Pencil

Pencil Design Tool

Pencil is a vector design tool that lives in your IDE and exposes a full MCP API for AI-driven design generation and modification. Designs are stored in .pen files (encrypted — only readable via MCP tools, never via Read/Grep).

Two ways to work with Pencil

1. MCP tools (live, interactive — use when a .pen file is open in the IDE)

Full read/write access to the active canvas. Prefer this for iterative design sessions.

2. CLI (pencil) (headless — use for batch/CI/file generation without the IDE)

pencil --out design.pen --prompt "Create a login page"   # generate new
pencil --in existing.pen --out modified.pen --prompt "Add sidebar"
pencil --export --in design.pen --out screenshot.png      # export PNG/JPEG/WEBP/PDF
pencil --tasks batch.json                                  # batch processing
pencil interactive -o output.pen                          # interactive MCP shell

Available models: claude-opus-4-6 (default), claude-sonnet-4-6, claude-haiku-4-5

Auth: pencil login (interactive) or PENCIL_CLI_KEY env var for CI.

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MCP Tool Reference

Rule: Never use Read/Grep/Glob on .pen files. Always use these MCP tools.

Orientation (start here)

get_editor_state()          → active file, selected nodes, canvas state
open_document("new")        → create blank .pen file
open_document("/path/to/file.pen")  → open existing file

Reading designs

batch_get(patterns, nodeIds)
  patterns  – text search across node properties (array of strings)
  nodeIds   – read specific nodes by ID (array of strings)

Making changes

batch_design(operations)    → one call, up to ~25 operations

Operation syntax (each line = one op):

foo = I("parentId", { type, x, y, width, height, ... })   # Insert
bar = C("sourceId", "parentId", { ...overrides })         # Copy/instance
     R("nodeId", { ...newProps })                          # Replace
     U("nodeId" or foo+"/childId", { ...props })          # Update
     D("nodeId")                                           # Delete

Design utilities

get_guidelines(category?, name?, params?)  → load style guides, design system rules
get_screenshot()                           → visual snapshot of current canvas
snapshot_layout()                          → layout tree with positions/sizes
find_empty_space_on_canvas()               → find room to place new nodes
search_all_unique_properties(prop)         → scan all unique values of a property
replace_all_matching_properties(prop, from, to)  → global find/replace on a property
get_variables()                            → read design tokens / theme variables
set_variables(vars)                        → update design tokens / theme variables
export_nodes(nodeIds, format)             → export specific nodes as PNG/SVG/etc

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.pen File Format (key concepts)

Every node requires:

• id — unique string (no forward slashes)
• type — frame, rectangle, ellipse, text, path, ref (component instance), group
• x, y — position (top-level nodes)

Layout uses flexbox properties: layout (none/vertical/horizontal), gap, padding, justifyContent, alignItems

Components: set reusable: true on a node to make it a component. Use type: "ref" to place instances.

Variables: document-wide design tokens that support themes (light/dark). Read/write via get_variables() / set_variables().

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Workflow patterns

Explore then modify

1. get_editor_state()              → understand what's open
2. batch_get(["Button", "Card"])   → find relevant nodes
3. get_screenshot()                → visual check
4. batch_design(...)               → make changes
5. get_screenshot()                → verify result

Generate from scratch

1. get_guidelines()                → load design system rules first
2. find_empty_space_on_canvas()    → where to place it
3. batch_design(...)               → insert nodes in one call

Bulk property changes

search_all_unique_properties("fill")                    → see all colors in use
replace_all_matching_properties("fill", "#FF0000", "#E53E3E")  → swap brand color

Design tokens / theming

get_variables()                    → read all tokens
set_variables({ "color-primary": "#6366F1" })          → update token

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CLI batch.json format

[
  { "out": "login.pen", "prompt": "Login page with email + password" },
  { "in": "base.pen", "out": "dark.pen", "prompt": "Apply dark theme" }
]

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Tips

• Call get_guidelines() before generating complex layouts — it loads design system constraints
• batch_design accepts up to ~25 operations per call; split large changes across multiple calls
• Use snapshot_layout() after inserting nodes to verify positions before taking a screenshot
• For CI pipelines, set PENCIL_CLI_KEY (from Pencil web app → Developer Keys) instead of pencil login
• The .pen format is JSON under the hood but encrypted — never try to read it directly