Author a canvas

Dry-run, fix analyzer warnings, commit with the echoed specHash. Preview URL on the web app. This is the first write call in the canonical sequence.

The flow

Write your HTML (use design-system classes, fire both completion calls).
Call canvas_create with dryRun: true.
Check the response for analyzer warnings and errors. Fix any severity: "error" issues.
Open the previewUrl in a browser. Verify the canvas looks right.
Call canvas_create again with dryRun: false and the echoed specHash.
Store the returned canvasId for skill_write.

Shell

# Step 1: dry-run
canvas_create({
name: "Evening reflection",
html: "<!doctype html><html>...</html>",
dryRun: true
})
# Returns: { specHash: "abc123...", previewUrl: "https://...", issues: [...] }
# Step 2: commit with the echoed specHash
canvas_create({
name: "Evening reflection",
html: "<!doctype html><html>...</html>",
dryRun: false,
specHash: "abc123..."
})
# Returns: { canvasId: "757c5bb0-...", previewUrl: "https://..." }

Analyzer warnings

The dry-run response includes an issues array. Common ones:

Missing SproutBridge.postMessage (error): the dual-completion wart. Add the legacy bridge call.
Hardcoded hex color (warning): use design-system tokens instead.
External script reference (error): CSP blocks it. Inline everything.
Canvas too large (error): 1MB limit. Compress images or simplify.

infoAlways read sprout://canvas/sdk before writing canvas HTML. The SDK reference has the latest completion call signatures and signal types.

The flow

Analyzer warnings

Further reading