Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
Schema coverage is 100%, so the baseline is 3. The description adds coordination semantics by noting that key, filename, mimeType are the values from prepare_image_upload, which is critical for correct invocation. Alt text is not described in the prose but is already explained in the schema.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.