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s3_generate_presigned_url

Generate a time-limited pre-signed URL for read-only access to an S3 object. Share the link for downloads; it expires after a set time.

Instructions

Generate a time-limited pre-signed URL granting read access to an S3 object. The URL is a bearer secret — anyone who possesses it can download the object until it expires. Reserved for dangerous guard mode. Confirm with the user before calling.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
keyYesObject key.
bucketYesBucket name.
regionNoRegion the bucket lives in (e.g. 'eu-central-1'). Omit unless you know it — it is resolved automatically. AWS, and OVH when no endpoint URL is configured (pass the region s3_list_buckets reports). Rejected for Hetzner, and for an OVH endpoint URL set by hand, whose region is fixed by that endpoint.
accountNoAccount label of the provider whose object storage to use; omit for that provider's default account.
providerYesStorage provider.
expires_inNoURL expiry in seconds (1–604800, default 3600).

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed2 schema fields changedv2.28.0
    • addedInput schema / properties / account
      Added value: +{
+  "description": "Account label of the provider whose object storage to use; omit for that provider's default account.",
+  "type": "string"
+}
    • changedInput schema / properties / region / description
      Previous value: -"Region the bucket lives in (e.g. 'eu-central-1'). Omit unless you know it — it is resolved automatically. AWS only; rejected for Hetzner/OVH, whose region is fixed by the configured endpoint URL."New value: +"Region the bucket lives in (e.g. 'eu-central-1'). Omit unless you know it — it is resolved automatically. AWS, and OVH when no endpoint URL is configured (pass the region s3_list_buckets reports). Rejected for Hetzner, and for an OVH endpoint URL set by hand, whose region is fixed by that endpoint."
  2. Changed1 schema field changedv2.24.2
    • addedInput schema / properties / region
      Added value: +{
+  "description": "Region the bucket lives in (e.g. 'eu-central-1'). Omit unless you know it — it is resolved automatically. AWS only; rejected for Hetzner/OVH, whose region is fixed by the configured endpoint URL.",
+  "type": "string"
+}
  3. Addedv2.24.0
  4. Removedv2.23.8
  5. First observedv1.0.0

TDQS

A4.2/5.0
Behavior4/5

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

No annotations are provided, so the description carries the full burden — and it delivers the key behavioral trait: the URL is a 'bearer secret' that anyone can use to download until expiry, plus the guard-mode gating. Missing are items like whether credentials/permissions are required to mint the URL or whether generation is logged, but the critical security profile is disclosed.

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?

Three short sentences, front-loaded with what it does, then the security consequence, then the gating prerequisite. No filler, every sentence earns its place.

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

Completeness4/5

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

For a 6-parameter tool with a fully documented schema and no output schema, the description covers purpose, security impact, and invocation prerequisites. The only gap is that it never says the URL itself is what comes back (though this is strongly implied by the tool name), which is minor given no output schema exists.

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% — provider enum, region resolution rules, bucket, key, account, and the expires_in range (1–604800) are all documented in the schema. The description adds no parameter meaning beyond that, so the baseline 3 applies.

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

Purpose5/5

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

States a specific verb and resource — 'Generate a time-limited pre-signed URL' — and scopes it tightly to 'read access to an S3 object'. This is clearly distinguishable from s3_download_object (which returns bytes) and the other s3_* siblings without opening any schema.

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

Usage Guidelines4/5

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

Gives explicit usage conditions: 'Reserved for dangerous guard mode. Confirm with the user before calling.' This tells the agent both the gating mode and the required confirmation step. It stops short of naming an alternative (e.g. use s3_download_object if you just need the object), so it falls short of a 5.

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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