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AWS MCP Server

by rishikavikondala
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s3_object_upload

Upload files to Amazon S3 buckets using base64-encoded content. Specify bucket name, object key, and file content for cloud storage management.

Instructions

Upload an object to S3

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
bucket_nameYesName of the S3 bucket
object_keyYesKey/path of the object in the bucket
file_contentYesBase64 encoded file content for upload

Implementation Reference

  • src/mcp_server_aws/server.py:155-159 (handler)
    Executes the S3 object upload by decoding base64 file content and using boto3 s3_client.upload_fileobj to upload to the specified bucket and key.
    elif name == "s3_object_upload":
    response = s3_client.upload_fileobj(
        io.BytesIO(base64.b64decode(arguments["file_content"])),
        arguments["bucket_name"],
        arguments["object_key"])
  • src/mcp_server_aws/tools.py:42-63 (schema)
    Defines the Tool object with input schema for s3_object_upload, specifying required parameters: bucket_name, object_key, and base64-encoded file_content.
    Tool(
    name="s3_object_upload",
    description="Upload an object to S3",
    inputSchema={
        "type": "object",
        "properties": {
            "bucket_name": {
                "type": "string",
                "description": "Name of the S3 bucket"
            },
            "object_key": {
                "type": "string",
                "description": "Key/path of the object in the bucket"
            },
            "file_content": {
                "type": "string",
                "description": "Base64 encoded file content for upload"
            }
        },
        "required": ["bucket_name", "object_key", "file_content"]
    }
),
  • src/mcp_server_aws/server.py:136-140 (registration)
    Registers the s3_object_upload tool (among others) by returning the list from get_aws_tools() in response to list_tools() calls.
    async def list_tools() -> list[Tool]:
    """List available AWS tools"""
    logger.debug("Handling list_tools request")
    return get_aws_tools()

Tool Definition Quality

C2.9/5.0
Behavior2/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations provided, the description carries full burden but only states the basic action. It doesn't disclose critical behaviors like authentication requirements, file size limits, overwrite policies, error conditions, or response format. For a write operation with potential side effects, this is a significant gap in transparency.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single, direct sentence with zero wasted words. It's front-loaded with the core action and resource, making it immediately scannable and efficient. Every word earns its place.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness2/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a write operation with no annotations and no output schema, the description is incomplete. It doesn't address mutation implications, success/failure responses, or operational constraints. Given the complexity of S3 uploads (e.g., permissions, encoding), more context is needed for safe and effective use.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the schema fully documents all three parameters. The description adds no additional parameter semantics beyond what's in the schema (e.g., it doesn't explain format expectations for object_key or constraints on file_content). Baseline 3 is appropriate when the schema does all the work.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the action ('Upload') and resource ('an object to S3'), making the purpose immediately understandable. However, it doesn't differentiate from sibling tools like 's3_object_delete' or 's3_object_read' beyond the obvious verb difference, missing an opportunity to clarify its specific role in the S3 toolset.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

No guidance is provided on when to use this tool versus alternatives. For example, it doesn't mention prerequisites like bucket existence, compare to batch operations, or indicate if it's for single-file uploads only. This leaves the agent to infer usage from the name alone.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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