Headless Blender, Manim and CadQuery for coding agents

A coding agent can make a Blender product shot, a CadQuery part, or a Manim explainer. Revision comes the next day, when someone asks for a warmer metal, a lower camera, or a wider bracket. There are two ways to let an agent drive these tools. SlopCamera takes the second way: it keeps each film as a short program that the agent edits and renders again.

Two ways to put an agent in front of Blender

One route is a live session. MCP for Blender (formerly blender-mcp) adds a socket server to a running Blender. An MCP server then relays the agent's commands to that socket: create an object, set a material, or run any Python with execute_blender_code. You watch the scene change in the viewport. This works well for exploring an idea. The project's README warns that the code tool can run arbitrary Python and tells you to save your work before you use it. It also offers an optional safe mode (BLENDER_MCP_SAFE_MODE=1) that checks each script first and blocks file access, running other programs, and network access.

A live session keeps its state inside the open Blender file and its history in the chat. To make the same shot a week later, you need that file or a replay of every call, in order. If neither the file nor the call history survives, the agent starts over. Unless the agent also saved each script it sent, the .blend file is the only record, and it is not a short program you can diff, review, or run again with a new parameter.

The other route treats the scene as a program. Blender, Manim, and CadQuery can all run from a script with no window: Blender has command-line rendering, Manim renders a named scene class from its CLI, and CadQuery is a Python library that builds solids and exports STEP. SlopCamera's studio commands take this route. The program is the thing you keep, and every render starts from it.

What the agent writes

The product example is a six-second camera move around a machined optical instrument in brass, rubber, and glass. Its source is three files:

• scene.py, 51 lines. It defines the materials, builds the geometry from primitives, and keys the camera and focus distance at frames 0, 72, and 144.
• studio_scene.py, 147 lines of helpers for materials, primitives, lights, and camera aim.
• source.json, one line that lists exactly those two files.

A job.json file sets the rest: 1280×720, frames 0 to 143 at 24 fps, Cycles with 32 samples and denoising, the AgX view transform, and two outputs, a PNG sequence and the saved .blend file.

That split is where the agent saves effort. It writes or edits a few dozen lines of scene code. SlopCamera handles the rest. It copies the listed files into a fixed bundle and starts Blender with no window and no user preferences. studio run checks that every declared frame and file was written. studio encode makes a video that it checks frame by frame against the PNGs, and studio assemble puts the clip in an ordinary video project. The agent does not rewrite render settings, file handling, or encoding each time. To start a new film, it picks one of seven starters:

slopcamera studio init product --template blender-product --json

The other starters are blender-character, blender-shaded-street, blender-cloth, blender-fluid, cadquery-bracket, and manim-lesson. None of them runs anything when created.

Change a material and render again

Say the brass reads too yellow. The starter's scene.py has the same brass line as the showcase example, but its default job is shorter (72 frames), so studio init does not give you the six-second film. The material is one line:

brass = material("Satin champagne brass", (0.56, 0.31, 0.105), metallic=0.88, roughness=0.23, texture=True)

The agent changes the color or the roughness and leaves the rest of the scene alone. It then runs the same sequence as the first time:

slopcamera studio bundle product/source.json --json
slopcamera studio plan product/job.json --json
slopcamera studio probe product/job.json \
  --blender-bin /Applications/Blender.app/Contents/MacOS/Blender --json
slopcamera studio run product/job.json \
  --blender-bin /Applications/Blender.app/Contents/MacOS/Blender \
  --allow-trusted-code --json
slopcamera studio inspect <studio-id> --json
slopcamera studio encode <studio-id> --output-id beauty --json

bundle returns a new bundleSha256. The agent copies that digest into job.json and gives the job a new ID:

"jobId": "studio_product_v2",
"bundleSha256": "<new digest>"

Reusing an old job ID with changed source is rejected as a conflict. plan reads the job without starting Blender, and probe loads the chosen Blender without running your scene, so mistakes show up before a long render. After the run, inspect rechecks the hashes of the source and every output.

A camera change works the same way. The camera path is three keyed positions in scene.py, and the lens and f-stop are arguments to one camera() call. The first render and the old files stay as they were, so you can compare the two versions.

The first native film tutorial walks through this loop with a one-second CPU preview at 320×180. It ends with studio assemble, which puts the clip in a video project for captions, sound, and other aspect ratios.

Change a part's dimensions with CadQuery

The CAD example builds a mounting bracket with counterbored holes, a central opening, and an isolation pad. Its default bracket is 100 mm wide. The variation job sets two parameters:

"parameters": { "widthMm": 132, "heightMm": 68 }

The same CadQuery source builds the larger part, and it rejects values outside its limits, such as a width below 50 mm or above 160 mm. The example measures both solids, exports STEP in millimeters and GLB in meters, and puts the GLB into a Blender scene for the film. The example's helper script, cad-variations.ts, reimports the baseline STEP file and checks that it gives back two valid solids with a relative volume error below 0.00001. That is a geometry check, not a structural or manufacturing check. The parametric design guide covers CAD work in more depth.

Keep Manim visuals and sound apart

The geometry lesson is a ten-second portrait Manim render: a 3–4–5 triangle becomes 9 + 16 = 25 square tiles, with a presenter, typeset math, and a caption rail. Scene timings, gestures, and captions live in the lesson source.

SlopCamera's Manim driver renders silent visuals and rejects audio in the scene source. Narration, music, and effects go in the video project instead. When the voice-over changes, the agent edits the project. It does not render the math again. The math explainer guide covers this path from studio init lesson --template manim-lesson.

Know what runs on your machine

• studio run runs the bundled Python as your user. There is no operating-system sandbox. Blender starts with scripts embedded in .blend files turned off and with a clean environment, and your AI provider keys are not passed to it. That limits what the run inherits. It does not contain what the Python itself can do.
• Every run needs --allow-trusted-code on that command. Choosing a Blender or Python path does not grant it, and a stored approval cannot stand in for it. Read the source first, especially anything downloaded.
• You install the engines. SlopCamera does not install or upgrade Blender, CadQuery, or Manim. The examples were made with Blender 5.2.1 LTS, CadQuery 2.8.0, and Manim Community 0.21.0 on macOS arm64. Other versions and platforms need their own testing.
• A GPU request fails if no GPU is available. The product job asks for Cycles on the GPU. To render on the CPU, set device: "cpu" in a new job.
• A failed or interrupted run is never started again on its own. studio inspect and studio reconcile show what happened before you choose a new job.

The native engines reference lists the full rules.

Where a live session fits

A live MCP session suits sketching in an open Blender window, poking at an existing .blend, or asking questions about a scene. Scene source you keep suits work that will be revised or rendered again: product shots with several color variants, parts whose dimensions change, and lessons whose narration will be rewritten. You can use both. Explore live, then have the agent write the scene you settled on as a scene.py.

For how this fits the rest of SlopCamera, read why SlopCamera, the native film guide, and the films section of the techniques reference. Remotion alternatives for coding agents compares MCP for Blender with other tools. The guide also covers cloth and fluid caches, character rigs, color and alpha masters, and imported models.

Limits

The same source can render slightly differently on another machine, because Blender builds, GPU drivers, and codecs vary. The saved record names the Blender executable, its hash, and the observed package versions. It does not hash every system library, font, or add-on. Physics settings in the cloth and fluid starters are not proof of a correct simulation; review the baked frames. Native control rigs and IK stay in Blender; a portable GLB export and a rendered MP4 do not carry them. Durable workflows that include a native job need the Bun package or a source checkout, not a copied standalone executable. The studio commands do not capture screen or camera recordings.

The install steps use release 3.9.2.