chat : tolerate dropped tool-call wrapper in tag-based formats

[jamincollins]

Overview

Several chat templates use a tagged tool-call format where <tool_call> is the wrapper around an inner <function=name>...</function> block (Qwen3-Coder, Nemotron, Seed-OSS, and others). Testing found that some models (see below) sometimes emit the well-formed inner block but skip the outer wrapper. Before this change, that was rejected entirely - the whole response fell back to plain text, dropping the tool call.

What changed (common/chat-auto-parser-generator.cpp):

• The wrapper's open/close tags are now optional in the grammar rather than mandatory - the inner function tag stays mandatory, so this can't misfire on unrelated text
• The scan that looks for "where does the tool call start" now also checks the inner function tag as a fallback, not just the wrapper, so a dropped wrapper doesn't cause the whole response to be swallowed as content
• The same leniency was applied to both of the two sibling code paths that share this exact pattern (tag_tagged and tag_json formats), for consistency

Test coverage (tests/test-chat.cpp): five new cases covering the dropped-wrapper scenario, an asymmetric case (open tag missing, close tag present), and a parallel-calls case where only one of two calls is missing its wrapper - across both affected code paths.

Additional information

• Model: unsloth/Qwen3-Coder-30B-A3B-Instruct-GGUF
• Setup: served via llama-server with --jinja (native template handling), dual NVIDIA GPU (RTX 3080 + RTX 3060, 12GB each)
• Failure rate: measured over batches of 20-30 identical requests - roughly 1 in 3 requests (~33%) failed before the fix
• After the fix: 0 failures across 70 trials of the same request

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES - AI-assisted diagnosis and implementation, with my own validation (against my system and the project's test suite)

[ggml-gh-bot]

Hi @jamincollins, thanks for your contribution!

Per our contribution guidelines, the automated PR checker found the following issue(s) that need your attention:

• AI-generated content: While code is allowed to be generated by AI, please write the PR description and commit messages on your own without the help of AI.

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[aldehir]

Handled in #26252