feat: wire chat UI to Responses API with streaming

Add ChatController that proxies POST /api/chat to the local Responses API
(localhost:8317/v1/responses) with SSE streaming. Client reads tokens via
SetBrowserResponseStreamingEnabled and renders them incrementally. Includes
thinking indicator, input disabled during streaming, and error handling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

openspec/changes/archive/2026-04-04-wire-responses-api/tasks.md

@@ -0,0 +1,29 @@
+## 1. Shared Models
+
+- [x] 1.1 Create ChatRequest.cs in ChatAgent.Shared/Models with a Messages list property
+
+## 2. API Backend
+
+- [x] 2.1 Add appsettings.json to ChatAgent.Api with ResponsesApi:BaseUrl and ResponsesApi:Model
+- [x] 2.2 Register an HttpClient for the Responses API proxy in Api Program.cs
+- [x] 2.3 Create ChatController with POST /api/chat that proxies to the Responses API with streaming
+- [x] 2.4 Parse Responses API SSE stream, extract response.output_text.delta events, re-emit as simplified SSE to client
+
+## 3. Client Streaming
+
+- [x] 3.1 Add a streaming SendChatAsync method to ChatApiClient that uses SetBrowserResponseStreamingEnabled and HttpCompletionOption.ResponseHeadersRead
+- [x] 3.2 Parse the simplified SSE stream line-by-line, yielding text deltas
+
+## 4. Chat Page Updates
+
+- [x] 4.1 Replace hardcoded response in Chat.razor with a call to ChatApiClient.SendChatAsync
+- [x] 4.2 Append tokens to the assistant message incrementally with StateHasChanged after each delta
+- [x] 4.3 Add a thinking indicator shown until the first token arrives
+- [x] 4.4 Disable input field and send button while streaming is in progress
+- [x] 4.5 Handle errors — display error message if API call fails
+- [x] 4.6 Auto-scroll during streaming (not just at the end)
+
+## 5. Verify
+
+- [x] 5.1 Run dotnet build to confirm no errors
+- [ ] 5.2 Manually verify: send a message, see streaming response from Claude

openspec/changes/wire-responses-api/tasks.md

@@ -1,29 +0,0 @@
-## 1. Shared Models
-
-- [ ] 1.1 Create ChatRequest.cs in ChatAgent.Shared/Models with a Messages list property
-
-## 2. API Backend
-
-- [ ] 2.1 Add appsettings.json to ChatAgent.Api with ResponsesApi:BaseUrl and ResponsesApi:Model
-- [ ] 2.2 Register an HttpClient for the Responses API proxy in Api Program.cs
-- [ ] 2.3 Create ChatController with POST /api/chat that proxies to the Responses API with streaming
-- [ ] 2.4 Parse Responses API SSE stream, extract response.output_text.delta events, re-emit as simplified SSE to client
-
-## 3. Client Streaming
-
-- [ ] 3.1 Add a streaming SendChatAsync method to ChatApiClient that uses SetBrowserResponseStreamingEnabled and HttpCompletionOption.ResponseHeadersRead
-- [ ] 3.2 Parse the simplified SSE stream line-by-line, yielding text deltas
-
-## 4. Chat Page Updates
-
-- [ ] 4.1 Replace hardcoded response in Chat.razor with a call to ChatApiClient.SendChatAsync
-- [ ] 4.2 Append tokens to the assistant message incrementally with StateHasChanged after each delta
-- [ ] 4.3 Add a thinking indicator shown until the first token arrives
-- [ ] 4.4 Disable input field and send button while streaming is in progress
-- [ ] 4.5 Handle errors — display error message if API call fails
-- [ ] 4.6 Auto-scroll during streaming (not just at the end)
-
-## 5. Verify
-
-- [ ] 5.1 Run dotnet build to confirm no errors
-- [ ] 5.2 Manually verify: send a message, see streaming response from Claude

openspec/specs/chat-streaming/spec.md

@@ -0,0 +1,55 @@
+## Purpose
+
+Define the streaming AI response pipeline — backend proxy to the Responses API, SSE delivery to the WASM client, configuration, and error handling.
+
+## Requirements
+
+### Requirement: Chat endpoint proxies to Responses API
+
+The API backend SHALL expose `POST /api/chat` that accepts a list of messages and proxies the request to the local Responses API at a configurable base URL using the `POST /v1/responses` endpoint.
+
+#### Scenario: Successful proxy request
+
+- **WHEN** the client sends a POST to `/api/chat` with a message list
+- **THEN** the API forwards the messages to the Responses API with the configured model and returns the response
+
+### Requirement: Streaming response delivery
+
+The API backend SHALL stream the Responses API's SSE events back to the WASM client as `text/event-stream`, forwarding `response.output_text.delta` events so the client can render tokens incrementally.
+
+#### Scenario: Tokens stream to client
+
+- **WHEN** the Responses API emits `response.output_text.delta` events
+- **THEN** the backend forwards each delta as an SSE event to the client containing the text fragment
+
+#### Scenario: Stream completes
+
+- **WHEN** the Responses API emits `response.completed`
+- **THEN** the backend signals stream completion to the client
+
+### Requirement: Configurable proxy target
+
+The Responses API base URL and model name SHALL be configurable via `appsettings.json` in the API project, not hardcoded.
+
+#### Scenario: Configuration read at startup
+
+- **WHEN** the API starts
+- **THEN** it reads `ResponsesApi:BaseUrl` and `ResponsesApi:Model` from configuration
+
+### Requirement: Client streams from backend
+
+The WASM client SHALL call `POST /api/chat` with `SetBrowserResponseStreamingEnabled(true)` and `HttpCompletionOption.ResponseHeadersRead`, then iterate the SSE stream to update the UI token by token.
+
+#### Scenario: Client reads streaming response
+
+- **WHEN** the client sends a chat request
+- **THEN** it reads the response stream incrementally and appends each text delta to the assistant message in real time
+
+### Requirement: Error propagation
+
+If the Responses API returns an error or is unreachable, the API backend SHALL return an appropriate HTTP error status and the client SHALL display the error to the user.
+
+#### Scenario: Proxy unreachable
+
+- **WHEN** the Responses API is not running
+- **THEN** the client displays an error message instead of an assistant response

openspec/specs/chat-ui/spec.md

@@ -42,14 +42,33 @@ The chat page SHALL provide a text input area at the bottom of the page where th
 - **WHEN** the user attempts to send an empty or whitespace-only message
 - **THEN** nothing is sent and no message is added
 
-### Requirement: Hardcoded response
+#### Scenario: Input disabled during streaming
 
-In this phase, the assistant SHALL reply with a hardcoded message to every user input. This stubs the AI integration point for future phases.
+- **WHEN** the assistant is currently streaming a response
+- **THEN** the input field and send button are disabled until streaming completes
 
-#### Scenario: Bot replies to any input
+### Requirement: Thinking indicator
+
+The chat page SHALL show a visual indicator while waiting for the first token from the assistant.
+
+#### Scenario: Indicator shown during wait
+
+- **WHEN** the user sends a message and the assistant has not yet started streaming
+- **THEN** a thinking indicator (e.g., animated dots) is shown in the assistant message area
+
+#### Scenario: Indicator replaced by content
+
+- **WHEN** the first token arrives from the stream
+- **THEN** the thinking indicator is replaced by the streamed text
+
+### Requirement: Streaming AI response
+
+The assistant SHALL reply with a real AI response streamed from the backend API. Tokens appear incrementally as they arrive.
+
+#### Scenario: Bot replies with streamed AI response
 
 - **WHEN** the user sends any message
-- **THEN** the assistant replies with a hardcoded response (e.g., "This is a placeholder response. AI integration coming soon!")
+- **THEN** the assistant message appears and grows token by token as the stream delivers text
 
 ### Requirement: Auto-scroll