diff --git a/apps/desktop/src/settings/DesktopClientSettings.test.ts b/apps/desktop/src/settings/DesktopClientSettings.test.ts
index ea2a80010124..f6622049ab49 100644
--- a/apps/desktop/src/settings/DesktopClientSettings.test.ts
+++ b/apps/desktop/src/settings/DesktopClientSettings.test.ts
@@ -34,6 +34,7 @@ const clientSettings: ClientSettings = {
   contextWindowMeterEnabled: false,
   composerCollapseOnScroll: true,
   dismissedProviderUpdateNotificationKeys: [],
+  diffFilesCollapsed: true,
   diffIgnoreWhitespace: true,
   diffLayout: "stacked",
   environmentIdentificationMode: "artwork",
diff --git a/apps/web/src/clientPersistenceStorage.test.ts b/apps/web/src/clientPersistenceStorage.test.ts
index a86177b48eb3..17e531918ab1 100644
--- a/apps/web/src/clientPersistenceStorage.test.ts
+++ b/apps/web/src/clientPersistenceStorage.test.ts
@@ -114,6 +114,21 @@ describe("clientPersistenceStorage", () => {
     expect(settings).not.toHaveProperty("diffWordWrap");
   });
 
+  it("keeps the default diff file state across reloads and defaults it to expanded", async () => {
+    const testWindow = getTestWindow();
+    const { readBrowserClientSettings, writeBrowserClientSettings } =
+      await import("./clientPersistenceStorage");
+
+    testWindow.localStorage.setItem("t3code:client-settings:v1", JSON.stringify({}));
+    expect(readBrowserClientSettings()?.diffFilesCollapsed).toBe(false);
+
+    writeBrowserClientSettings({ ...DEFAULT_CLIENT_SETTINGS, diffFilesCollapsed: true });
+    expect(readBrowserClientSettings()?.diffFilesCollapsed).toBe(true);
+
+    writeBrowserClientSettings({ ...DEFAULT_CLIENT_SETTINGS, diffFilesCollapsed: false });
+    expect(readBrowserClientSettings()?.diffFilesCollapsed).toBe(false);
+  });
+
   it("keeps the diff layout across reloads and defaults it to stacked", async () => {
     const testWindow = getTestWindow();
     const { readBrowserClientSettings, writeBrowserClientSettings } =
diff --git a/apps/web/src/components/DiffPanel.tsx b/apps/web/src/components/DiffPanel.tsx
index d764b9c6a9e2..ca0bdaa7b4d5 100644
--- a/apps/web/src/components/DiffPanel.tsx
+++ b/apps/web/src/components/DiffPanel.tsx
@@ -226,10 +226,6 @@ export default function DiffPanel({
     ? `${routeThreadRef.environmentId}:${routeThreadRef.threadId}:${reviewSectionId}`
     : null;
   const codeViewMountKey = `${collapseScopeKey ?? reviewSectionId}:${codeViewRevision}`;
-  const collapsedDiffFileKeys =
-    collapsedDiffFiles.scopeKey === collapseScopeKey
-      ? collapsedDiffFiles.fileKeys
-      : EMPTY_COLLAPSED_DIFF_FILE_KEYS;
   const reviewSectionTitle = selectedTurn
     ? `Turn ${selectedCheckpointTurnCount ?? "?"}`
     : selectedGitScope === "unstaged"
@@ -425,6 +421,17 @@ export default function DiffPanel({
       })),
     [renderableFiles],
   );
+  const defaultCollapsedDiffFileKeys = useMemo(
+    () =>
+      settings.diffFilesCollapsed
+        ? new Set(renderableFileEntries.map((file) => file.fileKey))
+        : EMPTY_COLLAPSED_DIFF_FILE_KEYS,
+    [renderableFileEntries, settings.diffFilesCollapsed],
+  );
+  const collapsedDiffFileKeys =
+    collapsedDiffFiles.scopeKey === collapseScopeKey
+      ? collapsedDiffFiles.fileKeys
+      : defaultCollapsedDiffFileKeys;
   const codeViewFiles = useMemo(
     () =>
       renderableFileEntries.map(({ fileDiff, fileKey, fileVersion }) => {
@@ -462,14 +469,16 @@ export default function DiffPanel({
       if (!file) return;
       if (file.collapsed) {
         setCollapsedDiffFiles((current) => {
-          const next = new Set(current.scopeKey === collapseScopeKey ? current.fileKeys : []);
+          const next = new Set(
+            current.scopeKey === collapseScopeKey ? current.fileKeys : defaultCollapsedDiffFileKeys,
+          );
           next.delete(file.fileKey);
           return { scopeKey: collapseScopeKey, fileKeys: next };
         });
       }
       requestTreeReveal(file.fileKey);
     },
-    [codeViewFiles, collapseScopeKey, requestTreeReveal],
+    [codeViewFiles, collapseScopeKey, defaultCollapsedDiffFileKeys, requestTreeReveal],
   );
 
   const openDiffFile = useCallback(
@@ -503,7 +512,9 @@ export default function DiffPanel({
   const toggleDiffFileCollapsed = useCallback(
     (fileKey: string) => {
       setCollapsedDiffFiles((current) => {
-        const next = new Set(current.scopeKey === collapseScopeKey ? current.fileKeys : []);
+        const next = new Set(
+          current.scopeKey === collapseScopeKey ? current.fileKeys : defaultCollapsedDiffFileKeys,
+        );
         if (next.has(fileKey)) {
           next.delete(fileKey);
         } else {
@@ -512,21 +523,21 @@ export default function DiffPanel({
         return { scopeKey: collapseScopeKey, fileKeys: next };
       });
     },
-    [collapseScopeKey],
+    [collapseScopeKey, defaultCollapsedDiffFileKeys],
   );
 
   const toggleDiffFileCollapse = useCallback(() => {
     setCodeViewRevision((current) => current + 1);
     setCollapsedDiffFiles((current) => {
       const currentKeys =
-        current.scopeKey === collapseScopeKey ? current.fileKeys : EMPTY_COLLAPSED_DIFF_FILE_KEYS;
+        current.scopeKey === collapseScopeKey ? current.fileKeys : defaultCollapsedDiffFileKeys;
 
       return {
         scopeKey: collapseScopeKey,
         fileKeys: toggleAllDiffFiles(diffFileKeys, currentKeys),
       };
     });
-  }, [collapseScopeKey, diffFileKeys]);
+  }, [collapseScopeKey, defaultCollapsedDiffFileKeys, diffFileKeys]);
 
   const selectTurn = (turnId: TurnId) => {
     if (!routeThreadRef) return;
diff --git a/apps/web/src/components/pullRequest/PullRequestCodeTab.tsx b/apps/web/src/components/pullRequest/PullRequestCodeTab.tsx
index b77a3711d90f..b0bc3fdb8e22 100644
--- a/apps/web/src/components/pullRequest/PullRequestCodeTab.tsx
+++ b/apps/web/src/components/pullRequest/PullRequestCodeTab.tsx
@@ -485,7 +485,11 @@ function PullRequestCodeTab({
           groupAt(anchor.side, anchor.line).draft = true;
         }
 
-        const collapsed = isFileDiffCollapsed(fileKey, foldOverride, toggledFiles);
+        const collapsed = isFileDiffCollapsed(
+          fileKey,
+          foldOverride ?? (settings.diffFilesCollapsed ? "folded" : "expanded"),
+          toggledFiles,
+        );
 
         const annotations: ReviewAnnotation[] = [...groups.values()].map((group) => ({
           side: toViewerSide(group.side),
@@ -538,6 +542,7 @@ function PullRequestCodeTab({
       foldOverride,
       pendingComments,
       placedThreadIds,
+      settings.diffFilesCollapsed,
       toggledFiles,
     ],
   );
diff --git a/apps/web/src/components/pullRequest/pullRequestDiff.logic.ts b/apps/web/src/components/pullRequest/pullRequestDiff.logic.ts
index a286a4552cb6..e818e989d200 100644
--- a/apps/web/src/components/pullRequest/pullRequestDiff.logic.ts
+++ b/apps/web/src/components/pullRequest/pullRequestDiff.logic.ts
@@ -30,8 +30,8 @@ export type DiffFoldOverride = "expanded" | "folded" | null;
  * A diff arrives a slice at a time, so the reader's own choices are kept as the difference from
  * what the toolbar last said rather than as the set of folded files: a file that has not loaded
  * yet cannot be in a set, and would otherwise land expanded moments after the reader folded
- * everything. Files start expanded so opening the Code tab immediately shows the change; the
- * reader can still fold individual files or the whole diff from the toolbar.
+ * everything. The caller supplies the saved default until the toolbar overrides it; individual
+ * files can still be toggled independently.
  */
 export function isFileDiffCollapsed(
   fileKey: string,
diff --git a/apps/web/src/components/settings/SettingsPanels.tsx b/apps/web/src/components/settings/SettingsPanels.tsx
index 8a78d4138c6b..98684f50721b 100644
--- a/apps/web/src/components/settings/SettingsPanels.tsx
+++ b/apps/web/src/components/settings/SettingsPanels.tsx
@@ -533,6 +533,9 @@ export function useSettingsRestore(onRestored?: () => void) {
         : []),
       ...(settings.wordWrap !== DEFAULT_UNIFIED_SETTINGS.wordWrap ? ["Word wrap"] : []),
       ...getChangedTypographySettingLabels(settings),
+      ...(settings.diffFilesCollapsed !== DEFAULT_UNIFIED_SETTINGS.diffFilesCollapsed
+        ? ["Default diff file state"]
+        : []),
       ...(settings.diffIgnoreWhitespace !== DEFAULT_UNIFIED_SETTINGS.diffIgnoreWhitespace
         ? ["Diff whitespace changes"]
         : []),
@@ -608,6 +611,7 @@ export function useSettingsRestore(onRestored?: () => void) {
       settings.addProjectBaseDirectory,
       settings.defaultThreadEnvMode,
       settings.newWorktreesStartFromOrigin,
+      settings.diffFilesCollapsed,
       settings.diffIgnoreWhitespace,
       settings.diffLayout,
       settings.proactivePanelsEnabled,
@@ -706,6 +710,7 @@ export function useSettingsRestore(onRestored?: () => void) {
       diffColorScheme: DEFAULT_UNIFIED_SETTINGS.diffColorScheme,
       timestampFormat: DEFAULT_UNIFIED_SETTINGS.timestampFormat,
       wordWrap: DEFAULT_UNIFIED_SETTINGS.wordWrap,
+      diffFilesCollapsed: DEFAULT_UNIFIED_SETTINGS.diffFilesCollapsed,
       diffIgnoreWhitespace: DEFAULT_UNIFIED_SETTINGS.diffIgnoreWhitespace,
       diffLayout: DEFAULT_UNIFIED_SETTINGS.diffLayout,
       proactivePanelsEnabled: DEFAULT_UNIFIED_SETTINGS.proactivePanelsEnabled,
@@ -2304,6 +2309,48 @@ export function GeneralSettingsPanel() {
             />
           }
         />
+        <SettingsRow
+          {...searchableSetting("default-diff-file-state")}
+          description="Start with files expanded or collapsed when opening diffs or a pull request's Code tab."
+          resetAction={
+            settings.diffFilesCollapsed !== DEFAULT_UNIFIED_SETTINGS.diffFilesCollapsed ? (
+              <SettingResetButton
+                label="default diff file state"
+                onClick={() =>
+                  updateSettings({
+                    diffFilesCollapsed: DEFAULT_UNIFIED_SETTINGS.diffFilesCollapsed,
+                  })
+                }
+              />
+            ) : null
+          }
+          control={
+            <Select
+              value={settings.diffFilesCollapsed ? "collapsed" : "expanded"}
+              onValueChange={(value) => {
+                if (value === "expanded" || value === "collapsed") {
+                  updateSettings({ diffFilesCollapsed: value === "collapsed" });
+                }
+              }}
+            >
+              <SelectTrigger
+                size="sm"
+                className="w-full sm:w-40"
+                aria-label="Default diff file state"
+              >
+                <SelectValue>{settings.diffFilesCollapsed ? "Collapsed" : "Expanded"}</SelectValue>
+              </SelectTrigger>
+              <SelectPopup align="end" alignItemWithTrigger={false}>
+                <SelectItem hideIndicator value="expanded">
+                  Expanded
+                </SelectItem>
+                <SelectItem hideIndicator value="collapsed">
+                  Collapsed
+                </SelectItem>
+              </SelectPopup>
+            </Select>
+          }
+        />
         <SettingsRow
           {...searchableSetting("diff-layout")}
           description="Show diffs stacked or side by side. The toggle in the diff toolbar changes this too."
diff --git a/apps/web/src/components/settings/settingsSearch.ts b/apps/web/src/components/settings/settingsSearch.ts
index 60ef9760af93..45354c25220c 100644
--- a/apps/web/src/components/settings/settingsSearch.ts
+++ b/apps/web/src/components/settings/settingsSearch.ts
@@ -247,6 +247,12 @@ export const SETTINGS_SEARCH_ITEMS = [
     to: "/settings/general",
     searchTerms: ["diff ignore spaces edits default"],
   },
+  {
+    id: "default-diff-file-state",
+    title: "Default diff file state",
+    to: "/settings/general",
+    searchTerms: ["collapsed expanded collapse expand files pull request pr code tab"],
+  },
   {
     id: "diff-layout",
     title: "Diff layout",
diff --git a/packages/contracts/src/settings.test.ts b/packages/contracts/src/settings.test.ts
index 2fb6ead09c16..48f147be051e 100644
--- a/packages/contracts/src/settings.test.ts
+++ b/packages/contracts/src/settings.test.ts
@@ -164,6 +164,20 @@ describe("ClaudeSettings auto-compaction", () => {
   });
 });
 
+describe("ClientSettings default diff file state", () => {
+  it("keeps files expanded when existing settings omit the preference", () => {
+    expect(decodeClientSettings({}).diffFilesCollapsed).toBe(false);
+  });
+
+  it.each([true, false])("preserves a saved collapsed preference of %s", (diffFilesCollapsed) => {
+    const settings = decodeClientSettings({ diffFilesCollapsed });
+    expect(encodeClientSettings(settings).diffFilesCollapsed).toBe(diffFilesCollapsed);
+    expect(decodeClientSettingsPatch({ diffFilesCollapsed }).diffFilesCollapsed).toBe(
+      diffFilesCollapsed,
+    );
+  });
+});
+
 describe("ClientSettings diff colors", () => {
   it("keeps red and green for existing settings without a saved palette", () => {
     expect(decodeClientSettings({}).diffColorScheme).toBe("red-green");
diff --git a/packages/contracts/src/settings.ts b/packages/contracts/src/settings.ts
index 6b931f4ef3ea..b420b737ee0a 100644
--- a/packages/contracts/src/settings.ts
+++ b/packages/contracts/src/settings.ts
@@ -344,6 +344,7 @@ export const ClientSettingsSchema = Schema.Struct({
   dismissedProviderUpdateNotificationKeys: Schema.Array(TrimmedNonEmptyString).pipe(
     Schema.withDecodingDefault(Effect.succeed([])),
   ),
+  diffFilesCollapsed: Schema.Boolean.pipe(Schema.withDecodingDefault(Effect.succeed(false))),
   diffIgnoreWhitespace: Schema.Boolean.pipe(Schema.withDecodingDefault(Effect.succeed(true))),
   diffLayout: DiffLayout.pipe(Schema.withDecodingDefault(Effect.succeed(DEFAULT_DIFF_LAYOUT))),
   environmentIdentificationMode: EnvironmentIdentificationMode.pipe(
@@ -1432,6 +1433,7 @@ export const ClientSettingsPatch = Schema.Struct({
   confirmThreadArchive: Schema.optionalKey(Schema.Boolean),
   confirmThreadDelete: Schema.optionalKey(Schema.Boolean),
   confirmThreadUnpin: Schema.optionalKey(Schema.Boolean),
+  diffFilesCollapsed: Schema.optionalKey(Schema.Boolean),
   diffIgnoreWhitespace: Schema.optionalKey(Schema.Boolean),
   diffLayout: Schema.optionalKey(DiffLayout),
   environmentIdentificationMode: Schema.optionalKey(EnvironmentIdentificationMode),