Canvas Integration¶

Prerequisites

This guide assumes you've completed the Quick Start and have a working summon config check.

Every summon-claude session gets a persistent markdown canvas — a dedicated tab in its Slack channel that Claude can read and write. Unlike the message thread (which can be compacted away), the canvas survives conversation history changes and gives Claude a place to store structured notes, status, and work output.

How canvases work¶

When a session authenticates to a Slack channel, summon-claude creates (or finds) a canvas for that channel. The canvas lives as a tab in the channel, visible to anyone in the workspace.

Canvas tab in channel header

All canvas state is stored locally in SQLite first. A background worker syncs the local state to Slack on a 2-second dirty delay, then again on a 60-second heartbeat interval. This means:

Claude's canvas writes are instant locally
Slack reflects changes within a few seconds
If Slack is temporarily unreachable, changes queue and sync when connectivity returns

Free plan limitation

Slack free plans allow one canvas per channel. summon-claude respects this — if a canvas already exists for a channel, it reuses it rather than trying to create a second one.

Canvas templates¶

summon-claude ships templates for each agent profile. The template chosen depends on what kind of session is starting.

Standard agent canvas¶

Used for regular summon start sessions:

Session Info — name, model, start time
Current Task — what Claude is working on right now
Notes — free-form markdown for Claude to accumulate context

PM agent canvas¶

Used for PM sessions (started via summon project up):

Project — project name and description
Active Work — table of current child sessions with status and task
Completed Work — summary of finished sessions
Notes — PM-level notes and decisions

PM canvas with session status and scheduled jobs

Global PM canvas¶

Used for a top-level PM coordinating multiple projects.

Scribe canvas¶

Used for the scribe monitoring agent:

Recent Signals — items from the latest scan
Active Items — ongoing events and threads
Suppressed — items seen but filtered below importance threshold

MCP tools¶

Claude can interact with canvases through three MCP tools. These tools are available in all sessions that have a canvas — not just PM sessions.

`summon_canvas_read`¶

Read the current canvas markdown:

summon_canvas_read()
# Returns the full canvas markdown for the current session's channel

summon_canvas_read(channel="C01234567")
# Read another session's canvas (cross-channel access)

Cross-channel access: Claude can read canvases from other sessions in the same workspace, subject to a scope guard. The scope guard verifies that the requesting user authenticated both sessions (via authenticated_user_id) — Claude cannot read canvases belonging to other users' sessions.

`summon_canvas_write`¶

Replace the entire canvas with new markdown:

summon_canvas_write(markdown="# My Notes\n\nUpdated content here...")

Limits:

Maximum 100,000 characters
Whitespace-only content is rejected
Replaces the full canvas body — use summon_canvas_update_section for partial updates

`summon_canvas_update_section`¶

Update a single named section within the canvas:

summon_canvas_update_section(
  heading="Current Task",
  content="Refactoring the auth middleware — see PR #142"
)

The heading parameter is the section heading text (without # characters). summon-claude finds the section with that heading and replaces its content. If the section does not exist, it is created as a new ## section at the end of the canvas.

Limits:

Maximum 100,000 characters for the full canvas after update
The heading parameter must not be empty or consist only of # characters

This is the preferred tool for structured updates — the PM agent uses it to update the Active Work table without touching other sections of the canvas.

Sync behavior¶

Event	When it happens
Write via MCP tool	Stored in SQLite immediately
First Slack sync	Within 2 seconds of the write (dirty delay)
Subsequent syncs	Every 60 seconds (heartbeat)
Sync failure	Backs off to 300-second intervals after 3 consecutive failures
Sync recovery	Resets to 60-second interval on next success

If sync fails repeatedly, summon-claude logs an error. The local canvas state is never lost — it syncs as soon as connectivity is restored.

Viewing the canvas¶

The canvas is always visible as a tab in the Slack channel. You can also ask Claude to read and summarize it at any time:

What's on our canvas right now?

Or read it directly in Slack by clicking the canvas tab in the channel header.