gdal-fable

A cleanroom recreation of the GDAL 3.13 gdal command line interface, written by an AI model that could run the real thing but never see its source.

Not affiliated with OSGeo or GDAL, and not a substitute for either. If you need geospatial tooling, use GDAL.

What this is

Produced autonomously by Fable 5 on ProgramBench task osgeo__gdal.0847f12, over 8 days and 10 review passes. The only reference available was an execute-only binary: GDAL 3.13.0dev-cbc00e6b71, trimmed to 11 drivers with no GEOS. No source, no headers, no build system, no tests, no network. Every behavior here was recovered by running that binary and comparing bytes.

Write-up: What it Takes for Coding Agents to Complete Large Software Tasks

Result

Held-out behavioral suite	1,184 / 1,319 (89.8%), 90.3% on the official metric
Same model, single agent, no validation system	35.8%
Size	115,163 lines of C++17 in 104 files, 125 command paths
Upstream surface reached through this CLI	~600k lines of GDAL's 1.9M

It shares no code with GDAL

Measured across all 104 files here against all 2,952 upstream C/C++ files at commit 0847f12:

Measure	Result
Verbatim shared line runs (4+ non-trivial lines)	0
Code 7-gram overlap, string literals stripped	4.18%, all generic C++ idiom
Shared string literals (12+ chars)	21.2%, i.e. GDAL's error messages, reproduced on purpose

No GDAL API or convention appears anywhere: no CPLError, no GDALDataset as a type, no poDS/papsz. Where those names occur, they are inside string literals, because the reference prints them.

Four libraries are linked rather than rewritten, because byte-identical output is impossible without them and the reference links the same ones: PROJ 8.2.1 (all CRS text comes from the system proj.db), libdeflate, zstd, libwebp. libtiff, libgeotiff, libjpeg, libpng and expat were all available and went unused; the TIFF container, GeoTIFF keys, JPEG codec, XML and JSON parsers are original.

Layout

Path
src/	the implementation; engine.cpp parses, handlers_*.cpp implement, dataset.h is the raster model
spec/	6.2 MB of output captured from the reference, embedded into the binary at build time. Required to compile.
tests/	the model's own differential harness plus ~10,000 pinned cases; runs the reference and this build in twin sandboxes and byte-compares
NOTES.md	5,000 lines of derived behavioral laws and their evidence. The most interesting file here.
README.task.md	the product description handed to the model, harness-supplied

Building

Linux x86_64 with AVX, and nothing else without edits:

./compile.sh   # make + g++ -std=c++17 -> ./executable

Makefile hardcodes absolute paths for libproj, libdeflate, libzstd and libwebp; src/handlers_vector_grid.cpp uses AVX intrinsics (matching upstream's grid path requires _mm256_rcp_ps and its 12-bit reciprocal error); PROJ must be 8.2.1 or CRS text drifts. tests/run_diff.py needs the reference binary, which is not distributable, so the differential suite cannot run outside the benchmark container. The pinned cases still read as a behavioral specification.

Known gaps: multidimensional (mdim info modes, mdim convert subsetting), verbs used as steps inside pipeline, and hidden option aliases such as --z-field that appear in no help output and so cannot be probed. Geometry verbs (buffer, dissolve, simplify, clip by geometry) refuse with ERROR 6: GEOS support not enabled., which is what the reference does.

Provenance

The first commit, task scaffold, contains everything the task placed in the workspace before the model started: six files byte-identical to GDAL at 0847f12 plus a paraphrased product description. Everything else is in the second commit. LICENSE.TXT stays at its original path because the reference's --license reads it at runtime; it describes GDAL, not this code. spec/ contains bytes captured from an MIT-licensed GDAL build.

Two license files sit in this repository, covering different things:

File	Covers
COPYING.md	MIT. The original code in src/, tests/ and tools/.
LICENSE.TXT	GDAL's own terms, from the task scaffold. Kept at this path because the reference reads it at runtime for --license.