# AGENTS.md — Build guide for Fathom HQ ## Project scope Import monthly trial balances, map accounts into a reviewed chart, calculate a small set of explicitly defined KPIs and export a board-report snapshot. Catalogue verdict: kinda. A KPI dashboard over accounting exports is buildable, but Fathom's value is accounting integrations, consolidation, forecasting, polished board reports, and accountant workflows. 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 - Do not publish, spend money, contact customers, delete source data or run irreversible migrations without the project owner's authorization.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 ImportPeriod, SourceAccount, MappingVersion, Balance and MetricDefinition; every report is pinned to source/mapping versions and debits/credits follow one documented sign convention. - 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. - Start with CSV and selected formulas, not full accounting integration or forecasts. Explain currency, date basis and adjustments; tax/accounting interpretation requires qualified review. - 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 ImportPeriod, SourceAccount, MappingVersion, Balance and MetricDefinition; every report is pinned to source/mapping versions and debits/credits follow one documented sign convention. - Project rule — scope and recovery: Start with CSV and selected formulas, not full accounting integration or forecasts. Explain currency, date basis and adjustments; tax/accounting interpretation requires qualified review. - Project rule — acceptance: Import an unbalanced period and an unmapped account; block final report status and show the reconciliation difference instead of presenting a plausible margin. - 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. - Recommended skill: [pdf](https://github.com/anthropics/skills/blob/main/skills/pdf/SKILL.md) — inspect PDF source/extraction or generated report layout in the workflow; check the skill license and required PDF/OCR binaries. 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: Import monthly trial balances, map accounts into a reviewed chart, calculate a small set of explicitly defined KPIs and export a board-report snapshot. - Share a synthetic example export and the acceptance walkthrough; keep real customer, health, financial and source data private: Import an unbalanced period and an unmapped account; block final report status and show the reconciliation difference instead of presenting a plausible margin. - State the limits before asking someone to replace their existing tool: Start with CSV and selected formulas, not full accounting integration or forecasts. Explain currency, date basis and adjustments; tax/accounting interpretation requires qualified review. ## Engineering roadmap 1. Phase 1 — Pin the working slice and create its example input: Import monthly trial balances, map accounts into a reviewed chart, calculate a small set of explicitly defined KPIs and export a board-report snapshot. 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 ImportPeriod, SourceAccount, MappingVersion, Balance and MetricDefinition; every report is pinned to source/mapping versions and debits/credits follow one documented sign convention. 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: Start with CSV and selected formulas, not full accounting integration or forecasts. Explain currency, date basis and adjustments; tax/accounting interpretation requires qualified review. 5. Phase 5 — Walk through this concrete acceptance case and preserve its exported evidence: Import an unbalanced period and an unmapped account; block final report status and show the reconciliation difference instead of presenting a plausible margin. Finish the README and backup/restore instructions; report unfinished capabilities explicitly. ## Paid-product capabilities outside this build - accounting integrations - consolidation - templates - forecasting - report builder - advisor workflows - reliability ## Implementation prompt Build the following focused alternative to Fathom HQ. This is a deliberately limited personal or small-team substitute, not parity with the paid service. WORKING SLICE Import monthly trial balances, map accounts into a reviewed chart, calculate a small set of explicitly defined KPIs and export a board-report snapshot. 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 ImportPeriod, SourceAccount, MappingVersion, Balance and MetricDefinition; every report is pinned to source/mapping versions and debits/credits follow one documented sign convention. 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 Start with CSV and selected formulas, not full accounting integration or forecasts. Explain currency, date basis and adjustments; tax/accounting interpretation requires qualified review. ACCEPTANCE SCENARIO Import an unbalanced period and an unmapped account; block final report status and show the reconciliation difference instead of presenting a plausible margin. 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.