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BlenderMCP

by naab007
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Python
Local

start_blender

Launch Blender as a managed subprocess, optionally opening a .blend file, running in headless mode, or executing a Python expression. Auto-detects the Blender executable for AI-driven 3D workflows.

Instructions

Launch Blender as a managed subprocess.

Parameters:

  • blend_file: Path to a .blend file to open on startup (optional)

  • blender_exe: Full path to the Blender executable. Auto-detected if omitted (checks BLENDER_EXE env var, PATH, then common install locations).

  • background: True = headless mode (--background), no UI. Useful for rendering.

  • wait_for_addon: Wait up to 30 s for the BlenderMCP addon socket to become reachable on port 9876 (default True). Set False for background jobs that don't use the addon.

  • python_expr: Optional Python expression passed to Blender via --python-expr, e.g. "import bpy; bpy.ops.wm.quit_blender()" for scripted batch runs.

Blender console output goes to blender_mcp_blender.log in the temp folder; it must never share this process stdio, which carries the MCP transport.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
backgroundNo
blend_fileNo
blender_exeNo
python_exprNo
wait_for_addonNo

Schema Changelog

Changes observed during successful MCP inspections.

  1. First observedv1.5.5

TDQS

A4.1/5.0
Behavior3/5

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

No annotations are provided, so the description carries the full burden. It does disclose that console output goes to a log file and that stdio is not shared, which is useful, but it omits lifecycle behaviors such as repeated launches, cleanup, or failure modes.

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 compact and well-structured with clear parameter labels and explanatory notes. No redundant or irrelevant content is present.

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

Completeness4/5

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

The description covers invocation, parameter semantics, and an important side effect (log file location). It does not mention return values, but no output schema exists, so that is acceptable. It could be slightly improved by referencing how to verify successful startup via sibling tools like get_blender_status.

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?

Even though schema coverage is 0%, the description explains all five parameters with defaults and examples. This fully compensates for the missing schema descriptions.

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 specific action, 'Launch Blender as a managed subprocess', which clearly distinguishes it from sibling tools like close_blender and get_blender_status. It is concise and immediately understandable.

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

Usage Guidelines3/5

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

The usage is implied by the action verb and parameter hints, but there is no explicit guidance on when to choose this tool over alternatives or any stated exclusions. It could be improved by noting that this tool is the prerequisite for other Blender operations.

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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