# AGENTS.md — Build guide for CapCut

## Project scope
Import a local clip, generate editable captions, trim through a non-destructive timeline and choose a social aspect ratio. Preview safe caption areas and render an explicit standard video preset.

Catalogue verdict: kinda. A captioner/trimmer/exporter is buildable, but CapCut's value is mobile editing polish, effects/templates, asset library, TikTok workflow, and real-time rendering.
Use the implementation prompt below to define the deliverable. Complete each phase's acceptance checks before extending the scope.

## Working agreement
- Inspect the repository and its existing instructions before choosing paths, dependencies or commands. Keep one coherent stack and explain changes to the proposed architecture.
- Plan a vertical slice that accepts a real input and produces the useful output described below. Persist only the state the prompt calls for; respect memory-only and upstream-managed workflows. Use fixtures only when they are clearly labelled.
- After scaffolding, document the actual install, development, check and build commands in README and keep them synchronized with the package or project manifest. Do not report commands as successful unless they ran.
- Work in small steps. At handoff, list implemented flows, checks actually performed, remaining blockers, and any credentials or provider setup the owner must supply.
- Do not publish, spend money, contact customers, delete source data or run irreversible migrations without the project owner's authorization.

## Prerequisites
- A Python virtual environment, writable input/output directories and sufficient disk for both originals and outputs. Bind the service to localhost. Install FFmpeg and confirm codec support for the intended inputs.
- Implementation components: Python, FastAPI and server-rendered HTML with HTMX for a local interface. SQLite for manifests and job state, with an explicit worker process and immutable source files. FFmpeg/ffprobe for explicit media operations and browser previews; never interpolate user filenames into shell commands.
- Scope boundary: mobile app quality; effects/templates

## Stack and architecture
- Python, FastAPI and server-rendered HTML with HTMX for a local interface.
- SQLite for manifests and job state, with an explicit worker process and immutable source files.
- FFmpeg/ffprobe for explicit media operations and browser previews; never interpolate user filenames into shell commands.
- Domain model: video projects, source tracks, edit intervals, timed captions, aspect-ratio crops and render presets

## Security and data integrity
- Bound file sizes and processing time, reject path traversal, and use argument arrays for subprocesses. Treat imported text as data and redact confidential source content from logs.
- Correctness boundary: Caption timing follows the edited timeline rather than original timecodes; source clips are immutable and unsupported codecs produce a useful error.
- Save a job manifest with input hash, parameters and state. Write to temporary outputs, then atomically finalize only successful results; resume unfinished jobs without replacing originals.
- Export sources, manifests and outputs with checksums. Keep failed-job diagnostics and allow retry into a new output path; restore the database and file directory together.

## Agent implementation rules
- Project rule — data model: video projects, source tracks, edit intervals, timed captions, aspect-ratio crops and render presets
- Project rule — preserve this invariant: Caption timing follows the edited timeline rather than original timecodes; source clips are immutable and unsupported codecs produce a useful error.
- Project rule — acceptance evidence: Trim five seconds from the start and confirm spoken captions shift accordingly; a variable-frame-rate source exports with aligned audio and a playable duration.

## Optional agent skills and references
- Optional external skill: [modern-python](https://github.com/trailofbits/skills/blob/main/plugins/modern-python/skills/modern-python/SKILL.md) — Set up Python projects with pyproject.toml, dependency management, linting, typing and automated checks. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
- Optional external skill: [sharp-edges](https://github.com/trailofbits/skills/blob/main/plugins/sharp-edges/skills/sharp-edges/SKILL.md) — Review security-sensitive APIs and configuration for dangerous defaults and easy-to-misuse interfaces. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
- Optional external skill: [web-design-guidelines](https://github.com/vercel-labs/agent-skills/blob/main/skills/web-design-guidelines/SKILL.md) — Review web interfaces for accessibility, keyboard focus, forms, navigation and interaction quality. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.

Read the linked SKILL.md and its dependencies before adding a skill. Select only the skills matching this project's runtime and task; their documentation does not supply API access, credentials or approval to perform external actions. Pin the reviewed revision where the tool supports it. Follow the chosen agent's documented project-level installation mechanism.

## Distribution ideas
These are optional planning notes. Obtain the owner's approval before publishing or contacting anyone.
- Demonstrate the actual CapCut-inspired workflow with owned or clearly labeled sample data: Import a local clip, generate editable captions, trim through a non-destructive timeline and choose a social aspect ratio. Preview safe caption areas and render an explicit standard video preset.
- Publish a reproducible walkthrough with this observable result: Trim five seconds from the start and confirm spoken captions shift accordingly; a variable-frame-rate source exports with aligned audio and a playable duration.
- Explain who can operate this scoped tool, its setup and ongoing costs, and these remaining product gaps: mobile app quality; effects/templates Avoid guaranteed savings, performance scores or implied endorsement.

## Engineering roadmap
1. Phase 1 — Scope and fixtures. Implement this bounded workflow: Import a local clip, generate editable captions, trim through a non-destructive timeline and choose a social aspect ratio. Preview safe caption areas and render an explicit standard video preset. Record prerequisites, select representative user-owned fixtures and document the unsupported features: mobile app quality; effects/templates
2. Phase 2 — Durable model. Model video projects, source tracks, edit intervals, timed captions, aspect-ratio crops and render presets Add migrations or a versioned document format, explicit validation, stable IDs and a visible import-error report. Preserve this rule: Caption timing follows the edited timeline rather than original timecodes; source clips are immutable and unsupported codecs produce a useful error.
3. Phase 3 — Complete the first useful path. Implement the workflow's input, review and output interface, with clear controls and explicit empty/error states. Save a job manifest with input hash, parameters and state. Write to temporary outputs, then atomically finalize only successful results; resume unfinished jobs without replacing originals.
4. Phase 4 — Permissions and integration failure. Bound file sizes and processing time, reject path traversal, and use argument arrays for subprocesses. Treat imported text as data and redact confidential source content from logs. Request integration credentials and permissions only for the enabled feature; show a disconnected state instead of mock results.
5. Phase 5 — Portable handoff. Export sources, manifests and outputs with checksums. Keep failed-job diagnostics and allow retry into a new output path; restore the database and file directory together. Include setup, operating limits, fixture walkthrough and shutdown/restart instructions in the README.
6. Phase 6 — Acceptance scenarios. Trim five seconds from the start and confirm spoken captions shift accordingly; a variable-frame-rate source exports with aligned audio and a playable duration. Repeat the workflow after restart and with a denied permission or unavailable dependency; show recoverable failure rather than a success placeholder.

## Paid-product capabilities outside this build
- mobile app quality
- effects/templates
- asset library
- timeline editor
- platform integrations
- rendering performance

## Implementation prompt
WORKING SLICE
Import a local clip, generate editable captions, trim through a non-destructive timeline and choose a social aspect ratio. Preview safe caption areas and render an explicit standard video preset.

Build this scoped CapCut-inspired workflow with a documented data model and visible failure states.

Architecture
- Python, FastAPI and server-rendered HTML with HTMX for a local interface.
- SQLite for manifests and job state, with an explicit worker process and immutable source files.
- FFmpeg/ffprobe for explicit media operations and browser previews; never interpolate user filenames into shell commands.

Prerequisites and limits
A Python virtual environment, writable input/output directories and sufficient disk for both originals and outputs. Bind the service to localhost. Install FFmpeg and confirm codec support for the intended inputs.
Outside this release: mobile app quality; effects/templates

Data model and correctness
video projects, source tracks, edit intervals, timed captions, aspect-ratio crops and render presets
Invariant: Caption timing follows the edited timeline rather than original timecodes; source clips are immutable and unsupported codecs produce a useful error.
Save a job manifest with input hash, parameters and state. Write to temporary outputs, then atomically finalize only successful results; resume unfinished jobs without replacing originals.

Security and privacy
Bound file sizes and processing time, reject path traversal, and use argument arrays for subprocesses. Treat imported text as data and redact confidential source content from logs.

Recovery and export
Export sources, manifests and outputs with checksums. Keep failed-job diagnostics and allow retry into a new output path; restore the database and file directory together.

Implementation order
1. Phase 1 — Scope and fixtures. Implement this bounded workflow: Import a local clip, generate editable captions, trim through a non-destructive timeline and choose a social aspect ratio. Preview safe caption areas and render an explicit standard video preset. Record prerequisites, select representative user-owned fixtures and document the unsupported features: mobile app quality; effects/templates
2. Phase 2 — Durable model. Model video projects, source tracks, edit intervals, timed captions, aspect-ratio crops and render presets Add migrations or a versioned document format, explicit validation, stable IDs and a visible import-error report. Preserve this rule: Caption timing follows the edited timeline rather than original timecodes; source clips are immutable and unsupported codecs produce a useful error.
3. Phase 3 — Complete the first useful path. Implement the workflow's input, review and output interface, with clear controls and explicit empty/error states. Save a job manifest with input hash, parameters and state. Write to temporary outputs, then atomically finalize only successful results; resume unfinished jobs without replacing originals.
4. Phase 4 — Permissions and integration failure. Bound file sizes and processing time, reject path traversal, and use argument arrays for subprocesses. Treat imported text as data and redact confidential source content from logs. Request integration credentials and permissions only for the enabled feature; show a disconnected state instead of mock results.
5. Phase 5 — Portable handoff. Export sources, manifests and outputs with checksums. Keep failed-job diagnostics and allow retry into a new output path; restore the database and file directory together. Include setup, operating limits, fixture walkthrough and shutdown/restart instructions in the README.
6. Phase 6 — Acceptance scenarios. Trim five seconds from the start and confirm spoken captions shift accordingly; a variable-frame-rate source exports with aligned audio and a playable duration. Repeat the workflow after restart and with a denied permission or unavailable dependency; show recoverable failure rather than a success placeholder.

Acceptance
Trim five seconds from the start and confirm spoken captions shift accordingly; a variable-frame-rate source exports with aligned audio and a playable duration.
Use real source data or clearly labeled fixtures. Explain unsupported input and provider failures; do not fabricate analytics, delivery receipts, accuracy claims or security guarantees.

Optional agent guidance
Optional external skill: [modern-python](https://github.com/trailofbits/skills/blob/main/plugins/modern-python/skills/modern-python/SKILL.md) — Set up Python projects with pyproject.toml, dependency management, linting, typing and automated checks. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
Optional external skill: [sharp-edges](https://github.com/trailofbits/skills/blob/main/plugins/sharp-edges/skills/sharp-edges/SKILL.md) — Review security-sensitive APIs and configuration for dangerous defaults and easy-to-misuse interfaces. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
Optional external skill: [web-design-guidelines](https://github.com/vercel-labs/agent-skills/blob/main/skills/web-design-guidelines/SKILL.md) — Review web interfaces for accessibility, keyboard focus, forms, navigation and interaction quality. Review its instructions and compatibility before use; it does not grant deployment, data-access or publication permission.
Project rule — data model: video projects, source tracks, edit intervals, timed captions, aspect-ratio crops and render presets
Project rule — preserve this invariant: Caption timing follows the edited timeline rather than original timecodes; source clips are immutable and unsupported codecs produce a useful error.
Project rule — acceptance evidence: Trim five seconds from the start and confirm spoken captions shift accordingly; a variable-frame-rate source exports with aligned audio and a playable duration.

## Completion evidence
Demonstrate the prompt's acceptance scenarios against the scoped workflow. Include setup from a clean checkout and failure recovery. Check persistence across restart and export/restore only for the state the prompt says to store; for memory-only tools, confirm that temporary content is discarded as specified. Record actual results and remaining limitations. A detailed plan alone does not establish a working replacement.