blender-host-sandbox-boundary

Blender Host/Sandbox Boundary

Classify every operation by the process that executes it. A path is usable only when that process can see it.

Execution surfaces

Blender MCP code executes in the host Blender process even though Hermes itself runs in the sandbox. Do not search the sandbox for the Blender executable, bpy, a host add-on, or a host path. Do not ask Blender to open /sandbox/....

Route the task

1. Read this skill before probing files or software.
2. Classify the task:
   • MCP-only: Blender can inspect, edit, export, render, or validate the requested result and write it to a task-owned host directory.
   • Sandbox-only: every input has already been uploaded and every tool and output path is sandbox-local.
   • Hybrid: a later step requires a file produced on the other surface.
3. Probe capabilities on their owning surface. Use Blender MCP for bpy, the active .blend, add-ons, OVRTX, OVPhysX, and host file existence. Use sandbox tools only for sandbox paths and commands.
4. Execute one surface-local milestone at a time and verify its artifacts on that same surface.
5. At a hybrid boundary, stop and return TRANSFER_REQUIRED. Hermes cannot run the host-side nemohermes transfer command from inside its sandbox.

In the normal raw-Blender profile, use blender-python-api-verification for unknown Blender behavior: search the sandbox-local offline reference, then verify version-sensitive API names through live Blender RNA over MCP. The bounded handoff profile cannot run arbitrary reference or inspection code; use only its exposed typed operations and report unsupported capabilities.

Prefer bounded workflow tools

When the blender-workflow MCP server is available, use its typed tools before Blender's generic execute-code tool:

• capability_probe before claiming USD, OVRTX, OVPhysX, or SimReady support;
• scene_inventory with a task-owned host JSON path for complete scene detail;
• usd_export for a source-preserving Blender export and hash receipt;
• usd_inspect for composed-stage metadata, exact prim counts, dependencies, and physics-schema counts;
• artifact_receipts immediately before reporting any host file as created.

When a typed tool is discovered through tool_search, call tool_describe before its first invocation. Search results contain descriptions, not the full input schema, so never guess or rename argument fields. The path contract is:

• scene_inventory: optional output_path;
• usd_export: required output_path, optional selected_objects_only;
• usd_inspect: required input_path, optional output_path;
• artifact_receipts: required paths array.

If a tool returns invalid_arguments, call tool_describe, correct the named arguments, and retry once. This is a request validation failure, not evidence that Blender or the MCP server is unreachable.

A workflow operation exists only when tool_search or tool_describe exposes that exact operation. If the bounded tool set has no authoring, packaging, or execution operation required by the prompt, report that milestone as blocked. Search once for a missing operation. If no exact match is returned, stop discovery and report the blocker; do not search again with synonyms. Do not synthesize nested tool_call requests for disabled terminal or file tools, edit an installed skill to create a helper, or repeat USD inspection under new filenames; none of those actions add the missing capability.

When capability_probe reports that a required schema owner or authoring runtime is unsupported, finish only independent read-only milestones already underway, verify their receipts, and then return BLOCKED. Do not attempt dependent overlays, packaging, or validation claims.

Once scene_inventory or usd_inspect writes its detailed JSON and returns a successful receipt, treat that evidence milestone as complete. Do not try to read the host JSON with sandbox file tools, Blender resource URIs, or raw scene tools. The compact typed result is the conversation summary; the host JSON is for independent verification and downstream bounded operations.

Keep tool results compact. Write detailed inventories to task-owned host JSON artifacts and return only counts, paths, sizes, hashes, and blockers in the conversation. Never dump a full scene or stage inventory into the model context. Never author a long Python program inside an MCP JSON argument when a bounded workflow tool exists.

The typed tools intentionally do not emulate SimReady. If capability_probe does not find an enabled supported SimReady add-on, report its authoring and validation milestones as blocked. Do not invent custom properties and call them SimReady or nonvisual-material schema fields.

Transfer receipt

Return this compact structure instead of pretending a file is visible on both sides:

{
  "status": "TRANSFER_REQUIRED",
  "direction": "host_to_sandbox|sandbox_to_host",
  "source_surface": "blender_host|hermes_sandbox",
  "source_path": "exact path on the source surface",
  "destination_path": "requested path on the destination surface",
  "sha256": "hash measured on the source surface",
  "next_milestone": "what becomes possible after transfer"
}

After the external operator confirms the transfer, verify the destination hash before continuing.

Failure rules

• If a tool reports BLOCKED or says the user denied a command, stop that workflow. Do not retry, rephrase, or seek the same outcome with another sandbox command.
• Do not use find /, inspect restricted host-looking directories from the sandbox, or install a second Blender to compensate for wrong-surface access.
• A missing sandbox blender, bpy, add-on, native runtime, or host file does not prove that the host capability is missing. Probe it through Blender MCP.
• If a required capability is unavailable on its owning surface, return BLOCKED with the exact probe and error. Do not fabricate USD schemas, semantic buffers, simulation results, renders, files, or validation passes.
• Preserve the source scene and source assets. Use a task-owned copy or output directory for mutating evaluations.
• A final narrative is not artifact evidence. Report a host output only when artifact_receipts returns present with a nonzero size and SHA-256.

Closeout

Report the selected topology, every execution surface used, host and sandbox paths separately, artifact hashes measured on their owning surfaces, completed transfers, remaining transfer receipts, capability blockers, and any claim that was not independently verified.