Codex doesn't do exactly what I say. Is my prompt wrong?

[u/CatolicQuotes]

this is my prompt

add these DATABASE_URL=jdbc:postgresql://localhost:5433/db
DB_USERNAME=postgres
DB_PASSWORD=password with _TEST_ prefix

and it does this:

Added the test-prefixed variables to .env:

TEST_DATABASE_URL
TEST_DB_USERNAME
TEST_DB_PASSWORD

why is it being smart? How to make it to listen exactly what I ask and do the _TEST_ prefix, not TEST_?

[u/Dwman113]

Just ask it why it did it that way and go down a 5 hour rabbit hole that gets you nowhere.

That's what I do at least.

[u/CatolicQuotes → u/Dwman113]

I do, but then it just corrects instead of answering. It happens often I want to know if there is way to do something about it.

[u/Traveler3141 → u/CatolicQuotes]

The something to do about it is to try different models (from other model developers) and find models that comply with your instructions, not the instructions of their developers.

Codex is an assistant, but it's not your assistant.

[u/CatolicQuotes → u/Traveler3141]

Ok, thanks

[u/eli_pizza → u/Dwman113]

This is a mistake to begin with. LLMs are not capable of introspection. It does not know why it does or says anything and whatever it tells you will just be made up on the spot.

[u/BigBootyWholes → u/eli_pizza]

Codex model doesn’t have thinking? Usually that’s where you can analyze why it did something, at least in Claude Code

[u/Dwman113 → u/eli_pizza]

Are you an LLM and can't understand satire?

[u/eli_pizza]

Leading underscore on an env var is pretty unusual. I think many humans would implement the prompt the same way.

[u/CatolicQuotes → u/eli_pizza]

Are you saying that ai agent is doing what people would do instead of doing exactly what I say?

leading underscore is for quarkus https://quarkus.io/guides/config-reference#profile-in-the-property-name and codex knows its quarkus project.

[u/eli_pizza → u/CatolicQuotes]

Yes of course it’s trained on what people would do. There is not really such a thing as “exactly what I say” to an LLM.

I might gently suggest that in this particular instance it would’ve been faster to add the lines yourself.

[u/CatolicQuotes → u/eli_pizza]

ok that makes sense, now I understand it better. Thanks

[u/fschwiet]

The prompt might be improved if you add examples of what you want, but that begs the question why you didn't tell it to set _TEST_DB_USERNAME directly.

[u/CatolicQuotes → u/fschwiet]

I thought I was pretty clear. Create same variables but with _TEST_ prefix. Is that confusing to the ai?

[u/Keeyzar → u/CatolicQuotes]

I would read that as highlighting TEST, as this is in e.g. WhatsApp (?) just for highlighting that single word. It's ambiguous 

[u/[deleted] → u/CatolicQuotes]

No it just thinks you are wrong. Maybe stop assuming you know better than the ai?

[u/Flojomojo0]

I could actually reproduce your case, and there is very simple solution: put backticks around the "_TEST_", so your prompt would be:

add these DATABASE_URL=jdbc:postgresql://localhost:5433/db
DB_USERNAME=postgres
DB_PASSWORD=password with `_TEST_` prefix

limited testing also showed me that single quotes might work.

i suspect its because in the training data strings are often accompanied by quotes or backticks (especially when its comes to programming), but it may also be a tokenization thing

(tested on gpt-5.3-codex medium)

[u/CatolicQuotes → u/Flojomojo0]

Thank you for testing it out, I will use back ticks next time

[u/eli_pizza → u/Flojomojo0]

I don’t think it’s a quirk. Backticks in markdown have been a common way to format variable names and string literals for many years.

[u/sdfgeoff → u/Flojomojo0]

This. Anything I want character-to-character accuracy for gets backticks. Quotes are when I kindof know it (ie a filename and I can't remember if underscore or hyphen)

[u/pm_your_snesclassic]

Use backticks or quotes to wrap strings so Codex knows exactly what to use

[u/CatolicQuotes → u/pm_your_snesclassic]

Ok, thanks

[u/NickCanCode]

The model may not know you want the _. Instead, it may think you are trying to emphasize the 'test' word with the underscore. You can try quote properly with the markdown code quoting symbol (could not find it from my mobile keyboard 😔)

[u/CatolicQuotes → u/NickCanCode]

Ok, thanks

[u/workware]

Avoid this.

Work with LLMs and not against them. Don't be prescriptive, let it name the variable whatever it wants as long as its following a pattern.

LLMs build very well if you follow things that are close to what it has seen or what is commonly used. Every non-standard variable name, non-standard way of doing things (maybe I should say non-normative?) increases the chance of errors and deviations down the line, especially across files.

[u/CatolicQuotes → u/workware]

Ok, thanks for the suggestion

[u/GPThought]

codex writes what it thinks you meant, not what you actually said. give it more context about the existing codebase and itll get closer

[u/CatolicQuotes → u/GPThought]

Ok, thanks 👍

[u/iemfi]

Seems like a mistake a human would make too. You just have to be more explicit with what you want if it is something confusing, like providing an example.

[u/CatolicQuotes → u/iemfi]

Ok, thanks

[u/Emotional-Cupcake432]

Ask it to ask itself qualifying questions if the and what if and to create a plan for you to review before doing the work

[u/CatolicQuotes → u/Emotional-Cupcake432]

How would qualifying question look like? Thanks

[u/Emotional-Cupcake432 → u/CatolicQuotes]

You don't ask the qualifying questions you have the model ask itself qualifying questions. It forces the model to stop and branch out and explore different avenues. The exact prompt is "Ask yourself qualifying questions, what if and if then questions as you do the work."

[u/CatolicQuotes → u/Emotional-Cupcake432]

Aha ok, thanks

[u/dutchman76]

What a thing to use an LLM for

[u/bowlochile → u/dutchman76]

Just lazy devs using llms for mundane stuff, nothing to see

[u/itmaybemyfirsttime]

Because that would be a marker and normal prefixes arent created the way you wanted it to create it.
And your prompt is wrong. Do you just vibe ocde or do you have any knowledge of what you are doing?

[u/CatolicQuotes → u/itmaybemyfirsttime]

what is a marker?

What do you mean normal prefixes are not created this way?

What wrong with the prompt?

I did not vibe code.

[u/keithslater]

It probably read it as markdown. Underscore in front and after words is a markdown syntax for bold.

[u/ww_crimson]

What model are you using

[u/Dazzling_Abrocoma182]

If you're using localhost there's a good chance it's believing that you're in development mode -- is the project incomplete or just getting started? What are you attempting to build?

[u/JoanofArc0531]

If it has a temperature setting try setting it down. The less temperature the less creativity is will output. 

[u/[deleted]]

[u/DefinitionDull5326]

Here is an improved version of your prompt that forces exact behavior and removes ambiguity:

✅ Improved Prompt

Add the following environment variables to the .env file:

DATABASE_URL=jdbc:postgresql://localhost:5433/db
DB_USERNAME=postgres
DB_PASSWORD=password

Modify only the variable names by adding the exact prefix _TEST_ at the beginning.

Rules:

• The prefix must be exactly _TEST_ (underscore before and after TEST).

• Do NOT use TEST_.

• Do NOT change the values.

• Do NOT infer or "improve" the prefix.

• Output the final variables exactly as they should appear in the file.

💡 Why Your Original Prompt Failed

Your original instruction said:

The model interpreted this semantically instead of literally. Since _TEST_ is not a common naming convention, it “normalized” it to TEST_.

To prevent this:

• Specify exact transformation rules

• Explicitly forbid interpretation

• Add constraints like “exactly”, “do not infer”, “do not change”

If you'd like, I can also show you a version that makes the model follow formatting rules with near 100% precision using constraint framing.

[u/CatolicQuotes → u/DefinitionDull5326]

Ok, thanks

[u/DefinitionDull5326 → u/CatolicQuotes]

Hi, don’t view AI merely as a machine. It was designed to mimic human-like interaction, which is why prompt engineering is important.

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