diff --git a/examples/plugin-session/index.md b/examples/plugin-session/index.md
new file mode 100644
index 000000000..821c851cc
--- /dev/null
+++ b/examples/plugin-session/index.md
@@ -0,0 +1,5 @@
+---
+title:  Testing
+---
+
+# Testing
diff --git a/examples/plugin-session/myst.yml b/examples/plugin-session/myst.yml
new file mode 100644
index 000000000..0490629ec
--- /dev/null
+++ b/examples/plugin-session/myst.yml
@@ -0,0 +1,17 @@
+# See docs at: https://mystmd.org/guide/frontmatter
+version: 1
+project:
+  id: 70921a1f-f53a-440c-9f03-9023b8e9b815
+  # title:
+  # description:
+  # keywords: []
+  # authors: []
+  github: https://github.com/jupyter-book/mystmd
+  # To autogenerate a Table of Contents, run "myst init --write-toc"
+plugins:
+  - plugin.mjs
+site:
+  template: book-theme
+  # options:
+  #   favicon: favicon.ico
+  #   logo: site_logo.png
diff --git a/examples/plugin-session/plugin.mjs b/examples/plugin-session/plugin.mjs
new file mode 100644
index 000000000..be4d3a03b
--- /dev/null
+++ b/examples/plugin-session/plugin.mjs
@@ -0,0 +1,18 @@
+function sessionTransform(opts, utils) {
+  return async (mdast) => {
+    console.log('hello', utils.unstableSession);
+  };
+}
+
+// Declare a transform plugin
+const sessionTransformPlugin = {
+  plugin: sessionTransform,
+  stage: 'document',
+};
+
+const plugin = {
+  name: 'Session Transform Plugin',
+  transforms: [sessionTransformPlugin],
+};
+
+export default plugin;
diff --git a/examples/plugin-tagged/apples.md b/examples/plugin-tagged/apples.md
new file mode 100644
index 000000000..1f1ce2beb
--- /dev/null
+++ b/examples/plugin-tagged/apples.md
@@ -0,0 +1,17 @@
+---
+title: Apples
+description: A crisp and classic orchard fruit.
+date: 2024-01-15
+doi: 10.5555/apples.2024
+thumbnail: apples.png
+authors:
+  - name: Johnny Appleseed
+    affiliations:
+      - Orchard Institute
+tags:
+  - fruit
+---
+
+# Apples
+
+Apples are a fruit.
diff --git a/examples/plugin-tagged/apples.png b/examples/plugin-tagged/apples.png
new file mode 100644
index 000000000..4dc3fd6c7
Binary files /dev/null and b/examples/plugin-tagged/apples.png differ
diff --git a/examples/plugin-tagged/bananas.md b/examples/plugin-tagged/bananas.md
new file mode 100644
index 000000000..81574196a
--- /dev/null
+++ b/examples/plugin-tagged/bananas.md
@@ -0,0 +1,16 @@
+---
+title: Bananas
+description: A soft and sweet tropical favorite.
+date: 2024-06-08
+thumbnail: bananas.png
+authors:
+  - name: Bunch Plantain
+    affiliations:
+      - Tropical Fruit Lab
+tags:
+  - fruit
+---
+
+# Bananas
+
+Bananas are a fruit.
diff --git a/examples/plugin-tagged/bananas.png b/examples/plugin-tagged/bananas.png
new file mode 100644
index 000000000..58f65fc27
Binary files /dev/null and b/examples/plugin-tagged/bananas.png differ
diff --git a/examples/plugin-tagged/index.md b/examples/plugin-tagged/index.md
new file mode 100644
index 000000000..beafafdd8
--- /dev/null
+++ b/examples/plugin-tagged/index.md
@@ -0,0 +1,24 @@
+---
+title: Tagged Pages
+---
+
+# Tagged Pages
+
+This example demonstrates a directive transform plugin that lists every page in
+the project carrying a given tag. The plugin reads the project-wide page list
+from the (unstable) session API.
+
+## Fruit pages
+
+```{tagged} fruit
+```
+
+## Citrus pages
+
+```{tagged} citrus
+```
+
+## Pages that do not exist
+
+```{tagged} vegetable
+```
diff --git a/examples/plugin-tagged/myst.yml b/examples/plugin-tagged/myst.yml
new file mode 100644
index 000000000..e9a84cd82
--- /dev/null
+++ b/examples/plugin-tagged/myst.yml
@@ -0,0 +1,13 @@
+# See docs at: https://mystmd.org/guide/frontmatter
+version: 1
+project:
+  title: Tagged Pages Plugin Example
+  plugins:
+    - plugin.mjs
+  toc:
+    - file: index.md
+    - file: apples.md
+    - file: oranges.md
+    - file: bananas.md
+site:
+  template: book-theme
diff --git a/examples/plugin-tagged/oranges.md b/examples/plugin-tagged/oranges.md
new file mode 100644
index 000000000..9e3c814b7
--- /dev/null
+++ b/examples/plugin-tagged/oranges.md
@@ -0,0 +1,18 @@
+---
+title: Oranges
+description: A juicy citrus packed with vitamin C.
+date: 2024-03-22
+doi: 10.5555/oranges.2024
+thumbnail: oranges.png
+authors:
+  - name: Clementine Citrus
+    affiliations:
+      - Citrus Research Center
+tags:
+  - fruit
+  - citrus
+---
+
+# Oranges
+
+Oranges are a citrus fruit.
diff --git a/examples/plugin-tagged/oranges.png b/examples/plugin-tagged/oranges.png
new file mode 100644
index 000000000..34b157e69
Binary files /dev/null and b/examples/plugin-tagged/oranges.png differ
diff --git a/examples/plugin-tagged/plugin.mjs b/examples/plugin-tagged/plugin.mjs
new file mode 100644
index 000000000..aff8f09cf
--- /dev/null
+++ b/examples/plugin-tagged/plugin.mjs
@@ -0,0 +1,105 @@
+// A node type used as a placeholder by the directive, later replaced by the
+// project-stage transform once project-wide page information is available.
+const PAGE_LIST_NODE = 'taggedPageList';
+
+/**
+ * `{tagged} <tag>` directive
+ *
+ * Emits a placeholder node carrying the requested tag. The actual cards are
+ * filled in by the transform below, which has access to the full project via
+ * the (unstable) session API.
+ */
+const taggedDirective = {
+  name: 'tagged',
+  doc: 'Show a grid of cards for every page in the project that has the given tag.',
+  arg: {
+    type: String,
+    required: true,
+    doc: 'The tag to filter project pages by.',
+  },
+  run(data) {
+    return [{ type: PAGE_LIST_NODE, tag: `${data.arg}`.trim() }];
+  },
+};
+
+/** Build a card node for a single page. */
+function makeCard(page) {
+  const children = [];
+
+  // Thumbnail (rendered at the top of the card via the header slot)
+  const thumbnail = page.thumbnailOptimized ?? page.thumbnail;
+  if (thumbnail) {
+    children.push({
+      type: 'header',
+      children: [{ type: 'image', url: thumbnail, alt: page.title ?? '' }],
+    });
+  }
+
+  // Title
+  const title = page.title ?? page.slug ?? page.filename ?? 'Untitled';
+  children.push({ type: 'cardTitle', children: [{ type: 'text', value: title }] });
+
+  // Subtitle (body)
+  const subtitle = page.short_title ?? page.description;
+  if (subtitle) {
+    children.push({ type: 'paragraph', children: [{ type: 'text', value: subtitle }] });
+  }
+
+  // Author names (footer)
+  const authorNames = (page.authors ?? [])
+    .map((author) => author?.name)
+    .filter(Boolean)
+    .join(', ');
+  if (authorNames) {
+    children.push({
+      type: 'footer',
+      children: [{ type: 'emphasis', children: [{ type: 'text', value: `by ${authorNames}` }] }],
+    });
+  }
+
+  return { type: 'card', url: page.url, children };
+}
+
+/**
+ * Project-stage transform that replaces each placeholder node with a grid of
+ * cards for every page in the project carrying the requested tag.
+ */
+function taggedTransform(opts, utils) {
+  return async (mdast) => {
+    const { selectAll, unstableSession } = utils;
+    const placeholders = selectAll(PAGE_LIST_NODE, mdast);
+    if (placeholders.length === 0) return;
+    const pages = unstableSession?.project?.pages ?? [];
+    placeholders.forEach((node) => {
+      const { tag } = node;
+      const matching = pages.filter((page) => (page.tags ?? []).includes(tag));
+      delete node.tag;
+      if (matching.length === 0) {
+        node.type = 'paragraph';
+        node.children = [{ type: 'text', value: `No pages tagged "${tag}".` }];
+        return;
+      }
+      // Mutate the placeholder in place into a grid of cards
+      node.type = 'grid';
+      node.kind = 'listing';
+      node.columns = [1, 2, 2, 3];
+      node.children = matching.map((page) => makeCard(page));
+    });
+  };
+}
+
+const taggedTransformPlugin = {
+  name: 'Tagged pages cards',
+  // 'project' stage runs after all pages are processed, so every page's tags,
+  // frontmatter, and resolved url are available on the session.
+  stage: 'project',
+  plugin: taggedTransform,
+};
+
+const plugin = {
+  name: 'Tagged Pages Plugin',
+  directives: [taggedDirective],
+  transforms: [taggedTransformPlugin],
+};
+
+export default plugin;
diff --git a/packages/myst-cli/src/process/mdast.ts b/packages/myst-cli/src/process/mdast.ts
index fcd71cba4..aefdb4a47 100644
--- a/packages/myst-cli/src/process/mdast.ts
+++ b/packages/myst-cli/src/process/mdast.ts
@@ -1,6 +1,12 @@
 import path from 'node:path';
 import { tic } from 'myst-cli-utils';
-import type { GenericParent, PluginUtils, References } from 'myst-common';
+import type {
+  GenericParent,
+  References,
+  SessionPage,
+  SessionProject,
+  SessionSite,
+} from 'myst-common';
 import { fileError, fileWarn, RuleId, slugToUrl } from 'myst-common';
 import type { PageFrontmatter } from 'myst-frontmatter';
 import { SourceFileKind } from 'myst-spec-ext';
@@ -91,8 +97,6 @@ import {
 
 const LINKS_SELECTOR = 'link,card,linkBlock';
 
-const pluginUtils: PluginUtils = { select, selectAll };
-
 const htmlHandlers = {
   comment(h: any, node: any) {
     // Prevents HTML comments from showing up as text in web
@@ -108,6 +112,76 @@ export type TransformFn = (
   opts: Parameters<typeof transformMdast>[1],
 ) => Promise<void>;
 
+/**
+ * Build the project/site information exposed to plugins via the unstable session API.
+ *
+ * This gathers the list of all pages in the project (with their tags, slugs, and
+ * resolved urls) along with the current site configuration so that plugins, e.g. a
+ * directive that lists pages with a given tag, can operate on project-wide data.
+ *
+ * Page tags and urls are populated during `transformMdast`; for `project` stage
+ * transforms (running in `postProcessMdast`) all pages have been processed so this
+ * data is complete.
+ */
+function getSessionProject(session: ISession, projectPath?: string): SessionProject | undefined {
+  const state = session.store.getState();
+  const cache = castSession(session);
+  const siteConfig = selectors.selectCurrentSiteConfig(state);
+  const resolvedProjectPath = projectPath ?? selectors.selectCurrentProjectPath(state);
+  if (!resolvedProjectPath) return undefined;
+  const proj = selectors.selectLocalProject(state, resolvedProjectPath);
+  if (!proj) return undefined;
+  const projectSlug = siteConfig?.projects?.find((p) => p.path === resolvedProjectPath)?.slug;
+  const pages: SessionProject['pages'] = [];
+  // The full processed page frontmatter is available once a page has been through
+  // `transformMdast` (i.e. complete for `project` stage transforms). `selectFileInfo`
+  // is used as a fallback for the basics (e.g. during `document` stage transforms).
+  const pageFromFile = (file: string, slug?: string, level?: number): SessionPage => {
+    const fileInfo = selectors.selectFileInfo(state, file);
+    const frontmatter = cache.$getMdast(file)?.post?.frontmatter ?? {};
+    return {
+      title: fileInfo.title ?? undefined,
+      short_title: fileInfo.short_title ?? undefined,
+      description: fileInfo.description ?? undefined,
+      tags: fileInfo.tags ?? undefined,
+      date: fileInfo.date ?? undefined,
+      ...frontmatter,
+      slug,
+      url: fileInfo.url ?? undefined,
+      file,
+      filename: path.basename(file),
+      level,
+    };
+  };
+  // The project index page is tracked separately from the rest of the pages
+  pages.push(pageFromFile(proj.file, proj.index, 1));
+  proj.pages.forEach((tocEntry) => {
+    if ('file' in tocEntry) {
+      pages.push(pageFromFile(tocEntry.file, tocEntry.slug, tocEntry.level));
+    } else if ('url' in tocEntry) {
+      pages.push({ title: tocEntry.title, url: tocEntry.url, level: tocEntry.level });
+    }
+  });
+  const site: SessionSite | undefined = siteConfig
+    ? {
+        title: siteConfig.title,
+        description: siteConfig.description,
+        options: siteConfig.options,
+        nav: siteConfig.nav,
+        actions: siteConfig.actions,
+        domains: siteConfig.domains,
+        template: siteConfig.template,
+      }
+    : undefined;
+  return {
+    slug: projectSlug,
+    index: proj.index,
+    title: manifestTitleFromProject(session, resolvedProjectPath),
+    pages,
+    site,
+  };
+}
+
 export async function transformMdast(
   session: ISession,
   opts: {
@@ -220,9 +294,17 @@ export async function transformMdast(
     .use(inlineMathSimplificationPlugin, { replaceSymbol: false })
     .use(mathPlugin, { macros: frontmatter.math });
   // Load custom transform plugins
+  const documentSessionProject = getSessionProject(session, projectPath);
   session.plugins?.transforms.forEach((t) => {
     if (t.stage !== 'document') return;
-    pipe.use(t.plugin, undefined, pluginUtils);
+    pipe.use(t.plugin, undefined, {
+      select,
+      selectAll,
+      unstableSession: {
+        page: { slug: pageSlug, frontmatter },
+        project: documentSessionProject,
+      },
+    });
   });
 
   pipe
@@ -376,9 +458,17 @@ export async function postProcessMdast(
   await transformMystXRefs(session, vfile, mdast, frontmatter);
   await embedTransform(session, mdast, file, dependencies, state);
   const pipe = unified();
+  const projectSessionProject = getSessionProject(session, projectPath);
   session.plugins?.transforms.forEach((t) => {
     if (t.stage !== 'project') return;
-    pipe.use(t.plugin, undefined, pluginUtils);
+    pipe.use(t.plugin, undefined, {
+      select,
+      selectAll,
+      unstableSession: {
+        page: { slug: mdastPost.slug, frontmatter },
+        project: projectSessionProject,
+      },
+    });
   });
   await pipe.run(mdast, vfile);
 
diff --git a/packages/myst-common/src/index.ts b/packages/myst-common/src/index.ts
index e2147d0e8..4ec3191ef 100644
--- a/packages/myst-common/src/index.ts
+++ b/packages/myst-common/src/index.ts
@@ -51,6 +51,10 @@ export type {
   ValidatedMystPlugin,
   PluginOptions,
   PluginUtils,
+  SessionAPI,
+  SessionPage,
+  SessionProject,
+  SessionSite,
   TransformSpec,
   FrontmatterPart,
   FrontmatterParts,
diff --git a/packages/myst-common/src/types.ts b/packages/myst-common/src/types.ts
index e6643de66..d29beef7d 100644
--- a/packages/myst-common/src/types.ts
+++ b/packages/myst-common/src/types.ts
@@ -115,7 +115,62 @@ export type RoleSpec = {
 type Select = (selector: string, tree?: GenericParent) => GenericNode | null;
 type SelectAll = (selector: string, tree?: GenericParent) => GenericNode[] | null;
 
-export type PluginUtils = { select: Select; selectAll: SelectAll };
+/**
+ * Page information exposed to plugins via the unstable session API.
+ *
+ * This is the full page frontmatter (title, description, tags, date, authors,
+ * affiliations, contributors, doi, etc.) extended with the page's location
+ * within the site.
+ */
+export type SessionPage = PageFrontmatter & {
+  slug?: string;
+  /** Resolved url for the page within the site, e.g. `/my-page`. */
+  url?: string;
+  /** Full path to the source file on disk. */
+  file?: string;
+  /** Source file name, e.g. `my-page.md`. */
+  filename?: string;
+  /** Table of contents depth/level for the page. */
+  level?: number;
+};
+
+/** Minimal site configuration exposed to plugins via the unstable session API. */
+export type SessionSite = {
+  title?: string;
+  description?: string;
+  options?: Record<string, any>;
+  nav?: any[];
+  actions?: any[];
+  domains?: string[];
+  template?: string;
+};
+
+/** Minimal project information exposed to plugins via the unstable session API. */
+export type SessionProject = {
+  slug?: string;
+  index?: string;
+  title?: string;
+  pages: SessionPage[];
+  /** The current site configuration that this project belongs to. */
+  site?: SessionSite;
+};
+
+export type SessionAPI = {
+  page: {
+    slug?: string;
+    frontmatter: PageFrontmatter;
+  };
+  /**
+   * The current project, including the list of all pages and their tags, as
+   * well as the site configuration nested under `project.site`.
+   *
+   * This is only fully populated for `project` stage transforms; during
+   * `document` stage transforms other pages may not have been processed yet.
+   */
+  project?: SessionProject;
+};
+
+export type PluginUtils = { select: Select; selectAll: SelectAll; unstableSession: SessionAPI };
 export type PluginOptions = Record<string, any>;
 
 export type TransformSpec = {