feat: no add-downstream + on assets nothing in the workspace writes

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

frontend/src/lib/components/assets/AssetGraph/AssetGraphCanvas.svelte

@@ -623,6 +623,7 @@
 					error: a.error,
 					dbt: a.dbt,
 					onAddScript: onAddScriptForAsset,
+					neverWritten: a.never_written,
 					pathPrefix,
 					defaultPathSuffix,
 					producers: producersByAsset.get(`${a.kind}:${a.path}`) ?? [],

frontend/src/lib/components/assets/AssetGraph/AssetNode.svelte

@@ -156,6 +156,8 @@
 			foldedFixes?: NodeFix[]
 			/** The assets-only node for the scripts that build no asset. */
 			noAsset?: boolean
+			/** Nothing in the workspace writes it. */
+			neverWritten?: boolean
 			/** Card width in the assets-only view, sized to its text. */
 			width?: number
 			/** Assets-only view: deletes the producing script and trigger no other
@@ -219,8 +221,9 @@
 		}
 	}
 
-	// A red asset needs fixing where its error comes from, not a downstream step.
-	let showAdd = $derived(data.onAddScript != undefined && !data.error)
+	// A red asset needs fixing where its error comes from, not a downstream step;
+	// and a step downstream of what nothing writes would never run on its writes.
+	let showAdd = $derived(data.onAddScript != undefined && !data.error && !data.neverWritten)
 	let menuItems: Item[] = $derived(
 		data.onDeleteUpstream
 			? [

frontend/src/lib/components/assets/AssetGraph/types.ts

@@ -8,6 +8,9 @@ export interface AssetGraphAssetNode {
 	path: string
 	// Filled in client-side: the node is misconfigured (see `pipelineNodeErrors`).
 	error?: string
+	// Filled in client-side: nothing in the workspace writes it, so a step
+	// downstream of it would never run on its writes.
+	never_written?: boolean
 	// Fork workspaces only: 'fork' when this ducklake asset was materialized in
 	// the fork itself, 'deferred' when reads fall back to the parent workspace's
 	// current table via a defer view. Absent outside forks / for other kinds /

frontend/src/routes/(root)/(logged)/pipeline/[folder]/+page.svelte

@@ -1705,8 +1705,24 @@
 	// Assets an asset trigger waits on (explicit `on`, or an auto-triggering read)
 	// that nothing in the pipeline writes, checked against the whole workspace: an
 	// asset trigger fires on a write from anywhere.
+	// Every other asset nothing in the pipeline writes is looked up too, so an
+	// asset nothing writes anywhere can go without the "add downstream step" +.
+	let writtenInPipeline = $derived(
+		new Set(
+			shownGraph.edges
+				.filter((e) => e.access_type === 'w' || e.access_type === 'rw')
+				.map((e) => `${e.asset_kind}:${e.asset_path}`)
+		)
+	)
 	let unwrittenInPipeline = $derived(
-		[...pipelineNodeErrors(shownGraph, explicitOnByPath).unwrittenTriggerAssets]
+		[
+			...new Set([
+				...pipelineNodeErrors(shownGraph, explicitOnByPath).unwrittenTriggerAssets,
+				...shownGraph.assets
+					.filter((a) => !a.dbt && !writtenInPipeline.has(`${a.kind}:${a.path}`))
+					.map((a) => `${a.kind}:${a.path}`)
+			])
+		]
 			.sort()
 			.join('\n')
 	)
@@ -1768,8 +1784,16 @@
 			assets: g.assets
 				.filter((a) => !phantom.has(`${a.kind}:${a.path}`))
 				.map((a) => {
-					const error = errors.assets.get(`${a.kind}:${a.path}`)
-					return error ? { ...a, error } : a
+					const key = `${a.kind}:${a.path}`
+					const error = errors.assets.get(key)
+					// Only once the workspace lookup has answered: before, it is unknown.
+					const never_written =
+						!a.dbt &&
+						!writtenInPipeline.has(key) &&
+						!!writtenElsewhere.current &&
+						!writtenElsewhere.loading &&
+						!writtenElsewhere.current.has(key)
+					return error || never_written ? { ...a, error, never_written } : a
 				}),
 			runnables: g.runnables.map((r) => {
 				const draft = showDrafts ? pe.drafts.get(r.path)?.script : undefined