feat(mcp): wire page-level SET Title over MCP (ALTER PAGE)

Page-level SET (no `on` clause) previously rejected every property with a
generic "not yet supported". Wire SET Title onto the pg LightPage's top-level
`title` string (the same key CreatePage writes and pg_read_page returns), so
`alter page … set (Title = '…')` works over the MCP backend.

Url and the pop-up settings (PopupWidth/Height/Resizable/CloseAction) are not
part of the pg LightPage shape — its schema is additionalProperties:false, so
injecting those keys would make pg_patch_page reject the whole root-replace
patch (taking any bundled Title change with it). They are rejected at SET time,
before Save(), with an accurate message, so they never poison the content tree;
set them in Studio Pro.

Also refresh PED_MCP_CAPABILITIES.md (pg_write_page → pg_patch_page in the
ALTER PAGE paragraph, page-level SET Title documented) and capabilities.yaml.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ako

docs/03-development/PED_MCP_CAPABILITIES.md

@@ -260,20 +260,26 @@ use, so it has its own success check (contains "success").
 
 **ALTER PAGE** is a read-modify-write on the pg tree: `OpenPageForMutation` loads
 the page via `pg_read_page`, the mutator edits the in-memory tree, and `Save()`
-writes it back via `pg_write_page`. Supported in-place ops: INSERT (before/after a
-widget), DROP widget, REPLACE widget, SET DataSource, SET Layout, and **SET widget
-property** (`set (prop = value, …) on <widget>`) — plus the introspection the
-executor needs (FindWidget, WidgetScope, ParamScope, EnclosingEntity). The widget
-ref is the widget name (recursive tree search). The executor passes the AST
-position token (`"AFTER"`/`"BEFORE"`), so the mutator compares case-insensitively.
-SET maps the MDL property name (also case-insensitive) to its pg key: Class/Style
-→ the widget's `appearance`; Caption/Content/Label → the `ct:`-prefixed client
-templates; ButtonStyle → pg's normalized enum; TabIndex/RenderMode/Editable/Name →
-direct keys; Visible → a conditional-visibility expression. Not yet mapped: unknown
-SET properties, column INSERT/REPLACE/property, design properties,
-pluggable-property SET, and page variables — each returns a clear error.
-
-Pages use a **separate protocol**: `pg_write_page` / `pg_read_page` (PED is
+writes it back via `pg_patch_page` (a root-replace patch). Supported in-place ops:
+INSERT (before/after a widget), DROP widget, REPLACE widget, SET DataSource, SET
+Layout, **SET widget property** (`set (prop = value, …) on <widget>`), and
+**page-level SET Title** (`set (Title = '…')` with no `on` clause) — plus the
+introspection the executor needs (FindWidget, WidgetScope, ParamScope,
+EnclosingEntity). The widget ref is the widget name (recursive tree search). The
+executor passes the AST position token (`"AFTER"`/`"BEFORE"`), so the mutator
+compares case-insensitively. SET maps the MDL property name (also
+case-insensitive) to its pg key: Class/Style → the widget's `appearance`;
+Caption/Content/Label → the `ct:`-prefixed client templates; ButtonStyle → pg's
+normalized enum; TabIndex/RenderMode/Editable/Name → direct keys; Visible → a
+conditional-visibility expression. Page-level SET Title maps onto the LightPage's
+top-level `title` string. Not yet mapped: page-level Url and pop-up settings
+(PopupWidth/Height/Resizable/CloseAction) — the LightPage schema is
+`additionalProperties:false`, so they aren't on the pg shape and are rejected
+before `Save()` (set them in Studio Pro); also unknown SET properties, column
+INSERT/REPLACE/property, design properties, pluggable-property SET, and page
+variables — each returns a clear error.
+
+Pages use a **separate protocol**: `pg_patch_page` / `pg_read_page` (PED is
 forbidden for pages). The backend maps the executor's `pages.Page` (shell +
 LayoutCall slots + widget tree) onto the high-level page content. Note pg's
 container type is **`Pages$DivContainer`** (not `Container`); page reads

mdl/backend/mcp/capabilities.yaml

@@ -45,7 +45,7 @@ features:
   - key: pages
     feature: "Pages"
     available: true
-    note: "CREATE + ALTER (widget coverage grows per type)"
+    note: "CREATE + ALTER (widget coverage grows per type); ALTER page-level SET Title (maps onto the pg LightPage's top-level title). Url and pop-up settings (PopupWidth/Height/Resizable/CloseAction) are NOT on the pg LightPage shape (schema is additionalProperties:false) — set them in Studio Pro"
   - key: workflows
     feature: "Workflows"
     available: true

mdl/backend/mcp/page_mutator.go

@@ -339,11 +339,8 @@ func (m *mcpPageMutator) SetWidgetProperty(widgetRef, prop string, value any) er
 	if widgetRef == "" {
 		// Page-level SET (no ON clause): Title, Url, PopupWidth/PopupHeight/
 		// PopupResizable, etc. These map onto the pg page object itself, not a
-		// widget. Their pg content-tree representation hasn't been probed/mapped
-		// against pg_read_page yet, so reject honestly rather than guess at the
-		// key names (and avoid the misleading "widget \"\" not found"). The MPR
-		// engines support these via the shared pagemutator's applyPageLevelSetMut.
-		return fmt.Errorf("setting page-level property %q is not yet supported by the MCP backend", prop)
+		// widget.
+		return m.setPageLevelProperty(prop, value)
 	}
 	_, _, _, w, ok := findWidget(m.content, widgetRef)
 	if !ok {
@@ -390,6 +387,34 @@ func (m *mcpPageMutator) SetWidgetProperty(widgetRef, prop string, value any) er
 	return nil
 }
 
+// setPageLevelProperty applies a page-level SET (no widget target) onto the pg
+// LightPage content tree. Pages go through the pg_* protocol, not PED, so the
+// only page-level fields that can be set are the ones the LightPage exposes.
+//
+// The LightPage carries the page title as a plain top-level "title" string (the
+// same key CreatePage writes and pg_read_page returns). Url and the pop-up
+// settings (PopupWidth/Height/Resizable/CloseAction) are NOT part of the
+// LightPage shape — its schema is additionalProperties:false, so injecting those
+// keys would make pg_patch_page reject the whole root-replace patch (taking any
+// bundled Title change down with it). They are rejected here, before Save(), so
+// they never poison the content tree; set them in Studio Pro. (If a future pg
+// server exposes them on the LightPage, add the mapping here.)
+func (m *mcpPageMutator) setPageLevelProperty(prop string, value any) error {
+	switch strings.ToLower(prop) {
+	case "title":
+		s, ok := value.(string)
+		if !ok {
+			return fmt.Errorf("page Title must be a string, got %T", value)
+		}
+		m.content["title"] = s
+		return nil
+	default:
+		return fmt.Errorf("setting page-level property %q is not supported by the MCP backend "+
+			"(pages go through the pg_* tools, not PED; the pg LightPage exposes only the page Title at page level — "+
+			"set Url and pop-up settings in Studio Pro)", prop)
+	}
+}
+
 func toStr(v any) string {
 	if s, ok := v.(string); ok {
 		return s

mdl/backend/mcp/page_mutator_test.go

@@ -134,11 +134,35 @@ func TestPageMutator_SetLayoutAndUnsupported(t *testing.T) {
 	}
 }
 
-// Page-level SET (empty widgetRef) — Title / pop-up dimensions / etc. — is not
-// yet mapped onto the pg content tree. It must reject clearly rather than report
-// the misleading "widget \"\" not found" (issue #661).
-func TestPageMutator_PageLevelSet_Rejected(t *testing.T) {
-	for _, prop := range []string{"Title", "PopupWidth", "PopupHeight", "PopupResizable"} {
+// Page-level SET Title (empty widgetRef) maps onto the pg LightPage's top-level
+// "title" key — the same key CreatePage writes and pg_read_page returns.
+func TestPageMutator_PageLevelSet_Title(t *testing.T) {
+	m := newTestMutator()
+	if err := m.SetWidgetProperty("", "Title", "Edit Expense"); err != nil {
+		t.Fatalf("SET Title: %v", err)
+	}
+	if m.content["title"] != "Edit Expense" {
+		t.Fatalf("title = %v, want \"Edit Expense\"", m.content["title"])
+	}
+	// Lowercase from the executor must work too.
+	if err := m.SetWidgetProperty("", "title", "Lower"); err != nil {
+		t.Fatalf("SET title (lowercase): %v", err)
+	}
+	if m.content["title"] != "Lower" {
+		t.Fatalf("title = %v, want \"Lower\"", m.content["title"])
+	}
+	// A non-string Title is rejected (no silent coercion).
+	if err := m.SetWidgetProperty("", "Title", 42); err == nil {
+		t.Error("non-string Title should be rejected")
+	}
+}
+
+// Url and pop-up settings are not part of the pg LightPage shape (its schema is
+// additionalProperties:false), so they must reject clearly — before Save(), so
+// they never poison the content tree — rather than report the misleading
+// "widget \"\" not found" (issue #661).
+func TestPageMutator_PageLevelSet_Unsupported(t *testing.T) {
+	for _, prop := range []string{"Url", "PopupWidth", "PopupHeight", "PopupResizable", "PopupCloseAction"} {
 		m := newTestMutator()
 		err := m.SetWidgetProperty("", prop, 800)
 		if err == nil {
@@ -147,6 +171,10 @@ func TestPageMutator_PageLevelSet_Rejected(t *testing.T) {
 		if got := err.Error(); !contains(got, "page-level property") || !contains(got, "MCP backend") {
 			t.Errorf("%s: unclear error %q", prop, got)
 		}
+		// The unsupported key must NOT have leaked into the content tree.
+		if _, leaked := m.content[strings.ToLower(prop)]; leaked {
+			t.Errorf("%s: rejected property leaked into content tree", prop)
+		}
 	}
 }