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by io.github.molkex
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generate_video_between_frames

Generate the transition between TWO given frames: the video starts at the first image and ends at the second one, Flow invents everything in between.

This is Flow's "Кадры" with both slots filled — a different mechanism from
generate_video_from_image, which only pins the FIRST frame and lets the ending drift.
Use this when the ending matters: a logo that must resolve exactly, a character that
must land in a specific pose, two stills from a storyboard that need connecting.

Both images are raw base64 bytes, no data: URI prefix. The default model
(veo_3_1_interpolation_lite, 10 credits, 8 seconds) is the cheap one — transitions
are something you retry a few times, and defaulting to the 100-credit key would make
every experiment expensive.

beats: how many DISTINCT action phases the prompt describes — "ears flick back",
"tail lashes", "strikes the ball", "ball flies off" are four. Give it and the server
picks the clip length for you (about two phases per second) and says in the reply what
it picked and why. Measured 31.08.2026: the model STRETCHES whatever you describe over
whatever length you ask for, so one phase in four seconds comes out as slow motion, and
the same text at six seconds instead of four loses 15% of its movement. Adjectives are
not phases: "springy cartoon motion" adds nothing. Leave beats out and the length you
passed is used unchanged.

project_id: put the clip into a project made with create_project instead of the
account's default one.
beats: how many DISTINCT action phases the prompt describes. The server then picks
the clip length itself (about two phases per second) and says so in the reply. For a
first+last-frame transition keep it low: the two frames already fix the endpoints, and
asking for more seconds than the action needs only stretches it — measured 31.08.2026,
the same text at six seconds instead of four lost 15% of its movement.

Input Schema

NameRequiredDescriptionDefault
seedNo
beatsNo
aspectNoVIDEO_ASPECT_RATIO_LANDSCAPE
promptYes
project_idNo
include_previewNo
video_model_keyNoveo_3_1_interpolation_lite
end_image_base64Yes
start_image_base64Yes

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changed
    • addedInput schema / properties / seed
      Added value: +{
+  "anyOf": [
+    {
+      "type": "integer"
+    },
+    {
+      "type": "null"
+    }
+  ],
+  "default": null,
+  "title": "Seed"
+}
  2. First observed

TDQS

A4.5/5.0
Behavior5/5

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

Annotations only cover the safety profile (non-readonly, open-world, non-idempotent), and the description adds substantial context on top: images must be raw base64 with no data: URI prefix, the default model is the 10-credit/8-second cheap one versus a 100-credit key, and the model STRETCHES described action over the requested duration (with a measured example). This is exactly the kind of cost/behavior disclosure that helps an agent invoke the tool well.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness3/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The purpose and the beats semantics are well front-loaded, but the description is padded by near-verbatim duplication: the 'beats' paragraph and its 31.08.2026 stretching measurement appear twice, and the project_id sentence repeats the beats explanation. A tight edit would cut roughly a third of the text with no loss of meaning.

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

Completeness3/5

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

For a 9-parameter, 0%-schema-coverage tool with no output schema, the description covers the critical semantics (beats, cost/model choice, image encoding, project scoping) but silently omits seed, aspect, and include_preview, and never enumerates what the eleven video_model_key values mean beyond the default. Adequate, with clear remaining gaps.

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

Parameters4/5

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

Schema description coverage is 0%, so the description carries the full burden. It explains beats in depth (distinct action phases, ~two per second, server picks length, adjectives don't count), the two image params (raw base64, no prefix), and project_id (must come from create_project). It leaves seed, aspect, and include_preview entirely unexplained, so it doesn't fully close the 9-param gap.

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?

States a specific verb and resource ('generate the transition between TWO given frames') and immediately scopes it: video starts at first image, ends at second. It explicitly contrasts with the sibling generate_video_from_image ('only pins the FIRST frame'), so an agent can pick between them without opening either schema.

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?

Gives explicit when-to-use conditions with concrete examples ('a logo that must resolve exactly', 'two stills from a storyboard'), names the alternative tool, and explains the mechanism difference that selects it. Nothing about tool selection is left to inference.

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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