LangSmith Engine notifications

LangSmith Engine can notify you when it opens a new issue, links a new trace to an existing issue, or fails to complete a run. Deliver these notifications to a Slack channel, an HTTP webhook endpoint, or both. Each destination has its own event types and minimum priority, so you can route urgent issues to a paging webhook while sending every issue to a Slack channel.

Add a destination

Notification destinations are configured per tracing project. On the Engine page, click Configure Engine, then under Notifications click + Add destination. For each destination, choose:

Deliver to: Slack or Webhook. See Notify a Slack channel and Send to a webhook.
Notify when: The event types that trigger a notification.
Minimum priority: The lowest issue severity that triggers a notification.

To be alerted when a watched issue recurs, click Alert me via Slack on the issue, which opens the same Notifications section.

Event types

Event	Sent when
`issue.created`	Engine opens a new issue.
`issue.trace.added`	Engine links a new trace to an existing issue.
`issue.agent_run.failed`	An Engine run fails to complete.

This is the complete set of event types Engine sends today. New types may be added in the future. A destination created without an explicit list of event types receives only issue.created.

Severity filtering

The Minimum priority setting is stored as a severity_threshold from 0 to 3. For issue events, a notification is delivered only when the issue’s severity is less than or equal to the threshold. Lower numbers are more urgent.

Severity	Meaning
`0`	Urgent
`1`	High
`2`	Medium
`3`	Low

For example, a destination with severity_threshold: 1 receives events for URGENT (0) and HIGH (1) issues only. Severity thresholds do not apply to issue.agent_run.failed, because run-failure events are scoped to an Engine session rather than to a specific issue.

Notify a Slack channel

Connect a Slack workspace

Connecting a Slack workspace is an organization-level action you perform once, not per project. Connecting or disconnecting a workspace requires the organization:manage permission. In the LangSmith console, open Settings, go to your organization’s General settings, and under Slack click Connect Slack. Authorize the LangSmith app in Slack. You can connect more than one Slack workspace to an organization.

Add a Slack destination

On the Engine page, click Configure Engine, then click Add destination. Set the Deliver to field to Slack, then choose the workspace and channel under Channel.

Choose events and priority

Under Notify when, select which event types post a message to the channel. Under Minimum priority, choose the lowest severity that triggers a notification. Click Add destination to save.

LangSmith automatically joins the public channel you select. To post to a private channel, invite the LangSmith app to that channel in Slack first. Each Slack message includes the issue title, description, and severity, a View issue link back to LangSmith, and (for issue events) a chart of the issue’s recurrence over time. If a workspace’s connection becomes invalid, for example, the app is removed from Slack, its destinations stop delivering until you reconnect it from your organization’s General settings. Slack destinations post through LangSmith’s managed Slack app instead of sending the webhook payload, so signing secrets and custom headers do not apply.

Send to a webhook

Forward Engine events to your own incident-management, paging, or chat tooling. Add a destination and set the Deliver to field to Webhook. Enter a URL and, optionally, custom headers. Each delivery is signed so you can verify its authenticity.

Delivery

LangSmith sends a POST request with a JSON body to your webhook URL. The request uses Content-Type: application/json and includes any custom headers you attached to the destination.

Property	Value
Method	`POST`
Body	JSON, common envelope below
Scheme	`http://` and `https://` are accepted. `https://` is strongly recommended
Signature	`X-LangSmith-Signature` header, signed with the destination’s signing secret
Timeout	20 seconds per attempt
Attempts	Up to 4 attempts (1 initial plus 3 retries with exponential backoff) on transport errors, HTTP `408`, `425`, `429`, and any HTTP `5xx`. Other `4xx` responses are treated as permanent and are not retried
Response	Success is determined from the status code alone. Response bodies are ignored.

Retries deliver a byte-identical payload, including the same id. Dedupe on id so a retried delivery does not produce a duplicate downstream effect.

Custom headers

You can attach arbitrary headers to each destination (for example, Authorization: Bearer …) to authenticate the caller at your endpoint. Content-Type is always set by LangSmith and cannot be overridden.

Signing secret

Each destination has a signing secret. LangSmith uses this secret to sign the raw webhook request body and sends the result in the X-LangSmith-Signature header. The header value has this format:

sha256=<hex-encoded HMAC-SHA256 digest>

Verify the signature before parsing or acting on the payload. The HMAC input is the exact raw request body bytes, and the HMAC key is the destination’s signing secret. Do not parse and reserialize the JSON body before verification.

import hashlib
import hmac
from typing import Optional


def verify_langsmith_signature(
    *,
    body: bytes,
    signing_secret: str,
    signature_header: Optional[str],
) -> bool:
    if not signature_header or not signature_header.startswith("sha256="):
        return False

    expected = "sha256=" + hmac.new(
        signing_secret.encode("utf-8"),
        body,
        hashlib.sha256,
    ).hexdigest()

    return hmac.compare_digest(expected, signature_header)

import { createHmac, timingSafeEqual } from "node:crypto";

export function verifyLangSmithSignature({
  body,
  signingSecret,
  signatureHeader,
}: {
  body: Buffer;
  signingSecret: string;
  signatureHeader: string | undefined;
}) {
  if (!signatureHeader?.startsWith("sha256=")) {
    return false;
  }

  const expected = `sha256=${createHmac("sha256", signingSecret)
    .update(body)
    .digest("hex")}`;

  const expectedBytes = Buffer.from(expected);
  const actualBytes = Buffer.from(signatureHeader);

  return (
    expectedBytes.length === actualBytes.length &&
    timingSafeEqual(expectedBytes, actualBytes)
  );
}

Roll a signing secret

Roll a signing secret when it may have been exposed, or when your organization’s credential rotation policy requires a new secret. To roll a secret, open the destination row in Engine Settings, click Roll signing secret, and confirm. LangSmith generates a new signing secret and uses it for future webhook deliveries immediately. The previous secret stops signing deliveries as soon as the roll completes. After rolling the secret, update every consumer that verifies X-LangSmith-Signature with the new value.

Test your endpoint

Before pointing a real destination at your endpoint, send a sample payload to verify it accepts and acknowledges within the 20-second timeout:

curl -X POST https://your-endpoint.example.com/webhook \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $WEBHOOK_SECRET" \
  -d @sample-issue-created.json

Use the example body from issue.created as sample-issue-created.json. Verify that:

The custom Authorization header arrives and matches the secret you configured on the destination.
The handler persists the event keyed by its id so retries are deduped.
The handler returns 2xx before kicking off slow downstream work.

Security

Webhook URLs are validated when the destination is created and again at delivery time. Private and metadata IP ranges are blocked in SaaS. Both http:// and https:// are accepted; use https:// so the payload and any custom headers are not sent in cleartext.
LangSmith signs webhook bodies with the destination’s signing secret. Verify X-LangSmith-Signature before processing the payload.
You can also set custom headers on the destination, such as Authorization: Bearer …, for routing or additional authentication at your endpoint.
Dedupe on the event id so that a retried delivery does not cause a duplicate notification.

Best practices

Acknowledge fast. Respond with 2xx as soon as you have persisted the event. Move slow work (fan-out, paging, downstream API calls) onto a queue so your handler stays within the 20-second timeout.
Tolerate unknown event types. Ignore type values your handler does not recognize. New event types may be added without notice.
Tolerate new fields. Parse payloads with a permissive schema. New fields may be added to existing event types without notice.

Webhook payload reference

Webhook destinations receive the JSON payloads below. Slack destinations do not.

Event envelope

Every event delivered to your endpoint uses the same outer JSON shape.

Field	Type	Description
`id`	UUID	Unique identifier for this delivery. Stable across retries. Use it to dedupe.
`type`	string	Event type. One of `issue.created`, `issue.trace.added`, or `issue.agent_run.failed`.
`created`	integer	Unix seconds (UTC) when the event was enqueued.
`request_id`	UUID	Shared by every event fired from the same upstream action. See Batch coalescing.
`data`	object	Event payload. Always contains `data.object`. Contains `data.trace` only on `issue.trace.added` events.

Issue `data.object`

For issue.created and issue.trace.added, data.object is a snapshot of the issue. Treat it as the authoritative state of the issue at the time the event was generated.

Field	Type	Description
`id`	UUID	Issue ID.
`name`	string	Short title of the issue.
`description`	string	Human-readable description.
`severity`	integer	`0` (urgent) through `3` (low). See Severity filtering.
`tenant_id`	UUID	Workspace the issue belongs to.
`tenant_name`	string	Workspace display name.
`session_id`	UUID	Tracing project the issue belongs to.
`session_name`	string	Tracing project name.
`url`	string	Deep link to the issue in the LangSmith UI.

Run failure `data.object`

For issue.agent_run.failed, data.object describes the Engine run that failed.

Field	Type	Description
`tenant_id`	UUID	Workspace the run belongs to.
`tenant_name`	string	Workspace display name.
`session_id`	UUID	Tracing project the run belongs to.
`session_name`	string	Tracing project name.
`url`	string	Deep link to the LangSmith project in the UI.
`thread_id`	string	Engine thread ID.
`run_id`	string	Engine run ID. Omitted when unavailable.
`status`	string	Final run status.
`error_message`	string	Error text from the failed run. Omitted when unavailable.
`occurred_at`	string	RFC 3339 timestamp of when the failure occurred.

`data.trace`

data.trace is included only on issue.trace.added events.

Field	Type	Description
`run_id`	UUID	ID of the run that was linked to the issue.
`trace_id`	UUID	ID of the trace that contains the run.
`start_time`	string	RFC 3339 timestamp of when the run started.
`comment`	string \| null	Optional note recorded when the trace was linked. Omitted when empty.

Batch coalescing

A single upstream action can produce multiple webhook events. When Engine opens a new issue and attaches five traces to it, you receive one issue.created event and five issue.trace.added events, all sharing the same request_id. Use request_id to group these into a single downstream notification.

`issue.created`

Sent when LangSmith Engine creates a new issue. data.trace is omitted.

{
  "id": "b91c1f0e-7c4a-4f53-9d3e-9f1c8e7a2b10",
  "type": "issue.created",
  "created": 1747238400,
  "request_id": "0d2f4f6a-2a3a-4b6e-9b87-5d5b6e8c9a01",
  "data": {
    "object": {
      "id": "9a8b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d",
      "name": "Tool selection inconsistency",
      "description": "Agent repeatedly calls the search tool with identical arguments before terminating.",
      "severity": 1,
      "tenant_id": "11111111-2222-3333-4444-555555555555",
      "tenant_name": "Acme Workspace",
      "session_id": "66666666-7777-8888-9999-aaaaaaaaaaaa",
      "session_name": "prod-api",
      "url": "https://smith.langchain.com/o/11111111-2222-3333-4444-555555555555/projects/p/66666666-7777-8888-9999-aaaaaaaaaaaa?tab=5&issue=9a8b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d"
    }
  }
}

`issue.trace.added`

Sent when a new trace is linked to an existing issue. data.trace describes the linked trace.

{
  "id": "c02e3a4b-5c6d-7e8f-9a0b-1c2d3e4f5a6b",
  "type": "issue.trace.added",
  "created": 1747238410,
  "request_id": "0d2f4f6a-2a3a-4b6e-9b87-5d5b6e8c9a01",
  "data": {
    "object": {
      "id": "9a8b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d",
      "name": "Tool selection inconsistency",
      "description": "Agent repeatedly calls the search tool with identical arguments before terminating.",
      "severity": 1,
      "tenant_id": "11111111-2222-3333-4444-555555555555",
      "tenant_name": "Acme Workspace",
      "session_id": "66666666-7777-8888-9999-aaaaaaaaaaaa",
      "session_name": "prod-api",
      "url": "https://smith.langchain.com/o/11111111-2222-3333-4444-555555555555/projects/p/66666666-7777-8888-9999-aaaaaaaaaaaa?tab=5&issue=9a8b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d"
    },
    "trace": {
      "run_id": "f1e2d3c4-b5a6-9788-6655-44332211ffee",
      "trace_id": "abcdefab-1234-5678-9abc-def012345678",
      "start_time": "2026-05-14T12:30:00Z",
      "comment": "Reproduces the same tool-loop pattern."
    }
  }
}

`issue.agent_run.failed`

Sent when LangSmith Engine fails to complete a run. This event is session-scoped, so it does not include data.trace and does not use severity filtering.

{
  "id": "4d0e8db2-81e6-4491-b8e5-b13a8f5afc0d",
  "type": "issue.agent_run.failed",
  "created": 1747238500,
  "request_id": "f6bbd48a-0386-403d-9344-31051264b45f",
  "data": {
    "object": {
      "tenant_id": "11111111-2222-3333-4444-555555555555",
      "tenant_name": "Acme Workspace",
      "session_id": "66666666-7777-8888-9999-aaaaaaaaaaaa",
      "session_name": "prod-api",
      "url": "https://smith.langchain.com/o/11111111-2222-3333-4444-555555555555/projects/p/66666666-7777-8888-9999-aaaaaaaaaaaa",
      "thread_id": "thread-123",
      "run_id": "run-456",
      "status": "error",
      "error_message": "RuntimeError: missing API key",
      "occurred_at": "2026-05-14T12:45:00Z"
    }
  }
}

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Edit this page on GitHub or file an issue.

​Add a destination

​Event types

​Severity filtering

​Notify a Slack channel

​Send to a webhook

​Delivery

​Custom headers

​Signing secret

​Roll a signing secret

​Test your endpoint

​Security

​Best practices

​Webhook payload reference

​Event envelope

​Issue data.object

​Run failure data.object

​data.trace

​Batch coalescing

​issue.created

​issue.trace.added

​issue.agent_run.failed

Add a destination

Event types

Severity filtering

Notify a Slack channel

Send to a webhook

Delivery

Custom headers

Signing secret

Roll a signing secret

Test your endpoint

Security

Best practices

Webhook payload reference

Event envelope

Issue `data.object`

Run failure `data.object`

`data.trace`

Batch coalescing

`issue.created`

`issue.trace.added`

`issue.agent_run.failed`