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by healkeiser
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start_render

Execute render or file-writing nodes in Houdini, with foreground progress and cancellation, or background rendering. Reports output path and file presence.

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. overrides: {parm_name: value} for this render only, e.g. a draft {"resolutionx": 640, "resolutiony": 360}; a parm tuple takes a list. Put back afterwards, expressions and keyframes included, even if the render fails (overrides_applied, overrides_restored). Foreground only. A Karma LOP's "Wait for Render to Complete" is switched on for the call when it is off (foreground_forced).

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
node_pathYes
overridesNo
backgroundNo
frame_rangeNo

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed2 schema fields changedv2.24.1
    • addedInput schema / additionalProperties
      Added value: +false
    • addedInput schema / properties / overrides
      Added value: +{
+  "anyOf": [
+    {
+      "additionalProperties": true,
+      "type": "object"
+    },
+    {
+      "type": "null"
+    }
+  ],
+  "default": null,
+  "title": "Overrides"
+}
  2. First observedv0.1.0

TDQS

A4.7/5.0
Behavior5/5

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

No annotations are provided, so the description carries the full burden and does so richly: foreground holds the call until render finishes, the user can cancel, background returns immediately with status 'launched', overrides are restored even on failure (with overrides_applied/overrides_restored flags), and Karma LOP Wait-for-Render is forced on (foreground_forced).

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

Conciseness4/5

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

Front-loaded with the core action and organized with an Args section that maps cleanly to the schema. It is somewhat long, but nearly every sentence conveys non-obvious behavior, so little is wasted.

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?

Despite no output schema, the description explains what the result reports (output path written and whether anything is on disk) and covers failure/restoration edge cases, background launch behavior, and override forcing. Nothing an agent needs to invoke it correctly is missing.

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 compensate, and it does: node_path is any node with render() or an execute button; frame_range is [start, end] or [start, end, increment]; background explains the hython/hip semantics; overrides gives a concrete example and notes tuple-as-list and restoration semantics.

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?

States a specific verb and resource ('Execute any node that renders or writes files') and clarifies scope beyond just /out ROPs, naming LOP usdrender_rop, SOP ROP Geometry, and File Cache Save to Disk. An agent can distinguish this from siblings like setup_render, render_viewport, or render_sheet without opening a schema.

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?

Gives explicit foreground-vs-background guidance ('use it only when asked to keep working while a render runs') and warns not to poll the disk. It names get_render_progress as the companion for background status. It does not explicitly contrast against render_viewport/render_node_network/render_sheet, so sibling routing is slightly incomplete.

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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