diff --git a/.github/ISSUE_TEMPLATE/bug_report.yml b/.github/ISSUE_TEMPLATE/bug_report.yml
new file mode 100644
index 0000000..ed99c76
--- /dev/null
+++ b/.github/ISSUE_TEMPLATE/bug_report.yml
@@ -0,0 +1,74 @@
+name: Bug Report
+description: Report a bug in timekeeper-countdown
+title: "[Bug]: "
+labels: ["bug"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for reporting a bug! Please fill out the form below.
+
+  - type: textarea
+    id: description
+    attributes:
+      label: Bug Description
+      description: A clear and concise description of the bug.
+      placeholder: Describe the bug...
+    validations:
+      required: true
+
+  - type: textarea
+    id: reproduction
+    attributes:
+      label: Reproduction Steps
+      description: Steps to reproduce the behavior.
+      placeholder: |
+        1. Create a countdown with...
+        2. Call start()...
+        3. After 5 seconds...
+        4. See error...
+    validations:
+      required: true
+
+  - type: textarea
+    id: expected
+    attributes:
+      label: Expected Behavior
+      description: What you expected to happen.
+    validations:
+      required: true
+
+  - type: dropdown
+    id: package
+    attributes:
+      label: Affected Package
+      options:
+        - "@timekeeper-countdown/core"
+        - "@timekeeper-countdown/react"
+        - Both
+    validations:
+      required: true
+
+  - type: textarea
+    id: system-info
+    attributes:
+      label: System Information
+      description: Paste relevant environment details.
+      placeholder: |
+        - Package version:
+        - Node.js version:
+        - OS:
+        - Browser (if applicable):
+      render: text
+
+  - type: checkboxes
+    id: validations
+    attributes:
+      label: Validations
+      options:
+        - label: I have read the [Code of Conduct](https://github.com/eagle-head/timekeeper-countdown/blob/main/CODE_OF_CONDUCT.md)
+          required: true
+        - label: I have searched for [existing issues](https://github.com/eagle-head/timekeeper-countdown/issues)
+          required: true
+        - label: I have tested with the latest version
+          required: true
diff --git a/.github/ISSUE_TEMPLATE/config.yml b/.github/ISSUE_TEMPLATE/config.yml
new file mode 100644
index 0000000..ae7eadc
--- /dev/null
+++ b/.github/ISSUE_TEMPLATE/config.yml
@@ -0,0 +1,5 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Questions & Help
+    url: https://github.com/eagle-head/timekeeper-countdown/discussions
+    about: Ask questions and get help from the community.
diff --git a/.github/ISSUE_TEMPLATE/feature_request.yml b/.github/ISSUE_TEMPLATE/feature_request.yml
new file mode 100644
index 0000000..e9f6327
--- /dev/null
+++ b/.github/ISSUE_TEMPLATE/feature_request.yml
@@ -0,0 +1,54 @@
+name: Feature Request
+description: Suggest a new feature or improvement
+title: "[Feature]: "
+labels: ["enhancement"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for suggesting a feature! Please fill out the form below.
+
+  - type: textarea
+    id: motivation
+    attributes:
+      label: Problem or Motivation
+      description: What problem does this feature solve? Why is it needed?
+      placeholder: I'm always frustrated when...
+    validations:
+      required: true
+
+  - type: textarea
+    id: solution
+    attributes:
+      label: Proposed Solution
+      description: Describe the solution you'd like to see.
+    validations:
+      required: true
+
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives Considered
+      description: Any alternative solutions or workarounds you've considered?
+
+  - type: dropdown
+    id: package
+    attributes:
+      label: Related Package
+      options:
+        - "@timekeeper-countdown/core"
+        - "@timekeeper-countdown/react"
+        - New adapter
+        - Documentation
+    validations:
+      required: true
+
+  - type: checkboxes
+    id: validations
+    attributes:
+      label: Validations
+      options:
+        - label: I have read the [Code of Conduct](https://github.com/eagle-head/timekeeper-countdown/blob/main/CODE_OF_CONDUCT.md)
+          required: true
+        - label: I have searched for [existing issues](https://github.com/eagle-head/timekeeper-countdown/issues)
+          required: true
diff --git a/.github/PULL_REQUEST_TEMPLATE.md b/.github/PULL_REQUEST_TEMPLATE.md
new file mode 100644
index 0000000..b9694bf
--- /dev/null
+++ b/.github/PULL_REQUEST_TEMPLATE.md
@@ -0,0 +1,35 @@
+## Description
+
+<!-- What does this PR do? Why is it needed? -->
+
+Fixes #<!-- issue number -->
+
+## Type of Change
+
+<!-- Check the one that applies -->
+
+- [ ] Bug fix (non-breaking change that fixes an issue)
+- [ ] New feature (non-breaking change that adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to change)
+- [ ] Documentation update
+- [ ] Refactoring (no functional changes)
+- [ ] Chore (dependencies, CI, build, etc.)
+
+## How to Test
+
+<!-- Describe how reviewers can test your changes -->
+
+1.
+2.
+3.
+
+## Checklist
+
+- [ ] My code follows the project's code style
+- [ ] I have added/updated tests for my changes
+- [ ] `npm run lint` passes
+- [ ] `npm run typecheck` passes
+- [ ] `npm run test` passes
+- [ ] I have followed the [commit convention](.github/commit-convention.md)
+- [ ] I have added a changeset (`npm run changeset`) if this is a user-facing change
+- [ ] I have updated documentation if needed
diff --git a/.github/commit-convention.md b/.github/commit-convention.md
new file mode 100644
index 0000000..822a67f
--- /dev/null
+++ b/.github/commit-convention.md
@@ -0,0 +1,110 @@
+# Commit Convention
+
+This project follows [Conventional Commits](https://www.conventionalcommits.org/).
+
+## Format
+
+```
+<type>(<scope>): <subject>
+
+<body>
+
+<footer>
+```
+
+Only `<type>` and `<subject>` are required. Scope, body, and footer are optional.
+
+## Types
+
+| Type       | Description                          |
+| ---------- | ------------------------------------ |
+| `feat`     | New feature                          |
+| `fix`      | Bug fix                              |
+| `docs`     | Documentation only                   |
+| `style`    | Code style changes (no logic change) |
+| `refactor` | Code refactoring (no feature or fix) |
+| `perf`     | Performance improvement              |
+| `test`     | Adding or updating tests             |
+| `chore`    | Maintenance, dependencies, tooling   |
+| `ci`       | CI/CD configuration                  |
+| `build`    | Build system changes                 |
+
+## Scopes
+
+Scopes are optional. Use them when the change is specific to a package:
+
+| Scope   | When to use                              |
+| ------- | ---------------------------------------- |
+| `core`  | Changes to `@timekeeper-countdown/core`  |
+| `react` | Changes to `@timekeeper-countdown/react` |
+| `docs`  | Changes to the documentation site        |
+| `deps`  | Dependency updates                       |
+
+Omit the scope when the change spans multiple packages or is project-wide.
+
+## Rules
+
+- Use **imperative mood**: "add feature" not "added feature"
+- **Do not capitalize** the first letter of the subject
+- **No period** at the end of the subject
+- Subject line: **max 50 characters**
+- Body: wrap at **72 characters** per line
+- Separate subject from body with a **blank line**
+
+## Breaking Changes
+
+Indicate breaking changes with `!` before the colon:
+
+```
+feat(core)!: remove deprecated startFrom option
+```
+
+Or use the `BREAKING CHANGE:` footer:
+
+```
+feat(core): change snapshot format
+
+BREAKING CHANGE: snapshot.remaining is now in milliseconds instead of seconds
+```
+
+## Examples
+
+```
+feat(react): add onComplete callback to useCountdown
+```
+
+```
+fix(core): correct elapsed time calculation after resume
+```
+
+```
+docs: update getting started guide
+```
+
+```
+test(core): add edge case tests for pause/resume cycle
+```
+
+```
+refactor(core): extract tick logic into separate function
+```
+
+```
+chore(deps): bump vitest to v2
+```
+
+```
+ci: add Node 20 to test matrix
+```
+
+```
+perf(core): reduce unnecessary snapshot allocations
+```
+
+```
+style: apply prettier formatting to config files
+```
+
+```
+build: update tsup config for tree-shaking
+```
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..9722e9a
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,65 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  lint:
+    name: Lint & Format
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - run: npm ci
+      - run: npm run lint
+      - run: npm run format:check
+
+  typecheck:
+    name: Type Check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - run: npm ci
+      - run: npm run build --workspace @timekeeper-countdown/core
+      - run: npm run typecheck
+
+  test:
+    name: Test (Node ${{ matrix.node-version }})
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [18, 20]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: npm
+      - run: npm ci
+      - run: npm run test
+
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - run: npm ci
+      - run: npm run build
diff --git a/CODE_OF_CONDUCT.md b/CODE_OF_CONDUCT.md
new file mode 100644
index 0000000..15472ad
--- /dev/null
+++ b/CODE_OF_CONDUCT.md
@@ -0,0 +1,83 @@
+# Contributor Covenant 3.0 Code of Conduct
+
+## Our Pledge
+
+We pledge to make our community welcoming, safe, and equitable for all.
+
+We are committed to fostering an environment that respects and promotes the dignity, rights, and contributions of all individuals, regardless of characteristics including race, ethnicity, caste, color, age, physical characteristics, neurodiversity, disability, sex or gender, gender identity or expression, sexual orientation, language, philosophy or religion, national or social origin, socio-economic position, level of education, or other status. The same privileges of participation are extended to everyone who participates in good faith and in accordance with this Covenant.
+
+## Encouraged Behaviors
+
+While acknowledging differences in social norms, we all strive to meet our community's expectations for positive behavior. We also understand that our words and actions may be interpreted differently than we intend based on culture, background, or native language.
+
+With these considerations in mind, we agree to behave mindfully toward each other and act in ways that center our shared values, including:
+
+1. Respecting the **purpose of our community**, our activities, and our ways of gathering.
+2. Engaging **kindly and honestly** with others.
+3. Respecting **different viewpoints** and experiences.
+4. **Taking responsibility** for our actions and contributions.
+5. Gracefully giving and accepting **constructive feedback**.
+6. Committing to **repairing harm** when it occurs.
+7. Behaving in other ways that promote and sustain the **well-being of our community**.
+
+## Restricted Behaviors
+
+We agree to restrict the following behaviors in our community. Instances, threats, and promotion of these behaviors are violations of this Code of Conduct.
+
+1. **Harassment.** Violating explicitly expressed boundaries or engaging in unnecessary personal attention after any clear request to stop.
+2. **Character attacks.** Making insulting, demeaning, or pejorative comments directed at a community member or group of people.
+3. **Stereotyping or discrimination.** Characterizing anyone's personality or behavior on the basis of immutable identities or traits.
+4. **Sexualization.** Behaving in a way that would generally be considered inappropriately intimate in the context or purpose of the community.
+5. **Violating confidentiality**. Sharing or acting on someone's personal or private information without their permission.
+6. **Endangerment.** Causing, encouraging, or threatening violence or other harm toward any person or group.
+7. Behaving in other ways that **threaten the well-being** of our community.
+
+### Other Restrictions
+
+1. **Misleading identity.** Impersonating someone else for any reason, or pretending to be someone else to evade enforcement actions.
+2. **Failing to credit sources.** Not properly crediting the sources of content you contribute.
+3. **Promotional materials**. Sharing marketing or other commercial content in a way that is outside the norms of the community.
+4. **Irresponsible communication.** Failing to responsibly present content which includes, links or describes any other restricted behaviors.
+
+## Reporting an Issue
+
+Tensions can occur between community members even when they are trying their best to collaborate. Not every conflict represents a code of conduct violation, and this Code of Conduct reinforces encouraged behaviors and norms that can help avoid conflicts and minimize harm.
+
+When an incident does occur, it is important to report it promptly. To report a possible violation, please email **ekohn.npm@gmail.com**.
+
+Community Moderators take reports of violations seriously and will make every effort to respond in a timely manner. They will investigate all reports of code of conduct violations, reviewing messages, logs, and recordings, or interviewing witnesses and other participants. Community Moderators will keep investigation and enforcement actions as transparent as possible while prioritizing safety and confidentiality. In order to honor these values, enforcement actions are carried out in private with the involved parties, but communicating to the whole community may be part of a mutually agreed upon resolution.
+
+## Addressing and Repairing Harm
+
+If an investigation by the Community Moderators finds that this Code of Conduct has been violated, the following enforcement ladder may be used to determine how best to repair harm, based on the incident's impact on the individuals involved and the community as a whole. Depending on the severity of a violation, lower rungs on the ladder may be skipped.
+
+1. **Warning**
+   1. Event: A violation involving a single incident or series of incidents.
+   2. Consequence: A private, written warning from the Community Moderators.
+   3. Repair: Examples of repair include a private written apology, acknowledgement of responsibility, and seeking clarification on expectations.
+2. **Temporarily Limited Activities**
+   1. Event: A repeated incidence of a violation that previously resulted in a warning, or the first incidence of a more serious violation.
+   2. Consequence: A private, written warning with a time-limited cooldown period designed to underscore the seriousness of the situation and give the community members involved time to process the incident. The cooldown period may be limited to particular communication channels or interactions with particular community members.
+   3. Repair: Examples of repair may include making an apology, using the cooldown period to reflect on actions and impact, and being thoughtful about re-entering community spaces after the period is over.
+3. **Temporary Suspension**
+   1. Event: A pattern of repeated violation which the Community Moderators have tried to address with warnings, or a single serious violation.
+   2. Consequence: A private written warning with conditions for return from suspension. In general, temporary suspensions give the person being suspended time to reflect upon their behavior and possible corrective actions.
+   3. Repair: Examples of repair include respecting the spirit of the suspension, meeting the specified conditions for return, and being thoughtful about how to reintegrate with the community when the suspension is lifted.
+4. **Permanent Ban**
+   1. Event: A pattern of repeated code of conduct violations that other steps on the ladder have failed to resolve, or a violation so serious that the Community Moderators determine there is no way to keep the community safe with this person as a member.
+   2. Consequence: Access to all community spaces, tools, and communication channels is removed. In general, permanent bans should be rarely used, should have strong reasoning behind them, and should only be resorted to if working through other remedies has failed to change the behavior.
+   3. Repair: There is no possible repair in cases of this severity.
+
+This enforcement ladder is intended as a guideline. It does not limit the ability of Community Managers to use their discretion and judgment, in keeping with the best interests of our community.
+
+## Scope
+
+This Code of Conduct applies within all community spaces, and also applies when an individual is officially representing the community in public or other spaces. Examples of representing our community include using an official email address, posting via an official social media account, or acting as an appointed representative at an online or offline event.
+
+## Attribution
+
+This Code of Conduct is adapted from the Contributor Covenant, version 3.0, permanently available at [https://www.contributor-covenant.org/version/3/0/](https://www.contributor-covenant.org/version/3/0/).
+
+Contributor Covenant is stewarded by the Organization for Ethical Source and licensed under CC BY-SA 4.0. To view a copy of this license, visit [https://creativecommons.org/licenses/by-sa/4.0/](https://creativecommons.org/licenses/by-sa/4.0/)
+
+For answers to common questions about Contributor Covenant, see the FAQ at [https://www.contributor-covenant.org/faq](https://www.contributor-covenant.org/faq). Translations are provided at [https://www.contributor-covenant.org/translations](https://www.contributor-covenant.org/translations). Additional enforcement and community guideline resources can be found at [https://www.contributor-covenant.org/resources](https://www.contributor-covenant.org/resources). The enforcement ladder was inspired by the work of [Mozilla's code of conduct team](https://github.com/mozilla/inclusion).
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
new file mode 100644
index 0000000..d533d65
--- /dev/null
+++ b/CONTRIBUTING.md
@@ -0,0 +1,136 @@
+# Contributing to Timekeeper Countdown
+
+Thank you for your interest in contributing! All contributions are welcome — whether it's reporting a bug, suggesting a feature, improving documentation, or submitting code.
+
+Please read and follow our [Code of Conduct](CODE_OF_CONDUCT.md) in all interactions.
+
+## How to Contribute
+
+- **Report bugs** using the [bug report template](https://github.com/eagle-head/timekeeper-countdown/issues/new?template=bug_report.yml)
+- **Suggest features** using the [feature request template](https://github.com/eagle-head/timekeeper-countdown/issues/new?template=feature_request.yml)
+- **Improve documentation** in the `docs/` directory
+- **Submit pull requests** for bug fixes or features
+- **Review existing PRs** to help the maintainers
+
+## Development Setup
+
+### Prerequisites
+
+- [Node.js](https://nodejs.org/) 18 or later
+- npm (comes with Node.js)
+
+### Getting Started
+
+```bash
+# Clone the repository
+git clone https://github.com/eagle-head/timekeeper-countdown.git
+cd timekeeper-countdown
+
+# Install dependencies (always from the root — never inside individual packages)
+npm install
+```
+
+### Key Scripts
+
+| Command                                                | Description             |
+| ------------------------------------------------------ | ----------------------- |
+| `npm run build`                                        | Build all packages      |
+| `npm run test`                                         | Run all tests           |
+| `npm run lint`                                         | Lint all packages       |
+| `npm run typecheck`                                    | Type check all packages |
+| `npm run format:check`                                 | Check code formatting   |
+| `npm run test --workspace @timekeeper-countdown/core`  | Test core only          |
+| `npm run test --workspace @timekeeper-countdown/react` | Test react only         |
+
+## Project Structure
+
+```
+packages/
+├── core/    # @timekeeper-countdown/core — zero-dependency countdown engine
+│              (state machine, timer, formatter, testing utilities)
+├── react/   # @timekeeper-countdown/react — React hook adapter (useCountdown)
+└── docs/    # VitePress documentation site
+```
+
+## Development Workflow
+
+1. **Fork** the repository (or create a branch if you have write access)
+2. **Create a feature branch** from `main`:
+   ```bash
+   git checkout -b feat/my-feature
+   ```
+3. **Make your changes** and add or update tests as needed
+4. **Run the checks** before committing:
+   ```bash
+   npm run lint && npm run typecheck && npm run test
+   ```
+5. **Commit** using [Conventional Commits](#commit-convention)
+6. **Push** and open a pull request against `main`
+
+## Commit Convention
+
+We use [Conventional Commits](https://www.conventionalcommits.org/). The basic format is:
+
+```
+<type>(<scope>): <subject>
+```
+
+Common types: `feat`, `fix`, `docs`, `test`, `chore`, `refactor`, `ci`
+
+For the full guide with all types, scopes, and examples, see [.github/commit-convention.md](.github/commit-convention.md).
+
+## Pull Request Guidelines
+
+- Fill out the **PR template** completely
+- **Link related issues** using `fixes #123` or `closes #123`
+- Keep PRs **focused** — one feature or fix per PR
+- **Add tests** for new functionality
+- **Update documentation** if behavior changes
+- Add a **changeset** for user-facing changes:
+  ```bash
+  npm run changeset
+  ```
+  Follow the prompts to describe your change and its semver impact.
+- **CI must pass** before your PR can be merged
+
+## Code Style
+
+The project uses ESLint 9 and Prettier for linting and formatting. TypeScript strict mode is enabled. Run `npm run lint` and `npm run format:check` to verify your code.
+
+### Naming Conventions
+
+| Element                   | Convention                    | Example                                |
+| ------------------------- | ----------------------------- | -------------------------------------- |
+| Files                     | `kebab-case`                  | `countdown-engine.ts`                  |
+| Factory functions & types | `PascalCase`                  | `CountdownEngine`, `CountdownSnapshot` |
+| React hooks               | `camelCase` with `use` prefix | `useCountdown`                         |
+| Constants                 | `UPPER_SNAKE_CASE`            | `SECONDS_PER_MINUTE`                   |
+
+### Key Patterns
+
+- **Factory functions, not classes** — all constructs are called without `new`
+- **`_` prefix = private** — properties prefixed with `_` are mangled in production builds
+
+## Testing
+
+- **Framework:** [Vitest](https://vitest.dev/) with `globals: true`
+- **Test location:** `src/__tests__/*.test.ts` (core) or `src/__tests__/*.test.tsx` (react)
+- **Time mocking:** Use `vi.useFakeTimers()` in `beforeEach` and `vi.useRealTimers()` in `afterEach`
+- **React tests:** Use `renderHook` and `act` from `@testing-library/react`
+- **Custom test helpers:** Available from `@timekeeper-countdown/core/testing-utils` — includes `createFakeTimeProvider()`, `buildSnapshot()`, and assertion utilities
+
+Run tests for a specific package:
+
+```bash
+npm run test --workspace @timekeeper-countdown/core
+npm run test --workspace @timekeeper-countdown/react
+```
+
+## Reporting Issues
+
+- Use the **bug report** template for bugs — include clear reproduction steps
+- Use the **feature request** template for ideas and suggestions
+- **Search existing issues** before opening a new one
+- **One issue per bug or feature** — don't combine multiple topics
+
+Thank you for helping make Timekeeper Countdown better!
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..6e2910a
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2024 Eduardo Kohn
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
diff --git a/README.md b/README.md
index e9b1748..5546a89 100644
--- a/README.md
+++ b/README.md
@@ -50,13 +50,9 @@ Vitest powers the unit test suites in both packages. React tests use `@testing-l
 
 ## Contributing
 
-1. Fork or branch from `main`.
-2. Run `npm install` to fetch dependencies.
-3. Make changes and add or update tests when applicable.
-4. Ensure `npm run lint`, `npm run typecheck`, and relevant `npm run test --workspace ...` pass.
-5. Open a PR with a clear summary and link any related issues.
+We welcome contributions! Please read our [Contributing Guide](CONTRIBUTING.md) to get started.
 
-Please review the internal guidelines in [`AGENTS.md`](AGENTS.md) before raising a PR.
+By participating in this project, you agree to abide by our [Code of Conduct](CODE_OF_CONDUCT.md).
 
 ## License
 
diff --git a/packages/core/src/__tests__/countdown.test.ts b/packages/core/src/__tests__/countdown.test.ts
index 650b062..62e19d9 100644
--- a/packages/core/src/__tests__/countdown.test.ts
+++ b/packages/core/src/__tests__/countdown.test.ts
@@ -524,9 +524,7 @@ describe('Countdown - Error Handling', () => {
     });
 
     it('should throw error when onError is not a function', () => {
-      expect(() => Countdown(60, { onError: 'not a function' as any })).toThrowError(
-        'onError must be a function'
-      );
+      expect(() => Countdown(60, { onError: 'not a function' as any })).toThrowError('onError must be a function');
     });
   });
 
@@ -581,7 +579,9 @@ describe('Countdown - Error Handling', () => {
   describe('onError callback', () => {
     it('should call onError when onSnapshot throws', () => {
       const onError = vi.fn();
-      const onSnapshot = vi.fn(() => { throw new Error('snapshot boom'); });
+      const onSnapshot = vi.fn(() => {
+        throw new Error('snapshot boom');
+      });
       const countdown = Countdown(60, { onSnapshot, onError });
 
       countdown.start();
@@ -592,7 +592,9 @@ describe('Countdown - Error Handling', () => {
 
     it('should call onError when onStateChange throws', () => {
       const onError = vi.fn();
-      const onStateChange = vi.fn(() => { throw new Error('state boom'); });
+      const onStateChange = vi.fn(() => {
+        throw new Error('state boom');
+      });
       const countdown = Countdown(60, { onStateChange, onError });
 
       countdown.start();
@@ -603,7 +605,9 @@ describe('Countdown - Error Handling', () => {
 
     it('should normalize non-Error throws to Error instances', () => {
       const onError = vi.fn();
-      const onSnapshot = vi.fn(() => { throw 'raw string error'; });
+      const onSnapshot = vi.fn(() => {
+        throw 'raw string error';
+      });
       const countdown = Countdown(60, { onSnapshot, onError });
 
       countdown.start();
@@ -614,7 +618,9 @@ describe('Countdown - Error Handling', () => {
 
     it('should normalize non-Error throws from onStateChange to Error instances', () => {
       const onError = vi.fn();
-      const onStateChange = vi.fn(() => { throw 'raw string from state'; });
+      const onStateChange = vi.fn(() => {
+        throw 'raw string from state';
+      });
       const countdown = Countdown(60, { onStateChange, onError });
 
       countdown.start();
@@ -624,7 +630,9 @@ describe('Countdown - Error Handling', () => {
     });
 
     it('should not throw when onError is not provided and callbacks throw', () => {
-      const onSnapshot = vi.fn(() => { throw new Error('no handler'); });
+      const onSnapshot = vi.fn(() => {
+        throw new Error('no handler');
+      });
       const countdown = Countdown(60, { onSnapshot });
 
       countdown.start();
@@ -632,8 +640,12 @@ describe('Countdown - Error Handling', () => {
     });
 
     it('should not throw when onError itself throws', () => {
-      const onError = vi.fn(() => { throw new Error('handler also broken'); });
-      const onSnapshot = vi.fn(() => { throw new Error('snapshot error'); });
+      const onError = vi.fn(() => {
+        throw new Error('handler also broken');
+      });
+      const onSnapshot = vi.fn(() => {
+        throw new Error('snapshot error');
+      });
       const countdown = Countdown(60, { onSnapshot, onError });
 
       expect(() => countdown.start()).not.toThrow();
@@ -661,7 +673,9 @@ describe('Countdown - Error Handling', () => {
     it('should propagate errors thrown by the engine start method', async () => {
       vi.doMock('../runtime/timer', () => ({
         Timer: vi.fn(() => ({
-          start: vi.fn(() => { throw new Error('Timer start error'); }),
+          start: vi.fn(() => {
+            throw new Error('Timer start error');
+          }),
           stop: vi.fn(),
           reset: vi.fn(),
           destroy: vi.fn(),
@@ -686,7 +700,9 @@ describe('Countdown - Error Handling', () => {
         Timer: vi.fn(() => ({
           start: vi.fn(() => true),
           stop: vi.fn(),
-          reset: vi.fn(() => { throw new Error('Reset error'); }),
+          reset: vi.fn(() => {
+            throw new Error('Reset error');
+          }),
           destroy: vi.fn(),
           getTotalSeconds: vi.fn(() => 60),
           setSeconds: vi.fn(),
@@ -708,7 +724,9 @@ describe('Countdown - Error Handling', () => {
       vi.doMock('../format/formatter', () => ({
         Formatter: vi.fn(() => ({
           formatTime: vi.fn(() => ({ minutes: '00', seconds: '00' })),
-          formatSeconds: vi.fn(() => { throw new Error('Format seconds error'); }),
+          formatSeconds: vi.fn(() => {
+            throw new Error('Format seconds error');
+          }),
           formatMinutes: vi.fn(() => '00'),
           formatHours: vi.fn(() => '00'),
           formatDays: vi.fn(() => '00'),
@@ -868,6 +886,5 @@ describe('Countdown - Error Handling', () => {
 
       vi.doUnmock('../runtime/timer');
     });
-
   });
 });
diff --git a/packages/core/src/__tests__/timer.test.ts b/packages/core/src/__tests__/timer.test.ts
index 83cb2c8..1f6663f 100644
--- a/packages/core/src/__tests__/timer.test.ts
+++ b/packages/core/src/__tests__/timer.test.ts
@@ -104,7 +104,6 @@ describe('Timer - Happy Path', () => {
 
       expect(() => Timer(Number.MAX_SAFE_INTEGER + 1, events)).toThrow('initialSeconds exceeds maximum safe integer');
     });
-
   });
 
   describe('Start functionality', () => {
diff --git a/packages/core/src/api/countdown.ts b/packages/core/src/api/countdown.ts
index 55e74a4..de6fd77 100644
--- a/packages/core/src/api/countdown.ts
+++ b/packages/core/src/api/countdown.ts
@@ -49,7 +49,11 @@ export function Countdown(initialSeconds: number, options: CountdownOptions = {}
 
   const handleError = (error: Error) => {
     if (!onError) return;
-    try { onError(error); } catch { /* ignore handler errors */ }
+    try {
+      onError(error);
+    } catch {
+      /* ignore handler errors */
+    }
   };
 
   const notifySnapshot = (snapshot: CountdownSnapshot) => {
@@ -84,11 +88,21 @@ export function Countdown(initialSeconds: number, options: CountdownOptions = {}
   });
 
   return {
-    start: () => { engine.start(); },
-    pause: () => { engine.pause(); },
-    resume: () => { engine.resume(); },
-    reset: (newInitialSeconds?: number) => { engine.reset(newInitialSeconds); },
-    stop: () => { engine.stop(); },
+    start: () => {
+      engine.start();
+    },
+    pause: () => {
+      engine.pause();
+    },
+    resume: () => {
+      engine.resume();
+    },
+    reset: (newInitialSeconds?: number) => {
+      engine.reset(newInitialSeconds);
+    },
+    stop: () => {
+      engine.stop();
+    },
     getSeconds: () => formatter.formatSeconds(lastSnapshot.totalSeconds),
     getMinutes: () => formatter.formatMinutes(lastSnapshot.totalSeconds),
     getHours: () => formatter.formatHours(lastSnapshot.totalSeconds),
@@ -98,7 +112,11 @@ export function Countdown(initialSeconds: number, options: CountdownOptions = {}
     getCurrentState: () => lastSnapshot.state,
     getSnapshot: () => lastSnapshot,
     destroy: () => {
-      try { subscription.unsubscribe(); } finally { engine.destroy(); }
+      try {
+        subscription.unsubscribe();
+      } finally {
+        engine.destroy();
+      }
     },
   };
 }
diff --git a/packages/core/src/format/formatter.ts b/packages/core/src/format/formatter.ts
index 1e17493..9bbaa15 100644
--- a/packages/core/src/format/formatter.ts
+++ b/packages/core/src/format/formatter.ts
@@ -128,7 +128,10 @@ export function Formatter() {
 export const defaultFormatter = Formatter();
 
 export function formatTime(target: number): { minutes: string; seconds: string };
-export function formatTime(target: Pick<CountdownSnapshot, 'totalSeconds'> | null | undefined): { minutes: string; seconds: string };
+export function formatTime(target: Pick<CountdownSnapshot, 'totalSeconds'> | null | undefined): {
+  minutes: string;
+  seconds: string;
+};
 export function formatTime(target: FormatTarget): { minutes: string; seconds: string } {
   return defaultFormatter.formatTime(target);
 }
diff --git a/packages/react/src/use-countdown.ts b/packages/react/src/use-countdown.ts
index 03326bf..92ef9d6 100644
--- a/packages/react/src/use-countdown.ts
+++ b/packages/react/src/use-countdown.ts
@@ -99,7 +99,12 @@ export function useCountdown(initialSeconds: number, options: UseCountdownOption
   const pause = useCallback(() => invoke(engine => engine.pause(), false), [invoke]);
   const resume = useCallback(() => invoke(engine => engine.resume(), false), [invoke]);
   const stop = useCallback(() => invoke(engine => engine.stop(), false), [invoke]);
-  const setSeconds = useCallback((value: number): void => { invoke(engine => engine.setSeconds(value), undefined); }, [invoke]);
+  const setSeconds = useCallback(
+    (value: number): void => {
+      invoke(engine => engine.setSeconds(value), undefined);
+    },
+    [invoke]
+  );
   const reset = useCallback(
     (nextInitialSeconds?: number) => invoke(engine => engine.reset(nextInitialSeconds), false),
     [invoke]