diff --git a/packages/opencode/src/cli/cmd/tui/component/prompt/autocomplete-detect.ts b/packages/opencode/src/cli/cmd/tui/component/prompt/autocomplete-detect.ts
new file mode 100644
index 000000000..9b9584870
--- /dev/null
+++ b/packages/opencode/src/cli/cmd/tui/component/prompt/autocomplete-detect.ts
@@ -0,0 +1,34 @@
+import { stringIndexToWidth, widthToStringIndex } from "./offset"
+
+export type TriggerKind = "@" | "$" | "/"
+
+// Decide whether an autocomplete popup should open for the current input.
+//
+// `value` is the editor plainText (UTF-16) and `cursorWidth` is the editor's
+// display-width cursor offset (CJK = 2 columns). We convert the cursor to a
+// UTF-16 index before doing any string work, then report the trigger position
+// back in width coordinates so it matches the editor's extmark/cursor space.
+export function detectTrigger(value: string, cursorWidth: number): { kind: TriggerKind; index: number } | undefined {
+  if (cursorWidth === 0) return undefined
+
+  const cursorIndex = widthToStringIndex(value, cursorWidth)
+
+  // "/" command only when it is the very first character and nothing before the cursor is whitespace.
+  if (value.startsWith("/") && !value.slice(0, cursorIndex).match(/\s/)) {
+    return { kind: "/", index: 0 }
+  }
+
+  // Nearest "@" (files) or "$" (agents) before the cursor with no whitespace in between.
+  const text = value.slice(0, cursorIndex)
+  const idx = Math.max(text.lastIndexOf("@"), text.lastIndexOf("$"))
+  if (idx === -1) return undefined
+
+  const kind: TriggerKind = idx === text.lastIndexOf("$") ? "$" : "@"
+  const before = idx === 0 ? undefined : value[idx - 1]
+  const between = text.slice(idx)
+  if ((before === undefined || /\s/.test(before)) && !between.match(/\s/)) {
+    return { kind, index: stringIndexToWidth(value, idx) }
+  }
+
+  return undefined
+}
diff --git a/packages/opencode/src/cli/cmd/tui/component/prompt/autocomplete.tsx b/packages/opencode/src/cli/cmd/tui/component/prompt/autocomplete.tsx
index bcb427c11..b80134552 100644
--- a/packages/opencode/src/cli/cmd/tui/component/prompt/autocomplete.tsx
+++ b/packages/opencode/src/cli/cmd/tui/component/prompt/autocomplete.tsx
@@ -17,6 +17,8 @@ import { useTerminalDimensions } from "@opentui/solid"
 import { Locale } from "@/util"
 import type { PromptInfo } from "./history"
 import { useFrecency } from "./frecency"
+import { detectTrigger } from "./autocomplete-detect"
+import { charAfterCursor } from "./offset"
 
 function removeLineRange(input: string) {
   const hashIndex = input.lastIndexOf("#")
@@ -158,8 +160,7 @@ export function Autocomplete(props: {
     const input = props.input()
     const currentCursorOffset = input.cursorOffset
 
-    const charAfterCursor = props.value.at(currentCursorOffset)
-    const needsSpace = charAfterCursor !== " "
+    const needsSpace = charAfterCursor(props.value, currentCursorOffset) !== " "
     const append = prefix + text + (needsSpace ? " " : "")
 
     input.cursorOffset = store.index
@@ -531,32 +532,12 @@ export function Autocomplete(props: {
           return
         }
 
-        // Check if autocomplete should reopen (e.g., after backspace deleted a space)
-        const offset = props.input().cursorOffset
-        if (offset === 0) return
-
-        // Check for "/" at position 0 - reopen slash commands
-        if (value.startsWith("/") && !value.slice(0, offset).match(/\s/)) {
-          show("/")
-          setStore("index", 0)
-          return
-        }
-
-        // Check for "@" (files) or "$" (agents) trigger - find the nearest one before
-        // the cursor with no whitespace between it and the cursor.
-        const text = value.slice(0, offset)
-        const atIdx = text.lastIndexOf("@")
-        const dollarIdx = text.lastIndexOf("$")
-        const idx = Math.max(atIdx, dollarIdx)
-        if (idx === -1) return
-
-        const trigger = idx === dollarIdx ? "$" : "@"
-        const between = text.slice(idx)
-        const before = idx === 0 ? undefined : value[idx - 1]
-        if ((before === undefined || /\s/.test(before)) && !between.match(/\s/)) {
-          show(trigger)
-          setStore("index", idx)
-        }
+        // Check if autocomplete should reopen (e.g., after backspace deleted a space).
+        // detectTrigger works in width coordinates so CJK before/after the trigger stays correct.
+        const trigger = detectTrigger(value, props.input().cursorOffset)
+        if (!trigger) return
+        show(trigger.kind)
+        setStore("index", trigger.index)
       },
       onKeyDown(e: KeyEvent) {
         if (store.visible) {
diff --git a/packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx b/packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx
index f748fe179..f9fe899bb 100644
--- a/packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx
+++ b/packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx
@@ -16,7 +16,7 @@ import { MessageID, PartID } from "@/session/schema"
 import { createStore, produce, unwrap } from "solid-js/store"
 import { useKeybind } from "@tui/context/keybind"
 import { usePromptHistory, type PromptInfo } from "./history"
-import { assign } from "./part"
+import { assign, expandPlaceholders } from "./part"
 import { usePromptStash } from "./stash"
 import { DialogStash } from "../dialog-stash"
 import { type AutocompleteRef, Autocomplete } from "./autocomplete"
@@ -1091,23 +1091,20 @@ export function Prompt(props: PromptProps) {
     }
 
     const messageID = MessageID.ascending()
-    let inputText = store.prompt.input
 
-    // Expand pasted text inline before submitting
-    const allExtmarks = input.extmarks.getAllForTypeId(promptPartTypeId)
-    const sortedExtmarks = allExtmarks.sort((a: { start: number }, b: { start: number }) => b.start - a.start)
-
-    for (const extmark of sortedExtmarks) {
-      const partIndex = store.extmarkToPartIndex.get(extmark.id)
-      if (partIndex !== undefined) {
+    // Expand pasted text inline before submitting. Extmark offsets are
+    // display-width based while plainText is UTF-16, so expandPlaceholders
+    // bridges the two coordinate systems (otherwise CJK content desyncs them).
+    const marks = input.extmarks
+      .getAllForTypeId(promptPartTypeId)
+      .flatMap((extmark: { id: number; start: number; end: number }) => {
+        const partIndex = store.extmarkToPartIndex.get(extmark.id)
+        if (partIndex === undefined) return []
         const part = store.prompt.parts[partIndex]
-        if (part?.type === "text" && part.text) {
-          const before = inputText.slice(0, extmark.start)
-          const after = inputText.slice(extmark.end)
-          inputText = before + part.text + after
-        }
-      }
-    }
+        if (part?.type !== "text" || !part.text) return []
+        return [{ start: extmark.start, end: extmark.end, text: part.text }]
+      })
+    const inputText = expandPlaceholders(store.prompt.input, marks)
 
     // Filter out text parts (pasted content) since they're now expanded inline
     const nonTextParts = store.prompt.parts.filter((part) => part.type !== "text")
diff --git a/packages/opencode/src/cli/cmd/tui/component/prompt/offset.ts b/packages/opencode/src/cli/cmd/tui/component/prompt/offset.ts
new file mode 100644
index 000000000..7420832bf
--- /dev/null
+++ b/packages/opencode/src/cli/cmd/tui/component/prompt/offset.ts
@@ -0,0 +1,45 @@
+// The editor (@opentui/core) tracks cursor/extmark positions as display-WIDTH
+// offsets: a wide CJK character counts as 2 columns. The plainText we slice in
+// JS is a UTF-16 string where that same character is 1 unit. These helpers
+// translate between the two coordinate systems so the two never get mixed.
+// Inputs are assumed to sit on character (code-point) boundaries, which is all
+// the editor ever emits; an offset landing inside a wide char rounds up to the
+// next boundary.
+
+// The editor advances its offset by 1 for a newline and 2 for a tab, but
+// Bun.stringWidth returns 0 for both, so we special-case them to stay aligned
+// with the editor. (Pasted "\r" never reaches here — paste input is normalized
+// to "\n" and the editor itself maps "\r" to "\n".)
+function charWidth(ch: string): number {
+  if (ch === "\n") return 1
+  if (ch === "\t") return 2
+  return Bun.stringWidth(ch)
+}
+
+export function widthToStringIndex(text: string, widthOffset: number): number {
+  let width = 0
+  let index = 0
+  for (const ch of text) {
+    if (width >= widthOffset) break
+    width += charWidth(ch)
+    index += ch.length
+  }
+  return index
+}
+
+export function stringIndexToWidth(text: string, stringIndex: number): number {
+  let width = 0
+  let index = 0
+  for (const ch of text) {
+    if (index >= stringIndex) break
+    width += charWidth(ch)
+    index += ch.length
+  }
+  return width
+}
+
+// The character immediately after a width-based cursor offset, or undefined at
+// end of input. Used to decide whether an inserted mention needs a trailing space.
+export function charAfterCursor(text: string, cursorWidth: number): string | undefined {
+  return text.at(widthToStringIndex(text, cursorWidth))
+}
diff --git a/packages/opencode/src/cli/cmd/tui/component/prompt/part.ts b/packages/opencode/src/cli/cmd/tui/component/prompt/part.ts
index 8cdcef606..57c10f572 100644
--- a/packages/opencode/src/cli/cmd/tui/component/prompt/part.ts
+++ b/packages/opencode/src/cli/cmd/tui/component/prompt/part.ts
@@ -1,5 +1,6 @@
 import { PartID } from "@/session/schema"
 import type { PromptInfo } from "./history"
+import { widthToStringIndex } from "./offset"
 
 type Item = PromptInfo["parts"][number]
 
@@ -14,3 +15,22 @@ export function assign(part: Item): Item & { id: PartID } {
     id: PartID.ascending(),
   }
 }
+
+// Editor extmark offsets are display-WIDTH based (a wide CJK char counts as 2),
+// while plainText is a JS UTF-16 string (a CJK char is 1 unit). widthToStringIndex
+// converts a width offset into the matching UTF-16 string index so .slice lines up.
+// Replace each placeholder span (given in width-based offsets) in the editor
+// plainText with its real pasted content. Marks are applied right-to-left so
+// earlier offsets stay valid as the string is rewritten.
+export function expandPlaceholders(
+  plainText: string,
+  marks: { start: number; end: number; text: string }[],
+): string {
+  return [...marks]
+    .sort((a, b) => b.start - a.start)
+    .reduce((text, mark) => {
+      const start = widthToStringIndex(text, mark.start)
+      const end = widthToStringIndex(text, mark.end)
+      return text.slice(0, start) + mark.text + text.slice(end)
+    }, plainText)
+}
diff --git a/packages/opencode/test/cli/cmd/tui/autocomplete-detect.test.ts b/packages/opencode/test/cli/cmd/tui/autocomplete-detect.test.ts
new file mode 100644
index 000000000..ff8807b74
--- /dev/null
+++ b/packages/opencode/test/cli/cmd/tui/autocomplete-detect.test.ts
@@ -0,0 +1,55 @@
+import { describe, expect, test } from "bun:test"
+import { detectTrigger } from "../../../../src/cli/cmd/tui/component/prompt/autocomplete-detect"
+
+// cursorWidth is the editor's display-width cursor offset (CJK = 2 columns).
+// detectTrigger inspects the plainText and returns the trigger kind plus the
+// trigger's position expressed in the SAME width coordinate (matching store.index),
+// or undefined when nothing should open.
+describe("detectTrigger", () => {
+  test("returns undefined at start of input", () => {
+    expect(detectTrigger("", 0)).toBeUndefined()
+    expect(detectTrigger("@foo", 0)).toBeUndefined()
+  })
+
+  test("detects a leading slash command", () => {
+    expect(detectTrigger("/hel", 4)).toEqual({ kind: "/", index: 0 })
+  })
+
+  test("detects @ trigger in pure ascii", () => {
+    // "hi @fo" cursor at end (width 6)
+    expect(detectTrigger("hi @fo", 6)).toEqual({ kind: "@", index: 3 })
+  })
+
+  test("detects $ trigger in pure ascii", () => {
+    expect(detectTrigger("run $ag", 7)).toEqual({ kind: "$", index: 4 })
+  })
+
+  test("returns width-based index when CJK precedes the trigger", () => {
+    // "你好 @fo" — 你好 width 4, space width 1, so @ sits at width index 5 (string index 3)
+    // cursor at end: width = 5 + 3 = 8
+    expect(detectTrigger("你好 @fo", 8)).toEqual({ kind: "@", index: 5 })
+  })
+
+  test("returns width-based index for a $ trigger preceded by CJK", () => {
+    // "你好 $ag" — same geometry as the @ case but for the agent trigger.
+    expect(detectTrigger("你好 $ag", 8)).toEqual({ kind: "$", index: 5 })
+  })
+
+  test("does not over-read past the cursor when CJK follows the trigger", () => {
+    // "你好 @x尾巴", cursor right after "@x": 你好(4)+space(1)+@x(2) = width 7 (string index 5).
+    // There is no whitespace between @ and the cursor, so it must still trigger,
+    // and must NOT be fooled by the trailing "尾巴" after the cursor.
+    expect(detectTrigger("你好 @x尾巴", 7)).toEqual({ kind: "@", index: 5 })
+  })
+
+  test("does not trigger when whitespace sits between trigger and cursor", () => {
+    // "你 @ x" — @ is preceded by a space (valid start) but a space sits between @ and the cursor.
+    // 你(2)+space(1)+@(1)+space(1)+x(1) = width 6
+    expect(detectTrigger("你 @ x", 6)).toBeUndefined()
+  })
+
+  test("does not trigger when char before @ is non-whitespace", () => {
+    // "a@fo" — '@' is glued to 'a', not a fresh mention
+    expect(detectTrigger("a@fo", 4)).toBeUndefined()
+  })
+})
diff --git a/packages/opencode/test/cli/cmd/tui/offset.test.ts b/packages/opencode/test/cli/cmd/tui/offset.test.ts
new file mode 100644
index 000000000..9433abff3
--- /dev/null
+++ b/packages/opencode/test/cli/cmd/tui/offset.test.ts
@@ -0,0 +1,101 @@
+import { describe, expect, test } from "bun:test"
+import {
+  charAfterCursor,
+  stringIndexToWidth,
+  widthToStringIndex,
+} from "../../../../src/cli/cmd/tui/component/prompt/offset"
+
+// The editor uses display-width offsets (a wide CJK char counts as 2 columns)
+// while plainText is a JS UTF-16 string (a CJK char is 1 unit). These helpers
+// translate between the two coordinate systems.
+describe("offset conversion", () => {
+  test("widthToStringIndex maps a width offset to a UTF-16 index", () => {
+    // "你好" is width 4 but 2 UTF-16 units
+    expect(widthToStringIndex("你好world", 4)).toBe(2)
+    expect(widthToStringIndex("你好world", 6)).toBe(4) // 你好wo
+    expect(widthToStringIndex("hello", 3)).toBe(3) // ascii: width == index
+  })
+
+  test("stringIndexToWidth maps a UTF-16 index to a width offset", () => {
+    expect(stringIndexToWidth("你好world", 2)).toBe(4) // 你好 -> width 4
+    expect(stringIndexToWidth("你好world", 4)).toBe(6) // 你好wo
+    expect(stringIndexToWidth("hello", 3)).toBe(3)
+  })
+
+  test("the two conversions round-trip on character boundaries", () => {
+    const text = "前缀@x后缀"
+    for (let i = 0; i <= text.length; i++) {
+      const width = stringIndexToWidth(text, i)
+      expect(widthToStringIndex(text, width)).toBe(i)
+    }
+  })
+
+  test("counts a newline as width 1 (matching the editor, not Bun.stringWidth)", () => {
+    // The editor advances its width offset by 1 per "\n", but Bun.stringWidth("\n")
+    // is 0. The converters must follow the editor, otherwise every newline before an
+    // offset desyncs the two coordinate systems by one.
+    // "你好\n[" — 你好=4, \n=1, so "[" sits at width 5 / string index 3
+    expect(stringIndexToWidth("你好\n[", 3)).toBe(5)
+    expect(widthToStringIndex("你好\n[", 5)).toBe(3)
+  })
+
+  test("round-trips across newlines and CJK together", () => {
+    const text = "你好\n世界\nA"
+    let index = 0
+    for (const ch of text) {
+      const width = stringIndexToWidth(text, index)
+      expect(widthToStringIndex(text, width)).toBe(index)
+      index += ch.length
+    }
+  })
+
+  test("counts a tab as width 2 (matching the editor, not Bun.stringWidth)", () => {
+    // The editor advances its width offset by 2 per "\t", but Bun.stringWidth("\t")
+    // is 0. Pasted code often contains tabs, so the converters must follow the editor.
+    // "ab\tc" — ab=2, \t=2, so "c" sits at width 4 / string index 3
+    expect(stringIndexToWidth("ab\tc", 3)).toBe(4)
+    expect(widthToStringIndex("ab\tc", 4)).toBe(3)
+  })
+
+  test("round-trips across tabs, newlines and CJK together", () => {
+    const text = "你好\t世界\nA\tB"
+    let index = 0
+    for (const ch of text) {
+      const width = stringIndexToWidth(text, index)
+      expect(widthToStringIndex(text, width)).toBe(index)
+      index += ch.length
+    }
+  })
+
+  test("round-trips across supplementary-plane (emoji) code-point boundaries", () => {
+    // 😀 is one code point but 2 UTF-16 units and display width 2. The converters
+    // are only contracted to agree on real code-point boundaries (the editor never
+    // emits an offset that splits a surrogate pair), so iterate by code point.
+    const text = "a😀好b"
+    let index = 0
+    for (const ch of text) {
+      const width = stringIndexToWidth(text, index)
+      expect(widthToStringIndex(text, width)).toBe(index)
+      index += ch.length
+    }
+  })
+})
+
+describe("charAfterCursor", () => {
+  test("reads the char right after the cursor in pure ascii", () => {
+    // "ab cd", cursor (width 2) sits before the space
+    expect(charAfterCursor("ab cd", 2)).toBe(" ")
+  })
+
+  test("reads the correct char when CJK precedes the cursor", () => {
+    // "你好 x" — cursor after 你好 (width 4) must land on the space, not be shifted
+    // by the width/UTF-16 mismatch.
+    expect(charAfterCursor("你好 x", 4)).toBe(" ")
+    // cursor after "你好 " (width 5) lands on "x"
+    expect(charAfterCursor("你好 x", 5)).toBe("x")
+  })
+
+  test("returns undefined at end of input", () => {
+    expect(charAfterCursor("你好", 4)).toBeUndefined()
+  })
+})
diff --git a/packages/opencode/test/cli/cmd/tui/prompt-part.test.ts b/packages/opencode/test/cli/cmd/tui/prompt-part.test.ts
index 326d3e624..580f9894b 100644
--- a/packages/opencode/test/cli/cmd/tui/prompt-part.test.ts
+++ b/packages/opencode/test/cli/cmd/tui/prompt-part.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, test } from "bun:test"
 import type { PromptInfo } from "../../../../src/cli/cmd/tui/component/prompt/history"
-import { assign, strip } from "../../../../src/cli/cmd/tui/component/prompt/part"
+import { assign, expandPlaceholders, strip } from "../../../../src/cli/cmd/tui/component/prompt/part"
 
 describe("prompt part", () => {
   test("strip removes persisted ids from reused file parts", () => {
@@ -45,3 +45,53 @@ describe("prompt part", () => {
     })
   })
 })
+
+describe("expandPlaceholders", () => {
+  // Editor extmark offsets are display-WIDTH based (CJK = 2 columns), while the
+  // plain text is a JS UTF-16 string (CJK = 1 unit). expandPlaceholders must
+  // bridge the two coordinate systems.
+
+  test("expands a single ascii placeholder", () => {
+    const result = expandPlaceholders("[Pasted ~3 lines] ", [
+      { start: 0, end: "[Pasted ~3 lines]".length, text: "line1\nline2\nline3" },
+    ])
+    expect(result).toBe("line1\nline2\nline3 ")
+  })
+
+  test("does not leave residue or swallow content when preceded by CJK", () => {
+    // User typed "你好" (width 4, but string index 2) then pasted.
+    // plainText shown in editor: "你好[Pasted ~3 lines] "
+    // extmark.start is width-based = 4, extmark.end = 4 + 17 = 21
+    const result = expandPlaceholders("你好[Pasted ~3 lines] ", [
+      { start: 4, end: 4 + "[Pasted ~3 lines]".length, text: "line1\nline2\nline3" },
+    ])
+    expect(result).toBe("你好line1\nline2\nline3 ")
+  })
+
+  test("handles multiple placeholders interleaved with CJK", () => {
+    const v1 = "[Pasted ~3 lines]"
+    const v2 = "[Pasted ~2 lines]"
+    // plainText: "你好" + v1 + " " + "世界" + v2 + " " + "末"
+    // widths: 你好=4 -> v1 start 4, end 4+17=21; after v1+space width=18 => 22; 世界=4 => 26; v2 start 26 end 43
+    const plain = `你好${v1} 世界${v2} 末`
+    const result = expandPlaceholders(plain, [
+      { start: 4, end: 4 + v1.length, text: "AAA" },
+      { start: 26, end: 26 + v2.length, text: "BBB" },
+    ])
+    expect(result).toBe("你好AAA 世界BBB 末")
+  })
+
+  test("expands a placeholder that sits on its own line after CJK", () => {
+    const v = "[Pasted ~3 lines]"
+    // User typed CJK on line 1, pasted on line 2, typed CJK on line 3:
+    //   "你好\n" + v + " " + "\n世界"
+    // Editor offsets count "\n" as width 1: 你好=4, \n=5, so the placeholder starts at width 5.
+    const plain = `你好\n${v} \n世界`
+    const result = expandPlaceholders(plain, [{ start: 5, end: 5 + v.length, text: "CONTENT" }])
+    expect(result).toBe("你好\nCONTENT \n世界")
+  })
+
+  test("returns input unchanged when there are no placeholders", () => {
+    expect(expandPlaceholders("你好世界", [])).toBe("你好世界")
+  })
+})