diff --git a/apps/web/src/components/DiffPanel.browser.test.tsx b/apps/web/src/components/DiffPanel.browser.test.tsx
index 503f9b60c1b5d..604889da5bb68 100644
--- a/apps/web/src/components/DiffPanel.browser.test.tsx
+++ b/apps/web/src/components/DiffPanel.browser.test.tsx
@@ -94,6 +94,7 @@ vi.mock("./DiffFilePathCopyButton", () => ({ DiffFilePathCopyButton: () => null
 
 import DiffPanel from "./DiffPanel";
 import { useDiffPanelSessionStore, useDiffPanelStore } from "~/diffPanelStore";
+import { selectThreadRightPanelState, useRightPanelStore } from "~/rightPanelStore";
 
 const cwd = "/project-worktree";
 const errorMessage = "Could not find a common commit between 'origin/orphan' and HEAD.";
@@ -126,6 +127,7 @@ function setup(failedBaseRef: string | null) {
       branchBaseRefByThreadKey: {},
     });
     useDiffPanelSessionStore.setState({ collapsedFileKeysByScopeKey: {} });
+    useRightPanelStore.setState({ byThreadKey: {}, threadPanelVisibilityByThreadKey: {} });
   };
   reset();
   state.diffFilesCollapsed = false;
@@ -233,6 +235,36 @@ it.each([
   },
 );
 
+it("opens a repository-relative file from a nested project without a remote", async () => {
+  using context = setup("origin/orphan");
+  state.thread = { ...state.thread!, worktreePath: null };
+  const path = "project/back\\slash.txt";
+  const metadata = preview();
+  const source = {
+    ...metadata.sources[0]!,
+    files: [{ path, previousPath: null, additions: 1, deletions: 0 }],
+  };
+  state.previews.set("main", Atom.make(AsyncResult.success({ ...metadata, sources: [source] })));
+  state.status = Atom.make(
+    AsyncResult.success({
+      ...AsyncResult.getOrElse(context.registry.get(state.status), () => {
+        throw new Error("Missing status");
+      }),
+      workingTree: { rootPath: "/", files: [], insertions: 0, deletions: 0 },
+    }),
+  );
+  useDiffPanelStore.getState().selectBranchBaseRef(context.threadRef, "main");
+  await using screen = await render(
+    <RegistryContext.Provider value={context.registry}>
+      <DiffPanel composerDraftTarget={context.threadRef} workspaceMutationId={null} visible />
+    </RegistryContext.Provider>,
+  );
+  await userEvent.click(screen.getByText(path, { exact: true }));
+  expect(
+    selectThreadRightPanelState(useRightPanelStore.getState().byThreadKey, context.threadRef),
+  ).toMatchObject({ activeSurfaceId: "file:back\\slash.txt" });
+});
+
 it.each([false, true])(
   "expands an unloaded file on the first click with default collapse %s",
   async (collapsed) => {
diff --git a/apps/web/src/components/DiffPanel.tsx b/apps/web/src/components/DiffPanel.tsx
index 71c197b6ce363..2ec6d51997ace 100644
--- a/apps/web/src/components/DiffPanel.tsx
+++ b/apps/web/src/components/DiffPanel.tsx
@@ -254,9 +254,6 @@ export default function DiffPanel({
       : null,
   );
   const activeCwd = activeThread?.worktreePath ?? activeProject?.workspaceRoot;
-  const activeRepositoryRoot = activeThread?.worktreePath
-    ? undefined
-    : activeProject?.repositoryIdentity?.rootPath;
   const serverConfig = useAtomValue(
     serverEnvironment.configValueAtom(activeThread?.environmentId ?? null),
   );
@@ -274,6 +271,9 @@ export default function DiffPanel({
         })
       : null,
   );
+  const activeRepositoryRoot =
+    gitStatusQuery.data?.workingTree.rootPath ??
+    (activeThread?.worktreePath ? undefined : activeProject?.repositoryIdentity?.rootPath);
   const diffSelection = useDiffPanelStore((state) =>
     selectThreadDiffPanelSelection(state.byThreadKey, routeThreadRef),
   );
diff --git a/apps/web/src/components/chat/TimelineAssistantRows.tsx b/apps/web/src/components/chat/TimelineAssistantRows.tsx
index 1d03161e83014..b62f6f8e4b10f 100644
--- a/apps/web/src/components/chat/TimelineAssistantRows.tsx
+++ b/apps/web/src/components/chat/TimelineAssistantRows.tsx
@@ -32,6 +32,11 @@ import { readThreadTurnItems } from "../../state/entities";
 import { useV2ItemSupport } from "../../state/v2ItemSupport";
 import { TimelineRowCtx, TimelineRowActivityCtx, type TimelineRow } from "./timelineRowContext";
 import { MESSAGE_HEADING_LEVEL, MessageAuthorHeading } from "./TimelineRowParts";
+import { useAtomValue } from "@effect/atom-react";
+import { AsyncResult, Atom } from "effect/reactivity";
+import { vcsEnvironment } from "../../state/vcs";
+
+const NO_VCS_STATUS = Atom.make(AsyncResult.initial());
 
 export function AssistantTimelineRow({ row }: { row: Extract<TimelineRow, { kind: "message" }> }) {
   const ctx = use(TimelineRowCtx);
@@ -460,6 +465,15 @@ function AssistantChangedFilesSectionInner({
       : null,
   );
   const onFileContextMenu = useFileContextMenuHandler(ctx.activeThreadEnvironmentId);
+  const repositoryRoot = useAtomValue(
+    ctx.workspaceRoot
+      ? vcsEnvironment.status({
+          environmentId: ctx.activeThreadEnvironmentId,
+          input: { cwd: ctx.workspaceRoot },
+        })
+      : NO_VCS_STATUS,
+    (status) => AsyncResult.getOrElse(status, () => null)?.workingTree.rootPath,
+  );
 
   return (
     <ChangedFilesCard
@@ -479,9 +493,10 @@ function AssistantChangedFilesSectionInner({
             filePath,
             workspaceRoot: ctx.workspaceRoot,
             repositoryRoot:
-              thread?.worktreePath == null
+              repositoryRoot ??
+              (thread?.worktreePath == null
                 ? activeProject?.repositoryIdentity?.rootPath
-                : undefined,
+                : undefined),
           },
           event,
         )
diff --git a/apps/web/src/diffFileActions.test.ts b/apps/web/src/diffFileActions.test.ts
index f7faf72ef8bd3..8f7b2bc597b79 100644
--- a/apps/web/src/diffFileActions.test.ts
+++ b/apps/web/src/diffFileActions.test.ts
@@ -45,15 +45,39 @@ describe("openDiffFilePrimaryAction", () => {
     ).toBe("frontend/Dockerfile");
   });
 
-  it("handles Windows roots and mixed diff separators", () => {
-    expect(
-      resolveDiffPathForWorkspace({
-        filePath: "Frontend/src\\index.ts",
-        workspaceRoot: "C:\\repo\\frontend",
-        repositoryRoot: "C:\\repo",
-      }),
-    ).toBe("src/index.ts");
-  });
+  it.each([
+    {
+      filePath: "Frontend/src\\index.ts",
+      workspaceRoot: "C:\\repo\\frontend",
+      repositoryRoot: "C:\\repo",
+      expected: "src/index.ts",
+    },
+    {
+      filePath: "Frontend\\src\\index.ts",
+      workspaceRoot: "C:\\repo\\frontend",
+      repositoryRoot: "C:\\repo",
+      expected: "src/index.ts",
+    },
+    {
+      filePath: "frontend/back\\slash.txt",
+      workspaceRoot: "/repo/frontend",
+      repositoryRoot: "/repo",
+      expected: "back\\slash.txt",
+    },
+    {
+      filePath: "back/slash/a.ts",
+      workspaceRoot: "/repo/back\\slash",
+      repositoryRoot: "/repo",
+      expected: null,
+    },
+  ])(
+    "resolves $filePath in $workspaceRoot",
+    ({ filePath, workspaceRoot, repositoryRoot, expected }) => {
+      expect(resolveDiffPathForWorkspace({ filePath, workspaceRoot, repositoryRoot })).toBe(
+        expected,
+      );
+    },
+  );
 
   // regression: #6174 opening diff files broke for repositories at a filesystem root
   it.each([
@@ -89,6 +113,7 @@ describe("openDiffFilePrimaryAction", () => {
     "backend/server.ts",
     "frontend2/app.ts",
     "frontend/../secret.ts",
+    "frontend/..\\secret.ts",
     "C:secret.ts",
     "/repo/backend/server.ts",
     "/repo/frontend/../backend/server.ts",
diff --git a/apps/web/src/diffFileActions.ts b/apps/web/src/diffFileActions.ts
index c6a3fa3d436fc..046baa4ce5b64 100644
--- a/apps/web/src/diffFileActions.ts
+++ b/apps/web/src/diffFileActions.ts
@@ -17,16 +17,22 @@ interface OpenDiffFilePrimaryActionInput {
   readonly openInEditor: (targetPath: string) => void;
 }
 
-function normalizedRelativePathSegments(filePath: string): ReadonlyArray<string> | null {
+function normalizedRelativePathSegments(
+  filePath: string,
+  windowsRoot: boolean,
+): ReadonlyArray<string> | null {
   if (filePath.startsWith("/") || isWindowsAbsolutePath(filePath) || /^[a-zA-Z]:/.test(filePath)) {
     return null;
   }
 
-  const segments = filePath
-    .replaceAll("\\", "/")
-    .split("/")
-    .filter((segment) => segment.length > 0 && segment !== ".");
-  if (segments.length === 0 || segments.includes("..")) return null;
+  // On POSIX a backslash is part of a file name; Windows treats it as a separator
+  // (tool-call patches can name files with either), so reject ".." either way.
+  const parts = filePath.split(/[\\/]/);
+  if (parts.includes("..")) return null;
+  const segments = (windowsRoot ? parts : filePath.split("/")).filter(
+    (segment) => segment.length > 0 && segment !== ".",
+  );
+  if (segments.length === 0) return null;
   return segments;
 }
 
@@ -40,16 +46,13 @@ function repositoryRelativeWorkspaceSegments(
   const normalizedRepositoryRoot = normalizeProjectPathForComparison(repositoryRoot);
   if (normalizedWorkspaceRoot === normalizedRepositoryRoot) return [];
 
-  const separator = normalizedRepositoryRoot.includes("\\") ? "\\" : "/";
+  const separator = isWindowsAbsolutePath(normalizedRepositoryRoot) ? "\\" : "/";
   const repositoryPrefix = normalizedRepositoryRoot.endsWith(separator)
     ? normalizedRepositoryRoot
     : `${normalizedRepositoryRoot}${separator}`;
   if (!normalizedWorkspaceRoot.startsWith(repositoryPrefix)) return null;
 
-  return normalizedWorkspaceRoot
-    .slice(repositoryPrefix.length)
-    .split(/[\\/]+/)
-    .filter(Boolean);
+  return normalizedWorkspaceRoot.slice(repositoryPrefix.length).split(separator).filter(Boolean);
 }
 
 export function resolveDiffPathForWorkspace(input: {
@@ -57,14 +60,18 @@ export function resolveDiffPathForWorkspace(input: {
   readonly workspaceRoot: string | undefined;
   readonly repositoryRoot: string | undefined;
 }): string | null {
+  const windowsRoot = [input.workspaceRoot, input.repositoryRoot].some(
+    (root) => root !== undefined && isWindowsAbsolutePath(root),
+  );
   // Claude and ACP tool-call diffs name files by absolute path.
   if (isAbsolutePath(input.filePath)) {
     const relativePath = workspaceRelativeFilePath(input.filePath, input.workspaceRoot);
-    const segments = relativePath === null ? null : normalizedRelativePathSegments(relativePath);
+    const segments =
+      relativePath === null ? null : normalizedRelativePathSegments(relativePath, windowsRoot);
     return segments ? segments.join("/") : null;
   }
 
-  const fileSegments = normalizedRelativePathSegments(input.filePath);
+  const fileSegments = normalizedRelativePathSegments(input.filePath, windowsRoot);
   if (!fileSegments) return null;
 
   const workspaceSegments = repositoryRelativeWorkspaceSegments(