docs(maintainers): adding block kit types instructions

AGENTS.md

@@ -281,6 +281,78 @@ Defined in `.github/workflows/ci-build.yml`, runs on push to `main`, all PRs, an
 3. Add tests mirroring the module structure
 4. Validate: `./scripts/run_validation.sh`
 
+### Adding a New Block Kit Type
+
+Block Kit models live in `slack_sdk/models/blocks/` across three files:
+
+| File | Contents |
+| --- | --- |
+| `blocks.py` | Layout blocks (`SectionBlock`, `ActionsBlock`, `HeaderBlock`, etc.) |
+| `block_elements.py` | Interactive elements (`ButtonElement`, `StaticSelectElement`, `DatePickerElement`, etc.) |
+| `basic_components.py` | Composition objects (`TextObject`, `Option`, `ConfirmObject`, etc.) |
+
+All types are exported from `slack_sdk/models/blocks/__init__.py`.
+
+#### Base class hierarchy
+
+```
+JsonObject
+├── Block                      → layout blocks
+├── BlockElement               → non-interactive elements
+│   └── InteractiveElement     → elements with action_id
+│       └── InputInteractiveElement → elements usable inside InputBlock
+├── TextObject                 → PlainTextObject, MarkdownTextObject
+├── Option / OptionGroup
+└── ConfirmObject, etc.
+```
+
+Choose the base class that matches the type you're adding.
+
+#### Steps
+
+1. **Define the class** in the appropriate file. Follow this pattern:
+
+   ```python
+   class MyNewBlock(Block):
+       type = "my_new_block"
+
+       @property
+       def attributes(self) -> Set[str]:
+           return super().attributes.union({"text", "optional_field"})
+
+       def __init__(self, *, text: Union[str, dict, TextObject], optional_field: Optional[str] = None, block_id: Optional[str] = None, **others: dict):
+           super().__init__(type=self.type, block_id=block_id)
+           show_unknown_key_warning(self, others)
+           self.text = TextObject.parse(text, default_type=PlainTextObject.type)
+           self.optional_field = optional_field
+
+       @JsonValidator("text must be provided")
+       def _validate_text(self):
+           return self.text is not None
+   ```
+
+   Key conventions:
+   - Set `type` class attribute to the Slack API type string
+   - Override `attributes` to return the set of JSON field names for serialization
+   - Call `super().__init__()` with `type=self.type`
+   - Call `show_unknown_key_warning(self, others)` to log unexpected kwargs
+   - Use `TextObject.parse()`, `ConfirmObject.parse()`, and `BlockElement.parse()` for nested composition objects
+   - Add `@JsonValidator` decorators for constraints (required fields, max lengths, enum values, date formats)
+   - Use `@EnumValidator` for fields restricted to a set of values
+
+2. **Register for deserialization:**
+   - **Elements:** Automatic — `BlockElement.parse()` discovers subclasses at runtime via `__subclasses__()`. No manual step needed.
+   - **Blocks:** Manual — add an `elif` clause in `Block.parse()` (in `blocks.py`) mapping the type string to the new class.
+
+3. **Export the class** — add it to the imports and `__all__` list in `slack_sdk/models/blocks/__init__.py`.
+
+4. **Add tests** in `tests/slack_sdk/models/test_blocks.py`. Cover:
+   - Round-trip: `input_dict == MyNewBlock(**input_dict).to_dict()`
+   - Validation: required fields raise `SlackObjectFormationError` when missing
+   - Constraints: max lengths, enum values, format patterns
+
+5. **Validate:** `./scripts/run_validation.sh`
+
 ### Fixing a Bug
 
 1. Write a test that reproduces the bug