# AGENTS.md — Build guide for Mortgage Coach ## Project scope Compare two to four user-entered loan scenarios with amortization schedules, total paid over chosen horizons and an assumptions-first printout. Catalogue verdict: kinda. The math here is not exotic: amortization schedules, PMI drop-off, points breakeven, net cost after N years, rent versus buy. An agent can build a clean side-by-side scenario comparator with charts and a printable one-pager in a single session, and for a person modeling their own refinance that is genuinely enough. What you cannot one-shot is the part that makes it a business tool: pulling live loan scenarios out of the LOS or pricing engine, compliance-reviewable output your employer will actually let you send to a borrower, and the delivery layer that tracks whether the client opened the presentation. If you are a licensed originator, your DIY version is a scratchpad, not something you put in front of a client. If you are the borrower trying to sanity check what your lender sent you, the DIY version is arguably better because you control the assumptions. Use the implementation prompt below to define the deliverable. Complete each phase's acceptance checks before extending the scope. ## Working agreement - Inspect the repository and its existing instructions before choosing paths, dependencies or commands. Keep one coherent stack and explain changes to the proposed architecture. - Plan a vertical slice that accepts a real input and produces the useful output described below. Persist only the state the prompt calls for; respect memory-only and upstream-managed workflows. Use fixtures only when they are clearly labelled. - After scaffolding, document the actual install, development, check and build commands in README and keep them synchronized with the package or project manifest. Do not report commands as successful unless they ran. - Work in small steps. At handoff, list implemented flows, checks actually performed, remaining blockers, and any credentials or provider setup the owner must supply. - Do not publish, spend money, contact customers, delete source data or run irreversible migrations without the project owner's authorization. ## Prerequisites - Runtime and tools: TypeScript, React, Node, SQLite and decimal.js for explicit decimal calculations. - Before starting: A representative CSV with known totals, currency and date conventions, and an agreed calculation specification. ## Stack and architecture - TypeScript, React, Node, SQLite and decimal.js for explicit decimal calculations - Data design: Store Scenario, Principal, RateAssumption, Fee and AmortizationRow; distinguish financed fees from cash costs and define rounding, prepayments and horizon explicitly. - Setup: A representative CSV with known totals, currency and date conventions, and an agreed calculation specification ## Security and data integrity - Preserve imported source rows and units, use decimal arithmetic with stated rounding, and retain correction history. Missing values are unknown rather than zero; reports describe records and assumptions rather than giving advice. - This is an educational calculator, not a loan quote or financial advice. Do not model adjustable rates, taxes or insurance implicitly; missing costs must be disclosed, not treated as guaranteed savings. - Keep secrets outside client bundles and exported projects; document what leaves the device and make retention/deletion controls visible. ## Agent implementation rules - Project rule — domain: Store Scenario, Principal, RateAssumption, Fee and AmortizationRow; distinguish financed fees from cash costs and define rounding, prepayments and horizon explicitly. - Project rule — scope and recovery: This is an educational calculator, not a loan quote or financial advice. Do not model adjustable rates, taxes or insurance implicitly; missing costs must be disclosed, not treated as guaranteed savings. - Project rule — acceptance: Compare a zero-interest loan and a conventional fixed-rate loan with an extra payment; schedules reconcile principal to zero without divide-by-zero or negative remaining balances. - Project rule — delivery: document real setup commands and permissions; do not claim a build, accuracy level, performance result or security certification that has not been demonstrated. ## Optional agent skills and references - Recommended skill: [web-design-guidelines](https://github.com/vercel-labs/agent-skills/blob/main/skills/web-design-guidelines/SKILL.md) — review keyboard access, focus, validation, error recovery and the readable work/review interface or HTML report. Follow the maintainer's installation instructions and match its requirements to the chosen runtime. - Recommended skill: [sharp-edges](https://github.com/trailofbits/skills/blob/main/plugins/sharp-edges/skills/sharp-edges/SKILL.md) — review configuration and API defaults against the app-specific invariants and recovery boundaries above; this is not a security certification. Follow the maintainer's installation instructions and match its requirements to the chosen runtime. Read the linked SKILL.md and its dependencies before adding a skill. Select only the skills matching this project's runtime and task; their documentation does not supply API access, credentials or approval to perform external actions. Pin the reviewed revision where the tool supports it. Follow the chosen agent's documented project-level installation mechanism. ## Distribution ideas These are optional planning notes. Obtain the owner's approval before publishing or contacting anyone. - Demonstrate this working slice using synthetic or explicitly authorized non-sensitive examples: Compare two to four user-entered loan scenarios with amortization schedules, total paid over chosen horizons and an assumptions-first printout. - Share a synthetic example export and the acceptance walkthrough; keep real customer, health, financial and source data private: Compare a zero-interest loan and a conventional fixed-rate loan with an extra payment; schedules reconcile principal to zero without divide-by-zero or negative remaining balances. - State the limits before asking someone to replace their existing tool: This is an educational calculator, not a loan quote or financial advice. Do not model adjustable rates, taxes or insurance implicitly; missing costs must be disclosed, not treated as guaranteed savings. ## Engineering roadmap 1. Phase 1 — Pin the working slice and create its example input: Compare two to four user-entered loan scenarios with amortization schedules, total paid over chosen horizons and an assumptions-first printout. Confirm setup: A representative CSV with known totals, currency and date conventions, and an agreed calculation specification. 2. Phase 2 — Implement persistence and write-time invariants before decorating the UI: Store Scenario, Principal, RateAssumption, Fee and AmortizationRow; distinguish financed fees from cash costs and define rounding, prepayments and horizon explicitly. 3. Phase 3 — Connect the working view to real saved state. Preserve imported source rows and units, use decimal arithmetic with stated rounding, and retain correction history. Missing values are unknown rather than zero; reports describe records and assumptions rather than giving advice. 4. Phase 4 — Expose the app-specific limits and recovery path in context: This is an educational calculator, not a loan quote or financial advice. Do not model adjustable rates, taxes or insurance implicitly; missing costs must be disclosed, not treated as guaranteed savings. 5. Phase 5 — Walk through this concrete acceptance case and preserve its exported evidence: Compare a zero-interest loan and a conventional fixed-rate loan with an extra payment; schedules reconcile principal to zero without divide-by-zero or negative remaining balances. Finish the README and backup/restore instructions; report unfinished capabilities explicitly. ## Paid-product capabilities outside this build - Integrations with loan origination systems, pricing engines and CRMs, so every scenario is typed in by hand - Compliance-vetted presentation output and audit trail, which is the entire reason a regulated lender licenses this instead of using a spreadsheet - Borrower engagement tracking: who opened the presentation, how long they looked, which option they clicked - Polished branded video and mobile presentation formats that agents and borrowers are used to receiving - Someone else maintaining the edge cases: ARMs, buydowns, MI structures, state-specific cost lines ## Implementation prompt Build the following focused alternative to Mortgage Coach. This is a deliberately limited personal or small-team substitute, not parity with the paid service. WORKING SLICE Compare two to four user-entered loan scenarios with amortization schedules, total paid over chosen horizons and an assumptions-first printout. SETUP AND ARCHITECTURE Use TypeScript, React, Node, SQLite and decimal.js for explicit decimal calculations. Prerequisites: A representative CSV with known totals, currency and date conventions, and an agreed calculation specification. Before integrating anything, record actual versions and permissions, plus model files or provider limits only where used, in the README; make unavailable dependencies visible rather than simulating success. DOMAIN MODEL AND INVARIANTS Store Scenario, Principal, RateAssumption, Fee and AmortizationRow; distinguish financed fees from cash costs and define rounding, prepayments and horizon explicitly. IMPLEMENTATION CONTRACT Preserve imported source rows and units, use decimal arithmetic with stated rounding, and retain correction history. Missing values are unknown rather than zero; reports describe records and assumptions rather than giving advice. Provide an input/setup view, the main work view, and a review/export view appropriate to this workflow. Preserve the last saved state if a job or save fails. Include empty, loading, permission-denied, partial and retryable-error states. Log identifiers and error categories without secret values or unnecessary private content. APP-SPECIFIC BOUNDARY AND RECOVERY This is an educational calculator, not a loan quote or financial advice. Do not model adjustable rates, taxes or insurance implicitly; missing costs must be disclosed, not treated as guaranteed savings. ACCEPTANCE SCENARIO Compare a zero-interest loan and a conventional fixed-rate loan with an extra payment; schedules reconcile principal to zero without divide-by-zero or negative remaining balances. Also reopen the app after an interrupted operation, confirm the saved record/export remains inspectable, and document the recovery action. These are implementation acceptance requirements, not a claim that this guide has been tested. DELIVERY Deliver a runnable repository with migrations or project-format versioning, a non-sensitive example, environment/permission setup, the exact manual acceptance steps, and a backup/export-and-restore walkthrough. Implement the working slice before optional integrations; list any deferred paid-product capabilities honestly. Do not add capabilities outside the working slice just to resemble the original product. PROJECT RULES FOR AGENTS.md Keep the domain invariants above executable at the write boundary. Propose scope changes before adding providers or permissions. Never fabricate source evidence, publish results, identity matches or successful delivery. Preserve user originals and require an explicit confirmation for destructive changes or external publication. ## Completion evidence Demonstrate the prompt's acceptance scenarios against the scoped workflow. Include setup from a clean checkout and failure recovery. Check persistence across restart and export/restore only for the state the prompt says to store; for memory-only tools, confirm that temporary content is discarded as specified. Record actual results and remaining limitations. A detailed plan alone does not establish a working replacement.