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start_span

Begin a tracing span to monitor and log an operation. Returns a span ID to close with end_span when the work completes.

Instructions

Open a new tracing span. Returns the span_id to pass to end_span().

Args:
    name:       Human-readable span label (e.g. 'stage_1_bootstrap', 'tool:search_library').
    kind:       Span type — 'internal' | 'tool' | 'agent' | 'llm'. Default: 'internal'.
    session_id: Session identifier (from session_bootstrap). Optional.
    run_id:     FK to agent_runs.run_id. Optional.
    parent_id:  Parent span_id for nested spans. Optional.
    tags:       JSON string of extra key/value metadata. Optional.

Returns the span_id string — pass it to end_span() when the work is done.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
nameYes
kindNointernal
session_idNo
run_idNo
parent_idNo
tagsNo

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
resultYes
Behavior4/5

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

With no annotations, the description fully discloses the behavior: it opens a span and returns a span_id. It does not mention side effects or destruction, which is appropriate for a tracing tool. The description adds useful context about parameter meanings and defaults.

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

Conciseness5/5

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

The description is extremely concise: one summary line, a structured Args list with clear labels, and a brief return note. Every sentence adds value without redundancy. It is front-loaded with the core purpose.

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

Completeness5/5

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

Given the tool's 6 parameters and no annotations, the description covers all necessary aspects: what it does, all parameters with types and defaults, the return value, and integration with end_span. It is complete for an agent to correctly invoke the tool.

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

Parameters5/5

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

Schema coverage is 0%, but the description provides rich semantics for all 6 parameters, including examples (e.g., 'stage_1_bootstrap' for name), enumeration of kind options, and explanations of optional fields like parent_id and tags. This far exceeds the schema's bare titles and defaults.

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 clearly states 'Open a new tracing span' with the verb 'open' and resource 'span', and explicitly returns the span_id for use with end_span. This distinguishes it from sibling tools like end_span, log_span, and get_spans by focusing on creation.

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

Usage Guidelines4/5

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

The description explains the parameters in detail and hints at usage with end_span, but does not explicitly state when to use this tool versus alternatives (e.g., log_span for logging without span structure). The guidance is clear but not exhaustive.

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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