[Bug]: multi_grep drops positive constraints and degrades OR results

[Kh05ifr4nD]

Which fff frontend?

MCP server (fff-mcp)

has logs

Not applicable: this is deterministic incorrect output without a server error.

Description

Summary

multi_grep does not apply positive file constraints. It can therefore return matches outside the requested scope. Its documented OR behavior also collapses when any requested pattern has no match; the plain-grep fallback masks the zero result but returns only the first successful pattern.

Reproduced on fff-mcp 0.10.1; current main contains the same code path. I found no existing issue covering this behavior.

Sanitized reproduction

Use any indexed repository containing:

scope-a/one.txt   alpha
scope-a/two.txt   beta
scope-b/three.txt alpha

Positive directory constraint:

{
  "patterns": ["alpha"],
  "constraints": "scope-a/"
}

Expected: only scope-a/one.txt.

Actual: matches from both scopes. The same occurs with a positive glob or exact-file constraint. A negative constraint is applied correctly. Ordinary grep with the equivalent inline query (scope-a/ alpha) is scoped correctly.

OR control:

{
  "patterns": ["alpha", "definitely_missing_token"],
  "constraints": "scope-a/"
}

Expected: the alpha match, because patterns is documented as OR.

Actual: the core multi-pattern search returns zero, then the server reports a plain-grep fallback for one pattern. Because the fallback returns after the first successful pattern, it cannot preserve the union for a larger pattern set.

Root cause

In crates/fff-mcp/src/server.rs, multi_grep_inner parses the standalone constraints string with AiGrepConfig, then passes only parsed_constraints.constraints to the picker. A lone positive path/glob is retained as grep text rather than classified as a constraint, leaving the constraint slice empty. The fallback reconstructs an inline query correctly, but exits after the first successful pattern.

Impact

This is a silent scope violation: callers can believe a directory, glob, or exact-file restriction was enforced when it was not. Multi-pattern results can also be incomplete while appearing successful through fallback output.

Suggested regression tests

• Positive directory constraint
• Positive extension/glob constraint
• Positive exact-file constraint
• Negative constraint
• OR with one matching and one missing pattern
• OR with multiple matching patterns plus one missing pattern

A useful invariant is: multi_grep(patterns, constraint) must equal the deduplicated union of individually scoped grep calls for the same patterns and constraint.

[gustav-fff]

[triage-bot] LINKED PR:

Tracked by #739 (OPEN) — #739

PR adds an explicit parse_constraints entry point in fff-query-parser so a standalone positive path/glob/exact-file token stays classified as a constraint instead of collapsing into fuzzy grep text. Fallback path in fff-mcp is rewritten to aggregate a deduplicated OR union across all patterns with per-pattern offsets and an opaque composite cursor, so a single missing pattern no longer drops the rest. Formatter and native cursor behavior unchanged.

Honk-Honk 🪿