Imagine your ai agent fixing its own fu*ckups, here's how the rewind actually works

Siddham Jain — Sun, 26 Jul 2026 14:14:54 +0000

When an AI agent fails in production, you get told that it failed. You almost never get to
answer the only question that matters: what if the third agent had made a different call?
You can't replay the run. You can't isolate the step that broke it. You can't test a fix without
re-running the whole thing and hoping.

I spent a hackathon building Rewind — a "time machine" for multi-agent runs — to see if
SigNoz's traces plus LangGraph's checkpointer could actually answer that question. They can. This
post is the honest version: how the replay works, and the three things that nearly broke it.

The setup: a crew that ships a subtly wrong fix

The observed workload is a four-agent coding crew (Planner → Coder → Tester → Reviewer) that
fixes a GitHub-style issue. The issue is deliberately ambiguous:

"An order line for more than 10 units should receive a 10% discount."

The tests, however, require the discount at exactly 10 units. A weak model reads the spec
literally and writes:

if quantity > 10:            # what the weak model wrote
    subtotal *= 0.90

and the boundary test fails with a very real assertion:

def test_bulk_discount_boundary():
    assert cart_total([(10.0, 10)]) == 90.0
E   assert 100.0 == 90.0

That's the failure I want to reproduce and fix from a trace, not by re-reading logs.

Making every agent step a span

I instrument the crew two ways. openinference-instrumentation-langchain auto-captures the
LangChain/LangGraph internals, and a tiny custom context manager wraps each node so the span
carries exactly the attributes I'll want to query later:

@contextmanager
def node_span(config, name, inputs):
    with tracer.start_as_current_span(f"crew.{name}") as span:
        span.set_attribute("rewind.node", name)
        span.set_attribute("rewind.thread_id", thread_id(config))
        span.set_attribute("rewind.input", short(inputs))
        yield span

def record_llm(span, model, resp):
    usage = resp.usage_metadata or {}
    span.set_attribute("gen_ai.request.model", model)
    span.set_attribute("gen_ai.usage.input_tokens", usage.get("input_tokens", 0))
    span.set_attribute("gen_ai.usage.output_tokens", usage.get("output_tokens", 0))
    span.set_attribute("rewind.cost_usd", round(cost, 6))

I used OpenTelemetry's GenAI semantic conventions
(gen_ai.request.model, gen_ai.usage.*_tokens) for the model/token fields so they're not
bespoke, and a rewind. namespace for the app-specific bits. Spans export over OTLP/HTTP to
SigNoz's ingester on :4318. The rewind.thread_id is the thread that stitches an entire run
together — remember it, it's the whole trick.

Here's what lands in SigNoz — every planner/coder/tester/reviewer step, queryable:

Gotcha #1: getting SigNoz up when Docker Desktop is gone

I deployed SigNoz with Foundry (casting.yaml → casting.yaml.lock → cast), which is a
genuinely clean one-command install. But my machine had no Docker Desktop, and the leftovers
fought me: docker pointed at a dangling symlink, docker compose was a broken cli-plugin, and
Foundry's cast died on docker-credential-desktop executable not found. The fixes, in order:

brew install colima docker and colima start — a lightweight Docker engine, no Desktop.
relink the compose plugin into ~/.docker/cli-plugins/.
delete the credsStore: "desktop" line from ~/.docker/config.json so Foundry could authenticate to the registry.

None of this is in a tutorial. If you're on a clean Mac without Docker Desktop, Colima + fixing
config.json is the path.

Reading the telemetry back out of SigNoz

Here's the part I care about most: Rewind doesn't keep its own trace store. To render a run's
timeline, the backend queries SigNoz for its own spans via the v5 query API:

POST /api/v5/query_range
{
  "requestType": "raw",
  "compositeQuery": {"queries": [{"type": "builder_query", "spec": {
    "signal": "traces",
    "filter": {"expression":
      "service.name = 'rewind-crew' AND rewind.thread_id = 'run-64c55532' AND rewind.node EXISTS"},
    "selectFields": [
      {"name": "rewind.node",  "fieldContext": "attribute"},
      {"name": "rewind.cost_usd", "fieldContext": "attribute"},
      {"name": "gen_ai.usage.input_tokens", "fieldContext": "attribute"}
    ]
  }}]}
}

The filter.expression string syntax is the thing to get right in v5 — it's a mini query
language (AND, =, EXISTS), and fieldContext: "attribute" vs "resource" matters for
whether SigNoz finds your field. Once that clicked, the same query grouped by rewind.node
became a reliability heatmap across every run: pass rate, average cost per node, and where
failures surface. Across 24 seeded runs mine sat at a 25% pass rate with the failures localizing
to the Tester and Reviewer nodes — which is exactly right, because that's where a wrong > vs
>= gets caught.

Gotcha #2: the replay is real because LangGraph makes it real

The reproduce-and-fork isn't a simulation, and that's the honest core of the project. The crew
writes to a LangGraph SqliteSaver checkpointer, and the backend shares that same file. So to
fork a run I use LangGraph's native time-travel:

history = list(graph.get_state_history(config))     # every checkpoint of the run
snap = pick_snapshot_before(history, node="coder")  # rewind to just before the coder
graph.update_state(snap.config, values=overrides)   # change model / guardrail
graph.invoke(None, fork_config)                      # re-run everything downstream, live

The nodes after the checkpoint actually re-execute with the new state, and the forked run emits
its own spans — so it becomes a second trace in SigNoz, not a diffed copy. In Rewind's UI you
can see the reproduced timeline, the exact code the model wrote, and the real pytest output that
failed:

10` endraw , the failing pytest, and the fork panel" width="800" height="556">

When I fork the failing run with a spec guardrail and a stronger model, the coder rewrites the
line as if quantity >= 10, and the same pytest that was red goes green. Failed and fixed, side
by side, both backed by real traces — and the fix costs a few more tokens, which the cost deltas
make honest.

Gotcha #3: know when not to depend on the live query

SigNoz v0.134 moved its auth around (login is under /api/v2/sessions, not where older guides
say), and I burned time before making a call I'd defend: the checkpointer is the source of
truth for the timeline; the SigNoz query is enrichment. If the query path is configured, the
timeline gets real tokens/cost from SigNoz and every card deep-links to the live trace; if it
isn't, the app still reproduces and forks from the checkpointer. A demo that hard-depends on one
network call to a freshly-booted observability stack is a demo that dies on stage. Degrade
gracefully.

The same telemetry drives a dashboard and a failure-rate alert, both built programmatically via
the SigNoz API — the alert plots the failure count against a threshold, straight from the Query
Builder:

What I'd tell my past self

Pick your span attributes before you write a single query. I renamed fields twice because I hadn't decided that rewind.thread_id was the join key. Design the schema first.
fieldContext is not optional trivia. In the v5 query API, marking a field attribute vs resource is the difference between data and an empty result set.
Dashboard widgets need a layout. I posted widgets with no id/layout and got a dashboard that rendered the empty "Welcome" state — the panels were stored but never placed on the grid.
Let the framework do the replay. I almost hand-rolled a "re-run from step N" engine. LangGraph's get_state_history + update_state + invoke(None, cfg) already is that engine, and it's the reason the fork is trustworthy instead of a mock.
Two proofs beat one. pytest is the hard gate (it can't be argued with); an LLM-as-judge score rides along as a soft quality signal on the timeline. Different jobs, both useful.

Conclusion

Rewind reproduces a production agent failure, forks one decision, and proves the fix with a real
passing test — and none of it required a custom trace store or a faked replay. SigNoz's spans
already held the full I/O and cost; LangGraph's checkpointer already held the executable state.
The work was joining them on one thread id. If you're building agents, that pairing —
OpenTelemetry traces for what happened, a checkpointer for do it again differently — is worth
stealing.

Code: github.com/siddham-jain/rewind. Built for the
Agents of SigNoz hackathon.

DEV Community: Siddham Jain