Cursor: using site context carefully

AI Summary: Cursor leverages semantic codebase embeddings, @Docs external documentation indices, and rule-based system instructions (.cursor/rules). To maximize code generation accuracy and prevent hallucinated APIs, developers should index curated /llms.txt endpoints directly in Cursor, isolate domain rules into focused .mdc files, and enforce test-driven verification before file writes.

#The Cursor Ingestion Architecture

Cursor does not treat external context as a simple string prompt. Its internal pipeline merges four distinct retrieval systems:

[Developer Prompt / Intent]
            │
            ▼
┌─────────────────────────────────────────────────────────────┐
│                Cursor Agent Reasoning Loop                  │
│                                                             │
│  1. Semantic Codebase Search (Local Merkle tree embeddings) │
│  2. Active File State & LSP Diagnostic Errors               │
│  3. Custom Workspace Directives (.cursor/rules/*.mdc)       │
│  4. External Documentation (@Docs fetched from /llms.txt)   │
└─────────────────────────────┬───────────────────────────────┘
                              │
                              ▼
        [Multi-File Unified Diff / Composer Execution]

When you point Cursor's @Docs feature to an external site that serves a bloated HTML documentation portal, Cursor's crawler must strip navigation, execute JavaScript, and guess content boundaries. In contrast, pointing @Docs directly to an https://example.com/llms.txt endpoint enables clean, instantaneous vector indexing with zero DOM interference.

#Modern Rule Configuration: From Monolithic .cursorrules to Modular .mdc

In modern Cursor versions, the monolithic root .cursorrules file has been superseded by modular rule files located in .cursor/rules/*.mdc. This prevents context bloat by applying rules only when specific file patterns are matched.

#Example: .cursor/rules/api-contracts.mdc

---
description: API communication and typing conventions
globs: ["src/api/**/*.ts", "src/hooks/**/*.ts"]
alwaysApply: false
---

# API Integration & Data Contracts

- Always use the generated OpenAPI types from `@/types/api.generated.ts`.
- Never invent query parameters: verify against the local `llms.txt` documentation index.
- All mutating endpoints (`POST`, `PUT`, `DELETE`) must include an `Idempotency-Key` header.
- Error handling must match the standard `{ code: string; message: string; details?: unknown }` schema.

By setting alwaysApply: false and defining specific globs, the agent loads this rule only when editing API files, preserving valuable context tokens during UI component refactoring.

#Production Workflow: The 3-Step Agent Verification Loop

When instructing Cursor Composer or Chat to execute non-trivial multi-file changes:

#1. The Orientation Prompt (Read-Only)

Never ask Cursor to edit code on the first turn. Force it to formulate an execution plan:

Inspect the repository structure and the indexed @Docs for Acme API.
Do NOT edit any files yet.
1. List the files you will need to modify.
2. State the key architectural assumptions you are making.
3. Detail the verification tests you will execute to validate the change.

#2. Guardrails Against Phantom Dependencies

AI coding tools frequently invent non-existent utility libraries (e.g. import { formatIsoDate } from 'date-fns-utils'). Enforce this hard invariant in your workspace rules:

• "Before importing any external library, verify its presence in package.json. If missing, inspect whether native language built-ins can accomplish the task before proposing npm install."

#3. Test-First Execution

Instruct Cursor to create or update unit tests first, run the test suite via terminal tool execution, and iterate until green:

Execute the planned refactor.
Run `pnpm test` via the terminal tool after modifying files.
If any test fails, analyze the assertion error, fix the root cause, and re-run.
Do not consider the task complete until the entire test suite exits with code 0.

#Comparative Context Matrix in Cursor

#Related guidance

To configure your site's documentation for @Docs ingestion, study the llms.txt specification, learn how to optimize editor performance in Cursor IDE Optimization, and review Claude Code Context Architecture.

#References

• Cursor Official Documentation: Custom Rules & @Docs: Official guide to configuring .cursor/rules and managing documentation indexes.
• The /llms.txt Specification Proposal: The standardized format for agent documentation ingestion.

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