# AGENTS.md — Build guide for Fathom Analytics

## Project scope
Collect a deliberately small first-party event payload and show page, referrer and goal reports for one site. Separate event-time and ingestion-time windows and expose retention and bot-filter assumptions.

Catalogue verdict: yes. A simple privacy analytics clone is very buildable; the paid service is hosted compliance posture, uptime, data retention, and polished reporting.
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- Do not publish, spend money, contact customers, delete source data or run irreversible migrations without the project owner's authorization.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
- A supported Node release, PostgreSQL, HTTPS for a shared deployment, and a backup destination. Begin with one workspace and explicit owner/member permissions.
- Implementation components: Next.js App Router and TypeScript for server-rendered pages and validated mutations. PostgreSQL with Drizzle migrations; Better Auth sessions for a small private workspace.
- Scope boundary: managed infrastructure; privacy/compliance messaging

## Stack and architecture
- Next.js App Router and TypeScript for server-rendered pages and validated mutations.
- PostgreSQL with Drizzle migrations; Better Auth sessions for a small private workspace.
- Domain model: registered sites, bounded events, route/referrer dimensions, goals and hourly aggregation windows

## Security and data integrity
- Authorize every record read and mutation on the server using its workspace membership; validate payloads, protect mutations against CSRF, and escape user-authored HTML.
- Correctness boundary: Cookie-free does not automatically mean anonymous or legally compliant; never store full query strings or persistent visitor fingerprints by default.
- Commit related database changes in one transaction. Use version checks for competing edits and an outbox for external notifications; retry delivery independently of saving the record.
- Export versioned JSON plus attachments and a readable CSV summary. Restore into a separate database and compare record IDs and attachment checksums before switching.

## Agent implementation rules
- Project rule — data model: registered sites, bounded events, route/referrer dimensions, goals and hourly aggregation windows
- Project rule — preserve this invariant: Cookie-free does not automatically mean anonymous or legally compliant; never store full query strings or persistent visitor fingerprints by default.
- Project rule — acceptance evidence: A replayed event ID is counted once; an event arriving after an aggregation pass is reflected in a corrected report with its original time window.

## Optional agent skills and references
- Optional external skill: [vercel-react-best-practices](https://github.com/vercel-labs/agent-skills/blob/main/skills/react-best-practices/SKILL.md) — Improve React and Next.js data fetching, rendering, bundle size and server performance. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
- Optional external skill: [supabase-postgres-best-practices](https://github.com/supabase/agent-skills/blob/main/skills/supabase-postgres-best-practices/SKILL.md) — Review PostgreSQL schemas, queries, indexes, pooling, concurrency and row-level security. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
- Optional external skill: [web-design-guidelines](https://github.com/vercel-labs/agent-skills/blob/main/skills/web-design-guidelines/SKILL.md) — Review web interfaces for accessibility, keyboard focus, forms, navigation and interaction quality. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.

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 the actual Fathom Analytics-inspired workflow with owned or clearly labeled sample data: Collect a deliberately small first-party event payload and show page, referrer and goal reports for one site. Separate event-time and ingestion-time windows and expose retention and bot-filter assumptions.
- Publish a reproducible walkthrough with this observable result: A replayed event ID is counted once; an event arriving after an aggregation pass is reflected in a corrected report with its original time window.
- Explain who can operate this scoped tool, its setup and ongoing costs, and these remaining product gaps: managed infrastructure; privacy/compliance messaging Avoid guaranteed savings, performance scores or implied endorsement.

## Engineering roadmap
1. Phase 1 — Scope and fixtures. Implement this bounded workflow: Collect a deliberately small first-party event payload and show page, referrer and goal reports for one site. Separate event-time and ingestion-time windows and expose retention and bot-filter assumptions. Record prerequisites, select representative user-owned fixtures and document the unsupported features: managed infrastructure; privacy/compliance messaging
2. Phase 2 — Durable model. Model registered sites, bounded events, route/referrer dimensions, goals and hourly aggregation windows Add migrations or a versioned document format, explicit validation, stable IDs and a visible import-error report. Preserve this rule: Cookie-free does not automatically mean anonymous or legally compliant; never store full query strings or persistent visitor fingerprints by default.
3. Phase 3 — Complete the first useful path. Implement the workflow's input, review and output interface, with clear controls and explicit empty/error states. Commit related database changes in one transaction. Use version checks for competing edits and an outbox for external notifications; retry delivery independently of saving the record.
4. Phase 4 — Permissions and integration failure. Authorize every record read and mutation on the server using its workspace membership; validate payloads, protect mutations against CSRF, and escape user-authored HTML. Request integration credentials and permissions only for the enabled feature; show a disconnected state instead of mock results.
5. Phase 5 — Portable handoff. Export versioned JSON plus attachments and a readable CSV summary. Restore into a separate database and compare record IDs and attachment checksums before switching. Include setup, operating limits, fixture walkthrough and shutdown/restart instructions in the README.
6. Phase 6 — Acceptance scenarios. A replayed event ID is counted once; an event arriving after an aggregation pass is reflected in a corrected report with its original time window. Repeat the workflow after restart and with a denied permission or unavailable dependency; show recoverable failure rather than a success placeholder.

## Paid-product capabilities outside this build
- managed infrastructure
- privacy/compliance messaging
- email reports
- uptime
- support
- billing

## Implementation prompt
WORKING SLICE
Collect a deliberately small first-party event payload and show page, referrer and goal reports for one site. Separate event-time and ingestion-time windows and expose retention and bot-filter assumptions.

Build this scoped Fathom Analytics-inspired workflow with a documented data model and visible failure states.

Architecture
- Next.js App Router and TypeScript for server-rendered pages and validated mutations.
- PostgreSQL with Drizzle migrations; Better Auth sessions for a small private workspace.

Prerequisites and limits
A supported Node release, PostgreSQL, HTTPS for a shared deployment, and a backup destination. Begin with one workspace and explicit owner/member permissions.
Outside this release: managed infrastructure; privacy/compliance messaging

Data model and correctness
registered sites, bounded events, route/referrer dimensions, goals and hourly aggregation windows
Invariant: Cookie-free does not automatically mean anonymous or legally compliant; never store full query strings or persistent visitor fingerprints by default.
Commit related database changes in one transaction. Use version checks for competing edits and an outbox for external notifications; retry delivery independently of saving the record.

Security and privacy
Authorize every record read and mutation on the server using its workspace membership; validate payloads, protect mutations against CSRF, and escape user-authored HTML.

Recovery and export
Export versioned JSON plus attachments and a readable CSV summary. Restore into a separate database and compare record IDs and attachment checksums before switching.

Implementation order
1. Phase 1 — Scope and fixtures. Implement this bounded workflow: Collect a deliberately small first-party event payload and show page, referrer and goal reports for one site. Separate event-time and ingestion-time windows and expose retention and bot-filter assumptions. Record prerequisites, select representative user-owned fixtures and document the unsupported features: managed infrastructure; privacy/compliance messaging
2. Phase 2 — Durable model. Model registered sites, bounded events, route/referrer dimensions, goals and hourly aggregation windows Add migrations or a versioned document format, explicit validation, stable IDs and a visible import-error report. Preserve this rule: Cookie-free does not automatically mean anonymous or legally compliant; never store full query strings or persistent visitor fingerprints by default.
3. Phase 3 — Complete the first useful path. Implement the workflow's input, review and output interface, with clear controls and explicit empty/error states. Commit related database changes in one transaction. Use version checks for competing edits and an outbox for external notifications; retry delivery independently of saving the record.
4. Phase 4 — Permissions and integration failure. Authorize every record read and mutation on the server using its workspace membership; validate payloads, protect mutations against CSRF, and escape user-authored HTML. Request integration credentials and permissions only for the enabled feature; show a disconnected state instead of mock results.
5. Phase 5 — Portable handoff. Export versioned JSON plus attachments and a readable CSV summary. Restore into a separate database and compare record IDs and attachment checksums before switching. Include setup, operating limits, fixture walkthrough and shutdown/restart instructions in the README.
6. Phase 6 — Acceptance scenarios. A replayed event ID is counted once; an event arriving after an aggregation pass is reflected in a corrected report with its original time window. Repeat the workflow after restart and with a denied permission or unavailable dependency; show recoverable failure rather than a success placeholder.

Acceptance
A replayed event ID is counted once; an event arriving after an aggregation pass is reflected in a corrected report with its original time window.
Use real source data or clearly labeled fixtures. Explain unsupported input and provider failures; do not fabricate analytics, delivery receipts, accuracy claims or security guarantees.

Optional agent guidance
Optional external skill: [vercel-react-best-practices](https://github.com/vercel-labs/agent-skills/blob/main/skills/react-best-practices/SKILL.md) — Improve React and Next.js data fetching, rendering, bundle size and server performance. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
Optional external skill: [supabase-postgres-best-practices](https://github.com/supabase/agent-skills/blob/main/skills/supabase-postgres-best-practices/SKILL.md) — Review PostgreSQL schemas, queries, indexes, pooling, concurrency and row-level security. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
Optional external skill: [web-design-guidelines](https://github.com/vercel-labs/agent-skills/blob/main/skills/web-design-guidelines/SKILL.md) — Review web interfaces for accessibility, keyboard focus, forms, navigation and interaction quality. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
Project rule — data model: registered sites, bounded events, route/referrer dimensions, goals and hourly aggregation windows
Project rule — preserve this invariant: Cookie-free does not automatically mean anonymous or legally compliant; never store full query strings or persistent visitor fingerprints by default.
Project rule — acceptance evidence: A replayed event ID is counted once; an event arriving after an aggregation pass is reflected in a corrected report with its original time window.

## 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.