Debugging,
rewound.
Stop guessing what went wrong. Rewind your Next.js session and inspect every state change — with absolute clarity.
Read the docs ›App Router apps are hard to debug because the story is scattered.
State spans the boundary
Some state lives on the server, some on the client. One interaction means reasoning across two execution models at once.
Rendering is streamed
RSC payloads arrive in chunks over time. The UI at tick 8 isn't the UI at tick 11 — and nothing shows you the difference.
Cache behavior is invisible
updateTag and revalidateTag imply different freshness. Whether the user actually saw fresh data is never surfaced.
No tool unifies the layers
Logs, traces, metrics, and firewall rules each see one slice. None replay the causal chain end to end.
One debugger, six planes, one timeline.
Observability shows one slice. Runtime metadata shows another. Flightrec unifies all six planes against a single scrubber — the difference versus traces, metrics, or generic session replay.
The bugs that don't show up in logs.
App Router failures hide between layers. Flightrec puts the whole causal chain on one timeline, so the root cause is where you'd expect it.
The dashboard that stayed stale
An edit flow used the wrong invalidation. Flightrec shows updateTag fired but no tracked node changed — an orphaned invalidation, not the read-your-own-writes you intended.
The sign-in that lost the cookie
A redirect through nested layouts left auth half-applied. The timeline lines up the cookie write, the 303, and the tree reconcile so the partial-stale state is obvious.
The mutation that ran twice
A double-submit fired the Server Action twice. Two invocations sit side by side on the timeline with their args and outcomes — no more guessing from logs.
Capture both sides. Normalize. Replay deterministically.
Server and client-side capture feed a normalizer, which persists locally and replays in the inspector. No shadow React runtime — a deterministic replay graph. MCP enriches; it's never the source of truth.
export function register() { // captures Server Actions, cache, RSC, headers registerFlightrec({ mode: 'normal' }) }
const bundle = await session.export() // → ses_8f31a0.frec (zip + manifest + diffs) await bundle.download()
Light enough to run always-on.
A debugger you can't afford to run is a demo, not a tool. Flightrec's normal mode targets a bounded, dev-only overhead envelope — benchmarked the way OpenTelemetry recommends.
Representative targets, not measured results — validated by the Phase-4 benchmark harness. Cross-tool numbers are directional; each tool records a different surface.
Adjacent tools each see one slice.
Replay.io pioneered time-travel debugging — for client JavaScript. Sentry replays the DOM; Vercel ships traces; Next.js MCP exposes metadata; OpenTelemetry captures spans. None replay causality across Server Actions, cache, RSC, and reconciliation.
| Capability | Flightrec this | Replay.io client JS | Sentry DOM replay | Vercel Obs. traces | Next.js MCP metadata | OTel spans |
|---|---|---|---|---|---|---|
| Session-level time-travel replay | ● | ● | ◐ | — | — | — |
| Server Action causality | ● | — | — | — | meta | ◐ |
| Cache invalidation semantics | ● | — | — | — | — | — |
| RSC / Flight payload frames | ● | — | — | — | — | — |
| Client tree reconciliation diff | ● | ◐ | ◐ | — | — | — |
| Cookie / header mutation trail | ● | — | — | ◐ | — | ◐ |
| Source-mapped to your code | ● | ● | ● | ◐ | ● | — |
| Works offline from a bundle | ● | — | — | — | — | ● |
| Agent-queryable (MCP) traces | ● | — | — | — | ● | — |
● full · ◐ partial · — none. Reflects each tool's primary design intent.
Start free. Scale when your team does.
The core SDK, inspector, and trace format are open source and free forever. Pay only for cloud collaboration.
For solo debugging, fully local.
- Local capture, all six planes
- .frec export / import
- Inspector PWA
- Cache-semantics classifier
- Community support
For teams shipping fast.
- Everything in Free
- Cloud sync + shared trace links
- 90-day retention
- Flightrec MCP server
- AI session summaries & bug reports
- Priority support
For orgs at scale.
- SSO / SAML + audit logs
- Self-host / BYO storage
- Data residency
- SLA + dedicated support
- Security review & DPA
Start free. No credit card required.
Built in the open. Watch it take shape.
The demo above runs on synthetic data today — the recorder, inspector, and MCP server are landing phase by phase.