[Bug] XMLAdapter inserting wrong markers

[enitrat]

What happened?

The XMLAdapter inserts a [[ ## completed ## ]] instead of having no completion tag at all (or a tag)
Also, the User message: is formatted using ChatAdapter format for the input fields instead of XML format.

Reading the full prompt when using XMLAdapter, we see the wrong tags used:

[2025-09-30T23:08:40.684097]

System message:

Your input fields are:
1. `query` (str):
Your output fields are:
1. `answer` (str):
All interactions will be structured in the following way, with the appropriate values filled in.

<query>
{query}
</query>

<answer>
{answer}
</answer>

[[ ## completed ## ]]
In adhering to this structure, your objective is:
        Given the fields `query`, produce the fields `answer`.


User message:

[[ ## query ## ]]
when was Marie Curie born

Respond with the corresponding output fields wrapped in XML tags`<answer>`, and then end with the `<completed>` tag.


Response:

<query>
when was Marie Curie born
</query>

<answer>
Marie Curie was born on November 7, 1867.
</answer>

[[ ## completed ## ]]

Although it says Respond with the corresponding output fields wrapped in XML tags, and then end with the tag., there's no <completed> tag, instead there's a [[ ## completed ## ]] inherited from the base ChatAdapter.

Steps to reproduce

from dspy.adapters import XMLAdapter

dspy.configure(lm=dspy.LM("gemini/gemini-flash-latest", max_tokens=30000, cache=False), adapter=XMLAdapter())
prog2 = dspy.Predict("query: str -> answer: str")
prog2(query="when was Marie Curie born")
dspy.inspect_history(n=1)

output:

DSPy version

3.0.3

[TomeHirata]

Thanks for the report. Please take a look at my reply here.

[chenmoneygithub]

I am actually confused where it grabs the knowledge of [[ ## completed ## ]], as the entire prompt doesn't mention [[ ## completed ## ]] at all.

Speaking of XMLAdapter, the current implementation doesn't really make much difference in the quality, we have a pending PR #8482 that introduces the desired behavior, but unfortunately we haven't yet gained bandwidth to land it.

@isaacbmiller would you have some bandwidth to take a look at #8482 and help land it?

[TomeHirata]

@chenmoneygithub [[ ## completed ##]] is mentioned in the system instruction; it comes from the format_assistant_message_content method of ChatAdapter. The current system instruction of XMLAdapter is like this:

Your input fields are:
question (str): Your output fields are:
reasoning (str):
answer (str): All interactions will be structured in the following way, with the appropriate values filled in.

<question> {question} </question> <reasoning> {reasoning} </reasoning> <answer> {answer} </answer>
[[ ## completed ## ]]  <- "completed" is mentioned here

In adhering to this structure, your objective is: Given the fields question, produce the fields answer.

[chenmoneygithub]

@TomeHirata Good catch, it's now a placeholder string in my brain that I read it as void😅

[isaacbmiller]

I should have bandwidth this week/early next to review this!