docs(01): capture phase context

.planning/phases/01-core-pipeline/01-CONTEXT.md

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+# Phase 1: Core Pipeline - Context
+
+**Gathered:** 2026-04-11
+**Status:** Ready for planning
+
+<domain>
+## Phase Boundary
+
+Single CLI command that takes a humming WAV file and an instrument name, and produces an instrument rendition via ACE-Step XL-SFT cover mode. Output audibly follows the pitch contour and rhythmic timing of the input. No multi-instrument batch, no config files, no advanced error handling — just the core end-to-end flow.
+
+</domain>
+
+<decisions>
+## Implementation Decisions
+
+### CLI invocation design
+- Invoked as `python hum2inst.py input.wav --instrument piano`
+- Python script directly, no installation step
+- `--instrument` as a named CLI flag (not positional)
+- `--output` flag optional, defaults to `./output/` directory
+- Use Python argparse for argument parsing (gives --help for free)
+
+### ACE-Step generation parameters
+- Default cover_strength in the high fidelity range (0.8-1.0)
+- `--strength` flag exposed in Phase 1 so users can experiment immediately
+- Duration matches input WAV length by default; `--duration` flag to override
+- Caption auto-built from instrument name (e.g., "piano cover of a melody") — no custom prompt flag in Phase 1
+
+### Output behavior
+- Output filename includes instrument and timestamp (exact format at Claude's discretion)
+- On generation failure or silence: print clear error message, exit with non-zero code
+- No auto-play — just save and print the output path
+- No silent failures
+
+### Pipeline architecture
+- Single `hum2inst.py` script — no module splitting in Phase 1
+- Assume CUDA GPU is available; fail with clear message if no GPU detected
+- Move existing experimental scripts (midi_to_audio.py, musicgen_melody.py) to an `/archive` folder
+
+### Claude's Discretion
+- ACE-Step invocation method (import Python API vs subprocess call — choose based on what ACE-Step exposes)
+- Progress/feedback during generation (print statements, progress bar, or similar — pick what's appropriate)
+- Exact output filename format (instrument + timestamp pattern)
+- Exact cover_strength default value within the 0.8-1.0 range
+
+</decisions>
+
+<specifics>
+## Specific Ideas
+
+- ACE-Step XL-SFT cover mode is the generation backend — this is established from prior experimentation
+- The `ace-step/` directory already exists in the project root with the model code
+- User wants high melodic fidelity as the default — the pipeline should prioritize faithful melody reproduction over creative interpretation
+
+</specifics>
+
+<deferred>
+## Deferred Ideas
+
+None — discussion stayed within phase scope
+
+</deferred>
+
+---
+
+*Phase: 01-core-pipeline*
+*Context gathered: 2026-04-11*