for revenue growth. The shift-left strategy involves creating and documenting specifications prior to development, promoting predictability and reducing errors, and is supported by tools like OpenAPI, which help standardize API behaviors. Ultimately, well-documented APIs, such as those from successful companies like Airbnb and eBay, drive significant business value by improving partner experiences, reducing customer churn, and increasing revenue potential.