Validation boundaries

Slackblocks checks the shared model and its declared contextual rules. Successful local construction is not a Slack API receipt: permissions, enabled features, receiving surfaces and write-path rules still apply. The implementations also have different native normalization behavior.

The following rules were reviewed against official Slack references and minimal blocks.validate view probes on 2026-10-13. The sanitized requests and responses are retained in spec/evidence/SB-013-validation-boundary.json. Those probes establish structural validation results, not views.open or message posting results.

Field or combination	Library guarantee	Evidence and boundary
Plain-text input min_length and max_length	Each retains its documented scalar range; when both are supplied, minimum must not exceed maximum. Equality, zero minimum and omitted bounds are supported.	This is an explicit library consistency rule derived from the minimum/maximum meanings. Slack's structural validator accepted the reversed pair 20/10; this stricter library rule prevents an unsatisfiable input.
External select min_query_length, single and multi	An explicitly supplied threshold must be nonnegative; zero is preserved.	Slack's select and multi-select docs describe a character count. The validator accepted zero and rejected minus one for both families.
Static select options and option_groups, single and multi	Each supplied list must contain at least one item; the existing maximum and mutual-exclusion rules still apply.	Empty lists were rejected with min_items: 1; corresponding one-item lists passed. This does not make an external options-response payload subject to static-menu constraints.
Date picker initial_date	The string must have the ASCII YYYY-MM-DD shape.	The date-picker reference specifies this format. The validator rejected a prose date but accepted 2028-02-30. Python's existing calendar parser additionally rejects that impossible date; the other implementations preserve it. Validate calendar/business constraints in the application.
Time picker initial_time	The string must be an ASCII, zero-padded 24-hour time from 00:00 through 23:59.	The time-picker reference specifies HH:mm; the validator rejected 9:00 and 25:99, and accepted 23:59.
Image-element alt_text	The existing maximum remains; an empty string is permitted.	The image reference does not establish a positive minimum. Empty and nonempty controls both passed structural validation.

Invalid ordered bounds and negative query thresholds use out-of-range; malformed dates/times use type-mismatch; empty static lists use length-exceeded. Go, TypeScript, Java, C#, Ruby, Rust, PHP, Elixir and Haskell expose a field path with these categories. Python exposes the corresponding RangeError, TypeMismatchError or LengthError class and a diagnostic message; its existing calendar parser may raise ValueError for an impossible date.

These checks deliberately change previously accepted invalid configurations. Remove an optional field to omit it. Native factories that accept an explicit empty static list now reject it, including the TypeScript legacy factories that previously omitted it. TypeScript .options([]) records an empty list on a fresh builder and is rejected; on a populated builder it appends nothing. Java varargs setters treat zero arguments as no additions and leave an unset field omitted. The shared contract does not infer new calendar-validity or positive image-alt-text minimum rules from an application failure.

Elixir constructors and immutable changes preserve omitted (nil) fields and explicit zero thresholds, reject supplied empty static lists, and validate the same rules on checked JSON decoding and serialization. Its date picker checks the documented shape; application code owns calendar and business constraints.

Haskell factories and checked builders enforce the same rules. Its typed fields retain explicit zero and empty lists for validation; optional Nothing omits a field. decode validates the receiving model after checking raw JSON syntax.