Document prompt character limit for agents (#58817)

Author: belaltaher8

content/copilot/how-tos/use-copilot-agents/coding-agent/create-custom-agents.md

@@ -89,7 +89,7 @@ To configure your {% data variables.copilot.agent_profile %}:
 1. If creating an organization or enterprise level agent on {% data variables.product.github %}, you can use the `mcp-servers` property to optionally configure MCP servers that will be available only to this agent to extend its capabilities.
 1. If you are creating and using the {% data variables.copilot.agent_profile %} in {% data variables.product.prodname_vscode_shortname %}, JetBrains IDEs, Eclipse, or Xcode, you can also use the `model` property to control which AI model the agent should use.
 1. Optionally, set the `target` property to either `vscode` or `github-copilot` if you want to only use the agent in a specific environment. The agent will be available in both environments if you omit the property.
-1. Write the agent's prompt. Define the agent's behavior, expertise, and instructions in the Markdown content below the YAML frontmatter.
+1. Write the agent's prompt. Define the agent's behavior, expertise, and instructions in the Markdown content below the YAML frontmatter. The prompt can be a maximum of 30,000 characters.
 
 ## Example {% data variables.copilot.agent_profiles %}
 

content/copilot/reference/custom-agents-configuration.md

@@ -31,6 +31,8 @@ The following table outlines the properties that are supported in repository-lev
 
 {% endrowheaders %}
 
+Define the agent's behavior, expertise, and instructions in the Markdown content below the YAML frontmatter. The prompt can be a maximum of 30,000 characters.
+
 > [!NOTE]
 > The `model`, `argument-hint`, and `handoffs` properties from {% data variables.product.prodname_vscode_shortname %} and other IDE {% data variables.copilot.custom_agents_short %} are currently not supported for {% data variables.copilot.copilot_coding_agent %} on {% data variables.product.prodname_dotcom_the_website %}. They are ignored to ensure compatibility. For more information on {% data variables.copilot.copilot_custom_agent_short %} file structure in {% data variables.product.prodname_vscode_shortname %}, see [{% data variables.copilot.custom_agents_caps_short %} in {% data variables.product.prodname_vscode_shortname %}](https://code.visualstudio.com/docs/copilot/customization/custom-agents#_custom-agent-file-structure) in the {% data variables.product.prodname_vscode_shortname %} documentation.