Agent UI Server

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Component: Agent UI Server - FastAPI backend for the desktop chat application
Module: gaia.ui
Import: from gaia.ui.server import create_app
Source: src/gaia/ui/

Overview

The Agent UI Server is a FastAPI-based backend that powers the GAIA Chat desktop application. It provides REST API endpoints for session management, real-time chat with SSE streaming, document library management, and system health monitoring. Key Design Decisions:

SQLite for zero-configuration persistence (WAL mode for concurrent reads)
FastAPI for automatic OpenAPI docs and Pydantic validation
SSE (Server-Sent Events) for streaming chat responses
Hybrid document model — global library with per-session attachment
Shared database between CLI and desktop app

Module Structure

src/gaia/ui/
  __init__.py          # Package init
  server.py            # FastAPI app factory and endpoints
  database.py          # ChatDatabase class (SQLite)
  models.py            # Pydantic request/response models
  sse_handler.py       # SSE output handler for streaming agent events
  tunnel.py            # Tunnel manager for mobile access via ngrok

Server (`server.py`)

App Factory

def create_app(db_path: str = None) -> FastAPI:

Creates a configured FastAPI application with:

CORS middleware (allows all origins for local use)
Database initialization
All API endpoint registration
Static file serving (for web frontend, when dist/ build exists)

Default port: 4200

Endpoint Registration

All endpoints are registered as closures within create_app(), sharing the db instance via closure scope. The database is also stored on app.state.db for test access.

Streaming Implementation

Chat streaming uses a thread-based producer/consumer pattern:

A background thread calls AgentSDK.send_stream() synchronously
Chunks are placed into a queue.Queue
The async generator polls the queue and yields SSE events
On completion, the full response is saved to the database

# SSE event format
data: {"type": "chunk", "content": "partial text"}
data: {"type": "done", "message_id": 42, "content": "full response"}
data: {"type": "error", "content": "error description"}

Helper Functions

Function	Description
`_session_to_response(session)`	Convert DB dict to `SessionResponse`
`_message_to_response(msg)`	Convert DB dict to `MessageResponse` (parses JSON `rag_sources`)
`_doc_to_response(doc)`	Convert DB dict to `DocumentResponse`
`_sanitize_document_path(user_path)`	Validate and sanitize user-provided file paths (null bytes, extension allowlist, path resolution)
`_sanitize_static_path(base_dir, user_path)`	Validate URL paths for static file serving stay within base directory
`_validate_file_path(filepath)`	Check path is absolute, no null bytes, and has allowed extension
`_compute_file_hash(filepath)`	SHA-256 hash for document deduplication
`_index_document(filepath)`	Index file via RAG SDK, returns chunk count
`_get_chat_response(db, session, request)`	Non-streaming LLM response
`_stream_chat_response(db, session, request)`	SSE streaming generator

Database (`database.py`)

Schema

Four tables with foreign key relationships:

documents          -- Global document library
sessions           -- Chat conversations
session_documents  -- Many-to-many attachment relationship
messages           -- Conversation messages with optional RAG metadata

Configuration

Foreign keys: Enabled (PRAGMA foreign_keys = ON)
Journal mode: WAL (PRAGMA journal_mode = WAL)
Thread safety: check_same_thread=False (FastAPI runs across threads)
Row factory: sqlite3.Row for dict-like access

Transaction Management

@contextmanager
def _transaction(self):
    """Execute operations atomically."""
    try:
        yield
        self._conn.commit()
    except Exception:
        self._conn.rollback()
        raise

Document Deduplication

Documents are deduplicated by SHA-256 file hash. If a document with the same hash already exists, the existing record is returned with an updated last_accessed_at timestamp.

Document Upload Paths

The server exposes two document upload endpoints, each for a different source of truth:

Endpoint	Input	Used By	Storage
`POST /api/documents/upload-path`	JSON `{filepath}` — absolute server-side path	File browser (user picks an existing file)	Indexes in place; server does NOT own the file
`POST /api/documents/upload`	`multipart/form-data` file blob	Drag-and-drop in browser mode / modern Electron	File is persisted under `~/.gaia/documents/<uuid>.<ext>` and owned by the server

The blob endpoint exists because browser File objects never expose a filesystem path, and since Electron 32 File.path has been removed as well (see issue #728). It streams the upload with a running size check (capped at 20 MB), computes a SHA-256 hash, short-circuits on dedup by hash, and only then promotes the file into the managed directory via an atomic os.replace. On failure, partial and orphan files are cleaned up. When a server-owned document is deleted (DELETE /api/documents/{id}), the on-disk file under ~/.gaia/documents/ is also unlinked as a best-effort cleanup. User-owned files from upload-path are left untouched.

Cascade Deletes

Deleting a session cascades to its messages and session_documents entries
Deleting a document cascades to session_documents entries

Models (`models.py`)

All 14 Pydantic models:

Model	Type	Used By
`SystemStatus`	Response	`GET /api/system/status`
`CreateSessionRequest`	Request	`POST /api/sessions`
`UpdateSessionRequest`	Request	`PUT /api/sessions/{session_id}`
`SessionResponse`	Response	Session endpoints
`SessionListResponse`	Response	`GET /api/sessions`
`ChatRequest`	Request	`POST /api/chat/send`
`ChatResponse`	Response	`POST /api/chat/send` (non-streaming)
`SourceInfo`	Nested	RAG source citations
`MessageResponse`	Response	`GET /api/sessions/{session_id}/messages`
`MessageListResponse`	Response	`GET /api/sessions/{session_id}/messages`
`DocumentResponse`	Response	Document endpoints
`DocumentListResponse`	Response	`GET /api/documents`
`DocumentUploadRequest`	Request	`POST /api/documents/upload-path`
`UploadFile` (multipart)	Request	`POST /api/documents/upload` (blob)
`AttachDocumentRequest`	Request	`POST /api/sessions/{session_id}/documents`

Dependencies

Required

Package	Purpose
`fastapi`	Web framework
`uvicorn`	ASGI server
`pydantic`	Request/response validation
`httpx`	Async HTTP client (Lemonade health checks)

Optional

Package	Purpose
`psutil`	Memory availability check in system status
`gaia.chat.sdk`	LLM communication (AgentSDK)
`gaia.rag.sdk`	Document indexing (RAGSDK)

Error Handling

Error	HTTP Code	Behavior
Session not found	404	`"Session not found"`
Document not found	404	`"Document not found"` or `"File not found: {path}"`
Path is not a file	400	`"Path is not a file"`
Unsupported export format	400	`"Unsupported format: {format}"`
LLM error (non-streaming)	200	Error message in response text
LLM error (streaming)	200	SSE error event: `{"type": "error", "content": "..."}`
Database locked	500	SQLite WAL mode minimizes this

Security Considerations

CORS: Uses an allowlist of localhost origins plus ngrok regex. The standalone server (python -m gaia.ui.server) binds to localhost by default. Note: gaia chat --ui binds to 0.0.0.0 for Electron/browser access
No authentication: Designed for single-user local use. Do not expose to untrusted networks
File path validation: The upload-path endpoint sanitizes all user-provided paths:
- Null byte rejection (prevents path injection)
- Path resolution to absolute canonical paths (eliminates .. traversal)
- File extension allowlist (only document/code file types accepted)
- See _sanitize_document_path() in server.py
Static file serving: URL paths are validated to stay within the dist/ directory via _sanitize_static_path(), preventing directory traversal
SQL injection: Prevented by parameterized queries throughout
No telemetry: No data is sent externally

Core Framework

SDKs

Infrastructure

Code Infrastructure

Tool Mixins

Packaging

Agents & Apps

Overview

Module Structure

Server (`server.py`)

App Factory

Endpoint Registration

Streaming Implementation

Helper Functions

Database (`database.py`)

Schema

Configuration

Transaction Management

Document Deduplication

Document Upload Paths

Cascade Deletes

Models (`models.py`)

Dependencies

Required

Optional

Error Handling

Security Considerations

Core Framework

SDKs

Infrastructure

Code Infrastructure

Tool Mixins

Packaging

Agents & Apps

​Overview

​Module Structure

​Server (server.py)

​App Factory

​Endpoint Registration

​Streaming Implementation

​Helper Functions

​Database (database.py)

​Schema

​Configuration

​Transaction Management

​Document Deduplication

​Document Upload Paths

​Cascade Deletes

​Models (models.py)

​Dependencies

​Required

​Optional

​Error Handling

​Security Considerations

Overview

Module Structure

Server (`server.py`)

App Factory

Endpoint Registration

Streaming Implementation

Helper Functions

Database (`database.py`)

Schema

Configuration

Transaction Management

Document Deduplication

Document Upload Paths

Cascade Deletes

Models (`models.py`)

Dependencies

Required

Optional

Error Handling

Security Considerations