Output is decoded bit-exactly by libdav1d (ffmpeg -c:v libdav1d, checked in tests/ffmpeg_oracle.rs when ffmpeg is installed) and by FilmCraft's spec-derived filmcraft-av1 decoder (tests/roundtrip.rs): every decoded frame equals the encoder's own reconstruction, so prediction never drifts.

What is implemented

Stream: Main profile (seq_profile 0), 4:2:0, 8 or 10 bit, BT.709 colour description, limited or full range. Sequence header with one operating point; seq_level_idx given or the lowest Annex A level whose MaxPicSize / MaxH/VSize / MaxDisplayRate (and, in bitrate mode, Main tier MaxBitrate) fit. Order hints, film grain, superres, CDEF, loop restoration, intra edge filter, filter intra, inter-intra, masked / distance compound, warped motion and dual filter are disabled in the sequence header.
Packets: one temporal unit per frame as stored in MP4 / Matroska samples: OBUs with size fields, no temporal delimiter; key frames start with the sequence header OBU, followed by one OBU_FRAME (frame header + tile group). Every frame is shown, in input order. Encoder::av1c() returns the AV1CodecConfigurationRecord (also the WebM CodecPrivate).
Frames: a KEY_FRAME every keyint frames, INTER_FRAMEs otherwise. Inter frames use slot 0 (the previous reconstruction) for all seven references and refresh slot 0 only, so LAST_FRAME is always the previous frame. CDFs adapt per symbol and are saved at the end of every frame; inter frames load them from LAST_FRAME (primary_ref_frame = 0). One tile, TX_MODE_LARGEST, reduced_tx_set, no segmentation, no delta q / lf.
Deblocking: the normative loop filter (7.14) runs on the reconstruction; the frame level (the same for both luma directions and both chroma planes, no deltas, sharpness 0) is searched per frame for the lowest luma error.
Tiles: the multi-symbol arithmetic encoder is the exact inverse of the spec's symbol decoder (8.2), including the exit process padding; CDF adaptation as in 8.2.6.
Partitions: 64x64 superblocks, PARTITION_NONE / PARTITION_SPLIT down to 8x8 chosen by a rate-distortion cost (SSE + λ·estimated bits); blocks crossing the frame edge are split as the syntax requires. 64x64 blocks are only used as inter skip blocks.
Intra: DC, V, H, Paeth, smooth, smooth-V, smooth-H for luma and chroma, ranked by SATD with a rate-distortion choice among the best three; the spec's edge preparation and above-right / below-left availability.
Inter: single reference (LAST_FRAME), NEWMV / NEARESTMV / NEARMV / GLOBALMV chosen from the spec's motion vector prediction stack (7.10.2, mirrored exactly); motion search from neighbour / co-located / parent candidates, a whole-sample diamond search and half- then quarter-sample refinement (allow_high_precision_mv = 0); the normative 8-tap EIGHTTAP prediction for luma and chroma; skip blocks; per-block intra / inter choice.
Residual: one transform per plane and block (4x4 ... 32x32): for intra luma DCT_DCT or the mode's preferred ADST combination (rate-distortion choice over the two best modes), the mode-dependent type for intra chroma, DCT_DCT for inter blocks; dead-zone quantisation with the spec's Dc_Qlookup / Ac_Qlookup; the normative dequantisation and inverse transforms in the reconstruction loop; full coefficient coding (all_zero, tx type, eob_pt, eob_extra, coeff_base_eob, coeff_base, coeff_br, dc_sign, Golomb) with the exact contexts.
Rate control: ConstantQ(base_q_idx); Bitrate { kbps } adapts base_q_idx per frame towards the average target (key frames get a lower quantizer and a larger share; the first key frame is searched in a few passes).

Limits

One tile: width and height up to 4096 and at most 2304 superblocks (4096x2304 samples).
No CDEF or loop restoration, no compound prediction, no rectangular or 4xN blocks, no directional intra modes other than V / H, no CfL or palette, no transform-size splits, so compression is below mature encoders.
4:2:0 only (Main profile), no 12 bit.

Specification and provenance

Implemented from the AV1 Bitstream & Decoding Process Specification, version 1.0.0 with Errata 1 (AOMediaCodec, 2019; the edition FilmCraft's decoder follows) and the AV1 Codec ISO Media File Format Binding (v1.2.0) for av1C.

The constant tables (default CDFs, scan orders, quantizer lookup tables, interpolation filter taps, smooth weights, block / transform size tables, ...) and the decoder-side processes the encoder must reproduce exactly (inverse transforms, intra and inter prediction rounding, context derivations, motion vector prediction) are mirrored from FilmCraft's first-party, spec-derived filmcraft-av1 decoder (MIT OR Apache-2.0), which is also the round-trip oracle. No source of libaom, dav1d, rav1e, SVT-AV1, ffmpeg or any other third-party encoder or decoder was consulted; ffmpeg / libdav1d is used only as an external test oracle.