Agent-Skills.md

Agent Skills: Correct: Input options before their -i

Complete FFmpeg command syntax reference covering option ordering, input vs output options, stream specifiers, and position-sensitive options. PROACTIVELY activate for: (1) Command syntax questions, (2) Option placement issues, (3) Input vs output option confusion, (4) Stream specifier syntax, (5) -ss/-t/-to position questions, (6) Global vs per-file options, (7) Multiple input/output handling, (8) Option order errors. Provides: Correct option placement rules, input-only vs output-only options, position-sensitive option behavior, stream specifier syntax, common mistakes and fixes.

UncategorizedID: josiahsiegel/claude-plugin-marketplace/ffmpeg-command-syntax

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josiahsiegel/claude-plugin-marketplace
JosiahSiegelLicense: MIT
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plugins/ffmpeg-core/skills/ffmpeg-command-syntax/SKILL.md

Skill Metadata

Name
ffmpeg-command-syntax
Description
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CRITICAL: FFmpeg Option Ordering Rules

The most common FFmpeg mistake is putting options in the wrong place. Options in FFmpeg are position-sensitive and apply to the NEXT file specified after them.

The Golden Rule

ffmpeg [global_options] {[input_options] -i input}... {[output_options] output}...

Key principle: Options are applied to the next file. They are reset between files.

Option Categories at a Glance

| Category | Where it goes | Notable members | |----------|--------------|-----------------| | Global | First, before any -i | -y, -n, -v, -hide_banner, -filter_complex, -init_hw_device | | Input | Between previous output (or start) and the next -i | -ss, -t, -to, -re, -stream_loop, -hwaccel, -itsoffset | | Output | After all -i, before the matching output file | -c:v, -c:a, -crf, -preset, -vf, -af, -map, -movflags |

The full per-category catalog (every option, with descriptions) lives in references/complete-option-reference.md.

Global Options (Quick Subset)

ffmpeg -y -hide_banner -v warning -i input.mp4 output.mp4

Common global flags: -y, -n, -v/-loglevel, -stats, -progress, -report, -hide_banner, -filter_complex, -filter_complex_threads, -init_hw_device, -filter_hw_device.

Input Options (Quick Subset)

Placed immediately before the -i they apply to. They affect how the file is read/decoded.

Most-used: -ss, -t, -to, -itsoffset, -itsscale, -re, -readrate, -stream_loop, -hwaccel, -hwaccel_device, -hwaccel_output_format, -c:v/-c:a (as decoder), -r/-s/-pix_fmt (raw inputs), -accurate_seek, -thread_queue_size.

# Correct: Input options before their -i
ffmpeg -ss 00:01:00 -t 30 -i input.mp4 output.mp4

# Wrong: -ss after -i becomes an OUTPUT (slow) seek
ffmpeg -i input.mp4 -ss 00:01:00 output.mp4

Hardware acceleration must always go before -i:

ffmpeg -hwaccel cuda -hwaccel_output_format cuda -i input.mp4 \
  -c:v h264_nvenc output.mp4

Output Options (Quick Subset)

Placed after all inputs and before the output file they apply to. They affect how the file is encoded/written.

Most-used: -c:v/-c:a/-c:s (as encoder), -b:v/-b:a, -crf, -qp, -preset, -tune, -profile:v, -level, -r/-s/-aspect/-pix_fmt, -vf/-af, -map, -ss/-t/-to (output forms), -fs, -frames:v/-frames:a, -movflags, -metadata, -disposition, -shortest, -an/-vn/-sn.

ffmpeg -i input.mp4 -c:v libx264 -crf 23 -preset medium output.mp4

Options That Switch Behavior by Position

Some options have completely different meanings depending on whether they appear before or after -i.

-ss (Seek/Start Time) - Position Critical

# BEFORE -i (INPUT): Fast keyframe seek, then decode to exact position with -accurate_seek (default)
ffmpeg -ss 00:01:00 -i input.mp4 -t 30 -c copy output.mp4

# AFTER -i (OUTPUT): Frame-accurate but slow - decodes everything, discards until target
ffmpeg -i input.mp4 -ss 00:01:00 -t 30 -c:v libx264 output.mp4

# BOTH (recommended): coarse seek + fine seek
ffmpeg -ss 00:00:55 -i input.mp4 -ss 00:00:05 -t 30 -c:v libx264 output.mp4

Note: -ss before -i resets timestamps to 0. Use -copyts to preserve originals.

-t and -to

# -t BEFORE -i: limits how much of input to READ
ffmpeg -t 60 -i input.mp4 -c copy output.mp4

# -t AFTER -i: limits output DURATION
ffmpeg -i input.mp4 -t 60 -c copy output.mp4

# -ss + -to: with timestamp reset, -to is relative to new zero
ffmpeg -ss 00:01:00 -i input.mp4 -to 00:00:30 output.mp4  # 30s clip
ffmpeg -ss 00:01:00 -copyts -i input.mp4 -to 00:01:30 output.mp4  # also 30s clip

-r (Frame Rate)

# BEFORE -i: input frame rate (raw/image sequences)
ffmpeg -r 30 -i frame_%04d.png output.mp4

# AFTER -i: output frame rate (adds/drops frames)
ffmpeg -i input.mp4 -r 24 output.mp4

-s (Size)

# BEFORE -i: raw input size
ffmpeg -s 1920x1080 -pix_fmt yuv420p -i raw.yuv output.mp4

# AFTER -i: scales output (prefer -vf scale instead)
ffmpeg -i input.mp4 -s 1280x720 output.mp4

-c / -codec

# BEFORE -i: DECODER selection
ffmpeg -c:v h264_cuvid -i input.mp4 -c:v h264_nvenc output.mp4

# AFTER -i: ENCODER selection (common case)
ffmpeg -i input.mp4 -c:v libx264 -c:a aac output.mp4

Stream Specifiers (Quick Form)

Target specific streams with -option[:specifier]:

ffmpeg -i input.mkv -c:v libx264 -c:a aac -c:s mov_text output.mp4
ffmpeg -i a.mp4 -i b.mp4 -map 1:0 -c copy output.mp4

Common specifiers: :v, :V, :a, :s, :d, :t, :v:0, :a:1, :0, :m:key:value, :p:0. Full table and per-stream examples in references/stream-specifiers.md.

Multiple Inputs and Outputs

# Per-input options
ffmpeg \
  -ss 10 -i first.mp4 \
  -ss 5 -i second.mp4 \
  -filter_complex "[0:v][1:v]overlay" output.mp4

# Per-output options (they RESET between outputs)
ffmpeg -i input.mp4 \
  -c:v libx264 -crf 23 output_h264.mp4 \
  -c:v libx265 -crf 28 output_h265.mp4

# WRONG: second output silently gets no encoding options
ffmpeg -i input.mp4 -c:v libx264 -crf 23 out1.mp4 out2.mp4

Common Mistakes and Fixes

1. Input option after -i

# WRONG: -hwaccel after -i has no effect
ffmpeg -i input.mp4 -hwaccel cuda -c:v h264_nvenc output.mp4
# CORRECT
ffmpeg -hwaccel cuda -i input.mp4 -c:v h264_nvenc output.mp4

2. Output option before -i

# WRONG: -c:v before -i tries to choose a decoder
ffmpeg -c:v libx264 -i input.mp4 output.mp4
# CORRECT
ffmpeg -i input.mp4 -c:v libx264 output.mp4

3. Expecting options to persist across outputs

# WRONG
ffmpeg -i input.mp4 -c:v libx264 -crf 23 out1.mp4 out2.mp4
# CORRECT
ffmpeg -i input.mp4 \
  -c:v libx264 -crf 23 out1.mp4 \
  -c:v libx264 -crf 23 out2.mp4

4. Wrong -ss position for accuracy

# Fast but may snap to keyframe
ffmpeg -ss 00:05:00 -i input.mp4 -t 30 -c copy output.mp4
# Accurate but slow
ffmpeg -i input.mp4 -ss 00:05:00 -t 30 -c:v libx264 output.mp4
# Fast AND accurate
ffmpeg -ss 00:04:55 -i input.mp4 -ss 5 -t 30 -c:v libx264 output.mp4

5. Mixing inputs and outputs

# WRONG
ffmpeg -i input1.mp4 -c:v libx264 output1.mp4 -i input2.mp4 output2.mp4
# CORRECT
ffmpeg -i input1.mp4 -i input2.mp4 \
  -map 0 -c:v libx264 output1.mp4 \
  -map 1 -c:v libx264 output2.mp4

6. -t vs -to with -ss timestamp reset

ffmpeg -ss 00:01:00 -i input.mp4 -to 00:00:30 output.mp4         # 30s clip
ffmpeg -ss 00:01:00 -copyts -i input.mp4 -to 00:01:30 output.mp4 # 30s clip
ffmpeg -ss 00:01:00 -i input.mp4 -t 00:00:30 output.mp4          # 30s clip

7. -vf vs -filter_complex

# Single-input filter (per-output)
ffmpeg -i input.mp4 -vf "scale=1280:720" output.mp4

# Multi-input filter (global)
ffmpeg -i video.mp4 -i overlay.png \
  -filter_complex "[0:v][1:v]overlay=10:10" output.mp4

# WRONG: -vf cannot reference multiple inputs
ffmpeg -i video.mp4 -i overlay.png -vf "overlay=10:10" output.mp4

Command Structure Examples

# Basic transcode
ffmpeg -i input.mp4 -c:v libx264 -crf 23 -c:a aac output.mp4

# With input seeking
ffmpeg -ss 60 -i input.mp4 -t 30 -c:v libx264 output.mp4

# Hardware acceleration
ffmpeg -hwaccel cuda -hwaccel_output_format cuda -i input.mp4 \
  -c:v h264_nvenc -preset p4 output.mp4

# Multiple inputs with filters
ffmpeg -i main.mp4 -i overlay.png \
  -filter_complex "[0:v][1:v]overlay=10:10[v]" \
  -map "[v]" -map 0:a -c:v libx264 -c:a copy output.mp4

# Real-time streaming
ffmpeg -re -i input.mp4 -c:v libx264 -f flv rtmp://server/live/stream

# Full structure
ffmpeg \
  -y -hide_banner \
  -hwaccel cuda -ss 10 -i input1.mp4 \
  -stream_loop -1 -i input2.mp4 \
  -filter_complex "[0:v][1:v]overlay[v]" \
  -map "[v]" -map 0:a \
  -c:v h264_nvenc -b:v 5M \
  -c:a aac -b:a 192k \
  -movflags +faststart \
  output.mp4

Quick Placement Cheatsheet

| Option | Before -i | After -i | Notes | |--------|-------------|------------|-------| | -ss | Fast seek | Accurate seek | Before = keyframe, After = decode | | -t | Input limit | Output limit | Read vs write | | -to | Input end | Output end | Affected by timestamp reset | | -r | Input FPS | Output FPS | Raw / output conversion | | -s | Input size | Output size | Raw / scaling | | -c:v/-c:a | Decoder | Encoder | Before = decode | | -hwaccel, -re, -itsoffset, -stream_loop | Required | N/A | Input only | | -vf, -af, -map, -crf, -preset, -movflags | N/A | Required | Output only | | -y, -v, -filter_complex | Global | Global | Place first |

Extended Reference Map

For exhaustive lookups, see the companion reference files:

External References