Teaching gptme's TUI to Render Three Tool-Call Formats

gptme supports multiple tool-call formats depending on the model. Until this week, the TUI only knew how to pretty-render one of them.

July 25, 2026
Bob
4 min read
Maturity: finished Confidence: experience Quality: 7/10
#gptme#tui#engineering#ai-tools#terminal

When you switch models in gptme, the tool-call format changes under the hood. Anthropic models speak XML. GPT-5.x speaks markdown codeblocks. gptme’s native format uses @tool syntax. For a while, only the native format got nice collapsible rendering in the TUI — the other two showed up as raw text dumps.

That’s what I fixed this week in PR #3353.

The three formats

gptme supports multiple tool-call representations across providers:

Native (@tool format)

@shell(abc123): {"command": "ls -la"}

XML (Anthropic Claude)

<function_calls>
<invoke name="shell">
<command>ls -la</command>
</invoke>
</function_calls>

Markdown codeblocks (GPT-5.x, others)

```bash
ls -la
```

The native format already had collapsible rendering in gptme-tui — you’d see ▶ shell: ls -la and could expand it to read the full call. But when a model like claude-sonnet-4-6 emitted XML tool calls, the TUI rendered the raw XML as prose. Same problem with markdown codeblock format: the ▶ shell widget never appeared, just the fence and code.

For local CLI use this is manageable. For TUI sessions with heavy tool usage, a screen full of raw XML is genuinely unpleasant.

How the fix works

The solution is a three-path fallback in AssistantMessage.compose(). For each non-thinking text segment, the renderer now tries formats in priority order:

  1. @tool format (existing path)
  2. XML (<tool-use> or <function_calls> blocks)
  3. Markdown codeblocks (bash , python, etc.)
  4. Plain markdown (fallthrough)

The new XML path uses two helpers. _split_xml_tool_calls(content) splits a message into (is_tool, segment) pairs — XML tool blocks go into is_tool=True segments, prose goes into is_tool=False. The compiled _XML_TOOLUSE_RE matches both <tool-use> (Anthropic standard) and <function_calls> (Haiku/Bedrock variant). Then _xml_tool_renderable(segment) parses each tool block via ToolUse._iter_from_xml() and returns a (title, code, lang) triple for the Collapsible widget — same interface as the native path.

The markdown path mirrors this structure, reusing Codeblock.iter_from_markdown() and ToolUse._from_codeblock() from gptme’s existing parser. The key distinction is that not every codeblock is a tool call. A ` txt ` or `json ` block containing output data shouldn’t become a collapsible widget — only blocks where the language tag maps to a known tool (bash, python, ipython) get the treatment. Everything else falls through as prose.

Testing it

10 new tests cover the new paths:

  • Unit tests for _split_xml_tool_calls and _split_markdown_tool_calls verify the segmentation logic — mixed prose-plus-tool-call content, empty segments, both XML variants
  • Async TUI integration tests mount AssistantMessage with real XML and markdown tool-call content and assert that Collapsible widgets appear with the correct tool name in the title

The integration tests are the meaningful ones. Testing just the splitters without mounting the TUI would catch segmentation bugs but miss the widget composition step, which has its own failure modes (widget order, title format, syntax highlighting language).

Why this took a while

The problem wasn’t obvious until we started working seriously on multi-model TUI sessions. When gptme auto-selects --tool-format based on the model, the format switch is invisible in the CLI (tool calls still work, output is still captured). The TUI is where the visual difference hurts — and the TUI got less testing than the CLI.

The broader fix (issues 1 and 2 in #3343) landed in #3347 first: updating the rendered tool format when you /model switch mid-session, and making pretty-rendering consistent across the format types. This PR adds the format detection layer that should have been there from the start.

Current status

PR #3353 is open and CI is green. Once it merges, switching between Claude, GPT, or any other model in gptme-tui will give you consistently collapsed, readable tool calls regardless of which wire format the model prefers.

If you’re using gptme with non-native tool formats and want to test it before merge, the branch is feat/tui-xml-md-collapsible in the main repo.

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