diff --git a/apps/desktop/src/settings/DesktopClientSettings.test.ts b/apps/desktop/src/settings/DesktopClientSettings.test.ts
index 51c3d31daae85..df61117c46883 100644
--- a/apps/desktop/src/settings/DesktopClientSettings.test.ts
+++ b/apps/desktop/src/settings/DesktopClientSettings.test.ts
@@ -45,6 +45,7 @@ const clientSettings: ClientSettings = {
   fontFamilyComposer: "",
   fontFamilySans: "",
   fontFamilyTerminal: "",
+  fontSizeChat: null,
   fontSizeCode: 13,
   fontSizeInterface: 16,
   fontSizePrompt: 14,
diff --git a/apps/web/src/appearanceFonts.test.ts b/apps/web/src/appearanceFonts.test.ts
new file mode 100644
index 0000000000000..d027447c4074b
--- /dev/null
+++ b/apps/web/src/appearanceFonts.test.ts
@@ -0,0 +1,21 @@
+import { describe, expect, it } from "vite-plus/test";
+
+import { chatFontScale } from "./appearanceFonts";
+
+describe("chatFontScale", () => {
+  it("follows the interface when unset or equal to it", () => {
+    expect(chatFontScale(null, 16)).toBeNull();
+    expect(chatFontScale(16, 16)).toBeNull();
+  });
+
+  it("scales chat text relative to the interface size", () => {
+    expect(chatFontScale(20, 16)).toBe(1.25);
+    expect(chatFontScale(12, 16)).toBe(0.75);
+    expect(chatFontScale(18, 12)).toBe(1.5);
+  });
+
+  it("clamps out-of-range sizes before scaling", () => {
+    expect(chatFontScale(40, 16)).toBe(20 / 16);
+    expect(chatFontScale(20, 40)).toBeNull();
+  });
+});
diff --git a/apps/web/src/appearanceFonts.ts b/apps/web/src/appearanceFonts.ts
index c816836ae8aed..3a3228769075f 100644
--- a/apps/web/src/appearanceFonts.ts
+++ b/apps/web/src/appearanceFonts.ts
@@ -64,6 +64,7 @@ export interface AppearanceFontPreferences {
   readonly code: string;
   readonly composer: string;
   readonly sizeInterface: number;
+  readonly sizeChat: number | null;
   readonly sizePrompt: number;
   readonly sizeCode: number;
   readonly smoothing: boolean;
@@ -93,6 +94,12 @@ export function applyAppearanceFontVariables(
   }
 
   root.style.fontSize = `${clampInterfaceFontSize(preferences.sizeInterface)}px`;
+  const chatScale = chatFontScale(preferences.sizeChat, preferences.sizeInterface);
+  if (chatScale === null) {
+    root.style.removeProperty("--chat-font-size-ratio");
+  } else {
+    root.style.setProperty("--chat-font-size-ratio", String(chatScale));
+  }
   root.style.setProperty("--font-size-prompt", `${clampPromptFontSize(preferences.sizePrompt)}px`);
   const code = clampCodeFontSize(preferences.sizeCode);
   root.style.setProperty("--font-size-code", `${code}px`);
@@ -119,6 +126,18 @@ function clampInterfaceFontSize(value: number): number {
   );
 }
 
+/**
+ * Chat text keeps its rem-based sizes, so a chat size is applied as a ratio to
+ * the interface size rather than as pixels. Null means chat follows the
+ * interface.
+ */
+export function chatFontScale(sizeChat: number | null, sizeInterface: number): number | null {
+  if (sizeChat === null) return null;
+  const chat = clampInterfaceFontSize(sizeChat);
+  const base = clampInterfaceFontSize(sizeInterface);
+  return chat === base ? null : chat / base;
+}
+
 function clampPromptFontSize(value: number): number {
   return clampFontSize(value, MIN_PROMPT_FONT_SIZE, MAX_PROMPT_FONT_SIZE, DEFAULT_PROMPT_FONT_SIZE);
 }
diff --git a/apps/web/src/components/chat/MessagesTimeline.tsx b/apps/web/src/components/chat/MessagesTimeline.tsx
index 19b1aeb55a5d5..9d863b0c09334 100644
--- a/apps/web/src/components/chat/MessagesTimeline.tsx
+++ b/apps/web/src/components/chat/MessagesTimeline.tsx
@@ -1046,7 +1046,7 @@ const ConversationTimeline = memo(function ConversationTimeline({
     <TimelineRowCtx value={sharedState}>
       <div className="messages-timeline-row-frame">
         <div
-          className="@container/chat-lane chat-content-lane overflow-x-clip"
+          className="@container/chat-lane chat-content-lane chat-text-scope overflow-x-clip"
           data-timeline-root="true"
         >
           <TimelineRowContent row={item} />
diff --git a/apps/web/src/components/settings/SettingsPanels.logic.ts b/apps/web/src/components/settings/SettingsPanels.logic.ts
index f7d60578989a6..1fdd6998ba91a 100644
--- a/apps/web/src/components/settings/SettingsPanels.logic.ts
+++ b/apps/web/src/components/settings/SettingsPanels.logic.ts
@@ -79,6 +79,7 @@ type TypographySettings = Pick<
   | "fontFamilyCode"
   | "fontFamilyTerminal"
   | "fontSizeInterface"
+  | "fontSizeChat"
   | "fontSizePrompt"
   | "fontSizeCode"
   | "fontSizeTerminal"
@@ -90,6 +91,7 @@ export function getChangedTypographySettingLabels(settings: TypographySettings):
     settings.fontSizeInterface !== DEFAULT_UNIFIED_SETTINGS.fontSizeInterface
       ? ["Interface font"]
       : []),
+    ...(settings.fontSizeChat !== DEFAULT_UNIFIED_SETTINGS.fontSizeChat ? ["Chat font size"] : []),
     ...(settings.fontFamilyComposer !== DEFAULT_UNIFIED_SETTINGS.fontFamilyComposer ||
     settings.fontSizePrompt !== DEFAULT_UNIFIED_SETTINGS.fontSizePrompt
       ? ["Prompt font"]
diff --git a/apps/web/src/components/settings/SettingsPanels.tsx b/apps/web/src/components/settings/SettingsPanels.tsx
index ec1e510ad160f..0912233621e37 100644
--- a/apps/web/src/components/settings/SettingsPanels.tsx
+++ b/apps/web/src/components/settings/SettingsPanels.tsx
@@ -635,6 +635,7 @@ export function useSettingsRestore(onRestored?: () => void) {
       fontFamilyCode: DEFAULT_UNIFIED_SETTINGS.fontFamilyCode,
       fontFamilyTerminal: DEFAULT_UNIFIED_SETTINGS.fontFamilyTerminal,
       fontSizeInterface: DEFAULT_UNIFIED_SETTINGS.fontSizeInterface,
+      fontSizeChat: DEFAULT_UNIFIED_SETTINGS.fontSizeChat,
       fontSizePrompt: DEFAULT_UNIFIED_SETTINGS.fontSizePrompt,
       fontSizeCode: DEFAULT_UNIFIED_SETTINGS.fontSizeCode,
       fontSizeTerminal: DEFAULT_UNIFIED_SETTINGS.fontSizeTerminal,
diff --git a/apps/web/src/components/settings/TypographySettings.tsx b/apps/web/src/components/settings/TypographySettings.tsx
index f991a49bbd439..7282ef81f6415 100644
--- a/apps/web/src/components/settings/TypographySettings.tsx
+++ b/apps/web/src/components/settings/TypographySettings.tsx
@@ -82,6 +82,64 @@ function InterfaceFontRow({ preview }: { preview?: ReactNode }) {
   );
 }
 
+const FOLLOW_INTERFACE_VALUE = "interface";
+
+function ChatFontSizeRow() {
+  const settings = useScopedSettings();
+  const updateSettings = useUpdateScopedSettings();
+  const value = settings.fontSizeChat;
+  return (
+    <SettingsRow
+      {...searchableSetting("chat-font-size")}
+      description="Your messages and the agent's replies. Follows the interface size by default."
+      resetAction={
+        value !== DEFAULT_UNIFIED_SETTINGS.fontSizeChat ? (
+          <SettingResetButton
+            label="chat font size"
+            onClick={() => updateSettings({ fontSizeChat: DEFAULT_UNIFIED_SETTINGS.fontSizeChat })}
+          />
+        ) : null
+      }
+      control={
+        <Select
+          value={value === null ? FOLLOW_INTERFACE_VALUE : String(value)}
+          onValueChange={(next) => {
+            if (next === FOLLOW_INTERFACE_VALUE) {
+              updateSettings({ fontSizeChat: null });
+              return;
+            }
+            const parsed = Number(next);
+            if (
+              Number.isInteger(parsed) &&
+              parsed >= MIN_INTERFACE_FONT_SIZE &&
+              parsed <= MAX_INTERFACE_FONT_SIZE
+            ) {
+              updateSettings({ fontSizeChat: parsed });
+            }
+          }}
+        >
+          <SelectTrigger size="sm" className="w-28 shrink-0" aria-label="Chat font size">
+            <SelectValue>{value === null ? "Interface" : `${value} px`}</SelectValue>
+          </SelectTrigger>
+          <SelectPopup align="end" alignItemWithTrigger={false}>
+            <SelectItem hideIndicator value={FOLLOW_INTERFACE_VALUE}>
+              Interface
+            </SelectItem>
+            {Array.from(
+              { length: MAX_INTERFACE_FONT_SIZE - MIN_INTERFACE_FONT_SIZE + 1 },
+              (_, index) => MIN_INTERFACE_FONT_SIZE + index,
+            ).map((px) => (
+              <SelectItem hideIndicator key={px} value={String(px)}>
+                {px} px
+              </SelectItem>
+            ))}
+          </SelectPopup>
+        </Select>
+      }
+    />
+  );
+}
+
 function PromptFontRow() {
   const settings = useScopedSettings();
   const updateSettings = useUpdateScopedSettings();
@@ -254,6 +312,7 @@ function FontSettingsGroup() {
   return (
     <>
       <InterfaceFontRow />
+      <ChatFontSizeRow />
       <PromptFontRow />
       <CodeFontRow />
       <TerminalFontRow />
@@ -267,6 +326,7 @@ function SimpleFontRows() {
   return (
     <>
       <InterfaceFontRow preview={<PromptFontPreview />} />
+      <ChatFontSizeRow />
       <CodeFontRow
         title="Monospace font"
         description="Code blocks, diffs, file previews, and the terminal."
diff --git a/apps/web/src/components/settings/settingsSearch.ts b/apps/web/src/components/settings/settingsSearch.ts
index b621143836feb..9886037d4122a 100644
--- a/apps/web/src/components/settings/settingsSearch.ts
+++ b/apps/web/src/components/settings/settingsSearch.ts
@@ -285,6 +285,12 @@ export const SETTINGS_SEARCH_ITEMS = [
     to: "/settings/appearance",
     searchTerms: ["typography family size system sans"],
   },
+  {
+    id: "chat-font-size",
+    title: "Chat font size",
+    to: "/settings/appearance",
+    searchTerms: ["typography text size conversation messages replies responses timeline"],
+  },
   {
     id: "prompt-font",
     title: "Prompt font",
diff --git a/apps/web/src/index.css b/apps/web/src/index.css
index 814cfa484dcd6..bdfda25d8cf98 100644
--- a/apps/web/src/index.css
+++ b/apps/web/src/index.css
@@ -1784,6 +1784,19 @@ code {
   background: var(--app-scrollbar-thumb-hover);
 }
 
+/* Chat font size. Tailwind's text utilities read these tokens, so rescaling
+   them here resizes conversation text without touching chrome, the prompt, or
+   code, which sizes itself in pixels through --font-size-code. The root only
+   carries the ratio; scaling applies inside the scope, so markdown elsewhere
+   (prompt, file previews) keeps the interface size. */
+.chat-text-scope {
+  --chat-font-scale: var(--chat-font-size-ratio, 1);
+  --text-xs: calc(0.75rem * var(--chat-font-scale, 1));
+  --text-sm: calc(0.875rem * var(--chat-font-scale, 1));
+  --text-base: calc(1rem * var(--chat-font-scale, 1));
+  --text-lg: calc(1.125rem * var(--chat-font-scale, 1));
+}
+
 /* Chat markdown rendering */
 
 .chat-markdown > :first-child {
@@ -1822,21 +1835,21 @@ code {
 }
 
 :is(.chat-markdown, .composer-tiptap) h1 {
-  font-size: 1.25rem;
+  font-size: calc(1.25rem * var(--chat-font-scale, 1));
 }
 
 :is(.chat-markdown, .composer-tiptap) h2 {
-  font-size: 1.125rem;
+  font-size: calc(1.125rem * var(--chat-font-scale, 1));
 }
 
 :is(.chat-markdown, .composer-tiptap) h3 {
-  font-size: 1rem;
+  font-size: calc(1rem * var(--chat-font-scale, 1));
 }
 
 :is(.chat-markdown, .composer-tiptap) h4,
 :is(.chat-markdown, .composer-tiptap) h5,
 :is(.chat-markdown, .composer-tiptap) h6 {
-  font-size: 0.875rem;
+  font-size: calc(0.875rem * var(--chat-font-scale, 1));
 }
 
 :is(.chat-markdown, .composer-tiptap) h6 {
diff --git a/apps/web/src/routes/__root.tsx b/apps/web/src/routes/__root.tsx
index 3d73c01104a24..8baecb0869c12 100644
--- a/apps/web/src/routes/__root.tsx
+++ b/apps/web/src/routes/__root.tsx
@@ -313,6 +313,7 @@ function FontAppearanceSync() {
   const fontFamilyCode = useClientSettings((settings) => settings.fontFamilyCode);
   const fontFamilyComposer = useClientSettings((settings) => settings.fontFamilyComposer);
   const fontSizeInterface = useClientSettings((settings) => settings.fontSizeInterface);
+  const fontSizeChat = useClientSettings((settings) => settings.fontSizeChat);
   const fontSizePrompt = useClientSettings((settings) => settings.fontSizePrompt);
   const fontSizeCode = useClientSettings((settings) => settings.fontSizeCode);
   const fontSmoothing = useClientSettings((settings) => settings.fontSmoothing);
@@ -323,6 +324,7 @@ function FontAppearanceSync() {
       code: fontFamilyCode,
       composer: fontFamilyComposer,
       sizeInterface: fontSizeInterface,
+      sizeChat: fontSizeChat,
       sizePrompt: fontSizePrompt,
       sizeCode: fontSizeCode,
       smoothing: fontSmoothing,
@@ -332,6 +334,7 @@ function FontAppearanceSync() {
     fontFamilyComposer,
     fontFamilySans,
     fontSizeCode,
+    fontSizeChat,
     fontSizeInterface,
     fontSizePrompt,
     fontSmoothing,
diff --git a/packages/contracts/src/settings.ts b/packages/contracts/src/settings.ts
index 9f84636d12bea..2e8dde1376ced 100644
--- a/packages/contracts/src/settings.ts
+++ b/packages/contracts/src/settings.ts
@@ -401,6 +401,10 @@ export const ClientSettingsSchema = Schema.Struct({
   fontSizePrompt: PromptFontSize.pipe(
     Schema.withDecodingDefault(Effect.succeed(DEFAULT_PROMPT_FONT_SIZE)),
   ),
+  // Null follows the interface size.
+  fontSizeChat: Schema.NullOr(InterfaceFontSize).pipe(
+    Schema.withDecodingDefault(Effect.succeed(null)),
+  ),
   fontSizeCode: CodeFontSize.pipe(
     Schema.withDecodingDefault(Effect.succeed(DEFAULT_CODE_FONT_SIZE)),
   ),
@@ -1710,6 +1714,7 @@ export const ClientSettingsPatch = Schema.Struct({
   onboardingCompletedAt: Schema.optionalKey(Schema.NullOr(Schema.String)),
   fontSizeInterface: Schema.optionalKey(InterfaceFontSize),
   fontSizePrompt: Schema.optionalKey(PromptFontSize),
+  fontSizeChat: Schema.optionalKey(Schema.NullOr(InterfaceFontSize)),
   fontSizeCode: Schema.optionalKey(CodeFontSize),
   fontSizeTerminal: Schema.optionalKey(TerminalFontSize),
   fontFamilyCode: Schema.optionalKey(FontFamilyPreference),