InMemoryVectorStore.delete(None) does not delete all documents

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.documents import Document
from langchain_core.embeddings import Embeddings
from langchain_core.vectorstores import InMemoryVectorStore


class FakeEmbeddings(Embeddings):
    def embed_documents(self, texts: list[str]) -> list[list[float]]:
        return [[1.0, 0.0] for _ in texts]

    def embed_query(self, text: str) -> list[float]:
        return [1.0, 0.0]


vectorstore = InMemoryVectorStore(FakeEmbeddings())

vectorstore.add_documents(
    [Document(page_content="document A")],
    ids=["doc-a"],
)

print("IDs before delete:", list(vectorstore.store))
vectorstore.delete(ids=None)
print("IDs after delete:", list(vectorstore.store))

Error Message and Stack Trace (if applicable)

Description

VectorStore.delete() documents that passing ids=None should delete all vectors. However, InMemoryVectorStore.delete() only performs deletion when ids is truthy.

The implementation currently uses:

def delete(
    self,
    ids: Sequence[str] | None = None,
    **kwargs: Any,
) -> None:
    if ids:
        for identifier in ids:
            self.store.pop(identifier, None)

### 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_