MorphaReels

Split caption line

split_caption_line

Destructive

Split one caption line into two at a COMPOSITION frame strictly inside its window (in the editor this is the playhead). The right half is a full clone — style, band geometry, weld — and the text divides at the word gap nearest the split point (a single-word line keeps its text on the left; the right half starts empty). A welded line stays welded on both halves (the frame converts to the clip's source timeline); a standalone line splits its block. Returns { left, right, splitFrame }. Retime the halves afterwards with set_layer_block; fix the wording with set_layer_text.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "elementId": "<elementId>",
      "atFrame": "<atFrame>",
      "projectId": "<projectId>"
    },
    "name": "split_caption_line"
  }
}

Input Schema

Name	Required	Description
`atFrame`	Yes	Composition frame to split at — must be strictly inside the line's on-timeline window (see describe_video / inspect_layers for windows).
`elementId`	Yes	The caption line to split, "text.<id>".
`projectId`	Yes	Opaque project id (a v4 UUID, from list_projects/create_project). Selects which existing project this call mutates.

Output Schema

Name	Required	Description
`ok`	Yes	Whether the call succeeded.
`data`	No	The payload, shaped by the tool.
`note`	No	What to do next when not ready.
`error`	No	Why it failed.
`status`	No	For cache-backed readers: whether the answer was ready.
`editorUrl`	No	Opens this project in the editor.

Schema Changelog

Changes observed during successful MCP inspections.

Changed2 schema fields changedSep 17, 2026

removedInput schema / properties / anonymousToken

Removed value: -{
-  "description": "The token create_anonymous_account returned. This connection has no Morpha key, so every call carries it.",
-  "type": "string"
-}

changedInput schema / required

Previous value: -[
-  "elementId",
-  "atFrame",
-  "projectId",
-  "anonymousToken"
-]New value: +[
+  "elementId",
+  "atFrame",
+  "projectId"
+]

Changed1 schema field changedSep 15, 2026

changedOutput schema / (root)

Previous value: -nullNew value: +{
+  "description": "The result envelope every Morpha tool returns.",
+  "properties": {
+    "data": {
+      "description": "The payload, shaped by the tool.",
+      "type": [
+        "object",
+        "array",
+        "string",
+        "number",
+        "boolean",
+        "null"
+      ]
+    },
+    "editorUrl": {
+      "description": "Opens this project in the editor.",
+      "type": "string"
+    },
+    "error": {
+      "description": "Why it failed.",
+      "type": "string"
+    },
+    "note": {
+      "description": "What to do next when not ready.",
+      "type": "string"
+    },
+    "ok": {
+      "description": "Whether the call succeeded.",
+      "type": "boolean"
+    },
+    "status": {
+      "description": "For cache-backed readers: whether the answer was ready.",
+      "enum": [
+        "ready",
+        "not-ready"
+      ],
+      "type": "string"
+    }
+  },
+  "required": [
+    "ok"
+  ],
+  "type": "object"
+}

Changed2 schema fields changedSep 11, 2026

addedInput schema / properties / anonymousToken

Added value: +{
+  "description": "The token create_anonymous_account returned. This connection has no Morpha key, so every call carries it.",
+  "type": "string"
+}

changedInput schema / required

Previous value: -[
-  "elementId",
-  "atFrame",
-  "projectId"
-]New value: +[
+  "elementId",
+  "atFrame",
+  "projectId",
+  "anonymousToken"
+]

First observedSep 7, 2026

TDQS

A4.5/5.0

Behavior5/5

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

The description goes well beyond the destructiveHint annotation, detailing the exact cloning behavior (style, band geometry, weld), the text-division rule (nearest word gap, single-word line handling), the weld behavior (stays welded, frame converts to source timeline), and the return value. This provides the agent with deep insight into what will happen, far beyond the annotation's simple destructive hint.

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 description is dense but every sentence adds value: it states the action, the split rule, the cloning, the weld behavior, the return, and follow-up actions. It is front-loaded with the essential purpose and then details edge cases. No fluff or repetition.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a mutation tool with an output schema and a destructive hint, the description covers all essential behavioral aspects: what happens to each half, how text divides, how welds behave, what is returned, and what to do next. An agent has all the information needed to call it correctly and anticipate the result.

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?

The schema covers 100% of parameters with descriptions, so the baseline is 3. The description does not add extra semantic detail about the parameters themselves beyond what the schema already states (e.g., atFrame must be strictly inside window). It does mention the splitFrame return, but that's not parameter semantics. No need to compensate for gaps since the schema is thorough.

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 states a specific verb ('Split'), a specific resource ('one caption line'), and the exact condition ('at a COMPOSITION frame strictly inside its window'). It clearly distinguishes from the sibling 'merge_caption_lines' by describing the opposite operation, and it details the outcome (right half is a clone, text division rule).

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

Usage Guidelines4/5

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

The description clearly implies when to use this tool: when you need to split a caption line at a specific frame. It also provides follow-up guidance ('Retime the halves afterwards with set_layer_block; fix the wording with set_layer_text'), which helps the agent plan subsequent actions. However, it doesn't explicitly mention when NOT to use it or alternative tools like merge_caption_lines, though the purpose is self-evident.

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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