diff --git a/components/simple-transcription/index.js b/components/simple-transcription/index.js
index f9525978..e4e5d3cf 100644
--- a/components/simple-transcription/index.js
+++ b/components/simple-transcription/index.js
@@ -969,10 +969,10 @@ export default class SimpleTranscriptionInterface extends HTMLElement {
   /**
    * Resolve each item in `page.items` to a full Annotation via the vault.
    * Vault fetches for AnnotationPages return children as bare `{id, type}`
-   * refs — downstream tools need hydrated targets/selectors/bodies. Returns
-   * a shallow copy of the page with the items array replaced; errors on
-   * individual items fall back to the original ref so partial hydration
-   * still produces a usable payload.
+   * refs — downstream tools that opt into the hydrated payload need hydrated
+   * targets/selectors/bodies. Returns a shallow copy of the page with the
+   * items array replaced; errors on individual items fall back to the
+   * original ref so partial hydration still produces a usable payload.
    */
   async #hydratePageItems(page) {
     if (!Array.isArray(page?.items) || page.items.length === 0) return page
@@ -983,10 +983,81 @@ export default class SimpleTranscriptionInterface extends HTMLElement {
     return { ...page, items }
   }
 
-  async #buildTPENContext() {
+  /**
+   * Find the layer page entry that matches the active page in the URL. Used
+   * to source the column ordering and the canvas list for the lean
+   * TPEN_CONTEXT payload.
+   */
+  #getActiveLayerPage() {
+    const pageInQuery = TPEN.screen?.pageInQuery
+    if (!pageInQuery) return null
+    return TPEN.activeProject?.layers
+      ?.flatMap(layer => layer.pages || [])
+      .find(p => p.id?.split('/').pop() === pageInQuery) ?? null
+  }
+
+  #getActiveLayerCanvases() {
+    const pageInQuery = TPEN.screen?.pageInQuery
+    if (!pageInQuery) return []
+    const layer = TPEN.activeProject?.layers
+      ?.find(l => l.pages?.some(p => p.id?.split('/').pop() === pageInQuery))
+    return layer?.pages?.map(p => ({ id: p.target, label: p.label })) ?? []
+  }
+
+  /**
+   * Lean boot payload sent to every iframe tool on load. Carries identity
+   * fields and URIs only — tools that need annotation bodies should fetch
+   * `annotationPage` directly, and tools that need the fully-populated
+   * project or page should send `REQUEST_POPULATED_PROJECT` /
+   * `REQUEST_POPULATED_PAGE`.
+   *
+   * Note: each entry in `canvases` is `{ id, label }` where `id` is the
+   * **canvas IRI** for that page (i.e. `page.target`), not the page IRI
+   * itself. This matches Compare-Pages' usage (it fetches the IRI as a IIIF
+   * canvas). Tools that need the page id should derive it from the canvas
+   * IRI or request the populated page.
+   */
+  #buildTPENContext() {
+    const project = TPEN.activeProject ?? null
+    const layerPage = this.#getActiveLayerPage()
     return {
       type: 'TPEN_CONTEXT',
-      project: TPEN.activeProject ?? null,
+      project: project ? {
+        id: project._id ?? project.id ?? null,
+        label: project.label ?? null,
+        slug: project.slug ?? null
+      } : null,
+      manifest: project?.manifest?.[0] ?? null,
+      canvas: this.#canvas?.id ?? this.#canvas?.['@id'] ?? null,
+      annotationPage: this.#page?.id ?? null,
+      currentLineId: this.#getCurrentLineId(),
+      columns: layerPage?.columns ?? [],
+      canvases: this.#getActiveLayerCanvases()
+    }
+  }
+
+  /**
+   * Reply payload for `REQUEST_POPULATED_PROJECT`. Carries the full active
+   * project (layers, pages, columns, members, …). Lean `TPEN_CONTEXT` only
+   * carries project identity, so tools that need the full graph must ask
+   * for it explicitly.
+   */
+  #buildPopulatedProject() {
+    return {
+      type: 'TPEN_POPULATED_PROJECT',
+      project: TPEN.activeProject ?? null
+    }
+  }
+
+  /**
+   * Reply payload for `REQUEST_POPULATED_PAGE`. Carries the active page
+   * with items resolved to full Annotations via the vault, the full canvas
+   * object, and the current line id. TPEN-Prompts uses this for
+   * prompt-template rendering.
+   */
+  async #buildPopulatedPage() {
+    return {
+      type: 'TPEN_POPULATED_PAGE',
       page: await this.#hydratePageItems(this.#page),
       canvas: this.#canvas ?? null,
       currentLineId: this.#getCurrentLineId()
@@ -998,10 +1069,11 @@ export default class SimpleTranscriptionInterface extends HTMLElement {
     targetWindow.postMessage(message, this._iframeOrigin)
   }
 
-  async #sendTPENContextToTool(targetWindow = this.#activeToolIframe?.contentWindow) {
-    this.#postToTool(await this.#buildTPENContext(), targetWindow)
-  }
-
+  // The TPEN ID token is the most sensitive message in this protocol. It is
+  // user-gated (only sent in reply to REQUEST_TPEN_ID_TOKEN), posted to the
+  // iframe origin only, and surfaced via a toast so the user always sees the
+  // grant. Do not send it unprompted, broadcast it with targetOrigin '*', or
+  // log it.
   #sendIdTokenToTool(targetWindow = this.#activeToolIframe?.contentWindow) {
     const idToken = TPEN.getAuthorization()
 
@@ -1095,37 +1167,11 @@ export default class SimpleTranscriptionInterface extends HTMLElement {
       this._iframeOrigin = new URL(tool.url).origin
 
       iframe.addEventListener('load', () => {
-        const target = iframe.contentWindow
-        this.#sendTPENContextToTool(target)
-
-        this.#postToTool({
-          type: 'MANIFEST_CANVAS_ANNOTATIONPAGE_ANNOTATION',
-          manifest: TPEN.activeProject?.manifest?.[0] ?? '',
-          canvas: this.#canvas?.id ?? this.#canvas?.['@id'] ?? '',
-          annotationPage: this.#page?.id ?? '',
-          annotation: TPEN.activeLineIndex >= 0
-            ? this.#page?.items?.[TPEN.activeLineIndex]?.id ?? null
-            : null,
-          columns: TPEN.activeProject?.layers
-            ?.flatMap(layer => layer.pages || [])
-            .find(p => p.id?.split('/').pop() === TPEN.screen?.pageInQuery)?.columns || []
-        }, target)
-
-        this.#postToTool({
-          type: 'CANVASES',
-          canvases: TPEN.activeProject?.layers
-            ?.find(layer => layer.pages?.some(p => p.id?.split('/').pop() === TPEN.screen?.pageInQuery))
-            ?.pages?.flatMap(p => ({ id: p.target, label: p.label })) ?? []
-        }, target)
-
-        this.#postToTool({ type: 'CURRENT_LINE_INDEX', lineId: this.#getCurrentLineId() }, target)
+        this.#postToTool(this.#buildTPENContext(), iframe.contentWindow)
       })
 
       const sendLineSelection = () => {
-        const currentLineId = this.#getCurrentLineId()
-        this.#postToTool({ type: 'UPDATE_CURRENT_LINE', currentLineId })
-
-        this.#postToTool({ type: 'CURRENT_LINE_INDEX', lineId: currentLineId })
+        this.#postToTool({ type: 'UPDATE_CURRENT_LINE', currentLineId: this.#getCurrentLineId() })
       }
       this.#toolCleanup.onEvent(TPEN.eventDispatcher, 'tpen-transcription-previous-line', sendLineSelection)
       this.#toolCleanup.onEvent(TPEN.eventDispatcher, 'tpen-transcription-next-line', sendLineSelection)
@@ -1143,39 +1189,45 @@ export default class SimpleTranscriptionInterface extends HTMLElement {
     this.checkMagnifierVisibility?.()
   }
 
-  #handleToolMessages(event) {
+  async #handleToolMessages(event) {
     // Validate message origin if iframe origin is set
     if (this._iframeOrigin && event.origin !== this._iframeOrigin) {
       return
     }
 
-    if (event.data?.type === 'REQUEST_TPEN_ID_TOKEN') {
-      this.#sendIdTokenToTool(event.source)
-      return
-    }
-    
-    // Handle incoming messages from tools
-    const lineId = event.data?.lineId ?? event.data?.lineid ?? event.data?.annotation // handle different casing and properties
-
-    if (!lineId) return
-
-    // Handle all line navigation message types
-    if (event.data?.type === "CURRENT_LINE_INDEX" || 
-        event.data?.type === "RETURN_LINE_ID" || 
-        event.data?.type === "SELECT_ANNOTATION" ||
-        event.data?.type === "NAVIGATE_TO_LINE") {
-      // Tool is telling us to navigate to a specific line
-      // Line ID might be full URI or just the ID part
-      const lineIndex = this.#page?.items?.findIndex(item => {
-        const itemId = item.id ?? item['@id']
-        // Match either full ID or just the last part after the last slash
-        return itemId === lineId || itemId?.endsWith?.(`/${lineId}`) || itemId?.split?.('/').pop() === lineId
-      })
-      
-      if (lineIndex !== undefined && lineIndex !== -1) {
-        TPEN.activeLineIndex = lineIndex
-        this.updateLines()
+    const type = event.data?.type
+    if (!type) return
+
+    try {
+      if (type === 'REQUEST_TPEN_ID_TOKEN') {
+        this.#sendIdTokenToTool(event.source)
+        return
       }
+
+      if (type === 'REQUEST_POPULATED_PROJECT') {
+        this.#postToTool(this.#buildPopulatedProject(), event.source)
+        return
+      }
+
+      if (type === 'REQUEST_POPULATED_PAGE') {
+        this.#postToTool(await this.#buildPopulatedPage(), event.source)
+        return
+      }
+
+      if (type === 'NAVIGATE_TO_LINE') {
+        const lineId = event.data?.lineId
+        if (!lineId) return
+        const lineIndex = this.#page?.items?.findIndex(item => {
+          const itemId = item.id ?? item['@id']
+          return itemId === lineId || itemId?.endsWith?.(`/${lineId}`) || itemId?.split?.('/').pop() === lineId
+        })
+        if (lineIndex !== undefined && lineIndex !== -1) {
+          TPEN.activeLineIndex = lineIndex
+          this.updateLines()
+        }
+      }
+    } catch (err) {
+      console.error(`[simple-transcription] tool message handler threw on type=${type}`, err)
     }
   }
 }
diff --git a/components/transcription-block/index.js b/components/transcription-block/index.js
index 8e8e4cc8..4a420ce5 100644
--- a/components/transcription-block/index.js
+++ b/components/transcription-block/index.js
@@ -155,16 +155,11 @@ export default class TranscriptionBlock extends HTMLElement {
             if (typeof index === 'number') this.scheduleLineSave(index)
         })
 
-        // Window message handler for external tool communication
+        // Window message handler for external tool communication. Line
+        // navigation is owned by simple-transcription; this listener only
+        // handles UPDATE_LINE_TEXT (e.g. Line-Breaking, Preview-Transcription
+        // pushing edited line text into the active transcription block).
         this.renderCleanup.onWindow('message', (event) => {
-            if (event.data?.type === "RETURN_LINE_ID") {
-                const lineIndex = this.#page.items.findIndex(item => item.id === event.data.lineId)
-                if (lineIndex !== -1) {
-                    this.moveToLine(lineIndex, 'next')
-                    this.updateTranscriptionUI()
-                }
-            }
-
             if (event.data?.type === "UPDATE_LINE_TEXT") {
                 if (typeof event.data.lineIndex === 'number') {
                     this.shadowRoot.querySelector('.transcription-input').value = event.data.text