Pattern Critic

Systematically review pattern code for violations of Common Tools documentation rules and gotchas.

Workflow

• Read the pattern file to review

• Check each category below against the code

• Output results in the checklist format (see Output Format)

• For any [FAIL], include the line number and fix

Violation Categories

1. Module Scope Violations

Check that these are NOT inside the pattern body:

Violation Fix

handler() defined inside pattern Move to module scope, or use action() instead

lift() immediately invoked (lift(...)(args) ) Use computed() or define lift at module scope

Helper functions defined inside pattern Move to module scope

Allowed inside patterns: computed() , action() , .map() callbacks, JSX event handlers.

2. Reactivity Violations

Violation Fix

NAME: computed(() => someProp)

text ${someProp}`` NAME: computed(() =>
text ${someProp})

Writable.of(reactiveValue)

Initialize empty, set in handler/action

.get() on computed/lift result Access directly (only Writable has .get())

items.filter(...) inline in JSX Wrap in computed() outside JSX

items.sort(...) inline in JSX Wrap in computed() outside JSX

Nested computed with outer scope vars Pre-compute with lift or outer computed

lift() closing over reactive deps Pass deps as explicit params

Cells from composed patterns in ifElse Wrap in local computed()

3. Conditional Rendering

Violation Fix

onClick inside computed()

Move button outside, use disabled attr

Note: Ternaries work fine in JSX - the transformer auto-converts them to ifElse() . Both {show ? <Element /> : null} and {ifElse(show, ...)} are valid.

4. Type System

Violation Fix

Array without Default<T[], []>

Add default to prevent undefined

Missing Writable<> for .set() /.push()

Add Writable<T> to input type

Map or Set in cell data Use plain objects/arrays (serialization)

Custom id property for identity Use equals() function instead

5. Binding

Violation Fix

checked={item.done}

$checked={item.done} (add $ prefix)

value={title}

$value={title} (add $ prefix)

$checked={item} (whole item) $checked={item.done} (bind property)

Wrong event name Use onct-send , onct-input , onct-change

6. Style Syntax

Element Type Required Syntax

HTML (div , span ) Object: style={{ backgroundColor: "#fff" }}

Custom (ct-* ) String: style="background-color: #fff;"

Violation Fix

String style on HTML Convert to object syntax

Object style on ct-* Convert to string syntax

kebab-case props on ct-* Use camelCase: allowCustom not allow-custom

7. Handler Binding

Violation Fix

onClick={addItem({ title: "x", items })}

Event data comes at runtime, bind state only

Creating handlers inside .map()

Create handler once at module/pattern scope

8. Stream/Async

Violation Fix

Stream.of()

Doesn't exist. Bound handler IS the stream

.subscribe() on stream Doesn't exist. Return stream from pattern

async/await in handlers Use fetchData() (blocks UI otherwise)

await generateText(...)

Reactive, not a promise. Use .result

await generateObject(...)

Reactive, not a promise. Use .result

9. LLM Integration

Violation Fix

Array as root schema for generateObject Wrap in object: { items: T[] }

Missing /// <cts-enable /> directive Add at top of file

Prompt derived from agent-written cells Causes infinite loop. Use separate cells

Invalid model name format Use vendor:model (e.g., anthropic:claude-sonnet-4-5 )

10. Performance

Violation Fix

Handler created per-item in .map()

Create handler once, bind with item

Expensive computation inside loop Pre-compute outside, reference result

11. Action vs Handler Choice

Key Principle: Prefer action() by default. Use handler() only when you need to bind different data to different handler instantiations.

Violation Fix

handler() at module scope not used in .map() or multi-binding scenario Convert to action() inside pattern body

handler() when all instantiations use same data Convert to action()

action() inside .map() creating new action per item Use handler() at module scope with binding

When to use action() (the default):

• Handler is specific to this pattern

• Closes over pattern-scope variables (inputs, Writables, computeds, wish results)

• All instantiations use the same closed-over data

• Examples: button clicks, form submissions, modal open/close, state toggles

When to use handler() :

• You need different data bound per instantiation

• Common scenarios:

• .map() loops where each item needs its own binding

• Reusable handlers shared across multiple patterns

• Same handler definition called with different bindings in different places

• Examples: list item clicks, per-row actions in tables, delete buttons per item

Decision Question: Does this handler need different data bound to different instantiations?

• YES → Use handler() at module scope, bind with item-specific data

• NO → Use action() inside pattern body, close over what you need

12. Design Review

Check the domain model quality:

Check What to look for

Clear entity boundaries Each pattern represents one concept (Card, Column, Board)

Actions match user intent Handler names reflect what user wants (addCard, moveCard, removeCard)

Unidirectional data flow Parent owns state, children receive props

Normalized state No duplicate data, single source of truth

Self-documenting types Type names and fields are clear without comments

Appropriate granularity Not too fine (trivial patterns) or too coarse (god patterns)

13. Regression Check (for updates only)

When reviewing changes to existing code:

Check What to verify

Tests still pass Run existing tests after changes

Type signatures preserved Or intentionally changed with migration path

Handlers still work Existing functionality not broken

No unintended side effects Changes scoped to intended area

Output Format

Pattern Review: [filename]

1. Module Scope

• [PASS] No handler() inside pattern
• [FAIL] lift() immediately invoked (line 23) Fix: Use computed() or move lift to module scope

2. Reactivity

• [PASS] NAME properly wrapped
• [FAIL] Writable.of(deck.name) uses reactive value (line 15) Fix: Initialize empty, set in action()

3. Conditional Rendering

• [PASS] Using ifElse() correctly
• [N/A] No conditional rendering

[...continue for all categories...]

11. Action vs Handler Choice

• [PASS] Actions used for pattern-specific handlers
• [FAIL] handler() used but not needed for multi-binding (line 45) Fix: Convert to action() inside pattern body

12. Design Review

• [PASS] Clear entity boundaries
• [WARN] Handler names could be clearer (moveCard vs reorderCard)
• [PASS] Unidirectional data flow

13. Regression Check (if updating)

• [PASS] Existing tests pass
• [N/A] No type signature changes

Summary

• Passed: 22
• Failed: 3
• Warnings: 1
• N/A: 2

Priority Fixes

1. [Line 15] Writable.of() with reactive value
2. [Line 23] lift() inside pattern
3. [Line 45] Missing $ prefix on binding

Documentation References

• docs/development/debugging/README.md

• Error reference table

• docs/development/debugging/gotchas/

• Individual gotcha files

• docs/common/components/COMPONENTS.md

• UI components and binding

• docs/common/capabilities/llm.md

• LLM integration

Quick Patterns

Correct action() Usage (Default Choice)

// action() inside pattern body - closes over pattern variables export default pattern<MyInput, MyOutput>(({ items, title }) => { const menuOpen = Writable.of(false);

// Action closes over menuOpen - no binding needed const toggleMenu = action(() => menuOpen.set(!menuOpen.get()));

// Action closes over items - no binding needed const addItem = action(() => items.push({ title: title.get() }));

return { [UI]: ( <> <ct-button onClick={toggleMenu}>Menu</ct-button> <ct-button onClick={addItem}>Add</ct-button> </> ), items, }; });

Correct handler() Usage (Only for Multi-Binding)

// handler() at module scope - will be bound with different items in .map() const deleteItem = handler<void, { item: Writable<Item>; items: Writable<Item[]> }>( (_, { item, items }) => { const list = items.get(); items.set(list.filter(i => i !== item)); } );

export default pattern<MyInput, MyOutput>(({ items }) => ({ [UI]: ( <ul> {items.map((item) => ( <li> {item.name} {/* Each item gets its own binding */} <ct-button onClick={deleteItem({ item, items })}>Delete</ct-button> </li> ))} </ul> ), items, }));

Correct Reactive NAME

export default pattern<Input>(({ deck }) => ({ NAME: computed(() => Study: ${deck.name}), // ... }));

Correct Conditional Rendering

// Both are valid - ternaries auto-transform to ifElse() {showDetails ? <div>Details content</div> : null} {ifElse(showDetails, <div>Details content</div>, null)}

Correct Style Syntax

<div style={{ display: "flex", gap: "1rem" }}> <ct-vstack style="flex: 1; padding: 1rem;"> Content </ct-vstack> </div>