What AI coding tools are good and bad at

How the models behind these tools work, the three ways you use them, and where they help or hurt, so you know when to reach for one and when to think first.

Large language model

Also called LLM, model, foundation model

What: The model behind every AI coding tool: a program trained on huge amounts of text and public code that predicts the most likely next piece of text, called a token, given everything before it.
Why: It explains both the magic and the mistakes. The model produces code that looks like code it has seen, which is usually right for common tasks and plausibly wrong for unusual ones.

Watch out

Thinking the model looked something up or ran the code: unless the tool actually ran a command or read a file, it predicted the answer.

Hallucination

Also called confabulation, made-up API, invented function

What: A confident answer that is simply wrong: a function, option, file path or package that does not exist, or a version detail from before the model was trained.
Why: Hallucinated code often compiles in your head and fails at runtime. Checking names against the official docs, the type checker and the tests is how you catch it.
Example: A tool suggests df.drop_nulls() for a pandas DataFrame. It sounds right, but pandas calls it dropna(); drop_nulls belongs to Polars.

Watch out

Trusting details about a library version newer than the model, instead of opening that version's docs.

Context window

Also called context, tokens, context limit

What: Everything the model can see at once: your messages, its replies, the files it read, command output and the tool's own instructions. It has a fixed size measured in tokens.
Why: The model only knows what is in the window. When it fills up, tools summarise or drop older parts, and quality falls, so managing context is a core skill.

Watch out

Keeping one long conversation for unrelated tasks, so old, irrelevant detail crowds out what matters now.

Completion, chat and agent

Also called autocomplete, ghost text, agent mode, inline suggestions

What: The three ways you use these tools: completions suggest the next lines as you type, chat answers questions about code, and an agent reads files, edits several of them and runs commands to finish a task.
Why: Each needs a different level of checking. A two-line completion is easy to read; an agent that changed twelve files and ran a migration needs a proper review.

Watch out

Reviewing agent changes with the same quick glance you give a one-line completion.

Where AI helps most

What: Well-trodden work with a clear check: boilerplate, syntax you half remember, explaining unfamiliar code, first drafts of tests, mechanical refactors covered by tests, and searching a codebase you are new to.
Why: Handing over this work saves time without hiding anything important, because you can verify the result quickly and you already know what good looks like.
Example: Asking for a script that renames 300 files by date is a good fit: the task is common and you can check the result on a copy of the folder.

Where AI struggles

What: Work that depends on facts the model cannot see or rarely saw: your business rules, subtle concurrency, security decisions, new library versions, performance under real load, and large, vague tasks with no clear definition of done.
Why: These are the places where a plausible answer is most dangerous, so they need your design thinking first and much closer review afterwards.

Watch out

Asking an agent to "build the whole app" and then trying to understand thousands of lines after the fact.

Automation bias

Also called over-trust, rubber-stamping

What: The human habit of trusting output from a confident machine more than we should, and approving it with less care than we would give a colleague's code.
Why: Most AI coding incidents are review failures, not tool failures. Naming the bias makes it easier to slow down for the changes that deserve it.

Watch out

Clicking approve on the tenth permission prompt without reading it because the first nine were fine.

GitHub Copilot

What: GitHub's AI assistant: inline suggestions and chat in VS Code, JetBrains IDEs and Visual Studio, agents in the editor, the Copilot CLI in your terminal, code review and a cloud agent on GitHub. Copilot Free has monthly limits and verified students can get Copilot Student.
Why: It sits where many teams already work, in the editor and on pull requests, so you meet it at university and at work. Knowing each surface lets you pick the right one.

Watch out

Assuming every feature is on every plan: check the plans page before building a workflow around one.

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