ExperimentalMarkdownSyntaxTextSplitter mutates results returned by previous split_text calls

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_text_splitters import ExperimentalMarkdownSyntaxTextSplitter


splitter = ExperimentalMarkdownSyntaxTextSplitter()

first = splitter.split_text("first document\n")
first_before = [document.page_content for document in first]

second = splitter.split_text("second document\n")
first_after = [document.page_content for document in first]

print("same list:", first is second)
print("first result before second call:", first_before)
print("first result after second call:", first_after)

Error Message and Stack Trace (if applicable)

No exception is raised. The previously returned list is silently mutated.

Description

ExperimentalMarkdownSyntaxTextSplitter.split_text() returns its internal self.chunks list directly.

At the beginning of the next split_text() call, the implementation executes self.chunks.clear(). Because the caller's previously returned value and self.chunks refer to the same list object, processing a second document clears and replaces the result returned for the first document.

Actual output:

same list: True
first result before second call: ['first document\n']
first result after second call: ['second document\n']

### System Info

System Information
------------------
> OS:  Windows
> OS Version:  10.0.26200
> Python Version:  3.13.3 (main, May 30 2025, 05:37:00) [MSC v.1943 64 bit (AMD64)]

Package Information
-------------------
> langchain_core: 1.6.3
> langsmith: 0.12.5
> langchain_classic: 1.0.8
> langchain_huggingface: 1.2.2
> langchain_protocol: 0.0.19
> langchain_text_splitters: 1.1.2

Optional packages not installed
-------------------------------
> deepagents
> deepagents-cli

Other Dependencies
------------------
> anyio: 4.15.1
> distro: 1.9.0
> httpx: 0.28.1
> httpx2: 2.13.0
> huggingface-hub: 1.31.0
> jsonpatch: 1.33
> orjson: 3.12.0
> packaging: 26.3
> pydantic: 2.13.5
> pyyaml: 6.0.3
> requests: 2.34.2
> requests-toolbelt: 1.0.0
> sniffio: 1.3.1
> sqlalchemy: 2.0.54
> tenacity: 9.1.4
> tokenizers: 0.23.2
> typing-extensions: 4.16.0
> uuid-utils: 0.17.1
> websockets: 17.1
> xxhash: 4.0.1
> zstandard: 0.25.0

### Social handles (optional)

_No response_