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Cobalt Capture

by com.cobaltcapture
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add_screenshot

Append one screenshot, with an optional title and note, to the end of a review. The image is stored in Cobalt and shown in the review. Three ways to supply it: page_url, a public page that Cobalt renders in a headless browser (viewport 'desktop' at 1280px or 'mobile' at 390px, full_page for the whole scroll, hide_selectors for elements to hide; common cookie banners are hidden automatically), which also returns the rendered image; image_url, an image already uploaded to https://cobaltcapture.com/api/upload; or image, a small base64 image. This tool only appends; it never changes an existing item (replace_screenshot does that).

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
imageNoA small base64-encoded PNG, JPEG or WebP (data: URL prefix accepted). Intended for images under about 100 KB; larger screenshots are better uploaded and passed as image_url.
titleNoOptional short caption for the screenshot.
reviewYesThe review slug or URL from create_review.
commentNoOptional note describing what this screen shows or what's wrong with it.
page_urlNoA public http(s) page for Cobalt to render and screenshot server-side. Pages behind a login can't be rendered. Becomes the item's source_url unless one is passed. Takes 10 to 30 seconds; a site that blocks automated browsers returns an error.
viewportNoWith page_url: 'desktop' (1280x800, default) or 'mobile' (390x844 phone, 2x, touch).
full_pageNoWith page_url: capture the entire scrollable page instead of the first viewport. Default false. Pages that scroll inside an inner container come back viewport-sized.
image_urlNoAn image URL on Cobalt's own image host, as returned by https://cobaltcapture.com/api/upload (multipart field `file`, returns {url, key}) or by the editor's window.cobaltGrab(). External URLs are refused.
source_urlNoOptional URL of the page the screenshot was taken on.
claim_tokenYesThe claim_token from create_review (authorizes the write). For a review the human has SAVED to their account, pass the agent token they generated on the review page (Export & more → Hand to your agent) here instead.
hide_selectorsNoWith page_url: extra CSS selectors to hide before the shot, e.g. ['#newsletter-modal', '.chat-widget']. Common cookie and consent banners are hidden on every render.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed6 schema fields changed
    • changedInput schema / properties / full_page / description
      Previous value: -"With page_url: capture the entire scrollable page instead of the first viewport. Default false. Prefer the fold first; use full_page for one overview shot. Sites that scroll inside an inner container (some app-style marketing pages) come back fold-sized regardless — if that happens, say so rather than retrying."New value: +"With page_url: capture the entire scrollable page instead of the first viewport. Default false. Pages that scroll inside an inner container come back viewport-sized."
    • changedInput schema / properties / hide_selectors / description
      Previous value: -"With page_url: extra CSS selectors to hide before the shot (e.g. ['#newsletter-modal', '.chat-widget']) when the returned image shows something still covering the page. Common cookie/consent banners are already removed on every render."New value: +"With page_url: extra CSS selectors to hide before the shot, e.g. ['#newsletter-modal', '.chat-widget']. Common cookie and consent banners are hidden on every render."
    • changedInput schema / properties / image / description
      Previous value: -"For SMALL images only (< ~100 KB) that you already hold as base64 text: a base64-encoded PNG/JPEG/WebP (data: URL prefix accepted). Not the path for screenshots — emitting one as tool-call text is slow and error-prone, and an image you received as an image cannot be transcribed. Upload it and pass image_url instead."New value: +"A small base64-encoded PNG, JPEG or WebP (data: URL prefix accepted). Intended for images under about 100 KB; larger screenshots are better uploaded and passed as image_url."
    • changedInput schema / properties / image_url / description
      Previous value: -"The image URL returned by Cobalt's own upload: POST the file as multipart field `file` to https://cobaltcapture.com/api/upload (returns {url, key}) — ideally from inside the script that captured it — or use the browser hook window.cobaltGrab(). Must be on Cobalt's own image host; external URLs are refused."New value: +"An image URL on Cobalt's own image host, as returned by https://cobaltcapture.com/api/upload (multipart field `file`, returns {url, key}) or by the editor's window.cobaltGrab(). External URLs are refused."
    • changedInput schema / properties / page_url / description
      Previous value: -"A public http(s) page for Cobalt to render and screenshot server-side — the path when you have no browser or screenshot tool of your own (chat clients, text-only agents). Public pages only: nothing behind a login you signed into in your own session. Becomes the item's source_url unless you pass one. Takes 10–30 s (bot checks are waited through); a site that hard-blocks automated browsers returns a clear error — report that as a coverage gap, don't retry."New value: +"A public http(s) page for Cobalt to render and screenshot server-side. Pages behind a login can't be rendered. Becomes the item's source_url unless one is passed. Takes 10 to 30 seconds; a site that blocks automated browsers returns an error."
    • changedInput schema / properties / viewport / description
      Previous value: -"With page_url: 'desktop' (1280x800, default) or 'mobile' (390x844 phone, 2x, touch) — the way to test the phone fold."New value: +"With page_url: 'desktop' (1280x800, default) or 'mobile' (390x844 phone, 2x, touch)."
  2. Changed1 schema field changed
    • removedInput schema / properties / replace_item
      Removed value: -{
-  "description": "RETAKE: the 1-based number of an existing screenshot item to overwrite in place with this capture (same position, pins cleared, title/comment replaced only if given). Use it when the image that came back wasn't the shot. Not for notes.",
-  "minimum": 1,
-  "type": "integer"
-}
  3. First observed

TDQS

A4.7/5.0
Behavior5/5

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

Annotations only cover the safety profile (readOnlyHint=false, destructiveHint=false, openWorldHint=true); the description adds a great deal on top: append-only semantics, a 10-30 second render latency for page_url, the failure mode of sites blocking headless browsers, automatic cookie-banner hiding, and that page_url becomes source_url unless overridden. That is exactly the behavioral context an agent needs before invoking.

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

Conciseness4/5

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

The purpose and the append-vs-replace distinction are front-loaded, and the middle sentence is a dense but well-organized enumeration of the three input methods. It runs long with nested parentheticals, but nearly every clause carries operational information rather than filler.

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 an 11-parameter mutation tool with no output schema, the description covers authorization (claim_token), input modes, latency, and error behavior, and even notes that page_url returns the rendered image. What it does not describe is the shape of the success response (item id, position in the review), which leaves a small gap given there is no output schema to fall back on.

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

Parameters4/5

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

Schema coverage is 100%, so the baseline is 3, but the description adds genuine cross-parameter meaning the schema lacks: which three parameters are alternative image sources, that page_url populates source_url unless one is passed, and that claim_token can be either the create_review token or the human's exported agent token for a saved review.

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?

The first sentence gives a precise verb+resource+effect: append one screenshot to the end of a review. It then explicitly distinguishes itself from replace_screenshot ('This tool only appends; it never changes an existing item'), so an agent can route between the two without opening either schema.

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

Usage Guidelines5/5

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

It enumerates the three mutually alternative supply paths (page_url, image_url, image) with the conditions that select each, and names the sibling to use for the modify case. It also states when-not: login-gated pages can't be rendered, external image URLs are refused, and oversized base64 images should be uploaded instead.

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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