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by healkeiser
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start_render

Execute any Houdini node that renders or writes files, with optional frame range and background mode. Returns written output path and disk status so failed or empty renders are visible.

Instructions

Execute any node that renders or writes files.

Foreground by default: Houdini shows its own progress dialog and the user can cancel. The call holds until the render finishes, however long that is; a client that hands a long call to a background task notifies you with the verdict. Do nothing else in Houdini meanwhile and never poll the disk.

Not just /out ROPs: a LOP usdrender_rop (which is how Solaris renders), a SOP ROP Geometry, or a File Cache's Save to Disk all work, because what matters is whether the node can be executed rather than its category.

The result reports the output path it wrote to and whether anything is actually on disk there, so a render that succeeds and writes nowhere is visible instead of silent.

Args: node_path: Any node with a render() or an 'execute' button. frame_range: [start, end] or [start, end, increment]. background: Render in a separate hython on the saved hip and return at once with status "launched"; get_render_progress reports the process, its log tail and the files. The user sees no progress in Houdini, so use it only when asked to keep working while a render runs.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
node_pathYes
backgroundNo
frame_rangeNo

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv0.1.0

TDQS

A5/5.0
Behavior5/5

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

With no annotations, the description carries full burden. It transparently discloses that the call blocks until render completion, shows a progress dialog, can be canceled by the user, and that background mode spawns a separate hython process returning immediately with status 'launched.' It also explains that the result reports output path and disk presence, making silent failures visible. This is exhaustive behavioral disclosure.

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 long but every sentence earns its place. It starts with the core purpose, then details blocking behavior, node support, result reporting, and parameters in a logical order. No filler—all content is directly actionable for an agent.

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?

For a complex tool that executes renders, the description covers all necessary operational context: blocking vs background, node type support, output reporting, and parameter syntax. It even notes when not to use background. There is no missing information that would prevent an agent from calling it correctly.

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 description coverage is 0%, so the description must fully explain parameters. It does: node_path is any node with a render() or 'execute' button; frame_range is [start, end] or [start, end, increment]; background renders in a separate hython on the saved hip and returns at once with status 'launched.' This adds critical semantic meaning beyond the bare type definitions.

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 states a clear verb and resource: 'Execute any node that renders or writes files.' It explicitly distinguishes itself from sibling tools like render_viewport or get_render_progress by clarifying scope (any executable node, not just ROPs) and behavior (holds until finish). This gives an agent a precise mental model of what the tool does.

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

Usage Guidelines5/5

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

The description provides explicit when-to-use guidance: it explains foreground vs background behavior, when background should be used ('only when asked to keep working while a render runs'), and instructs the agent to 'Do nothing else in Houdini meanwhile and never poll the disk.' It also clarifies which node categories are valid, preventing misuse.

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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