# Web API calls, pagination and error handling

*Slack Developer Study Guide · Design Your Workflows: Spotlight on Slack's APIs · Last updated October 21, 2026 · https://www.certifyforce.com/study-guide/slack-developer/design-your-workflows-spotlight-on-slacks-apis/web-api-calls-pagination-and-error-handling*

How to call Slack Web API methods correctly: RPC-style method URLs, token placement, JSON body rules, reading ok and error instead of the HTTP status, cursor pagination that stops only on an empty next_cursor, conversation types and scopes, and fixing common chat.postMessage errors.

- Certification: Salesforce Slack Developer
- Exam section: Design Your Workflows: Spotlight on Slack's APIs
- Study guide: https://www.certifyforce.com/study-guide/slack-developer

**Official exam objective:**

> Implement the Web API in your app.

## What the exam expects

- Address a Web API method by its RPC-style URL and send its arguments in a format Slack accepts.
- Apply the JSON body rules: explicit `Content-type: application/json`, bearer token in the `Authorization` header, one argument style per request, an object rather than an array.
- Detect failure from the `ok` boolean and the `error` code, not from the HTTP status, and tell errors from warnings.
- Write a cursor pagination loop that ends only when `next_cursor` is empty, null or missing.
- Get the right conversations from `conversations.list` with the `types` argument and matching scopes, and fix `not_in_channel` and missing notification text on `chat.postMessage`.

Key terms: **method** (a Web API operation such as `conversations.create`), **bearer token** (the token sent as `Authorization: Bearer xoxb-...`), **`ok`/`error`** (the success flag and machine-readable failure code in every response), **cursor** (the `next_cursor` pointer to the next page), **conversation types** (`public_channel`, `private_channel`, `mpim`, `im`).

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Method URL | `https://slack.com/api/METHOD_FAMILY.method`, not REST | [Slack Web API](https://docs.slack.dev/apis/web-api/) |
| Token placement | `Authorization` header preferred; never a query parameter | [Slack Web API](https://docs.slack.dev/apis/web-api/) |
| JSON body token | Must be a bearer token in the header, not a JSON attribute | [Slack Web API](https://docs.slack.dev/apis/web-api/) |
| Array body | Returns `json_not_object` | [Slack Web API](https://docs.slack.dev/apis/web-api/) |
| Success check | Top-level `ok`; `error` on failure; `warning` with `ok: true` | [Slack Web API](https://docs.slack.dev/apis/web-api/) |
| End of pages | `next_cursor` empty, null or missing | [Pagination](https://docs.slack.dev/apis/web-api/pagination) |
| Page size | Recommend 100-200; maximum 1000, may vary per method | [Pagination](https://docs.slack.dev/apis/web-api/pagination) |
| `conversations.list` default | `types` defaults to `public_channel` | [conversations.list method](https://docs.slack.dev/reference/methods/conversations.list) |
| Private channels | Need `groups:read` | [Using the Conversations API](https://docs.slack.dev/apis/web-api/using-the-conversations-api) |
| Posting without joining | `chat:write.public` for public channels | [chat.postMessage method](https://docs.slack.dev/reference/methods/chat.postMessage) |
| `text` with blocks | Fallback for notifications and screen readers | [chat.postMessage method](https://docs.slack.dev/reference/methods/chat.postMessage) |

## Check yourself

- Why does a Web API call that returns HTTP 200 still need inspection?
- When is a cursor-paginated collection finished?
- Which error does a JSON array body return?
- Why might `conversations.list` return only public channels for a bot invited to private ones?
- When does a bot get `not_in_channel`, and what are the two fixes?

## Sources

- [Slack Web API](https://docs.slack.dev/apis/web-api/)
- [Pagination](https://docs.slack.dev/apis/web-api/pagination)
- [chat.postMessage method](https://docs.slack.dev/reference/methods/chat.postMessage)
- [Using the Conversations API](https://docs.slack.dev/apis/web-api/using-the-conversations-api)
- [conversations.list method](https://docs.slack.dev/reference/methods/conversations.list)
- [conversations.history method](https://docs.slack.dev/reference/methods/conversations.history)
- [Installing via OAuth authorization code flow](https://docs.slack.dev/authentication/installing-with-oauth)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 8 linked practice questions, and unlimited timed mock exams are included with the Slack Developer pack (USD $19): https://www.certifyforce.com/practice-exams