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

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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.
Gzip/base64 compression and metadata_transport diagnostics are an unreleased prototype. Production retains the 3,800-byte event-payload budget until live Slack acceptance and deployment.
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.

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Validate realistic extremes

  1. Read the selected template’s input schema.
  2. Test longest expected text, empty arrays, and maximum expected row counts.
  3. Run POST /v1/templates/validate with the complete blocks and representative sample_data.
  4. Inspect the generated output and measured interactivity metadata.
  5. 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 Interactive metadata that already fits stays in its existing format. Previously sent messages remain readable. Compression does not raise Slack text or block limits, and incompressible data can still exceed the metadata budget. validate reports raw interactivity.metadata_bytes and metadata_transport diagnostics: encoding, actual wire bytes, and bytes with both log references reserved. Sample-data estimates can differ from an actual send; the sending runtime checks the final metadata before debit or dispatch.