CPDD: Adapting an autoregressive DNA language model for bidirectional phage genome modeling

Autoregressive DNA language models generate sequences ef[fi]ciently from left to right, but many genome-analysis and sequence-editing problems are naturally conditioned on both [fl]anks. We introduce CreatiPhage, a 144.5M-parameter autoregressive DNA language model for bacteriophages, and its diffusion extension, CreatiPhage Diffusion DNA-LM (CPDD). CPDD is a 147.5M-parameter bidirectional discrete diffusion model initialized from the six-epoch CreatiPhage checkpoint. CPDD retains the pretrained Transformer, block attention residuals, and Engram conditional memory, while introducing non-causal attention, a MASK-aware base-5 Engram, centered depthwise convolution, and position-aligned nucleotide readout. Training combines masked and GIDD-style uniformly substituted bases with nucleotide- and amino-acid-level losses over a 10 kb, 32 kb, and whole-source context curriculum. On a screened panel of 522 phage CDSs, CPDD achieves higher random-mask restoration accuracy than a comparable diffusion model and performs well above codon-level references. In a mixed-corruption test, it restores 42.4% of visible incorrect bases. Frozen CPDD representations also support infA [fi]tness prediction and seven-class phage virion protein subtype classi[fi]cation. Controlled evaluations further characterize how restoration depends on visible context and identify opportunities to improve longer-range sequence recovery. Together, these results support scaffold-conditioned sequence modeling as a step towards modular design of phage genomes.