Premiere Pro MCP Server

Compute Mask Fit Motion

compute_mask_fit_motion

Read-onlyIdempotent

Calculates Motion Scale and Position to fit a still image's subject inside an existing Rounded Crop or Crop mask using a supplied subject box, without changing Premiere.

Instructions

Inspect only. Compute the Motion Scale (%) and Position that place a still image's subject inside an existing Rounded Crop, Crop, or similar mask effect. Reads the sequence frame size, the source frame size, current Motion values, and the mask effect's parameters, then solves the geometry deterministically from a caller-supplied subject box (fractions of the source image, for example head-top to chin). No image analysis and no changes to Premiere. Apply the result with set_clip_scale and set_clip_position (or set_effect_property), then verify with capture_frame. Assumes Rotation 0 and uniform scale; mask geometry is treated as fixed in the sequence frame (mask_space 'sequence').

Input Schema

Name	Required	Description
`node_id`	Yes	Node ID of the video clip that holds the still image and its Motion effect.
`subject`	Yes	Subject box in the SOURCE image as fractions of the source frame (0..1), for example top = head-top and bottom = chin or collar.
`fit_axis`	No	Fit the subject's height to placement top/bottom (default) or its width to placement left/right.
`placement`	No	Where the subject should sit inside the mask, as fractions of the mask's bounding box. Defaults: top 0.15, bottom 0.85, center_x 0.5 (height fit); left 0.15, right 0.85, center_y 0.5 (width fit).
`mask_space`	No	Where the mask geometry lives. 'sequence' (default): the mask stays fixed in the sequence frame while Motion moves the image (mask on an adjustment layer or nest, or an effect that renders after Motion). 'clip': the mask moves with the clip's Motion, so Motion cannot reframe the subject inside it and the tool returns an error explaining that.
`mask_effect`	No	Display name or match name of the mask effect (case-insensitive). Defaults to 'Rounded Crop'. Built-in 'Crop' Left/Top/Right/Bottom percentages are supported too.
`mask_node_id`	No	Node ID of the clip that carries the mask effect when it is not the image clip, for example an adjustment layer or nest above it. Defaults to node_id.
`source_width`	No	Source image width in pixels. Overrides the value read from project metadata; required with source_height when Premiere does not report it.
`mask_override`	No	Mask bounding box as fractions of the SEQUENCE frame. Skips reading mask parameters; use it when the effect's parameters cannot be interpreted.
`source_height`	No	Source image height in pixels. Overrides the value read from project metadata.
`source_prescale`	No	Extra scale Premiere applies before Motion, for example when Scale to Frame Size is on (sequence height / source height for a letterboxed fit). Defaults to 1.
`expected_cep_target`	No	Optional reviewed CEP sequence/clip identity and exact tick ranges. Checked before every generated command; UXP refuses this CEP-only guard.
`expected_project_path`	No	Optional absolute saved-project path. Every generated CEP command refuses before its body when the active project differs or is unsaved. UXP commands refuse this CEP-only guard; local-only tools do not inspect Premiere. Omit to keep existing behavior.

Output Schema

Name	Required	Description
`ok`	Yes	Whether the tool completed successfully.
`data`	No	Tool-specific result data when ok is true; on failure, diagnostic detail when the tool provides it.
`tool`	Yes	The registered MCP tool name.
`error`	No	Failure detail when ok is false.
`receipt`	No	Versioned common mutation receipt; success never implies rendered verification.
`idempotency`	No	Present when idempotency_key deduplicated this call: replayed is true when the original outcome was returned instead of running again.

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedv1.25.0Oct 24, 2026

addedOutput schema / properties / idempotency

Added value: +{
+  "additionalProperties": true,
+  "description": "Present when idempotency_key deduplicated this call: replayed is true when the original outcome was returned instead of running again.",
+  "type": "object"
+}

Changed3 schema fields changedv1.24.0Oct 12, 2026

addedInput schema / properties / expected_cep_target

Added value: +{
+  "additionalProperties": false,
+  "anyOf": [
+    {
+      "required": [
+        "sequence_id"
+      ]
+    },
+    {
+      "required": [
+        "source_item"
+      ]
+    }
+  ],
+  "description": "Optional reviewed CEP sequence/clip identity and exact tick ranges. Checked before every generated command; UXP refuses this CEP-only guard.",
+  "properties": {
+    "backend": {
+      "const": "cep",
+      "description": "Backend namespace for these IDs.",
+      "type": "string"
+    },
+    "clip": {
+      "additionalProperties": false,
+      "description": "Optional exact reviewed timeline clip.",
+      "properties": {
+        "effect": {
+          "additionalProperties": false,
+          "description": "Reviewed effect and parameter identity checked before mutation.",
+          "properties": {
+            "display_name": {
+              "description": "Optional exact host display name.",
+              "maxLength": 512,
+              "minLength": 1,
+              "type": "string"
+            },
+            "match_name": {
+              "description": "Effect match name from host inspection.",
+              "maxLength": 512,
+              "minLength": 1,
+              "type": "string"
+            },
+            "parameter_index": {
+              "description": "Inspected zero-based parameter index.",
+              "maximum": 255,
+              "minimum": 0,
+              "type": "integer"
+            },
+            "parameter_name": {
+              "description": "Exact inspected parameter name.",
+              "maxLength": 512,
+              "minLength": 1,
+              "type": "string"
+            }
+          },
+          "required": [
+            "match_name",
+            "parameter_index",
+            "parameter_name"
+          ],
+          "type": "object"
+        },
+        "end_ticks": {
+          "description": "Exact expected exclusive timeline end ticks.",
+          "pattern": "^-?\\d{1,30}$",
+          "type": "string"
+        },
+        "in_ticks": {
+          "description": "Exact expected source In ticks.",
+          "pattern": "^-?\\d{1,30}$",
+          "type": "string"
+        },
+        "node_id": {
+          "description": "Stable CEP node ID.",
+          "maxLength": 512,
+          "minLength": 1,
+          "type": "string"
+        },
+        "out_ticks": {
+          "description": "Exact expected exclusive source Out ticks.",
+          "pattern": "^-?\\d{1,30}$",
+          "type": "string"
+        },
+        "project_item_id": {
+          "description": "Expected source project-item node ID.",
+          "maxLength": 512,
+          "minLength": 1,
+          "type": "string"
+        },
+        "start_ticks": {
+          "description": "Exact expected timeline start ticks.",
+          "pattern": "^-?\\d{1,30}$",
+          "type": "string"
+        },
+        "track_index": {
+          "description": "Expected zero-based track index; identity still checked.",
+          "maximum": 255,
+          "minimum": 0,
+          "type": "integer"
+        },
+        "track_type": {
+          "description": "Expected media track group.",
+          "enum": [
+            "video",
+            "audio"
+          ],
+          "type": "string"
+        }
+      },
+      "required": [
+        "node_id",
+        "track_type",
+        "track_index"
+      ],
+      "type": "object"
+    },
+    "project_document_id": {
+      "description": "Exact Premiere project document ID observed during review.",
+      "maxLength": 512,
+      "minLength": 1,
+      "type": "string"
+    },
+    "sequence_id": {
+      "description": "Stable expected active sequence ID.",
+      "maxLength": 512,
+      "minLength": 1,
+      "type": "string"
+    },
+    "source_item": {
+      "additionalProperties": false,
+      "description": "Reviewed source identity for a source-only operation.",
+      "properties": {
+        "media_path": {
+          "description": "Optional exact source-media path.",
+          "maxLength": 4096,
+          "minLength": 1,
+          "type": "string"
+        },
+        "node_id": {
+          "description": "Exact project-item node ID.",
+          "maxLength": 512,
+          "minLength": 1,
+          "type": "string"
+        }
+      },
+      "required": [
+        "node_id"
+      ],
+      "type": "object"
+    }
+  },
+  "required": [
+    "backend"
+  ],
+  "type": "object"
+}

addedInput schema / properties / expected_project_path

Added value: +{
+  "description": "Optional absolute saved-project path. Every generated CEP command refuses before its body when the active project differs or is unsaved. UXP commands refuse this CEP-only guard; local-only tools do not inspect Premiere. Omit to keep existing behavior.",
+  "maxLength": 4096,
+  "minLength": 1,
+  "type": "string"
+}

addedOutput schema / properties / receipt

Added value: +{
+  "additionalProperties": true,
+  "description": "Versioned common mutation receipt; success never implies rendered verification.",
+  "type": "object"
+}

Changed1 schema field changedv1.18.6Sep 30, 2026

changedOutput schema / properties / data / description

Previous value: -"Tool-specific result data when ok is true."New value: +"Tool-specific result data when ok is true; on failure, diagnostic detail when the tool provides it."

Addedv1.18.0Sep 23, 2026

TDQS

A4.6/5.0

Behavior5/5

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

Beyond the readOnly/idempotent annotations, the description discloses that the tool reads sequence size, source size, current Motion values, and mask parameters, and solves deterministically from a caller-supplied box. It states the modeling assumptions (Rotation 0, uniform scale, mask fixed in sequence frame) and one explicit failure mode, which the annotations alone would not convey.

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 scope flag 'Inspect only' is front-loaded, followed immediately by what is computed, then inputs read, then how to apply. Five sentences carry distinct load with no repetition of the schema or of the title.

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?

For a 13-parameter geometry solver with an output schema, the description covers purpose, inputs read, assumptions, and the follow-up workflow, so an agent can call it correctly. Minor gaps remain: behavior when the node is not a still or the mask effect is not found, and no note that the CEP/path guard parameters are optional no-ops for this local computation.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so every parameter already carries an explanation; the description mostly restates them at a high level (subject box as fractions, mask_space 'sequence' default). It adds a little framing ('head-top to chin') but no new syntax, units, or interaction rules beyond the schema, so the baseline of 3 applies.

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 names a specific verb and output: compute Motion Scale (%) and Position to fit a still's subject inside an existing mask effect. It explicitly marks the tool as 'Inspect only' and 'No image analysis and no changes to Premiere', which separates it cleanly from the mutating siblings in the list (set_clip_scale, set_clip_position, crop_clip).

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

Usage Guidelines5/5

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

It states the situation that selects this tool (placing a still's subject inside a Rounded Crop/Crop) and names the exact downstream alternatives for applying the result (set_clip_scale, set_clip_position, set_effect_property) plus verification via capture_frame. The mask_space='clip' case is called out as an error condition, so the agent knows when the tool cannot be used.

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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