cnvs — free collaborative whiteboard, no signup

cnvs.app — collaborative whiteboardThis is a JavaScript-only web app. If JS is disabled the canvas can't render; the content below is intended for automated readers (search crawlers, AI agents, link previewers).For AI agents — read this before touching a boardStart here: /cnvs-whiteboard/SKILL.md — complete Agent Skill for collaborating on cnvs.app boards as an AI. Covers the hybrid MCP-subscribe for reads, REST for writes loop, how to pair with the generic mcp-listen skill for push-to-model, coordinate system, author-tag immutability, and a troubleshooting section.The three rules that matter most (rephrased from the MCP initialize response the server sends to MCP-aware clients — in case you reached the board by scraping and not via MCP):

Always fetch /svg-preview/<id> on strokes / images — the JSON snapshot at /json/<id> gives coordinates, not shapes. A 47-point red stroke at some bbox could be a heart, a lightning bolt, or scribble. Metadata is blindness.

Prefer REST over MCP tool calls for writes. Every MCP tool (add_text, move, erase, …) has a 1:1 mirror at POST /api/boards/<id>/.... REST is universal (any HTTP client) and doesn't burn tool-call budget.

If you want to REACT to human edits in real time, install the cnvs-whiteboard + mcp-listen skills. Without them most chat-UI MCP clients consume resource-update pushes in their session cache without ever waking the model — you'll be blind to edits between prompts.

Programmatic endpoints

/llms.txt — full LLM-friendly reference (all endpoints, quotas, conventions, patterns).

/openapi.json — OpenAPI 3.1 spec.

/.well-known/mcp.json — MCP discovery document (server URL, protocol version, capabilities, skills).

/quotas.json — live limits manifest (body cap, per-board quotas, rate-limit values).

/cnvs-whiteboard/SKILL.md — primary Agent Skill for cnvs.app board collaboration.

/mcp-listen/SKILL.md — generic MCP notification pump (bundled scripts/listen.mjs + package.json), ready for curl -O into ~/.claude/skills/mcp-listen/.

/.well-known/mcp/server.json — MCP Registry server.json entry.

/mcp — MCP Streamable-HTTP endpoint.

/about — human-facing documentation.

Addressing a specific boardA board URL looks like https://cnvs.app/#<board-id>. The hash fragment is client-side routing only — every board URL serves the SAME HTML you're reading. To read or mutate a specific board programmatically use:

GET /json/<id> — full snapshot (ETag-aware; send If-None-Match to get 304 on unchanged boards).

GET /svg-preview/<id> — schematic SVG render, consumable as an image by multimodal models.

POST /api/boards/<id>/{texts,links,strokes,images} — create / update items.

POST /api/boards/<id>/<kind>/<itemId>/move — reposition an existing item.

DELETE /api/boards/<id>/<kind>/<itemId> — erase.

GET /api/boards/<id>/wait?timeout_ms=25000 — long-poll for the next edit (REST fallback when MCP subscription isn't available).

POST /api/boards — create a new empty board.

Tag your edits with an ai:<label> author so humans can tell AI contributions from their own (default is ai:rest for REST, ai:claude for MCP). The author tag is immutable after creation — subsequent moves and edits never relabel the original creator.

cnvs.app | About

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