fix(rerank): derive the doc cap from tokens (measured floor 1.92), cap the query (#24)

docs_mcp/server.py

@@ -293,13 +293,35 @@ def _rrf_fuse(rankings: list[list[str]], k: int = RRF_K) -> list[str]:
     return sorted(scores, key=lambda d: scores[d], reverse=True)
 
 
-# Per-doc character cap when sending to the reranker. jina-reranker-v2-base
-# accepts up to ~1024 tokens PER QUERY+DOC PAIR (n_ctx_train) and rejects
-# the WHOLE BATCH if any one pair exceeds it. Truncating each doc to
-# ~2000 chars (≈ 500-700 tokens) leaves headroom for the query + chat
-# template overhead. The truncation is reranking-only — full chunk text
-# still goes back to the LLM caller.
-RERANK_DOC_MAX_CHARS = 2000
+
+# --- Reranker input budget (see docs-mcp-template + retrieval-lore.md) ------
+# jina-reranker-v2 is a BERT cross-encoder with bert.context_length=1024 and
+# LEARNED ABSOLUTE position embeddings: 1024 is a HARD ceiling, not something a
+# bigger llama.cpp --ubatch-size can lift. llama.cpp 500s the ENTIRE batch if
+# any (query, doc) pair exceeds it, so ONE oversized chunk silently drops that
+# whole query to fused order — the same ~90%->~62% P@1 cliff as the sidecar
+# being off the network, and just as quiet.
+#
+# A char cap is only a PROXY for tokens. Derive it from this corpus's measured
+# chars-per-token FLOOR (tokenise real chunks via {RERANK_URL}/tokenize and
+# take the min). Measured floor for this corpus: 1.92 — identifier-dense variety/trial
+# tables tokenise far worse than prose. The old flat 2000-char cap produced docs
+# of up to 1042 tokens here, i.e. over the ceiling before the query was even
+# prepended; 16% of sampled chunks exceeded 1000 tokens.
+RERANK_CTX_TOKENS = int(os.environ.get("RERANK_CTX_TOKENS", "1024"))
+RERANK_CHARS_PER_TOKEN = float(os.environ.get("RERANK_CHARS_PER_TOKEN", "1.90"))
+RERANK_QUERY_MAX_CHARS = int(os.environ.get("RERANK_QUERY_MAX_CHARS", "300"))
+# Margin covers [CLS]/[SEP] framing plus slack, because chars-per-token is an
+# ESTIMATE from a sample. Budget to ~94%, never to exactly 1024.
+_RERANK_MARGIN_TOKENS = int(os.environ.get("RERANK_MARGIN_TOKENS", "64"))
+_RERANK_QUERY_TOKENS = int(RERANK_QUERY_MAX_CHARS / RERANK_CHARS_PER_TOKEN) + 1
+RERANK_DOC_MAX_CHARS = max(
+    256,
+    int(
+        (RERANK_CTX_TOKENS - _RERANK_QUERY_TOKENS - _RERANK_MARGIN_TOKENS)
+        * RERANK_CHARS_PER_TOKEN
+    ),
+)
 
 
 def _rerank(query: str, candidates: list[tuple[str, str]]) -> list[str] | None:
@@ -329,6 +351,7 @@ def _rerank(query: str, candidates: list[tuple[str, str]]) -> list[str] | None:
     # Truncate each doc to fit the per-pair token budget. jina-reranker
     # rejects the entire batch on any oversize doc.
     docs = [(text[:RERANK_DOC_MAX_CHARS] if text else "") for _cid, text in candidates]
+    query = query[:RERANK_QUERY_MAX_CHARS]
     ids = [cid for cid, _ in candidates]
 
     try: