feat(slack): use native markdown_text for outgoing messages#440
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dancer
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May 5, 2026
Slack now natively renders markdown via the `markdown_text` parameter on
chat.postMessage / postEphemeral / update / scheduleMessage and via
response_url payloads. The adapter passes markdown through directly instead
of converting to mrkdwn.
- Tables, headings, code fences, blockquotes, and nested lists render
natively in Slack instead of falling back to ASCII / mrkdwn.
- `string` and `{ raw }` messages still go to `text` (preserves literal `*`).
- `{ markdown }` and `{ ast }` messages go to `markdown_text` (12k char limit).
- `renderWithTableBlocks`, `toBlocksWithTable`, `mdastTableToSlackBlock`,
and the AST→mrkdwn renderer (`fromAst` / `nodeToMrkdwn`) are removed.
- `SlackMarkdownConverter` alias is removed; use `SlackFormatConverter`.
- `renderFormatted(ast)` now returns standard markdown (was mrkdwn).
- Incoming `message` events still arrive as mrkdwn and are parsed unchanged.
Net -473 lines across markdown.ts and the five sender call sites.
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This was referenced May 29, 2026
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Summary
Slack now natively supports markdown via the
markdown_textfield onchat.postMessage/postEphemeral/update/scheduleMessage. Previously the adapter converted outgoing markdown to mrkdwn and synthesized Block Kittableblocks (with ASCII fallback) to work around mrkdwn's lack of table support.This PR passes markdown straight through on the Slack Web API methods that support
markdown_text. Tables, headings, fenced code blocks, blockquotes, and nested lists now render natively in those Slack messages instead of degrading.Interactive
response_urlpayloads are the one exception: e2e testing showed Slack rejectsmarkdown_textthere with{"ok":false,"error":"no_text"}, so response_url edits intentionally keep a small mrkdwn text fallback.Builds on my prev PR #349, which started using Slack's native markdown rendering for streamed tables.
What changes
{ markdown }and{ ast }messages on Slack Web API methods ->markdown_textfieldresponse_urledits -> mrkdwntextfallbackstringand{ raw }messages -> stilltext(preserves literal*/_)SlackMarkdownConverteralias (useSlackFormatConverter)renderFormatted(ast)now returns standard markdown instead of mrkdwnmessageevents are still parsed as mrkdwn — unchangedTest plan
toSlackPayload, response_url fallback rendering, and response_url edit payloadspackages/integration-tests/src/slack.test.ts) updated formarkdown_textwire formatpnpm --filter @chat-adapter/slack typecheckpnpm checkpnpm --filter @chat-adapter/slack buildmarkdown_text; response_url fallback uses mrkdwntextBefore / after screenshots:
