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fix(retrieval): report keyword-only search when the index has no vectors - #298
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Indexing stores a vector for every chunk, but if .vera/vectors.db was deleted or emptied, opening the vector store recreated an empty one and vector search returned no hits without an error. Search then returned BM25-only results with no warning on the CLI or in MCP, and vera update reported the index as up to date. Hybrid search now treats BM25 hits over an empty vector store as a semantic fallback, so the CLI warns and MCP adds its degraded-search notice telling the agent to re-index.
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The title clearly and concisely describes the main change: reporting keyword-only search when the index has no vectors. |
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Devin Review found 1 potential issue.
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| (Ok(bm25), Ok(vector)) => { | ||
| // Indexing stores a vector for every chunk, so BM25 hits over an | ||
| // empty vector store mean the vector database was lost or emptied. | ||
| if index_count == 0 && !bm25.is_empty() { |
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🟡 Missing vectors go unreported without keyword hits
When a query has no keyword hits, semantic_fallback stays unset even if the index has lost every vector. Semantic-only matches vanish silently, and MCP reports no degradation.
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Hybrid search gets keyword hits from BM25 and semantic hits from the vector store. The index can contain chunks even when a keyword query matches none of them. Here, an empty vector database sets the fallback reason only when BM25 finds a hit, so the same damaged index appears healthy for other queries. The MCP notice depends on this fallback reason in degraded_search_notices.
Example: Delete vectors.db from an index containing a function named allocate_page, then search for memory allocation strategy when those words have no BM25 matches. Search returns [] without a fallback notice instead of identifying the missing semantic index.
Recommended fix: Detect a missing vector database using stored chunk existence, such as chunk_count, independently of whether this query produces BM25 hits. Add a no-keyword-hit regression test for the direct and retained-store search paths.
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All reported issues were addressed across 4 files
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A query with no keyword hits still loses its semantic matches when the vector database is gone, so check the metadata chunk count instead of the BM25 results. Only a confirmed zero vector count triggers the check, so a failed count is not reported as a missing database.
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Actionable comments posted: 1
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Review comments at @crates/vera-core/src/retrieval/hybrid.rs:
- Line 862: Update the `(Err(bm25_err), Ok(vector))` branch guarded by
`vectors_missing` to return a search failure instead of `Ok` with empty vector
results, or ensure the required missing-vector notice is preserved before
returning. Do not leave `timings.semantic_fallback` unset while reporting empty
success.
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crates/vera-core/src/retrieval/hybrid.rscrates/vera-core/src/retrieval/hybrid_tests.rs
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Routing the missing-vector case through the vector error arm also covers a BM25 failure in the same query, which now reports both failures instead of returning an empty success.
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1 issue found across 1 file (changes from recent commits).
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<file name="crates/vera-core/src/retrieval/hybrid.rs">
<violation number="1" location="crates/vera-core/src/retrieval/hybrid.rs:852">
P2: This branch discards any successful vector results based on an earlier count snapshot. During `vera update`, metadata chunks are committed before vectors, so a search can observe zero vectors, then find newly committed vectors and incorrectly return BM25-only results. Recheck the vector state after the search or synchronize the snapshot with the vector query before forcing this error.</violation>
</file>
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| .map(|(_, embed_elapsed)| *embed_elapsed); | ||
| charge_vector_span(&mut timings, vector_start.elapsed(), embed_elapsed); | ||
| let vector_results = vector_outcome.map(|(results, _)| results); | ||
| let vector_results = if vectors_missing { |
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P2: This branch discards any successful vector results based on an earlier count snapshot. During vera update, metadata chunks are committed before vectors, so a search can observe zero vectors, then find newly committed vectors and incorrectly return BM25-only results. Recheck the vector state after the search or synchronize the snapshot with the vector query before forcing this error.
Prompt for AI agents
Check if this issue is valid — if so, understand the root cause and fix it. When an issue isn't valid or won't be fixed in this PR, reply in its thread with the reason and then resolve the thread. At crates/vera-core/src/retrieval/hybrid.rs, line 852:
<comment>This branch discards any successful vector results based on an earlier count snapshot. During `vera update`, metadata chunks are committed before vectors, so a search can observe zero vectors, then find newly committed vectors and incorrectly return BM25-only results. Recheck the vector state after the search or synchronize the snapshot with the vector query before forcing this error.</comment>
<file context>
@@ -849,7 +849,13 @@ async fn search_hybrid_inner(
.map(|(_, embed_elapsed)| *embed_elapsed);
charge_vector_span(&mut timings, vector_start.elapsed(), embed_elapsed);
- let vector_results = vector_outcome.map(|(results, _)| results);
+ let vector_results = if vectors_missing {
+ Err(VectorSearchError::StorageError(anyhow::anyhow!(
+ "the vector database is missing or empty; re-index to restore vector search"
</file context>
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Not changing this. The flag is only set for a confirmed zero vector count, and that same count clamps the vector fetch to zero, so the vector arm can only return an empty list here; no vector hits are discarded. The only effect is the notice, which is accurate for that query: it ran keyword-only.
Follow-up to #297. If
.vera/vectors.dbis deleted or emptied,VectorStore::openrecreates an empty store, vector search returnsOk([]), and search silently returns BM25-only results (no stderr warning, no MCP notice;vera updatealso reports the index as up to date).Indexing always stores a vector per chunk, so a confirmed zero vector count over an index that has chunks now sets
semantic_fallbackwith "the vector database is missing or empty; re-index to restore vector search". The CLI warns and MCP surfaces it through the existing notice block. The chunk check runs only when the vector count is zero, and a failed vector count is not reported as a missing database.missing_vector_database_is_reported_as_semantic_fallbackdeletesvectors.dbfrom a built index and checks a query with keyword hits and one without.vera mcpreturns the notice after deletingvectors.db; aftervera index .no warning.Summary by CodeRabbit