Configuration

MifBridge is configured with environment variables. Each is read by the process that owns that piece: the Unreal editor, Blender, or the MCP server. The editor also has a few console variables. On purpose, there is no endpoint that changes configuration.

The Unreal editor

Variable	Default	What it does
MIF_BRIDGE_TOKEN	dev	Shared secret. Every request must send it in the X-Mif-Token header. Change it before real use, on both the editor and the MCP server.
MIF_BRIDGE_PORT	8791	The port the bridge listens on. It always binds loopback only.
MIF_BRIDGE_WRITE_MODE	scratch	The starting write mode: read, scratch or full. See Safety and Write Modes.
MIF_CHANGE_CAP	25	From 1.2: how many things one call may change before it answers a list to commit instead. 0 turns the cap off. A project's own setting wins. See the change cap.
MIF_BRIDGE_NO_UPDATE_CHECK	unset	1 turns off the daily update check for every project. Editor Preferences ▸ Plugins ▸ Mif Bridge ▸ Check for Updates does the same per engine version.
MIF_BRIDGE_AUTO_DECLINE_RECOVERY	off	1 declines the editor's crash-recovery prompt at startup. This discards unsaved recovery data, so use it only on disposable automation projects.

The editor reads these from its own process environment when it starts, so a change takes effect at the next launch. To make one stick on Windows:

setx MIF_BRIDGE_TOKEN "your-secret-here"

Then restart the editor, and the MCP client, so each picks up the new value.

Console variables

Variable	Default	What it does
mif.BridgeAutoOpen	1	Opens the MifBridge panel a moment after the editor starts. 0 turns it off; the panel stays available under Window.
mif.ModalGuardServe	1	From 1.2: keeps the bridge answering while a modal dialog is open, refusing every call except the dialog's own. 0 only records the dialog. See MifBridge.
mif.SourceControl.SessionChangelist	0	With a revision control provider connected, 1 puts the files the bridge checks out into a changelist of their own for the session, instead of the default one.
mif.BridgeJournal	1	The crash journal: one record per call, flushed to disk before the call runs. Leave it on. A crash journal you have to enable before the crash is off when you need it.

The MCP server

Variable	Default	What it does
MIF_BRIDGE_URL	http://127.0.0.1:8791/api	Where the editor's API is. Point it at another port for a second editor. Unset, it follows MIF_BRIDGE_PORT when that is set.
MIF_BRIDGE_TOKEN	dev	Must match the editor's.
MIF_BRIDGE_TIMEOUT	30	Per-request timeout in seconds. Raise it if compiles are slow.
MIF_BRIDGE_DEBUG	unset	1 logs requests and responses to stderr.
MIF_AGENT	the client's own name	A name for the caller, sent as the X-Mif-Agent header and shown after each call's endpoint in the panel's Activity tab. Unset, the name the MCP client gives for itself is used. Self-reported, and not checked.
MIF_TOOL_PACKS	lean	Which tool packs to load. Unset (or lean), only find_tools, mif_call and run_python are registered, and mif_call reaches every other tool. Name packs, for example core,world, to load their tools directly (core comes with any named pack), or all for every tool. See MCP Server.
MIF_GENERATIVE	unset	off switches the generative bl_gen_* tools off for every project the server touches, whatever a project's own mode says. See Using MifBridge Without Generative AI.
MIF_EDITOR_EXE	unset	The editor binary mif_editor starts when a call names no engine. Unset, it is found from the project's engine association.
MIF_BLENDER_HOST	127.0.0.1	The Blender add-on's address.
MIF_BLENDER_PORT	8792	The port a session starts on. From 1.2, a MifBlender on another port is found anyway, and bl_status switches to it.
MIF_BLENDER_TOKEN	falls back to MIF_BRIDGE_TOKEN	One secret for both backends, unless you want two.
MIF_BLENDER_CONNECT_TIMEOUT	3	How long to wait to reach Blender at all, so a closed Blender reports "refused" quickly instead of hanging.
MIF_BLENDER_PROBE_TIMEOUT	5	bl_status only: the cheap "is it alive" call.
MIF_BLENDER_TIMEOUT	180	Per Blender operation. Geometry work can take a while.

The timeout ladder

The Blender add-on abandons a job after 150 seconds, and that has to stay below MIF_BLENDER_TIMEOUT. Then Blender gives up first and the socket carries a real error.

The other way round, the server drops the connection while Blender goes on changing the scene, on behalf of a caller that has already been told the operation failed. If you raise one timeout, raise the other and keep the add-on's lower.

The Blender add-on

The add-on's Token and Port are preferences in Edit ▸ Preferences ▸ Add-ons ▸ MifBlender. They must agree with MIF_BLENDER_TOKEN and MIF_BLENDER_PORT on the MCP server. It binds 127.0.0.1 only, and there is deliberately no setting to bind anything wider.

Its other preferences are Main-thread job timeout (150 seconds; see the timeout ladder above), Allow run_python (on by default; see MifBlender), Allow generative tools (ComfyUI) (on by default; off refuses every generation operation in that Blender), Start server automatically, Log to console, Write the call journal to disk, and the Viewport HUD (off by default).

The add-on also reads two variables from Blender's own environment:

Variable	Default	What it does
MIF_BLENDER_UNDO	on	0 stops the add-on recording undo steps. See MifBlender.
MIF_CHANGE_CAP	25	The change cap for a .blend that has none of its own, as on the Unreal side.
MIF_COMFY_HOST	127.0.0.1:8188	Where the generation operations (gen_image, gen_texture, gen_mesh, gen_asset) find the local ComfyUI server they drive.

Ports

Port	Owner	Notes
8791	MifBridge, your first Unreal editor	The default.
8792	MifBlender, the Blender add-on	Reserved. Do not point an editor here.
8793–8799	leave clear	Headroom for the two above.
8801 and up	additional Unreal editors	For a second project's editor, set MIF_BRIDGE_PORT=8801, and point that MCP server's MIF_BRIDGE_URL at it.
9876	the third-party blender-mcp add-on	Not ours. Avoided so both can be installed side by side.

Why this matters more than it looks. A port collision does not look like a collision. Put a second Unreal editor on 8792 and the Blender tools connect, something answers, and every check anyone would run passes. But they are now speaking Blender's socket protocol to an HTTP server, and nothing works. Move the editor, not the add-on: the editor's port is one variable, while the add-on's lives in two places that must agree. MifBridge logs a warning at startup if it is configured for 8792 or 9876.