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

Annotations establish the mutation profile (readOnlyHint=false, idempotentHint=false, destructiveHint=false), and the description adds real value beyond them: the source preserves its id, labels and citations, and the return value carries a new word count. It stops short of noting any re-indexing or citation-refresh behavior, so it is not exhaustive.

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?

Three tight sentences, front-loaded with the append semantics before the alternative and the return note. The trailing 'POST /sources/append' is marginally useful API detail but not wasteful.

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?

With no output schema, the description helpfully states what comes back (the updated source with its new word count), and the append-vs-add distinction is covered. For a simple 3-parameter mutation with full annotation coverage, this is nearly complete; only the effect on indexing/citations over time is unstated.

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 description coverage is 100%, so all three parameters are already documented (including that title is an optional heading). The description restates the title/optional-heading point rather than adding syntax or constraint detail, so baseline 3 applies.

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?

States a specific verb ('Add text to the end') and resource ('an existing source'), scopes it to pasted text or web pages, and explicitly contrasts with notebook_add_sources for new sources. An agent can distinguish it from siblings like notebook_add_sources without opening the schema.

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?

Explicitly routes the agent: use this to append to an existing source, use notebook_add_sources to add a separate source. The condition that selects the alternative is stated directly, leaving nothing to inference.

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