Build a MinIO Object Storage MCP Server for Agentic Document Retrieval in 2026

AI agents that need to read documents, images, or datasets from object storage face a binary choice: store S3 credentials in the agent's context (catastrophic if compromised) or deny file access entirely (useless for data-heavy workflows). Neither option works in production.

This FastMCP MinIO server generates presigned URLs on demand. When an agent needs to read a file, it calls get_file_url with the bucket and key. The server generates a time-limited presigned URL (default 5 minutes) that grants read-only access to that specific object. The agent downloads the file via the presigned URL without ever receiving MinIO credentials. Every access is logged, every URL expires, and credential exposure is zero.

Architecture

Claude / Cursor Agent
        │
        ▼ MCP Protocol
┌───────────────────┐
│  MinIO MCP Server │
│  (FastMCP + S3)   │
├───────────────────┤
│ • Presigned URLs   │
│ • Metadata Search  │
│ • Scoped Access    │
│ • Audit Logging    │
└────────┬──────────┘
         │ S3 API (HTTPS)
         ▼
┌───────────────────┐
│  MinIO / S3       │
│  • Buckets        │
│  • Objects        │
│  • Versioning     │
└───────────────────┘

File Structure

minio-mcp-server/
├── src/
│   ├── server.ts          # FastMCP server with MinIO tools
│   ├── minio-client.ts    # MinIO S3 client wrapper
│   ├── presigner.ts       # Presigned URL generation
│   └── metadata.ts        # Object metadata indexing
├── config.yaml
├── package.json
└── tsconfig.json

FastMCP Server

// src/server.ts
import { FastMCP } from "fastmcp";
import { z } from "zod";
import { MinIOClientWrapper } from "./minio-client.js";
import { Presigner } from "./presigner.js";
import { MetadataIndex } from "./metadata.js";

const minioEndpoint = process.env.MINIO_ENDPOINT || "localhost:9000";
const minioAccessKey = process.env.MINIO_ACCESS_KEY || "";
const minioSecretKey = process.env.MINIO_SECRET_KEY || "";
const defaultTtl = parseInt(process.env.PRESIGN_TTL || "300"); // 5 min

const minio = new MinIOClientWrapper(minioEndpoint, minioAccessKey, minioSecretKey);
const presigner = new Presigner(minio, defaultTtl);
const metadata = new MetadataIndex(minio);

const server = new FastMCP({
  name: "minio-documents",
  version: "1.0.0",
});

// Tool: Get presigned download URL for a file
server.tool(
  "get_file_url",
  "Get a time-limited presigned URL to download a file from MinIO",
  {
    bucket: z.string().describe("Bucket name"),
    key: z.string().describe("Object key/path"),
    ttl_seconds: z.number().optional().default(300).describe("URL expiry in seconds (max 3600)"),
  },
  async ({ bucket, key, ttl_seconds }) => {
    const effectiveTtl = Math.min(ttl_seconds, 3600);

    try {
      const url = await presigner.getDownloadUrl(bucket, key, effectiveTtl);
      const info = await minio.statObject(bucket, key);

      return {
        content: [{
          type: "text",
          text: JSON.stringify({
            url,
            expires_in: effectiveTtl,
            bucket,
            key,
            size_bytes: info.size,
            content_type: info.metaData?.["content-type"] || "unknown",
            last_modified: info.lastModified?.toISOString(),
            warning: "URL expires automatically. Request a new URL if needed.",
          }, null, 2),
        }],
      };
    } catch (error) {
      return {
        content: [{ type: "text", text: `File not found or access denied: ${error}` }],
        isError: true,
      };
    }
  }
);

// Tool: Search files by metadata
server.tool(
  "search_files",
  "Search for files in MinIO by metadata tags and prefix",
  {
    bucket: z.string().describe("Bucket name to search"),
    prefix: z.string().optional().default("").describe("Key prefix filter (e.g., 'reports/2026/')"),
    tags: z.record(z.string()).optional().describe("Metadata tag filters (e.g., {'department': 'engineering'})"),
    max_results: z.number().optional().default(20).describe("Maximum results to return"),
  },
  async ({ bucket, prefix, tags, max_results }) => {
    try {
      const results = await metadata.search(bucket, prefix, tags, max_results);

      return {
        content: [{
          type: "text",
          text: JSON.stringify({
            bucket,
            prefix,
            filters: tags,
            count: results.length,
            files: results.map((r) => ({
              key: r.key,
              size: r.size,
              last_modified: r.lastModified?.toISOString(),
              tags: r.tags,
            })),
          }, null, 2),
        }],
      };
    } catch (error) {
      return {
        content: [{ type: "text", text: `Search failed: ${error}` }],
        isError: true,
      };
    }
  }
);

// Tool: Upload file with metadata
server.tool(
  "upload_file",
  "Upload a file to MinIO with metadata tags",
  {
    bucket: z.string().describe("Bucket name"),
    key: z.string().describe("Object key/path"),
    content_base64: z.string().describe("File content as base64"),
    content_type: z.string().optional().default("application/octet-stream").describe("MIME type"),
    tags: z.record(z.string()).optional().default({}).describe("Metadata tags"),
  },
  async ({ bucket, key, content_base64, content_type, tags }) => {
    try {
      const buffer = Buffer.from(content_base64, "base64");

      await minio.putObject(bucket, key, buffer, buffer.length, {
        "Content-Type": content_type,
        ...Object.fromEntries(Object.entries(tags).map(([k, v]) => [`x-amz-meta-${k}`, v])),
      });

      return {
        content: [{
          type: "text",
          text: JSON.stringify({
            uploaded: true,
            bucket,
            key,
            size_bytes: buffer.length,
            content_type,
            tags,
          }, null, 2),
        }],
      };
    } catch (error) {
      return {
        content: [{ type: "text", text: `Upload failed: ${error}` }],
        isError: true,
      };
    }
  }
);

// Tool: List buckets
server.tool(
  "list_buckets",
  "List all accessible MinIO buckets",
  {},
  async () => {
    try {
      const buckets = await minio.listBuckets();
      return {
        content: [{
          type: "text",
          text: JSON.stringify({
            buckets: buckets.map((b) => ({
              name: b.name,
              created: b.creationDate?.toISOString(),
            })),
          }, null, 2),
        }],
      };
    } catch (error) {
      return {
        content: [{ type: "text", text: `Failed to list buckets: ${error}` }],
        isError: true,
      };
    }
  }
);

// Tool: Get file metadata (without downloading)
server.tool(
  "get_file_metadata",
  "Get metadata and tags for a file without downloading it",
  {
    bucket: z.string().describe("Bucket name"),
    key: z.string().describe("Object key/path"),
  },
  async ({ bucket, key }) => {
    try {
      const info = await minio.statObject(bucket, key);
      return {
        content: [{
          type: "text",
          text: JSON.stringify({
            bucket,
            key,
            size_bytes: info.size,
            content_type: info.metaData?.["content-type"],
            last_modified: info.lastModified?.toISOString(),
            etag: info.etag,
            tags: Object.fromEntries(
              Object.entries(info.metaData || {}).filter(([k]) => k.startsWith("x-amz-meta-"))
            ),
          }, null, 2),
        }],
      };
    } catch (error) {
      return {
        content: [{ type: "text", text: `Metadata fetch failed: ${error}` }],
        isError: true,
      };
    }
  }
);

server.start({ transport: "stdio" });

Presigned URL Security

// src/presigner.ts
import { Client } from "minio";

export class Presigner {
  private client: Client;
  private defaultTtl: number;

  constructor(client: Client, defaultTtl: number) {
    this.client = client;
    this.defaultTtl = defaultTtl;
  }

  async getDownloadUrl(
    bucket: string,
    key: string,
    ttlSeconds?: number
  ): Promise<string> {
    const expiry = ttlSeconds || this.defaultTtl;
    return this.client.presignedGetObject(bucket, key, expiry);
  }

  async getUploadUrl(
    bucket: string,
    key: string,
    ttlSeconds?: number
  ): Promise<string> {
    const expiry = ttlSeconds || this.defaultTtl;
    return this.client.presignedPutObject(bucket, key, expiry);
  }
}

Configuration

# config.yaml
minio:
  endpoint: localhost:9000
  use_ssl: true
  access_key: ${MINIO_ACCESS_KEY}
  secret_key: ${MINIO_SECRET_KEY}
  default_ttl: 300
  max_ttl: 3600
  allowed_buckets:
    - agent-documents
    - agent-reports
    - agent-datasets
  denied_buckets:
    - admin-backups
    - system-logs

mcp:
  name: minio-documents
  transport: stdio

// .cursor/mcp.json
{
  "mcpServers": {
    "minio-documents": {
      "command": "node",
      "args": ["dist/server.js"],
      "env": {
        "MINIO_ENDPOINT": "minio.internal:9000",
        "MINIO_ACCESS_KEY": "agent-reader",
        "MINIO_SECRET_KEY": "your-secret-key",
        "PRESIGN_TTL": "300"
      }
    }
  }
}

Security Hardening

1. Dedicated MinIO User: Create a MinIO service account with read/write access only to agent-allowed buckets. Never use the root admin credentials.
2. Bucket Policies: Apply bucket-level policies that restrict the agent service account to specific prefixes (e.g., agent-documents/reports/*).
3. Presigned URL TTL: Enforce maximum TTL of 3600 seconds. Shorter TTLs (300s) reduce the window for URL interception.
4. Audit Logging: Enable MinIO audit logging to a write-only destination. Every presignedGetObject call generates an audit event.
5. Content Validation: Validate uploaded content types against an allowlist. Reject executable files (.exe, .sh, .bat) to prevent agent-generated malware.

Last tested: August 2026 with TypeScript 5.5, FastMCP 1.2.0, MinIO 8.0, minio-js 8.0, and Node v22.

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