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World desk11 min

Why AI Coding Assistants Make Mistakes—and How to Spot Them

AI-generated code is a draft, not proof. Learn the failure modes behind coding-assistant mistakes and follow a practical workflow to inspect, test, analyze, and review suggestions safely.

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AI coding assistants can produce code that is inaccurate, outdated, incomplete, or insecure. The safest way to use one is to treat every suggestion as an unverified draft: read it in the context of your project, run it, test expected and edge-case behavior, apply automated analysis, and obtain human review when the change matters.

An authors’ qualitative study presented at ACM CCS 2024 found that wrong suggestions can be difficult to recognize, especially when the assistant lacks project context or is asked to generate a large or complex change.

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What an AI coding assistant can get wrong

A mistake is more than a syntax error that prevents a file from compiling. An assistant may return code that looks idiomatic and passes a narrow happy-path test while still violating the requirement, failing in production, or weakening a security boundary.

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  • Incorrect logic: The code runs but computes the wrong result, mishandles a state transition, or silently drops an error.
  • Stale knowledge: It uses an API, configuration key, library behavior, or framework pattern that has changed or is not available in your pinned version.
  • Hallucinated details: It invents a package, method, option, file, or capability that does not exist.
  • Incomplete implementation: The visible function works, but required validation, cleanup, retries, transactions, logging, or failure handling is absent.
  • Context mismatch: Names, types, data formats, architectural conventions, and security assumptions do not match the surrounding repository.
  • Security weakness: Input validation, authorization, safe secret handling, escaping, or other protections are omitted or applied incorrectly.

These failure modes matter because generated code is often persuasive. A developer who assumes that a detailed explanation represents a proof may overlook a defect that a compiler, test, or reviewer would have exposed.

Why does AI-generated code have mistakes?

The model may not have enough project context

An assistant generally sees only the prompt, the files or symbols supplied to it, and whatever repository context its integration makes available. It may not know an implicit contract in another module, a database invariant, a deployment restriction, or a team convention. The authors’ qualitative study presented at ACM CCS 2024 reports that assistants often lacked enough surrounding code to produce a dependable change.

Context can also be technically present but practically unusable. A long repository contains more information than a model can attend to reliably, and similarly named functions may have different guarantees. The result can be code that is locally plausible but incompatible with the system around it.

Larger and more complex requests increase the surface for failure

The authors’ qualitative study presented at ACM CCS 2024 describes lower-quality output for larger code blocks and more complex problems. Every additional requirement creates another opportunity for an assumption to be missed: multiple error paths, concurrency, data migration order, backward compatibility, performance limits, or interactions between services.

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Generating a small, well-specified function is not equivalent to generating an entire feature. A response can satisfy the visible outline while omitting the less obvious work needed to integrate it safely.

Training information can be outdated or incomplete

Assistants learn patterns from existing material, but a pattern that was common in older documentation may be deprecated, insecure, or incompatible with the version in your project. The authors’ qualitative study presented at ACM CCS 2024 records reports of outdated information and suggestions that did not work as intended. It does not establish one universal cause or a current error rate for every assistant.

Prediction is not the same as verification

The generated text is selected because it is a likely continuation of the prompt and surrounding code, not because the assistant has compiled it, executed it against your data, or proved its security properties. A likely-looking continuation can therefore contain an invented API or a subtle logical error.

Security assumptions are easy to omit

Security controls are often cross-cutting and are not always visible in a short code request. The authors’ qualitative study presented at ACM CCS 2024 reports that participants raised concerns that generated code could omit protections or introduce vulnerabilities. Those reports show risks developers encounter or anticipate; they are not evidence that all AI-generated code is insecure or that a particular percentage is vulnerable.

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What the available studies actually show

Two studies are useful for understanding the evidence, but neither supplies a universal accuracy score.

  • Microsoft Research published a 2025 study of 30 programmers on uncertainty highlighting in AI code completions.
  • An authors’ qualitative study presented at ACM CCS 2024 interviewed or discussed practices with 27 software professionals about security and AI assistants.

The second study records experiences with inaccurate and outdated output, hallucinations, insufficient context, and weaker results on larger or more complex requests. It also describes developers manually inspecting suggestions, executing or adapting them, using software tests, applying static analysis or fuzzing, and seeking peer review.

Because these are qualitative samples, the counts are sample sizes, not estimates of how often all assistants fail. The sources do not establish a current cross-model benchmark, rank products, or provide a market-wide error rate. Apply the findings as reasons for a checking process, not as a numerical prediction about the next suggestion you receive.

Why confidence displays are not proof

The Microsoft Research paper compared three conditions: no highlighting, highlights for tokens with the lowest model-generation likelihood, and highlights for tokens predicted most likely to be edited by a programmer. Highlights based on predicted human edits led to faster task completion and more targeted edits in that study. Highlights based only on generation probability did not outperform the no-highlight baseline.

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Participants preferred cues that were granular, informative, interpretable, and not overwhelming. This is evidence about particular display designs in one study, not evidence that a confidence indicator can certify generated code as correct. Treat any uncertainty cue as a prompt to inspect, never as an approval signal.

How can I tell if AI-generated code is wrong?

Use a repeatable sequence. The order matters: understand the change before relying on automated results, then combine behavioral, analytical, security, and human checks.

  1. Read the suggestion in context. Compare every changed line with the requirement and the surrounding code. Check names, types, return values, error behavior, API versions, data formats, and side effects. Look for unexplained files, dependencies, permissions, or configuration changes. A plausible explanation is not proof.
  2. Trace normal and abnormal paths. Write down what should happen for valid input, empty input, malformed input, duplicate requests, timeouts, missing records, and partial failure. Follow the generated control flow for each case. Confirm that resources are released and errors are propagated or handled deliberately.
  3. Run the code in an appropriate environment. Use the project’s normal build, lint, test, and integration commands rather than an isolated snippet whenever possible. Compile or type-check against the versions actually pinned by the repository. A clean build catches some problems, but it cannot establish that the behavior is correct.
  4. Test expected behavior and edge cases. Add or run unit tests for the contract, then exercise boundary values and failure paths. Verify outputs, state changes, emitted events, and persisted data, not merely that the function returns without throwing.
  5. Apply automated analysis matched to the risk. Static analysis can flag suspicious patterns, type errors, unreachable branches, and some security issues. Fuzzing can expose crashes and unexpected behavior by supplying many generated inputs. These checks complement tests; neither proves the absence of defects.
  6. Obtain human review before consequential use. Ask a reviewer who understands the component and its threat model to inspect the diff and the tests before merging. Peer review is a normal engineering control, not a guarantee that every defect will be found.
  7. Record assumptions and unresolved questions. Document which version, input contract, permissions, and failure policy the code relies on. If an assumption cannot be verified, stop short of treating the suggestion as production-ready.

Which verification method should you use?

Different checks reveal different classes of failure. Select several rather than expecting one tool or person to find everything.

Method Best at detecting When it fits What it cannot establish alone
Manual inspection Context mismatches, missing requirements, questionable assumptions, and anomalous changes Immediately after generation and whenever the diff changes behavior or access It cannot reliably simulate every input or prove that an unfamiliar API behaves as assumed
Compilation or type checking Syntax errors, missing symbols, incompatible types, and some interface mistakes Every change in a compiled or typed project A program can compile while producing wrong results or unsafe side effects
Unit and integration tests Behavioral regressions and violations of specified contracts under selected scenarios After implementation and before merge or release Untested paths, unrealistic fixtures, and incorrect test expectations can hide defects
Static analysis and security scanning Suspicious patterns, data-flow problems, some defects, and known classes of weakness As an automated gate or local check appropriate to the language and risk A clean report does not prove business logic, configuration, or dependency safety
Fuzzing Crashes, parser errors, hangs, and unexpected behavior under varied or malformed input Parsers, protocol handlers, validation code, and other input-heavy components It does not replace requirements-based tests or prove that accepted output is correct
Peer review Design trade-offs, security assumptions, maintainability, and missing context Before merging changes with material operational, financial, privacy, or safety impact Reviewers can miss defects, especially when tests and requirements are weak

Give security-sensitive code a separate review

Security deserves more than a general “does this work?” check. Before accepting generated code that handles untrusted input, identity, permissions, secrets, payments, personal data, or network requests, inspect the assumptions explicitly.

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  • Input validation: Confirm that type, length, encoding, range, and allowed values are checked at the trust boundary. Reject malformed input rather than relying on a later operation to fail safely.
  • Authorization: Verify that every sensitive action checks the caller’s permission for the specific resource, not merely that the caller is authenticated.
  • Secret handling: Ensure tokens, passwords, and keys are not hard-coded, logged, returned in errors, or exposed to clients. Use the project’s approved secret-management path.
  • Output and query safety: Check escaping, parameterization, and serialization where data crosses into a browser, shell, database, template, or command interpreter.
  • Failure behavior: Make sure exceptions, retries, and timeouts do not leak sensitive details, bypass a control, duplicate an operation, or leave partial state.
  • Dependencies and permissions: Examine every new package, native module, permission, and configuration change. Remove anything the requirement does not need.

The authors’ qualitative study presented at ACM CCS 2024 supports this extra attention but does not provide a complete vulnerability checklist or test result for each class of weakness. Use your organization’s secure-development guidance and threat model for the component.

How to reduce mistakes before asking for code

Verification is mandatory, but a precise request can reduce avoidable ambiguity.

Supply the smallest useful context

Include the relevant interfaces, data shapes, error conventions, supported runtime and dependency versions, and a short description of invariants. Point to the existing function or test that defines expected behavior. Do not assume the assistant can infer contracts from a filename.

Break a feature into reviewable changes

Ask for one cohesive function, adapter, test, or migration at a time. After each piece, compile and test it before requesting the next. Smaller diffs make omissions and unintended side effects easier to see; they do not eliminate the need for review.

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State constraints and failure cases

Specify whether an operation must be idempotent, transactional, bounded in time, backward-compatible, or safe for concurrent callers. List invalid and adversarial inputs. Ask the assistant to identify assumptions and propose tests, then verify those proposals independently.

Pin versions and verify APIs

Tell the assistant the exact language, framework, and library versions that the repository uses. Check every unfamiliar method against the versioned documentation or source in your environment. Never add a dependency solely because the assistant named it.

Request tests, not just implementation

A generated test can contain the same mistaken assumption as the generated implementation, but requiring a test exposes the intended contract and gives you an executable starting point. Review the assertions to ensure they would fail for a broken implementation.

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Protect confidential code and data

Before pasting proprietary source, credentials, customer records, unpublished designs, or regulated information into an external assistant, check your organization’s policy and the service’s approved data-handling arrangement. Participants in the authors’ qualitative security study presented at ACM CCS 2024 described privacy and data-leakage concerns and noted that organizational rules affect whether and how assistants are used.

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  • Use an approved workspace and account rather than an unmanaged personal account.
  • Remove secrets and minimize identifying or proprietary data before sharing a snippet.
  • Keep a record of what was sent when policy or compliance requires it.
  • Do not assume that a private repository automatically makes every assistant integration acceptable.

If policy forbids external processing, keep the code and data within the permitted environment or do not use the assistant for that task.

Three practical examples of the checking loop

An input parser

An assistant may produce a parser that handles a normal sample but loops on an empty delimiter or allocates memory proportional to an attacker-controlled length. Read the bounds and termination conditions, compile it with the project’s version, add tests for empty, oversized, malformed, and non-UTF input, then fuzz the parser. A peer should review resource limits and error behavior before the parser handles untrusted data.

An authorization endpoint

Generated code may check that a user is logged in but forget to verify ownership of the requested record. Trace the request from authentication to the database query, test access to another user’s identifier, inspect error responses and logs, and have a reviewer examine the threat model. A passing happy-path test is not evidence that object-level authorization is correct.

A data migration

A migration can appear syntactically valid while losing data when rerun or when a row violates an assumption. Inspect transaction and rollback behavior, test it against a copy containing nulls and duplicates, verify it is safe to retry, and review the deployment order with someone familiar with the production schema. Static checks cannot substitute for a realistic migration rehearsal.

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Match the review effort to the consequence

Change risk Minimum sensible checks Escalate when
Low: isolated utility with no external input or sensitive state Context inspection, compile or type-check, focused unit tests, and a small diff The utility becomes shared infrastructure or its assumptions are unclear
Medium: shared service logic, persistence, integrations, or user-visible behavior All low-risk checks plus edge-case tests, static analysis, and peer review Failures affect many users, data integrity, or availability
High: authentication, authorization, secrets, payments, personal data, migrations, or safety-critical behavior Threat-focused inspection, comprehensive tests, static and security analysis, realistic staging validation, and specialist peer review Requirements, ownership, or policy are not clear; pause rather than accepting a plausible draft

A professional standard for AI-assisted code

Participant P14 in the authors’ qualitative study presented at ACM CCS 2024 said: "I just think there are vulnerabilities and there are things that it doesn’t know. Currently, and at least for the next five years, I think all code written by AI will need to be gone over by a professional." This is an anonymized individual’s opinion and forecast, not a conclusion proved by the study. Its practical warning is nevertheless consistent with the evidence: generated code needs an accountable engineer who understands the requirement, validates the behavior, and accepts responsibility for the change.

The most reliable habit is simple: inspect the diff, execute it, test it beyond the happy path, apply checks suited to the risk, and involve a human reviewer when the consequences justify one. No confidence label, polished explanation, or isolated successful test can replace that loop.

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