# AGENTS.md — Build guide for Calday

## Project scope
Publish a simple booking link with one appointment duration, guest timezone selection and explicit date overrides. Confirm a calendar event and deliver cancellation and rescheduling links.

Catalogue verdict: yes. Calday's solo core is compact: build a personal booking links tool that reads one calendar, publishes availability, reserves a slot, and writes the event back. A competent builder can reach a useful personal version in one sitting, while the paid product mainly wins on OAuth, calendar reliability.
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- Do not publish, spend money, contact customers, delete source data or run irreversible migrations without the project owner's authorization.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
- Google OAuth credentials, a registered HTTPS callback and calendar access granted by the owner. Confirm the provider scopes and publishing requirements before adding guests.
- 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. One Google Calendar OAuth adapter, a timezone-aware date library, and an outbox worker for email/ICS delivery.
- Scope boundary: race-free booking under load; team routing and round robin

## 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.
- One Google Calendar OAuth adapter, a timezone-aware date library, and an outbox worker for email/ICS delivery.
- Domain model: booking page slugs, appointment durations, weekly availability, date overrides and booking states

## Security and data integrity
- Encrypt refresh tokens with a separately managed key; expose busy intervals rather than event titles publicly. Hash guest management tokens and bound their lifetime.
- Correctness boundary: Date overrides take precedence over weekly rules; local daylight-saving gaps never become bookable instants.
- Store UTC instants plus the intended IANA timezone. Reserve locally in a transaction, recheck provider availability, create with a stable external ID, then reconcile timeouts before retrying.
- 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: booking page slugs, appointment durations, weekly availability, date overrides and booking states
- Project rule — preserve this invariant: Date overrides take precedence over weekly rules; local daylight-saving gaps never become bookable instants.
- Project rule — acceptance evidence: Close a normally available date and remove its slots; a guest in another timezone receives an ICS file for the same absolute appointment.

## Optional agent skills and references
- Optional external skill: [gws-calendar](https://github.com/googleworkspace/cli/blob/main/skills/gws-calendar/SKILL.md) — Use Google Calendar calendars, events, recurrence, free/busy queries and integration discovery through the gws CLI. 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 Calday-inspired workflow with owned or clearly labeled sample data: Publish a simple booking link with one appointment duration, guest timezone selection and explicit date overrides. Confirm a calendar event and deliver cancellation and rescheduling links.
- Publish a reproducible walkthrough with this observable result: Close a normally available date and remove its slots; a guest in another timezone receives an ICS file for the same absolute appointment.
- Explain who can operate this scoped tool, its setup and ongoing costs, and these remaining product gaps: race-free booking under load; team routing and round robin Avoid guaranteed savings, performance scores or implied endorsement.

## Engineering roadmap
1. Phase 1 — Scope and fixtures. Implement this bounded workflow: Publish a simple booking link with one appointment duration, guest timezone selection and explicit date overrides. Confirm a calendar event and deliver cancellation and rescheduling links. Record prerequisites, select representative user-owned fixtures and document the unsupported features: race-free booking under load; team routing and round robin
2. Phase 2 — Durable model. Model booking page slugs, appointment durations, weekly availability, date overrides and booking states Add migrations or a versioned document format, explicit validation, stable IDs and a visible import-error report. Preserve this rule: Date overrides take precedence over weekly rules; local daylight-saving gaps never become bookable instants.
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. Store UTC instants plus the intended IANA timezone. Reserve locally in a transaction, recheck provider availability, create with a stable external ID, then reconcile timeouts before retrying.
4. Phase 4 — Permissions and integration failure. Encrypt refresh tokens with a separately managed key; expose busy intervals rather than event titles publicly. Hash guest management tokens and bound their lifetime. 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. Close a normally available date and remove its slots; a guest in another timezone receives an ICS file for the same absolute appointment. 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
- race-free booking under load
- team routing and round robin
- reliable reminders and timezone edge cases
- calendar-provider edge cases and timezone correctness
- multi-calendar conflict handling

## Implementation prompt
WORKING SLICE
Publish a simple booking link with one appointment duration, guest timezone selection and explicit date overrides. Confirm a calendar event and deliver cancellation and rescheduling links.

Build this scoped Calday-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.
- One Google Calendar OAuth adapter, a timezone-aware date library, and an outbox worker for email/ICS delivery.

Prerequisites and limits
Google OAuth credentials, a registered HTTPS callback and calendar access granted by the owner. Confirm the provider scopes and publishing requirements before adding guests.
Outside this release: race-free booking under load; team routing and round robin

Data model and correctness
booking page slugs, appointment durations, weekly availability, date overrides and booking states
Invariant: Date overrides take precedence over weekly rules; local daylight-saving gaps never become bookable instants.
Store UTC instants plus the intended IANA timezone. Reserve locally in a transaction, recheck provider availability, create with a stable external ID, then reconcile timeouts before retrying.

Security and privacy
Encrypt refresh tokens with a separately managed key; expose busy intervals rather than event titles publicly. Hash guest management tokens and bound their lifetime.

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: Publish a simple booking link with one appointment duration, guest timezone selection and explicit date overrides. Confirm a calendar event and deliver cancellation and rescheduling links. Record prerequisites, select representative user-owned fixtures and document the unsupported features: race-free booking under load; team routing and round robin
2. Phase 2 — Durable model. Model booking page slugs, appointment durations, weekly availability, date overrides and booking states Add migrations or a versioned document format, explicit validation, stable IDs and a visible import-error report. Preserve this rule: Date overrides take precedence over weekly rules; local daylight-saving gaps never become bookable instants.
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. Store UTC instants plus the intended IANA timezone. Reserve locally in a transaction, recheck provider availability, create with a stable external ID, then reconcile timeouts before retrying.
4. Phase 4 — Permissions and integration failure. Encrypt refresh tokens with a separately managed key; expose busy intervals rather than event titles publicly. Hash guest management tokens and bound their lifetime. 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. Close a normally available date and remove its slots; a guest in another timezone receives an ICS file for the same absolute appointment. Repeat the workflow after restart and with a denied permission or unavailable dependency; show recoverable failure rather than a success placeholder.

Acceptance
Close a normally available date and remove its slots; a guest in another timezone receives an ICS file for the same absolute appointment.
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: [gws-calendar](https://github.com/googleworkspace/cli/blob/main/skills/gws-calendar/SKILL.md) — Use Google Calendar calendars, events, recurrence, free/busy queries and integration discovery through the gws CLI. 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: booking page slugs, appointment durations, weekly availability, date overrides and booking states
Project rule — preserve this invariant: Date overrides take precedence over weekly rules; local daylight-saving gaps never become bookable instants.
Project rule — acceptance evidence: Close a normally available date and remove its slots; a guest in another timezone receives an ICS file for the same absolute appointment.

## 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.