fix(agent): erase double-width characters cleanly at the prompt

.github/workflows/ci.yml

@@ -0,0 +1,41 @@
+name: CI
+
+# Run the test suite on every pull request, and again on main once a change
+# has landed. Read-only: this workflow never publishes anything, so it holds
+# no write scopes of any kind.
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+# A new push to a branch supersedes the run still in flight for it.
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: pytest (Python ${{ matrix.python-version }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false # one version failing should not hide the other's result
+      matrix:
+        # The lowest version the project supports, and the newest it can run
+        # on, so a break in either direction shows up here.
+        python-version: ["3.13", "3.14"]
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          fetch-depth: 0 # full history + tags so hatch-vcs can derive the version
+      - name: Install uv
+        uses: astral-sh/setup-uv@08807647e7069bb48b6ef5acd8ec9567f424441b # v8.1.0
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Run tests
+        # --frozen installs exactly what uv.lock pins and fails if the lock has
+        # drifted from pyproject.toml, so CI tests the declared dependencies.
+        run: uv run --frozen pytest

docs/changelogs/0.5.x.md

@@ -4,6 +4,13 @@ All notable changes in the **0.5.x** release series are documented here.
 
 ## [Unreleased]
 
+### Fixed
+- Backspacing a double-width character (CJK, emoji) at the `You>` prompt no
+  longer leaves half of its glyph on screen. The prompt now goes through
+  `readline`, which redraws the line, instead of letting the terminal erase
+  one column per backspace for a character that occupies two. Arrow-key
+  editing and in-session history come with it.
+
 ## [0.5.0] - 2026-07-29
 
 ### Added

src/nanopycodeagent/agent.py

@@ -14,6 +14,16 @@
 import os
 from importlib.metadata import PackageNotFoundError, version
 
+try:
+    # Importing readline routes input() through a line editor that redraws the
+    # whole line. Without it the tty erases one column per backspace, which
+    # leaves half of a double-width character (CJK, emoji) on screen even
+    # though it is gone from the buffer. Editing and history come along for
+    # the ride. Not available on every platform, so the import is optional.
+    import readline  # noqa: F401
+except ImportError:  # pragma: no cover - platform without readline
+    pass
+
 import anthropic
 from anthropic.types import MessageParam, ToolResultBlockParam, ToolUseBlock
 
@@ -105,7 +115,11 @@ def run() -> None:
     messages: list[MessageParam] = []
     while True:
         try:
-            user_input = input("\nYou> ").strip()
+            # The blank line before the prompt is printed separately: readline
+            # measures the prompt to place the cursor, and a newline inside it
+            # throws that off.
+            print()
+            user_input = input("You> ").strip()
         except (EOFError, KeyboardInterrupt):
             print()
             break

tests/helpers.py

@@ -100,15 +100,20 @@ def patch_client_and_input(monkeypatch, *, client, inputs):
     The config file is isolated by the autouse ``_isolate_config`` fixture in
     conftest.py, so ``run()`` sees no config file unless a test writes one to
     ``settings.SETTINGS_PATH``.
+
+    Returns the list that records the prompt of each ``input()`` call.
     """
     monkeypatch.setattr(anthropic, "Anthropic", lambda *a, **k: client)
 
     answers = iter(inputs)
+    prompts = []
 
     def fake_input(prompt=""):
+        prompts.append(prompt)
         try:
             return next(answers)
         except StopIteration as exc:  # safety net: behaves like Ctrl-D
             raise EOFError from exc
 
     monkeypatch.setattr("builtins.input", fake_input)
+    return prompts

tests/test_agent.py

@@ -5,6 +5,8 @@
 then assert on captured stdout and the recorded calls.
 """
 
+import pytest
+
 from nanopycodeagent import agent
 
 from helpers import (
@@ -145,6 +147,28 @@ def test_multi_turn_accumulates_history(monkeypatch, capsys):
     ]
 
 
+def test_readline_is_imported_for_line_editing():
+    # Without readline, input() leaves erasing to the tty, which clears one
+    # column per backspace and so strands half of a double-width character.
+    pytest.importorskip("readline")
+    assert hasattr(agent, "readline")
+
+
+def test_prompt_carries_no_embedded_newline(monkeypatch, capsys):
+    messages = FakeMessages([])
+    client = FakeClient(messages)
+    prompts = patch_client_and_input(monkeypatch, client=client, inputs=["/exit"])
+
+    agent.run()
+
+    # readline measures the prompt to place the cursor, so the blank line
+    # between turns is printed on its own rather than embedded in the prompt.
+    assert prompts == ["You> "]
+    # The banner's last line is still followed by that blank line. (input() is
+    # faked here, so the prompt itself never reaches stdout.)
+    assert "or /exit to quit.\n\n" in capsys.readouterr().out
+
+
 def test_blank_input_is_skipped(monkeypatch, capsys):
     messages = FakeMessages([])
     client = FakeClient(messages)

tests/test_line_editing.py

@@ -0,0 +1,169 @@
+"""End-to-end check that backspace erases wide characters cleanly.
+
+Erasing a double-width character (CJK, emoji) used to leave half its glyph on
+screen: the tty clears one column per backspace while the character occupies
+two, so the buffer emptied but the display did not. Showing that needs a real
+terminal, so this test drives the agent over a pty and replays everything it
+writes onto a model of a terminal line — the assertion is on what the user
+would see, not on the escape sequences a particular readline build emits.
+
+The child never reaches the API: the session ends at ``/exit``.
+"""
+
+import locale
+import os
+import select
+import sys
+import time
+import unicodedata
+
+import pytest
+
+
+def _ctype_is_utf8() -> bool:
+    """Whether this environment's character type is UTF-8.
+
+    readline only treats a multibyte character as a single unit under a UTF-8
+    ctype, so under, say, an explicit ``LC_ALL=C`` there is nothing sensible
+    to assert about erasing a CJK character.
+    """
+    if not hasattr(locale, "nl_langinfo"):  # not POSIX
+        return False
+    return "utf-8" in locale.nl_langinfo(locale.CODESET).replace("_", "-").lower()
+
+
+pytestmark = [
+    pytest.mark.skipif(not hasattr(os, "fork"), reason="needs a pty (POSIX only)"),
+    pytest.mark.skipif(not _ctype_is_utf8(), reason="needs a UTF-8 locale"),
+]
+
+CHILD = "from nanopycodeagent.agent import run; run()"
+PROMPT = b"You> "
+
+
+def render(stream: str) -> str:
+    """Replay a terminal write stream and return the last line it leaves.
+
+    Enough of a terminal to judge this: printable characters (one cell wide,
+    or two for East Asian wide/fullwidth ones), carriage return, backspace,
+    and the two escapes a line editor redraws with — ``ESC[<n>G`` to move to
+    an absolute column and ``ESC[K`` to erase to end of line.
+    """
+    cells: list[str] = []  # one entry per column; "" continues a wide glyph
+    col = 0
+    i = 0
+    while i < len(stream):
+        char = stream[i]
+        if char == "\x1b" and stream[i + 1 : i + 2] == "[":
+            end = i + 2
+            while end < len(stream) and not stream[end].isalpha():
+                end += 1
+            params, final = stream[i + 2 : end], stream[end : end + 1]
+            if final == "G":
+                col = max(int(params or "1") - 1, 0)
+            elif final == "K":
+                del cells[col:]
+            i = end + 1
+            continue
+        if char == "\r":
+            col = 0
+        elif char == "\n":
+            cells, col = [], 0  # only the line the prompt sits on matters
+        elif char == "\b":
+            col = max(col - 1, 0)
+        else:
+            width = 2 if unicodedata.east_asian_width(char) in "WF" else 1
+            cells.extend(" " for _ in range(col + width - len(cells)))
+            cells[col] = char
+            for offset in range(1, width):
+                cells[col + offset] = ""
+            col += width
+        i += 1
+    return "".join(cells).rstrip()
+
+
+def drive(keystrokes: bytes, home) -> str:
+    """Run the agent on a pty and type ``keystrokes`` at the prompt.
+
+    Returns everything the child wrote up to that point — the snapshot is
+    taken before ``/exit`` is sent, so it is exactly the screen the user is
+    looking at mid-edit.
+    """
+    import pty
+    import termios
+
+    # The autouse _isolate_config fixture has already cleared the ambient
+    # ANTHROPIC_* vars; HOME is redirected so the child cannot read the
+    # developer's ~/.nanoPyCodeAgent/settings.json either.
+    env = {**os.environ, "HOME": str(home), "ANTHROPIC_API_KEY": "sk-test"}
+
+    pid, fd = pty.fork()
+    if pid == 0:  # child: replaced by exec, or dies trying
+        try:
+            os.execve(sys.executable, [sys.executable, "-c", CHILD], env)
+        finally:
+            os._exit(1)
+
+    attrs = termios.tcgetattr(fd)
+    attrs[0] |= termios.IUTF8  # multibyte-aware ERASE, as a real terminal has
+    termios.tcsetattr(fd, termios.TCSANOW, attrs)
+
+    output = b""
+
+    def drain(seconds: float) -> bool:
+        """Read what is ready within ``seconds``; False once the child is done."""
+        nonlocal output
+        if not select.select([fd], [], [], seconds)[0]:
+            return True  # nothing to read yet, but the child is still up
+        try:
+            chunk = os.read(fd, 4096)
+        except OSError:  # child exited and closed its end
+            return False
+        if not chunk:
+            return False
+        output += chunk
+        return True
+
+    def wait_for(marker: bytes, timeout: float = 10.0) -> None:
+        deadline = time.monotonic() + timeout
+        while marker not in output and time.monotonic() < deadline:
+            if not drain(0.1):
+                return
+
+    def settle(quiet: float = 0.3, timeout: float = 5.0) -> None:
+        """Read until the child has written nothing for ``quiet`` seconds."""
+        deadline = time.monotonic() + timeout
+        while time.monotonic() < deadline:
+            before = len(output)
+            if not drain(quiet) or len(output) == before:
+                return
+
+    try:
+        wait_for(PROMPT)
+        assert PROMPT in output, f"never reached the prompt: {output!r}"
+        os.write(fd, keystrokes)
+        settle()
+        screen = output.decode("utf-8", "replace")
+        os.write(fd, b"/exit\r")
+        wait_for(b"Bye!", timeout=5.0)
+    finally:
+        os.close(fd)
+        os.waitpid(pid, 0)
+    return screen
+
+
+def test_backspace_clears_a_wide_character(tmp_path):
+    # Type two CJK characters, then erase both.
+    screen = drive("你好".encode() + b"\x7f\x7f", tmp_path)
+
+    # Nothing of either character is left beside the prompt. Before the fix
+    # two backspaces cleared two columns of the four they had filled, so
+    # "You> 你" stayed on screen after the input was already empty.
+    assert render(screen) == PROMPT.decode().rstrip()
+
+
+def test_backspace_keeps_the_characters_it_should(tmp_path):
+    # Erasing one of three leaves the other two intact and correctly placed.
+    screen = drive("中文字".encode() + b"\x7f", tmp_path)
+
+    assert render(screen) == "You> 中文"