read_spanRetrieve a span's summary and original messages by run ID and index after archived context is compacted. Long spans are truncated at 20,000 characters.
The summary and original messages of one span, by run id and index (as search_spans prints them). Long spans are cut at 20000 characters.
| Name | Required | Description | Default |
|---|---|---|---|
| run | Yes | ||
| index | Yes |
Changes observed during successful MCP inspections.
v0.10.0Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds genuinely new behavior beyond annotations: the 20000-character truncation of long spans and the fact that both summary and original messages come back.
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?
Two tight sentences with zero filler. The primary content statement is front-loaded and the truncation caveat follows immediately, so an agent gets value in the first read.
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 two-parameter read tool with no output schema, the description covers what is returned, how to obtain the identifiers, and the truncation bound. The only real gap is error/out-of-range behavior, which is minor given the read-only, idempotent annotations.
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 0%, so the description must carry the load. It identifies 'run id and index' and points at search_spans as the way to obtain them, which is more than the bare schema offers, but it does not clarify index ordering/type or what happens for an out-of-range index.
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 names the exact resource ('one span') and its payload ('summary and original messages'), and ties it to the sibling search_spans which produces the identifiers. It lacks an explicit verb, but the resource and scope are unambiguous enough to separate it from guardian_status and search_spans.
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 parenthetical '(as search_spans prints them)' implies this tool is the follow-up read after search_spans surfaces a run/index pair, which is useful routing. However, it never states when to use this tool versus re-running search_spans or when not to call it, so the guidance is inferred rather than explicit.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.