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

Annotations are minimal (readOnlyHint=false, destructiveHint=false, openWorldHint=false), so the description carries the burden. It discloses that PNG is printed while PDF is only filed with rendered=false, that fileUrl is preferred over base64 due to context/color concerns, and the 20MB limit. This adds meaningful behavioral context beyond annotations. It doesn't mention side effects like overwriting existing letterheads, but for this tool the key behaviors are covered.

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

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

About 5 sentences covering the essential behavior, source selection, rendering limitations, and sibling routing. The bilingual structure is somewhat redundant, but each sentence adds information. The description is longer than ideal but not bloated; the front-loaded functional statement is first.

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

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

For a complex 7-parameter upload with no output schema, the description explains the critical decisions: source priority, PNG vs PDF, exactly-one constraint, and when to use letter_design_save. The 'reasoning' parameter is undocumented and width/height semantics are thin, but the description covers the high-stakes aspects of the tool's behavior.

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

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

Schema coverage is 86% (6 of 7 parameters described). The description adds semantics beyond the schema by explaining the source selection logic ('genau eine von beiden'), the PNG/PDF rendering difference, and why fileUrl is preferred. However, widthMm/heightMm/displayName are only described as 'optional' in both schema and description without further guidance, and the 'reasoning' parameter is completely undocumented. Still, the core parameter semantics are substantially enriched.

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

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

The description states a specific verb ('Lädt ... hoch und speichert'), a resource ('Briefkopf'), and target ('Partnerprofil'), and distinguishes from letter_design_save and blocks. It clearly explains what this tool does and what it does not do. The sibling differentiation is explicit with 'dann letter_design_save' and 'der Brieftext bleibt den blocks vorbehalten.'

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

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

The description explicitly states the source preference (fileUrl bevorzugt), the mutual exclusivity ('genau eine von beiden'), and when to use letter_design_save for rebuilding letters. It gives clear context for what belongs here versus the blocks. This is explicit when-to-use guidance with alternatives named.

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