query_graph: inbound anti-join unsupported — the README's own dead-code example is rejected, and the OPTIONAL MATCH workaround silently returns 0 rows

[melihemreguler]

Summary

There is no way to express "nodes with no incoming edge" in query_graph. The form the README documents is rejected, and the natural workaround returns zero rows without an error.

The documented example does not work

README, Supported Cypher section:

EXISTS { (n)-[:TYPE]->() } (single-hop existence — great for dead-code, e.g. WHERE NOT EXISTS { (f)<-[:CALLS]-() })

That exact expression is rejected:

WHERE NOT EXISTS { MATCH (f)<-[:CALLS]-() }
-> unsupported EXISTS pattern — only the single-hop form '(var)-[:TYPE]->()' is supported

All three inbound syntaxes rejected

Query fragment	Error
WHERE NOT (f)<-[:CALLS]-()	unexpected operator at pos 31
WHERE NOT EXISTS((f)<-[:CALLS]-())	expected '{' after EXISTS
WHERE NOT EXISTS { MATCH (f)<-[:CALLS]-() }	unsupported EXISTS pattern …

The outbound direction works as documented: WHERE NOT EXISTS { (f)-[:CALLS]->() } -> 2,949 leaf functions.

Silent-failure workaround

MATCH (f:Function)
OPTIONAL MATCH (f)<-[:CALLS]-(c)
WITH f, count(c) AS fan_in
WHERE fan_in = 0
RETURN f.name
-> 0 rows, no error

Expected: the ~291 zero-fan-in functions that search_graph(max_degree=0, exclude_entry_points=true) does return.

Impact

Dead-code detection is a headline feature. It is reachable via search_graph(max_degree=0) but not via Cypher, while the README advertises the Cypher form. Three possible fixes, in descending preference: support the inbound single-hop form; or make the OPTIONAL MATCH + WITH count() = 0 path work; or at minimum make that path fail loudly and correct the README example.

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Environment

• Version: codebase-memory-mcp 0.9.0 (release binary, darwin-arm64)
• OS: macOS 26.5.2, Apple Silicon
• Repo under test: DeusData/codebase-memory-mcp @ af3ffbd (21,143 nodes / 123,103 edges)
• Verified identically through both the MCP server and cli mode (outputs byte-identical)

[DeusData]

Thanks for testing both the documented form and the apparent workaround.

On current main the Cypher parser has no executable EXISTS-subquery/anti-join representation for this shape. OPTIONAL MATCH is parsed, but predicate evaluation and row preservation are separate paths; it is therefore not a safe equivalent for an inbound anti-join. That confirms the README example and engine contract are out of sync, with the dangerous behavior being the silent zero-row workaround.

I have routed this to 0.9.1-rc at priority/high under the query correctness work. Thank you for verifying the workaround rather than only reporting the parse error.