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extract_frame

Idempotent

Extract one frame at an exact timestamp from the original source video at native resolution. Use when keyframes miss the exact moment or full detail is needed.

Instructions

Re-extract ONE frame at an exact timestamp from the ORIGINAL source video at native resolution, with an optional crop {x, y, w, h} in source pixels. Use when the stored keyframes miss the instant (they capture scene changes + a 1 fps floor) or when you need full-resolution detail. Slower than get_frames — it decodes the source file, which must still exist at its recorded path. When NOT to use: normal browsing — get_frames serves stored keyframes instantly without touching the source. Examples:

  • keyframes sit at 12:31 and 12:38 but the flash happened at 12:34.5 → extract_frame(job_id, at_ms=754500)

  • extract_frame(job_id="...", at_ms=754500, crop={"x":800,"y":40,"w":400,"h":120}) — zoom into the toast text

  • tiny UI text unreadable in the 1568px keyframe → extract_frame at the same ms for native resolution

  • verify a one-frame glitch: extract_frame at 12300, 12400, 12500 and compare

  • OCR missed small text → extract_frame with a tight crop, then read the returned image

  • crop coordinates are SOURCE pixels (a Retina screen recording may be 2940x1912) — not keyframe scale

  • source file moved or deleted → clear error; stored keyframes via get_frames still work

  • audio-only job → always errors: there is no video stream to decode

  • anti-example: "show me around 5:00" → get_frames(job_id, at_ms=300000); extract_frame is for exact instants

  • anti-example: scanning a range frame by frame → get_frames(start_ms, end_ms) first, refine once after

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
cropNo
at_msYes
job_idYes
Behavior5/5

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

Disclosures beyond annotations: slower than get_frames, decodes source file, requirement that source file exists at recorded path, and error conditions. No contradiction with annotations (readOnlyHint=false, idempotentHint=true are consistent with a non-destructive, idempotent extraction).

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?

Well-structured: first sentence defines core purpose, followed by usage guidance, examples, and edge cases. All sentences are informative; no redundancy.

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?

Covers all necessary aspects: purpose, when to use/avoid, parameter details, error cases, and examples. No output schema exists, but the description sufficiently describes the tool's behavior and limitations.

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

Parameters5/5

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

Schema has 0% description coverage, but the description fully compensates: explains 'at_ms' as exact timestamp in ms, 'crop' as optional object with {x,y,w,h} in source pixels, and that crop coordinates are source pixels not keyframe scale. Adds semantics not in schema.

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 clearly states the tool extracts one frame at an exact timestamp from the original source video at native resolution, with optional crop. It distinguishes from sibling 'get_frames' by specifying it uses the original source for exact timestamps and full resolution.

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?

Explicitly states when to use (keyframes miss instant or need full resolution) and when not to use (normal browsing via get_frames). Provides multiple examples and anti-examples, plus error scenarios (source moved/deleted, audio-only).

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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