---

⚡ The Problem

Building production AI agents requires automated testing and continuous integration (CI). But running multi-step agents in tests creates three massive bottlenecks:

1. Flaky CI Builds: LLMs are non-deterministic. Even at temperature=0, small prompt variations break downstream assertions.
2. Exorbitant API Costs: Re-running an 8-step agent across 50 pull requests burns thousands of dollars in LLM API credits.
3. Debug Nightmare: When an agent fails at Step 5, you cannot reproduce the bug locally without paying to re-run Steps 1 through 4.

Rerunner solves this by providing a zero-overhead, tamper-evident cassette recorder that intercepts httpx traffic natively across the Anthropic Claude SDK, OpenAI SDK, and custom agents.

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🚀 Key Features

• $0 CI Replays: Freeze agent runs into append-only JSONL cassettes. Replay in 0ms without hitting external LLM APIs.
• Fork from Step N: Replay Steps 1..4 from the cassette for free, and execute Step 5 live against the model with your new prompt or bug fix.
• Tolerant Request Matching: Automatically strips volatile nonces, millisecond timestamps, request IDs, and SDK metadata headers (x-stainless-*).
• Side-Effect Safety: The @side_effect decorator prevents destructive external actions (database writes, Slack webhooks, payments) during replays.
• Cryptographic Integrity: SHA-256 tamper-evident hash chaining ensures recorded cassettes have not been altered.
• Native Pytest Integration: Seamless @pytest.mark.rerunner marker and fixtures.

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📊 Comparison

Capability	rerunner	vcrpy	LangSmith	LangGraph
Primary Focus	Deterministic CI Replay	Generic HTTP	Web Observability	Orchestration
LLM Streaming (SSE) Support	✅ Yes	❌ Breaks	✅ Yes	N/A
Fork-from-Step Execution	✅ Yes	❌ No	❌ No	⚠️ Graph-locked
Side-Effect Stubbing	✅ Yes	❌ No	❌ No	❌ No
Zero Infrastructure / $0 Cost	✅ Local JSONL	✅ Local YAML	❌ Cloud / Paid	❌ Variable
Framework Agnostic	✅ Any httpx SDK	✅ HTTP	⚠️ Complex	❌ Locked

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📦 Installation

pip install rerunner

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🛠️ Quickstart

1. Recording an Agent Session

Rerunner transparently attaches to standard SDK clients via httpx.Client(transport=...):

import anthropic
import httpx
from rerunner import CassetteTransport

# 1. Record live interactions to a cassette
transport = CassetteTransport("session.jsonl", mode="record")
client = anthropic.Anthropic(http_client=httpx.Client(transport=transport))

response = client.messages.create(
    model="claude-3-5-sonnet-20241022",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Analyze quarterly revenue report."}]
)
print(response.content[0].text)

2. Deterministic $0 Replay in CI

# 2. Replay locally or in CI without spending API tokens or waiting on network
transport = CassetteTransport("session.jsonl", mode="replay")
client = anthropic.Anthropic(http_client=httpx.Client(transport=transport))

# Instant response, zero cost, completely reproducible
response = client.messages.create(
    model="claude-3-5-sonnet-20241022",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Analyze quarterly revenue report."}]
)

3. Forking from Step 3

from rerunner import fork

# Replay steps 1 through 3 from cassette; run step 4 live with your modified prompt
with fork("session.jsonl", step=3) as session:
    client = session.get_client()
    # Steps 1..3 replayed from tape
    # Step 4 executed live against model

Compressed Cassettes

For long-running agent workflows, use the .jsonl.gz extension to store cassettes in gzip-compressed form:

from rerunner import CassetteTransport

transport = CassetteTransport("session.jsonl.gz", mode="record")

Rerunner automatically compresses .jsonl.gz cassettes when recording and decompresses them when replaying. The same CLI commands and SHA-256 integrity verification work with both .jsonl and .jsonl.gz files:

rerunner inspect session.jsonl.gz
rerunner verify session.jsonl.gz
rerunner play session.jsonl.gz

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🛡️ Guarding Destructive Side-Effects

Ensure your replayed agents don't charge cards or send duplicate Slack messages:

from rerunner import side_effect

@side_effect(name="post_slack_alert")
def send_slack_notification(channel: str, message: str) -> bool:
    # Runs live during recording. During replay, safely returns recorded result!
    return slack_client.chat_postMessage(channel=channel, text=message)

---

🧪 Pytest Integration

Mark your unit tests with @pytest.mark.rerunner:

import pytest

@pytest.mark.rerunner("tests/fixtures/agent_run.jsonl")
def test_agent_decision(rerunner_transport):
    client = openai.OpenAI(http_client=httpx.Client(transport=rerunner_transport))
    completion = client.chat.completions.create(
        model="gpt-4o",
        messages=[{"role": "user", "content": "Evaluate risk factor"}]
    )
    assert "low risk" in completion.choices[0].message.content.lower()

Run test suite in replay mode (0 network calls, $0 cost)

pytest

Re-record all cassettes when prompts change intentionally

pytest --rerunner-record

For a reusable GitHub Actions workflow that restores recorded cassettes before
replay tests, see the [GitHub Actions cassette caching recipe](docs/recipes/github_actions_caching.md).

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## ⚡ 1-Line Client Wrappers

Wrap your existing SDK instances with zero configuration:

```python
import anthropic
import openai
from rerunner.integrations import wrap_anthropic, wrap_openai

# Wrap Anthropic client
client_claude = wrap_anthropic(anthropic.Anthropic(), "agent_run.jsonl", mode="replay")

# Wrap OpenAI client
client_openai = wrap_openai(openai.OpenAI(), "agent_run.jsonl", mode="replay")

LangChain & LangGraph Integration

from rerunner.integrations import RerunnerCallbackHandler

handler = RerunnerCallbackHandler("langgraph_agent.jsonl", mode="replay")
agent_executor.invoke({"input": "Fetch stock price"}, config={"callbacks": [handler]})

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🔒 Enterprise Secret & PII Sanitization

Never accidentally commit API keys or auth tokens to GitHub. Rerunner automatically scrubs:

• Anthropic API keys (sk-ant-...) $\rightarrow$ [REDACTED_ANTHROPIC_KEY]
• OpenAI API keys (sk-proj-..., sk-...) $\rightarrow$ [REDACTED_OPENAI_KEY]
• Authorization headers (Bearer ...) $\rightarrow$ Bearer [REDACTED_TOKEN]
• AWS access keys (AKIA...) $\rightarrow$ [REDACTED_AWS_KEY_ID]

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💻 CLI Commands

# Calculate CI cost reductions and token savings
rerunner stats session.jsonl

# Visually play back agent execution steps in terminal
rerunner play session.jsonl

# Inspect cassette details and step sequences
rerunner inspect session.jsonl

# Cryptographically verify tamper-evident hash chain
rerunner verify session.jsonl

# Compare prompt and response divergences between two cassettes
rerunner diff session_v1.jsonl session_v2.jsonl

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🤝 Contributing

Contributions from the community and frontier AI research teams are warmly welcomed. Please review CONTRIBUTING.md to set up your development environment.

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📜 License

Rerunner is licensed under the Apache 2.0 License.