# Sessions, workflows & traces (/docs/concepts/sessions-and-traces)
When your app runs, TokenPolice records each LLM call as a **span**. The spans from one run
form a tree — that tree is a **trace**. Wrapping your entrypoint gives that tree a root and a
name, so the dashboard shows "the support-agent run" instead of a pile of loose calls.
## A trace is one run [#a-trace-is-one-run]
Every LLM call becomes a span. On its own, a call is a lone span. Wrap the code around it and
those calls nest **under** an anchor span — one trace for the whole run, with the model calls
as children. That's what lets the dashboard total the cost of a run, not just of a call.
## Giving the tree a root [#giving-the-tree-a-root]
You get that root by wrapping the code that handles one run. Whichever wrapper you use, it
carries the same identity tags ([user\_id, paid\_plan, session\_id](/docs/concepts/identity))
down to every call inside.
In practice you'll reach for `@tp.workflow` most of the time — it wraps a function as one
trace and pulls identity straight from its arguments. Wrap a *region* of code instead, or
label a sub-part by its shape, with `session` / `agent` / `chain`. The four wrappers and when
to use which are in [Wrapping your app](/docs/sdk/wrapping).
```python
@tp.workflow(name="support_agent")
def run_agent(user_id: str, session_id: str, query: str):
# both calls below are children of one "support_agent" trace
plan = client.chat.completions.create(...)
reply = client.chat.completions.create(...)
return reply
```
## Threading multi-turn conversations [#threading-multi-turn-conversations]
One run is one trace. A back-and-forth conversation is *many* runs — so to see them as one
thread, reuse the same `session_id` across turns. Each turn is still its own trace, but they
share a session, and the dashboard groups them into a single conversation.
Nested scopes inherit their parent's `session_id` and trace automatically — you only set the
id at the top. Same id across turns → one conversation; a new id → a new one. See the
[stable-session\_id contract](/docs/concepts/identity#the-stable-session_id-contract).
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