diff --git a/docs/proposals/single-editor-hierarchy.md b/docs/proposals/single-editor-hierarchy.md
index 658765a..ab3a99e 100644
--- a/docs/proposals/single-editor-hierarchy.md
+++ b/docs/proposals/single-editor-hierarchy.md
@@ -1,6 +1,6 @@
 # Proposal: one editor for a whole workflow hierarchy
 
-**Status:** proposal. Nothing here is implemented.
+**Status:** phases 1 and 2 are implemented (in-place navigation, the root's run, and the view in the chat's context). The GLSP-MCP root/view split, phases 3 to 5 and the outline are not.
 **Affects:** dialogram (most of it), wfpy (a hierarchy export), wfpy-ide (one
 profile flag).
 
@@ -197,13 +197,25 @@ comes from the same export, and is the cheap part of it: names, files and sizes.
 
 ## Phasing
 
-1. **In-place navigation.** The profile flag, drill-down / breadcrumb / "Used
-   By" in place, the host's per-editor trail and refresh context, and "Open in
-   its own editor".
-2. **The root runs, the chat knows the view.** Root fields in the diagram
-   context, and Run, Rerun from Here and ⟲ from the root. The run driver and
-   refresh keyed by the editor. The trail in the chat's turn context, and the
-   GLSP-MCP root/view split.
+1. **In-place navigation.** *Done.* `clientBehavior.nestedNavigation`
+   (`'in-place'` / `'new-editor'`, default `'new-editor'`). Drill-down and the
+   breadcrumb stay in the editor for any file. "Used By" navigates in place to
+   an ancestor in the trail or to a workflow of the editor's own document, and
+   opens a caller in another file as a new root with no trail. The client's
+   navigation stack is keyed by the editor's root. The host keys the refresh
+   context by the editor (from the request's client) and remembers the shown
+   file, so a save, an external change or a live preview refreshes the view
+   shown. For another file it reloads from disk, since live-preview content is
+   the root's text. A node menu item opens a nested workflow's file in its own
+   editor.
+2. **The root runs, the chat knows the view.** *Done, except the GLSP-MCP
+   split.* The diagram context carries `rootSourceUri`, `rootWorkflowName` and
+   the trail. ▶ Run, ⟲ and "Rerun from Here" use the root, and inside a nested
+   view "Rerun from Here" reruns the root-level instance the view is in. The
+   refresh during and after a run reloads the editor's current view (file,
+   workflow, trail) rather than jumping to the root. The chat panel sends the
+   view's trail with the selection and each message, and the turn context names
+   the nested view and the file defining it.
 3. **Editing nested views.** Secondary documents saved after undo/redo, the root
    editor watching every file in its trail, and the concurrency behaviour
    tested.
diff --git a/packages/diagram-client/src/chat-panel-integrated.ts b/packages/diagram-client/src/chat-panel-integrated.ts
index 2e9eeb7..83d395c 100644
--- a/packages/diagram-client/src/chat-panel-integrated.ts
+++ b/packages/diagram-client/src/chat-panel-integrated.ts
@@ -160,6 +160,20 @@ interface CommandEntry {
  * Communication with the extension host happens over the GLSP vscode-messenger
  * channel (the diagram webview's raw postMessage is owned by GLSP).
  */
+
+/**
+ * The view on screen, as a trail from the editor's root, when it is not the
+ * root itself.
+ *
+ * The chat is the root's for the whole hierarchy, so without this the agent
+ * cannot tell that the diagram shows a nested workflow -- nor which instance
+ * the selected nodes belong to.
+ */
+export function currentViewTrail(): Array<{ sourceUri: string; workflowName: string; workflowInstanceName?: string }> | undefined {
+  const trail = (globalThis as any).__calDiagramContext?.trail;
+  return Array.isArray(trail) && trail.length > 1 ? trail : undefined;
+}
+
 @injectable()
 export class ChatPanel implements IDiagramStartup, ISelectionListener {
   /** Run after the diagram model is ready so the webview DOM exists. */
@@ -258,7 +272,7 @@ export class ChatPanel implements IDiagramStartup, ISelectionListener {
    */
   selectionChanged(_root: unknown, selectedElements: string[]): void {
     this.selectedNodeIds = selectedElements ?? [];
-    this.sendToHost('chat.selection', { selectedNodeIds: this.selectedNodeIds });
+    this.sendToHost('chat.selection', { selectedNodeIds: this.selectedNodeIds, viewTrail: currentViewTrail() });
     if (this.initialized) this.update();
   }
 
@@ -811,6 +825,7 @@ export class ChatPanel implements IDiagramStartup, ISelectionListener {
       sessionId: this.currentSessionId,
       mode: this.currentMode,
       selectedNodeIds: this.selectedNodeIds,
+      viewTrail: currentViewTrail(),
     });
 
     if (!ok) {
diff --git a/packages/diagram-client/src/editing-action-handlers.ts b/packages/diagram-client/src/editing-action-handlers.ts
index b266962..1f25fb9 100644
--- a/packages/diagram-client/src/editing-action-handlers.ts
+++ b/packages/diagram-client/src/editing-action-handlers.ts
@@ -1,8 +1,10 @@
 import { inject, injectable, optional } from 'inversify';
 import { EditorContextService, IActionDispatcher, IGridManager, TYPES } from '@eclipse-glsp/client';
-import { Action as GlspAction, ApplyLabelEditOperation, ICommand, IActionHandler } from '@eclipse-glsp/sprotty';
+import { Action as GlspAction, ApplyLabelEditOperation, ICommand, IActionHandler, NavigateToExternalTargetAction } from '@eclipse-glsp/sprotty';
 import { VscodeUi, type VscodeQuickPickItem } from './vscode-ui';
 import { clientBehavior, commandId, settingsNamespace } from './profile';
+import { rerunActorFor } from './navigation-ui';
+import { NETWORK_NAME_ARG, OPEN_DIAGRAM_ARG } from './network-navigation-target';
 import { EXECUTION_OVERLAY_ACTION_KIND, type ExecutionOverlayActionPayload } from '@dialogram/shared';
 
 export type WorkflowWorkspaceEntity = {
@@ -87,6 +89,21 @@ export namespace WorkflowRerunFromHereAction {
     }
 }
 
+export namespace WorkflowOpenInOwnEditorAction {
+    export const KIND = 'dialogram.openInOwnEditor';
+
+    export interface Action extends GlspAction {
+        kind: typeof KIND;
+        uri: string;
+        workflowName: string;
+    }
+
+    export function is(action: unknown): action is Action {
+        return !!action && typeof action === 'object' && (action as any).kind === KIND
+            && typeof (action as any).uri === 'string' && typeof (action as any).workflowName === 'string';
+    }
+}
+
 export namespace WorkflowRenameEntityOperation {
     export const KIND = 'dialogram.renameEntity';
 
@@ -1198,7 +1215,14 @@ export class WorkflowRerunFromHereActionHandler implements IActionHandler {
             return;
         }
         const context = (globalThis as any).__calDiagramContext as
-            | { sourceUri?: string; workflowName?: string; runDir?: string }
+            | {
+                sourceUri?: string;
+                workflowName?: string;
+                runDir?: string;
+                rootSourceUri?: string;
+                rootWorkflowName?: string;
+                trail?: Array<{ workflowInstanceName?: string }>;
+            }
             | undefined;
         // The menu offers it only for a run that can be resumed; anything the
         // run driver then finds wrong with it, the driver reports.
@@ -1206,11 +1230,39 @@ export class WorkflowRerunFromHereActionHandler implements IActionHandler {
             console.warn('[workflow] Rerun from Here: no resumable run is shown');
             return;
         }
+        // The root's run, at the root's step: inside a nested view, the node is
+        // rerun by rerunning the instance it is in.
         void VscodeUi.instance.executeCommand(commandId('runWorkflow'), [{
-            sourceUri: context.sourceUri,
-            workflowName: context.workflowName,
+            sourceUri: context.rootSourceUri ?? context.sourceUri,
+            workflowName: context.rootWorkflowName ?? context.workflowName,
             resumeFrom: context.runDir,
-            resumeAtActor: action.entityName
+            resumeAtActor: rerunActorFor(action.entityName, context.trail ?? [])
         }]);
     }
 }
+
+
+/**
+ * "Open in Its Own Editor": open a nested workflow's file as a root of its own.
+ *
+ * No trail goes with it, on purpose: the new editor's root is that file, with
+ * its own chat and runs, not a view of this editor's hierarchy.
+ */
+@injectable()
+export class WorkflowOpenInOwnEditorActionHandler implements IActionHandler {
+    @inject(TYPES.IActionDispatcher)
+    protected readonly actionDispatcher!: IActionDispatcher;
+
+    handle(action: GlspAction): ICommand | GlspAction | void {
+        if (!WorkflowOpenInOwnEditorAction.is(action)) {
+            return;
+        }
+        void this.actionDispatcher.dispatch(NavigateToExternalTargetAction.create({
+            uri: action.uri,
+            args: {
+                [OPEN_DIAGRAM_ARG]: true,
+                [NETWORK_NAME_ARG]: action.workflowName
+            }
+        }) as never);
+    }
+}
diff --git a/packages/diagram-client/src/navigation-ui.ts b/packages/diagram-client/src/navigation-ui.ts
index 6546b16..21ae160 100644
--- a/packages/diagram-client/src/navigation-ui.ts
+++ b/packages/diagram-client/src/navigation-ui.ts
@@ -135,6 +135,51 @@ function isQueueTraceVisible(): boolean {
     }
 }
 
+/** Whether a nested workflow in another file opens in this editor, not its own. */
+export function navigatesInPlace(): boolean {
+    return clientBehavior().nestedNavigation === 'in-place';
+}
+
+/**
+ * The workflow this editor runs: the root of its hierarchy, not the view.
+ *
+ * Navigating in place, the root is the first crumb, the editor's own document.
+ * Otherwise a stack can start in another editor's file (a drill-down passes its
+ * trail along), so the root is the first crumb in this editor's document -- the
+ * top of what this editor shows. Without a stack, the view is the root.
+ */
+export function rootOfStack(
+    stack: Array<{ sourceUri: string; workflowName: string }>,
+    editorSourceUri: string | undefined,
+    inPlace: boolean,
+    shown: { sourceUri: string; workflowName: string }
+): { sourceUri: string; workflowName: string } {
+    if (stack.length === 0) {
+        return shown;
+    }
+    if (inPlace) {
+        return { sourceUri: stack[0].sourceUri, workflowName: stack[0].workflowName };
+    }
+    const editorKey = editorSourceUri ? normalizeSourceUriKey(editorSourceUri) : shown.sourceUri;
+    const first = stack.find(crumb => crumb.sourceUri === editorKey);
+    return first ? { sourceUri: first.sourceUri, workflowName: first.workflowName } : shown;
+}
+
+/**
+ * The node a "Rerun from Here" reruns, as the run's trace names it.
+ *
+ * The trace has a step per firing of the root workflow's own actors. A node
+ * inside a nested view is not one of them: the instance the view is inside,
+ * at the root's level, is -- rerunning it reruns the node.
+ */
+export function rerunActorFor(
+    entityName: string,
+    stack: Array<{ workflowInstanceName?: string }>
+): string {
+    const top = stack.slice(1).find(crumb => typeof crumb.workflowInstanceName === 'string' && crumb.workflowInstanceName.trim() !== '');
+    return top?.workflowInstanceName?.trim() ?? entityName;
+}
+
 /** The body class that shows the queue-size badges on the edges. */
 export const DEBUG_EXPANDED_CLASS = 'workflow-debug-expanded';
 
@@ -215,6 +260,27 @@ export class WorkflowNavigationUi {
         @inject(EditorContextService) private readonly editorContext: EditorContextService
     ) {}
 
+    /**
+     * The navigation stack a view belongs to.
+     *
+     * Navigating in place, every view in this editor -- in any file -- is a view
+     * of one hierarchy, rooted at the editor's own document, so they share its
+     * stack. Opening a nested file in its own editor, the stack is the shown
+     * file's, as it always was.
+     */
+    private stackKey(sourceUri: string): string {
+        const root = this.editorContext.sourceUri;
+        return normalizeSourceUriKey(navigatesInPlace() && root ? root : sourceUri);
+    }
+
+    private getStack(sourceUri: string): NavigationCrumb[] | undefined {
+        return this.stacksBySourceUri.get(this.stackKey(sourceUri));
+    }
+
+    private setStack(sourceUri: string, stack: NavigationCrumb[]): void {
+        this.stacksBySourceUri.set(this.stackKey(sourceUri), stack);
+    }
+
     buildNavigationTrail(
         currentSourceUri: string | undefined,
         targetSourceUri: string,
@@ -223,7 +289,7 @@ export class WorkflowNavigationUi {
     ): NavigationCrumb[] {
         const currentSourceKey = currentSourceUri ? normalizeSourceUriKey(currentSourceUri) : undefined;
         const targetSourceKey = normalizeSourceUriKey(targetSourceUri);
-        const fromCurrent = currentSourceKey ? (this.stacksBySourceUri.get(currentSourceKey) ?? []) : [];
+        const fromCurrent = currentSourceKey ? (this.getStack(currentSourceKey) ?? []) : [];
         const trail = [...fromCurrent];
 
         if (trail.length === 0 && currentSourceKey) {
@@ -252,20 +318,20 @@ export class WorkflowNavigationUi {
         const sourceKey = normalizeSourceUriKey(sourceUri);
 
         if (Array.isArray(trail) && trail.length > 0) {
-            this.stacksBySourceUri.set(sourceKey, this.normalizeTrail(trail));
+            this.setStack(sourceKey, this.normalizeTrail(trail));
             return;
         }
 
-        const stack = this.stacksBySourceUri.get(sourceKey) ?? [];
+        const stack = this.getStack(sourceKey) ?? [];
         if (stack.length === 0) {
-            this.stacksBySourceUri.set(sourceKey, [{ sourceUri: sourceKey, workflowName }]);
+            this.setStack(sourceKey, [{ sourceUri: sourceKey, workflowName }]);
             return;
         }
         const last = stack[stack.length - 1];
         if (last?.sourceUri === sourceKey && last?.workflowName === workflowName) {
             return;
         }
-        this.stacksBySourceUri.set(sourceKey, [...stack, { sourceUri: sourceKey, workflowName }]);
+        this.setStack(sourceKey, [...stack, { sourceUri: sourceKey, workflowName }]);
     }
 
     onModelChanged(root: unknown): void {
@@ -327,7 +393,20 @@ export class WorkflowNavigationUi {
 
         const incomingTrail = this.parseTrail(args[NAV_TRAIL_ARG]);
         const nextStack = this.reconcileStack(sourceUri, selected, entryWorkflows, incomingTrail);
-        this.stacksBySourceUri.set(sourceUri, nextStack);
+        this.setStack(sourceUri, nextStack);
+
+        // What ▶ Run, "Rerun from Here" and the stepper's ⟲ act on: the root of
+        // this editor's hierarchy, whatever view is on screen.
+        const hierarchyRoot = rootOfStack(nextStack, this.editorContext.sourceUri, navigatesInPlace(), { sourceUri, workflowName: selected });
+        try {
+            Object.assign((globalThis as any).__calDiagramContext ?? {}, {
+                rootSourceUri: hierarchyRoot.sourceUri,
+                rootWorkflowName: hierarchyRoot.workflowName,
+                trail: nextStack
+            });
+        } catch {
+            // ignore
+        }
 
         const runtimeProfile = typeof args['wf:runtimeProfile'] === 'string' ? args['wf:runtimeProfile'] : undefined;
 
@@ -362,7 +441,7 @@ export class WorkflowNavigationUi {
             }
         }
 
-        const current = this.stacksBySourceUri.get(sourceUri) ?? [];
+        const current = this.getStack(sourceUri) ?? [];
         if (current.length === 0) {
             return [{ sourceUri, workflowName: selected }];
         }
@@ -418,7 +497,7 @@ export class WorkflowNavigationUi {
             }
             this.setStoredEntryWorkflow(sourceUri, picked);
             const nextTrail = [{ sourceUri, workflowName: picked }];
-            this.stacksBySourceUri.set(sourceUri, nextTrail);
+            this.setStack(sourceUri, nextTrail);
             void this.actionDispatcher.dispatch(this.requestModel(sourceUri, picked, undefined, nextTrail));
         });
     }
@@ -508,7 +587,7 @@ export class WorkflowNavigationUi {
             btn.className = 'workflow-wf-breadcrumb';
             const displayName = entry.workflowInstanceName || entry.workflowName;
             btn.textContent = displayName;
-            const isCrossFile = entry.sourceUri !== currentSourceKey;
+            const isCrossFile = entry.sourceUri !== currentSourceKey && !navigatesInPlace();
             btn.title = idx === stack.length - 1
                 ? `Current: ${displayName}`
                 : (isCrossFile ? `Open ${displayName} in referenced file` : `Go back to ${displayName}`);
@@ -516,8 +595,8 @@ export class WorkflowNavigationUi {
             btn.addEventListener('click', () => {
                 const nextTrail = stack.slice(0, idx + 1);
                 const next = nextTrail[nextTrail.length - 1];
-                this.stacksBySourceUri.set(next.sourceUri, nextTrail);
-                if (next.sourceUri === currentSourceKey) {
+                this.setStack(next.sourceUri, nextTrail);
+                if (next.sourceUri === currentSourceKey || navigatesInPlace()) {
                     void this.actionDispatcher.dispatch(this.requestModel(next.sourceUri, next.workflowName, undefined, nextTrail, selectedRunId));
                     return;
                 }
@@ -801,7 +880,7 @@ export class WorkflowNavigationUi {
         }
         this.setStoredEntryWorkflow(sourceUri, picked);
         const nextTrail = [{ sourceUri, workflowName: picked }];
-        this.stacksBySourceUri.set(sourceUri, nextTrail);
+        this.setStack(sourceUri, nextTrail);
         void this.actionDispatcher.dispatch(this.requestModel(sourceUri, picked, undefined, nextTrail, meta.selectedRunId));
     }
 
@@ -827,7 +906,29 @@ export class WorkflowNavigationUi {
         const nextTrail = existingParentIndex >= 0
             ? stack.slice(0, existingParentIndex + 1)
             : [{ sourceUri: selectedParent.sourceUri, workflowName: selectedParent.workflowName }];
-        this.stacksBySourceUri.set(selectedParent.sourceUri, nextTrail);
+        if (navigatesInPlace()) {
+            const editorRoot = this.editorContext.sourceUri ? normalizeSourceUriKey(this.editorContext.sourceUri) : sourceUri;
+            // An ancestor already in this editor's trail, or a workflow of the
+            // editor's own document: a view of this editor, shown in place.
+            if (existingParentIndex >= 0 || selectedParent.sourceUri === editorRoot) {
+                this.setStack(sourceUri, nextTrail);
+                void this.actionDispatcher.dispatch(this.requestModel(selectedParent.sourceUri, selectedParent.workflowName, undefined, nextTrail, meta.selectedRunId));
+                return;
+            }
+            // A caller in another file is a root of its own: its own editor, and
+            // no trail of this one, which keeps its root, chat and run.
+            void this.actionDispatcher.dispatch(
+                NavigateToExternalTargetAction.create({
+                    uri: selectedParent.sourceUri,
+                    args: {
+                        [OPEN_DIAGRAM_ARG]: true,
+                        [NETWORK_NAME_ARG]: selectedParent.workflowName
+                    }
+                })
+            );
+            return;
+        }
+        this.setStack(selectedParent.sourceUri, nextTrail);
         if (selectedParent.sourceUri === sourceUri) {
             void this.actionDispatcher.dispatch(this.requestModel(sourceUri, selectedParent.workflowName, undefined, nextTrail, meta.selectedRunId));
             return;
@@ -912,11 +1013,13 @@ export class WorkflowNavigationUi {
         container.appendChild(status);
         container.appendChild(next);
 
+        // The run is the root's, so a resume is too, whatever view is shown.
+        const context = (globalThis as any).__calDiagramContext;
         const resumeRequest = stepperResumeRequest(
             queueTrace,
             clientBehavior().resumeAtStep === true,
-            sourceUri,
-            meta.selected
+            typeof context?.rootSourceUri === 'string' ? context.rootSourceUri : sourceUri,
+            typeof context?.rootWorkflowName === 'string' ? context.rootWorkflowName : meta.selected
         );
         if (resumeRequest) {
             const resume = document.createElement('button');
diff --git a/packages/diagram-client/src/network-navigation-mouse-listener.ts b/packages/diagram-client/src/network-navigation-mouse-listener.ts
index fc6675e..c8ba4fb 100644
--- a/packages/diagram-client/src/network-navigation-mouse-listener.ts
+++ b/packages/diagram-client/src/network-navigation-mouse-listener.ts
@@ -9,7 +9,7 @@ import {
 } from '@eclipse-glsp/sprotty';
 import { inject, injectable } from 'inversify';
 import { WorkflowDiagramTypes, WorkflowDiagramMetadata } from '@dialogram/shared';
-import { WorkflowNavigationUi } from './navigation-ui';
+import { navigatesInPlace, WorkflowNavigationUi } from './navigation-ui';
 import { clientBehavior } from './profile';
 import {
     buildCrossFileNavigationTarget,
@@ -164,8 +164,13 @@ export class WorkflowNetworkNavigationMouseListener extends MouseListener implem
             ? navTrail[0]?.workflowName?.trim() || currentWorkflowName
             : undefined;
 
-        // Same-file: switch diagram model within the current editor.
-        if (normalizedCurrentSourceUri && normalizedReferencedUri === normalizedCurrentSourceUri) {
+        // Same file, or any file when navigating in place: switch the diagram
+        // model within the current editor. The request names the defining file;
+        // the trail keeps the editor's root at its head.
+        if (
+            (normalizedCurrentSourceUri && normalizedReferencedUri === normalizedCurrentSourceUri)
+            || navigatesInPlace()
+        ) {
             this.workflowNavUi.noteNavigate(referencedUri, targetNetworkName, navTrail);
             const includeGraphSourceFallback = useGraphSourceNavigation
                 && !!normalizedCurrentSourceUri
diff --git a/packages/diagram-client/src/profile.ts b/packages/diagram-client/src/profile.ts
index 2c252db..63e5236 100644
--- a/packages/diagram-client/src/profile.ts
+++ b/packages/diagram-client/src/profile.ts
@@ -29,6 +29,10 @@ type DiagramClientBehavior = {
     acpConnectors?: AcpConnectorInfo[];
     /** Whether a run can be resumed at a step of its queue trace. */
     resumeAtStep?: boolean;
+    /** Where a nested workflow defined in another file opens: `in-place` shows it in
+     *  this editor, as a view of the editor's root (one editor per hierarchy);
+     *  `new-editor` (the default) opens the file in its own editor. */
+    nestedNavigation?: 'in-place' | 'new-editor';
 };
 
 export type AcpConnectorInfo = {
diff --git a/packages/diagram-client/src/stock-features.module.ts b/packages/diagram-client/src/stock-features.module.ts
index cfdbbda..194580e 100644
--- a/packages/diagram-client/src/stock-features.module.ts
+++ b/packages/diagram-client/src/stock-features.module.ts
@@ -33,6 +33,8 @@ import {
     WorkflowPromptRenameEntityActionHandler,
     WorkflowRerunFromHereAction,
     WorkflowRerunFromHereActionHandler,
+    WorkflowOpenInOwnEditorAction,
+    WorkflowOpenInOwnEditorActionHandler,
     WorkflowEditParametersAction,
     WorkflowEditParametersActionHandler,
     WorkflowEditAnnotationsAction,
@@ -102,6 +104,7 @@ export const workflowFeaturesModule = new ContainerModule((bind, unbind, isBound
     configureActionHandler(context, WorkflowToggleGridAction.KIND, WorkflowToggleGridActionHandler);
     configureActionHandler(context, WorkflowPromptRenameEntityAction.KIND, WorkflowPromptRenameEntityActionHandler);
     configureActionHandler(context, WorkflowRerunFromHereAction.KIND, WorkflowRerunFromHereActionHandler);
+    configureActionHandler(context, WorkflowOpenInOwnEditorAction.KIND, WorkflowOpenInOwnEditorActionHandler);
     configureActionHandler(context, WorkflowEditParametersAction.KIND, WorkflowEditParametersActionHandler);
     configureActionHandler(context, WorkflowEditAnnotationsAction.KIND, WorkflowEditAnnotationsActionHandler);
     configureActionHandler(context, WorkflowShowWorkspaceEntitiesAction.KIND, WorkflowShowWorkspaceEntitiesActionHandler);
diff --git a/packages/diagram-client/src/tool-palette-header-controls.ts b/packages/diagram-client/src/tool-palette-header-controls.ts
index f18fb90..2d96478 100644
--- a/packages/diagram-client/src/tool-palette-header-controls.ts
+++ b/packages/diagram-client/src/tool-palette-header-controls.ts
@@ -8,6 +8,9 @@ type DiagramContext = {
     runtimeProfile?: string;
     namespaceName?: string;
     queueTraceVisible?: boolean;
+    /** The root of the editor's hierarchy: what runs (navigation-ui `rootOfStack`). */
+    rootSourceUri?: string;
+    rootWorkflowName?: string;
 };
 
 const QUEUE_TRACE_VISIBLE_STORAGE_KEY = queueTraceVisibleStorageKey();
@@ -186,7 +189,12 @@ function patchHeaderTools(headerTools: HTMLElement): void {
             void (async () => {
                 const ctx = getDiagramContext();
                 if (ctx?.sourceUri && ctx?.workflowName) {
-                    await exec(commandId('runWorkflow'), [ctx]);
+                    // The root of the hierarchy runs, not the nested view on screen.
+                    await exec(commandId('runWorkflow'), [{
+                        ...ctx,
+                        sourceUri: ctx.rootSourceUri ?? ctx.sourceUri,
+                        workflowName: ctx.rootWorkflowName ?? ctx.workflowName
+                    }]);
                 } else {
                     // Fallback: host command uses active diagram tab.
                     await exec(commandId('runWorkflow'));
diff --git a/packages/diagram-client/test/container-parity.test.ts b/packages/diagram-client/test/container-parity.test.ts
index 9ff263d..3f25731 100644
--- a/packages/diagram-client/test/container-parity.test.ts
+++ b/packages/diagram-client/test/container-parity.test.ts
@@ -15,11 +15,12 @@
  *
  * The baseline is no longer a pure capture of that commit: features added since
  * the split are appended to it deliberately, and the fixture currently carries
- * four additions — `IEdgeRouter -> LibavoidEdgeRouter` (the client-side live
+ * five additions — `IEdgeRouter -> LibavoidEdgeRouter` (the client-side live
  * routing tier), `ChangeBoundsTool -> WorkflowChangeBoundsTool` (the mouse-drag
  * threshold) and `IDiagramStartup -> WorkflowPaletteIconStartup` (draws the
- * palette icons a product contributed), and the `dialogram.rerunFromHere` action handler
- * (resumes the shown run before a node's last firing) — plus one change: `label:boundary:type` is bound to
+ * palette icons a product contributed), the `dialogram.rerunFromHere` action handler
+ * (resumes the shown run before a node's last firing) and the `dialogram.openInOwnEditor`
+ * one (opens a nested workflow's file as a root of its own) — plus one change: `label:boundary:type` is bound to
  * the non-editable `BoundaryLabel`, because editing it renamed the port.
  * The oracle still does its job: it fails on any binding this composition gains
  * or loses, and updating the fixture is the deliberate act of accepting one.
diff --git a/packages/diagram-client/test/fixtures/container-parity.baseline.json b/packages/diagram-client/test/fixtures/container-parity.baseline.json
index a20b690..9bb2fc0 100644
--- a/packages/diagram-client/test/fixtures/container-parity.baseline.json
+++ b/packages/diagram-client/test/fixtures/container-parity.baseline.json
@@ -34,6 +34,11 @@
         "kind": "dialogram.rerunFromHere",
         "op": "actionHandler"
     },
+    {
+        "handler": "WorkflowOpenInOwnEditorActionHandler",
+        "kind": "dialogram.openInOwnEditor",
+        "op": "actionHandler"
+    },
     {
         "handler": "WorkflowShowAgentSkillsActionHandler",
         "kind": "dialogram.showAgentSkills",
diff --git a/packages/diagram-client/test/in-place-navigation.test.ts b/packages/diagram-client/test/in-place-navigation.test.ts
new file mode 100644
index 0000000..f354f9c
--- /dev/null
+++ b/packages/diagram-client/test/in-place-navigation.test.ts
@@ -0,0 +1,104 @@
+/**
+ * Navigating a hierarchy in place: one editor, anchored to its root.
+ *
+ * Whatever view is shown -- a nested workflow, in any file -- the editor's
+ * root is what runs, what a resume resumes, and what "Rerun from Here" reruns
+ * in. A nested workflow can still be opened as a root of its own, on purpose.
+ */
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { VscodeUi } from '../src/vscode-ui';
+import { navigatesInPlace, rerunActorFor, rootOfStack } from '../src/navigation-ui';
+import {
+    WorkflowOpenInOwnEditorActionHandler,
+    WorkflowRerunFromHereActionHandler
+} from '../src/editing-action-handlers';
+
+const TOP = 'file:///w/top.py';
+const CHILD = 'file:///w/layers/block.py';
+const stack = [
+    { sourceUri: TOP, workflowName: 'top' },
+    { sourceUri: CHILD, workflowName: 'block', workflowInstanceName: 'b2' },
+    { sourceUri: CHILD, workflowName: 'leaf', workflowInstanceName: 'l1' }
+];
+
+afterEach(() => {
+    (VscodeUi as any)._instance = undefined;
+    delete (globalThis as any).diagramIdentifier;
+    delete (globalThis as any).__calDiagramContext;
+});
+
+describe('whether the editor navigates in place', () => {
+    it('is the product’s choice, off by default', () => {
+        expect(navigatesInPlace()).toBe(false);
+        (globalThis as any).diagramIdentifier = { clientBehavior: { nestedNavigation: 'in-place' } };
+        expect(navigatesInPlace()).toBe(true);
+    });
+});
+
+describe('the root of an editor’s hierarchy', () => {
+    const shown = { sourceUri: CHILD, workflowName: 'leaf' };
+
+    it('is the first crumb, navigating in place', () => {
+        expect(rootOfStack(stack, TOP, true, shown)).toEqual({ sourceUri: TOP, workflowName: 'top' });
+    });
+
+    it('is the first crumb in the editor’s own file otherwise', () => {
+        // An editor opened on block.py from top.py carries top.py's trail: its
+        // own top is the first crumb in block.py.
+        expect(rootOfStack(stack, CHILD, false, shown)).toEqual({ sourceUri: CHILD, workflowName: 'block' });
+    });
+
+    it('is the view when there is no stack', () => {
+        expect(rootOfStack([], TOP, true, shown)).toEqual(shown);
+    });
+});
+
+describe('what "Rerun from Here" reruns', () => {
+    it('is the node itself at the root’s level', () => {
+        expect(rerunActorFor('review', [{ sourceUri: TOP, workflowName: 'top' } as any])).toBe('review');
+    });
+
+    it('is the root-level instance a nested view is inside', () => {
+        expect(rerunActorFor('leafNode', stack)).toBe('b2');
+    });
+
+    it('resumes the root’s run from a nested view', () => {
+        const executeCommand = vi.fn(async () => undefined);
+        (VscodeUi as any)._instance = { executeCommand };
+        (globalThis as any).diagramIdentifier = {
+            commandIds: { runWorkflow: 'product.runWorkflow' },
+            clientBehavior: { resumeAtStep: true }
+        };
+        (globalThis as any).__calDiagramContext = {
+            sourceUri: CHILD,
+            workflowName: 'leaf',
+            runDir: '/w/wf-out/r1',
+            rootSourceUri: TOP,
+            rootWorkflowName: 'top',
+            trail: stack
+        };
+
+        new WorkflowRerunFromHereActionHandler().handle({ kind: 'dialogram.rerunFromHere', entityName: 'leafNode' } as any);
+
+        expect(executeCommand).toHaveBeenCalledWith('product.runWorkflow', [{
+            sourceUri: TOP,
+            workflowName: 'top',
+            resumeFrom: '/w/wf-out/r1',
+            resumeAtActor: 'b2'
+        }]);
+    });
+});
+
+describe('Open in Its Own Editor', () => {
+    it('opens the defining file as a root of its own: no trail goes with it', () => {
+        const handler = new WorkflowOpenInOwnEditorActionHandler();
+        const dispatch = vi.fn();
+        (handler as any).actionDispatcher = { dispatch };
+
+        handler.handle({ kind: 'dialogram.openInOwnEditor', uri: CHILD, workflowName: 'block' } as any);
+
+        const navigate = dispatch.mock.calls[0][0];
+        expect(navigate.uri).toBe(CHILD);
+        expect(navigate.args).toEqual({ 'cal:openDiagram': true, 'cal:networkName': 'block' });
+    });
+});
diff --git a/packages/diagram-server/src/server/context-menu-item-provider.ts b/packages/diagram-server/src/server/context-menu-item-provider.ts
index 03451da..5476102 100644
--- a/packages/diagram-server/src/server/context-menu-item-provider.ts
+++ b/packages/diagram-server/src/server/context-menu-item-provider.ts
@@ -20,6 +20,7 @@ const PROMPT_LABEL_EDIT_KIND = 'dialogram.promptLabelEdit';
 const EDIT_PARAMETERS_KIND = 'dialogram.editParameters';
 const PROMPT_RENAME_ENTITY_KIND = 'dialogram.promptRenameEntity';
 const RERUN_FROM_HERE_KIND = 'dialogram.rerunFromHere';
+const OPEN_IN_OWN_EDITOR_KIND = 'dialogram.openInOwnEditor';
 const RESET_EDGE_ROUTES_KIND = 'dialogram.resetEdgeRoutes';
 const REROUTE_EDGES_AVOID_OVERLAPS_KIND = 'dialogram.rerouteEdgesAvoidOverlaps';
 const LAYOUT_BOUNDARY_FLOW_KIND = 'dialogram.layoutBoundaryFlow';
@@ -136,6 +137,12 @@ function workflowDefinitionTargetFromRoot(rootElement: unknown, fallbackSourceUr
     return undefined;
 }
 
+/** Whether two source URIs name the same file, ignoring query and fragment. */
+function sameSourceFile(a: string, b: string): boolean {
+    const strip = (uri: string): string => uri.trim().replace(/[?#].*$/, '');
+    return strip(a) === strip(b);
+}
+
 function displayNameFromQualifiedName(qualifiedName: string): string {
     const afterDots = qualifiedName.split('.').pop() ?? qualifiedName;
     const parts = afterDots.split('__');
@@ -289,6 +296,30 @@ export class WorkflowContextMenuItemProvider extends ContextMenuItemProvider {
             elementType === WorkflowDiagramTypes.NODE_EXTERNAL_ACTOR
         ) {
             const entityName = elementArgs?.[WorkflowDiagramMetadata.ENTITY_NAME];
+            // A nested workflow defined in another file can be opened as a root
+            // of its own: its own editor, chat and runs. Navigating in place, a
+            // double-click shows it here instead, so this is the way to the old
+            // behaviour on purpose.
+            const referencedUri = elementArgs?.[WorkflowDiagramMetadata.REFERENCED_URI];
+            const referencedName = elementArgs?.[WorkflowDiagramMetadata.REFERENCED_ENTITY_NAME]
+                ?? elementArgs?.[WorkflowDiagramMetadata.ENTITY_TYPE];
+            if (
+                elementArgs?.[WorkflowDiagramMetadata.IS_NETWORK_INSTANCE] === true
+                && typeof referencedUri === 'string' && referencedUri.trim() !== ''
+                && typeof referencedName === 'string' && referencedName.trim() !== ''
+                && !sameSourceFile(referencedUri, sourceUri)
+            ) {
+                items.push({
+                    id: 'dialogram.openInOwnEditor',
+                    label: 'Open in Its Own Editor',
+                    sortString: 'a8',
+                    actions: [{
+                        kind: OPEN_IN_OWN_EDITOR_KIND,
+                        uri: referencedUri,
+                        workflowName: displayNameFromQualifiedName(referencedName.trim())
+                    } as any]
+                });
+            }
             // The run the overlay shows can be resumed at a step: offer to
             // resume it just before this node's last firing, so it fires again.
             const rootArgs = (this.modelState.root as any)?.args as Args | undefined;
diff --git a/packages/diagram-server/test/open-in-own-editor.test.ts b/packages/diagram-server/test/open-in-own-editor.test.ts
new file mode 100644
index 0000000..fc856f8
--- /dev/null
+++ b/packages/diagram-server/test/open-in-own-editor.test.ts
@@ -0,0 +1,52 @@
+/**
+ * "Open in Its Own Editor" on a nested workflow defined in another file.
+ *
+ * Navigating in place, a double-click shows a nested workflow inside this
+ * editor. This item is the way to open its file as a root of its own instead.
+ * It is offered only where that means something: a workflow instance whose
+ * definition is in a different file from the one shown.
+ */
+import { describe, expect, it } from 'vitest';
+import { WorkflowDiagramMetadata, WorkflowDiagramTypes } from '@dialogram/shared';
+import { WorkflowContextMenuItemProvider } from '../src/server/context-menu-item-provider';
+
+const SHOWN = 'file:///w/top.py';
+
+function itemsFor(args: Record<string, unknown>): any[] {
+    const root: any = { id: 'root', type: 'graph', args: {} };
+    const node: any = { id: 'n', type: WorkflowDiagramTypes.NODE_NETWORK, args, parent: root };
+    const provider: any = new WorkflowContextMenuItemProvider();
+    provider.modelState = { sourceUri: SHOWN, root, index: { find: () => node } };
+    return provider.getItems(['n'], { x: 0, y: 0 });
+}
+
+const instance = (referencedUri: string) => ({
+    [WorkflowDiagramMetadata.ENTITY_NAME]: 'b2',
+    [WorkflowDiagramMetadata.IS_NETWORK_INSTANCE]: true,
+    [WorkflowDiagramMetadata.REFERENCED_URI]: referencedUri,
+    [WorkflowDiagramMetadata.REFERENCED_ENTITY_NAME]: 'layers.block.block'
+});
+
+describe('Open in Its Own Editor', () => {
+    it('opens a workflow defined in another file, by its name', () => {
+        const item = itemsFor(instance('file:///w/layers/block.py')).find(i => i.id === 'dialogram.openInOwnEditor');
+
+        expect(item).toBeDefined();
+        expect(item.actions).toEqual([{
+            kind: 'dialogram.openInOwnEditor',
+            uri: 'file:///w/layers/block.py',
+            workflowName: 'block'
+        }]);
+    });
+
+    it('is not offered for a workflow defined in the file shown', () => {
+        const ids = itemsFor(instance(SHOWN)).map(i => i.id);
+        expect(ids).not.toContain('dialogram.openInOwnEditor');
+    });
+
+    it('is not offered for a node that is not a workflow instance', () => {
+        const ids = itemsFor({ ...instance('file:///w/layers/block.py'), [WorkflowDiagramMetadata.IS_NETWORK_INSTANCE]: false })
+            .map(i => i.id);
+        expect(ids).not.toContain('dialogram.openInOwnEditor');
+    });
+});
diff --git a/packages/extension-core/src/api.ts b/packages/extension-core/src/api.ts
index 0323c43..36dd0d2 100644
--- a/packages/extension-core/src/api.ts
+++ b/packages/extension-core/src/api.ts
@@ -295,6 +295,10 @@ export interface DiagramClientBehavior {
    */
   paletteIcons?: Record<string, { dark: string; light?: string }>;
   nodeFamilies?: NodeFamilySpec[];
+  /** Where a nested workflow defined in another file opens: `in-place` shows it in
+   *  this editor, as a view of the editor's root (one editor per hierarchy);
+   *  `new-editor` (the default) opens the file in its own editor. */
+  nestedNavigation?: 'in-place' | 'new-editor';
   /**
    * Whether the host has a chat backend behind this diagram.
    *
diff --git a/packages/extension-core/src/extension/chat/chat-runtime.ts b/packages/extension-core/src/extension/chat/chat-runtime.ts
index 79b4c27..cdad878 100644
--- a/packages/extension-core/src/extension/chat/chat-runtime.ts
+++ b/packages/extension-core/src/extension/chat/chat-runtime.ts
@@ -91,6 +91,41 @@ export interface ChatRuntimeConfig {
   postTurnHook?: (file: string, text: string) => Promise<void>;
 }
 
+/** One step of the trail from the root workflow down to the view on screen. */
+export interface ViewCrumb {
+  sourceUri?: string;
+  workflowName: string;
+  workflowInstanceName?: string;
+}
+
+/**
+ * Tell the agent which nested workflow the diagram is showing.
+ *
+ * The chat session is the root's for the whole hierarchy: the agent reads the
+ * root file and its graph. Navigating in place, the diagram can show a nested
+ * workflow, possibly defined in another file -- and the selected node ids are
+ * that view's. Nothing when the root itself is on screen.
+ */
+export function viewContextText(trail: ViewCrumb[] | undefined): string | undefined {
+  if (!trail || trail.length < 2) {
+    return undefined;
+  }
+  const path = trail
+    .map((crumb, i) =>
+      i === 0
+        ? crumb.workflowName
+        : `${crumb.workflowInstanceName ?? crumb.workflowName} (${crumb.workflowName})`,
+    )
+    .join(" › ");
+  const shown = trail[trail.length - 1];
+  const file = shown.sourceUri ? decodeURIComponent(shown.sourceUri.replace(/^file:\/\//, "")) : undefined;
+  return (
+    `The diagram is showing a nested workflow, not the root: ${path}.` +
+    (file ? ` It is defined in ${file}.` : "") +
+    " Selected nodes belong to this view."
+  );
+}
+
 export class ChatRuntime {
   private readonly acp = new ACPClientService();
   private readonly sessions: SessionManager;
@@ -110,6 +145,8 @@ export class ChatRuntime {
   private pendingModel: string | undefined;
   /** Latest diagram selection (node ids) per file, fed to the turn context. */
   private readonly selectionByFile = new Map<string, string[]>();
+  /** The nested view on screen per file, when it is not the root (see `viewContextText`). */
+  private readonly viewTrailByFile = new Map<string, ViewCrumb[]>();
   private readonly output: vscode.OutputChannel;
   /** Tears down the ACP -> webview event forwarding registered in the ctor. */
   private readonly acpForwardingDisposer: () => void;
@@ -151,6 +188,10 @@ export class ChatRuntime {
     }
     this.acp.setTurnContextBlocksProvider(async (file) => {
       const blocks: any[] = [];
+      const view = file ? viewContextText(this.viewTrailByFile.get(file)) : undefined;
+      if (view) {
+        blocks.push({ type: "text", text: view });
+      }
       if (file && this.config.selectionContext !== false) {
         const selected = this.selectionByFile.get(file) ?? [];
         if (selected.length > 0) {
@@ -467,6 +508,17 @@ export class ChatRuntime {
     await this.sendModels(uri);
   }
 
+  private setViewTrail(file: string, raw: unknown): void {
+    const trail = Array.isArray(raw)
+      ? raw.filter((c: any) => c && typeof c.workflowName === "string")
+      : [];
+    if (trail.length > 1) {
+      this.viewTrailByFile.set(file, trail as ViewCrumb[]);
+    } else {
+      this.viewTrailByFile.delete(file);
+    }
+  }
+
   private fileFor(uri: string): string {
     return vscode.Uri.parse(uri).fsPath;
   }
@@ -650,11 +702,13 @@ export class ChatRuntime {
             ? data.selectedNodeIds.map(String)
             : [],
         );
+        this.setViewTrail(file, data?.viewTrail);
         return;
       case "chat.sendMessage": {
         if (Array.isArray(data?.selectedNodeIds)) {
           this.selectionByFile.set(file, data.selectedNodeIds.map(String));
         }
+        this.setViewTrail(file, data?.viewTrail);
         await this.ensureStarted(cwd);
         // Fall back to the current session if the panel didn't pass one.
         let sessionId: string | undefined =
diff --git a/packages/extension-core/src/extension/diagram/diagram-editor-provider.ts b/packages/extension-core/src/extension/diagram/diagram-editor-provider.ts
index d4b65a3..5b365f4 100644
--- a/packages/extension-core/src/extension/diagram/diagram-editor-provider.ts
+++ b/packages/extension-core/src/extension/diagram/diagram-editor-provider.ts
@@ -54,7 +54,7 @@ export class WorkflowEditorProvider extends GlspEditorProvider {
     private uriToClientId = new Map<string, string>();
     /** Per-URI webview handles, for the profile's raw `postToWebview` channel. */
     private uriToWebview = new Map<string, vscode.Webview>();
-    private uriToRefreshContext = new Map<string, { networkName?: string; navTrail?: string; queueTraceVisible?: boolean; runId?: string }>();
+    private uriToRefreshContext = new Map<string, { networkName?: string; navTrail?: string; queueTraceVisible?: boolean; runId?: string; shownSourceUri?: string }>();
     private uriToLiveOverlaySignature = new Map<string, string | undefined>();
 
     /**
@@ -566,17 +566,28 @@ export class WorkflowEditorProvider extends GlspEditorProvider {
     ): void {
         const refreshContext = this.getRefreshContext(sourceUri);
 
+        // The editor refreshes the view it shows. Navigating in place that can be
+        // a workflow in another file than the editor's own document: refresh that
+        // file, from disk -- live-preview `content` is the editor document's text,
+        // which is not the shown file's.
+        const shownSourceUri = refreshContext?.shownSourceUri;
+        const showsOtherFile = typeof shownSourceUri === 'string'
+            && this.canonicalizeUriString(shownSourceUri) !== this.canonicalizeUriString(sourceUri);
+        const effectiveOptions = showsOtherFile && options.content !== undefined
+            ? { forceReloadFromDisk: true }
+            : options;
+
         // Create a RequestModelAction to refresh the diagram
         const action = RequestModelAction.create({
             requestId: 'refresh-' + Date.now(),
             options: {
-                sourceUri,
+                sourceUri: showsOtherFile ? shownSourceUri : sourceUri,
                 diagramType: this.diagramType,
                 ...(typeof refreshContext?.queueTraceVisible === 'boolean' ? { queueTraceVisible: refreshContext.queueTraceVisible } : {}),
                 ...(refreshContext?.networkName ? { networkName: refreshContext.networkName } : {}),
                 ...(refreshContext?.navTrail ? { 'wf:navTrail': refreshContext.navTrail } : {}),
                 ...(refreshContext?.runId ? { [RUN_ID_ARG]: refreshContext.runId } : {}),
-                ...options
+                ...effectiveOptions
             }
         });
 
@@ -587,7 +598,7 @@ export class WorkflowEditorProvider extends GlspEditorProvider {
 
     setRefreshContext(
         documentUri: vscode.Uri | string,
-        context: { networkName?: string; navTrail?: string; queueTraceVisible?: boolean; runId?: string }
+        context: { networkName?: string; navTrail?: string; queueTraceVisible?: boolean; runId?: string; shownSourceUri?: string }
     ): void {
         const key = this.canonicalizeUriString(documentUri);
         const next = {
@@ -595,7 +606,7 @@ export class WorkflowEditorProvider extends GlspEditorProvider {
             ...context
         };
 
-        if (!next.networkName && !next.navTrail && typeof next.queueTraceVisible !== 'boolean' && !next.runId) {
+        if (!next.networkName && !next.navTrail && typeof next.queueTraceVisible !== 'boolean' && !next.runId && !next.shownSourceUri) {
             this.uriToRefreshContext.delete(key);
             return;
         }
@@ -603,7 +614,7 @@ export class WorkflowEditorProvider extends GlspEditorProvider {
         this.uriToRefreshContext.set(key, next);
     }
 
-    getRefreshContext(documentUri: vscode.Uri | string): { networkName?: string; navTrail?: string; queueTraceVisible?: boolean; runId?: string } | undefined {
+    getRefreshContext(documentUri: vscode.Uri | string): { networkName?: string; navTrail?: string; queueTraceVisible?: boolean; runId?: string; shownSourceUri?: string } | undefined {
         return this.uriToRefreshContext.get(this.canonicalizeUriString(documentUri));
     }
 
@@ -1024,6 +1035,16 @@ export class WorkflowEditorProvider extends GlspEditorProvider {
      * Resolve the GLSP client/session id for a given CAL document URI.
      * Useful for dispatching actions when the diagram tab is not the active webview.
      */
+    /** The document an editor's client was opened on: its root. */
+    getDocumentUriForClientId(clientId: string): string | undefined {
+        for (const [uri, id] of this.uriToClientId) {
+            if (id === clientId) {
+                return uri;
+            }
+        }
+        return undefined;
+    }
+
     getClientIdForDocumentUri(uri: vscode.Uri): string | undefined {
         return this.uriToClientId.get(this.canonicalizeUriString(uri));
     }
diff --git a/packages/extension-core/src/extension/diagram/glsp-activation.ts b/packages/extension-core/src/extension/diagram/glsp-activation.ts
index 5d935af..f8db054 100644
--- a/packages/extension-core/src/extension/diagram/glsp-activation.ts
+++ b/packages/extension-core/src/extension/diagram/glsp-activation.ts
@@ -863,7 +863,14 @@ export async function activateGlspIntegration(
                 };
                 if (typeof sourceUri === 'string') {
                     const finalOptions = (patched.action.options as any) ?? {};
-                    editorProvider?.setRefreshContext(sourceUri, {
+                    // Keyed by the editor the request came from, not the file it
+                    // asks for: navigating in place, an editor shows workflows of
+                    // other files, and refreshes them as views of its own root.
+                    const editorDocumentUri = typeof (message as any).clientId === 'string'
+                        ? editorProvider?.getDocumentUriForClientId((message as any).clientId)
+                        : undefined;
+                    editorProvider?.setRefreshContext(editorDocumentUri ?? sourceUri, {
+                        shownSourceUri: sourceUri,
                         ...(typeof finalOptions.networkName === 'string' && finalOptions.networkName.trim() !== ''
                             ? { networkName: finalOptions.networkName.trim() }
                             : {}),
@@ -1422,14 +1429,21 @@ function registerCalDiagramCommands(
                 runOutput.appendLine(`[wf-lang live] SKIP ${kind === 'agentContextOnly' ? 'agent-ctx' : 'full'} refresh: provider=true clientId=undefined`);
                 return;
             }
+            // The run is the root's; the view may be a nested workflow under it,
+            // in another file. Refresh the view the editor shows -- its file,
+            // workflow and trail -- or a run would pull it back to the root.
+            const view = editorProvider.getRefreshContext(uri);
             const refreshAction = RequestModelAction.create({
                 requestId: (kind === 'agentContextOnly' ? 'refresh-agent-ctx-' : 'refresh-during-run-') + Date.now(),
                 options: {
-                    sourceUri: uri.toString(),
+                    sourceUri: view?.shownSourceUri ?? uri.toString(),
                     diagramType: WORKFLOW_DIAGRAM_TYPE,
                     ...(kind === 'agentContextOnly' ? { agentContextOnly: true } : {}),
                     queueTraceVisible: getQueueTraceVisible(),
-                    ...(networkName ? { networkName } : {})
+                    ...(view?.networkName
+                        ? { networkName: view.networkName }
+                        : (networkName ? { networkName } : {})),
+                    ...(view?.navTrail ? { [NAV_TRAIL_ARG]: view.navTrail } : {})
                 }
             });
             runOutput.appendLine(`[wf-lang live] dispatching ${kind === 'agentContextOnly' ? 'agent-ctx-only refresh' : 'full refresh'}`);
diff --git a/packages/extension-core/test/chat-view-context.test.ts b/packages/extension-core/test/chat-view-context.test.ts
new file mode 100644
index 0000000..fe97cb9
--- /dev/null
+++ b/packages/extension-core/test/chat-view-context.test.ts
@@ -0,0 +1,25 @@
+/**
+ * The chat is the root's for the whole hierarchy; the agent is told which
+ * nested workflow the diagram shows, so "this" and the selected nodes mean the
+ * view on screen, not the root.
+ */
+import { describe, expect, it } from 'vitest';
+import { viewContextText } from '../src/extension/chat/chat-runtime';
+
+describe('the view on screen, in the chat’s turn context', () => {
+    it('says nothing when the root is on screen', () => {
+        expect(viewContextText(undefined)).toBeUndefined();
+        expect(viewContextText([{ workflowName: 'top' }])).toBeUndefined();
+    });
+
+    it('names the path from the root, the instance, and the file defining it', () => {
+        const text = viewContextText([
+            { sourceUri: 'file:///w/top.py', workflowName: 'top' },
+            { sourceUri: 'file:///w/layers/block.py', workflowName: 'block', workflowInstanceName: 'b2' }
+        ]);
+
+        expect(text).toContain('top › b2 (block)');
+        expect(text).toContain('/w/layers/block.py');
+        expect(text).toContain('Selected nodes belong to this view');
+    });
+});
diff --git a/packages/extension-core/test/diagram-editor-provider-in-place-refresh.test.ts b/packages/extension-core/test/diagram-editor-provider-in-place-refresh.test.ts
new file mode 100644
index 0000000..0a87b67
--- /dev/null
+++ b/packages/extension-core/test/diagram-editor-provider-in-place-refresh.test.ts
@@ -0,0 +1,87 @@
+/**
+ * An editor refreshes the view it shows, which navigating in place can be a
+ * workflow in another file than its own document.
+ *
+ * The refresh context is kept per editor (its own document) and remembers the
+ * shown file. A refresh -- after a save, an external change, a live preview --
+ * reloads that file at the view's workflow and trail. Live-preview content is
+ * the editor document's text, so for another file the view reloads from disk.
+ */
+import { describe, expect, it, vi } from 'vitest';
+import * as vscode from 'vscode';
+import type { DiagramProfile } from '../src/api';
+
+vi.mock('@eclipse-glsp/vscode-integration', () => ({
+    GlspEditorProvider: class {
+        onDidChangeCustomDocument: unknown;
+        constructor(protected readonly glspVscodeConnector: any) {
+            this.onDidChangeCustomDocument = glspVscodeConnector?.onDidChangeCustomDocument;
+        }
+    },
+    GlspVscodeConnector: class {}
+}));
+
+const { WorkflowEditorProvider } = await import('../src/extension/diagram/diagram-editor-provider');
+
+const ROOT = vscode.Uri.file('/w/top.py');
+const CHILD = vscode.Uri.file('/w/layers/block.py');
+
+function makeProvider() {
+    const dispatched: Array<{ action: any; clientId: string }> = [];
+    const connector = {
+        onDidChangeCustomDocument: undefined,
+        dispatchAction: (action: any, clientId: string) => dispatched.push({ action, clientId })
+    } as any;
+    const context = { subscriptions: [] } as unknown as vscode.ExtensionContext;
+    const provider: any = new WorkflowEditorProvider(context, connector, {} as unknown as DiagramProfile);
+    provider.uriToClientId.set(provider.canonicalizeUriString(ROOT), 'client-top');
+    return { provider, dispatched };
+}
+
+const trail = JSON.stringify([
+    { sourceUri: ROOT.toString(), workflowName: 'top' },
+    { sourceUri: CHILD.toString(), workflowName: 'block', workflowInstanceName: 'b2' }
+]);
+
+describe('refreshing an editor that navigates in place', () => {
+    it('finds the editor’s document from its client', () => {
+        const { provider } = makeProvider();
+        expect(provider.getDocumentUriForClientId('client-top')).toBe(provider.canonicalizeUriString(ROOT));
+        expect(provider.getDocumentUriForClientId('nobody')).toBeUndefined();
+    });
+
+    it('reloads the shown file at the view’s workflow and trail', () => {
+        const { provider, dispatched } = makeProvider();
+        provider.setRefreshContext(ROOT, { shownSourceUri: CHILD.toString(), networkName: 'block', navTrail: trail });
+
+        provider.dispatchModelRefresh('client-top', provider.canonicalizeUriString(ROOT), { forceReloadFromDisk: true });
+
+        const options = dispatched[0].action.options;
+        expect(dispatched[0].clientId).toBe('client-top');
+        expect(options.sourceUri).toBe(CHILD.toString());
+        expect(options.networkName).toBe('block');
+        expect(options['wf:navTrail']).toBe(trail);
+    });
+
+    it('reloads another file from disk rather than with the editor document’s text', () => {
+        const { provider, dispatched } = makeProvider();
+        provider.setRefreshContext(ROOT, { shownSourceUri: CHILD.toString(), networkName: 'block' });
+
+        provider.dispatchModelRefresh('client-top', provider.canonicalizeUriString(ROOT), { content: 'top.py text' });
+
+        const options = dispatched[0].action.options;
+        expect(options.content).toBeUndefined();
+        expect(options.forceReloadFromDisk).toBe(true);
+    });
+
+    it('refreshes its own document as before when that is what it shows', () => {
+        const { provider, dispatched } = makeProvider();
+        provider.setRefreshContext(ROOT, { shownSourceUri: ROOT.toString(), networkName: 'top' });
+
+        provider.dispatchModelRefresh('client-top', provider.canonicalizeUriString(ROOT), { content: 'top.py text' });
+
+        const options = dispatched[0].action.options;
+        expect(options.sourceUri).toBe(provider.canonicalizeUriString(ROOT));
+        expect(options.content).toBe('top.py text');
+    });
+});
diff --git a/packages/extension-core/test/network-navigation-mouse-listener.test.ts b/packages/extension-core/test/network-navigation-mouse-listener.test.ts
index 5e1c67f..2fb435b 100644
--- a/packages/extension-core/test/network-navigation-mouse-listener.test.ts
+++ b/packages/extension-core/test/network-navigation-mouse-listener.test.ts
@@ -17,7 +17,8 @@ vi.mock('@eclipse-glsp/client', () => ({
 }));
 
 vi.mock('../../diagram-client/src/navigation-ui', () => ({
-    WorkflowNavigationUi: class {}
+    WorkflowNavigationUi: class {},
+    navigatesInPlace: () => (globalThis as any).diagramIdentifier?.clientBehavior?.nestedNavigation === 'in-place'
 }));
 
 vi.mock('@eclipse-glsp/sprotty', () => ({
@@ -142,4 +143,54 @@ describe('WorkflowNetworkNavigationMouseListener', () => {
         const actions = listener.doubleClick(target, {} as MouseEvent);
         expect(actions).toEqual([]);
     });
-});
\ No newline at end of file
+});
+
+/**
+ * A nested workflow defined in another file: a new editor by default, a view in
+ * this editor when the product navigates in place.
+ */
+describe('cross-file drill-down', () => {
+    const root = 'file:///workspace/top.py';
+    const child = 'file:///workspace/layers/block.py';
+    const navTrail = [
+        { sourceUri: root, workflowName: 'top' },
+        { sourceUri: child, workflowName: 'block', workflowInstanceName: 'b2' }
+    ];
+
+    function drillDown(nestedNavigation?: string) {
+        const listener = new WorkflowNetworkNavigationMouseListener();
+        (listener as any).editorContext = { sourceUri: root, diagramType: 'workflow-diagram' };
+        (listener as any).workflowNavUi = { buildNavigationTrail: () => navTrail, noteNavigate: () => {} };
+        (globalThis as any).diagramIdentifier = {
+            clientBehavior: nestedNavigation ? { nestedNavigation } : {}
+        };
+        (globalThis as any).__calDiagramContext = { workflowName: 'top' };
+        return listener.doubleClick({
+            type: WorkflowDiagramTypes.NODE_NETWORK,
+            args: {
+                [WorkflowDiagramMetadata.IS_NETWORK_INSTANCE]: true,
+                [WorkflowDiagramMetadata.REFERENCED_URI]: child,
+                [WorkflowDiagramMetadata.REFERENCED_ENTITY_NAME]: 'block',
+                'wf:entityInstanceName': 'b2'
+            }
+        } as any, {} as MouseEvent);
+    }
+
+    it('opens a new editor by default', () => {
+        const actions = drillDown();
+        expect(actions).toHaveLength(1);
+        expect((actions[0] as any).kind).toBe('navigateToExternalTarget');
+    });
+
+    it('shows the nested workflow in this editor when navigating in place', () => {
+        const actions = drillDown('in-place');
+
+        expect(actions).toHaveLength(1);
+        expect(RequestModelAction.is(actions[0] as any)).toBe(true);
+        const request = actions[0] as any;
+        // The defining file is what is rendered; the trail keeps the root first.
+        expect(request.options.sourceUri).toBe(child);
+        expect(request.options.networkName).toBe('block');
+        expect(JSON.parse(request.options[NAV_TRAIL_ARG])[0]).toEqual({ sourceUri: root, workflowName: 'top' });
+    });
+});
diff --git a/packages/sidecar-toolkit/src/sidecar-diagram-profile.ts b/packages/sidecar-toolkit/src/sidecar-diagram-profile.ts
index ed6d98d..fb7b37e 100644
--- a/packages/sidecar-toolkit/src/sidecar-diagram-profile.ts
+++ b/packages/sidecar-toolkit/src/sidecar-diagram-profile.ts
@@ -57,6 +57,10 @@ export interface SidecarClientBehavior {
     networkNavigationLabels?: boolean;
     noneSentinel?: string;
     scriptInterpreterCommands?: string[];
+    /** Where a nested workflow defined in another file opens: `in-place` shows it in
+     *  this editor, as a view of the editor's root (one editor per hierarchy);
+     *  `new-editor` (the default) opens the file in its own editor. */
+    nestedNavigation?: 'in-place' | 'new-editor';
 }
 
 /** The 24 consumer-owned command ids a sidecar diagram profile carries. */