How to Write Project Guidance Files That Make AI Coding Assistants More Effective

When using AI coding assistants in project development, many teams notice the same pattern: for similar tasks, sometimes the generated code is well structured and precisely scoped, while other times the assistant misunderstands business goals, modifies unrelated files, or introduces implementations that do not match the current state of the project. The key difference is often not only model capability, but also whether the project provides a clear, stable, executable guidance document.

This type of document is usually placed in the root directory of the code repository or another agreed directory. It explains project background, directory structure, development standards, common commands, testing requirements, and collaboration boundaries. It is not a simple manual, nor a place to pile up all available knowledge. It is a working contract that helps the AI coding assistant establish project context.

1. Clarify What the Guidance File Should Solve

A high-quality project guidance file mainly addresses the following problems.

1. Help the Assistant Understand the Project Quickly

AI coding assistants can usually read files in the current repository, but without explicit project instructions, they must infer the project purpose, module relationships, and runtime behavior. This inference process can lead to problems such as:

• Treating example directories as formal business code;
• Mistaking an old implementation for the current standard;
• Not knowing the entry files and core modules;
• Not knowing which directories are allowed to be modified;
• Being unable to determine whether a configuration file contains sensitive content.

Therefore, the guidance file should first provide an overall map of the project, including the project purpose, main modules, startup entry points, and relationships between key dependencies.

2. Standardize How Code Is Modified

The same feature can be implemented in multiple ways, but a project usually accepts only one of them. For example, the project may require a unified error-handling approach, interface layering style, state management pattern, or log format. If these constraints are not clearly documented, the assistant may generate code that is technically runnable but does not comply with engineering standards.

The guidance file should turn abstract requirements into executable rules, such as:

• New business logic must be placed in the specified layer;
• Controllers are only responsible for receiving parameters and returning results;
• Data access must go through a unified repository interface;
• Sensitive configuration must not be written directly into business code;
• After modifying an interface, validation and tests must be updated accordingly.

3. Reduce Repeated Communication Costs

If every conversation requires repeating “which files to check first,” “which directories not to modify,” and “what the test command is,” development efficiency will clearly decline. Writing stable and frequently used requirements into the guidance file allows the assistant to follow project conventions automatically at the start of every task.

2. Where to Place the File

The location of project guidance files should follow the principle: the closer to the task, the more specific the rules.

1. Global Rules at the Root

The root-level file is suitable for recording project-wide common content, such as:

• Project goals and technical boundaries;
• Top-level directory descriptions;
• Global coding standards;
• Build, test, and check methods;
• Validation that must be completed before submission;
• Security and privacy requirements.

This content should be as stable as possible. Avoid placing details of a temporary requirement in this file.

2. Local Rules in Subdirectories

When a project contains multiple relatively independent modules, local guidance files can be added to module directories. Local files are suitable for explaining:

• The responsibilities of the current module;
• The internal directory structure of the module;
• Module-specific naming rules;
• Module-specific testing methods;
• Boundaries that the module must not cross.

For example, frontend modules, server-side modules, and data processing modules may have different code standards. Putting all content into the root directory will make the file too long and can also cause general rules to conflict with local rules.

3. Avoid Duplication and Conflicts

Guidance files at multiple levels must maintain a clear inheritance relationship. If the root directory states that “all interfaces must go through unified validation,” subdirectories should not contain requirements with the opposite meaning. The following priority order is recommended:

1. Local rules closer to the current file;
2. General rules in the project root directory;
3. Explicit requirements in the current task;
4. The assistant’s default coding habits.

For rules that may be ambiguous, state the priority directly instead of letting the assistant guess.

3. Recommended Content Structure

A practical guidance file can use the following structure.

1. Project Overview

Explain what the project does, who it mainly serves, and what technical boundaries exist at the current stage. The content should not be too long, but it must answer three questions:

• What problem does this project solve?
• Which modules make up the main functionality?
• Which capabilities are currently out of scope?

Example:

The project processes internal business data and consists of three parts: the API service, the admin console, and scheduled jobs.
The API service handles business requests, the admin console handles operational configuration, and the scheduled jobs handle data synchronization.
At the current stage, do not add new cross-module communication mechanisms; reuse existing service-layer capabilities first.

2. Directory Structure

Do not just list directory names. Explain the responsibility of each directory and identify which directories contain generated artifacts, temporary files, or external resources.

src/core       Core business rules; domain logic only
src/api        Request entry points and parameter conversion
src/repository Data access wrappers
src/shared     Shared capabilities reused across modules
tests          Automated tests
scripts        Local development and CI scripts

If certain directories must not be modified directly, state this explicitly:

Build output directories are generated by scripts and must not be edited directly.
Dependency cache directories are not business code; do not modify their contents when troubleshooting.

3. Development Workflow

The development workflow should describe “what to do first, what to do next, and how to validate at the end,” rather than only saying “run tests after modification.”

A recommended format is:

1. First read the task-related modules and existing tests;
2. Prefer reusing existing utility functions and service interfaces;
3. Modify only the minimum set of files required to complete the task;
4. Run local tests first, then run full checks;
5. Report modified files, implementation approach, test results, and remaining risks.

This workflow can effectively reduce the problem of the assistant rewriting large areas of code directly.

4. Coding Standards

Coding standards should prioritize what the project truly cares about. Do not copy an entire general textbook into the file. Key areas include:

• Naming conventions;
• File organization;
• Function complexity control;
• Exception handling strategy;
• Log and error message formats;
• Appropriate use of comments;
• Type definition requirements;
• Async task handling approach.

Instead of writing “code should be elegant,” write rules that can be judged:

Public functions must declare input and output types.
Business exceptions must use the project's unified exception type; do not throw plain strings directly.
Logs must not contain passwords, tokens, complete identity information, or raw request content.
When adding branch logic, prefer adding corresponding unit tests.

4. Turn Vague Requirements into Executable Instructions

The hardest input for an AI coding assistant is usually not an explicit command, but a vague expression such as “optimize the code,” “refactor according to best practices,” or “make the page faster.” These descriptions lack scope, goals, and acceptance criteria.

A better approach is to use the structure “action + scope + constraints + result.”

Not Recommended

Optimize the user module and improve code quality.

Recommended

Only inspect the query flow in the user module; do not change public interfaces or the database schema.
Prioritize reducing duplicate queries and duplicate conversion logic.
Keep the existing response fields unchanged.
After completion, add boundary-condition tests and explain the behavior differences before and after the change.

This expression tells the assistant:

• What the modification scope is;
• What must not change;
• Which problems should be solved first;
• How the result will be accepted.

5. Document Commands and Their Purpose

The guidance file can record build, test, format check, and local startup commands, but it should not list only the commands themselves. Each command should explain its applicable scenario, execution cost, and expected result.

Local unit tests: validate a single module; suitable after each change.
Full test suite: validate before commit; longer execution time.
Static checks: find type, formatting, and potential errors.
Build check: confirm the code can produce a formal build artifact.

If commands differ across systems, document them separately to prevent the assistant from generating commands that cannot run in the current environment. When commands involve environment variables, credentials, or internal addresses, use placeholders and do not write real sensitive information into the repository.

6. Set Safety Boundaries for AI Coding Assistants

The more efficient automated code modification becomes, the more important it is to define boundaries. Consider adding the following constraints to the guidance file.

1. File Modification Boundaries

By default, modify only files directly related to the current task.
Do not delete files, perform bulk renames, or upgrade core dependencies without confirmation.
If the change scope needs to expand, list the reasons and impact first.

2. Data and Credential Protection

Do not read, output, or commit real passwords, access tokens, certificate private keys, or production data.
Use sanitized samples during debugging.
If sensitive information has already entered version history, stop spreading it immediately and prompt for manual handling.

3. High-Risk Operation Confirmation

When an operation involves deleting data, modifying the database schema, overwriting configuration, publishing build artifacts, or making bulk changes, the assistant should first explain the impact scope and wait for explicit confirmation. The guidance file should list these operations as actions that “must be manually confirmed.”

7. Do Not Turn the File into an Encyclopedia

A longer guidance file is not necessarily better. Too much content creates three problems:

• Key rules are buried under large amounts of background information;
• Outdated content is difficult to maintain in a timely manner;
• Different rules are more likely to conflict.

Keep only information that is high-frequency, stable, and mandatory. Specific business solutions, temporary requirements, meeting notes, and long interface documents should be placed in other dedicated documents, with short indexes provided in the guidance file.

A layered approach can be used:

• Root file: global rules and workflow;
• Module files: local structure and special constraints;
• Topic documents: architecture design, interface descriptions, and business background;
• Task instructions: goals and acceptance criteria for the current requirement.

8. How to Validate Whether the Guidance File Works

You can test the file through several practical tasks:

1. Ask the assistant to explain the project structure and check whether it identifies the core modules;
2. Ask the assistant to create a modification plan for a small feature and observe whether it respects directory boundaries;
3. Ask the assistant to modify existing logic and confirm whether it reuses existing project patterns;
4. Ask the assistant to run tests and check whether it selects the correct commands;
5. Deliberately propose a high-risk operation and verify whether it proactively warns and requests confirmation.

If the assistant repeatedly makes the same mistake, do not only correct it temporarily in the conversation. Turn stable corrections into rules. For example, if it always modifies generated directories directly, explicitly state in the file that “generated directories must not be edited, and specify the location of the source files.”

9. Continuous Maintenance Matters More Than the First Draft

Projects continue to evolve, and the guidance file must be updated accordingly. Maintain it promptly in the following situations:

• The directory structure changes;
• The build or test process is adjusted;
• Coding standards are upgraded uniformly;
• New security restrictions are added;
• Existing commands stop working;
• The team finds that the assistant repeatedly produces the same type of error.

The guidance file can be included in code review, with rule changes submitted together with code structure changes. Outdated content should be deleted promptly rather than continuously stacked with supplementary explanations.

Conclusion

The key to using AI coding assistants efficiently is not writing the longest possible instruction document, but establishing a clear, executable, verifiable set of project rules. A good guidance file should tell the assistant what the project is, where the code lives, how modifications should be made, what must not be touched, and how to prove that a change is correct after completion.

In practice, start with a minimal version: first add the project overview, directory structure, common commands, coding constraints, and safety boundaries, then iterate continuously based on problems found in actual use. After several rounds of task validation, the guidance file will gradually become part of the project development workflow, helping the team reduce repeated communication and improve the accuracy, controllability, and delivery efficiency of code modifications.