LangSmith observes.
Runback re-executes.
LangSmith is an excellent trace viewer and dataset manager for LangChain apps. It shows what happened. It can't re-execute the step, gate your CI pipeline, or produce a signed audit record.
| Capability | LangSmith | Runback | Note |
|---|---|---|---|
| Read and search traces | ✓ | ✓ | |
| Dataset and eval management | ✓ | ✓ | |
| Re-execute any step from captured inputs | ✗ | ✓ | Runback's core capability — bisect in log₂ tries |
| CI release gate — policy blocks the deploy | ✗ | ✓ | |
| Signed, tamper-evident audit export | ✗ | ✓ | SHA-256 chain; verify without Runback |
| Self-host, data never leaves your perimeter | partial | ✓ | LangSmith is managed SaaS; partial self-host available |
| Regulatory mappings (EU AI Act · CPS 230 · NIST) | ✗ | ✓ |
Most teams use both — LangSmithfor day-to-day trace browsing, Runback for incident replay, CI gating, and the compliance artifact. The two don't overlap until the moment an agent misbehaves in production and someone asks you to prove it.
Try the replay on a real incident.
No signup. Walk a failing production run step by step and see what the model saw.