format-for

One Markdown input → clean output for GitHub, Slack, or Linear.

You don’t need to know the input’s dialect. Pass Markdown that might mix Linear fences, Slack <url|label> links/mentions, and GFM. format‑for parses once and renders target‑aware output with predictable, safe degradations and explicit warnings.

Contents

• Install
• Quick start
• Example: one input → three outputs
• Concepts
• API
• Target behavior highlights
• Recipes
• Warnings and safety
• Performance and idempotency
• Contributing
• License

Install

This package is ESM‑first with a CJS fallback and works in Node 22+ and Bun 1.2+.

bun add @charlie-labs/format-for
# or
npm i @charlie-labs/format-for
# or
yarn add @charlie-labs/format-for
# or
pnpm add @charlie-labs/format-for

Quick start

import { formatFor } from '@charlie-labs/format-for';

const md = `
+++ Summary

Collapsible content

+++

See @riley in #dev and <https://example.com|site>.
`;

const gh = await formatFor.github(md);
const slack = await formatFor.slack(md);
const linear = await formatFor.linear(md);

Outputs for the snippet above (exact values):

GitHub

<details>
<summary>Summary</summary>

Collapsible content

</details>

See @riley in #dev and [site](https://example.com).

Slack

*Summary*
> Collapsible content
See @riley in #dev and <https://example.com|site>.

Linear

+++ Summary

Collapsible content

+++

See @riley in #dev and [site](https://example.com).

Prefer to inject live Slack/Linear defaults (real users/channels, org/team autolinks)? Use the factory:

import {
  createFormatFor,
  createEnvDefaultsProvider,
} from '@charlie-labs/format-for';

const ff = createFormatFor({
  defaults: createEnvDefaultsProvider({
    // optional: override cache namespace and token/TTL; when not provided,
    // SLACK_BOT_TOKEN and LINEAR_API_KEY env vars are used by default
    // namespace: 'my-app:format-for:v1',
    // slack: { token: 'xoxb-…', ttlMs: 10 * 60_000 },
    // linear: { apiKey: 'lin_api_…', ttlMs: 60 * 60_000 },
  }),
});

const out = await ff.slack('Ping @riley in #dev');

Example: one input → three outputs

Below is a realistic mixed‑syntax input (taken from our test fixtures), followed by the exact strings returned for each target.

Input (Markdown, mixed Slack/Linear/GFM)

# Project Alpha: Auth flow update (fixture input)

Short summary: We are migrating the auth callback. FYI <!here> see <https://charlie-labs.slack.com/archives/C12345/p1726800000000|auth-discussion>. Ping <@U02AAAAAA> and <#C02OPS|ops>. Old flow ~deprecated~.

+++ Decisions

- Keep email-first login; remove ~magic-link-only~ path.
- Links: Markdown [spec](https://spec.commonmark.org) and Slack form <https://example.com|Docs>.
- Include a bare URL too: <https://example.org>.

+++ Edge cases

- Safari ITP and cookies.
- If user is SSO-only, show a link back.
- Mention special <!channel> to alert during rollout.

+++

- Table and images should still render cross-platform.

+++

## Tasks

1. Backend
   - Add POST /v2/auth/verify
     - Increase rate limit from 30 to 60
2. Frontend
   - Update callback handler
   - Add retry UI

> Note: rollout starts Monday. Coordinate with #release and @riley.

### Code sample

```ts
// `+++` inside code should NOT create a details block.
export function normalizeEmail(s: string): string {
  return s.trim().toLowerCase();
}
```

### Cases

| Case        | Expected |
| ----------- | -------- |
| Valid email | 200      |
| Bad token   | 401      |

![diagram](https://example.com/flow.png)

HTML allowed tags: <u>Important</u> and <sup>2</sup>.<br>
HTML disallowed inline in a paragraph (to exercise Linear's allowlist): <video src="/noop"></video>

<div>Standalone HTML block that should be stripped on Slack/Linear</div>

Footnote ref[^1].

[^1]: This is a footnote with a Slack user <@U0FOOT> and a Slack link <https://ex.com|ex>.

GitHub output (exact value)

# Project Alpha: Auth flow update (fixture input)

Short summary: We are migrating the auth callback. FYI @here see [auth-discussion](https://charlie-labs.slack.com/archives/C12345/p1726800000000). Ping @U02AAAAAA and #ops. Old flow ~~deprecated~~.

<details>
<summary>Decisions</summary>

- Keep email-first login; remove ~~magic-link-only~~ path.
- Links: Markdown [spec](https://spec.commonmark.org) and Slack form [Docs](https://example.com).
- Include a bare URL too: <https://example.org>.

<details>
<summary>Edge cases</summary>

- Safari ITP and cookies.
- If user is SSO-only, show a link back.
- Mention special @channel to alert during rollout.
</details>

- Table and images should still render cross-platform.
</details>

## Tasks

1. Backend
   - Add POST /v2/auth/verify
     - Increase rate limit from 30 to 60
2. Frontend
   - Update callback handler
   - Add retry UI

> Note: rollout starts Monday. Coordinate with #release and @riley.

### Code sample

```ts
// `+++` inside code should NOT create a details block.
export function normalizeEmail(s: string): string {
  return s.trim().toLowerCase();
}
```

### Cases

| Case        | Expected |
| ----------- | -------- |
| Valid email | 200      |
| Bad token   | 401      |

![diagram](https://example.com/flow.png)

HTML allowed tags: <u>Important</u> and <sup>2</sup>.<br>
HTML disallowed inline in a paragraph (to exercise Linear's allowlist): <video src="/noop"></video>

<div>Standalone HTML block that should be stripped on Slack/Linear</div>

Footnote ref[^1].

[^1]: This is a footnote with a Slack user @U0FOOT and a Slack link [ex](https://ex.com).

Linear output (exact value)

# Project Alpha: Auth flow update (fixture input)

Short summary: We are migrating the auth callback. FYI @here see [auth-discussion](https://charlie-labs.slack.com/archives/C12345/p1726800000000). Ping @U02AAAAAA and #ops. Old flow ~~deprecated~~.

+++ Decisions

- Keep email-first login; remove ~~magic-link-only~~ path.
- Links: Markdown [spec](https://spec.commonmark.org) and Slack form [Docs](https://example.com).
- Include a bare URL too: <https://example.org>.

+++ Edge cases

- Safari ITP and cookies.
- If user is SSO-only, show a link back.
- Mention special @channel to alert during rollout.

+++

- Table and images should still render cross-platform.

+++

## Tasks

1. Backend
   - Add POST /v2/auth/verify
     - Increase rate limit from 30 to 60
2. Frontend
   - Update callback handler
   - Add retry UI

> Note: rollout starts Monday. Coordinate with #release and @riley.

### Code sample

```ts
// `+++` inside code should NOT create a details block.
export function normalizeEmail(s: string): string {
  return s.trim().toLowerCase();
}
```

### Cases

| Case        | Expected |
| ----------- | -------- |
| Valid email | 200      |
| Bad token   | 401      |

![diagram](https://example.com/flow.png)

HTML allowed tags: <u>Important</u> and <sup>2</sup>.<br>
HTML disallowed inline in a paragraph (to exercise Linear's allowlist):

Footnote ref[^1].

[^1]: This is a footnote with a Slack user @U0FOOT and a Slack link [ex](https://ex.com).

Slack output (exact value)

_Project Alpha: Auth flow update (fixture input)_

Short summary: We are migrating the auth callback. FYI <!here> see <https://charlie-labs.slack.com/archives/C12345/p1726800000000|auth-discussion>. Ping <@U02AAAAAA> and <#C02OPS|ops>. Old flow ~deprecated~.

_Decisions_

> • Keep email-first login; remove ~magic-link-only~ path.
> • Links: Markdown <https://spec.commonmark.org|spec> and Slack form <https://example.com|Docs>.
> • Include a bare URL too: <https://example.org|https://example.org>.
>
> _Edge cases_
>
> > • Safari ITP and cookies.
> > • If user is SSO-only, show a link back.
> > • Mention special <!channel> to alert during rollout.
> > • Table and images should still render cross-platform.
> > _Tasks_

1. Backend
   • Add POST /v2/auth/verify
   → Increase rate limit from 30 to 60

2. Frontend
   • Update callback handler
   • Add retry UI

> Note: rollout starts Monday. Coordinate with #release and @riley.

_Code sample_

```
// `+++` inside code should NOT create a details block.
export function normalizeEmail(s: string): string {
  return s.trim().toLowerCase();
}
```

_Cases_

```
Case | Expected
Valid email | 200
Bad token | 401
```

<https://example.com/flow.png|diagram>

HTML allowed tags: &lt;u&gt;Important&lt;/u&gt; and &lt;sup&gt;2&lt;/sup&gt;.&lt;br&gt;
HTML disallowed inline in a paragraph (to exercise Linear's allowlist): &lt;video src="/noop"&gt;&lt;/video&gt;

Footnote ref^[1].

Footnotes:
[1] This is a footnote with a Slack user <@U0FOOT> and a Slack link <https://ex.com|ex>.

Concepts

• Parse once → canonical mdast → render per‑target.
• Predictable degradations with explicit warnings (e.g., Slack tables → code blocks; math → code; images → links; Linear strips disallowed HTML).
• Idempotent output per target; code/inline code is never changed by autolinks or formatting.
• Autolinks and mentions are deterministic and local by default; optional env‑backed defaults provider hydrates Slack users/channels and Linear org/team/user data when tokens are present.

API

formatFor

• formatFor.github(input, options?)
• formatFor.slack(input, options?)
• formatFor.linear(input, options?)

Each returns a Promise<string> with the formatted value for that target.

Factory and env defaults

import {
  createFormatFor,
  createEnvDefaultsProvider,
} from '@charlie-labs/format-for';

const ff = createFormatFor({
  defaults: createEnvDefaultsProvider({
    // optional: override token/TTL/namespace; env fallbacks used by default
    // slack: { token: 'xoxb-…', ttlMs: 10 * 60_000 },
    // linear: { apiKey: 'lin_api_…', ttlMs: 60 * 60_000 },
  }),
});

const out = await ff.github('Ref ENG-123 and say hi to @riley');

FormatOptions (v1)

type FormatOptions = {
  maps?: {
    slack?: {
      users?: Record<string, { id: string; label?: string }>;
      channels?: Record<string, { id: string; label?: string }>;
    };
    linear?: { users?: Record<string, { url: string; label?: string }> };
  };
  autolinks?: Partial<
    Record<
      'github' | 'slack' | 'linear',
      Array<{
        pattern: RegExp;
        urlTemplate: string;
        labelTemplate?: string;
      }>
    >
  >;
  warnings?: {
    mode?: 'console' | 'silent';
    onWarn?: (message: string) => void;
  };
  target?: {
    slack?: {
      lists?: { maxDepth?: number }; // default: 2
      images?: { style?: 'link' | 'url'; emptyAltLabel?: string }; // defaults: style 'link'; emptyAltLabel 'image'
    };
    github?: { breaks?: 'two-spaces' | 'backslash' }; // default: 'two-spaces'
    // Linear options are intentionally not exposed in v1
  };
};

Notes

• Autolinks are normalized to global regex; when provider + caller rules collide, the caller wins.
• Linear’s HTML allowlist is fixed internally: details, summary, u, sub, sup, br.

Target behavior highlights

• GitHub
  • details nodes render as <details><summary>…</summary>…</details> HTML.
  • Hard breaks use two spaces by default (configurable to backslash).
  • Preserves task list state on bare marker lines.
• Slack
  • Headings become bold lines; quotes have readable spacing.
  • Lists deeper than maxDepth flatten with a single warning per render.
  • Images emit as <url|label> links (or bare URLs) with a warning; style is configurable.
  • Tables → fenced code with a warning; inline/display math → code/code blocks with warnings.
  • Footnotes become ^[n] plus appended refs; all HTML stripped with a warning.
• Linear
  • +++ Summary → collapsible; disallowed HTML is stripped while keeping surrounding text.
  • Slack/Linear mentions normalize to links/plain text as appropriate.

Recipes

• Autolink Linear issues (multiple team keys):

  const rules = [
    {
      pattern: /\b(BOT-\d+)\b/g,
      urlTemplate: 'https://linear.app/charlie/issue/$0',
    },
    {
      pattern: /\b(ENG-\d+)\b/g,
      urlTemplate: 'https://linear.app/charlie/issue/$0',
    },
  ];
  await formatFor.github(text, {
    autolinks: { github: rules, linear: rules, slack: rules },
  });

• Route warnings to your logger and silence console output:

  await formatFor.slack(md, {
    warnings: { mode: 'silent', onWarn: (m) => log.warn(m) },
  });

• Preserve GitHub backslash hard breaks:

  await formatFor.github(md, { target: { github: { breaks: 'backslash' } } });

• Use live Slack/Linear defaults (if tokens exist):

  const ff = createFormatFor({ defaults: createEnvDefaultsProvider() });
  const text = await ff.slack('See @riley in #dev');

Warnings and safety

• Slack: HTML stripped; images/tables/math/footnotes degrade with clear warnings; link labels are sanitized.
• Linear: strict inline HTML allowlist; disallowed tags are removed and paragraphs preserved; warnings are emitted.
• Code blocks and inline code are never altered by autolinks or formatting passes.

Control warnings with warnings.mode and warnings.onWarn.

Performance and idempotency

• One parse; lightweight renderers; no network calls unless you opt in via the factory.
• Formatting is idempotent for a given target (running twice yields the same string).

Contributing

Dev commands:

bun install
bun run typecheck
bun run lint
bun run test

Fixtures live under src/markdown/__tests__/__fixtures__/. To regenerate example outputs locally, run: bun scripts/gen-fixtures.ts.

License

MIT