How to parse structured data from an AI's response? (text + tool call XML)

[mddd (mdddj)]

I'm building a system where an AI model can invoke tools by generating a specific text format in its response. I need to reliably parse this tool-call information, which is often mixed with natural language.

Here's a concrete example of the interaction:

User Input:

Help me generate a test file.

AI's Response:

Okay, I will call the file writing tool,
<write_to_file>
    <path>/Users/ldd/1.txt</path>
    <content>some text</content>
</write_to_file>

So, the main question is: what's the best strategy to process these kinds of mixed responses and parse them into my structured data classes (TextContent or ToolUse)?

Additionally, I've noticed that some less advanced models tend to wrap the tool-call XML in a Markdown code block, like this:

Okay, I will call the file writing tool.

```xml
<write_to_file>
    <path>/Users/ldd/1.txt</path>
    <content>some text</content>
</write_to_file>

This is bad (GoogleModels.Gemini2_0Flash)

This is good (GoogleModels.Gemini2_5Pro)

To provide more context, here are the Kotlin data classes I am using to represent the assistant's message. My goal is to parse the AI's text response into one of these structures.

// Represents the content of a message from the AI assistant.
// The sealed interface means a message content can only be one of the types defined below (TextContent or ToolUse).
sealed interface AssistantMessageContent {
    // A flag to indicate if this is a partial message, e.g., from a streaming response.
    var partial: Boolean
}

// Represents a simple text response.
data class TextContent(
    var content: String,
    override var partial: Boolean
) : AssistantMessageContent

// Represents a request from the AI to use a tool.
// This is the structure I need to populate from the AI's XML-like output.
data class ToolUse(
    val name: ToolUseName, // The name of the tool, e.g., "write_to_file"
    val params: MutableMap<ToolParamName, String>, // A map of parameters for the tool
    override var partial: Boolean,
    val id: String = UUID.randomUUID().toString() // A unique ID for this specific tool call
) : AssistantMessageContent

[Antonii (antoniibelyshev)]

It's better to use the native tool capability rather than asking the model to generate tool messages using xml. It can be done by passing a tool registry to an agent or by passing tools parameters to the executor.execute method (depending on weather you are using an AIAgent or calling an executor directly).

If you'll provide a code snippet how you call the model, I can help you update it to use the native tool calls.