Add WhatsApp adapter with proper architectural foundation

[KlementMultiverse]

Problem

The Chat SDK supports 7 platforms (Slack, Teams, Google Chat, Discord, Telegram, GitHub, Linear) but WhatsApp is missing despite clear demand—a WhatsApp adapter PR was previously rejected, and WhatsApp is now a critical business messaging platform.

More fundamentally, there's no clear architectural pattern for adding new adapters. Issue #178 identifies this as a gap: the Gateway/Transport Listener Adapter Interface needs definition before scaling to more platforms.

Why this matters

1. Business case: WhatsApp has 500M+ active business accounts and is the primary messaging platform in many regions. Users building bots for these markets need WhatsApp support.

2. Architecture clarity: Each new adapter follows ad-hoc patterns. Without a formal interface, maintainers must review each adapter's design individually. This slows contribution velocity and creates inconsistent behavior across platforms.

3. Test coverage gap: The test matrix in the README shows feature parity by platform, but there's no validation that new adapters actually implement the promised interface. A WhatsApp adapter currently has no specification to conform to.

What good looks like

Part 1: Define the Adapter Interface

Create docs/ADAPTER_INTERFACE.md that specifies:

• Required event handlers: onNewMention(), onSubscribedMessage(), onReaction(), etc. with TypeScript signatures
• Platform capability flags: Each adapter declares what it supports (e.g., supportsModals: false for WhatsApp)
• Message formatting rules: How JSX cards map to platform-specific formats (Block Kit vs Adaptive Cards vs WhatsApp templates)
• State management contract: How adapters interact with state parameter from Chat constructor
• Error handling expectations: Required vs optional error types

Part 2: WhatsApp Adapter Implementation

With the interface defined, implement @chat-adapter/whatsapp that:

• Uses the official WhatsApp Business API (Cloud API)
• Implements core handlers: mentions, messages, reactions, media uploads
• Explicitly marks unsupported features in docs (no modals, no native emoji reactions)
• Includes adapter tests validating conformance to ADAPTER_INTERFACE.md
• Updates the platform matrix in README with accurate feature support

Acceptance criteria

• ADAPTER_INTERFACE.md exists with formal TypeScript interface definitions
• Existing adapters (Slack, Teams, Discord) are audited against the interface and any gaps documented
• @chat-adapter/whatsapp implements the interface with passing tests
• README platform matrix updated with WhatsApp feature support
• New adapters can follow the documented pattern without design review overhead

Reference

Related: #178 (architecture), #147 (messenger adapter pattern)

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Contributed by Klement Gunndu

[haydenbleasel]

@KlementMultiverse #179 was closed in favor of #102 which we're actively working on

[KlementMultiverse]

Withdrawing this issue. Apologies for any inconvenience.