#40460 core: mustache inverted section `{{^key}}` nested inside a `{{#key}}` list section misrenders or raises `IndexError`

Submission checklist

This is a bug, not a usage question.
I added a clear and descriptive title that summarizes this issue.
I used the GitHub search to find a similar question and didn't find it.
I am sure that this is a bug in LangChain rather than my code.
The bug is not resolved by updating to the latest stable version of LangChain (or the specific integration package).
This is not related to the langchain-community package.
I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.

Package (Required)

langchain
langchain-openai
langchain-anthropic
langchain-classic
langchain-core
langchain-model-profiles
langchain-tests
langchain-text-splitters
langchain-chroma
langchain-deepseek
langchain-exa
langchain-fireworks
langchain-groq
langchain-huggingface
langchain-mistralai
langchain-nomic
langchain-ollama
langchain-openrouter
langchain-perplexity
langchain-qdrant
langchain-xai
Other / not sure / general

Related Issues / PRs

No response

Reproduction Steps / Example Code (Python)

from langchain_core.prompts import ChatPromptTemplate

# 1. Silently wrong output
prompt = ChatPromptTemplate.from_messages(
    [("human", "{{#items}}[{{^items}}none{{/items}}{{name}}]{{/items}}")],
    template_format="mustache",
)
print(repr(prompt.invoke({"items": [{"name": "a"}, {"name": "b"}]}).messages[0].content))
# Actual:   '[[]'
# Expected: '[a][b]'

# 2. Crash
prompt = ChatPromptTemplate.from_messages(
    [("human", "{{#items}}{{^items}}none{{/items}}{{/items}}Done")],
    template_format="mustache",
)
prompt.invoke({"items": [{"name": "a"}, {"name": "b"}]})
# Actual:   IndexError: list index out of range
# Expected: 'Done'

# The same happens when calling `langchain_core.utils.mustache.render` directly, e.g. `render("{{#items}}[{{^items}}none{{/items}}{{.}}]{{/items}}END", {"items": [1, 2]})` raises `IndexError` instead of returning `'[1][2]END'`.

Error Message and Stack Trace (if applicable)

Description

Error:

  File ".../langchain_core/prompts/string.py", line 122, in mustache_formatter
    return mustache.render(template, kwargs)
  File ".../langchain_core/utils/mustache.py", line 525, in render
    current_scope = scopes[0]
IndexError: list index out of range

When a section's value is a list, render() gathers the tokens up to the matching {{/key}} and tracks nesting with tags_with_same_key. The counter is incremented only for ("section", key) tokens, but decremented for every ("end", key) token. An inverted section with the same key ({{^items}}...{{/items}}) therefore never increments the counter, yet its closing tag decrements it, so the loop treats the inner {{/items}} as the end of the outer section.

The loop body is truncated, the remaining tokens render once outside the loop, and the extra end token pops a scope that was never pushed, which surfaces as IndexError or as silently wrong output depending on the template. Nesting an inverted section with the same key inside a section is valid mustache.

The depth counting was introduced for nested regular sections (#23747), but inverted sections were not covered, and the existing unit tests only exercise a top-level {{^key}}.

Proposed fix: count ("inverted section", key) as an opening tag alongside ("section", key) in that loop, and add regression tests for both render() and ChatPromptTemplate(template_format="mustache"). I'd be glad to open a PR with the fix and tests if a maintainer assigns this to me.

System Info

langchain-core 1.6.3 (also current master)
Python 3.10.16
macOS 26.6 (arm64)

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