_estimate_mtp_overhead_bytes is also reached for the separate-drafter spec modes
(draft-simple / draft-eagle3) through _user_draft_via_extras. Those modes load a
small distinct drafter with its own KV -- already counted in the draft KV +
weights -- and keep no duplicated full target context; only MTP runs a second
context over the target model's own KV geometry (llama.cpp ctx_tgt). Charging the
~main-KV-sized f16 copy there over-reserved by tens of GiB on an MLA model and
needlessly shrank the advertised context, the same under-advertising #6312 set
out to fix.
Thread mtp_keeps_target_ctx through _estimate_mtp_overhead_bytes (True for MTP,
False for separate-drafter modes) and derive _engaged_is_mtp at the fit call site
so the target copy is added only when the engaged mode is actually MTP. MLA + MTP
(GLM-5.2 / DeepSeek / Kimi) is unchanged, so the GLM-5.2 OOM fix is preserved;
non-MLA and the draft-simple / draft-eagle3 paths no longer pay the copy.
test_mtp_mla_target_ctx.py adds a case asserting the separate-drafter reserve
collapses to the draft KV (no target copy) while the default MTP path keeps it.
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