diff --git a/packages/studio/src/App.tsx b/packages/studio/src/App.tsx
index 5a33390801..f2a9313e6d 100644
--- a/packages/studio/src/App.tsx
+++ b/packages/studio/src/App.tsx
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "./utils/projectRouting";
 import { useState, useCallback, useRef, useMemo, useLayoutEffect } from "react";
 import type { LeftSidebarHandle, SidebarTab } from "./components/sidebar/LeftSidebar";
 import { useRenderQueue } from "./components/renders/useRenderQueue";
@@ -326,9 +327,10 @@ export function StudioApp() {
   const renderClipContent = useRenderClipContent({
     projectIdRef: fileManager.projectIdRef,
     compIdToSrc,
-    activePreviewUrl: activeCompPath
-      ? `/api/projects/${projectId}/preview/comp/${activeCompPath}`
-      : null,
+    activePreviewUrl:
+      activeCompPath && projectId
+        ? buildProjectApiPath(projectId, `/preview/comp/${activeCompPath}`)
+        : null,
     effectiveTimelineDuration,
   });
   const compositionDimensions = useCompositionDimensions();
diff --git a/packages/studio/src/captions/hooks/useCaptionSync.ts b/packages/studio/src/captions/hooks/useCaptionSync.ts
index 072b4829f3..0523ab4e8f 100644
--- a/packages/studio/src/captions/hooks/useCaptionSync.ts
+++ b/packages/studio/src/captions/hooks/useCaptionSync.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 import { useCallback, useRef } from "react";
 import { useCaptionStore } from "../store";
 import { useMountEffect } from "../../hooks/useMountEffect";
@@ -92,7 +93,7 @@ export function useCaptionSync(projectId: string | null) {
     const seqAtSave = editSeqRef.current;
     const overrides = buildOverrides(state.model);
 
-    fetch(`/api/projects/${pid}/files/${encodeURIComponent("caption-overrides.json")}`, {
+    fetch(buildProjectApiPath(pid, `/files/${encodeURIComponent("caption-overrides.json")}`), {
       method: "PUT",
       headers: { "Content-Type": "text/plain", ...studioWriteHeaders() },
       body: JSON.stringify(overrides, null, 2),
@@ -171,7 +172,7 @@ export function useCaptionSync(projectId: string | null) {
     let data: { content?: string };
     try {
       const res = await fetch(
-        `/api/projects/${pid}/files/${encodeURIComponent("caption-overrides.json")}`,
+        buildProjectApiPath(pid, `/files/${encodeURIComponent("caption-overrides.json")}`),
       );
       if (!res.ok) return; // no overrides file yet — normal
       data = await res.json();
diff --git a/packages/studio/src/components/MediaPreview.tsx b/packages/studio/src/components/MediaPreview.tsx
index 9547a7f617..9803b8731a 100644
--- a/packages/studio/src/components/MediaPreview.tsx
+++ b/packages/studio/src/components/MediaPreview.tsx
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../utils/projectRouting";
 import { useState } from "react";
 import { IMAGE_EXT, VIDEO_EXT, AUDIO_EXT } from "../utils/mediaTypes";
 
@@ -28,7 +29,7 @@ function MediaErrorPanel({ name, filePath }: { name: string; filePath: string })
 }
 
 export function MediaPreview({ projectId, filePath }: { projectId: string; filePath: string }) {
-  const serveUrl = `/api/projects/${projectId}/preview/${filePath}`;
+  const serveUrl = buildProjectApiPath(projectId, `/preview/${filePath}`);
   const name = filePath.split("/").pop() ?? filePath;
   // Keyed by path so switching to another file clears a previous failure.
   const [failedPath, setFailedPath] = useState<string | null>(null);
diff --git a/packages/studio/src/components/editor/domEditingLayers.ts b/packages/studio/src/components/editor/domEditingLayers.ts
index 3b24e2ec98..77805259b4 100644
--- a/packages/studio/src/components/editor/domEditingLayers.ts
+++ b/packages/studio/src/components/editor/domEditingLayers.ts
@@ -1,3 +1,4 @@
+import { probeSourceElement } from "./probeSourceElement";
 import type { PatchOperation } from "../../utils/sourcePatcher";
 import {
   resolveEditingAffordances,
@@ -281,31 +282,6 @@ export function resolveDomEditCapabilities(args: {
   ).capabilities;
 }
 
-async function probeSourceElement(
-  projectId: string,
-  sourceFile: string,
-  target: { id?: string; hfId?: string; selector?: string; selectorIndex?: number },
-): Promise<boolean> {
-  try {
-    const response = await fetch(
-      `/api/projects/${projectId}/file-mutations/probe-element/${encodeURIComponent(sourceFile)}`,
-      {
-        method: "POST",
-        headers: { "Content-Type": "application/json" },
-        body: JSON.stringify({ target }),
-      },
-    );
-    if (!response.ok) return true;
-    const data = await response.json();
-    if (data && typeof data === "object" && "exists" in data && data.exists === false) {
-      return false;
-    }
-    return true;
-  } catch {
-    return true;
-  }
-}
-
 // fallow-ignore-next-line complexity
 export async function resolveDomEditSelection(
   startEl: HTMLElement | null,
diff --git a/packages/studio/src/components/editor/probeSourceElement.ts b/packages/studio/src/components/editor/probeSourceElement.ts
new file mode 100644
index 0000000000..32d56496e3
--- /dev/null
+++ b/packages/studio/src/components/editor/probeSourceElement.ts
@@ -0,0 +1,29 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
+
+export async function probeSourceElement(
+  projectId: string,
+  sourceFile: string,
+  target: { id?: string; hfId?: string; selector?: string; selectorIndex?: number },
+): Promise<boolean> {
+  try {
+    const response = await fetch(
+      buildProjectApiPath(
+        projectId,
+        `/file-mutations/probe-element/${encodeURIComponent(sourceFile)}`,
+      ),
+      {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ target }),
+      },
+    );
+    if (!response.ok) return true;
+    const data = await response.json();
+    if (data && typeof data === "object" && "exists" in data && data.exists === false) {
+      return false;
+    }
+    return true;
+  } catch {
+    return true;
+  }
+}
diff --git a/packages/studio/src/components/editor/propertyPanelFill.tsx b/packages/studio/src/components/editor/propertyPanelFill.tsx
index 6f6c303602..3e4d4a5323 100644
--- a/packages/studio/src/components/editor/propertyPanelFill.tsx
+++ b/packages/studio/src/components/editor/propertyPanelFill.tsx
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 import { useMemo, useRef, useState } from "react";
 import { Plus, RotateCcw, X } from "../../icons/SystemIcons";
 import {
@@ -158,7 +159,7 @@ export function ImageFillField({
             {selectedAsset && (
               <div className="overflow-hidden rounded-xl border border-neutral-800 bg-neutral-900/80">
                 <img
-                  src={`/api/projects/${projectId}/preview/${selectedAsset}`}
+                  src={buildProjectApiPath(projectId, `/preview/${selectedAsset}`)}
                   alt={selectedAsset.split("/").pop() ?? selectedAsset}
                   className="h-28 w-full object-contain bg-neutral-950/80"
                 />
diff --git a/packages/studio/src/components/feedback/projectProvenance.ts b/packages/studio/src/components/feedback/projectProvenance.ts
index 0bc8b4fd36..f8bfa6325a 100644
--- a/packages/studio/src/components/feedback/projectProvenance.ts
+++ b/packages/studio/src/components/feedback/projectProvenance.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 // ---------------------------------------------------------------------------
 // How this project came to exist, and roughly what shape it is.
 //
@@ -53,7 +54,9 @@ export async function captureProjectProvenance(
   if (!scaffolded) return;
 
   try {
-    const res = await fetch(`/api/projects/${projectId}/files/${encodeURIComponent(CONFIG_FILE)}`);
+    const res = await fetch(
+      buildProjectApiPath(projectId, `/files/${encodeURIComponent(CONFIG_FILE)}`),
+    );
     if (!res.ok) return;
     // The route answers with an envelope, not the file: {filename, content,
     // version}. The config is the `content` string inside it.
diff --git a/packages/studio/src/components/nle/NLEContext.tsx b/packages/studio/src/components/nle/NLEContext.tsx
index d71d7ef526..6285e87c60 100644
--- a/packages/studio/src/components/nle/NLEContext.tsx
+++ b/packages/studio/src/components/nle/NLEContext.tsx
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 import { useContext, useState, useCallback, useRef, useEffect, type ReactNode } from "react";
 import { useTimelinePlayer, usePlayerStore } from "../../player";
 import type { TimelineElement } from "../../player";
@@ -185,7 +186,7 @@ export function NLEProvider({
     setCompositionSourceMap(emptyMap);
     onCompIdToSrcChangeRef.current?.(emptyMap);
 
-    fetch(`/api/projects/${projectId}/files/index.html`, {
+    fetch(buildProjectApiPath(projectId, `/files/index.html`), {
       signal: controller.signal,
     })
       .then((r) => {
diff --git a/packages/studio/src/components/nle/useCompositionStack.ts b/packages/studio/src/components/nle/useCompositionStack.ts
index feb947c42d..47e23350f3 100644
--- a/packages/studio/src/components/nle/useCompositionStack.ts
+++ b/packages/studio/src/components/nle/useCompositionStack.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 // Composition drill-down stack management for NLEContext/EditorShell
 import { useState, useCallback, useRef, useEffect } from "react";
 import { usePlayerStore } from "../../player";
@@ -29,7 +30,7 @@ export function useCompositionStack({
     {
       id: "master",
       label: "Master",
-      previewUrl: `/api/projects/${projectId}/preview`,
+      previewUrl: buildProjectApiPath(projectId, `/preview`),
     },
   ]);
 
@@ -89,7 +90,10 @@ export function useCompositionStack({
             .split("/")
             .pop()
             ?.replace(/\.html$/, "") || resolvedPath;
-        const previewUrl = `/api/projects/${projectId}/preview/comp/${encodePreviewPath(resolvedPath)}`;
+        const previewUrl = buildProjectApiPath(
+          projectId,
+          `/preview/comp/${encodePreviewPath(resolvedPath)}`,
+        );
         return [...prev, { id: resolvedPath, label, previewUrl }];
       });
     },
@@ -103,7 +107,7 @@ export function useCompositionStack({
     const master: CompositionLevel = {
       id: "master",
       label: "Master",
-      previewUrl: `/api/projects/${projectId}/preview`,
+      previewUrl: buildProjectApiPath(projectId, `/preview`),
     };
     if (activeCompositionPath === "index.html") {
       usePlayerStore.getState().setElements([]);
@@ -116,7 +120,10 @@ export function useCompositionStack({
       // panel highlighted the row, so the canvas and timeline stayed on
       // index.html and edits landed in the root file.
       const label = activeCompositionPath.replace(/^compositions\//, "").replace(/\.html$/, "");
-      const previewUrl = `/api/projects/${projectId}/preview/comp/${encodePreviewPath(activeCompositionPath)}`;
+      const previewUrl = buildProjectApiPath(
+        projectId,
+        `/preview/comp/${encodePreviewPath(activeCompositionPath)}`,
+      );
       usePlayerStore.getState().setElements([]);
       updateCompositionStack((prev) => {
         if (prev[prev.length - 1]?.id === activeCompositionPath) return prev;
diff --git a/packages/studio/src/components/renders/RenderQueueItem.tsx b/packages/studio/src/components/renders/RenderQueueItem.tsx
index 4b0209e559..f11a040f59 100644
--- a/packages/studio/src/components/renders/RenderQueueItem.tsx
+++ b/packages/studio/src/components/renders/RenderQueueItem.tsx
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 import { memo, useCallback, useState } from "react";
 import { VideoFrameThumbnail } from "../ui/VideoFrameThumbnail";
 import { Button } from "../ui/Button";
@@ -36,7 +37,7 @@ export const RenderQueueItem = memo(function RenderQueueItem({
   const [confirmingDelete, setConfirmingDelete] = useState(false);
 
   // Direct file URL — serves from disk, survives server restarts
-  const fileSrc = `/api/projects/${projectId}/renders/file/${job.filename}`;
+  const fileSrc = buildProjectApiPath(projectId, `/renders/file/${job.filename}`);
 
   const handleOpen = useCallback(() => {
     window.open(fileSrc, "_blank");
diff --git a/packages/studio/src/components/renders/useRenderQueue.ts b/packages/studio/src/components/renders/useRenderQueue.ts
index 15d1348803..704ea828d5 100644
--- a/packages/studio/src/components/renders/useRenderQueue.ts
+++ b/packages/studio/src/components/renders/useRenderQueue.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 import { useState, useEffect, useCallback, useRef, useMemo } from "react";
 import type { CanvasResolution } from "@hyperframes/parsers";
 import { trackStudioRenderStart } from "../../telemetry/events";
@@ -125,7 +126,7 @@ export function useRenderQueue(
   const loadRenders = useCallback(async () => {
     if (!projectId) return;
     try {
-      const res = await fetch(`/api/projects/${projectId}/renders`);
+      const res = await fetch(buildProjectApiPath(projectId, `/renders`));
       if (!res.ok) {
         setLoadError(`Couldn't load render history (server error ${res.status}).`);
         return;
@@ -261,7 +262,7 @@ export function useRenderQueue(
       }
       let res: Response;
       try {
-        res = await fetch(`/api/projects/${projectId}/render`, {
+        res = await fetch(buildProjectApiPath(projectId, `/render`), {
           method: "POST",
           headers: { "Content-Type": "application/json" },
           body: JSON.stringify(body),
diff --git a/packages/studio/src/components/sidebar/AssetsTab.tsx b/packages/studio/src/components/sidebar/AssetsTab.tsx
index aa8270dbf0..0ca716a507 100644
--- a/packages/studio/src/components/sidebar/AssetsTab.tsx
+++ b/packages/studio/src/components/sidebar/AssetsTab.tsx
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 // fallow-ignore-file code-duplication
 import { memo, useState, useCallback, useRef, useMemo, useEffect } from "react";
 import { SearchInput } from "../ui/SearchInput";
@@ -216,7 +217,7 @@ export const AssetsTab = memo(function AssetsTab({
   useEffect(() => {
     if (manifest404Ref.current.has(projectId)) return;
     let cancelled = false;
-    fetch(`/api/projects/${projectId}/preview/.media/manifest.jsonl`)
+    fetch(buildProjectApiPath(projectId, `/preview/.media/manifest.jsonl`))
       .then((r) => {
         if (!r.ok) {
           manifest404Ref.current.add(projectId);
diff --git a/packages/studio/src/components/sidebar/CompositionsTab.tsx b/packages/studio/src/components/sidebar/CompositionsTab.tsx
index 47a9e61a17..173c9e4d4d 100644
--- a/packages/studio/src/components/sidebar/CompositionsTab.tsx
+++ b/packages/studio/src/components/sidebar/CompositionsTab.tsx
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 import { memo, useCallback, useEffect, useRef, useState } from "react";
 import { buildCompositionThumbnailUrl } from "../../player/components/CompositionThumbnail";
 import { setPreviewMediaMuted } from "../../player/lib/timelineIframeHelpers";
@@ -173,7 +174,7 @@ function CompCard({
     setLivePreviewLoaded(false);
   };
   const name = comp.replace(/^compositions\//, "").replace(/\.html$/, "");
-  const previewUrl = `/api/projects/${projectId}/preview/comp/${comp}`;
+  const previewUrl = buildProjectApiPath(projectId, `/preview/comp/${comp}`);
   const thumbnailUrl = buildCompositionThumbnailUrl({
     previewUrl,
     seekTime: THUMBNAIL_SEEK_TIME_SECONDS,
diff --git a/packages/studio/src/components/storyboard/FramePoster.tsx b/packages/studio/src/components/storyboard/FramePoster.tsx
index d0baaa595e..5f27139cc4 100644
--- a/packages/studio/src/components/storyboard/FramePoster.tsx
+++ b/packages/studio/src/components/storyboard/FramePoster.tsx
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 import { useEffect, useState } from "react";
 import { buildCompositionThumbnailUrl } from "../../player/components/CompositionThumbnail";
 
@@ -51,7 +52,7 @@ export function FramePoster({
     );
   }
   let url = buildCompositionThumbnailUrl({
-    previewUrl: `/api/projects/${projectId}/preview/comp/${src}`,
+    previewUrl: buildProjectApiPath(projectId, `/preview/comp/${src}`),
     seekTime: seconds,
     duration: 0,
     origin: window.location.origin,
diff --git a/packages/studio/src/hooks/useAskAgentModal.ts b/packages/studio/src/hooks/useAskAgentModal.ts
index f88f3f2613..3b3691e518 100644
--- a/packages/studio/src/hooks/useAskAgentModal.ts
+++ b/packages/studio/src/hooks/useAskAgentModal.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../utils/projectRouting";
 import { useState, useCallback, useRef, useEffect } from "react";
 import { copyTextToClipboard } from "../utils/clipboard";
 import { readTagSnippetByTarget } from "../utils/sourcePatcher";
@@ -53,7 +54,7 @@ export function useAskAgentModal({
       const targetPath = selection.sourceFile || activeCompPath || "index.html";
       try {
         const response = await fetch(
-          `/api/projects/${pid}/files/${encodeURIComponent(targetPath)}`,
+          buildProjectApiPath(pid, `/files/${encodeURIComponent(targetPath)}`),
         );
         if (!response.ok) return;
 
diff --git a/packages/studio/src/hooks/useCaptionDetection.ts b/packages/studio/src/hooks/useCaptionDetection.ts
index 243af89015..e7fce63c47 100644
--- a/packages/studio/src/hooks/useCaptionDetection.ts
+++ b/packages/studio/src/hooks/useCaptionDetection.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../utils/projectRouting";
 import { useEffect, useRef } from "react";
 import { useCaptionStore } from "../captions/store";
 import { acceptStudioRuntimeMessage } from "../player/lib/runtimeProtocol";
@@ -107,7 +108,7 @@ export function useCaptionDetection({
 
       activating = true;
       const srcPath = captionSrcPath;
-      fetch(`/api/projects/${projectId}/files/${encodeURIComponent(srcPath)}`)
+      fetch(buildProjectApiPath(projectId, `/files/${encodeURIComponent(srcPath)}`))
         .then((r) => r.json())
         .then((data: { content?: string }) => {
           if (!data.content || !doc || !win || useCaptionStore.getState().isEditMode) return;
diff --git a/packages/studio/src/hooks/useCompositionContentLoader.ts b/packages/studio/src/hooks/useCompositionContentLoader.ts
index 7d2549b507..09994a3edc 100644
--- a/packages/studio/src/hooks/useCompositionContentLoader.ts
+++ b/packages/studio/src/hooks/useCompositionContentLoader.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../utils/projectRouting";
 import { useCallback } from "react";
 
 /**
@@ -20,9 +21,10 @@ export function useCompositionContentLoader({
 }) {
   return useCallback(
     (comp: string) => {
+      if (!projectId) return;
       setActiveCompPath(comp.endsWith(".html") ? comp : null);
       setEditingFile({ path: comp, content: null });
-      fetch(`/api/projects/${projectId}/files/${comp}`)
+      fetch(buildProjectApiPath(projectId, `/files/${encodeURIComponent(comp)}`))
         .then(async (r) => {
           if (!r.ok) throw new Error(`Failed to load ${comp} (${r.status})`);
           return r.json();
diff --git a/packages/studio/src/hooks/useDomEditCommits.ts b/packages/studio/src/hooks/useDomEditCommits.ts
index b425ddb0d2..7b77d62d4a 100644
--- a/packages/studio/src/hooks/useDomEditCommits.ts
+++ b/packages/studio/src/hooks/useDomEditCommits.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../utils/projectRouting";
 import { useCallback, useRef } from "react";
 import { findUnsafeDomPatchValues } from "@hyperframes/core/studio-api/finite-mutation";
 import { FONT_EXT } from "../utils/mediaTypes";
@@ -127,11 +128,11 @@ export function useDomEditCommits({
           FONT_EXT.test(path) &&
           fontFamilyFromAssetPath(path).toLowerCase() === family.toLowerCase(),
       );
-      if (!asset) return null;
+      if (!asset || !projectId) return null;
       return {
         family: fontFamilyFromAssetPath(asset),
         path: asset,
-        url: `/api/projects/${projectId}/preview/${asset}`,
+        url: buildProjectApiPath(projectId, `/preview/${asset}`),
       };
     },
     [fileTree, projectId, importedFontAssetsRef],
@@ -161,7 +162,7 @@ export function useDomEditCommits({
       };
 
       const readResponse = await fetch(
-        `/api/projects/${pid}/files/${encodeURIComponent(targetPath)}`,
+        buildProjectApiPath(pid, `/files/${encodeURIComponent(targetPath)}`),
       );
       if (!readResponse.ok) {
         throw await createStudioSaveHttpError(readResponse, `Failed to read ${targetPath}`);
@@ -219,7 +220,7 @@ export function useDomEditCommits({
       domEditSaveTimestampRef.current = Date.now();
 
       const patchResponse = await fetch(
-        `/api/projects/${pid}/file-mutations/patch-element/${encodeURIComponent(targetPath)}`,
+        buildProjectApiPath(pid, `/file-mutations/patch-element/${encodeURIComponent(targetPath)}`),
         {
           method: "POST",
           headers: { "Content-Type": "application/json", ...studioWriteHeaders() },
diff --git a/packages/studio/src/hooks/useElementLifecycleOps.ts b/packages/studio/src/hooks/useElementLifecycleOps.ts
index df5de796de..464a05ed62 100644
--- a/packages/studio/src/hooks/useElementLifecycleOps.ts
+++ b/packages/studio/src/hooks/useElementLifecycleOps.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../utils/projectRouting";
 import { useCallback } from "react";
 import { usePlayerStore } from "../player";
 import {
@@ -152,7 +153,10 @@ export function useElementLifecycleOps({
         // selection runs to hundreds of members — the file ended up correct, but
         // only after long enough that Delete looked like it had done nothing.
         const removeResponse = await fetch(
-          `/api/projects/${pid}/file-mutations/remove-elements/${encodeURIComponent(targetPath)}`,
+          buildProjectApiPath(
+            pid,
+            `/file-mutations/remove-elements/${encodeURIComponent(targetPath)}`,
+          ),
           {
             method: "POST",
             headers: { "Content-Type": "application/json", ...studioWriteHeaders() },
diff --git a/packages/studio/src/hooks/useFileTree.ts b/packages/studio/src/hooks/useFileTree.ts
index adc078de69..f78126c6ab 100644
--- a/packages/studio/src/hooks/useFileTree.ts
+++ b/packages/studio/src/hooks/useFileTree.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../utils/projectRouting";
 import { useState, useCallback, useEffect, useMemo } from "react";
 import { FONT_EXT } from "../utils/mediaTypes";
 import { fontFamilyFromAssetPath, type ImportedFontAsset } from "../components/editor/fontAssets";
@@ -22,7 +23,7 @@ export function useFileTree({ projectId, projectIdRef }: UseFileTreeOptions) {
     }
     let cancelled = false;
     setFileTreeLoaded(false);
-    fetch(`/api/projects/${projectId}`)
+    fetch(buildProjectApiPath(projectId))
       .then((r) => r.json())
       .then((data: { files?: string[]; dir?: string; compositions?: string[] }) => {
         if (cancelled) return;
@@ -47,7 +48,7 @@ export function useFileTree({ projectId, projectIdRef }: UseFileTreeOptions) {
   const refreshFileTree = useCallback(async () => {
     const pid = projectIdRef.current;
     if (!pid) return;
-    const res = await fetch(`/api/projects/${pid}`);
+    const res = await fetch(buildProjectApiPath(pid));
     const data = await res.json();
     if (data.files) setFileTree(data.files);
   }, [projectIdRef]);
@@ -62,12 +63,12 @@ export function useFileTree({ projectId, projectIdRef }: UseFileTreeOptions) {
 
   const fontAssets = useMemo<ImportedFontAsset[]>(
     () =>
-      assets
+      (projectId ? assets : [])
         .filter((asset) => FONT_EXT.test(asset))
         .map((asset) => ({
           family: fontFamilyFromAssetPath(asset),
           path: asset,
-          url: `/api/projects/${projectId}/preview/${asset}`,
+          url: projectId ? buildProjectApiPath(projectId, `/preview/${asset}`) : "",
         })),
     [assets, projectId],
   );
diff --git a/packages/studio/src/hooks/useGroupCommits.ts b/packages/studio/src/hooks/useGroupCommits.ts
index 1ac24bd14e..532adff463 100644
--- a/packages/studio/src/hooks/useGroupCommits.ts
+++ b/packages/studio/src/hooks/useGroupCommits.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../utils/projectRouting";
 import { useCallback } from "react";
 import {
   readProjectFileContent,
@@ -73,7 +74,7 @@ async function commitStructuralMutation(
 
   deps.domEditSaveTimestampRef.current = Date.now();
   const mutateResponse = await fetch(
-    `/api/projects/${pid}/file-mutations/${route}/${encodeURIComponent(targetPath)}`,
+    buildProjectApiPath(pid, `/file-mutations/${route}/${encodeURIComponent(targetPath)}`),
     {
       method: "POST",
       headers: { "Content-Type": "application/json", ...studioWriteHeaders() },
diff --git a/packages/studio/src/hooks/useLintModal.ts b/packages/studio/src/hooks/useLintModal.ts
index 399fb75957..a2e234f215 100644
--- a/packages/studio/src/hooks/useLintModal.ts
+++ b/packages/studio/src/hooks/useLintModal.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../utils/projectRouting";
 import { useState, useCallback, useEffect, useRef, useMemo } from "react";
 import type { LintFinding } from "../components/LintModal";
 import { usePlayerStore } from "../player";
@@ -35,7 +36,7 @@ export function useLintModal(projectId: string | null, refreshKey?: number) {
       if (!projectId) return;
       if (!opts?.background) setLinting(true);
       try {
-        const res = await fetch(`/api/projects/${projectId}/lint`);
+        const res = await fetch(buildProjectApiPath(projectId, `/lint`));
         const data = await res.json();
         const parsed = ((data.findings ?? []) as RawFinding[]).map(parseFinding);
         if (opts?.background) {
diff --git a/packages/studio/src/hooks/useRenderClipContent.ts b/packages/studio/src/hooks/useRenderClipContent.ts
index 7d4f7b1579..3d3a821c13 100644
--- a/packages/studio/src/hooks/useRenderClipContent.ts
+++ b/packages/studio/src/hooks/useRenderClipContent.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../utils/projectRouting";
 import { useCallback, type ReactNode } from "react";
 import { createElement } from "react";
 import { CompositionThumbnail, VideoThumbnail } from "../player";
@@ -16,7 +17,7 @@ export function normalizeCompositionSrc(
 ): string {
   try {
     const parsed = new URL(compSrc, origin);
-    const previewPrefix = `/api/projects/${projectId}/preview/`;
+    const previewPrefix = buildProjectApiPath(projectId, `/preview/`);
     if (parsed.pathname.startsWith(previewPrefix)) {
       return parsed.pathname.slice(previewPrefix.length);
     }
@@ -35,7 +36,7 @@ function resolvePreviewRelative(
   if (!src) return null;
   try {
     const parsed = new URL(src, origin);
-    const base = new URL(`/api/projects/${pid}/preview/`, origin).pathname;
+    const base = new URL(buildProjectApiPath(pid, `/preview/`), origin).pathname;
     return parsed.pathname.startsWith(base)
       ? decodeURIComponent(parsed.pathname.slice(base.length))
       : null;
@@ -77,7 +78,7 @@ function renderAudioClip(
   // returns the DECODED path, so it must be re-encoded here.
   const encodedRelative = srcRelative ? encodePreviewPath(srcRelative) : null;
   const waveformUrl = encodedRelative
-    ? `/api/projects/${pid}/waveform/${encodedRelative}`
+    ? buildProjectApiPath(pid, `/waveform/${encodedRelative}`)
     : undefined;
   const { start, end } = trimFractions(el);
   return createElement(AudioWaveform, {
@@ -146,7 +147,7 @@ export function useRenderClipContent({
       // instead of capturing the master at a time when the comp is fading in.
       if (compSrc) {
         return createElement(CompositionThumbnail, {
-          previewUrl: `/api/projects/${pid}/preview/comp/${encodePreviewPath(compSrc)}`,
+          previewUrl: buildProjectApiPath(pid, `/preview/comp/${encodePreviewPath(compSrc)}`),
           label: "",
           labelColor: style.label,
 
@@ -225,7 +226,7 @@ export function useRenderClipContent({
 
       if (htmlPreviewEligible) {
         return createElement(CompositionThumbnail, {
-          previewUrl: `/api/projects/${pid}/preview`,
+          previewUrl: buildProjectApiPath(pid, `/preview`),
           label: "",
           labelColor: style.label,
 
diff --git a/packages/studio/src/hooks/useTimelineDeleteOps.ts b/packages/studio/src/hooks/useTimelineDeleteOps.ts
index cd47b9adcf..505067f035 100644
--- a/packages/studio/src/hooks/useTimelineDeleteOps.ts
+++ b/packages/studio/src/hooks/useTimelineDeleteOps.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../utils/projectRouting";
 // Timeline clip deletion: the marquee/multi path and the single-clip wrapper
 // the context menu uses. Extracted verbatim from useTimelineEditing.ts to keep
 // it under the studio 600-line cap, following useTimelineAssetDropOps.
@@ -76,7 +77,10 @@ export function useTimelineDeleteOps({
           }
 
           const removeResponse = await fetch(
-            `/api/projects/${pid}/file-mutations/remove-element/${encodeURIComponent(targetPath)}`,
+            buildProjectApiPath(
+              pid,
+              `/file-mutations/remove-element/${encodeURIComponent(targetPath)}`,
+            ),
             {
               method: "POST",
               headers: { "Content-Type": "application/json", ...studioWriteHeaders() },
diff --git a/packages/studio/src/player/components/Player.tsx b/packages/studio/src/player/components/Player.tsx
index 94ce3964d4..50ca3794ee 100644
--- a/packages/studio/src/player/components/Player.tsx
+++ b/packages/studio/src/player/components/Player.tsx
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 import { forwardRef, useEffect, useRef, useState } from "react";
 import { isLottieAnimationLoaded } from "@hyperframes/core/runtime/lottie-readiness";
 import { useMountEffect } from "../../hooks/useMountEffect";
@@ -158,6 +159,10 @@ export const Player = forwardRef<HTMLIFrameElement, PlayerProps>(
       const container = containerRef.current;
       if (!container) return;
 
+      const previewSource =
+        directUrl || (projectId ? buildProjectApiPath(projectId, "/preview") : null);
+      if (!previewSource) return;
+
       let canceled = false;
       let cleanup: (() => void) | undefined;
 
@@ -167,10 +172,7 @@ export const Player = forwardRef<HTMLIFrameElement, PlayerProps>(
 
         // Create the web component imperatively to avoid JSX custom-element typing.
         const player = document.createElement("hyperframes-player") as HyperframesPlayerElement;
-        const srcUrl = new URL(
-          directUrl || `/api/projects/${projectId}/preview`,
-          window.location.origin,
-        );
+        const srcUrl = new URL(previewSource, window.location.origin);
         applyPreviewVariablesToUrl(srcUrl);
         const src = srcUrl.pathname + srcUrl.search;
         const retryPreview = () => {
diff --git a/packages/studio/src/player/components/thumbnailUtils.ts b/packages/studio/src/player/components/thumbnailUtils.ts
index 0a55cb7d55..7a1732d8c1 100644
--- a/packages/studio/src/player/components/thumbnailUtils.ts
+++ b/packages/studio/src/player/components/thumbnailUtils.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "../../utils/projectRouting";
 /** Rendered height of a timeline-clip thumbnail strip, in CSS px. */
 export const THUMBNAIL_CLIP_HEIGHT = 66;
 
@@ -113,7 +114,7 @@ export function resolveMediaPreviewUrl(
       return src;
     }
     if (!studioOrigin || parsed.origin !== studioOrigin) return src;
-    const previewPath = new URL(`/api/projects/${projectId}/preview/`, studioOrigin).pathname;
+    const previewPath = new URL(buildProjectApiPath(projectId, `/preview/`), studioOrigin).pathname;
     if (parsed.pathname.startsWith(previewPath)) return src;
     if (parsed.pathname.startsWith("/api/")) return src;
     try {
@@ -128,5 +129,8 @@ export function resolveMediaPreviewUrl(
     suffix = `${parsed.search}${parsed.hash}`;
   }
 
-  return `/api/projects/${projectId}/preview/${encodePreviewPath(relativePath.replace(/^\/+/, ""))}${suffix}`;
+  return buildProjectApiPath(
+    projectId,
+    `/preview/${encodePreviewPath(relativePath.replace(/^\/+/, ""))}${suffix}`,
+  );
 }
diff --git a/packages/studio/src/utils/blockInstaller.ts b/packages/studio/src/utils/blockInstaller.ts
index 4406381845..7255b4c50c 100644
--- a/packages/studio/src/utils/blockInstaller.ts
+++ b/packages/studio/src/utils/blockInstaller.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "./projectRouting";
 import type { RegistryItem } from "@hyperframes/core/registry";
 import type { TimelineElement } from "../player";
 import {
@@ -64,7 +65,7 @@ async function installRegistryItem({
   block: RegistryItem;
   compositionFile: string;
 } | null> {
-  const response = await fetch(`/api/projects/${projectId}/registry/install`, {
+  const response = await fetch(buildProjectApiPath(projectId, `/registry/install`), {
     method: "POST",
     headers: { "Content-Type": "application/json" },
     body: JSON.stringify({ blockName }),
diff --git a/packages/studio/src/utils/projectRouting.test.ts b/packages/studio/src/utils/projectRouting.test.ts
index 1f1e0badc3..9f022719e3 100644
--- a/packages/studio/src/utils/projectRouting.test.ts
+++ b/packages/studio/src/utils/projectRouting.test.ts
@@ -9,6 +9,15 @@ import {
 } from "./projectRouting";
 
 describe("project routing utilities", () => {
+  it.each(["C:", "C:demo", ".", "..", "../sessions", "a/b", "a\\b", "a\u0000b", "a\nb"])(
+    "rejects unsafe decoded project IDs: %s",
+    (id) => {
+      expect(parseProjectIdFromHash(`#project/${encodeURIComponent(id)}`)).toBeNull();
+      expect(() => buildProjectApiPath(id, "/files/index.html")).toThrow("Invalid project ID");
+      expect(() => buildProjectHash(id)).toThrow("Invalid project ID");
+    },
+  );
+
   it("decodes project ids from hash routes before building capture URLs", () => {
     vi.useFakeTimers();
     vi.setSystemTime(new Date("2026-05-01T12:00:00Z"));
diff --git a/packages/studio/src/utils/projectRouting.ts b/packages/studio/src/utils/projectRouting.ts
index faed3790d1..6291d665c8 100644
--- a/packages/studio/src/utils/projectRouting.ts
+++ b/packages/studio/src/utils/projectRouting.ts
@@ -5,6 +5,19 @@ export interface ProjectHashRoute {
   params: URLSearchParams;
 }
 
+/** Project names are single path segments, including when received from a hash or server. */
+export function isValidProjectId(value: string): boolean {
+  return (
+    value.length > 0 &&
+    value !== "." &&
+    value !== ".." &&
+    !value.includes(":") &&
+    !value.includes("/") &&
+    !value.includes("\\") &&
+    !Array.from(value).some((char) => char.charCodeAt(0) < 32)
+  );
+}
+
 function decodeHashProjectId(value: string): string {
   try {
     return decodeURIComponent(value);
@@ -28,6 +41,7 @@ function normalizeHashParams(
 }
 
 export function encodeProjectId(projectId: string): string {
+  if (!isValidProjectId(projectId)) throw new Error("Invalid project ID");
   return encodeURIComponent(projectId);
 }
 
@@ -47,9 +61,12 @@ export function parseProjectHashRoute(hash: string): ProjectHashRoute | null {
   const encodedProjectId = queryIndex >= 0 ? route.slice(0, queryIndex) : route;
   if (!encodedProjectId || encodedProjectId.includes("/")) return null;
 
+  const projectId = decodeHashProjectId(encodedProjectId);
+  if (!isValidProjectId(projectId)) return null;
+
   const rawParams = queryIndex >= 0 ? route.slice(queryIndex + 1) : "";
   return {
-    projectId: decodeHashProjectId(encodedProjectId),
+    projectId,
     params: new URLSearchParams(rawParams),
   };
 }
diff --git a/packages/studio/src/utils/razorSplitTransaction.test.ts b/packages/studio/src/utils/razorSplitTransaction.test.ts
index 567bbffe39..ea526da58c 100644
--- a/packages/studio/src/utils/razorSplitTransaction.test.ts
+++ b/packages/studio/src/utils/razorSplitTransaction.test.ts
@@ -103,7 +103,7 @@ describe("runAtomicCutTransaction", () => {
     const synchronize = vi.fn();
 
     const result = await runAtomicCutTransaction({
-      projectId: "launch/demo",
+      projectId: "launch#demo",
       intents: buildAtomicCutIntents([element()], 2, "index.html"),
       label: "Split timeline clip",
       writeProjectFile,
@@ -114,8 +114,8 @@ describe("runAtomicCutTransaction", () => {
 
     expect(requests.filter((request) => request.url.includes("split-batch"))).toHaveLength(1);
     expect(requests.map((request) => request.url)).toEqual([
-      "/api/projects/launch%2Fdemo/files/index.html",
-      "/api/projects/launch%2Fdemo/file-mutations/split-batch",
+      "/api/projects/launch%23demo/files/index.html",
+      "/api/projects/launch%23demo/file-mutations/split-batch",
     ]);
     const splitRequest = requests.find((request) => request.url.includes("split-batch"));
     const writeToken = new Headers(splitRequest?.headers).get("X-Hyperframes-Write-Token");
diff --git a/packages/studio/src/utils/studioFileHistory.ts b/packages/studio/src/utils/studioFileHistory.ts
index c055cb8709..7125221b88 100644
--- a/packages/studio/src/utils/studioFileHistory.ts
+++ b/packages/studio/src/utils/studioFileHistory.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "./projectRouting";
 import type { MutableRefObject } from "react";
 import type { EditHistoryKind } from "./editHistory";
 import { serializeStudioFileMutations } from "./studioFileMutationCoordinator";
@@ -52,7 +53,7 @@ interface SaveProjectFilesWithHistoryInput {
 }
 
 export async function readProjectFileContent(pid: string, path: string): Promise<string> {
-  const response = await fetch(`/api/projects/${pid}/files/${encodeURIComponent(path)}`);
+  const response = await fetch(buildProjectApiPath(pid, `/files/${encodeURIComponent(path)}`));
   if (!response.ok) {
     throw await createStudioSaveHttpError(response, `Failed to read ${path}`);
   }
diff --git a/packages/studio/src/utils/studioHelpers.ts b/packages/studio/src/utils/studioHelpers.ts
index 489bf8ea0b..8531768a55 100644
--- a/packages/studio/src/utils/studioHelpers.ts
+++ b/packages/studio/src/utils/studioHelpers.ts
@@ -1,3 +1,4 @@
+import { buildProjectApiPath } from "./projectRouting";
 import { isTypingTarget } from "./typingTarget";
 import type { TimelineElement } from "../player/store/playerStore";
 import type { DomEditSelection } from "../components/editor/domEditing";
@@ -306,7 +307,7 @@ export async function resolveDroppedAssetDuration(
 
   const media = document.createElement(kind === "video" ? "video" : "audio");
   media.preload = "metadata";
-  media.src = `/api/projects/${projectId}/preview/${assetPath}`;
+  media.src = buildProjectApiPath(projectId, `/preview/${assetPath}`);
 
   const duration = await new Promise<number>((resolve) => {
     const timeout = window.setTimeout(() => resolve(DEFAULT_TIMELINE_ASSET_DURATION[kind]), 3000);
@@ -343,7 +344,7 @@ export async function resolveDroppedAssetDimensions(
   kind: TimelineAssetKind,
 ): Promise<{ width: number; height: number } | null> {
   if (kind === "audio") return null;
-  const src = `/api/projects/${projectId}/preview/${assetPath}`;
+  const src = buildProjectApiPath(projectId, `/preview/${assetPath}`);
 
   if (kind === "image") {
     return new Promise((resolve) => {
diff --git a/packages/studio/src/utils/timelineCompositionInsert.test.ts b/packages/studio/src/utils/timelineCompositionInsert.test.ts
index e50970f8b0..9a853fad46 100644
--- a/packages/studio/src/utils/timelineCompositionInsert.test.ts
+++ b/packages/studio/src/utils/timelineCompositionInsert.test.ts
@@ -33,7 +33,7 @@ describe("commitTimelineCompositionInsertion", () => {
     const refresh = vi.fn();
 
     await commitTimelineCompositionInsertion({
-      projectId: "launch/demo",
+      projectId: "launch#demo",
       targetPath: "index.html",
       sourcePath: "headline.html",
       start: 4,
@@ -48,8 +48,8 @@ describe("commitTimelineCompositionInsertion", () => {
 
     expect(fetchMock).toHaveBeenCalledTimes(2);
     expect(fetchMock.mock.calls.map(([url]) => url)).toEqual([
-      "/api/projects/launch%2Fdemo/files/index.html",
-      "/api/projects/launch%2Fdemo/file-mutations/insert-composition/index.html",
+      "/api/projects/launch%23demo/files/index.html",
+      "/api/projects/launch%23demo/file-mutations/insert-composition/index.html",
     ]);
     expect(recordEdit).toHaveBeenCalledOnce();
     expect(writeFile).not.toHaveBeenCalled();
diff --git a/packages/studio/vite.adapter.projects.test.ts b/packages/studio/vite.adapter.projects.test.ts
new file mode 100644
index 0000000000..bcb9b6f651
--- /dev/null
+++ b/packages/studio/vite.adapter.projects.test.ts
@@ -0,0 +1,80 @@
+import { afterEach, describe, expect, it } from "vitest";
+import { mkdtempSync, mkdirSync, realpathSync, rmSync, symlinkSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join, win32 } from "node:path";
+import { isValidProjectId } from "./src/utils/projectRouting";
+import { createStudioApi } from "@hyperframes/studio-server";
+import type { ViteDevServer } from "vite";
+import { createProjectSignatureCache, createViteAdapter } from "./vite.adapter";
+
+const roots: string[] = [];
+afterEach(() => {
+  for (const root of roots.splice(0)) rmSync(root, { recursive: true, force: true });
+});
+
+function fixture() {
+  const root = mkdtempSync(join(tmpdir(), "hf-project-routing-"));
+  roots.push(root);
+  const data = join(root, "data");
+  const sessions = join(root, "sessions");
+  mkdirSync(data);
+  mkdirSync(sessions);
+  // Project resolution does not call Vite's module loader.
+  const adapter = createViteAdapter(
+    data,
+    {} as ViteDevServer,
+    createProjectSignatureCache({ compute: () => "test" }),
+  );
+  const app = createStudioApi(adapter);
+  return { root, data, sessions, adapter, app };
+}
+
+describe("Vite project resolution boundary", () => {
+  it.each(["C%3A", "C%3Ademo", "..%2Fsessions", "a%2Fb", "a%5Cb", "%2E%2E%2Fsessions", "a%00b"])(
+    "rejects a router-decoded unsafe ID: %s",
+    async (id) => {
+      const { app } = fixture();
+      expect((await app.request(`http://localhost/projects/${id}`)).status).toBe(404);
+    },
+  );
+
+  it("preserves valid names, session aliases, and explicitly listed symlink projects", async () => {
+    const { data, sessions, root, adapter, app } = fixture();
+    const id = "..Mañana #1 50%";
+    mkdirSync(join(data, id));
+    expect((await app.request(`http://localhost/projects/${encodeURIComponent(id)}`)).status).toBe(
+      200,
+    );
+    writeFileSync(join(sessions, "alias.json"), JSON.stringify({ projectId: id, title: "Title" }));
+    expect(adapter.resolveProject("alias")?.id).toBe(id);
+    const linked = join(root, "linked");
+    mkdirSync(linked);
+    symlinkSync(linked, join(data, "shortcut"), "junction");
+    expect(adapter.resolveProject("shortcut")?.dir).toBe(realpathSync(linked));
+  });
+
+  it("rejects drive-relative IDs that Windows resolves as root or sibling aliases", () => {
+    expect(win32.resolve("C:\\hf\\data", "C:")).toBe("C:\\hf\\data");
+    expect(win32.resolve("C:\\hf\\data", "C:demo")).toBe("C:\\hf\\data\\demo");
+    expect(isValidProjectId("C:")).toBe(false);
+    expect(isValidProjectId("C:demo")).toBe(false);
+  });
+
+  it.skipIf(process.platform === "win32")(
+    "does not discover POSIX directory names outside the portable ID contract",
+    async () => {
+      const { data, adapter } = fixture();
+      for (const id of ["valid", "bad\\name", "bad\nname", "C:demo"]) {
+        mkdirSync(join(data, id));
+        writeFileSync(join(data, id, "index.html"), "<html></html>");
+      }
+      expect((await adapter.listProjects()).map((project) => project.id)).toEqual(["valid"]);
+    },
+  );
+
+  it("rejects traversal in a session's project mapping", () => {
+    const { sessions, adapter } = fixture();
+    writeFileSync(join(sessions, "alias.json"), JSON.stringify({ projectId: "../sessions" }));
+    expect(adapter.resolveProject("alias")).toBeNull();
+  });
+});
diff --git a/packages/studio/vite.adapter.ts b/packages/studio/vite.adapter.ts
index d34346251e..13017fc6ca 100644
--- a/packages/studio/vite.adapter.ts
+++ b/packages/studio/vite.adapter.ts
@@ -1,3 +1,4 @@
+import { isValidProjectId } from "./src/utils/projectRouting";
 // Vite adapter that wires the shared Studio API to the local filesystem and build tools.
 
 import {
@@ -10,7 +11,7 @@ import {
   copyFileSync,
   unlinkSync,
 } from "node:fs";
-import { join, relative, resolve, isAbsolute, dirname } from "node:path";
+import { join, relative, resolve, isAbsolute, dirname, sep } from "node:path";
 import type { ViteDevServer } from "vite";
 import {
   type ResolvedProject,
@@ -30,7 +31,9 @@ function isPathWithin(parentDir: string, childPath: string): boolean {
   const childRelativePath = relative(resolve(parentDir), resolve(childPath));
   return (
     childRelativePath === "" ||
-    (!childRelativePath.startsWith("..") && !isAbsolute(childRelativePath))
+    (childRelativePath !== ".." &&
+      !childRelativePath.startsWith(`..${sep}`) &&
+      !isAbsolute(childRelativePath))
   );
 }
 
@@ -183,6 +186,7 @@ export function createViteAdapter(
       return readdirSync(dataDir, { withFileTypes: true })
         .filter(
           (d) =>
+            isValidProjectId(d.name) &&
             (d.isDirectory() || d.isSymbolicLink()) &&
             (existsSync(join(dataDir, d.name, "index.html")) ||
               existsSync(join(dataDir, d.name, `${d.name}.html`))),
@@ -201,15 +205,19 @@ export function createViteAdapter(
 
     // fallow-ignore-next-line complexity
     resolveProject(id: string) {
-      let projectDir = join(dataDir, id);
+      if (!isValidProjectId(id)) return null;
+      let projectDir = resolve(dataDir, id);
+      if (!isPathWithin(dataDir, projectDir)) return null;
       if (!existsSync(projectDir)) {
         const sessionsDir = resolve(dataDir, "../sessions");
-        const sessionFile = join(sessionsDir, `${id}.json`);
+        const sessionFile = resolve(sessionsDir, `${id}.json`);
+        if (!isPathWithin(sessionsDir, sessionFile)) return null;
         if (existsSync(sessionFile)) {
           try {
             const session = JSON.parse(readFileSync(sessionFile, "utf-8"));
-            if (session.projectId) {
-              projectDir = join(dataDir, session.projectId);
+            if (typeof session.projectId === "string" && isValidProjectId(session.projectId)) {
+              projectDir = resolve(dataDir, session.projectId);
+              if (!isPathWithin(dataDir, projectDir)) return null;
               if (existsSync(projectDir)) {
                 return {
                   id: session.projectId,