# LangChain Integration

> Drop Attestix into a LangChain agent with the BaseCallbackHandler. Real (not example) integration shipped in v0.3.0; v0.4.0-rc.2 release candidate. Every tool call, LLM call, and chain step gets hash-chained and signed.

Attestix ships with a first-party `BaseCallbackHandler` implementation that writes every LangChain event into the Attestix audit trail and auto-issues a Verifiable Credential for each chain completion.

## Install

```bash
pip install attestix[langchain]
```

## Minimal wiring

```python
from langchain.agents import AgentExecutor
from langchain_openai import ChatOpenAI
from attestix.integrations.langchain import AttestixCallback

callback = AttestixCallback(
    agent_id="attestix:f9bdb7a94ccb40f1",
    record_llm_tokens=True,
)

executor = AgentExecutor(
    agent=my_agent,
    tools=tools,
    callbacks=[callback],
)

result = executor.invoke({"input": "Run Q4 analysis"})
```

Every tool call, LLM call, and chain step is captured with the SHA-256 hash chain advancing each step.

## What the callback records

| Event | Captured |
|-------|----------|
| `on_chain_start` | agent id, chain name, input hash |
| `on_tool_start` | tool name, args hash, parent chain id |
| `on_tool_end` | output hash, duration, prev hash |
| `on_llm_start` | model name, prompt hash, token count |
| `on_llm_end` | completion hash, tokens used, cost estimate |
| `on_chain_end` | final output hash, total tokens, VC issuance |
| `on_chain_error` | exception type, stack hash |

## Auto-issued credentials

Set `issue_vc_on_success=True` (default) and Attestix issues a `ChainExecutionCredential` with:

- Claim: input hash, output hash, tool chain, duration.
- Proof: `Ed25519Signature2020` over the canonicalised JSON.
- Subject: the agent ID.

Verify it offline with any W3C VC verifier, or via:

```python
from attestix.services.credential_service import CredentialService

credential_svc = CredentialService()
credential_svc.verify_credential(credential_id)
```

## Delegating to sub-agents

If the chain spawns a sub-chain with its own agent, Attestix issues a UCAN delegation with capability attenuation:

```python
callback = AttestixCallback(
    agent_id=parent_id,
    child_agent_id=child_id,
    delegate_capabilities=["summarize", "redact"],
)
```

## Verify offline

```python
from attestix.services.credential_service import CredentialService

credential_svc = CredentialService()
ok = credential_svc.verify_credential(credential_id)
```

See the [offline verify guide](/docs/guides/offline-verify) for a full walkthrough.

## Sample repository

The [examples directory](https://github.com/VibeTensor/attestix/tree/main/examples) contains a complete LangChain agent with Attestix wired end-to-end.