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CallBus heads v0.1 for Qwen3-Embedding-8B

Two small learned heads that sit on top of Qwen3-Embedding-8B sentence embeddings for tool retrieval. The state head reads the request, the action head reads a tool; each computes x + MLP(x) (a skip head: 4096 → 1536 → 1536 → 4096, GELU, LayerNorm), then L2-normalises, and a tool's score is the cosine of the two. The heads start as the identity, so training only adds a correction on top of the base model.

Code, documentation and benchmarks: github.com/yasinyaman/callbus, yaman.dev/callbus; the package: pip install callbus.
File callbus-heads-qwen3-emb-8b-v0.1.npz, float16, 59.8 MB
sha256 f3c101251b9c23925e2925bc02c4492e6c3dea79bcfa1b56715affb20f9f72f0
Parameters 29.9M (both heads)
Format numpy .npz, loaded with allow_pickle=False; runs without torch (callbus.adapters.heads_np.NumpyHeads)
Source checkpoint qwen_full_skip_neg0_e5.pt (torch), exported with callbus heads export --dtype float16
License Apache-2.0, like callbus; see Training data for the data it was trained on

Serving (stored in the checkpoint's cfg, used as defaults by callbus)

  • Backbone: Qwen/Qwen3-Embedding-8B, last-token pooling, 4096-d, rows L2-normalised, truncate_prompt_tokens 8192. bf16 or FP8: with the backbone in FP8 (vLLM's --quantization fp8) every benchmark stays within a query or two of bf16 (ToolRet 53.94 / 47.27, LiveMCPBench 53.48, MCP-Zero 79.51).
  • Tools: callbus's documentation text. For ingested MCP and OpenAPI tools that is {"server", "name", "description", "inputSchema"} as JSON.
  • Requests: Instruct: {instruction}\nQuery: {request} (instruct_query). The default instruction, chosen among three on LiveMCPBench and MCP-Zero, is Given an agent's request for a tool, retrieve the MCP tool that fulfills it.

Training

  • Data: 206K request–tool pairs from ToolRet-Training-20w (mangopy/ToolRet-Training-20w). Pairs whose request equals a ToolRet benchmark request were dropped.
  • Objective: InfoNCE with in-batch negatives only. The dataset's mined negatives cost up to 10 points and were not used.
  • Optimisation: lr 1e-5, batch 512, 5 epochs; epoch 4 was picked on held-out training pairs.
  • The backbone stays frozen. Its vectors come from a cache, so training takes minutes.

Training data license

ToolRet-Training-20w states no license: its dataset card has no license field and no license text. The ToolRet code repository (mangopy/tool-retrieval-benchmark) is Apache-2.0, but that does not cover the data, and the data itself is drawn from earlier benchmarks with their own terms. The callbus maintainers publish these heads accepting that risk.

If you need clean data provenance, use Qwen3-Embedding-8B without heads (Apache-2.0; 1–3 points lower) or train your own heads on your data.

Results

All numbers are w/ inst, and each set is scored under its own protocol (benchmarks):

ToolRet NDCG@10 (micro / cat-macro) LiveMCPBench Recall@5 MCP-Zero top-1
Qwen3-Embedding-8B 51.11 / 46.54 50.82 78.19
+ these heads (torch .pt) 54.03 / 47.14 53.03 79.87
+ these heads (this .npz, numpy) 54.03 / 47.13 53.03 79.87
  • ToolRet has 44,453 tools and 7,961 queries.
  • LiveMCPBench has 525 tools and 94 tasks; the server name is in the tool text.
  • MCP-Zero has 2,792 tools and one LLM-written request per tool; the server name is in the tool text.
  • float16 against the torch checkpoint: projection cosine ≥ 0.999999, top-10 overlap ≥ 99.89% on all three sets.

Use

callbus search --data data/mytools "create an invoice for this customer" --clm-ckpt default --emb-model qwen3-emb
callbus eval --data data/toolret --scorer clm --clm-ckpt path/to/callbus-heads-qwen3-emb-8b-v0.1.npz \
  --emb-url http://127.0.0.1:8091/v1 --emb-model qwen3-emb --with-inst

--emb-model names the served Qwen3-Embedding-8B. Without it, search and serve would use the default backbone (callbus-emb-v0.2), which these heads do not fit: they stop and say so (a backbone named with --emb-model, and eval, get a warning). --clm-ckpt default reads CALLBUS_HEADS, else ~/.cache/callbus/heads/, else downloads the file and checks its sha256. The file is downloaded from https://huggingface.co/yasinyaman/callbus-heads-qwen3-emb-8b/resolve/v0.1/callbus-heads-qwen3-emb-8b-v0.1.npz. CALLBUS_HEADS_URL points the download at a mirror; the sha256 is checked all the same.

Limitations

  • English requests and tool texts only.
  • Trained on ToolRet's task mix. The gain is mostly on large ToolRet tasks and small elsewhere (out of domain +2.2 LiveMCPBench Recall@5, +1.7 MCP-Zero top-1, +0.2 LiveMCPBench NDCG@10).
  • The ToolRet gate threshold (50 cat-macro) is not reached.
  • The heads only fit Qwen3-Embedding-8B vectors. Another backbone needs its own heads.