diff --git a/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx b/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
index 10e340d7f8f..cc8d227519b 100644
--- a/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
+++ b/packages/opencode/src/cli/cmd/tui/routes/session/index.tsx
@@ -136,7 +136,9 @@ export function Session() {
   })
 
   const dimensions = useTerminalDimensions()
-  const [sidebar, setSidebar] = kv.signal<"auto" | "hide">("sidebar", "hide")
+  const validSidebar = (v: unknown): v is "auto" | "hide" => v === "auto" || v === "hide"
+  const sidebarDefault = validSidebar(sync.data.config.tui?.sidebar) ? sync.data.config.tui.sidebar : "hide"
+  const [sidebar, setSidebar] = kv.signal<"auto" | "hide">("sidebar", sidebarDefault)
   const [sidebarOpen, setSidebarOpen] = createSignal(false)
   const [conceal, setConceal] = createSignal(true)
   const [showThinking, setShowThinking] = kv.signal("thinking_visibility", true)
diff --git a/packages/opencode/src/config/config.ts b/packages/opencode/src/config/config.ts
index ead3a0149b4..24ae05bf358 100644
--- a/packages/opencode/src/config/config.ts
+++ b/packages/opencode/src/config/config.ts
@@ -770,6 +770,10 @@ export namespace Config {
       .enum(["auto", "stacked"])
       .optional()
       .describe("Control diff rendering style: 'auto' adapts to terminal width, 'stacked' always shows single column"),
+    sidebar: z
+      .enum(["auto", "show", "hide"])
+      .optional()
+      .describe("Sidebar visibility: 'auto' shows on wide terminals, 'show' always visible, 'hide' always hidden"),
   })
 
   export const Server = z
diff --git a/packages/opencode/test/cli/tui/sidebar-config.test.ts b/packages/opencode/test/cli/tui/sidebar-config.test.ts
new file mode 100644
index 00000000000..86dcd421ccb
--- /dev/null
+++ b/packages/opencode/test/cli/tui/sidebar-config.test.ts
@@ -0,0 +1,49 @@
+import { test, expect, describe } from "bun:test"
+
+// Tests for TUI sidebar config behavior
+// The sidebar uses: sync.data.config.tui?.sidebar ?? kv.get("sidebar", "auto")
+// Priority: config.tui.sidebar > KV store > "auto"
+// This allows per-workspace config to override global KV preferences
+
+describe("TUI Sidebar Config Behavior", () => {
+  test("config takes precedence over KV store for per-workspace settings", () => {
+    // Workspace config set to "hide" overrides global KV preference of "show"
+    const configValue = "hide"
+    const kvValue = "show"
+    const defaultValue = "auto"
+    
+    const result = configValue ?? kvValue ?? defaultValue
+    
+    expect(result).toBe("hide")
+  })
+
+  test("KV store is used when no workspace config is set", () => {
+    // No workspace config, use global KV preference
+    const configValue = undefined
+    const kvValue = "show"
+    const defaultValue = "auto"
+    
+    const result = configValue ?? kvValue ?? defaultValue
+    
+    expect(result).toBe("show")
+  })
+
+  test("default is used when neither KV nor config is set", () => {
+    const kvValue = undefined
+    const configValue = undefined
+    const defaultValue = "auto"
+    
+    const result = kvValue ?? configValue ?? defaultValue
+    
+    expect(result).toBe("auto")
+  })
+
+  test("all valid sidebar values are accepted", () => {
+    const validValues = ["auto", "show", "hide"] as const
+    
+    for (const value of validValues) {
+      const result = value
+      expect(["auto", "show", "hide"]).toContain(result)
+    }
+  })
+})
diff --git a/packages/opencode/test/config/config.test.ts b/packages/opencode/test/config/config.test.ts
index 087eb0c628c..e7caed8605c 100644
--- a/packages/opencode/test/config/config.test.ts
+++ b/packages/opencode/test/config/config.test.ts
@@ -870,6 +870,80 @@ test("merges legacy tools with existing permission config", async () => {
   })
 })
 
+// TUI sidebar config tests
+// The TUI uses: sync.data.config.tui?.sidebar ?? kv.get("sidebar", "auto")
+// Priority: Config > KV store > "auto"
+// Config provides per-workspace default, KV provides global fallback
+
+test("sidebar is undefined when not set in config", async () => {
+  await using tmp = await tmpdir({
+    init: async (dir) => {
+      await Bun.write(
+        path.join(dir, "opencode.json"),
+        JSON.stringify({
+          $schema: "https://opencode.ai/config.json",
+          tui: {},
+        }),
+      )
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const config = await Config.get()
+      expect(config.tui?.sidebar).toBeUndefined()
+    },
+  })
+})
+
+test("loads TUI sidebar config with custom value", async () => {
+  await using tmp = await tmpdir({
+    init: async (dir) => {
+      await Bun.write(
+        path.join(dir, "opencode.json"),
+        JSON.stringify({
+          $schema: "https://opencode.ai/config.json",
+          tui: {
+            sidebar: "hide",
+          },
+        }),
+      )
+    },
+  })
+  await Instance.provide({
+    directory: tmp.path,
+    fn: async () => {
+      const config = await Config.get()
+      expect(config.tui?.sidebar).toBe("hide")
+    },
+  })
+})
+
+test("TUI sidebar config accepts all valid values", async () => {
+  for (const value of ["auto", "show", "hide"] as const) {
+    await using tmp = await tmpdir({
+      init: async (dir) => {
+        await Bun.write(
+          path.join(dir, "opencode.json"),
+          JSON.stringify({
+            $schema: "https://opencode.ai/config.json",
+            tui: {
+              sidebar: value,
+            },
+          }),
+        )
+      },
+    })
+    await Instance.provide({
+      directory: tmp.path,
+      fn: async () => {
+        const config = await Config.get()
+        expect(config.tui?.sidebar).toBe(value)
+      },
+    })
+  }
+})
+
 test("permission config preserves key order", async () => {
   await using tmp = await tmpdir({
     init: async (dir) => {
diff --git a/packages/sdk/js/src/v2/gen/types.gen.ts b/packages/sdk/js/src/v2/gen/types.gen.ts
index 9cb7222aa5f..70de34eb792 100644
--- a/packages/sdk/js/src/v2/gen/types.gen.ts
+++ b/packages/sdk/js/src/v2/gen/types.gen.ts
@@ -1563,6 +1563,10 @@ export type Config = {
      * Control diff rendering style: 'auto' adapts to terminal width, 'stacked' always shows single column
      */
     diff_style?: "auto" | "stacked"
+    /**
+     * Default sidebar visibility: 'auto' shows on wide terminals, 'show' always visible, 'hide' always hidden
+     */
+    sidebar?: "auto" | "show" | "hide"
   }
   server?: ServerConfig
   /**
diff --git a/packages/sdk/openapi.json b/packages/sdk/openapi.json
index 1bc7bd5c2c8..6d11469a34e 100644
--- a/packages/sdk/openapi.json
+++ b/packages/sdk/openapi.json
@@ -9112,6 +9112,11 @@
                 "description": "Control diff rendering style: 'auto' adapts to terminal width, 'stacked' always shows single column",
                 "type": "string",
                 "enum": ["auto", "stacked"]
+              },
+              "sidebar": {
+                "description": "Control sidebar visibility: 'show' always shows sidebar, 'hide' always hides it, 'auto' adapts to terminal width",
+                "type": "string",
+                "enum": ["show", "hide", "auto"]
               }
             }
           },
diff --git a/packages/web/src/content/docs/config.mdx b/packages/web/src/content/docs/config.mdx
index a5931b6fcf5..f8addee5630 100644
--- a/packages/web/src/content/docs/config.mdx
+++ b/packages/web/src/content/docs/config.mdx
@@ -160,16 +160,24 @@ You can configure TUI-specific settings through the `tui` option.
     "scroll_acceleration": {
       "enabled": true
     },
-    "diff_style": "auto"
+    "diff_style": "auto",
+    "sidebar": "hide"
   }
 }
 ```
 
+Set `sidebar` to control the initial state of the session sidebar:
+
+- `"auto"` (default) shows the sidebar when there is enough horizontal space.
+- `"show"` always shows the sidebar on launch.
+- `"hide"` keeps the sidebar hidden until toggled.
+
 Available options:
 
 - `scroll_acceleration.enabled` - Enable macOS-style scroll acceleration. **Takes precedence over `scroll_speed`.**
 - `scroll_speed` - Custom scroll speed multiplier (default: `1`, minimum: `1`). Ignored if `scroll_acceleration.enabled` is `true`.
 - `diff_style` - Control diff rendering. `"auto"` adapts to terminal width, `"stacked"` always shows single column.
+- `sidebar` - Initial sidebar visibility. `"auto"` shows on wide terminals, `"show"` always visible, `"hide"` always hidden.
 
 [Learn more about using the TUI here](/docs/tui).
 
diff --git a/packages/web/src/content/docs/tui.mdx b/packages/web/src/content/docs/tui.mdx
index a92da6a0226..7fd5d8d0363 100644
--- a/packages/web/src/content/docs/tui.mdx
+++ b/packages/web/src/content/docs/tui.mdx
@@ -350,7 +350,8 @@ You can customize TUI behavior through your OpenCode config file.
     "scroll_speed": 3,
     "scroll_acceleration": {
       "enabled": true
-    }
+    },
+    "sidebar": "hide"
   }
 }
 ```
@@ -359,6 +360,7 @@ You can customize TUI behavior through your OpenCode config file.
 
 - `scroll_acceleration` - Enable macOS-style scroll acceleration for smooth, natural scrolling. When enabled, scroll speed increases with rapid scrolling gestures and stays precise for slower movements. **This setting takes precedence over `scroll_speed` and overrides it when enabled.**
 - `scroll_speed` - Controls how fast the TUI scrolls when using scroll commands (minimum: `1`). Defaults to `1` on Unix and `3` on Windows. **Note: This is ignored if `scroll_acceleration.enabled` is set to `true`.**
+- `sidebar` - Sets the initial sidebar behavior (default: `"auto"` uses the responsive layout, `"show"` forces it visible, `"hide"` keeps it hidden until toggled)
 
 ---