trim_messages returning invalid messages history #33245
Description
Checked other resources
- 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 read what a minimal reproducible example is (https://stackoverflow.com/help/minimal-reproducible-example).
- I posted a self-contained, minimal, reproducible example. A maintainer can copy it and run it AS IS.
Example Code
By running the following code
from langchain_core.messages import AIMessage, HumanMessage, ToolMessage
from langchain_core.messages.utils import trim_messages
messages = [
HumanMessage("What's the weather in Florida?"),
AIMessage(
[
{"type": "text", "text": "Let's check the weather in Florida"},
{
"type": "tool_use",
"id": "abc123",
"name": "get_weather",
"input": {"location": "Florida"},
},
],
tool_calls=[
{
"name": "get_weather",
"args": {"location": "Florida"},
"id": "abc123",
"type": "tool_call",
},
],
),
ToolMessage(
"It's sunny.",
name="get_weather",
tool_call_id="abc123",
),
HumanMessage("I see"),
AIMessage("Do you want to know anything else?"),
HumanMessage("No, thanks"),
AIMessage("You're welcome! Have a great day!"),
]
trimmed_messages = trim_messages(
messages,
strategy="last",
token_counter=len,
max_tokens=5,
)
for message in trimmed_messages:
message.pretty_print()
Output:
================================= Tool Message =================================
Name: get_weather
It's sunny.
================================ Human Message =================================
I see
================================== Ai Message ==================================
Do you want to know anything else?
================================ Human Message =================================
No, thanks
================================== Ai Message ==================================
You're welcome! Have a great day!
As you can see, the history is invalid, given that we have a ToolMessage without a corresponding AIMessage.
Error Message and Stack Trace (if applicable)
No response
Description
I'm trying to use trim_messages to select the last n messages while keeping the history valid. But I'm getting an invalid history as the returned value.
System Info
System Information
------------------
> OS: Darwin
> OS Version: Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:40 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T8132
> Python Version: 3.13.3 (main, May 30 2025, 05:45:55) [Clang 20.1.4 ]
Package Information
-------------------
> langchain_core: 0.3.77
> langchain: 0.3.27
> langsmith: 0.4.32
> langchain_text_splitters: 0.3.11
Optional packages not installed
-------------------------------
> langserve
Other Dependencies
------------------
> async-timeout<5.0.0,>=4.0.0;: Installed. No version info available.
> httpx<1,>=0.23.0: Installed. No version info available.
> jsonpatch<2.0.0,>=1.33.0: Installed. No version info available.
> langchain-anthropic;: Installed. No version info available.
> langchain-aws;: Installed. No version info available.
> langchain-azure-ai;: Installed. No version info available.
> langchain-cohere;: Installed. No version info available.
> langchain-community;: Installed. No version info available.
> langchain-core<1.0.0,>=0.3.72: Installed. No version info available.
> langchain-core<2.0.0,>=0.3.75: Installed. No version info available.
> langchain-deepseek;: Installed. No version info available.
> langchain-fireworks;: Installed. No version info available.
> langchain-google-genai;: Installed. No version info available.
> langchain-google-vertexai;: Installed. No version info available.
> langchain-groq;: Installed. No version info available.
> langchain-huggingface;: Installed. No version info available.
> langchain-mistralai;: Installed. No version info available.
> langchain-ollama;: Installed. No version info available.
> langchain-openai;: Installed. No version info available.
> langchain-perplexity;: Installed. No version info available.
> langchain-text-splitters<1.0.0,>=0.3.9: Installed. No version info available.
> langchain-together;: Installed. No version info available.
> langchain-xai;: Installed. No version info available.
> langsmith-pyo3>=0.1.0rc2;: Installed. No version info available.
> langsmith<1.0.0,>=0.3.45: Installed. No version info available.
> langsmith>=0.1.17: Installed. No version info available.
> openai-agents>=0.0.3;: Installed. No version info available.
> opentelemetry-api>=1.30.0;: Installed. No version info available.
> opentelemetry-exporter-otlp-proto-http>=1.30.0;: Installed. No version info available.
> opentelemetry-sdk>=1.30.0;: Installed. No version info available.
> orjson>=3.9.14;: Installed. No version info available.
> packaging<26.0.0,>=23.2.0: Installed. No version info available.
> packaging>=23.2: Installed. No version info available.
> pydantic<3,>=1: Installed. No version info available.
> pydantic<3.0.0,>=2.7.4: Installed. No version info available.
> pytest>=7.0.0;: Installed. No version info available.
> PyYAML<7.0.0,>=5.3.0: Installed. No version info available.
> PyYAML>=5.3: Installed. No version info available.
> requests-toolbelt>=1.0.0: Installed. No version info available.
> requests<3,>=2: Installed. No version info available.
> requests>=2.0.0: Installed. No version info available.
> rich>=13.9.4;: Installed. No version info available.
> SQLAlchemy<3,>=1.4: Installed. No version info available.
> tenacity!=8.4.0,<10.0.0,>=8.1.0: Installed. No version info available.
> typing-extensions<5.0.0,>=4.7.0: Installed. No version info available.
> vcrpy>=7.0.0;: Installed. No version info available.
> zstandard>=0.23.0: Installed. No version info available.