diff --git a/packages/app-bundle/manifest.json b/packages/app-bundle/manifest.json
index c4f0c7ffc..bde698ef1 100644
--- a/packages/app-bundle/manifest.json
+++ b/packages/app-bundle/manifest.json
@@ -152,7 +152,7 @@
     "packages/app/src/components/profile-popover.tsx": "eff742e90544455f13234a9351bb1b82189663cba2cd3172bdcea1a8a37284e8",
     "packages/app/src/components/prompt-input-clipboard-structure.test.ts": "946b718c2164da394602f26e4d9919d752da0d0b9bee714f345ba16c274542da",
     "packages/app/src/components/prompt-input-v2.tsx": "f0b637e46737e3c7345957a0167c878c8e9870a15bb850057665a4444ea91fe8",
-    "packages/app/src/components/prompt-input.tsx": "c127f81e2c0de4bdbc21a349473df39c623d8d7e2df8df0c03ea60bbef101a25",
+    "packages/app/src/components/prompt-input.tsx": "6eaaba582ed28b73aa7273ea5e33dd0a0b81255b8b8db2c572e1f369ec14ae86",
     "packages/app/src/components/prompt-project-selector.tsx": "0157c954b938bce2b5aca9a82f4a72338e361f4c9027e55a7fd5a14a1654d294",
     "packages/app/src/components/question-inbox.tsx": "5ce03a30b7110092ddffccf3bf38e6c1b1a79624f378c819be7019725c96478a",
     "packages/app/src/components/remote-create-arm.test.ts": "388132209e16b6f00d79ac7c700f8c2bfb5f80343eee34cee73451b32842d3f2",
@@ -194,6 +194,7 @@
     "packages/app/src/components/vault-panel.tsx": "1d9f06f42b94ffb94a28bfd6d2693a36c75ceb969bcd434081b4cd2fbf99665c",
     "packages/app/src/components/windows-app-menu.tsx": "262d085facace692084d79763a6bd8eba6c5ceb53b952a134e12c7e8a8a12946",
     "packages/app/src/components/workbench-panel.tsx": "d16f477044098842a214a2e16f9be719e7688a04bcb575889ca544988f23c690",
+    "packages/app/src/components/prompt-input-resize-structure.test.ts": "38244b103702afc4b45cd40176f338c2a632d64b8ee9a719ee2073b6d7aaf062",
     "packages/app/src/context/command.tsx": "388d6cb12b6dc43ecb0827e3fb918370cb9d16d96161a5990dec10fff05af172",
     "packages/app/src/context/concise-mode.test.ts": "534258a1d82d66bd8b482bb3a8ca6a24d508328505abd0f7f52e1b75cdf57fa2",
     "packages/app/src/context/concise-mode.ts": "b03ac887222543e590d31781c944e69aca97bff32a661751dd817d46bf4ead64",
@@ -1038,7 +1039,10 @@
     "packages/sdk/js/src/v2/gen/sdk.gen.ts": "6610638b4fb71bac498da98d232ef9adcd4b011b63e5e81a63fc6dc20d311b17",
     "packages/sdk/js/src/v2/gen/types.gen.ts": "20dfc17423861bd51c197fe6a2fcb6efbd7ad11853b28e0bb79844115256e729",
     "packages/session-ui/src/v2/components/prompt-input/attachments.ts": "ff66078f2edc9a67a4df0d7dfd76a49be5eab27ac332b303f3412721ef7268ea",
-    "packages/session-ui/src/v2/components/prompt-input/index.tsx": "5c5c852ad55ae759f5869fd2e48ade6da99a98b6ab3c6dc102f3e062898e3f60",
+    "packages/session-ui/src/v2/components/prompt-input/index.tsx": "f393de5a7600796df174c877ffd5af64139df7fc65294c2bb9023cdc29945f66",
+    "packages/session-ui/src/v2/components/prompt-input/resize-handle.tsx": "e0c47cd2c8140eda79d96562d311c7c5e2ff9c162695691e9f6acd2a8c1786a1",
+    "packages/session-ui/src/v2/components/prompt-input/resize.test.ts": "c049ad1cd60e5ac4ec779dd37ab4b2c8a6244b5b6cbbc59240d1c43a449cfabd",
+    "packages/session-ui/src/v2/components/prompt-input/resize.ts": "bf1323b437bd0cc342084ac9d21035c8ea6d12fcdf644b5587fd88453638f83c",
     "packages/opencode/src/server/routes/instance/httpapi/server.ts": "8f142f16accad5a5a7ca84507f5c95863416af9f85b11131e5cd97cb6b128d90",
     "packages/sdk/js/src/v2/gen/core/serverSentEvents.gen.ts": "9da41a900811e8526f3d6796341b787f7e3c64bdc9cc3a547977f05780c1f79f",
     "packages/opencode/src/server/routes/instance/httpapi/groups/experimental.ts": "944a0aee215a917e867fd9482f3c2bd40f49082bd3174c8a7d967ca40e34ec15",
@@ -1073,7 +1077,7 @@
   "extracted_at": "2026-10-29T13:09:53.406Z",
   "per_package": {
     "packages/app": {
-      "A": 289,
+      "A": 290,
       "M": 166,
       "D": 0
     },
@@ -1108,7 +1112,7 @@
       "D": 0
     },
     "packages/session-ui": {
-      "A": 49,
+      "A": 52,
       "M": 32,
       "D": 0
     },
@@ -1119,7 +1123,7 @@
     }
   },
   "counts": {
-    "overlay_total": 1048,
+    "overlay_total": 1052,
     "deletions": 7,
     "server_coupled": 0
   },
@@ -1310,6 +1314,7 @@
     "packages/app/src/components/vault-panel.tsx": "A",
     "packages/app/src/components/windows-app-menu.tsx": "M",
     "packages/app/src/components/workbench-panel.tsx": "A",
+    "packages/app/src/components/prompt-input-resize-structure.test.ts": "A",
     "packages/app/src/context/command.tsx": "M",
     "packages/app/src/context/concise-mode.test.ts": "A",
     "packages/app/src/context/concise-mode.ts": "A",
@@ -2155,6 +2160,9 @@
     "packages/sdk/js/src/v2/gen/types.gen.ts": "M",
     "packages/session-ui/src/v2/components/prompt-input/attachments.ts": "M",
     "packages/session-ui/src/v2/components/prompt-input/index.tsx": "M",
+    "packages/session-ui/src/v2/components/prompt-input/resize-handle.tsx": "A",
+    "packages/session-ui/src/v2/components/prompt-input/resize.test.ts": "A",
+    "packages/session-ui/src/v2/components/prompt-input/resize.ts": "A",
     "packages/opencode/src/server/routes/instance/httpapi/server.ts": "M",
     "packages/sdk/js/src/v2/gen/core/serverSentEvents.gen.ts": "M",
     "packages/opencode/src/server/routes/instance/httpapi/groups/experimental.ts": "M",
diff --git a/packages/app-bundle/overlay/packages/app/src/components/prompt-input-resize-structure.test.ts b/packages/app-bundle/overlay/packages/app/src/components/prompt-input-resize-structure.test.ts
new file mode 100644
index 000000000..8b9b961bf
--- /dev/null
+++ b/packages/app-bundle/overlay/packages/app/src/components/prompt-input-resize-structure.test.ts
@@ -0,0 +1,39 @@
+import { describe, expect, test } from "bun:test"
+import { readFileSync } from "node:fs"
+import { join } from "node:path"
+
+// amicode#612 — both composers mount the shared drag-to-resize handle and
+// apply the persisted height to the text area. Source-structure checks, in the
+// same style as prompt-input-clipboard-structure.test.ts: the behavior itself
+// is covered in session-ui's resize.test.ts, this pins the wiring.
+const v1 = readFileSync(join(import.meta.dir, "prompt-input.tsx"), "utf8")
+const v2 = readFileSync(
+  join(import.meta.dir, "../../../session-ui/src/v2/components/prompt-input/index.tsx"),
+  "utf8",
+)
+const handle = readFileSync(
+  join(import.meta.dir, "../../../session-ui/src/v2/components/prompt-input/resize-handle.tsx"),
+  "utf8",
+)
+
+describe("composer drag-to-resize wiring (#612)", () => {
+  test("the legacy composer mounts the handle and pins the scroller height from it", () => {
+    expect(v1).toContain("<ComposerResizeHandle")
+    expect(v1).toContain("createComposerResize(")
+    expect(v1).toContain("resize.height()")
+  })
+
+  test("the v2 composer mounts the handle and pins the editor height from it", () => {
+    expect(v2).toContain("<ComposerResizeHandle")
+    expect(v2).toContain("createComposerResize(")
+    expect(v2).toContain("composerHeightStyle(resize.height())")
+  })
+
+  test("the handle is an accessible, keyboard-operable horizontal separator", () => {
+    expect(handle).toContain('role="separator"')
+    expect(handle).toContain('aria-orientation="horizontal"')
+    expect(handle).toContain('tabindex="0"')
+    expect(handle).toContain("onKeyDown")
+    expect(handle).toContain("onDblClick")
+  })
+})
diff --git a/packages/app-bundle/overlay/packages/app/src/components/prompt-input.tsx b/packages/app-bundle/overlay/packages/app/src/components/prompt-input.tsx
index 95002e2b6..fada4f546 100644
--- a/packages/app-bundle/overlay/packages/app/src/components/prompt-input.tsx
+++ b/packages/app-bundle/overlay/packages/app/src/components/prompt-input.tsx
@@ -56,6 +56,7 @@ import { usePlatform } from "@/context/platform"
 import { createSessionTabs } from "@/pages/session/helpers"
 import { createTextFragment, getCursorPosition, setCursorPosition, setRangeEdge } from "./prompt-input/editor-dom"
 import { createPromptAttachments } from "./prompt-input/attachments"
+import { ComposerResizeHandle, createComposerResize } from "@opencode-ai/session-ui/v2/prompt-input/resize-handle"
 import { readClipboardImageViaBridge, readClipboardViaBridge } from "./prompt-input/clipboard-bridge"
 import { ACCEPTED_FILE_TYPES, pickAttachmentFiles } from "./prompt-input/files"
 import {
@@ -134,9 +135,17 @@ export const PromptInput: Component<PromptInputProps> = (props) => {
   let restoreEndOnFocus = true
   let savedCursor: number | null = null
 
+  // amicode#612: drag the composer's top edge to make the text area taller;
+  // shares behavior + persisted height with the v2 composer. The stored height
+  // is the TEXT area; this scroller also holds the 56px control-row inset below
+  // the text, so that is added when applying and removed when measuring.
   const mirror = { input: false }
   const inset = 56
   const space = `${inset}px`
+  const resize = createComposerResize(() => {
+    const h = scrollRef?.getBoundingClientRect().height
+    return h === undefined ? undefined : h - inset
+  })
 
   const scrollCursorIntoView = () => {
     const container = scrollRef
@@ -1482,6 +1491,7 @@ export const PromptInput: Component<PromptInputProps> = (props) => {
           [props.class ?? ""]: !!props.class,
         }}
       >
+        <ComposerResizeHandle resize={resize} gripClass="bg-text-weak" />
         <PromptDragOverlay
           type={store.draggingType}
           label={language.t(store.draggingType === "@mention" ? "prompt.dropzone.file.label" : "prompt.dropzone.label")}
@@ -1524,7 +1534,17 @@ export const PromptInput: Component<PromptInputProps> = (props) => {
           <div
             class="relative max-h-[240px] overflow-y-auto no-scrollbar"
             ref={(el) => (scrollRef = el)}
-            style={{ "scroll-padding-bottom": space }}
+            style={{
+              "scroll-padding-bottom": space,
+              // Pins the text area to the chosen height (overrides max-h-[240px]);
+              // unset keeps the stock auto-grow.
+              ...(resize.height() === undefined
+                ? {}
+                : {
+                    height: `${resize.height()! + inset}px`,
+                    "max-height": `${resize.height()! + inset}px`,
+                  }),
+            }}
           >
             <div
               data-component="prompt-input"
diff --git a/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/index.tsx b/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/index.tsx
index a3eba817f..3536b3e78 100644
--- a/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/index.tsx
+++ b/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/index.tsx
@@ -22,6 +22,7 @@ import type {
   PromptInputV2Suggestion,
 } from "./types"
 import type { PromptInputV2Interaction, PromptInputV2SelectControl } from "./interaction"
+import { ComposerResizeHandle, composerHeightStyle, createComposerResize } from "./resize-handle"
 import "./attachments.css"
 
 export type {
@@ -75,6 +76,9 @@ export function PromptInputV2(props: PromptInputV2Props) {
   const view = props.controller.view
   let editor: HTMLDivElement | undefined
   let localInput = false
+  // amicode#612: drag the form's top edge to make the text area taller. The
+  // chosen height persists (localStorage) across sessions; unset = stock sizing.
+  const resize = createComposerResize(() => editor?.getBoundingClientRect().height)
   const updateCursor = () => {
     if (!editor || !window.getSelection()?.isCollapsed) return
     props.controller.onCursor(promptInputV2Cursor(editor))
@@ -147,6 +151,8 @@ export function PromptInputV2(props: PromptInputV2Props) {
         onDragLeave={props.controller.onDragLeave}
         onDrop={props.controller.onDrop}
       >
+        <ComposerResizeHandle resize={resize} gripClass="bg-v2-text-text-faint" />
+
         <Show when={state.drag === "active"}>
           <div class="pointer-events-none absolute inset-0 z-20 grid place-items-center rounded-xl bg-v2-background-bg-base/90 text-v2-text-text-base">
             Drop files to attach
@@ -185,6 +191,7 @@ export function PromptInputV2(props: PromptInputV2Props) {
             autocomplete="off"
             class="relative z-10 block min-h-[60px] max-h-[180px] w-full overflow-y-auto whitespace-pre-wrap bg-transparent px-4 pt-4 pb-2 text-[13px] font-[440] leading-5 text-v2-text-text-base focus:outline-none empty:before:content-['\200B'] [&_[data-mention=file]]:text-syntax-property [&_[data-mention=agent]]:text-syntax-type [&_[data-mention=reference]]:text-syntax-keyword"
             classList={{ "font-mono!": state.mode === "shell", "opacity-50": props.disabled }}
+            style={composerHeightStyle(resize.height())}
             onInput={(event) => {
               const cursor = promptInputV2Cursor(event.currentTarget)
               const prompt = parsePromptInputV2Editor(event.currentTarget)
diff --git a/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/resize-handle.tsx b/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/resize-handle.tsx
new file mode 100644
index 000000000..20baa81f7
--- /dev/null
+++ b/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/resize-handle.tsx
@@ -0,0 +1,97 @@
+import { createSignal, onCleanup, onMount, type Accessor } from "solid-js"
+import {
+  COMPOSER_MIN_HEIGHT,
+  composerMaxHeight,
+  composerStorage,
+  createComposerResizer,
+  type ComposerResizer,
+} from "./resize"
+
+// Solid glue for the composer drag-to-resize handle (amicode#612). Shared by
+// the legacy `PromptInput` (app package) and `PromptInputV2`; all behavior
+// lives in ./resize.ts, this file is signals + markup only.
+
+export type ComposerResize = {
+  /** User-chosen text-area height in px, or undefined for the stock auto height. */
+  height: Accessor<number | undefined>
+  resizer: ComposerResizer
+}
+
+export function createComposerResize(measure: () => number | undefined): ComposerResize {
+  const [height, setHeight] = createSignal<number | undefined>(undefined)
+  const viewport = () => (typeof window === "undefined" ? 800 : window.innerHeight)
+  const resizer = createComposerResizer({
+    measure: () => measure() ?? COMPOSER_MIN_HEIGHT,
+    viewport,
+    storage: composerStorage(),
+    onChange: setHeight,
+  })
+  setHeight(resizer.height())
+
+  onMount(() => {
+    const refit = () => resizer.refit()
+    window.addEventListener("resize", refit)
+    onCleanup(() => window.removeEventListener("resize", refit))
+  })
+
+  return { height, resizer }
+}
+
+/** Inline style that pins the text area to the user's chosen height. */
+export function composerHeightStyle(height: number | undefined) {
+  if (height === undefined) return undefined
+  return { "min-height": `${height}px`, "max-height": `${height}px` }
+}
+
+export function ComposerResizeHandle(props: {
+  resize: ComposerResize
+  label?: string
+  /** Colour classes for the grip pill; the pill is only visible on hover/focus/drag. */
+  gripClass?: string
+}) {
+  const resizer = () => props.resize.resizer
+  const [dragging, setDragging] = createSignal(false)
+  const finish = () => {
+    resizer().end()
+    setDragging(false)
+  }
+
+  return (
+    <div
+      data-component="composer-resize-handle"
+      role="separator"
+      aria-orientation="horizontal"
+      aria-label={props.label ?? "Resize message box"}
+      aria-valuemin={COMPOSER_MIN_HEIGHT}
+      aria-valuemax={composerMaxHeight(typeof window === "undefined" ? 800 : window.innerHeight)}
+      aria-valuenow={props.resize.height()}
+      tabindex="0"
+      title="Drag to resize · double-click to reset"
+      class="group/resize absolute inset-x-0 top-0 z-30 flex h-2 cursor-ns-resize touch-none select-none items-start justify-center outline-none"
+      onPointerDown={(event) => {
+        if (event.button !== 0) return
+        event.preventDefault()
+        event.currentTarget.setPointerCapture(event.pointerId)
+        resizer().begin(event.clientY)
+        setDragging(true)
+      }}
+      onPointerMove={(event) => resizer().move(event.clientY)}
+      onPointerUp={finish}
+      onPointerCancel={finish}
+      onLostPointerCapture={finish}
+      onDblClick={() => resizer().reset()}
+      onKeyDown={(event) => {
+        if (resizer().key(event.key)) event.preventDefault()
+      }}
+    >
+      <div
+        class={`mt-0.5 h-1 w-10 rounded-full transition-opacity ${props.gripClass ?? "bg-current"}`}
+        classList={{
+          "opacity-0": !dragging(),
+          "group-hover/resize:opacity-40 group-focus-visible/resize:opacity-40": !dragging(),
+          "opacity-60": dragging(),
+        }}
+      />
+    </div>
+  )
+}
diff --git a/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/resize.test.ts b/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/resize.test.ts
new file mode 100644
index 000000000..c7b35f67a
--- /dev/null
+++ b/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/resize.test.ts
@@ -0,0 +1,160 @@
+import { describe, expect, test } from "bun:test"
+import {
+  COMPOSER_HEIGHT_KEY,
+  COMPOSER_KEY_STEP,
+  COMPOSER_MIN_HEIGHT,
+  clampComposerHeight,
+  composerMaxHeight,
+  createComposerResizer,
+  dragComposerHeight,
+  readComposerHeight,
+  writeComposerHeight,
+  type ComposerStorage,
+} from "./resize"
+
+function memoryStorage(initial: Record<string, string> = {}): ComposerStorage & { data: Record<string, string> } {
+  const data = { ...initial }
+  return {
+    data,
+    getItem: (k) => (k in data ? data[k]! : null),
+    setItem: (k, v) => void (data[k] = v),
+    removeItem: (k) => void delete data[k],
+  }
+}
+
+const VIEWPORT = 1000
+
+function make(storage?: ComposerStorage, rendered = 100) {
+  const seen: Array<number | undefined> = []
+  const resizer = createComposerResizer({
+    measure: () => rendered,
+    viewport: () => VIEWPORT,
+    storage,
+    onChange: (h) => seen.push(h),
+  })
+  return { resizer, seen }
+}
+
+describe("composer height math", () => {
+  test("clamps between the minimum and a fraction of the viewport", () => {
+    expect(composerMaxHeight(1000)).toBe(600)
+    expect(clampComposerHeight(10, 1000)).toBe(COMPOSER_MIN_HEIGHT)
+    expect(clampComposerHeight(5000, 1000)).toBe(600)
+    expect(clampComposerHeight(300, 1000)).toBe(300)
+  })
+
+  test("a tiny viewport never pushes the max below the minimum", () => {
+    expect(composerMaxHeight(50)).toBe(COMPOSER_MIN_HEIGHT)
+    expect(clampComposerHeight(500, 50)).toBe(COMPOSER_MIN_HEIGHT)
+  })
+
+  test("dragging the top edge up grows, down shrinks", () => {
+    expect(dragComposerHeight(100, 500, 400, VIEWPORT)).toBe(200)
+    expect(dragComposerHeight(200, 400, 450, VIEWPORT)).toBe(150)
+  })
+})
+
+describe("composer resizer", () => {
+  test("a drag from the rendered height reports growing heights and stops at the cap", () => {
+    const { resizer, seen } = make(undefined, 120)
+    expect(resizer.height()).toBeUndefined()
+    resizer.begin(500)
+    resizer.move(450)
+    expect(resizer.height()).toBe(170)
+    resizer.move(-9999)
+    expect(resizer.height()).toBe(600)
+    resizer.end()
+    expect(seen.at(-1)).toBe(600)
+  })
+
+  test("moves without a begin are ignored", () => {
+    const { resizer, seen } = make()
+    resizer.move(10)
+    expect(resizer.height()).toBeUndefined()
+    expect(seen).toEqual([])
+  })
+
+  test("the height is persisted on release and restored by a fresh resizer", () => {
+    const storage = memoryStorage()
+    const first = make(storage, 100).resizer
+    first.begin(500)
+    first.move(380)
+    expect(storage.data[COMPOSER_HEIGHT_KEY]).toBeUndefined()
+    first.end()
+    expect(storage.data[COMPOSER_HEIGHT_KEY]).toBe("220")
+
+    const second = make(storage, 100).resizer
+    expect(second.height()).toBe(220)
+  })
+
+  test("a later drag starts from the stored height, not the rendered one", () => {
+    const { resizer } = make(memoryStorage({ [COMPOSER_HEIGHT_KEY]: "300" }), 100)
+    resizer.begin(500)
+    resizer.move(400)
+    expect(resizer.height()).toBe(400)
+  })
+
+  test("a stored height larger than the current viewport allows is displayed clamped but kept in storage", () => {
+    const storage = memoryStorage({ [COMPOSER_HEIGHT_KEY]: "900" })
+    const { resizer } = make(storage)
+    expect(resizer.height()).toBe(600)
+    expect(storage.data[COMPOSER_HEIGHT_KEY]).toBe("900")
+  })
+
+  test("ArrowUp / ArrowDown on the handle resize in steps and persist; other keys are not handled", () => {
+    const storage = memoryStorage()
+    const { resizer } = make(storage, 100)
+    expect(resizer.key("ArrowUp")).toBe(true)
+    expect(resizer.height()).toBe(100 + COMPOSER_KEY_STEP)
+    expect(storage.data[COMPOSER_HEIGHT_KEY]).toBe(String(100 + COMPOSER_KEY_STEP))
+    expect(resizer.key("ArrowDown")).toBe(true)
+    expect(resizer.height()).toBe(100)
+    expect(resizer.key("Enter")).toBe(false)
+  })
+
+  test("keyboard shrink stops at the minimum", () => {
+    const { resizer } = make(undefined, COMPOSER_MIN_HEIGHT)
+    resizer.key("ArrowDown")
+    expect(resizer.height()).toBe(COMPOSER_MIN_HEIGHT)
+  })
+
+  test("reset returns to the stock auto height and forgets the stored value", () => {
+    const storage = memoryStorage({ [COMPOSER_HEIGHT_KEY]: "300" })
+    const { resizer, seen } = make(storage)
+    resizer.reset()
+    expect(resizer.height()).toBeUndefined()
+    expect(storage.data[COMPOSER_HEIGHT_KEY]).toBeUndefined()
+    expect(seen.at(-1)).toBeUndefined()
+  })
+})
+
+describe("composer height storage", () => {
+  test("ignores missing, non-numeric, and non-positive values", () => {
+    expect(readComposerHeight(undefined)).toBeUndefined()
+    expect(readComposerHeight(memoryStorage())).toBeUndefined()
+    expect(readComposerHeight(memoryStorage({ [COMPOSER_HEIGHT_KEY]: "banana" }))).toBeUndefined()
+    expect(readComposerHeight(memoryStorage({ [COMPOSER_HEIGHT_KEY]: "-5" }))).toBeUndefined()
+    expect(readComposerHeight(memoryStorage({ [COMPOSER_HEIGHT_KEY]: "0" }))).toBeUndefined()
+  })
+
+  test("a throwing storage (private mode / quota) never breaks resizing", () => {
+    const boom: ComposerStorage = {
+      getItem: () => {
+        throw new Error("denied")
+      },
+      setItem: () => {
+        throw new Error("full")
+      },
+      removeItem: () => {
+        throw new Error("denied")
+      },
+    }
+    expect(readComposerHeight(boom)).toBeUndefined()
+    expect(() => writeComposerHeight(boom, 200)).not.toThrow()
+    const { resizer } = make(boom, 100)
+    resizer.begin(500)
+    resizer.move(400)
+    resizer.end()
+    expect(resizer.height()).toBe(200)
+  })
+})
diff --git a/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/resize.ts b/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/resize.ts
new file mode 100644
index 000000000..a1cd80c36
--- /dev/null
+++ b/packages/app-bundle/overlay/packages/session-ui/src/v2/components/prompt-input/resize.ts
@@ -0,0 +1,143 @@
+// Drag-to-resize state for the chat composer (amicode#612).
+//
+// Pure and dependency-free so both composers (the legacy `PromptInput` in the
+// app package and `PromptInputV2` here) share ONE behavior and the logic is
+// testable without a DOM. The components own only the markup + a Solid signal;
+// this module owns clamping, the drag math, keyboard steps, and persistence.
+//
+// Semantics: the stored number is the editor's *visible text-area height* in
+// px. `undefined` means "never resized" — the composer keeps its stock
+// auto-grow-up-to-a-cap behavior, byte-identical to before.
+
+export const COMPOSER_HEIGHT_KEY = "amicode.composer.height"
+/** Smallest height the handle can shrink the text area to. */
+export const COMPOSER_MIN_HEIGHT = 60
+/** Largest height as a fraction of the viewport, so the timeline stays visible. */
+export const COMPOSER_MAX_VIEWPORT_FRACTION = 0.6
+/** Pixels moved per ArrowUp / ArrowDown press on the focused handle. */
+export const COMPOSER_KEY_STEP = 24
+
+export type ComposerStorage = Pick<Storage, "getItem" | "setItem" | "removeItem">
+
+export function composerMaxHeight(viewportHeight: number): number {
+  const max = Math.floor(viewportHeight * COMPOSER_MAX_VIEWPORT_FRACTION)
+  return Math.max(COMPOSER_MIN_HEIGHT, Number.isFinite(max) ? max : COMPOSER_MIN_HEIGHT)
+}
+
+export function clampComposerHeight(height: number, viewportHeight: number): number {
+  if (!Number.isFinite(height)) return COMPOSER_MIN_HEIGHT
+  return Math.round(Math.min(composerMaxHeight(viewportHeight), Math.max(COMPOSER_MIN_HEIGHT, height)))
+}
+
+/** Dragging the TOP edge up (smaller clientY) makes the composer taller. */
+export function dragComposerHeight(startHeight: number, startY: number, currentY: number, viewportHeight: number) {
+  return clampComposerHeight(startHeight + (startY - currentY), viewportHeight)
+}
+
+export function readComposerHeight(storage: ComposerStorage | undefined): number | undefined {
+  if (!storage) return undefined
+  try {
+    const raw = storage.getItem(COMPOSER_HEIGHT_KEY)
+    if (raw === null) return undefined
+    const value = Number(raw)
+    if (!Number.isFinite(value) || value <= 0) return undefined
+    return Math.round(value)
+  } catch {
+    return undefined
+  }
+}
+
+export function writeComposerHeight(storage: ComposerStorage | undefined, height: number | undefined) {
+  if (!storage) return
+  try {
+    if (height === undefined) storage.removeItem(COMPOSER_HEIGHT_KEY)
+    else storage.setItem(COMPOSER_HEIGHT_KEY, String(Math.round(height)))
+  } catch {
+    // Storage can be unavailable or full (private mode, webview quota); the
+    // resize still works for this session.
+  }
+}
+
+export function composerStorage(): ComposerStorage | undefined {
+  try {
+    return typeof localStorage === "undefined" ? undefined : localStorage
+  } catch {
+    return undefined
+  }
+}
+
+export type ComposerResizeOptions = {
+  /** Current rendered height of the text area, used as the drag origin. */
+  measure: () => number
+  viewport: () => number
+  storage?: ComposerStorage
+  /** Fired whenever the height (or its absence) changes. */
+  onChange: (height: number | undefined) => void
+}
+
+export type ComposerResizer = {
+  height: () => number | undefined
+  /** Re-clamp against the current viewport (call on window resize). */
+  refit: () => void
+  begin: (clientY: number) => void
+  move: (clientY: number) => void
+  end: () => void
+  dragging: () => boolean
+  /** ArrowUp/ArrowDown on the focused handle; returns true when handled. */
+  key: (key: string) => boolean
+  /** Back to the stock auto-height (double-click on the handle). */
+  reset: () => void
+}
+
+export function createComposerResizer(options: ComposerResizeOptions): ComposerResizer {
+  let height = readComposerHeight(options.storage)
+  let dragging = false
+  let startY = 0
+  let startHeight = 0
+
+  const apply = (next: number | undefined, persist: boolean) => {
+    if (next === height) return
+    height = next
+    if (persist) writeComposerHeight(options.storage, next)
+    options.onChange(next)
+  }
+
+  const current = () => (height === undefined ? options.measure() : clampComposerHeight(height, options.viewport()))
+
+  return {
+    height: () => (height === undefined ? undefined : clampComposerHeight(height, options.viewport())),
+    refit: () => {
+      if (height === undefined) return
+      const next = clampComposerHeight(height, options.viewport())
+      // Display-only clamp: keep the user's preferred value in storage so a
+      // bigger window restores it.
+      options.onChange(next)
+    },
+    begin: (clientY) => {
+      dragging = true
+      startY = clientY
+      startHeight = current()
+    },
+    move: (clientY) => {
+      if (!dragging) return
+      apply(dragComposerHeight(startHeight, startY, clientY, options.viewport()), false)
+    },
+    end: () => {
+      if (!dragging) return
+      dragging = false
+      // Persist once, on release, not on every pointermove.
+      if (height !== undefined) writeComposerHeight(options.storage, height)
+    },
+    dragging: () => dragging,
+    key: (key) => {
+      if (key !== "ArrowUp" && key !== "ArrowDown") return false
+      const delta = key === "ArrowUp" ? COMPOSER_KEY_STEP : -COMPOSER_KEY_STEP
+      apply(clampComposerHeight(current() + delta, options.viewport()), true)
+      return true
+    },
+    reset: () => {
+      dragging = false
+      apply(undefined, true)
+    },
+  }
+}