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

Beyond the readOnly/openWorld annotations, it discloses data-sourcing behavior (venue fetch when candles[] is omitted), caching of past sessions making repeats fast, the reproducibility/zero-COGS trade-off of passing candles[], the completed-session precondition, and the exact failure mode (error when a session has no candles). That is unusually rich behavioral context.

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?

Front-loads the core purpose, then the query phrasings, then the return payload, then the execution-mode caveats — a logical ordering with no filler. The enumerated return fields earn their space because no output schema exists.

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 7 parameters, no output schema, and a non-trivial data-fetch path, the description covers purpose, return contents, input modes, caching, and error behavior. An agent has everything needed to choose and invoke it correctly.

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 100%, so the baseline is 3; the description still adds real meaning by explaining why candles[] and prev_candles exist (network-free, deterministic runs) and by reinforcing session_date's completed-UTC-session constraint and its error consequence. Instrument/venue/timeframe semantics are left entirely to the schema.

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 and resource: analyzing where/how price opened relative to the prior session's value area, plus the named sub-outputs (open_location, open_type). An agent can distinguish it from neighbors like run_session_structure or run_value_migration from the framing alone, though no sibling is explicitly named.

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?

It gives concrete trigger phrasing ("how did BTC open today?" / "what does the open imply for the session?") and explains the two operating modes — let the venue fetch candles, or supply candles[] for a reproducible, network-free run. It lacks explicit when-not guidance or routing to an alternative sibling tool.

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