John's Essentials

generate_placeholder_image

Placeholder Image — Generate a placeholder image at any dimension with custom background colour and label text. [category: generate]

Input Schema

TableJSON Schema

Name	Required	Description	Default
`text`	No	Label text. Defaults to '<width> × <height>'.
`width`	No	Pixels wide. >4096 clamps to 4096; 0/omitted = 640. The default label text shows the FINAL clamped size.
`format`	No	Image file type to save as.	png
`height`	No	Pixels tall. >4096 clamps to 4096; 0/omitted = 480.
`bgColor`	No	Background hex color, with or without '#'. Field name is 'bgColor' — not 'bg_color'.	3B82F6
`fgColor`	No	Label text hex color. Field name is 'fgColor' — not 'text_color'.	FFFFFF

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedSep 26, 2026
- addedInput schema / properties / width / x-ui / pair_label
```
Added value: +"Size"
```
- addedInput schema / properties / width / x-ui / pair_with
```
Added value: +"height"
```
Changed2 schema fields changedSep 23, 2026
- addedInput schema / properties / height / x-ui
```
Added value: +{
+  "unit": "px"
+}
```
- addedInput schema / properties / width / x-ui
```
Added value: +{
+  "unit": "px"
+}
```

Changed2 schema fields changedSep 11, 2026

changedInput schema / properties / format / description

Previous value: -"Sets the response MIME (png/jpg/jpeg/webp). Unrecognized values get mislabeled as PNG — stick to the enum."New value: +"Image file type to save as."

changedInput schema / properties / format / enum

Previous value: -[
-  "png",
-  "jpg",
-  "jpeg",
-  "webp"
-]New value: +[
+  "png",
+  "jpg",
+  "webp"
+]

First observedSep 5, 2026

TDQS

A3.6/5.0

Behavior3/5

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

Annotations (readOnlyHint=false, destructiveHint=false) already signal this is a generating tool. The description adds that it produces placeholder images at arbitrary dimensions with custom colors and text, but it does not disclose output behavior such as how the image is returned or saved, or the clamping behavior for dimensions over 4096.

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 one-sentence description is concise and front-loaded with the tool's identity and action. The leading 'Placeholder Image' title is slightly redundant with the tool name, but the rest of the sentence is efficient.

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

Completeness3/5

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

For a six-parameter tool with no output schema, the description gives a solid high-level overview but omits the return/save behavior and expected output form. The rich parameter schema compensates for most invocation details, yet the missing output semantics leave a notable gap.

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 every parameter including defaults, clamping, and hex color expectations. The description only mentions 'background colour' and 'label text' generically, adding no parameter meaning beyond the schema.

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 description states a specific verb ('Generate') and resource ('placeholder image') with concrete customization dimensions, background color, and label text. This distinguishes it from sibling generation tools such as generate_barcode or generate_favicon, even without naming an alternative.

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

Usage Guidelines3/5

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

The term 'placeholder image' implies the intended use case, but the description provides no explicit when-to-use guidance, exclusions, or comparison to sibling tools like generate_favicon or photo_resize. Usage context must be inferred from the tool's name and category.

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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