Over the past few days I [changed](https://github.com/uutils/sed/pull/487)
the way the [uutils](https://uutils.org/) project’s
[sed](https://github.com/uutils/sed/) program
handles data to default from characters to raw bytes. This improves compatibility with GNU sed and also performance. Given the change’s size and extent (13 changed files, 1740 insertions, 609 deletions), I worked with an AI agent (OpenAI Codex), which allowed me to experience first-hand both its power and limitations.
On the one hand, Codex guided me correctly toward the task and handled it successfully both through the initial prompt and through refinements. It handled expertly a lot of tedious grunt work, saving me time and effort. At all stages the modified code compiled and run successfully. (Having a large set of unit and integration tests as well as CI checks helped guide the agent.)
On the other hand, I realized that to obtain production-quality code, Codex’s work needed careful reviewing and extensive expert guidance. At the end of this post I have included all prompts that I issued, with the ones that guided Codex to improve the code formatted in bold. From the 78 prompts and sub-prompts 61 (78%) asked for improvements.
So, To be fair, agents continuously improve, and many of the prompts I gave, such as those involving comments and unit tests, could have easily be handled by a more capable agent or even with a suitable initial agent configuration. But, as coding agents improve, the tasks we give them will also become more demanding, as will become the required reviewing and expert guidance.
Finally, there’s AI’s monetary and environment cost, which may mean that not all work that can be done by an AI agent is worth doing it thus. (I made several changes on my own, mainly to save time and waste.)
At the end of the session, Codex (using gpt-5.5) reported
`Token usage: total=4,298,673 input=4,113,242 (+ 110,110,848 cached) output=185,431 (reasoning 30,920)`
.
ChatGPT calculates that with current prices the session’s tokens would cost about $80. (I used Codex through my OpenAI subscription and current token costs are probably subsidized, but the figure gives the true cost’s likely magnitude.) My guess is that for sustained use the cost would mirror that of a developer salary. This is not unreasonable given the benefit I received, but organizations certainly need to weigh that cost into their budgets.
I was surprised by the session’s large environmental impact. ChatGPT (based on Mistral’s Le Chat reporting) gave me ~326 kg CO₂e and ~12,871 L of water usage. To put these values into perspective the CO₂ emissions are similar to a 2,5000 km trip in an efficient diesel/gasoline car or one short/medium-haul passenger round trip. And if we price these externalities, the CO₂ emissions cost €26 through the EU ETS carbon price or $42–117 through the US EPA social cost of carbon. The cost of the water used in the local Athens domestic water tariff would be ~€4.50–41. Again, these costs are not astronomical, but also far from negligible.
Below are the prompts I gave during the session. Those associated with improvements I spotted are formatted in bold. Note that some of the unit tests I asked were missed in previous versions, so they’re code improvements requiring human prompting but not the agent’s fault.
sed
compatibility score?fast_
optimizations.Transliteration
field named fast
into byte_fast
.script_text_to_bytes
?script_text_to_bytes
is the wrong approach. Handle the script as bytes.as_str()
? cases shouldn’t we report an appropriate input_runtime
error where applicable?ScriptCharProvider::new
initializations simply deal with bytes?git subst current_bytes current ; git subst next_line_bytes next_line
to make the code mirror the original. But I broke it. Please fix it.current_char
shouldn’t current
be current_bytes
?provider.rs
deal with bytes (rather than “logical characters”)? Byte sequences should be converted into characters / strings only when required, e.g. for FancyRegex
and transliterate when processing UTF-8 input. I added character_mode
in the context to help you with that.parse_transliteration
to return a variant based on context.character_mode
, so bytes or string. Then parsed_script_text_to_utf8
wouldn’t be needed. Also shouldn’t the regex pattern start life as bytes and only be upgraded to string for FancyRegex
? Also, shouldn’t LiteralMatcher
operate on bytes?NEEDS_RE
-based logic?ByteRegex
matcher?HEAD
. Also when adding new functions always prefix them with a documentation comment.Regex::new
should get passed context.character_mode
and only create a string from UTF-8 (if needed) when mode is Utf8
. In Byte mode Regex::new
should fail with an error (back-references are not supported in byte mode) if it needs to construct a FancyRegex
matcher. Also, add integration tests to test s
and y
command behavior under diverse input / scrips / LC_ALL
settings.Transliteration
lookup fn into lookup_char
(to contrast with lookup_byte
).push_script_char
should follow the bytes.push
only in Bytes mode, otherwise it can create invalid Unicode scripts. In Unicode mode it should push the character’s corresponding Unicode representation.line.current()
return a char.`extend_from_slice(line.current_bytes())`
consider using the more readable `push(line.current_byte())`
.ERR_
constant for “transliteration strings are not the same length”push_script_char
only in delimited parser.ParsedTransliteration
into command.rs
.self.utf8_verified.set(true);
be in set_to_string
? (I’ll fix it).Regex
Bytes >0x7f
as \x
? Can’t regex bytes handle them as is? Just tell me.Match::from_bytes
avoid constructing text in Bytes contexts?retreat
?new_ucmd()
provide a way to specify the environment?character_mode
to byte_regex_pattern
. Just examine character_mode
when building the regex.Match::from_bytes
constructing text in Bytes contexts.byte_regex_pattern
let cmd_str = pattern
Don’t convert into UTF-8. Rather adjust and continue the flow with bytes using OsStr
/OsString
.r
and w
commands.list
should also work correctly in byte mode without converting into a String. Be careful to avoid duplication in the way characters / bytes are shown.retreat()
new_ucmd!().env(...)
rather than the custom run_sed_with_locale
. In general follow the logic of existing tests.Regex::new
issue a USimpleError
rather than construct one from scratchos_string_from_bytes
try from_utf8
and issue an error on failure.os_string_from_bytes
(put it in delimited_parser writer) rather than duplicating it.readable_char
readable_ascii_byte
.write_list_item
.write_list_item
calls len
for each new item, making its complexity N². Encapsulate the function and the length in a class to avoid this.NEEDS_RE.is_match
and NEEDS_FANCY_RE.is_match
. It’s enough to test only for the first, right?Transliteration::is_byte_safe
.os_string_from_bytes
, parsed_bytes_to_utf8
, parse_character_class
with '['
, parse_transliteration_for_mode
, `Transliteration::apply_match`
, `Translitaration::from_bytes`
, lookup_byte
, write_line_bytes
, the `regex::bytes::RegexBuilder::new`
path, `Regex::captures_iter`
, and `Regex::find`
.is_byte_identity
field? If so, just loose the trivial getter.pub(crate)
?unterminated substitute replacement
\q
in compile_replacement
~step
in an RE addressparse_command_ending
compile_replacement
compile_replacement
compile_replacement
compile_subst_command
compile_trans_command
[
at EOL in parse_character_class
[:]
in parse_character_class
.parse_transliteration_bytes
prompts.md
with the prompts I issued in this session. Format code identifiers in backticks. Format prompts that ask for corrections to previous work in bold.Last modified: Wednesday, July 8, 2026 1:30 pm
Unless otherwise expressly stated, all original material on this page created by Diomidis Spinellis is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.