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+# Claude ACP Raw SDK Message Support Design
+
+- Date: 2026-04-14
+- Status: Approved (implementation pending)
+- Scope: Protocol support only (backend receives and forwards, frontend event layer receives)
+
+## 1. Background
+
+Upstream PR `agentclientprotocol/claude-agent-acp#527` was merged on **2026-04-13**.
+It introduces opt-in emission of Claude Code SDK raw stream messages via ACP extension notification:
+
+- Method: `"_claude/sdkMessage"`
+- Params shape:
+  - `sessionId: string`
+  - `message: object` (raw SDK message)
+
+Opt-in is configured via session `_meta`:
+
+```json
+{
+  "claudeCode": {
+    "emitRawSDKMessages": true
+  }
+}
+```
+
+User decision for this project:
+
+- Keep npm/registry at current version (`0.27.0`)
+- Implement **protocol support only**
+- Enable raw SDK messages for Claude by default with `true`
+
+## 2. Goals and Non-Goals
+
+### Goals
+
+1. For `AgentType::ClaudeCode`, send `_meta.claudeCode.emitRawSDKMessages = true` in session setup.
+2. Receive `"_claude/sdkMessage"` notifications in backend connection loop.
+3. Convert and forward notifications to frontend via existing `acp://event` event bridge.
+4. Extend frontend `AcpEvent` typing to accept this event without changing UI behavior.
+
+### Non-Goals
+
+1. No UI visualization for raw SDK messages in this change.
+2. No DB persistence for raw SDK messages.
+3. No behavior changes for non-Claude agents.
+4. No protocol generalization for all extension notifications in this iteration.
+
+## 3. Selected Approach
+
+Selected approach: **Claude-specific minimal integration**.
+
+Why:
+
+- Minimal risk and smallest change surface.
+- Matches explicit scope: protocol plumbing only.
+- Preserves current runtime and rendering behavior.
+
+Rejected for now:
+
+- Global extension-notification bus (larger scope, more noise, higher maintenance).
+- Persistence/debug history (extra storage and lifecycle complexity).
+
+## 4. Architecture Changes
+
+## 4.1 Backend session meta injection
+
+File: `src-tauri/src/acp/connection.rs`
+
+When constructing `NewSessionRequest` and `LoadSessionRequest`:
+
+- If `agent_type == AgentType::ClaudeCode`, set request `meta` with:
+  - `claudeCode.emitRawSDKMessages = true`
+
+This preserves opt-in semantics upstream while default-enabling for Claude in Codeg.
+
+## 4.2 Backend extension notification handling
+
+File: `src-tauri/src/acp/connection.rs`
+
+Current code mainly consumes typed `SessionNotification` updates. This change adds a path for untyped notifications:
+
+- Match dispatch notification method.
+- If method is `"_claude/sdkMessage"`, parse params:
+  - `sessionId`
+  - `message`
+- Emit new app event through `acp://event`.
+- Ignore parse failures and continue session loop.
+
+## 4.3 Backend event model extension
+
+File: `src-tauri/src/acp/types.rs`
+
+Add a new `AcpEvent` variant:
+
+- `ClaudeSdkMessage`
+  - `connection_id: String`
+  - `session_id: String`
+  - `message: serde_json::Value`
+
+This keeps payload raw and unopinionated.
+
+## 4.4 Frontend type and event switch support
+
+Files:
+
+- `src/lib/types.ts`
+- `src/contexts/acp-connections-context.tsx`
+
+Changes:
+
+- Extend TS `AcpEvent` union with:
+  - `type: "claude_sdk_message"`
+  - `connection_id: string`
+  - `session_id: string`
+  - `message: unknown`
+- Add a no-op `case "claude_sdk_message"` in event handling.
+
+No UI state mutation is required in this iteration.
+
+## 5. Data Flow
+
+1. Codeg starts Claude ACP session.
+2. Codeg sends `session/new` or `session/load` with `_meta.claudeCode.emitRawSDKMessages=true`.
+3. `claude-agent-acp` emits extension notifications `"_claude/sdkMessage"`.
+4. Codeg backend parses and maps to `AcpEvent::claude_sdk_message`.
+5. Event is pushed via existing `acp://event` channel.
+6. Frontend receives typed event and safely ignores it (for now).
+
+## 6. Error Handling and Compatibility
+
+1. If upstream ignores meta, session still works; feature simply produces no raw SDK events.
+2. If `"_claude/sdkMessage"` payload is malformed, log and ignore that notification.
+3. Do not fail prompt/session loops due to extension-message parse errors.
+4. Keep all existing typed `SessionUpdate` flows unchanged.
+5. Restrict meta injection to Claude only.
+
+## 7. Validation Plan
+
+Manual/protocol validation:
+
+1. Confirm session setup request includes `_meta.claudeCode.emitRawSDKMessages=true` for Claude.
+2. Confirm `"_claude/sdkMessage"` notifications are received and forwarded as `claude_sdk_message` events.
+3. Confirm malformed ext payloads do not break turns.
+4. Confirm non-Claude agents have unchanged behavior.
+
+Project checks:
+
+1. `pnpm eslint .`
+2. `pnpm build`
+3. `cd src-tauri && cargo check`
+4. `cd src-tauri && cargo check --bin codeg-server --no-default-features`
+
+## 8. Acceptance Criteria
+
+1. Claude connections default-enable raw SDK emission via session meta.
+2. Backend forwards raw SDK notifications into frontend event layer.
+3. Frontend compiles and receives new event type without UI regressions.
+4. No persistence and no visual rendering changes.
+5. No regressions on existing ACP message paths.