Retrieval-Augmented Self-Recall — Part 6: The Fine-Tune That Did Nothing, and Shipping It as an MCP Server

Part 6 (finale) of Retrieval-Augmented Self-Recall. Code: RE-call . Part 5: the gap threshold that didn't transfer . I fine-tuned the embedder on my own domain expecting a win. I measured it properly, on held-out queries. The improvement was exactly zero. Δ+0.00 MRR. Δ+0.00 nDCG@10. Not "small". Not "within noise". Zero. It's also the result I wanted, which takes some explaining. That's the first half of this post. The second half is how the whole engine ships, so an agent can actually use it. The fine-tune that did nothing After Part 5, the natural next question: if calibrating the threshold helps, would a better embedding help more? So I fine-tuned one on my domain. The setup: all-MiniLM-L6-v2 , OnlineContrastiveLoss on query/gold-chunk pairs, trained on the 14-document corpus. The result: Model Test MRR Test nDCG@10 Base 1.00 1.00 + Fine-tuned 1.00 1.00 Δ +0.00 +0.00 Zero lift. And that is the correct outcome, not a failed experiment. Here's the reasoning, because it's the whole point. The base model already scores a perfect MRR and nDCG@10 on this corpus. There is no headroom left to recover. The only ways to manufacture a "gain" from here would be dishonest ones: evaluate on the training set (and measure memorization, not retrieval), or artificially cripple the baseline so fine-tuning has something to fix. Reporting +0.00 is the honest read, and the honest read is that off-the-shelf embeddings already saturate this corpus. But the full result is more nuanced, and more useful. On a harder , opaque-jargon corpus — one where the base model genuinely struggles to map queries to the right chunks — the same fine-tuning gave +0.24 MRR . So the real conclusion isn't "fine-tuning doesn't work." It's: Fine-tuning helps when the base model doesn't already cover your vocabulary. When it does, you get nothing. Know which regime you're in before you spend the GPU hours. That's the value of a null result. "+0.00" told me my corpus was already well-covered by a general-purpose