diff --git a/apps/web/src/appInit.tsx b/apps/web/src/appInit.tsx
index b9312858e334b..3b852508b97a2 100644
--- a/apps/web/src/appInit.tsx
+++ b/apps/web/src/appInit.tsx
@@ -65,8 +65,16 @@ function startClientAppearance(): void {
     if (previous?.diffColorScheme !== settings.diffColorScheme) {
       root.dataset.diffColorScheme = settings.diffColorScheme;
     }
-    if (previous?.chatWidth !== settings.chatWidth) {
+    if (
+      previous?.chatWidth !== settings.chatWidth ||
+      previous.chatCustomWidthRem !== settings.chatCustomWidthRem
+    ) {
       root.dataset.chatWidth = settings.chatWidth;
+      if (settings.chatWidth === "custom") {
+        root.style.setProperty("--chat-content-max-width", `${settings.chatCustomWidthRem}rem`);
+      } else {
+        root.style.removeProperty("--chat-content-max-width");
+      }
     }
     if (previous?.appearanceContrast !== settings.appearanceContrast) {
       applyAppearanceContrast(root, settings.appearanceContrast);
diff --git a/apps/web/src/components/settings/AppearanceSettingsPanel.tsx b/apps/web/src/components/settings/AppearanceSettingsPanel.tsx
index 1c5812a2bda0d..f16efd9c7c791 100644
--- a/apps/web/src/components/settings/AppearanceSettingsPanel.tsx
+++ b/apps/web/src/components/settings/AppearanceSettingsPanel.tsx
@@ -7,9 +7,11 @@ import {
   type EffortDisplay,
   type EnvironmentIdentificationMode,
   MAX_APPEARANCE_CONTRAST,
+  MAX_CHAT_CUSTOM_WIDTH_REM,
   MAX_GLASS_OPACITY,
   MAX_PANEL_ANIMATION_DURATION_MS,
   MIN_APPEARANCE_CONTRAST,
+  MIN_CHAT_CUSTOM_WIDTH_REM,
   MIN_GLASS_OPACITY,
   MIN_PANEL_ANIMATION_DURATION_MS,
 } from "@t3tools/contracts/settings";
@@ -76,6 +78,7 @@ const CHAT_WIDTH_LABELS: Record<ChatWidth, string> = {
   comfortable: "Comfortable",
   wide: "Wide",
   full: "Full",
+  custom: "Custom",
 };
 
 const EFFORT_DISPLAY_LABELS: Record<EffortDisplay, string> = {
@@ -119,6 +122,14 @@ export function AppearanceSettingsPanel() {
     "--settings-slider-progress": `${appearanceContrastRatio * 100}%`,
     "--settings-slider-fill-offset": `${0.5 - appearanceContrastRatio}rem`,
   } as CSSProperties;
+  const chatCustomWidthRatio =
+    (settings.chatCustomWidthRem - MIN_CHAT_CUSTOM_WIDTH_REM) /
+    (MAX_CHAT_CUSTOM_WIDTH_REM - MIN_CHAT_CUSTOM_WIDTH_REM);
+  const chatCustomWidthLabel = `${settings.chatCustomWidthRem * settings.fontSizeInterface} px`;
+  const chatCustomWidthSliderStyle = {
+    "--settings-slider-progress": `${chatCustomWidthRatio * 100}%`,
+    "--settings-slider-fill-offset": `${0.5 - chatCustomWidthRatio}rem`,
+  } as CSSProperties;
   const panelAnimationDurationRatio =
     (settings.panelAnimationDurationMs - MIN_PANEL_ANIMATION_DURATION_MS) /
     (MAX_PANEL_ANIMATION_DURATION_MS - MIN_PANEL_ANIMATION_DURATION_MS);
@@ -246,10 +257,16 @@ export function AppearanceSettingsPanel() {
           {...searchableSetting("chat-width")}
           description="Set how wide messages and the composer can grow on large screens."
           resetAction={
-            settings.chatWidth !== DEFAULT_UNIFIED_SETTINGS.chatWidth ? (
+            settings.chatWidth !== DEFAULT_UNIFIED_SETTINGS.chatWidth ||
+            settings.chatCustomWidthRem !== DEFAULT_UNIFIED_SETTINGS.chatCustomWidthRem ? (
               <SettingResetButton
                 label="chat width"
-                onClick={() => updateSettings({ chatWidth: DEFAULT_UNIFIED_SETTINGS.chatWidth })}
+                onClick={() =>
+                  updateSettings({
+                    chatWidth: DEFAULT_UNIFIED_SETTINGS.chatWidth,
+                    chatCustomWidthRem: DEFAULT_UNIFIED_SETTINGS.chatCustomWidthRem,
+                  })
+                }
               />
             ) : null
           }
@@ -258,7 +275,12 @@ export function AppearanceSettingsPanel() {
               <Select
                 value={settings.chatWidth}
                 onValueChange={(value) => {
-                  if (value === "comfortable" || value === "wide" || value === "full")
+                  if (
+                    value === "comfortable" ||
+                    value === "wide" ||
+                    value === "full" ||
+                    value === "custom"
+                  )
                     updateSettings({ chatWidth: value });
                 }}
               >
@@ -269,11 +291,47 @@ export function AppearanceSettingsPanel() {
                   <SelectItem value="comfortable">Comfortable (default)</SelectItem>
                   <SelectItem value="wide">Wide</SelectItem>
                   <SelectItem value="full">Full</SelectItem>
+                  <SelectItem value="custom">Custom</SelectItem>
                 </SelectPopup>
               </Select>
             </div>
           }
-        />
+        >
+          {settings.chatWidth === "custom" ? (
+            <div className="flex justify-end pb-2">
+              <div className="flex w-full items-center gap-3 sm:w-52">
+                <output
+                  className="min-w-18 rounded-md bg-muted px-2 py-1 text-center font-mono text-xs font-medium tabular-nums text-foreground"
+                  htmlFor="chat-custom-width"
+                >
+                  {chatCustomWidthLabel}
+                </output>
+                <input
+                  aria-label="Custom chat width"
+                  aria-valuetext={`${chatCustomWidthLabel}`}
+                  className="settings-slider min-w-0 flex-1"
+                  id="chat-custom-width"
+                  max={MAX_CHAT_CUSTOM_WIDTH_REM}
+                  min={MIN_CHAT_CUSTOM_WIDTH_REM}
+                  onChange={(event) => {
+                    const chatCustomWidthRem = Number(event.currentTarget.value);
+                    if (
+                      Number.isInteger(chatCustomWidthRem) &&
+                      chatCustomWidthRem >= MIN_CHAT_CUSTOM_WIDTH_REM &&
+                      chatCustomWidthRem <= MAX_CHAT_CUSTOM_WIDTH_REM
+                    ) {
+                      updateSettings({ chatCustomWidthRem });
+                    }
+                  }}
+                  step={2}
+                  style={chatCustomWidthSliderStyle}
+                  type="range"
+                  value={settings.chatCustomWidthRem}
+                />
+              </div>
+            </div>
+          ) : null}
+        </SettingsRow>
         <ThreadDetailsItemsRow />
         <SettingsRow
           {...searchableSetting("effort-display")}
diff --git a/apps/web/src/components/settings/SettingsPanels.tsx b/apps/web/src/components/settings/SettingsPanels.tsx
index c4e3d27a07fa7..ace9d6b49a06b 100644
--- a/apps/web/src/components/settings/SettingsPanels.tsx
+++ b/apps/web/src/components/settings/SettingsPanels.tsx
@@ -433,7 +433,10 @@ export function useSettingsRestore(onRestored?: () => void) {
     ...(settings.diffColorScheme !== DEFAULT_UNIFIED_SETTINGS.diffColorScheme
       ? ["Diff colors"]
       : []),
-    ...(settings.chatWidth !== DEFAULT_UNIFIED_SETTINGS.chatWidth ? ["Chat width"] : []),
+    ...(settings.chatWidth !== DEFAULT_UNIFIED_SETTINGS.chatWidth ||
+    settings.chatCustomWidthRem !== DEFAULT_UNIFIED_SETTINGS.chatCustomWidthRem
+      ? ["Chat width"]
+      : []),
     ...(settings.effortDisplay !== DEFAULT_UNIFIED_SETTINGS.effortDisplay
       ? ["Effort display"]
       : []),
@@ -609,6 +612,7 @@ export function useSettingsRestore(onRestored?: () => void) {
       appearanceContrast: DEFAULT_UNIFIED_SETTINGS.appearanceContrast,
       diffColorScheme: DEFAULT_UNIFIED_SETTINGS.diffColorScheme,
       chatWidth: DEFAULT_UNIFIED_SETTINGS.chatWidth,
+      chatCustomWidthRem: DEFAULT_UNIFIED_SETTINGS.chatCustomWidthRem,
       effortDisplay: DEFAULT_UNIFIED_SETTINGS.effortDisplay,
       mathRenderingEnabled: DEFAULT_UNIFIED_SETTINGS.mathRenderingEnabled,
       timestampFormat: DEFAULT_UNIFIED_SETTINGS.timestampFormat,
diff --git a/apps/web/src/components/settings/settingsSearch.ts b/apps/web/src/components/settings/settingsSearch.ts
index aabfd0300eda5..d6aa5774c8c5a 100644
--- a/apps/web/src/components/settings/settingsSearch.ts
+++ b/apps/web/src/components/settings/settingsSearch.ts
@@ -323,7 +323,7 @@ export const SETTINGS_SEARCH_ITEMS = [
     id: "chat-width",
     title: "Chat width",
     to: "/settings/appearance",
-    searchTerms: ["wide full width column layout messages composer monitor"],
+    searchTerms: ["wide full custom width column layout messages composer monitor"],
   },
   {
     id: "thread-details-items",
diff --git a/apps/web/src/index.css b/apps/web/src/index.css
index 8acc312321b8f..52a48f86e3249 100644
--- a/apps/web/src/index.css
+++ b/apps/web/src/index.css
@@ -2254,7 +2254,8 @@ code {
 }
 
 /* Chat width setting. Comfortable keeps the default --chat-content-max-width,
-   which sizes the timeline lane, the composer, and ChatCanvas's width probe. */
+   which sizes the timeline lane, the composer, and ChatCanvas's width probe.
+   Custom sets it inline from appInit.tsx. */
 :root[data-chat-width="wide"] {
   --chat-content-max-width: 72rem;
 }
diff --git a/packages/contracts/src/settings.test.ts b/packages/contracts/src/settings.test.ts
index 859644a54c72f..1e50c5ad1b41a 100644
--- a/packages/contracts/src/settings.test.ts
+++ b/packages/contracts/src/settings.test.ts
@@ -291,6 +291,15 @@ describe("ClientSettings notifications", () => {
   });
 });
 
+describe("ClientSettings chat width", () => {
+  it("keeps a saved preset when existing settings omit the custom width", () => {
+    expect(decodeClientSettings({ chatWidth: "wide" })).toMatchObject({
+      chatWidth: "wide",
+      chatCustomWidthRem: 60,
+    });
+  });
+});
+
 describe("ClientSettings quit confirmation", () => {
   it.each([
     [true, "hold"],
diff --git a/packages/contracts/src/settings.ts b/packages/contracts/src/settings.ts
index 643d27296f34b..bfbc317c61459 100644
--- a/packages/contracts/src/settings.ts
+++ b/packages/contracts/src/settings.ts
@@ -324,9 +324,17 @@ export const LoadBalancingWeights = Schema.Record(
 
 export const DiffColorScheme = Schema.Literals(["red-green", "blue-orange"]);
 
-export const ChatWidth = Schema.Literals(["comfortable", "wide", "full"]);
+export const ChatWidth = Schema.Literals(["comfortable", "wide", "full", "custom"]);
 export type ChatWidth = typeof ChatWidth.Type;
 
+/** In rem so the cap scales with the interface font size. */
+export const MIN_CHAT_CUSTOM_WIDTH_REM = 46;
+export const MAX_CHAT_CUSTOM_WIDTH_REM = 96;
+export const ChatCustomWidthRem = Schema.Int.check(
+  Schema.isBetween({ minimum: MIN_CHAT_CUSTOM_WIDTH_REM, maximum: MAX_CHAT_CUSTOM_WIDTH_REM }),
+);
+const DEFAULT_CHAT_CUSTOM_WIDTH_REM: typeof ChatCustomWidthRem.Type = 60;
+
 /** How the composer shows reasoning effort: the level's name, or rising bars. */
 export const EffortDisplay = Schema.Literals(["label", "bars"]);
 export type EffortDisplay = typeof EffortDisplay.Type;
@@ -357,6 +365,9 @@ export const ClientSettingsSchema = Schema.Struct({
     Schema.withDecodingDefault(Effect.succeed("red-green" as const)),
   ),
   chatWidth: ChatWidth.pipe(Schema.withDecodingDefault(Effect.succeed("comfortable" as const))),
+  chatCustomWidthRem: ChatCustomWidthRem.pipe(
+    Schema.withDecodingDefault(Effect.succeed(DEFAULT_CHAT_CUSTOM_WIDTH_REM)),
+  ),
   effortDisplay: EffortDisplay.pipe(Schema.withDecodingDefault(Effect.succeed("label" as const))),
   loadBalancingEnabled: Schema.Boolean.pipe(Schema.withDecodingDefault(Effect.succeed(false))),
   loadBalancingWeights: LoadBalancingWeights.pipe(Schema.withDecodingDefault(Effect.succeed({}))),
@@ -1721,6 +1732,7 @@ export const ClientSettingsPatch = Schema.Struct({
   inAppNotificationsEnabled: Schema.optionalKey(Schema.Boolean),
   diffColorScheme: Schema.optionalKey(DiffColorScheme),
   chatWidth: Schema.optionalKey(ChatWidth),
+  chatCustomWidthRem: Schema.optionalKey(ChatCustomWidthRem),
   effortDisplay: Schema.optionalKey(EffortDisplay),
   loadBalancingEnabled: Schema.optionalKey(Schema.Boolean),
   loadBalancingWeights: Schema.optionalKey(LoadBalancingWeights),