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diff --git a/crates/mozart-core/src/installer/installed_repo.rs b/crates/mozart-core/src/installer/installed_repo.rs
new file mode 100644
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+++ b/crates/mozart-core/src/installer/installed_repo.rs
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+//! Lightweight stand-in for `Composer\Repository\InstalledRepository`.
+//!
+//! Composer's `InstalledRepository` is a composite over `LockArrayRepository`,
+//! `InstalledRepositoryInterface`, `RootPackageRepository`, and
+//! `PlatformRepository`. Mozart does not (yet) expose a unified repository
+//! abstraction, so this struct is the smallest layer we need to support the
+//! handful of commands that drive their behavior off
+//! `findPackagesWithReplacersAndProviders` (currently `check-platform-reqs`
+//! and `suggests`; later candidates: `depends`/`prohibits`, `audit`).
+//!
+//! The struct serves two roles:
+//!
+//! - As a lower-cased name set: callers `insert(name)` whatever they want
+//!   visible to `contains` / suggestion-filter logic.
+//! - As a candidate index: callers `add_candidate(InstalledCandidate)` and
+//!   then resolve a require name to the candidate(s) that satisfy it directly
+//!   or through a `provide` / `replace` link.
+
+use indexmap::IndexSet;
+use std::collections::BTreeMap;
+
+/// One installed package, in the shape `findPackagesWithReplacersAndProviders`
+/// needs. Mirrors the fields of `Composer\Package\PackageInterface` that the
+/// PHP implementation reads — name, version, provides, replaces.
+#[derive(Debug, Clone)]
+pub struct InstalledCandidate {
+    /// Lower-cased package name, used for matching.
+    pub name: String,
+    /// Original-case package name, used in user-facing output.
+    pub pretty_name: String,
+    /// Normalized version (what the constraint matcher consumes).
+    pub version: String,
+    /// Original-case version, used in user-facing output.
+    pub pretty_version: String,
+    /// `provide` map: target package name → constraint string.
+    pub provides: BTreeMap<String, String>,
+    /// `replace` map: target package name → constraint string.
+    pub replaces: BTreeMap<String, String>,
+}
+
+#[derive(Debug, Clone, Default)]
+pub struct InstalledRepoLite {
+    /// Lower-cased names of every package, plus every `provide`/`replace`
+    /// target that any candidate exposes. `contains` queries this set.
+    pub names: IndexSet<String>,
+    /// Full candidate records, in insertion order.
+    pub candidates: Vec<InstalledCandidate>,
+}
+
+impl InstalledRepoLite {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn insert(&mut self, name: &str) {
+        self.names.insert(name.to_lowercase());
+    }
+
+    pub fn contains(&self, name: &str) -> bool {
+        self.names.contains(&name.to_lowercase())
+    }
+
+    /// Add a full candidate record. Also inserts the candidate's own name and
+    /// every `provide` / `replace` target into the names set so `contains`
+    /// keeps reflecting all installed virtuals.
+    pub fn add_candidate(&mut self, candidate: InstalledCandidate) {
+        self.names.insert(candidate.name.clone());
+        for target in candidate.provides.keys().chain(candidate.replaces.keys()) {
+            self.names.insert(target.to_lowercase());
+        }
+        self.candidates.push(candidate);
+    }
+
+    /// Mirrors `Composer\Repository\InstalledRepository::findPackagesWithReplacersAndProviders`
+    /// without the optional constraint filter — callers in
+    /// `check-platform-reqs` apply their own per-link constraint check after
+    /// they have the candidate list. Returns each candidate at most once.
+    pub fn find_with_replacers_and_providers(&self, require: &str) -> Vec<&InstalledCandidate> {
+        let needle = require.to_lowercase();
+        let mut matches: Vec<&InstalledCandidate> = Vec::new();
+        for candidate in &self.candidates {
+            if candidate.name == needle {
+                matches.push(candidate);
+                continue;
+            }
+            let provides_or_replaces = candidate
+                .provides
+                .keys()
+                .chain(candidate.replaces.keys())
+                .any(|target| target.to_lowercase() == needle);
+            if provides_or_replaces {
+                matches.push(candidate);
+            }
+        }
+        matches
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn make_candidate(name: &str, version: &str) -> InstalledCandidate {
+        InstalledCandidate {
+            name: name.to_lowercase(),
+            pretty_name: name.to_string(),
+            version: version.to_string(),
+            pretty_version: version.to_string(),
+            provides: BTreeMap::new(),
+            replaces: BTreeMap::new(),
+        }
+    }
+
+    #[test]
+    fn insert_and_contains_lowercase() {
+        let mut repo = InstalledRepoLite::new();
+        repo.insert("Vendor/Pkg");
+        assert!(repo.contains("vendor/pkg"));
+        assert!(repo.contains("VENDOR/PKG"));
+    }
+
+    #[test]
+    fn add_candidate_registers_name_and_virtuals() {
+        let mut c = make_candidate("vendor/poly", "1.0.0");
+        c.provides.insert("ext-mbstring".into(), "1.0".into());
+        c.replaces.insert("ext-iconv".into(), "*".into());
+
+        let mut repo = InstalledRepoLite::new();
+        repo.add_candidate(c);
+
+        assert!(repo.contains("vendor/poly"));
+        assert!(repo.contains("ext-mbstring"));
+        assert!(repo.contains("ext-iconv"));
+    }
+
+    #[test]
+    fn find_returns_direct_match() {
+        let mut repo = InstalledRepoLite::new();
+        repo.add_candidate(make_candidate("php", "8.2.1"));
+        let hits = repo.find_with_replacers_and_providers("php");
+        assert_eq!(hits.len(), 1);
+        assert_eq!(hits[0].name, "php");
+    }
+
+    #[test]
+    fn find_returns_provider() {
+        let mut c = make_candidate("symfony/polyfill-mbstring", "1.30.0");
+        c.provides.insert("ext-mbstring".into(), "*".into());
+
+        let mut repo = InstalledRepoLite::new();
+        repo.add_candidate(c);
+
+        let hits = repo.find_with_replacers_and_providers("ext-mbstring");
+        assert_eq!(hits.len(), 1);
+        assert_eq!(hits[0].name, "symfony/polyfill-mbstring");
+    }
+
+    #[test]
+    fn find_returns_replacer() {
+        let mut c = make_candidate("vendor/forklift", "2.0.0");
+        c.replaces.insert("vendor/legacy".into(), "1.*".into());
+
+        let mut repo = InstalledRepoLite::new();
+        repo.add_candidate(c);
+
+        let hits = repo.find_with_replacers_and_providers("vendor/legacy");
+        assert_eq!(hits.len(), 1);
+        assert_eq!(hits[0].name, "vendor/forklift");
+    }
+
+    #[test]
+    fn find_returns_empty_when_unknown() {
+        let mut repo = InstalledRepoLite::new();
+        repo.add_candidate(make_candidate("php", "8.2.1"));
+        assert!(
+            repo.find_with_replacers_and_providers("ext-foobar")
+                .is_empty()
+        );
+    }
+
+    #[test]
+    fn find_includes_each_candidate_at_most_once() {
+        let mut c = make_candidate("vendor/poly", "1.0.0");
+        // Same target listed in both maps — should still only return one hit.
+        c.provides.insert("ext-x".into(), "*".into());
+        c.replaces.insert("ext-x".into(), "*".into());
+
+        let mut repo = InstalledRepoLite::new();
+        repo.add_candidate(c);
+
+        let hits = repo.find_with_replacers_and_providers("ext-x");
+        assert_eq!(hits.len(), 1);
+    }
+}