Limits and validation - LithoBlocks

Limits apply to the compiled output, so a short template can exceed them after expanding an array or substituting long text.

Read the current contract

GET /v1/templates/block-schema returns limits, supported block types, directive operators, and interactivity.limits. Use it as the current reference for text lengths, chart dimensions, card actions, carousel size, and metadata size.

Boundary	Current documented limit
Slack message	50 compiled blocks
Modal view	100 compiled blocks
Modal title and submit/close labels	24 characters
Interactive message metadata	3,800 bytes for action configuration and original send context together; space is reserved within Slack’s 4,000-byte total metadata limit for the wrapper and log references
Bulk-send destinations	1–100 per request
Queue-ingest events	Up to 100 per request
Destination timeout	1,000–60,000 ms; default 10,000 ms
Authoring requests	120 per minute per organization; a 429 includes `Retry-After`

The endpoint schema is authoritative for request size, field limits, and pagination. Subscription limits for active queues, modes, and event volume depend on the organization; inspect tier_limit errors and account settings rather than assuming a published plan table is current.

Validate realistic extremes

Read the selected template’s input schema.
Test longest expected text, empty arrays, and maximum expected row counts.
Run POST /v1/templates/validate with the complete blocks and representative sample_data.
Inspect the generated output and measured interactivity metadata.
Send to a test channel and check the actual Slack rendering.

Limit arrays before sending, split large reports, and link to the full source when the message should be a summary. A successful local validation does not establish that every future input will fit. Directives · Tables · Troubleshooting