diff --git a/packages/opencode/src/cli/cmd/tui/util/clipboard.ts b/packages/opencode/src/cli/cmd/tui/util/clipboard.ts
index be3cec14c6a2..12fd11e9bc13 100644
--- a/packages/opencode/src/cli/cmd/tui/util/clipboard.ts
+++ b/packages/opencode/src/cli/cmd/tui/util/clipboard.ts
@@ -34,13 +34,16 @@ const getClipboardy = lazy(async () => {
  * This allows clipboard operations to work over SSH by having
  * the terminal emulator handle the clipboard locally.
  */
-function writeOsc52(text: string): void {
-  if (!process.stdout.isTTY) return
+export function buildOsc52Sequence(text: string): string {
   const base64 = Buffer.from(text).toString("base64")
   const osc52 = `\x1b]52;c;${base64}\x07`
   const passthrough = process.env["TMUX"] || process.env["STY"]
-  const sequence = passthrough ? `\x1bPtmux;\x1b${osc52}\x1b\\` : osc52
-  process.stdout.write(sequence)
+  return passthrough ? `\x1bPtmux;${osc52}\x1b\\` : osc52
+}
+
+function writeOsc52(text: string): void {
+  if (!process.stdout.isTTY) return
+  process.stdout.write(buildOsc52Sequence(text))
 }
 
 export interface Content {
diff --git a/packages/opencode/test/cli/cmd/tui/util/clipboard.test.ts b/packages/opencode/test/cli/cmd/tui/util/clipboard.test.ts
new file mode 100644
index 000000000000..9901789a62d4
--- /dev/null
+++ b/packages/opencode/test/cli/cmd/tui/util/clipboard.test.ts
@@ -0,0 +1,40 @@
+import { describe, expect, test } from "bun:test"
+import { buildOsc52Sequence } from "../../../../../src/cli/cmd/tui/util/clipboard"
+
+function withTmux(value: string | undefined, fn: () => void): void {
+  const before = process.env["TMUX"]
+  if (value === undefined) delete process.env["TMUX"]
+  else process.env["TMUX"] = value
+  try {
+    fn()
+  } finally {
+    if (before === undefined) delete process.env["TMUX"]
+    else process.env["TMUX"] = before
+  }
+}
+
+describe("buildOsc52Sequence", () => {
+  test("returns plain OSC 52 without TMUX", () => {
+    withTmux(undefined, () => {
+      const result = buildOsc52Sequence("hello")
+      expect(result).toBe("\x1b]52;c;aGVsbG8=\x07")
+    })
+  })
+
+  test("returns DCS-wrapped OSC 52 with TMUX (no double ESC)", () => {
+    withTmux("ses", () => {
+      const result = buildOsc52Sequence("x")
+
+      expect(result.startsWith("\x1bPtmux;")).toBe(true)
+
+      const semicolonPos = result.indexOf(";")
+      expect(result.charCodeAt(semicolonPos + 1)).toBe(0x1b)
+      expect(result.charCodeAt(semicolonPos + 2)).toBe(0x5d)
+
+      expect(result.endsWith("\x1b\\")).toBe(true)
+
+      const escCount = Array.from(result).filter((c) => c === "\x1b").length
+      expect(escCount).toBe(3)
+    })
+  })
+})