Blob Storage Service

A lightweight MCP data broker that stores large binary payloads under a UUID, enabling MCP clients and agents to exchange massive results without flooding the LLM context window.

Architecture

MCP Client / LLM Agent
        │
        │  tool call (large result)
        ▼
   MCP Server  ──store_blob──▶  Blob Storage Service  ──▶  SQLite + local files
        │                              │
        │  returns UUID                │  retrieve_blob(uuid)
        ◀──────────────────────────────┘

The agent only ever sees a UUID token — not the raw bytes.

Project Structure

src/blob_storage/
├── __init__.py       # package metadata
├── config.py         # pydantic-settings (env / .env)
├── models.py         # SQLAlchemy ORM + Pydantic schemas
├── database.py       # async engine, session factory, init_db()
├── storage.py        # StorageBackend ABC + LocalFileBackend
├── service.py        # BlobService (orchestration)
├── api.py            # FastAPI REST API
└── mcp_server.py     # MCP server (HTTP Streamable, /mcp)

tests/
├── conftest.py       # shared fixtures
├── test_api.py       # API integration tests
└── test_service.py   # service unit tests

Quickstart

Install dependencies

uv sync

Start the REST API

uv run blob-api                            # defaults from config / .env
uv run blob-api --host 0.0.0.0 --port 9000 # override at runtime

Interactive docs available at http://<host>:<port>/docs.

Start the MCP server

uv run blob-mcp                            # defaults: 0.0.0.0:8001
uv run blob-mcp --host 0.0.0.0 --port 9001

The MCP endpoint is served at http://<host>:<port>/mcp.

Configuration

Settings are read from environment variables or a .env file in the project root.

Variable	Default	Description
STORAGE_DIR	./data/blobs	Directory for blob files
DATABASE_URL	sqlite+aiosqlite:///./data/metadata.db	SQLAlchemy async DB URL
DEFAULT_TTL_SECONDS	7776000	Default TTL — 90 days. Use 0 for no expiry.
MAX_BLOB_SIZE_BYTES	524288000	Max upload size (500 MB)
API_HOST	0.0.0.0	REST API bind address
API_PORT	8000	REST API port
MCP_HOST	0.0.0.0	MCP server bind address
MCP_PORT	8001	MCP server port
MCP_API_BASE_URL	http://127.0.0.1:8000	REST API URL used internally by the MCP server
SWEEP_INTERVAL_SECONDS	3600	How often the TTL cleanup job runs

CLI flags (--host, --port) take precedence over config/env for the respective server.

REST API Reference

Method	Path	Description
GET	/health	Liveness check
POST	/blobs	Store a blob
GET	/blobs/{uuid}	Download a blob
GET	/blobs/{uuid}/meta	Metadata only (no payload)
DELETE	/blobs/{uuid}	Delete a blob

Store a blob

Pass metadata as custom request headers:

Header	Default	Description
X-Mime-Type	application/octet-stream	MIME type of the payload
X-Ttl-Seconds	config default	Override TTL; 0 = never expires
X-Origin	—	Free-form source identifier
X-Tags	—	JSON object for arbitrary labels

curl -X POST http://localhost:8000/blobs \
  -H "Content-Type: application/octet-stream" \
  -H "X-Mime-Type: application/json" \
  -H "X-Origin: my-mcp-server" \
  --data-binary '{"key": "value"}'

Response:

{
  "uuid": "3fa85f64-5717-4562-b3fc-2c963f66afa6",
  "expires_at": "2026-05-30T16:42:07Z",
  "size_bytes": 16
}

MCP Integration

The MCP server uses the Streamable HTTP transport. Configure your MCP host to connect over HTTP rather than spawning a subprocess:

{
  "mcpServers": {
    "blob-storage": {
      "type": "http",
      "url": "http://127.0.0.1:8001/mcp"
    }
  }
}

Available tools

Tool	Input	Output
store_blob	data_b64, mime_type, ttl_seconds?, origin?, tags?	{ uuid, expires_at, size_bytes }
retrieve_blob	uuid	{ uuid, mime_type, data_b64 }
get_blob_meta	uuid	metadata JSON
delete_blob	uuid	{ deleted: uuid }

All binary data is base64-encoded in MCP tool calls (MCP messages are JSON/text).

Running Tests

uv run pytest tests/ -v

Extending the Storage Backend

To use S3 or MinIO in production, implement the StorageBackend abstract class in storage.py:

from blob_storage.storage import StorageBackend

class S3Backend(StorageBackend):
    async def store(self, blob_uuid: str, data: bytes) -> None: ...
    async def retrieve(self, blob_uuid: str) -> bytes: ...
    async def delete(self, blob_uuid: str) -> None: ...

Then inject it into BlobService and the _backend singleton in api.py.