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The reliable way to prevent duplicate code is to make reuse a required decision before implementation. Ask the agent to inspect the relevant behavior, search for existing functions, components, tests, and configuration, read the strongest matches, and explain whether one can be extended. Permit a new helper or abstraction only when the agent can state why the existing candidates do not fit, then review the final diff for parallel implementations and scope creep.
This workflow reduces accidental duplication without pretending that an instruction file can guarantee perfect results. Agents still need accurate repository context, project-specific conventions, tests in the local style, and human review of what they changed.
Why coding agents create duplicate or unnecessary code
An agent usually optimizes for completing the requested behavior from the context it can see. If it cannot find an existing implementation, misunderstands the architecture, or receives a vague request, it may create a new helper, component, service, fixture, or configuration path that overlaps with code already in the repository.
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- A new UI component that renders the same pattern as an existing component under a different name.
- A parallel test fixture or factory instead of extending the established one.
- A new abstraction introduced for a one-off change when a nearby function already provides the needed seam.
- Repeated configuration, scripts, data-loading logic, or error handling copied into a new module.
- Unrelated cleanup, renaming, or refactoring that expands the patch beyond the requested behavior.
The cost is cumulative. Two implementations can drift, tests may cover only one path, future agents may choose the wrong version, and maintainers must learn more concepts to make a small change. The solution is a sequence of explicit decisions rather than a single magic prompt.
The inspection-first workflow
Use the following sequence for every non-trivial change. It works as a standing repository rule and as a task-specific prompt.
- Define the requested behavior and boundaries. State what must change, what must not change, and how success will be verified.
- Ask for repository inspection before implementation. Have the agent identify the files involved, trace the current behavior, and search for existing code that may already solve the problem.
- Require a reuse decision. Ask it to name the best candidates, compare their fit, and choose one to extend or explicitly reject each candidate with a reason.
- Read the selected implementation and nearby examples. The agent should inspect callers, related modules, and reference tests before proposing edits.
- Implement the smallest compatible change. Follow local naming, layering, error handling, and dependency conventions instead of introducing a parallel pattern.
- Extend tests in the repository’s existing style. Inspect both implementation tests and neighboring reference tests before adding coverage.
- Review the diff for duplication and scope. Look for second helpers, competing abstractions, duplicate fixtures, new files that are not needed, and unrelated cleanup.
- Run the relevant checks and report uncertainty. The agent should state which tests and commands ran, what was not run, and any remaining compatibility concern.
This order matters. Asking for code first and review later gives the agent an incentive to defend a design it has already created. Inspection first makes reuse part of the implementation plan.
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Your first request should produce an investigation, not a patch. Ask the agent to:
- Locate the entry point for the requested behavior.
- Trace the relevant call path, including adapters, services, state, and error handling.
- Search by domain terms, exported symbols, filenames, route names, event names, and distinctive strings.
- Find existing tests, fixtures, factories, schemas, and configuration used by the same behavior.
- Read the most relevant matches rather than relying on search-result names.
- Report the files inspected and the evidence that each candidate is or is not applicable.
A useful first-turn request is: “Do not edit yet. Trace how this behavior currently works, search the repository for existing implementations and tests that cover it, and list the candidates you would reuse. Explain any gaps or ambiguity.”
Search should be semantic as well as textual. A function called normalizeInput may be the right implementation even if the request uses the phrase “clean user data.” Conversely, a similarly named function may belong to a different boundary or have incompatible assumptions. Reading the body, callers, and tests is what distinguishes a genuine reuse candidate from a misleading name match.
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Step 2: Make reuse an explicit decision
Do not leave reuse as an implied preference. Add a decision gate to the prompt or repository instructions:
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“Only generate new code if no suitable existing solution is found.”
This sentence is documented in Cursor’s reuse guidance. Treat it as a decision rule, not a guarantee. A suitable solution must satisfy the requested behavior and fit the repository’s current boundaries, conventions, and compatibility requirements.
Require a short candidate report before implementation:
- Candidate: file and symbol, or the relevant component, test fixture, or configuration block.
- What it already does: behavior demonstrated by its implementation and tests.
- Fit: why it matches the new requirement.
- Gap: the smallest change needed, if any.
- Decision: extend, call as-is, or reject.
- Reason for rejection: a concrete incompatibility such as a different data contract, lifecycle, performance requirement, or public API.
“I could not find anything” is not enough. The agent should describe the search terms and areas it inspected so you can tell whether the conclusion is credible.
How to decide between reuse and a new implementation
When an existing implementation is close but imperfect, compare it on three dimensions before adding anything:
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| Dimension | Questions to ask | Preferred outcome |
|---|---|---|
| Behavioral fit | Does it handle the required inputs, errors, security rules, and edge cases? | Extend it when the missing behavior belongs to the same responsibility. |
| Compatibility | Does using it preserve current APIs, layering, naming, and dependency conventions? | Reuse it when callers can adopt it without an incompatible contract. |
| Maintenance impact | Will one implementation remain the source of truth, or will a change create two versions to maintain? | Prefer one shared path unless separation is a deliberate boundary. |
A new implementation is justified when the existing code has a genuinely different responsibility or contract, cannot meet the requirement without breaking callers, or would create an unsafe coupling. Record that reason in the change description and, when useful, in a focused code comment. Do not create a second abstraction merely because the existing name is inconvenient or because a fresh design feels cleaner.
Step 3: Give the agent accurate repository context
Agents make better reuse decisions when the repository’s architecture and conventions are written down. Maintain a concise instruction file in the location supported by your editor or agent. Include information that is specific to the project and difficult to infer reliably from source alone:
- System boundaries, major packages, and the direction of dependencies.
- Important directories and the role of each one.
- Supported runtime, package manager, build system, and test commands.
- Where shared utilities, UI primitives, API clients, schemas, and fixtures live.
- Public API, backwards-compatibility, security, and migration expectations.
- Naming, formatting, error-handling, logging, and import conventions.
- Required checks before a change is complete.
- Rules for generated files, database changes, feature flags, and ownership boundaries.
Visual Studio Code’s guidance on configuring AI for a codebase recommends keeping instructions focused on important repository-specific knowledge and removing stale or duplicate rules. That maintenance is part of duplication prevention: contradictory instruction files can cause an agent to follow the wrong pattern or generate another version of an existing helper.
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A reusable repository instruction
Before adding a helper, component, service, configuration block, or test fixture, search the repository for existing code that serves the same purpose. Read the most relevant matches and reuse an appropriate implementation or extend it in place. If you add a new implementation, list the existing candidates you checked and explain why each one does not fit. Trace the current behavior before editing, follow nearby implementation and test conventions, keep the change within the requested scope, and run the relevant project checks.Adapt this rule to the actual project. Add real directory names, commands, and boundaries only when they are true; invented paths make the instruction less useful and can misdirect an agent.
Standing instructions versus task-specific prompts
Use both layers, but for different purposes.
| Approach | Persistence | Specificity | Maintenance cost | Best use |
|---|---|---|---|---|
| Repository instruction | Applies across tasks | Project-wide architecture and conventions | Requires periodic review as the codebase changes | Stable rules such as search-before-add, ownership boundaries, and required checks |
| Task-specific prompt | Applies only to one request | Exact feature, files, constraints, and acceptance criteria | Low after the task ends | Temporary scope, migration limits, compatibility details, or a particular reuse decision |
A standing rule should not contain assumptions that are true only for one ticket. A task prompt should repeat the relevant reuse gate when the change is risky, touches unfamiliar code, or has an unusually narrow scope. If the two conflict, resolve the conflict explicitly before asking for implementation.
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Step 4: Explore behavior before editing
Ask the agent to explain the current code path in plain language before it proposes a patch. The explanation should identify:
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- Where input enters the system and where output is consumed.
- Which module owns the behavior and which modules merely adapt it.
- Existing validation, authorization, caching, retries, and error translation.
- Side effects, transaction boundaries, asynchronous behavior, and lifecycle assumptions.
- Tests that demonstrate the expected behavior and known edge cases.
Visual Studio Code’s guide to exploring a codebase with an agent emphasizes investigating existing behavior and reviewing generated project notes before promoting them to shared documentation. Treat an agent’s summary as a navigation aid, not as authoritative architecture. Check its claims against the source before adding them to an instruction file. Otherwise, an incorrect summary can become a durable rule that encourages future duplication.
For a large repository, narrow the exploration deliberately: identify the package, entry point, and likely ownership boundary first, then expand only when imports, tests, or runtime flow require it. This reduces the chance that the agent invents a new cross-cutting abstraction simply because it has not found the existing one.
Step 5: Extend tests in the existing style
Tests are a frequent source of unnecessary code. An agent may create a new fixture, factory, mock, or test utility even though neighboring tests already show the accepted pattern. Before asking for test changes, require it to inspect:
- The implementation under test and its public entry point.
- Several nearby tests for the same module or feature.
- Shared fixtures, factories, builders, stubs, and setup hooks.
- Assertions used for equivalent success, failure, boundary, and authorization cases.
- The command and environment used to run the relevant test subset.
Anthropic’s Claude Code developer-use-case guidance illustrates this approach: read the implementation and reference tests, then match existing test conventions. “Matching” should include fixture lifecycle, naming, isolation, async handling, and assertion style, not just indentation.
Prefer extending a shared fixture when the new data is a legitimate variation of the same domain object. Create a separate fixture when the test represents a different contract or lifecycle and combining them would obscure intent. In either case, ask the agent to explain why a new test helper is necessary.
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Step 6: Keep the implementation within scope
Agents often identify genuine improvements while working. Those improvements can still be wrong for the current change. Set explicit boundaries:
- Name the behavior that must change.
- List files or subsystems that are out of scope, when known.
- Do not rename, reformat, migrate, or reorganize unrelated code.
- Do not introduce a general-purpose abstraction for a single call site unless reuse is already demonstrated.
- Separate required compatibility fixes from optional cleanup.
- Put follow-up ideas in a note or issue rather than silently adding them to the patch.
When the agent proposes a broader refactor, ask for two plans: the smallest change that satisfies the request and the broader redesign as a separate option. Implement the smaller plan unless the broader one is required for correctness, security, or an explicitly approved migration.
Step 7: Review the diff for duplication
Do not review only the changed lines. Inspect the complete diff and the new file list. Use this checklist:
- Does the patch add a helper, component, service, fixture, or configuration block that overlaps an existing one?
- Could an existing function be extended instead of copied?
- Does a new abstraction have more than one real consumer, or is it speculative?
- Are imports, routes, schemas, and exports duplicated under different names?
- Did the agent create a second error-handling, validation, or formatting path?
- Are tests using the established fixture and assertion conventions?
- Are generated files or snapshots changed only when required?
- Did unrelated formatting, renaming, dependency upgrades, or cleanup enter the diff?
- Does each new file have a clear owner and a reason tied to the requested behavior?
For every new symbol, ask: “Why is this not an extension or call to an existing symbol?” For every deleted or bypassed path, ask: “What behavior and tests prove that the old path is no longer needed?” These questions expose parallel implementations that a line-by-line review can miss.
Ask the agent to summarize the final diff in terms of behavior, files, reuse decisions, tests run, and known limitations. Compare that summary with the actual patch; a mismatch is a reason to investigate, not a reason to trust the summary.
Failure modes and how to correct them
| Symptom | Likely cause | Correction |
|---|---|---|
| A new utility duplicates an existing helper | The request was implemented before repository search, or search used only the user’s wording | Ask for symbol, domain-term, and call-site searches; merge or replace the duplicate and add regression tests. |
| The agent cites a similarly named module that cannot be reused | It read the filename but not the contract, callers, or tests | Require a behavioral comparison and a concrete rejection reason before allowing new code. |
| Several instruction files give conflicting rules | Project guidance was copied, allowed to become stale, or maintained in multiple places | Choose one authoritative location where possible, remove duplicate rules, and update architecture details. |
| A new test factory appears beside an existing one | The agent did not inspect neighboring reference tests | Show the relevant tests in the task prompt and require the new case to follow their fixture pattern. |
| The patch contains unrelated cleanup | Scope and completion criteria were not explicit | Request a minimal patch, revert unrelated edits, and record optional refactors separately. |
| The agent keeps proposing a new abstraction | Existing code has an unclear owner or awkward API | Ask it to trace ownership and present an extension plan; improve the existing boundary if that is the real issue. |
| A repository rule causes wrong edits | The rule contains stale paths, commands, or architectural assumptions | Verify the rule against current source and remove details that cannot be maintained accurately. |
Prompt templates you can use
Inspection-only prompt
Do not edit yet. Investigate the requested behavior in this repository. Trace the current flow, search for existing implementations, components, utilities, configuration, fixtures, and tests that may already cover it, and read the strongest matches. Report the files and symbols you inspected, the best reuse candidate, any incompatibility, and the smallest change you would make.Implementation prompt with a reuse gate
Implement the requested behavior within the stated scope. Reuse or extend an existing implementation whenever it fits. Before adding a new helper, component, service, abstraction, or test fixture, list the existing candidates you checked and explain why each is unsuitable. Follow nearby code and test conventions. Do not perform unrelated cleanup. After editing, report the reuse decision, changed files, tests and checks run, and anything not verified.Diff-review prompt
Review the complete diff for duplicate or unnecessary code. Check every new symbol and file against existing helpers, components, services, fixtures, configuration, and tests. Identify parallel implementations, speculative abstractions, unrelated cleanup, and scope violations. Recommend the smallest corrective edits, and distinguish required fixes from optional refactoring.Replace general phrases such as “the requested behavior” with the actual feature and acceptance criteria. If you know the relevant package or directory, name it; if you do not, ask the agent to discover it rather than inventing a path.
What this workflow can and cannot guarantee
Search-first instructions, repository context, reference-test review, and diff inspection make duplication easier to detect and less likely to be introduced. They do not prove that a codebase contains no duplicate logic, and they do not guarantee that an agent will understand every architectural boundary. An agent can miss a dynamically registered component, a generated source file, a convention documented outside the repository, or a semantically equivalent implementation with an unexpected name.
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A practical completion checklist
- The request and out-of-scope areas are explicit.
- The current behavior was traced before editing.
- Repository search covered domain terms, symbols, callers, tests, and configuration.
- The agent read the strongest existing candidates and reference tests.
- Reuse, extension, or rejection of each candidate is documented.
- Any new implementation has a concrete behavioral or compatibility justification.
- The change follows local architecture, naming, error handling, and test conventions.
- No duplicate helper, component, fixture, abstraction, or configuration path was added.
- The diff contains no unrelated cleanup or speculative refactor.
- Relevant tests and checks ran, and unverified areas are clearly reported.
- Repository instructions remain accurate, focused, and free of stale duplicates.
Make this checklist part of the agent’s completion report or pull-request template. Repetition in the process is useful; repetition in the code is what you are trying to prevent.
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