AI Coding Tool Status Line Configuration: From Script to Verification

Conclusion first: five steps in about 15 minutes

A status line is not UI decoration. It is a single-line text channel triggered by the tool and rendered by an external script. The practical implementation path has five steps:

1. Choose the fields and define their priority (no more than six).
2. Capture one real stdin sample.
3. Write the script: read stdin -> parse -> compute -> output one line.
4. Point statusLine to the script in the configuration file and run chmod +x.
5. Verify it with a checklist: fault tolerance, length, performance, security, and fallback behavior.

If all five items pass, the status line can remain stable over time. If any item is missing, problems will later appear as empty output, garbled text, missing fields, or terminal lag.

1. Decide which fields to show and which to omit

Status line space is limited. Too much information reduces readability. Fill in the table below before writing code.

Field	Data source	Priority	Truncation rule	Display example
Current directory	workspace.dir	P0	Keep the last 2 levels, <= 28 characters	app/api
Context usage percentage	context	P0	Integer percentage	ctx=21%
Model name	assistant.model	P1	<= 12 characters; ellipsis if longer	example-l…
Deep-thinking toggle	flags.thinking	P1	Short label	think=on
Agent/subtask state	flags.agent	P1	Short label	agent=sub
Interaction mode	session.mode	P2	<= 8 characters	plan
Input/output tokens	context	P2	Convert thousands to K/M	42.0K/3.1K
Window capacity	context.window_tokens	P3	Convert to K/M	200.0K
Session ID	session.id	P4	Hidden by default; shown for debugging	a1b2c3

The two P0 items must be retained on narrow terminals. The remaining fields should be dropped from low priority to high priority according to available width. Information such as reasoning effort and project name is worth showing only when the terminal width is greater than 100 columns.

2. Configuration entry and execution model

Most tools expose a configuration item named statusLine whose value is an executable command. On each refresh, the tool writes the current session context to the script’s standard input as JSON, reads the first line of the script’s standard output, and renders it in the terminal.

Configuration example:

{
  "statusLine": {
    "type": "command",
    "command": "python3 ~/.config/agentcli/statusline.py",
    "padding": 0
  }
}

Implementation steps:

mkdir -p ~/.config/agentcli
# Put the script at ~/.config/agentcli/statusline.py
chmod +x ~/.config/agentcli/statusline.py
# Test offline before connecting it to the tool
echo '{"workspace":{"dir":"/srv/app/api"}}' | python3 ~/.config/agentcli/statusline.py

Three hard constraints: output only one line, read the full input from stdin, and keep exit code 0. Some tools treat a non-zero exit code as rendering failure and fall back to the built-in status line. They may also display stderr to the user. Therefore, do not use a non-zero exit code for recoverable situations such as missing data.

3. Input sample and graceful degradation

Illustrative input structure. Field names vary by tool; use the actual fields provided by your tool.

{
  "session":    {"id": "s-8f2c", "mode": "plan"},
  "workspace":  {"name": "demo", "dir": "/srv/app/api"},
  "assistant":  {"model": "example-large-128k", "effort": "high"},
  "context":    {"input_tokens": 42000, "output_tokens": 3100, "window_tokens": 200000},
  "flags":      {"thinking": true, "agent": "none"}
}

Normal output: plan example-l… /app/api ctx=21% 42.0K/3.1K think=on

Each exception type should have a matching degraded output:

Exception	Detection condition	Degraded output	Exit code
Empty stdin	raw.strip() == ""	Status line: waiting for data	0
Parse failure	JSON raises an exception or root node is not an object	Status line: parse failed · /srv/app/api (keep cwd as fallback)	0
Missing field	Key does not exist	model=unknown dir=/srv/app/api ctx=?	0
Type error	context is a string, null, or negative value	Ratio segment falls back to ctx=?; other segments still render	0
Control characters or newlines	Value contains \n, \r, \t, or C0 control characters	Cleaned to spaces to keep a single line	0

Key principle: always output one usable line, rather than exiting with an error, failing silently, or outputting multiple lines.

4. Ready-to-run Python 3 example script

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""Terminal status line renderer: read JSON from stdin -> output one line to stdout."""
import json
import os
import shutil
import sys
import time
USE_COLOR = os.environ.get("STATUSLINE_COLOR", "auto") != "off"
DEBUG = os.environ.get("STATUSLINE_DEBUG") == "1"
RESET, DIM, OK, WARN, ERR = "\033[0m", "\033[2m", "\033[36m", "\033[33m", "\033[31m"
def log(msg):
    if not DEBUG:
        return
    try:
        with open(os.environ.get("STATUSLINE_LOG", "/tmp/statusline.log"), "a",
                  encoding="utf-8") as fh:
            fh.write("%.3f %s\n" % (time.time(), msg))
    except OSError:
        pass
def paint(text, color):
    return "%s%s%s" % (color, text, RESET) if USE_COLOR else text
def sanitize(value, default="unknown"):
    """Clean a value into single-line safe text: drop C0/C1 control chars and escape residue."""
    if isinstance(value, bool):
        value = "on" if value else "off"
    if isinstance(value, (int, float)):
        return str(value)
    if not isinstance(value, str):
        return default
    cleaned = "".join(ch for ch in value if ch == " " or 32 <= ord(ch) < 127 or ord(ch) > 159)
    cleaned = " ".join(cleaned.replace("\x1b", "").split())
    return cleaned or default
def truncate(text, limit):
    text = str(text)
    return text if limit <= 1 or len(text) <= limit else text[: limit - 1] + "…"
def short_path(path, levels=2, limit=28):
    norm = os.path.normpath(str(path or "")).replace("\\", "/")
    parts = [p for p in norm.split("/") if p and p != "."]
    return truncate("/".join(parts[-levels:]) if parts else norm, limit)
def fmt_tokens(n):
    if isinstance(n, bool) or not isinstance(n, (int, float)) or n < 0:
        return "?"
    if n < 1000:
        return "%d" % n
    if n < 1000000:
        return "%.1fK" % (n / 1000.0)
    return "%.1fM" % (n / 1000000.0)
def ratio(ctx):
    """Return (integer percentage or None, used tokens or None)."""
    if not isinstance(ctx, dict):
        return None, None
    try:
        used = int(ctx.get("input_tokens") or 0) + int(ctx.get("output_tokens") or 0)
        window = int(ctx.get("window_tokens") or 0)
    except (TypeError, ValueError):
        return None, None
    if window <= 0:
        return None, used
    return int(used * 100 // window), used
def build(segments):
    """Drop segments from low to high priority based on terminal width, keeping one line."""
    cols = shutil.get_terminal_size(fallback=(120, 24)).columns
    limit = int(os.environ.get("STATUSLINE_MAX_LEN", min(160, max(40, cols - 2))))
    keep = list(segments)  # already ordered from high to low priority
    while keep and len("  ".join(s[1] for s in keep)) > limit:
        dropped = keep.pop()
        log("dropped segment: %s" % dropped[0])
    return "  ".join(s[1] for s in keep)[:limit]
def render(data):
    get = lambda *keys: next((data[k] for k in keys if isinstance(data.get(k), dict)), {})
    sess, ws, ast_, ctx, flags = get("session"), get("workspace"), get("assistant"), get("context"), get("flags")
    pct, used = ratio(ctx)
    pct_text = "ctx=%s%%" % pct if pct is not None else "ctx=?"
    if pct is None:
        pct_color = DIM
    elif pct >= 90:
        pct_color = ERR
    elif pct >= 70:
        pct_color = WARN
    else:
        pct_color = OK
    segments = [
        ("dir", paint(sanitize(ws.get("dir"), os.getcwd()), OK)),
        ("pct", paint(pct_text, pct_color)),
        ("model", sanitize(ast_.get("model"))[:12]),
        ("think", "think=" + sanitize(flags.get("thinking"), "off")),
    ]
    if sanitize(flags.get("agent"), "none") != "none":
        segments.append(("agent", "agent=" + sanitize(flags.get("agent"))))
    if ast_.get("effort"):
        segments.append(("effort", sanitize(ast_.get("effort"))))
    if used is not None:
        segments.append(("tokens", "%s/%s" % (fmt_tokens(ctx.get("input_tokens")),
                                              fmt_tokens(ctx.get("output_tokens")))))
    if sess.get("mode"):
        segments.append(("mode", sanitize(sess.get("mode"))[:8]))
    if DEBUG and sess.get("id"):
        segments.append(("sid", DIM + truncate(sanitize(sess.get("id")), 8) + RESET))
    return build(segments)
def main():
    try:
        raw = sys.stdin.read()
    except Exception as exc:                      # still output a line even if reading fails
        log("stdin error: %r" % exc)
        print("Status line: waiting for data")
        return 0
    if not raw.strip():
        print("Status line: waiting for data")
        return 0
    try:
        data = json.loads(raw)
        if not isinstance(data, dict):
            raise ValueError("root is not object")
    except Exception as exc:
        log("parse failed: %r" % exc)
        print("Status line: parse failed · %s" % short_path(os.getcwd()))
        return 0
    try:
        line = render(data)
    except Exception as exc:                      # fallback: never print traceback to terminal
        log("render failed: %r" % exc)
        line = "Status line: render fallback · %s" % short_path(os.getcwd())
    sys.stdout.write(line.replace("\n", " ") + "\n")
    return 0
if __name__ == "__main__":
    sys.exit(main())

Exit code convention: 0 means “displayable text has been produced,” including all degradation branches. A non-zero exit may occur only if the script file itself is not executable or the interpreter is missing. In that case, the tool’s default status line takes over automatically.

Minimal Bash version when Python is unavailable. It can work, but it is not recommended for long-term use because string processing is fragile:

#!/usr/bin/env bash
input="$(cat)"
[ -z "$input" ] && { printf 'Status line: waiting for data\n'; exit 0; }
model="$(printf '%s' "$input" | grep -o '"model"[[:space:]]*:[[:space:]]*"[^"]*"' | head -n1 | sed 's/.*"\([^"]*\)"$/\1/')"
printf 'Status line: %s · %s\n' "${model:-unknown}" "$(basename "$PWD")"
exit 0

5. Single-line output, length control, and color toggles

Length and truncation rules should be implemented in the script, not judged visually:

• Total length limit: by default min(160, terminal columns - 2), overridable with STATUSLINE_MAX_LEN.
• Width detection: use shutil.get_terminal_size() (equivalent to tput cols at the lower level). Do not hardcode 80 columns.
• When the limit is exceeded, drop segments from right to left (lower priority first). Do not drop the P0 dir or ctx fields.
• Per-field limits: directory 28 characters / 2 levels, model name 12 characters, mode 8 characters, session ID 8 characters in debug mode.
• Before output, normalize all newlines with replace("\n", " ") to prevent multiple lines from breaking terminal layout.

Color has three states:

• STATUSLINE_COLOR=off: force plain text for CI logs, redirection, and terminals without color.
• auto (default): use tput colors to detect support. If it returns -1 or a value below 8, disable color automatically.
• Keep only three semantic levels: normal (cyan), warning >= 70% (yellow), danger >= 90% (red). Use \033[2m for auxiliary information.

6. Security and permissions checklist

The status line is called automatically and frequently by the tool. Treat it with least privilege. Check each item:

• Restrict script permissions to chmod 750 and ensure the owner is the current user, preventing replacement by other users.
• Read only necessary inputs (stdin and environment variables). Do not read credential files such as ~/.ssh, ~/.aws, or .env.
• Filter newline characters, \r, \t, C0/C1 control characters, and \x1b escape sequences to prevent terminal escape injection.
• Do not execute external commands such as git, curl, or npm. Do not make network requests or start background processes.
• Do not write session data to disk. If caching is required, write only to a self-owned subdirectory under /tmp and do not store sensitive fields.
• Normalize paths with os.path.normpath() to prevent path concatenation issues and separator errors.
• Add a timeout wrapper at the calling layer: timeout 0.5 python3 ~/.config/agentcli/statusline.py.
• Move complex statistics (token audits, cost accounting) to asynchronous pre-generated caches. The script should only read the cache.

7. Performance and refresh strategy

The execution time budget for a status line should be kept within 50 ms. Once it exceeds 200 ms, users may notice lag.

• Cache key design: key = sha1(session.id + model + mode + dir + output_tokens)[:8], stored at /tmp/statusline-cache/<key>.json, with a default TTL of 5 seconds.
• Cache only slow-changing fields (model name mapping tables, project names, path shortening results). The ctx ratio can be computed each time; it only requires two integer additions and one integer division.
• Refresh threshold: if the token count changes by less than 2%, reuse the previous percentage to avoid unnecessary redraws caused by number jitter.
• Timeout strategy: apply an outer timeout 0.5 and do not leave room for network or subprocess calls inside the script.
• Measurement: run time (echo "$SAMPLE" | python3 statusline.py >/dev/null) 20 consecutive times and take p95.

8. Cross-platform behavior and degradation

• Color: detect before coloring. If detection fails, output plain text.
• Paths: normalize with os.path.normpath() and display using /. Do not hardcode \ or drive letters. On Windows, both C:\a\b and /a/b can be correctly reduced to the last two levels.
• Encoding: declare UTF-8 in the first line of the script and use explicit encoding="utf-8" for reads and writes. Verify whether locale charmap is UTF-8.
• Width: if tput cols returns a narrow value such as 40 columns, keep only dir + ctx.

9. Troubleshooting: from symptom to root cause

What if the status line does not appear at all?

Run the following in order. Any failed step helps identify the layer:

ls -l ~/.config/agentcli/statusline.py                     # 1. File exists and is executable
python3 -c "import json,sys;print(sys.version)"            # 2. Interpreter is available
echo '{"workspace":{"dir":"/srv/app/api"}}' \
  | python3 ~/.config/agentcli/statusline.py; echo "exit=$?" # 3. stdin pipeline
grep -rn "statusLine" ~/.config/agentcli/settings.json     # 4. Configuration points to the script

If all four checks pass but there is still no output, check whether the tool requires restarting the session or whether the configuration item only takes effect in a new session.

What if Chinese characters or colors become garbled or appear as squares?

locale charmap                                        # Expect UTF-8
printf 'é' | iconv -f UTF-8 -t UTF-8 >/dev/null && echo utf8-ok
tput colors                                           # -1 means color is unsupported
STATUSLINE_COLOR=off python3 statusline.py <<< '{"assistant":{"model":"example-model"}}'

Garbled text is usually caused by “input is UTF-8 but the script reads it as ASCII” or “forcing \033[ output on a terminal without color support.” For color issues, use STATUSLINE_COLOR=off to narrow down the cause.

How do you diagnose missing fields or incorrect values?

STATUSLINE_DEBUG=1 STATUSLINE_LOG=/tmp/statusline.log python3 statusline.py < sample.json
cat /tmp/statusline.log     # Check dropped segment / parse failed records

If the log contains dropped segment: model, the issue is width-based dropping rather than missing data. If it contains parse failed, the field names passed by the tool may differ from your assumed structure. In that case, compare the real structure with python3 -m json.tool < sample.json and add alias keys in the get() helper inside render().

What if refresh is too slow or the terminal lags?

time (for i in $(seq 20); do python3 statusline.py < sample.json >/dev/null; done)

If p95 exceeds 200 ms, check in order: whether subprocess or network calls are used, whether paths and mapping tables are recomputed on every refresh, and whether an outer timeout 0.5 protection is missing.

General troubleshooting flow: first make the script output fixed text to confirm the call chain, then parse fields to confirm the data, and finally add color and formatting to confirm display. Remove layers one by one.

10. One-page acceptance checklist

• Key fields (directory, context ratio, model) display stably. After 30 minutes of continuous use, there are no blank outputs or crashes.
• Empty input, parse failure, missing field, and type error all produce degraded output. The exit code remains 0.
• Output is always a single line. In both 120-column and 40-column terminals, it does not wrap and does not truncate P0 fields.
• STATUSLINE_COLOR=off outputs plain text. When tput colors returns -1, no escape sequences remain.
• p95 time is below 200 ms. There are no subprocesses or network requests in the call chain.
• Script permissions are 750, credentials are not read, session data is not written, and an outer timeout exists.
• Troubleshooting commands and debug logs are reproducible. The script is under version control and field definitions are documented.

The value of status line configuration is not in stacking fields, but in stably and cheaply converting tool runtime state into readable information. Around five principles—clear fields, robust parsing, readable display, safe execution, and continuous maintenance—use the script and checklist above to turn each principle into verifiable actions. This small configuration can then become a long-term reusable development efficiency asset.