diff --git a/packages/tui/src/config/index.tsx b/packages/tui/src/config/index.tsx
index bde0bfb0718a..984ba350be60 100644
--- a/packages/tui/src/config/index.tsx
+++ b/packages/tui/src/config/index.tsx
@@ -19,6 +19,7 @@ export const PluginOptions = Schema.Record(Schema.String, Schema.Unknown)
 export const PluginSpec = Schema.Union([Schema.String, Schema.mutable(Schema.Tuple([Schema.String, PluginOptions]))])
 
 export const LeaderTimeoutDefault = 2000
+export const SidebarWidthDefault = 42
 export const LeaderTimeout = Schema.Int.check(Schema.isGreaterThan(0)).annotate({
   description: "Leader key timeout in milliseconds",
 })
@@ -51,6 +52,7 @@ export const Attention = Schema.Struct({
 }).annotate({ description: "Attention notification and sound settings" })
 
 const PromptSize = Schema.Int.check(Schema.isGreaterThan(0))
+const SidebarWidth = Schema.Int.check(Schema.isGreaterThan(0))
 export const Prompt = Schema.Struct({
   max_height: Schema.optional(PromptSize).annotate({ description: "Prompt textarea max height" }),
   max_width: Schema.optional(Schema.Union([PromptSize, Schema.Literal("auto")])).annotate({
@@ -72,10 +74,11 @@ export const Info = Schema.Struct({
   diff_style: Schema.optional(DiffStyle),
   cursor: Schema.optional(Cursor),
   mouse: Schema.optional(Schema.Boolean).annotate({ description: "Enable or disable mouse capture (default: true)" }),
+  sidebar_width: Schema.optional(SidebarWidth).annotate({ description: "Sidebar width in columns (default: 42)" }),
 })
 export type Info = Schema.Schema.Type<typeof Info>
 
-export type Resolved = Omit<Info, "attention" | "keybinds" | "leader_timeout" | "mouse" | "cursor"> & {
+export type Resolved = Omit<Info, "attention" | "keybinds" | "leader_timeout" | "mouse" | "cursor" | "sidebar_width"> & {
   attention: {
     enabled: boolean
     notifications: boolean
@@ -87,6 +90,7 @@ export type Resolved = Omit<Info, "attention" | "keybinds" | "leader_timeout" |
   keybinds: TuiKeybind.BindingLookupView
   leader_timeout: number
   mouse: boolean
+  sidebar_width: number
   cursor?: {
     style: "block" | "underline" | "line" | "default"
     blinking: boolean
@@ -126,6 +130,7 @@ export function resolve(input: Info, options: ResolveOptions): Resolved {
     }),
     leader_timeout: input.leader_timeout ?? LeaderTimeoutDefault,
     mouse: input.mouse ?? true,
+    sidebar_width: input.sidebar_width ?? SidebarWidthDefault,
     cursor: input.cursor
       ? {
           style: input.cursor.style ?? "block",
diff --git a/packages/tui/src/routes/session/index.tsx b/packages/tui/src/routes/session/index.tsx
index cbdaf0cfa0c7..1db58b9dfc31 100644
--- a/packages/tui/src/routes/session/index.tsx
+++ b/packages/tui/src/routes/session/index.tsx
@@ -54,6 +54,7 @@ import { DialogTimeline } from "./dialog-timeline"
 import { DialogForkFromTimeline } from "./dialog-fork-from-timeline"
 import { DialogSessionRename } from "../../component/dialog-session-rename"
 import { Sidebar } from "./sidebar"
+import { clampSidebarWidth } from "../../util/sidebar-width"
 import { SubagentFooter } from "./subagent-footer.tsx"
 import { filetype } from "../../util/filetype"
 import parsers from "../../parsers-config"
@@ -275,8 +276,9 @@ export function Session() {
     if (sidebar() === "auto" && wide()) return true
     return false
   })
+  const sidebarWidth = createMemo(() => clampSidebarWidth(tuiConfig.sidebar_width, dimensions().width))
   const showTimestamps = createMemo(() => timestamps() === "show")
-  const contentWidth = createMemo(() => dimensions().width - (sidebarVisible() ? 42 : 0) - 4)
+  const contentWidth = createMemo(() => dimensions().width - (sidebarVisible() ? sidebarWidth() : 0) - 4)
   const providers = createMemo(() => Model.index(sync.data.provider))
 
   const scrollAcceleration = createMemo(() => getScrollAcceleration(tuiConfig))
@@ -1339,7 +1341,7 @@ export function Session() {
           <Show when={sidebarVisible()}>
             <Switch>
               <Match when={wide()}>
-                <Sidebar sessionID={route.sessionID} />
+                <Sidebar sessionID={route.sessionID} width={sidebarWidth()} />
               </Match>
               <Match when={!wide()}>
                 <box
@@ -1351,7 +1353,7 @@ export function Session() {
                   alignItems="flex-end"
                   backgroundColor={RGBA.fromInts(0, 0, 0, 70)}
                 >
-                  <Sidebar sessionID={route.sessionID} />
+                  <Sidebar sessionID={route.sessionID} width={sidebarWidth()} />
                 </box>
               </Match>
             </Switch>
diff --git a/packages/tui/src/routes/session/sidebar.tsx b/packages/tui/src/routes/session/sidebar.tsx
index 0c5d2b313967..75cfc48f2df8 100644
--- a/packages/tui/src/routes/session/sidebar.tsx
+++ b/packages/tui/src/routes/session/sidebar.tsx
@@ -9,7 +9,7 @@ import { usePluginRuntime } from "../../plugin/runtime"
 import { getScrollAcceleration } from "../../util/scroll"
 import { WorkspaceLabel } from "../../component/workspace-label"
 
-export function Sidebar(props: { sessionID: string; overlay?: boolean }) {
+export function Sidebar(props: { sessionID: string; overlay?: boolean; width: number }) {
   const pluginRuntime = usePluginRuntime()
   const project = useProject()
   const sync = useSync()
@@ -27,7 +27,7 @@ export function Sidebar(props: { sessionID: string; overlay?: boolean }) {
     <Show when={session()}>
       <box
         backgroundColor={theme.backgroundPanel}
-        width={42}
+        width={props.width}
         height="100%"
         paddingTop={1}
         paddingBottom={1}
diff --git a/packages/tui/src/util/sidebar-width.ts b/packages/tui/src/util/sidebar-width.ts
new file mode 100644
index 000000000000..aceb1fdb8ace
--- /dev/null
+++ b/packages/tui/src/util/sidebar-width.ts
@@ -0,0 +1,5 @@
+import { SidebarWidthDefault } from "../config"
+
+export function clampSidebarWidth(configured: number | undefined, terminalWidth: number) {
+  return Math.max(20, Math.min(configured ?? SidebarWidthDefault, terminalWidth - 40, 100))
+}
diff --git a/packages/tui/test/config.test.tsx b/packages/tui/test/config.test.tsx
index 37fc0033e561..dc976b97bb00 100644
--- a/packages/tui/test/config.test.tsx
+++ b/packages/tui/test/config.test.tsx
@@ -7,6 +7,7 @@ import {
   Info,
   LeaderTimeoutDefault,
   PluginSpec,
+  SidebarWidthDefault,
   resolve,
   TuiConfigProvider,
   type Info as TuiConfigInfo,
@@ -45,6 +46,7 @@ test("validates config constraints", () => {
   expect(() => decodeInfo({ prompt: { max_width: 0 } })).toThrow()
   expect(() => decodeInfo({ scroll_speed: 0 })).toThrow()
   expect(() => decodeInfo({ cursor: { style: "beam" } })).toThrow()
+  expect(() => decodeInfo({ sidebar_width: 42.7 })).toThrow()
   expect(decodeInfo({ attention: { sounds: { unknown: "sound.wav" } } })).toEqual({ attention: { sounds: {} } })
 })
 
@@ -61,6 +63,7 @@ test("resolves host-neutral defaults", () => {
   })
   expect(config.leader_timeout).toBe(LeaderTimeoutDefault)
   expect(config.mouse).toBe(true)
+  expect(config.sidebar_width).toBe(SidebarWidthDefault)
   expect(config.keybinds.has("terminal.suspend")).toBe(true)
   expect(config.keybinds.has("session.list")).toBe(true)
   expect(config.cursor).toBeUndefined()
@@ -70,6 +73,7 @@ test("resolves overrides without mutating input", () => {
   const input: TuiConfigInfo = {
     theme: "custom",
     mouse: false,
+    sidebar_width: 56,
     leader_timeout: 750,
     attention: {
       enabled: true,
@@ -87,6 +91,7 @@ test("resolves overrides without mutating input", () => {
   expect(config).toMatchObject({
     theme: "custom",
     mouse: false,
+    sidebar_width: 56,
     leader_timeout: 750,
     attention: input.attention,
     cursor: { style: "block", blinking: false },
diff --git a/packages/tui/test/util/sidebar-width.test.ts b/packages/tui/test/util/sidebar-width.test.ts
new file mode 100644
index 000000000000..5576cf1684cc
--- /dev/null
+++ b/packages/tui/test/util/sidebar-width.test.ts
@@ -0,0 +1,28 @@
+import { describe, expect, test } from "bun:test"
+import { clampSidebarWidth } from "../../src/util/sidebar-width"
+
+describe("util.sidebar-width", () => {
+  test("passes through a configured width within bounds", () => {
+    expect(clampSidebarWidth(48, 120)).toBe(48)
+  })
+
+  test("clamps widths below the minimum to 20", () => {
+    expect(clampSidebarWidth(12, 120)).toBe(20)
+  })
+
+  test("clamps widths above the terminal content limit", () => {
+    expect(clampSidebarWidth(90, 120)).toBe(80)
+  })
+
+  test("clamps widths above the hard cap to 100", () => {
+    expect(clampSidebarWidth(120, 200)).toBe(100)
+  })
+
+  test("keeps the minimum when terminal bounds are inverted", () => {
+    expect(clampSidebarWidth(30, 55)).toBe(20)
+  })
+
+  test("defaults absent configuration to 42", () => {
+    expect(clampSidebarWidth(undefined, 120)).toBe(42)
+  })
+})