Multi-line text and rich text fields limits and restrictions

To understand why the STRING_TOO_LONG error occurs and how to work within Salesforce field limits, it is important to distinguish between multi-line text and rich text field types.

Multi-line Text Field Limits

Fields like CaseComment.Body have a documented limit of 4,000 bytes. However, the actual character limit can be less than 4,000, and varies depending on the character encoding used. The 4,000-character limit includes not just visible characters, but also any HTML encoding. For example, a single & HTML entity counts as 5 bytes but represents only 1 visible character.
The length limit in multi-line text fields is by default enforced at the character level in the Salesforce UI but at the byte level when inserting data via API. This discrepancy is the most common cause of the STRING_TOO_LONG error when inserting content that appears within limits.

Rich Text Field Limits

Rich text fields that contain images include the full image source tag (<img src="...">) in the character count. The image source tag — which can be dozens of characters long — counts against the character limit of the field. When a rich text field contains multiple images, the cumulative length of all image tags can cause STRING_TOO_LONG errors even when the visible text content is short.
See Rich Text Area Field Limitations — Salesforce Help for the full list of restrictions : https://help.salesforce.com/s/articleView?id=platform.fields_rich_text_area_limitations.htm&type=5