Tool calls fail because arguments come back as XML-like text

In agent mode, tool calls break with empty arguments or list/index errors when using a self-hosted compatible endpoint.

Difficulty

Moderate

Time

~15 min

Steps

Fixed in

—

Last checked

2026-10-15

No upstream fix: these steps work around it. Source: issue closed (not planned) without a merged fix. Last checked 2026-10-15.

Symptom

In agent mode, tool calls break with empty arguments or list/index errors when using a self-hosted compatible endpoint.

Evidence what you'll see

function.arguments holds <tool_call><function=...><parameter=...> text
JSON parsing returns {}

Cause

Some open-weight models were trained on an XML-style tool format. If the server doesn't convert it, the client gets XML where it expects JSON.

Fix steps

0 of 3 done

Step 1Call the endpoint directly and inspect function.arguments.
Step 2Turn on your inference server's tool-call parser for that model family so it returns JSON arguments.
Step 3Or pick a model or template that outputs JSON tool calls.

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Verify the fix worked

A tool-heavy request runs tools with the right arguments.

Did this fix it?

Sources and credit

Summarized in our own words from these public reports. Credit goes to their authors.

continue issue #11453 ↗

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