The BYO fit-check's Route-C weight-swap (pull.sh _swap_path) hardcoded
`drop_spec_config: True` on the premise "a generic repo carries no MTP
head." That's false for a fine-tune that PRESERVES the head — so any such
model (e.g. ThinkingCap-Qwen3.6-27B, whose AutoRound INT4 ships the full
15-tensor BF16 mtp head in model_mtp_bf16.safetensors) was silently served
MTP-off, losing spec-dec despite the head being present.
Fix: detect it. `deriver.detect_mtp_head(config, api)` returns True when the
config DECLARES the MTP layers (`mtp_num_hidden_layers` / nested
`num_nextn_predict_layers`) AND a dedicated mtp weights file is present —
ground-truth for the separate-file layout fine-tune re-quants use, using
signals the deriver already has in hand (no extra fetch). Exposed as an
additive `has_mtp_head` on der.profile; _swap_path sets
`drop_spec_config = not has_mtp_head`. An embedded-head repo (head baked
into the shards with no named file) still falls back to drop — conservative,
and the named-file layout is the norm.
Validated on real HF metadata: josefprusa/ThinkingCap AutoRound →
has_mtp_head=True (keep --speculative-config); a plain AWQ re-quant →
False (drop). test-pull-swap replaces its tautology check with a real
detect_mtp_head unit test (declares+file, plain, nested text_config,
declares-but-no-file). Pull-gate suite green.
Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01EfF565T9eSLaqGzidyJ1Pm
