feat(vom): two channels to deal with canvas

packages/dsh-plugin-browserskill/src/tools.ts

@@ -489,8 +489,9 @@ function defineBrowserOperations(deps: ToolDeps, register: DefinitionRegistrar):
       ? "Capture an indented aria-tree snapshot of the session's active tab. Interactive elements " +
         "carry @eN refs for browser_interact actions. Prefer browser_inspect action=observe for a richer semantic view."
       : "Produce a semantic VOM observation of the session's active tab (roles, states, perception " +
-        "probes) with @eN refs for browser_interact actions. Visual surface refs are screenshot-only; " +
-        "DOM refs support interaction. Read-only: never submits input.";
+        "probes) with @eN refs for browser_interact actions. Visual-only surfaces require image " +
+        "understanding and their refs are screenshot-only; keep using any DOM/AX semantics rendered " +
+        "alongside them. Read-only: never submits input.";
     register(
       defineTool({
         name,
@@ -755,8 +756,11 @@ function defineBrowserOperations(deps: ToolDeps, register: DefinitionRegistrar):
       name: "inspect.screenshot",
       description:
         "Capture a PNG screenshot of the session's active tab, or crop to a snapshot ref element. " +
-        "This is the supported operation for visual surface refs emitted by browser_observe. Returns " +
-        "the image itself when the deployment supports image input, otherwise a file path.",
+        "This is the supported operation for visual surface refs emitted by inspect.observe. Only use " +
+        "it to interpret a visual-only surface when you can inspect image output. If you lack multimodal " +
+        "or image-reading capability, tell the user to switch to a model with image understanding; do " +
+        "not guess content or coordinates. " +
+        "Returns the image itself when the deployment supports image input, otherwise a file path.",
       parameters: {
         session: SESSION_PARAM,
         ref: {

packages/vom/src/__tests__/render.test.ts

@@ -48,7 +48,7 @@ describe("renderVom single-layer page", () => {
     const out = renderVom(input);
 
     expect(out.text).toContain(
-      '@e1 surface "Data grid" [rendering=canvas; layers=2; visual-only; use: bsk screenshot --ref @e1]',
+      '@e1 surface "Data grid" [rendering=canvas; layers=2; visual-only; requires=image-understanding; use: bsk screenshot --ref @e1]',
     );
     expect(out.refs).toContainEqual(
       expect.objectContaining({
@@ -77,7 +77,7 @@ describe("renderVom single-layer page", () => {
     const out = renderVom(input);
 
     expect(out.text).toContain(
-      '@e1 surface "canvas visual surface" [rendering=canvas; visual-only; use: bsk screenshot --ref @e1]',
+      '@e1 surface "canvas visual surface" [rendering=canvas; visual-only; requires=image-understanding; use: bsk screenshot --ref @e1]',
     );
     expect(out.refs).toContainEqual(
       expect.objectContaining({

packages/vom/src/render.ts

@@ -819,7 +819,7 @@ function emitVisualEntries(parentId: number | null, depth: number, state: Render
     const ref = `e${state.nextRef}`;
     const label = cleaned(surface.label) ?? `${surface.renderingKind} visual surface`;
     const layers = surface.memberCount > 1 ? `; layers=${surface.memberCount}` : "";
-    const line = `${"  ".repeat(depth)}@${ref} surface ${JSON.stringify(label)} [rendering=${surface.renderingKind}${layers}; visual-only; use: bsk screenshot --ref @${ref}]`;
+    const line = `${"  ".repeat(depth)}@${ref} surface ${JSON.stringify(label)} [rendering=${surface.renderingKind}${layers}; visual-only; requires=image-understanding; use: bsk screenshot --ref @${ref}]`;
     const nextTokens = state.tokens + estimateTokens(line);
     if (nextTokens > state.maxTokens) {
       state.truncated = true;
@@ -872,24 +872,34 @@ function emitActiveScopeBlock(scope: ActiveScopeBlock, depth: number, state: Ren
   }
 }
 
+type RenderTask =
+  | { kind: "node"; node: VomNode; depth: number }
+  | { kind: "visual"; parentId: number | null; depth: number };
+
 function pushRenderChildren(
-  stack: Array<{ node: VomNode; depth: number }>,
+  stack: RenderTask[],
   children: readonly VomNode[],
   depth: number,
 ): void {
   for (let index = children.length - 1; index >= 0; index -= 1) {
-    stack.push({ node: children[index], depth });
+    stack.push({ kind: "node", node: children[index], depth });
   }
 }
 
 function renderTree(children: Map<number | null, VomNode[]>, state: RenderState): void {
-  const stack: Array<{ node: VomNode; depth: number }> = [];
+  const stack: RenderTask[] = [{ kind: "visual", parentId: null, depth: 1 }];
   pushRenderChildren(stack, children.get(null) ?? [], 1);
 
   while (stack.length > 0 && !state.stopped) {
-    const { node, depth } = stack.pop() as { node: VomNode; depth: number };
+    const task = stack.pop() as RenderTask;
+    if (task.kind === "visual") {
+      emitVisualEntries(task.parentId, task.depth, state);
+      continue;
+    }
+    const { node, depth } = task;
 
     if (!shouldRender(node)) {
+      stack.push({ kind: "visual", parentId: node.id, depth });
       pushRenderChildren(stack, children.get(node.id) ?? [], depth);
       continue;
     }
@@ -935,9 +945,9 @@ function renderTree(children: Map<number | null, VomNode[]>, state: RenderState)
       emitVisualEntries(node.id, depth + 1, state);
       continue;
     }
+    stack.push({ kind: "visual", parentId: node.id, depth: depth + 1 });
     pushRenderChildren(stack, children.get(node.id) ?? [], depth + 1);
   }
-  emitVisualEntries(parentId, depth, state);
 }
 
 function renderNodes(