feat(chat): add callbackUrl to buttons and modals

[dancer]

summary

add callbackUrl prop to Button and Modal. when a button is clicked or modal submitted, the SDK POSTs action data to the callback URL in addition to firing existing handlers. enables awaitable button/modal patterns with webhook-based workflow engines

• callback tokens stored server-side in state adapter (30-day TTL), only a short token goes in the platform payload
• original button value stored alongside the callback URL in state, keeping encoded values to 21 chars across all platforms
• works on Slack, Teams, Google Chat, WhatsApp, Telegram, and Discord
• Discord encodes value in custom_id (100 char limit, throws ValidationError if exceeded)
• Telegram's 64-byte callback_data limit accommodated by token-only encoding
• callback POST runs in parallel with handler execution, awaited at the end for serverless safety
• modal callback POST uses waitUntil to avoid blocking the submit response
• callbackUrl processed in thread.post(), postEphemeral(), schedule(), edit(), and all channel equivalents

test plan

• callback-url unit tests: encode/decode, processCardCallbackUrls (flat, nested sections, immutability), resolveCallbackUrl (stored, legacy format, missing), postToCallbackUrl (success, non-2xx, network error)
• chat.test.ts integration tests: action handler decode + POST, no-value token, handlers fire alongside callback, expired/unknown token preserves raw value, modal submit POST
• channel.test.ts: callback URL encoding on channel.post
• discord cards.test.ts: encode/decode round-trip, validation throws on overflow, card rendering with values

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
chat	Ready	Preview, Comment, Open in v0	May 8, 2026 0:17am
chat-sdk-nextjs-chat	Ready	Preview, Comment, Open in v0	May 8, 2026 0:17am

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	npm/​workflow@​4.2.4

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[visyat]

Looks good for the most part. The workflow example you added to the example app is very interesting – may be worth adding to the "Durable Chat Agents" guide as well. I would really like replay tests, and workflow integration tests ideally!!