Sinch Conversation API

Overview

One unified API to send and receive messages across SMS, WhatsApp, RCS, MMS, Viber Business, Facebook Messenger, Instagram, Telegram, KakaoTalk, KakaoTalkChat, LINE, WeChat, and Apple Messages for Business. The API transcodes between a generic message format and channel-specific formats automatically.

Auth: OAuth2 bearer token (recommended) or HTTP Basic (keyId:keySecret, testing only — heavily rate limited). See sinch-authentication for setup.

Key Concepts

Apps — Container for channel integrations. Each app has channels, webhooks, and a processing mode. Created via dashboard or API. Contacts — End-users with channel identities. Auto-created in CONVERSATION mode. Conversations — Message threads between app and contact. Only exist in CONVERSATION mode. Processing modes — DISPATCH (default): no contacts/conversations, for high-volume unidirectional SMS. CONVERSATION: auto-creates contacts/conversations, enables 2-way flows. Set per app. Message types — text_message, media_message, card_message, carousel_message, choice_message, list_message, template_message, location_message, contact_info_message. See Message Types. Channel fallback — Set channel_priority_order to try channels in sequence. SWITCHING_CHANNEL status indicates fallback. Delivery statuses — QUEUED_ON_CHANNEL → DELIVERED → READ, or FAILED. SWITCHING_CHANNEL when fallback occurs. Webhooks — Up to 5 per app. Default callback rate: 25/sec. 21 usable triggers — most common: MESSAGE_INBOUND, MESSAGE_DELIVERY, EVENT_INBOUND. See Callbacks & Webhooks for full trigger list. HMAC validation — Signature: HMAC-SHA256(rawBody + '.' + nonce + '.' + timestamp, secret). Headers: x-sinch-webhook-signature, -timestamp, -nonce, -algorithm. Templates — Pre-defined messages with parameter substitution. Managed at {region}.template.api.sinch.com (V2 only — V1 no longer accessible). See references/templates.md. Batch sending — Up to 1000 recipients with ${parameter} substitution. Base URL: {region}.conversationbatch.api.sinch.com. See references/batch.md.

Supported channels: SMS, WHATSAPP, RCS, MMS, VIBERBM, MESSENGER, INSTAGRAM, TELEGRAM, KAKAOTALK, KAKAOTALKCHAT, LINE, WECHAT, APPLEBC. Channel-specific details: SMS, WhatsApp, RCS, MMS. See Channel Support.

Getting Started

Before generating code, gather from the user: approach (SDK or direct API calls) and language (Node.js, Python, Java, .NET/C#, curl). Do not assume defaults.

When the user chooses SDK, fetch the relevant SDK reference page linked in Quick Reference for accurate method signatures. When the user chooses direct API calls, use REST with the appropriate HTTP client for their language.

Language	Package	Install
Node.js	@sinch/sdk-core	npm install @sinch/sdk-core
Java	com.sinch.sdk:sinch-sdk-java	Maven dependency (see below)
Python	sinch	pip install sinch
.NET	Sinch	dotnet add package Sinch

Java Maven dependency

Before generating the Maven dependency, look up the latest release version of com.sinch.sdk:sinch-sdk-java on Maven Central and use that version.

<dependency>
    <groupId>com.sinch.sdk</groupId>
    <artifactId>sinch-sdk-java</artifactId>
    <version>LATEST_VERSION</version>
</dependency>

Base URL

Regional — must match the Conversation API app region:

• US: https://us.conversation.api.sinch.com
• EU: https://eu.conversation.api.sinch.com
• BR: https://br.conversation.api.sinch.com

Send a Text Message (curl)

curl -X POST "https://us.conversation.api.sinch.com/v1/projects/{PROJECT_ID}/messages:send" \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer {ACCESS_TOKEN}" \
  -d '{
    "app_id": "YOUR_APP_ID",
    "recipient": {
      "identified_by": {
        "channel_identities": [{
          "channel": "SMS",
          "identity": "+15551234567"
        }]
      }
    },
    "message": {
      "text_message": {
        "text": "Hello from Sinch Conversation API!"
      }
    },
    "channel_properties": {
      "SMS_SENDER": "+15559876543"
    }
  }'

For SDK examples, see the SDK references in Quick Reference.

Common Patterns

• Channel fallback — Add channel_priority_order array and list all channel identities in recipient. See Messages API Reference.
• Recipient by contact ID — Use { "recipient": { "contact_id": "CONTACT_ID" } } instead of identified_by when the contact already exists.
• Rich messages — card_message, carousel_message, choice_message, list_message. See Message Types.
• WhatsApp templates — Required outside the 24h service window. Use template_message with an approved template. See references/templates.md.
• Webhooks — Register via POST /webhooks with target, target_type: "HTTP", and triggers array. Each webhook target URL must be unique per app — attempting to register a duplicate target returns 400 INVALID_ARGUMENT. See Webhooks API Reference.
• Transcode — POST /messages:transcode to preview how a message renders on a specific channel without actually sending it. Useful for testing rich messages.

Common Tasks

• Send a message — See Messages API Reference
• List messages — GET /v1/projects/{project_id}/messages (filter by messages_source)
• Events — POST /events:send for typing indicators, composing events
• Capability lookup — POST /capability:query (async; result via CAPABILITY webhook)
• Manage apps — See App API Reference
• Manage contacts — See Contact API Reference. Includes merge, getChannelProfile, identityConflicts.
• Manage conversations — See Conversation API Reference. Includes recent, stop, inject-message/event.
• Manage webhooks — See Webhooks API Reference
• Templates — See Template Management API and references/templates.md
• Batch send — See Batch API and references/batch.md

Executable Scripts

Bundled Node.js scripts in scripts/ for sending messages (SMS, RCS text/card/carousel/choice/media/location/template), listing messages, and webhook CRUD. All read credentials from environment variables and support --help.

export SINCH_PROJECT_ID="your-project-id"
export SINCH_KEY_ID="your-key-id"
export SINCH_KEY_SECRET="your-key-secret"
export SINCH_APP_ID="your-app-id"
export SINCH_REGION="us"  # us|eu|br, default: us

Examples:

• node scripts/sms/send_sms.cjs --to +15551234567 --message "Hello"
• node scripts/rcs/send_card.cjs --to +15551234567 --title "Sale" --image-url URL
• node scripts/webhooks/create_webhook.cjs --app-id APP_ID --target URL --triggers MESSAGE_INBOUND,MESSAGE_DELIVERY
• node scripts/common/list_messages.cjs --channel SMS --page-size 20

SDK & Code Generation

For SDK code examples, refer to the official SDK syntax references:

• Node.js SDK
• Python SDK
• Java SDK
• .NET SDK

For REST API payloads and field definitions, see:

• Messages API Reference
• Webhooks API Reference
• OpenAPI Spec (YAML)

Webhook trigger payloads: See references/webhooks/triggers/ for payload structure and key fields for all 21 trigger types.

Gotchas and Best Practices

• Use OAuth2 in production. Cache tokens (expire in ~1 hour). Never use Basic Auth in production.
• Rich messages transcoded to text on unsupported channels — test across target channels.
• Implement idempotent webhook handlers — Sinch retries with exponential backoff.
• Load credentials from environment variables. Never hardcode.
• Always set the region explicitly in the client configuration. SDK clients must configure the region — do not rely on SDK defaults. Region mismatches between the client and the app cause 404 errors. All Conversation API URLs are region-specific ({region}.conversation.api.sinch.com). If you get a 404, the most likely cause is a region mismatch — verify the app's region in the Sinch dashboard and configure the SDK/URL accordingly.
• SDK naming varies across languages. Do not assume that method names, property paths, or service accessors are the same between SDKs (e.g., Python and Node.js may use different singular/plural forms for the same service). Always fetch the SDK reference docs for the target language before generating code — see the links in Quick Reference.
• Error codes: 400 malformed or duplicate resource (e.g., webhook with same target already exists), 401 bad credentials, 403 no access/billing limit, 404 not found/region mismatch, 429 rate limit, 500/501/503 retry with backoff.
• Messages not delivered: Verify app region matches base URL region (mismatches cause 404). Check delivery status via webhook or GET /messages/{message_id}. WhatsApp: must be within 24h window or using an approved template. Channel fallback: SWITCHING_CHANNEL status means fallback occurred — each attempted channel may incur charges.
• Webhook not receiving callbacks: Verify target_type is HTTP, target URL must be publicly reachable and return 2xx, check triggers are correct — max 5 webhooks per app.
• Rate limits (429): 800 requests/second per project across most endpoints. 500,000-message ingress queue per app, drained at 20 msg/sec by default. Channel-specific limits also apply.

Links

• Authentication setup
• Getting Started Guide
• Conversation API Reference
• Message Types
• Channel Support
• Callbacks & Webhooks
• Processing Modes
• Node.js SDK
• Python SDK
• Java SDK
• .NET SDK
• OpenAPI Spec (YAML)
• LLMs.txt (full docs index)