Guide

New

Change Detection

Monitor any webpage for content changes. AlterLab checks your URLs on a schedule, compares snapshots, and notifies you when something changes.

Built on the Scheduler

Monitors use the same cron-based scheduling engine as Schedules. The same balance-based limits apply for check intervals and active monitor counts.

How It Works

Create

Define a monitor with a URL, diff mode, check interval, and optional CSS selectors to narrow focus.

Snapshot

At each scheduled time, AlterLab scrapes the page and stores a content snapshot with a hash for fast comparison.

Compare

The new snapshot is compared against the previous one using your chosen diff mode. If content changed, a change record is created.

Notify

When a change is detected (or on every check, depending on your notify_on setting), your webhook receives the diff.

Create a Monitor

POST

/api/v1/monitors

Bash

curl -X POST https://api.alterlab.io/api/v1/monitors \
  -H "Authorization: Bearer your_jwt_token" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "Competitor pricing page",
    "url": "https://competitor.com/pricing",
    "diff_mode": "semantic",
    "check_interval": "0 */6 * * *",
    "timezone": "America/New_York",
    "notify_on": "change",
    "webhook_url": "https://your-server.com/monitor-webhook",
    "options": {
      "wait_for": ".pricing-table",
      "timeout": 30
    }
  }'

Request Body

Parameter	Type	Required	Description
name	string	Yes	Human-readable name (1-255 chars)
url	string	Yes	URL to monitor for changes
diff_mode	string	No	`semantic` (default), `exact`, or `selector`
check_interval	string	No	Cron expression (default: `0 /6 * *`)
timezone	string	No	IANA timezone (default: `UTC`)
monitor_selectors	string[]	Conditional	CSS selectors to watch (required when `diff_mode=selector`, max 20)
notify_on	string	No	`change` (default) or `always`
formats	string[]	No	Output formats: text, markdown, json, html, rag (default: markdown, json)
options	object	No	Scrape options: wait_for, timeout, headers, cookies, proxy_country
webhook_url	string	No	URL to receive change notifications

Diff Modes

Choose how content is compared between snapshots. The diff mode determines what counts as a “change” and how the diff is formatted.

Semantic
Default

Compares the structural content of the page, ignoring whitespace changes, ad injection, and layout shifts. Best for tracking meaningful content updates like price changes, product descriptions, or article text.

JSON

{
  "diff_mode": "semantic"
}

Exact

Byte-level comparison. Any change in the rendered content triggers a diff. Use when you need to detect every change, including formatting and whitespace.

JSON

{
  "diff_mode": "exact"
}

Selector

Monitor specific elements using CSS selectors. Only the matched elements are compared between snapshots. Ideal for tracking a specific price, stock status, or headline without being triggered by unrelated page changes.

JSON

{
  "diff_mode": "selector",
  "monitor_selectors": [
    ".product-price",
    ".stock-status",
    "h1.product-title"
  ]
}

Selectors Required

When using diff_mode: selector, you must provide at least one CSS selector in monitor_selectors. Maximum 20 selectors, each up to 500 characters.

Check Intervals

Monitors use standard 5-field cron expressions. The same balance-based limits from the Scheduler apply.

Expression	Schedule
/15 * * *	Every 15 minutes
0 /6 * *	Every 6 hours (default)
0 9 * * *	Every day at 9:00 AM
0 8 * * 1-5	Weekdays at 8:00 AM
0 0 * * 0	Every Sunday at midnight

Minimum interval depends on your account balance. See Balance-Based Limits for details.

Manage Monitors

List Monitors

GET

/api/v1/monitors

Query parameters: limit (1-100, default 20), offset (default 0), active_only (default false).

Bash

curl https://api.alterlab.io/api/v1/monitors?active_only=true \
  -H "Authorization: Bearer your_jwt_token"

Update a Monitor

PATCH

/api/v1/monitors/{monitor_id}

Send only the fields you want to change. Changing check_interval or timezone recomputes next_run_at.

Bash

curl -X PATCH https://api.alterlab.io/api/v1/monitors/{monitor_id} \
  -H "Authorization: Bearer your_jwt_token" \
  -H "Content-Type: application/json" \
  -d '{
    "diff_mode": "selector",
    "monitor_selectors": [".price", ".availability"],
    "check_interval": "0 */2 * * *"
  }'

Pause & Resume

Bash

# Pause a monitor
curl -X POST https://api.alterlab.io/api/v1/monitors/{monitor_id}/pause \
  -H "Authorization: Bearer your_jwt_token"

# Resume (recomputes next_run_at, re-checks balance limits)
curl -X POST https://api.alterlab.io/api/v1/monitors/{monitor_id}/resume \
  -H "Authorization: Bearer your_jwt_token"

Delete a Monitor

DELETE

/api/v1/monitors/{monitor_id}

Soft-deletes the monitor (deactivates it). Returns 204 No Content.

Snapshots

Every check stores a snapshot of the page content. Use snapshots to review historical versions or compare content at different points in time.

GET

/api/v1/monitors/{monitor_id}/snapshots

Returns a paginated list of snapshots, most recent first. Query parameters: limit (1-100), offset.

JSON

{
  "snapshots": [
    {
      "id": "snap-uuid-...",
      "schedule_id": "monitor-uuid-...",
      "run_id": "run-uuid-...",
      "url": "https://competitor.com/pricing",
      "content_hash": "a1b2c3d4...",
      "metadata": { "status_code": 200, "content_length": 42350 },
      "created_at": "2026-03-24T12:00:05Z"
    }
  ],
  "total": 156
}

To retrieve the full content of a specific snapshot:

GET

/api/v1/monitors/{monitor_id}/snapshots/{snapshot_id}

Returns the snapshot with the full content field included.

Changes & Diffs

When a snapshot differs from the previous one, a change record is created. Each change includes the full diff and references both the previous and current snapshots.

GET

/api/v1/monitors/{monitor_id}/diffs

Returns a paginated list of detected changes, most recent first.

JSON

{
  "changes": [
    {
      "id": "change-uuid-...",
      "schedule_id": "monitor-uuid-...",
      "previous_snapshot_id": "snap-old-...",
      "current_snapshot_id": "snap-new-...",
      "url": "https://competitor.com/pricing",
      "change_type": "content_changed",
      "diff": {
        "added": ["Enterprise plan now $299/mo"],
        "removed": ["Enterprise plan now $199/mo"],
        "summary": "Price increase detected on Enterprise plan"
      },
      "notified": true,
      "created_at": "2026-03-24T12:00:06Z"
    }
  ],
  "total": 12
}

To retrieve a specific change:

GET

/api/v1/monitors/{monitor_id}/diffs/{change_id}

Notifications

Configure when your webhook is called using the notify_on parameter:

Value	Behavior
change	Webhook fires only when a diff is detected (default)
always	Webhook fires after every check, even when nothing changed

Tip

Use notify_on: "always" for uptime monitoring. Your webhook receives a call on every check, so you can detect when a page goes down even if the content hasn't changed.

Python Example

Python

import requests

API_URL = "https://api.alterlab.io/api/v1"
HEADERS = {
    "Authorization": "Bearer your_jwt_token",
    "Content-Type": "application/json",
}

# Create a price monitoring watch
monitor = requests.post(
    f"{API_URL}/monitors",
    headers=HEADERS,
    json={
        "name": "Competitor pricing tracker",
        "url": "https://competitor.com/pricing",
        "diff_mode": "selector",
        "monitor_selectors": [".price-card", ".plan-name"],
        "check_interval": "0 */6 * * *",
        "notify_on": "change",
        "webhook_url": "https://your-server.com/price-alerts",
    },
).json()

print(f"Monitor created: {monitor['id']}")
print(f"Next check: {monitor['next_run_at']}")

# List active monitors
monitors = requests.get(
    f"{API_URL}/monitors?active_only=true",
    headers=HEADERS,
).json()

for m in monitors["monitors"]:
    print(f"  {m['name']} — mode: {m['diff_mode']}, runs: {m['run_count']}")

# Check recent changes
changes = requests.get(
    f"{API_URL}/monitors/{monitor['id']}/diffs",
    headers=HEADERS,
).json()

for c in changes["changes"]:
    print(f"  [{c['created_at']}] {c['change_type']}")
    if "summary" in c["diff"]:
        print(f"    Summary: {c['diff']['summary']}")

# Get snapshot history
snapshots = requests.get(
    f"{API_URL}/monitors/{monitor['id']}/snapshots?limit=5",
    headers=HEADERS,
).json()

for s in snapshots["snapshots"]:
    print(f"  Snapshot {s['id'][:8]}... hash={s['content_hash'][:12]}")

Node.js Example

TYPESCRIPT

const API_URL = "https://api.alterlab.io/api/v1";
const headers = {
  Authorization: "Bearer your_jwt_token",
  "Content-Type": "application/json",
};

// Create a monitor with CSS selector tracking
const monitor = await fetch(`${API_URL}/monitors`, {
  method: "POST",
  headers,
  body: JSON.stringify({
    name: "Competitor pricing tracker",
    url: "https://competitor.com/pricing",
    diff_mode: "selector",
    monitor_selectors: [".price-card", ".plan-name"],
    check_interval: "0 */6 * * *",
    notify_on: "change",
    webhook_url: "https://your-server.com/price-alerts",
  }),
}).then((r) => r.json());

console.log(`Monitor: ${monitor.id}, next check: ${monitor.next_run_at}`);

// List active monitors
const list = await fetch(`${API_URL}/monitors?active_only=true`, {
  headers,
}).then((r) => r.json());

for (const m of list.monitors) {
  console.log(`  ${m.name} — ${m.diff_mode}, ${m.run_count} runs`);
}

// Get recent changes
const changes = await fetch(
  `${API_URL}/monitors/${monitor.id}/diffs`,
  { headers }
).then((r) => r.json());

for (const c of changes.changes) {
  console.log(`  [${c.created_at}] ${c.change_type}`);
}

// Browse snapshot history
const snapshots = await fetch(
  `${API_URL}/monitors/${monitor.id}/snapshots?limit=5`,
  { headers }
).then((r) => r.json());

for (const s of snapshots.snapshots) {
  console.log(`  Snapshot ${s.id.slice(0, 8)}... hash=${s.content_hash.slice(0, 12)}`);
}

Scheduler Cloud Storage Export

Last updated: March 2026

Guide

New

Change Detection

Monitor any webpage for content changes. AlterLab checks your URLs on a schedule, compares snapshots, and notifies you when something changes.

Built on the Scheduler

Monitors use the same cron-based scheduling engine as Schedules. The same balance-based limits apply for check intervals and active monitor counts.

How It Works

Create

Define a monitor with a URL, diff mode, check interval, and optional CSS selectors to narrow focus.

Snapshot

At each scheduled time, AlterLab scrapes the page and stores a content snapshot with a hash for fast comparison.

Compare

The new snapshot is compared against the previous one using your chosen diff mode. If content changed, a change record is created.

Notify

When a change is detected (or on every check, depending on your notify_on setting), your webhook receives the diff.

Create a Monitor

POST

/api/v1/monitors

Bash

curl -X POST https://api.alterlab.io/api/v1/monitors \
  -H "Authorization: Bearer your_jwt_token" \
  -H "Content-Type: application/json" \
  -d '{
    "name": "Competitor pricing page",
    "url": "https://competitor.com/pricing",
    "diff_mode": "semantic",
    "check_interval": "0 */6 * * *",
    "timezone": "America/New_York",
    "notify_on": "change",
    "webhook_url": "https://your-server.com/monitor-webhook",
    "options": {
      "wait_for": ".pricing-table",
      "timeout": 30
    }
  }'

Request Body

Parameter	Type	Required	Description
name	string	Yes	Human-readable name (1-255 chars)
url	string	Yes	URL to monitor for changes
diff_mode	string	No	`semantic` (default), `exact`, or `selector`
check_interval	string	No	Cron expression (default: `0 /6 * *`)
timezone	string	No	IANA timezone (default: `UTC`)
monitor_selectors	string[]	Conditional	CSS selectors to watch (required when `diff_mode=selector`, max 20)
notify_on	string	No	`change` (default) or `always`
formats	string[]	No	Output formats: text, markdown, json, html, rag (default: markdown, json)
options	object	No	Scrape options: wait_for, timeout, headers, cookies, proxy_country
webhook_url	string	No	URL to receive change notifications

Diff Modes

Choose how content is compared between snapshots. The diff mode determines what counts as a “change” and how the diff is formatted.

Semantic
Default

JSON

{
  "diff_mode": "semantic"
}

Exact

Byte-level comparison. Any change in the rendered content triggers a diff. Use when you need to detect every change, including formatting and whitespace.

JSON

{
  "diff_mode": "exact"
}

Selector

JSON

{
  "diff_mode": "selector",
  "monitor_selectors": [
    ".product-price",
    ".stock-status",
    "h1.product-title"
  ]
}

Selectors Required

When using diff_mode: selector, you must provide at least one CSS selector in monitor_selectors. Maximum 20 selectors, each up to 500 characters.

Check Intervals

Monitors use standard 5-field cron expressions. The same balance-based limits from the Scheduler apply.

Expression	Schedule
/15 * * *	Every 15 minutes
0 /6 * *	Every 6 hours (default)
0 9 * * *	Every day at 9:00 AM
0 8 * * 1-5	Weekdays at 8:00 AM
0 0 * * 0	Every Sunday at midnight

Minimum interval depends on your account balance. See Balance-Based Limits for details.

Manage Monitors

List Monitors

GET

/api/v1/monitors

Query parameters: limit (1-100, default 20), offset (default 0), active_only (default false).

Bash

curl https://api.alterlab.io/api/v1/monitors?active_only=true \
  -H "Authorization: Bearer your_jwt_token"

Update a Monitor

PATCH

/api/v1/monitors/{monitor_id}

Send only the fields you want to change. Changing check_interval or timezone recomputes next_run_at.

Bash

curl -X PATCH https://api.alterlab.io/api/v1/monitors/{monitor_id} \
  -H "Authorization: Bearer your_jwt_token" \
  -H "Content-Type: application/json" \
  -d '{
    "diff_mode": "selector",
    "monitor_selectors": [".price", ".availability"],
    "check_interval": "0 */2 * * *"
  }'

Pause & Resume

Bash

# Pause a monitor
curl -X POST https://api.alterlab.io/api/v1/monitors/{monitor_id}/pause \
  -H "Authorization: Bearer your_jwt_token"

# Resume (recomputes next_run_at, re-checks balance limits)
curl -X POST https://api.alterlab.io/api/v1/monitors/{monitor_id}/resume \
  -H "Authorization: Bearer your_jwt_token"

Delete a Monitor

DELETE

/api/v1/monitors/{monitor_id}

Soft-deletes the monitor (deactivates it). Returns 204 No Content.

Snapshots

Every check stores a snapshot of the page content. Use snapshots to review historical versions or compare content at different points in time.

GET

/api/v1/monitors/{monitor_id}/snapshots

Returns a paginated list of snapshots, most recent first. Query parameters: limit (1-100), offset.

JSON

{
  "snapshots": [
    {
      "id": "snap-uuid-...",
      "schedule_id": "monitor-uuid-...",
      "run_id": "run-uuid-...",
      "url": "https://competitor.com/pricing",
      "content_hash": "a1b2c3d4...",
      "metadata": { "status_code": 200, "content_length": 42350 },
      "created_at": "2026-03-24T12:00:05Z"
    }
  ],
  "total": 156
}

To retrieve the full content of a specific snapshot:

GET

/api/v1/monitors/{monitor_id}/snapshots/{snapshot_id}

Returns the snapshot with the full content field included.

Changes & Diffs

When a snapshot differs from the previous one, a change record is created. Each change includes the full diff and references both the previous and current snapshots.

GET

/api/v1/monitors/{monitor_id}/diffs

Returns a paginated list of detected changes, most recent first.

JSON

{
  "changes": [
    {
      "id": "change-uuid-...",
      "schedule_id": "monitor-uuid-...",
      "previous_snapshot_id": "snap-old-...",
      "current_snapshot_id": "snap-new-...",
      "url": "https://competitor.com/pricing",
      "change_type": "content_changed",
      "diff": {
        "added": ["Enterprise plan now $299/mo"],
        "removed": ["Enterprise plan now $199/mo"],
        "summary": "Price increase detected on Enterprise plan"
      },
      "notified": true,
      "created_at": "2026-03-24T12:00:06Z"
    }
  ],
  "total": 12
}

To retrieve a specific change:

GET

/api/v1/monitors/{monitor_id}/diffs/{change_id}

Notifications

Configure when your webhook is called using the notify_on parameter:

Value	Behavior
change	Webhook fires only when a diff is detected (default)
always	Webhook fires after every check, even when nothing changed

Tip

Use notify_on: "always" for uptime monitoring. Your webhook receives a call on every check, so you can detect when a page goes down even if the content hasn't changed.

Python Example

Python

import requests

API_URL = "https://api.alterlab.io/api/v1"
HEADERS = {
    "Authorization": "Bearer your_jwt_token",
    "Content-Type": "application/json",
}

# Create a price monitoring watch
monitor = requests.post(
    f"{API_URL}/monitors",
    headers=HEADERS,
    json={
        "name": "Competitor pricing tracker",
        "url": "https://competitor.com/pricing",
        "diff_mode": "selector",
        "monitor_selectors": [".price-card", ".plan-name"],
        "check_interval": "0 */6 * * *",
        "notify_on": "change",
        "webhook_url": "https://your-server.com/price-alerts",
    },
).json()

print(f"Monitor created: {monitor['id']}")
print(f"Next check: {monitor['next_run_at']}")

# List active monitors
monitors = requests.get(
    f"{API_URL}/monitors?active_only=true",
    headers=HEADERS,
).json()

for m in monitors["monitors"]:
    print(f"  {m['name']} — mode: {m['diff_mode']}, runs: {m['run_count']}")

# Check recent changes
changes = requests.get(
    f"{API_URL}/monitors/{monitor['id']}/diffs",
    headers=HEADERS,
).json()

for c in changes["changes"]:
    print(f"  [{c['created_at']}] {c['change_type']}")
    if "summary" in c["diff"]:
        print(f"    Summary: {c['diff']['summary']}")

# Get snapshot history
snapshots = requests.get(
    f"{API_URL}/monitors/{monitor['id']}/snapshots?limit=5",
    headers=HEADERS,
).json()

for s in snapshots["snapshots"]:
    print(f"  Snapshot {s['id'][:8]}... hash={s['content_hash'][:12]}")

Node.js Example

TYPESCRIPT

const API_URL = "https://api.alterlab.io/api/v1";
const headers = {
  Authorization: "Bearer your_jwt_token",
  "Content-Type": "application/json",
};

// Create a monitor with CSS selector tracking
const monitor = await fetch(`${API_URL}/monitors`, {
  method: "POST",
  headers,
  body: JSON.stringify({
    name: "Competitor pricing tracker",
    url: "https://competitor.com/pricing",
    diff_mode: "selector",
    monitor_selectors: [".price-card", ".plan-name"],
    check_interval: "0 */6 * * *",
    notify_on: "change",
    webhook_url: "https://your-server.com/price-alerts",
  }),
}).then((r) => r.json());

console.log(`Monitor: ${monitor.id}, next check: ${monitor.next_run_at}`);

// List active monitors
const list = await fetch(`${API_URL}/monitors?active_only=true`, {
  headers,
}).then((r) => r.json());

for (const m of list.monitors) {
  console.log(`  ${m.name} — ${m.diff_mode}, ${m.run_count} runs`);
}

// Get recent changes
const changes = await fetch(
  `${API_URL}/monitors/${monitor.id}/diffs`,
  { headers }
).then((r) => r.json());

for (const c of changes.changes) {
  console.log(`  [${c.created_at}] ${c.change_type}`);
}

// Browse snapshot history
const snapshots = await fetch(
  `${API_URL}/monitors/${monitor.id}/snapshots?limit=5`,
  { headers }
).then((r) => r.json());

for (const s of snapshots.snapshots) {
  console.log(`  Snapshot ${s.id.slice(0, 8)}... hash=${s.content_hash.slice(0, 12)}`);
}

Scheduler Cloud Storage Export

Last updated: March 2026

Change Detection

How It Works

Create

Snapshot

Compare

Notify

Create a Monitor

Request Body

Diff Modes

Semantic Default

Exact

Selector

Check Intervals

Manage Monitors

List Monitors

Update a Monitor

Pause & Resume

Delete a Monitor

Snapshots

Changes & Diffs

Notifications

Python Example

Node.js Example

Change Detection

How It Works

Create

Snapshot

Compare

Notify

Create a Monitor

Request Body

Diff Modes

Semantic Default

Exact

Selector

Check Intervals

Manage Monitors

List Monitors

Update a Monitor

Pause & Resume

Delete a Monitor

Snapshots

Changes & Diffs

Notifications

Python Example

Node.js Example

Semantic
Default

Semantic
Default