llm-trace

LLMs can read your code but they can't run it in their head. When something breaks, they end up asking you to paste errors and add console.log statements one at a time while they guess at what's going on.

llm-trace fixes this. It gives LLMs a way to instrument your code with structured traces, see actual runtime values, and figure out what went wrong without the back-and-forth.

Install

Install the debugging skill so your LLM tool knows how to use llm-trace. The skill will ask to npm install llm-trace into your project — it's the SDK that provides the trace(), span(), and checkpoint() calls for code instrumentation.

Claude Code:

/plugin marketplace add moondef/llm-trace
/plugin install debugging-with-llm-trace@llm-trace

Codex:

$skill-installer install https://github.com/moondef/llm-trace/tree/main/skills/debugging-with-llm-trace

Any tool (via npx):

npx skills add moondef/llm-trace

Or clone and copy manually:

git clone https://github.com/moondef/llm-trace.git
cp -r llm-trace/skills/debugging-with-llm-trace ~/.claude/skills/

Usage

Just ask your LLM to debug something. The skill handles the rest.

You say "this endpoint returns 500 sometimes" and it:

1. Starts a tracing session (llm-trace start)
2. Instruments the suspect code with trace(), span(), and checkpoint()
3. Runs the code to trigger the bug
4. Reads the trace — actual values at each step, which span failed, the error
5. Fixes the root cause based on what it saw
6. Cleans up instrumentation and stops the session

What It Captures

Three primitives, all the LLM needs:

import { trace } from "llm-trace";

await trace("checkout", async (handle) => {

  // span — time a step, nest arbitrarily
  await handle.span("load-cart", async (h) => {
    const cart = await db.getCart(userId);

    // checkpoint — snapshot runtime values
    h.checkpoint("cart", cart);

    return cart;
  });
});

trace(name, fn) — wraps a complete operation. Captures start, end, duration, errors.

handle.span(name, fn) — a timed step within a trace. Nests to any depth.

handle.checkpoint(name, data?) — snapshots a value at a point in time (truncated at 64KB).

Errors are captured automatically — if anything throws, the trace records the error and stack.

CLI

llm-trace start             # begin session
llm-trace list              # all traces (JSON by default)
llm-trace list --errors     # just failures
llm-trace show <id>         # full trace tree
llm-trace tail              # watch live
llm-trace stop              # end session, delete traces

Output is JSON by default (for LLM consumption). Add --human for readable output.

How It Works

The LLM writes trace() / span() / checkpoint() calls into your code. When the code runs, events stream over HTTP to a local server that writes .ndjson files. The LLM reads those files via the CLI. After debugging, everything gets cleaned up — traces are ephemeral.

No dependencies. No config. Nothing persisted after stop.

Configuration

Variable	Default	Description
LLM_TRACE_PORT	13579	HTTP server port

Contributing

Issues and PRs welcome at github.com/moondef/llm-trace.