diff --git a/src/lib/onboard/host-mount/host-mount.test.ts b/src/lib/onboard/host-mount/host-mount.test.ts
index bb45600b549..a2abe4a216e 100644
--- a/src/lib/onboard/host-mount/host-mount.test.ts
+++ b/src/lib/onboard/host-mount/host-mount.test.ts
@@ -45,7 +45,7 @@ describe("read-only host mount validation", () => {
   });
 
   it.each(["\u001b[31m", "\u202e", "\u2028", "\u2029"])(
-    "rejects missing, relative, symlinked, non-normalized, and terminal-control paths [%s]",
+    "rejects missing, unreadable, relative, symlinked, non-normalized, and terminal-control paths [%s]",
     (terminalControl) => {
       const source = workspaceTempDir();
       const symlink = `${source}-link`;
@@ -56,7 +56,7 @@ describe("read-only host mount validation", () => {
         "host directory must be an absolute path",
       );
       expect(() => parseReadOnlyHostMount(`${source}-missing:/sandbox/project`)).toThrow(
-        "does not exist",
+        "no such file or directory",
       );
       expect(() => parseReadOnlyHostMount(`${symlink}:/sandbox/project`)).toThrow(
         "must not contain symlinks",
@@ -65,6 +65,19 @@ describe("read-only host mount validation", () => {
         "normalized absolute path below /sandbox",
       );
 
+      const file = path.join(source, "alpha.txt");
+      fs.writeFileSync(file, "test");
+      let unreadable: unknown;
+      try {
+        parseReadOnlyHostMount(`${file}/child:/sandbox/project`);
+      } catch (error) {
+        unreadable = error;
+      }
+      expect((unreadable as NodeJS.ErrnoException | undefined)?.cause).toMatchObject({
+        code: "ENOTDIR",
+      });
+      expect(unreadable).toMatchObject({ message: expect.stringContaining("ENOTDIR") });
+
       let message = "";
       try {
         parseReadOnlyHostMount(`${source}:/sandbox/project${terminalControl}forged`);
@@ -108,7 +121,7 @@ describe("read-only host mount validation", () => {
     created.splice(created.indexOf(source), 1);
     expect(() =>
       normalizePersistedSandboxHostMounts([{ source, target: "/sandbox/project", readOnly: true }]),
-    ).toThrow("host directory does not exist");
+    ).toThrow("no such file or directory");
   });
 
   it("revalidates and compares unordered declarations before sandbox reuse", () => {
diff --git a/src/lib/state/registry/host-mount.ts b/src/lib/state/registry/host-mount.ts
index 0fe40f31485..2d81a5ab671 100644
--- a/src/lib/state/registry/host-mount.ts
+++ b/src/lib/state/registry/host-mount.ts
@@ -13,8 +13,19 @@ export function hasUnsafeHostMountTerminalText(value: string): boolean {
   return UNSAFE_TERMINAL_TEXT.test(value);
 }
 
-function failHostMount(value: string, detail: string): never {
-  throw new Error(`Invalid --host-mount '${value}': ${detail}`);
+function failHostMount(value: string, detail: string, cause?: unknown): never {
+  throw new Error(
+    `Invalid --host-mount '${value}': ${detail}`,
+    cause === undefined ? undefined : { cause },
+  );
+}
+
+function failUnreadableHostMountSource(value: string, source: string, error: unknown): never {
+  failHostMount(
+    value,
+    `host directory is not usable: ${source}. ${error instanceof Error ? error.message : String(error)}`,
+    error,
+  );
 }
 
 function assertNoSymlinkComponents(source: string, original: string): void {
@@ -25,8 +36,8 @@ function assertNoSymlinkComponents(source: string, original: string): void {
     let stat: fs.Stats;
     try {
       stat = fs.lstatSync(current);
-    } catch {
-      failHostMount(original, `host directory does not exist: ${source}`);
+    } catch (error) {
+      failUnreadableHostMountSource(original, source, error);
     }
     if (stat.isSymbolicLink()) {
       failHostMount(original, `host path must not contain symlinks: ${current}`);
@@ -52,8 +63,8 @@ export function parseReadOnlyHostMount(value: string): SandboxHostMount {
   let sourceStat: fs.BigIntStats;
   try {
     sourceStat = fs.statSync(source, { bigint: true });
-  } catch {
-    failHostMount(value, `host directory does not exist: ${source}`);
+  } catch (error) {
+    failUnreadableHostMountSource(value, source, error);
   }
   if (!sourceStat.isDirectory()) {
     failHostMount(value, `host path must be a directory: ${source}`);