Local-first visual debugger for Python agents.
One HTML file. No cloud, no account. See LLM calls, tool calls, branches, errors, tokens, and cost.

Part of the Loom stack — local HTML traces for long-running agent loops.
Stack products: loom-run (dev) · loom-ops (ops) — --trace via loom-runner or @trace.

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Loom stack

Overview: kroq86.github.io/loom-stack — packages, flow, audience, quick start.

The stack is a pyramid, not five equal frameworks. Tail-call optimization is the primitive, runner is the durable runtime, xray is the microscope, and the apps prove the stack in real workflows.

Layer	Project	Job
Primitive	loom-tailcalls	Make async recursive/state-machine loops stack-safe
Runtime kernel	loom-runner	Make those loops durable, resumable, idempotent
Microscope	flow-xray ← this repo	Show what actually happened in one offline HTML trace
Proof app	loom-run	Chat agent reference implementation
Proof app	loom-ops	Ops/runbook agent reference implementation

@tailrec agent loop  →  loom-runner run/resume  →  --trace trace.html
     (shape)                  (durability)              (flow-xray)

This repo is the microscope. It does not provide durability or agent orchestration; it shows what actually happened. Use standalone with @trace, or via loom-runner --trace / loom-run / loom-ops --trace — no cloud account required. Ecosystem: ECOSYSTEM.md

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from flow_xray import trace

@trace
def call_llm(prompt):
    return openai.chat(prompt)

@trace
def agent(query):
    plan = call_llm(f"plan: {query}")
    return call_llm(f"answer based on: {plan}")

result = trace.run(agent, "weather in Tokyo?")
result.to_html("trace.html")

Open trace.html — you get a local debug artifact with:

• LLM calls
• tool calls
• branches and nested steps
• errors and retries
• tokens and estimated cost
• graph, timeline, overview, and raw trace views

No server, no account, no log viewer — one local HTML file.

The main use case

Your agent's final answer looks fine, but you do not trust the middle.

You want to see:

• which LLM calls happened
• which tool calls ran
• where the run branched or nested
• where an error or retry happened
• where tokens and cost accumulated

flow-xray is built for that exact debugging job.

Who this is for

flow-xray is most useful for:

• LangGraph users who want local tracing without hosted tooling
• LangChain users debugging tool calls and intermediate steps
• developers building Python tool-calling agents
• people who want one local trace artifact they can inspect or share after redaction

Install

pip install flow-xray

30-second quickstart

Create demo_trace.py:

from flow_xray import trace

@trace
def fetch_profile(user_id):
    return {"id": user_id, "plan": "pro"}

@trace
def answer_question(user_id):
    profile = fetch_profile(user_id)
    return f"user {profile['id']} is on {profile['plan']}"

result = trace.run(answer_question, 42)
result.to_html("demo_trace.html")

Run it:

python demo_trace.py

Open demo_trace.html in your browser. You should see one root node (answer_question) and one child node (fetch_profile).

When to use what

Use trace.run(...) when

• you control the function call directly
• you want the clearest Python API
• you want full control over arguments and return values
• you are tracing inside tests, scripts, notebooks, or app code

from flow_xray import trace

@trace
def pipeline(x):
    return x * 2

result = trace.run(pipeline, 5)
result.to_html("pipeline.html")

Use flow-xray run ... when

• you already have a Python script on disk
• you want a quick local trace without editing much code
• the traced calls already happen at module scope when the file is executed

flow-xray run my_agent.py --html trace.html

Usage

Decorator + trace.run

from flow_xray import trace

@trace
def step_a(x):
    return x + 1

@trace
def pipeline(x):
    return step_a(x) * 2

result = trace.run(pipeline, 5)
result.to_html("pipeline.html")

CLI

flow-xray run my_agent.py --html trace.html

The script must use @trace on the functions you want captured.

CLI onboarding

flow-xray run executes the file inside a trace session, but it does not enter if __name__ == "__main__" blocks.

That means:

• if your traced function call happens at module scope, flow-xray run can capture it directly
• if your traced function call only happens inside main() guarded by if __name__ == "__main__", the CLI may produce 0 nodes

If you hit 0 nodes, use one of these fixes:

1. Move a traced demo call to module scope for local debugging.
2. Call trace.run(...) inside the script instead.
3. Keep using the Python API directly from a small wrapper script.

Example that works with flow-xray run:

from flow_xray import trace

@trace
def step():
    return 1

step()

Example that will usually show 0 nodes with flow-xray run:

from flow_xray import trace

@trace
def step():
    return 1

if __name__ == "__main__":
    step()

Async support

@trace works with async def out of the box. Use trace.arun(...) when you are already in async code:

import asyncio
from flow_xray import trace

@trace
async def call_api(query):
    await asyncio.sleep(0.1)  # simulate async I/O
    return {"answer": query}

@trace
async def agent(query):
    result = await call_api(query)
    return result["answer"]

result = await trace.arun(agent, "hello")
result.to_html("async_trace.html")

Token / cost tracking

Token usage and estimated cost are auto-extracted from OpenAI response objects, or you can set them manually:

@trace
def call_llm(prompt):
    resp = openai.chat.completions.create(...)
    trace.meta(model=resp.model,
               prompt_tokens=resp.usage.prompt_tokens,
               completion_tokens=resp.usage.completion_tokens)
    return resp.choices[0].message.content

Redaction and share-safe traces

Use decorator options when a trace may contain secrets or bulky payloads:

from flow_xray import trace

@trace(redact={"api_key", "authorization"}, capture_output=False)
def call_service(api_key, payload):
    ...

• redact={...} masks matching argument names and nested dict keys as [redacted]
• capture_output=False keeps the real return value in Python, but stores [redacted] in the HTML trace

Safety and sharing

flow-xray serializes trace data into the generated HTML file.

By default, that can include:

• function names
• bound inputs
• outputs
• exceptions
• attached trace.meta(...) fields
• token and cost metadata

Use the privacy controls when you want a trace that is safer to share:

• redact={...} hides matching input names and nested dict keys as [redacted]
• capture_output=False keeps the real Python return value, but writes [redacted] into the HTML trace

Treat trace files as local debugging artifacts unless you have explicitly reviewed or redacted their contents. A trace is generally safer to share when you have:

• redacted secrets, auth headers, API keys, or user data
• hidden sensitive outputs with capture_output=False
• checked the generated HTML once before posting it publicly

Why not LangSmith or plain logs?

Use flow-xray when you want

• local-first debugging
• one HTML file you can open immediately
• graph and timeline views, not only logs
• no account and no hosted tracing requirement

Use plain logs when you want

• quick event-level debugging
• a tiny amount of output
• no structured trace at all

Use hosted tracing tools when you want

• team dashboards
• long-term trace storage
• production monitoring
• centralized evaluation workflows

flow-xray is best when your question is:

What actually happened inside this Python agent run, locally, right now?

What you see

• Nodes = function calls (name + latency + tokens)
• Edges = caller → callee
• Green = OK, Red = error, Yellow = slow (>1s)
• Header = total nodes, latency, tokens, estimated cost
• Click a node → side panel shows inputs, output, error, timing, model, tokens, cost

Why this exists

Langfuse, Helicone, LangSmith — they give you timelines and logs.

But when your agent pipeline branches, retries, or chains 6 tools — you don't need another table. You need a graph.

flow-xray is not an agent framework. It's the layer below them — like Chrome DevTools is to browsers.

Docs

• Debug Python agent runs locally
• FAQ
• Using flow-xray with LangGraph
• flow-xray vs LangSmith

Compatibility

• Python 3.10, 3.11, 3.12, 3.13, 3.14 — tested
• Sync and async functions — both supported
• Any Python code — not limited to LLM calls; works with any function you decorate
• Frameworks — works alongside LangGraph, CrewAI, OpenAI SDK, or plain Python

How it works

@trace wraps functions (sync and async). When called inside a trace.run() session (or flow-xray run CLI), it records:

• function name
• bound arguments
• return value or exception
• wall-clock latency
• token usage and estimated cost (auto or manual)
• parent/child relationships (call stack → DAG)

result.to_html() embeds the trace as JSON in a self-contained HTML page that renders via WASM Graphviz (CDN, works offline after first load).

The trace viewer also includes search, zoom/reset controls, and copy-details for the currently selected node.

Also included

Scalar autodiff core (micrograd-style Value graph with DOT/JSON export and stepping debugger) lives under flow-xray dot CLI and from flow_xray import Value. See examples/ and plan.md.

License

MIT