Query threads using the SDK

Programmatically fetch and inspect multi-turn conversation threads from your LangSmith projects.

If you’re building a conversational agent or any multi-turn application, LangSmith automatically groups your runs into threads. Querying threads lets you replay full conversations, audit agent behavior across sessions, build analytics on conversation length and latency, and feed downstream workflows like fine-tuning and evaluation.

The SDK exposes two methods for working with threads:

Method	Use when
list_threads / listThreads	You want to browse all threads in a project
read_thread / readThread	You already know the thread ID and need its runs

How threads work

Each run you create can carry a thread_id in its metadata. LangSmith uses this to group runs into threads. The backend looks for thread_id in metadata (falling back to session_id or conversation_id).

We recommend using UUID v7 thread IDs. UUIDv7 embeds a timestamp, which preserves correct time-ordering of threads. The LangSmith SDK exports a uuid7 helper (Python v0.4.43+, JS v0.3.80+):

• Python: from langsmith import uuid7
• JS/TS: import { uuid7 } from 'langsmith'

If you’re using a tracing integration, pass thread_id in the run metadata:

Python

from langsmith import traceable, uuid7

THREAD_ID = str(uuid7())

@traceable(metadata={"thread_id": THREAD_ID})
def my_agent(user_message: str) -> str:
    ...

List all threads in a project

list_threads / listThreads fetches all threads in a project and groups their runs together. Results are sorted by most recent activity first.

Python

from langsmith import Client

client = Client()

threads = client.list_threads(project_name="my-project")

for thread in threads:
    print(thread["thread_id"])
    print(f"  {thread['count']} runs")
    print(f"  last active: {thread['max_start_time']}")

Results are sorted by most recent activity:

Output

conv-abc123
  3 runs
  last active: 2026-02-25T10:05:42+00:00
conv-def456
  1 runs
  last active: 2026-02-25T09:30:00+00:00

Parameters

Parameter	Type	Default	Description
project_name / projectName	string	—	Project name. Required if project_id is not set.
project_id / projectId	string	—	Project ID. Required if project_name is not set.
limit	int	all	Maximum number of threads to return.
offset	int	0	Number of threads to skip (for pagination).
filter	string	—	Filter expression applied when fetching runs, using LangSmith trace query syntax.
start_time / startTime	datetime / Date	1 day ago	Only include runs started after this time. Widen this window to surface older threads.

Return value

A list of thread objects, each containing:

Field	Type	Description
thread_id	string	The thread identifier.
runs	[Run](https://reference.langchain.com/python/langsmith/schemas/Run)[]	Root runs in this thread, sorted chronologically (oldest first).
count	int	Number of runs in this thread.
min_start_time	string | null	ISO timestamp of the earliest run.
max_start_time	string | null	ISO timestamp of the most recent run.

list_threads always returns root runs only. If you need child runs (e.g., tool calls, sub-chains), use read_thread instead, which accepts an is_root / isRoot parameter you can set to false.

Read runs for a single thread

When you already know the thread_id, use read_thread / readThread. It returns an iterator over the thread’s runs directly, without fetching all threads first.

Python

from langsmith import Client

client = Client()

for run in client.read_thread(
    thread_id="conv-abc123",
    project_name="my-project",
):
    print(run.id, run.name, run.start_time)

Unlike list_threads, each item here is a Run object directly — there is no grouping wrapper. Runs are returned in ascending chronological order by default.

Output

[
    Run(id=UUID("a1b2..."), name="my_agent", run_type="chain", status="success", start_time=datetime(2026, 2, 25, 10, 0, 0, tzinfo=utc), ...),
    Run(id=UUID("c3d4..."), name="my_agent", run_type="chain", status="success", start_time=datetime(2026, 2, 25, 10, 3, 11, tzinfo=utc), ...),
    Run(id=UUID("e5f6..."), name="my_agent", run_type="chain", status="error",   start_time=datetime(2026, 2, 25, 10, 5, 42, tzinfo=utc), ...),
]

Parameters

Parameter	Type	Default	Description
thread_id / threadId	string	—	Required. The thread to query.
project_name / projectName	string	—	Project name. Required if project_id is not set.
project_id / projectId	string | string[]	—	Project ID or list of IDs. Required if project_name is not set.
is_root / isRoot	bool	true	Return only root runs. Set to false to include child runs.
limit	int	all	Maximum number of runs to return.
filter	string	—	Additional filter expression (combined with the thread filter).
order	"asc" | "desc"	"asc"	Sort order. "asc" returns runs oldest-first (chronological).
select	string[]	all fields	Specific run fields to return, to reduce response size.

Return value

An iterator (Python) or async iterator (TypeScript) of Run objects.

Examples

Filter threads by run properties

Pass a filter expression to narrow results using LangSmith trace query syntax. For example, to surface only threads containing at least one failed run:

Python

threads = client.list_threads(
    project_name="my-project",
    filter='eq(status, "error")',
)

Look back further than 24 hours

By default, list_threads only surfaces threads with runs from the last day. Pass start_time to widen the window:

Python

import datetime

threads = client.list_threads(
    project_name="my-project",
    start_time=datetime.datetime.now(datetime.timezone.utc) - datetime.timedelta(days=2),
)

Reconstruct a conversation

Use read_thread with order="asc" to replay a conversation turn by turn:

Python

runs = list(
    client.read_thread(
        thread_id="conv-abc123",
        project_name="my-project",
        order="asc",
    )
)

for run in runs:
    user_msg = run.inputs.get("messages", [{}])[-1].get("content", "")
    assistant_msg = (run.outputs or {}).get("content", "")
    print(f"User:      {user_msg}")
    print(f"Assistant: {assistant_msg}")
    print()

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