langchain-fathom
A coherence read for LangChain agents, as an agent middleware.
A long-running agent loses coherence with its own decisions. It renames a field at one step, then writes the old name at a later one. It marks a record done that it never wrote. The run reports success, and the contradiction ships. langchain-fathom watches the agent run and, when it finishes, names the step where a later action contradicts an earlier commitment.
The middleware observes the agent and does not change what it does. It maps the tool calls in the message history to a committed-state op stream, sends that stream to the Fathom read, and reports the findings. The read is deterministic and needs no model access.
Install
pip install langchain-fathom
Use
from langchain.agents import create_agent
from langchain_fathom import FathomMiddleware
agent = create_agent(
model="gpt-5.5",
tools=[...],
middleware=[FathomMiddleware()],
)
result = agent.invoke({"messages": [...]})
By default the middleware logs any findings through the fathom logger. Two other modes fit a test suite or a pipeline.
FathomMiddleware(on_finding="raise") # raise FathomCoherenceError when a run is not coherent
FathomMiddleware(on_finding="store") # put the verdict on agent state under the "fathom" key
If the run is a rename or a migration, tell the read which token replaced which, so it also reports records left on the old value at the end.
FathomMiddleware(supersede=[("guest_id", "customer_id")])
What it finds
| Finding | The agent... |
|---|---|
stale_reference |
acts on a record it already removed or renamed away |
superseded_value |
writes a value it already replaced |
residual |
ends the run with a record still carrying a value it replaced elsewhere |
duplicate_commit |
adds an entity a collection already holds |
post_commit_mutation |
changes a record after committing it |
A coherent run returns coherent and the middleware reports nothing else.
Your own tool names
The read knows common state-writing tool names. For tools with your own names, map them once and pass the file.
FathomMiddleware(mapping_path="tools.json")
{"save_decision": {"op": "set", "kind": "decision", "key": "topic", "value": "text"},
"confirm_booking": {"op": "commit", "kind": "flight", "key": "booking"}}
The read and the repair
The middleware is the read. It tells you where coherence broke. Embedded Risk Analytics also runs a hosted service that acts on what the read finds and re-grounds the agent before the contradiction ships. Write to contact@embeddedriskanalytics.com for a key that lifts the demo rate limit, or to run the read on a workflow of your own.
Links
- fathom-read, the read and its adapters for LangGraph, CrewAI, Letta, OpenInference, DBOS, and coding-agent edit logs
- Coherence census, every framework the read has run on, with a trace per row
- Research and the paper, SSRN 6683578
MIT licensed.
1 maintainer
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