PERISCOPE — Read-only Fusion 360 MCP

Let a local AI inspect the Autodesk Fusion design you have open — without letting it touch anything.

PERISCOPE exposes the live Fusion session as structured, strictly read-only MCP tools: assembly tree, joints, rigid groups, assembly constraints, timeline health, parameters, current selection, viewport screenshots — plus direct measurement (measure, get_geometry_context) so answers come from evaluated geometry, not guesswork.

"List all joints, rigid groups, and constraints in my active design, and tell me which ones are in error or conflict." "Measure the minimum clearance between these two components." "What did I just select, and what drives its dimensions?"

Status: working, actively developed, Windows-verified. macOS paths exist but are untested. Expect rough edges; the tools report their own limits honestly.

Read-only means read-only

There is no write path in this codebase. No mutation tools, no "run Python in Fusion" escape hatch, no timeline rollback; the local bridge answers no HTTP method other than GET. This is a property of the code, enforced by a regression test that fails the build if a mutating call appears — not a runtime flag. It is safe to query mid-command, mid-design, without wondering what it might do.

How it works

Any MCP client (Claude Desktop, Claude Code, others)
        │  MCP over stdio
        ▼
 periscope_mcp            external Python process (official `mcp` SDK)
        │  HTTP GET, 127.0.0.1:8735, bearer token
        ▼
 Fusion add-in bridge     inside Fusion's embedded Python, stdlib only
        ▼
 the live Fusion session  (all API calls on Fusion's main thread)

Two processes, because the Fusion API only exists inside Fusion's interpreter and is not thread-safe. The bridge binds loopback only and requires a locally generated bearer token on every request.

Quick start

Requirements: Windows, Autodesk Fusion, Python 3.10+.

git clone https://github.com/VXNTedits/fusion360-mcp-periscope
cd fusion360-mcp-periscope
python install/install.py

Then in Fusion: Shift+S → Add-Ins tab → Periscope → Run (tick Run on Startup).

Verify from a shell:

python install/verify.py --probe

Register with Claude Desktop / Claude Code:

python install/register.py --show   # preview
python install/register.py --all

Any other stdio-MCP client — point it at:

{
  "command": "<repo>/server/.venv/Scripts/python.exe",
  "args": ["-m", "periscope_mcp"],
  "env": { "PYTHONPATH": "<repo>/server" }
}

Note: the registered command points into your clone — don't move the folder after installing.

The tools

Twelve, all annotated read-only. See FEATURES.md for details and AGENTS.md for AI-facing usage guidance.

fusion_health · get_session_info · get_assembly_tree · get_joints · get_rigid_groups · get_constraints · get_timeline · get_parameters · measure · get_geometry_context · get_selection · capture_viewport

Design principles

• Measured over inferred. Distances and extents come from Fusion's MeasureManager and evaluated B-Rep bounds. Every value is classified fusion_measured, fusion_reported, or derived (with the formula shown). Parameters are labeled as nominal inputs, because a 1000 mm extrusion parameter says nothing about what downstream features left behind.
• Honest gaps. "This design has none" and "the API cannot see this" are never the same answer. Coverage blocks, truncation notes, and inline error nodes make the difference explicit — a design with one broken feature stays inspectable, because the broken feature is usually what you're asking about.
• Boring security. Loopback-only bind (refuses to start otherwise), per-install token compared with hmac.compare_digest, browser Origin and DNS-rebinding Host checks, failing closed. Design data leaves the machine only as answers into your own AI conversation.

Limits (current)

• Inspects open Design documents; does not browse cloud files, drawings, or version history.
• Constraint relationship entities (which faces an existing constraint mates) are not readable — the Fusion API only exposes them after a timeline roll, which is a write, which PERISCOPE refuses. Names, health, error text, offsets, and timeline order are fully reported.
• Very large assemblies get bounded, explicitly-labeled partial results rather than hangs.
• One request at a time inside Fusion (by design); a modal dialog or long compute yields a clean fusion_busy.

Credits

The transport and threading patterns were informed by reading several MIT-licensed Fusion MCP projects, chiefly ndoo/fusion360-mcp-bridge and frankhommers/autodesk-fusion-mcp. Neither was forked: both are authoring tools with code-execution surfaces, and PERISCOPE's read-only diagnostics surface had no prior art to adapt.

License

MIT