diff --git a/app/src-tauri/src/marketplace/mod.rs b/app/src-tauri/src/marketplace/mod.rs
index 0c89ff9..39aa076 100644
--- a/app/src-tauri/src/marketplace/mod.rs
+++ b/app/src-tauri/src/marketplace/mod.rs
@@ -6,6 +6,7 @@ pub mod auth;
 pub mod catalog;
 pub mod diff;
 pub mod git;
+pub mod payload;
 pub mod tree;
 #[cfg(test)]
 pub(crate) mod test_support;
diff --git a/app/src-tauri/src/marketplace/payload.rs b/app/src-tauri/src/marketplace/payload.rs
new file mode 100644
index 0000000..879589e
--- /dev/null
+++ b/app/src-tauri/src/marketplace/payload.rs
@@ -0,0 +1,483 @@
+//! Builds the tar a project's container receives: every effective install's
+//! files, read from the cache at its pinned commit, plus `manifest.json` and a
+//! generated Claude Code catalog per marketplace that contributes plugins.
+//! Layout: see the Interface Contract in the plan / spec §4. The tar carries
+//! no directory entries — the sync script's extraction (plus its umask)
+//! creates them.
+
+use std::collections::{BTreeMap, BTreeSet};
+use std::path::Path;
+
+use serde_json::{json, Value};
+
+use super::catalog::{item_files, plugin_catalog_entry, rendered_hook_settings, ItemFile};
+use super::git;
+use super::tree::GitTree;
+use crate::models::marketplace::{
+    is_valid_commit, is_valid_item_key, marketplace_slug, ItemKind, Marketplace,
+    MarketplaceInstall, SkippedItem,
+};
+
+pub struct PayloadInput<'a> {
+    pub installs: &'a [MarketplaceInstall],
+    pub marketplaces: &'a [Marketplace],
+    /// data root used to find caches (see git::cache_path)
+    pub data_root: &'a Path,
+}
+
+pub struct Payload {
+    pub tar: Vec<u8>,
+    pub manifest: Value,
+    pub skipped: Vec<SkippedItem>,
+}
+
+/// A relative path from `item_files` is joined under a directory we chose, so
+/// it must not be able to climb out of it. The catalog already refuses such
+/// entries; this is the second line.
+fn safe_rel(rel: &str) -> bool {
+    !rel.is_empty()
+        && !rel.starts_with('/')
+        && !rel.contains('\\')
+        && rel
+            .split('/')
+            .all(|seg| !seg.is_empty() && seg != "." && seg != "..")
+}
+
+struct TarWriter {
+    builder: tar::Builder<Vec<u8>>,
+    mtime: u64,
+}
+
+impl TarWriter {
+    fn new() -> Self {
+        let mtime = std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .map(|d| d.as_secs())
+            .unwrap_or(0);
+        Self {
+            builder: tar::Builder::new(Vec::new()),
+            mtime,
+        }
+    }
+
+    fn file(&mut self, path: &str, data: &[u8], executable: bool) -> Result<(), String> {
+        let mut header = tar::Header::new_gnu();
+        header.set_size(data.len() as u64);
+        header.set_mode(if executable { 0o755 } else { 0o644 });
+        header.set_mtime(self.mtime);
+        header.set_entry_type(tar::EntryType::Regular);
+        self.builder
+            .append_data(&mut header, path, data)
+            .map_err(|e| format!("Could not add {path} to the marketplace payload: {e}"))
+    }
+
+    fn finish(self) -> Result<Vec<u8>, String> {
+        self.builder
+            .into_inner()
+            .map_err(|e| format!("Could not finish the marketplace payload: {e}"))
+    }
+}
+
+struct PluginGroup {
+    entries: Vec<Value>,
+    keys: Vec<String>,
+}
+
+/// Files of one install, validated for use as payload paths.
+fn install_files(
+    repo: &Path,
+    inst: &MarketplaceInstall,
+) -> Result<(GitTree, Vec<ItemFile>), String> {
+    let tree = GitTree::open(repo, &inst.commit)?;
+    let files = item_files(&tree, inst.kind, &inst.key)?;
+    if let Some(bad) = files.iter().find(|f| !safe_rel(&f.rel_path)) {
+        return Err(format!("contains an unsafe path ({})", bad.rel_path));
+    }
+    Ok((tree, files))
+}
+
+pub fn build_payload(input: &PayloadInput) -> Result<Payload, String> {
+    let mut tar = TarWriter::new();
+    let mut items: Vec<Value> = Vec::new();
+    let mut skipped: Vec<SkippedItem> = Vec::new();
+    let mut plugin_groups: BTreeMap<String, PluginGroup> = BTreeMap::new();
+    // Non-plugin items share one namespace in ~/.claude; plugins are namespaced
+    // by their per-marketplace catalog, so they never collide.
+    let mut taken: BTreeSet<(ItemKind, String)> = BTreeSet::new();
+
+    for inst in input.installs {
+        let label = format!("{}:{}", inst.kind.as_str(), inst.key);
+        let mut skip = |reason: String| {
+            skipped.push(SkippedItem {
+                item: label.clone(),
+                reason,
+            })
+        };
+
+        let Some(m) = input
+            .marketplaces
+            .iter()
+            .find(|m| m.id == inst.marketplace_id)
+        else {
+            skip("its marketplace has been removed".to_string());
+            continue;
+        };
+        if !is_valid_item_key(&inst.key) || !is_valid_commit(&inst.commit) {
+            skip("the saved install entry is invalid".to_string());
+            continue;
+        }
+        if inst.kind != ItemKind::Plugin && taken.contains(&(inst.kind, inst.key.clone())) {
+            skip(format!(
+                "another marketplace's {label} is already installed"
+            ));
+            continue;
+        }
+        let repo = git::cache_path(input.data_root, &m.id);
+        if !git::has_commit(&repo, &inst.commit) {
+            skip(format!(
+                "pinned commit {} is not in the local cache of \"{}\" — refresh the marketplace",
+                &inst.commit[..8],
+                m.name
+            ));
+            continue;
+        }
+        let (tree, files) = match install_files(&repo, inst) {
+            Ok(v) => v,
+            Err(e) => {
+                skip(e);
+                continue;
+            }
+        };
+
+        let key = &inst.key;
+        let mut item = json!({
+            "kind": inst.kind.as_str(),
+            "key": key,
+            "marketplace": m.id,
+            "commit": inst.commit,
+        });
+        match inst.kind {
+            ItemKind::Agent | ItemKind::Command => {
+                let dir = if inst.kind == ItemKind::Agent {
+                    "agents"
+                } else {
+                    "commands"
+                };
+                let Some(f) = files.first() else {
+                    skip("has no files".to_string());
+                    continue;
+                };
+                let path = format!("{dir}/{key}.md");
+                tar.file(&path, &f.data, false)?;
+                item["file"] = json!(path);
+            }
+            ItemKind::Skill | ItemKind::Hook => {
+                let dir = if inst.kind == ItemKind::Skill {
+                    format!("skills/{key}")
+                } else {
+                    format!("hooks/{key}")
+                };
+                if inst.kind == ItemKind::Hook {
+                    match rendered_hook_settings(&tree, key) {
+                        Ok(settings) => item["settings"] = settings,
+                        Err(e) => {
+                            skip(e);
+                            continue;
+                        }
+                    }
+                }
+                for f in &files {
+                    tar.file(&format!("{dir}/{}", f.rel_path), &f.data, f.executable)?;
+                }
+                item["dir"] = json!(dir);
+            }
+            ItemKind::Plugin => {
+                let mut entry = match plugin_catalog_entry(&tree, key) {
+                    Ok(e) => e,
+                    Err(e) => {
+                        skip(e);
+                        continue;
+                    }
+                };
+                entry["source"] = json!(format!("./{key}"));
+                let slug = marketplace_slug(&m.name, &m.id);
+                for f in &files {
+                    tar.file(
+                        &format!("plugins/{slug}/{key}/{}", f.rel_path),
+                        &f.data,
+                        f.executable,
+                    )?;
+                }
+                let group = plugin_groups
+                    .entry(slug.clone())
+                    .or_insert_with(|| PluginGroup {
+                        entries: Vec::new(),
+                        keys: Vec::new(),
+                    });
+                group.entries.push(entry);
+                group.keys.push(key.clone());
+                item["slug"] = json!(slug);
+            }
+        }
+        if inst.kind != ItemKind::Plugin {
+            taken.insert((inst.kind, key.clone()));
+        }
+        items.push(item);
+    }
+
+    let mut plugin_marketplaces = Vec::new();
+    for (slug, group) in plugin_groups {
+        let catalog = json!({
+            "name": format!("triple-c-{slug}"),
+            "owner": { "name": "Triple-C" },
+            "plugins": group.entries,
+        });
+        let bytes = serde_json::to_vec_pretty(&catalog).map_err(|e| e.to_string())?;
+        tar.file(
+            &format!("plugins/{slug}/.claude-plugin/marketplace.json"),
+            &bytes,
+            false,
+        )?;
+        plugin_marketplaces
+            .push(json!({ "slug": slug, "dir": format!("plugins/{slug}"), "plugins": group.keys }));
+    }
+
+    let manifest =
+        json!({ "version": 1, "items": items, "plugin_marketplaces": plugin_marketplaces });
+    let bytes = serde_json::to_vec_pretty(&manifest).map_err(|e| e.to_string())?;
+    tar.file("manifest.json", &bytes, false)?;
+
+    Ok(Payload {
+        tar: tar.finish()?,
+        manifest,
+        skipped,
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::marketplace::test_support::GitFixture;
+    use std::collections::HashMap;
+    use std::io::Read;
+
+    struct Entry {
+        data: Vec<u8>,
+        mode: u32,
+    }
+
+    fn unpack(tar_bytes: &[u8]) -> HashMap<String, Entry> {
+        let mut archive = tar::Archive::new(tar_bytes);
+        let mut out = HashMap::new();
+        for e in archive.entries().unwrap() {
+            let mut e = e.unwrap();
+            let path = e.path().unwrap().to_string_lossy().into_owned();
+            let mode = e.header().mode().unwrap();
+            let mut data = Vec::new();
+            e.read_to_end(&mut data).unwrap();
+            out.insert(path, Entry { data, mode });
+        }
+        out
+    }
+
+    fn market(id: &str) -> Marketplace {
+        Marketplace {
+            id: id.into(),
+            name: "Team Tools".into(),
+            url: "https://example.invalid/r.git".into(),
+            branch: None,
+            account_id: None,
+        }
+    }
+
+    fn inst(kind: ItemKind, key: &str, commit: &str) -> MarketplaceInstall {
+        MarketplaceInstall {
+            marketplace_id: "m1aaaaaaaa".into(),
+            kind,
+            key: key.into(),
+            commit: commit.into(),
+        }
+    }
+
+    /// Fetch the fixture into `<data>/marketplaces/m1aaaaaaaa.git`.
+    fn cache(fx: &GitFixture, data: &Path) {
+        let repo = git::cache_path(data, "m1aaaaaaaa");
+        git::fetch(&repo, &fx.url(), None, None).unwrap();
+    }
+
+    #[test]
+    fn every_kind_lands_at_its_contract_path() {
+        let Some(fx) = GitFixture::new() else { return };
+        let c = fx.with_all_kinds();
+        let data = tempfile::tempdir().unwrap();
+        cache(&fx, data.path());
+        let installs = vec![
+            inst(ItemKind::Agent, "code-reviewer", &c),
+            inst(ItemKind::Skill, "example-skill", &c),
+            inst(ItemKind::Command, "example-command", &c),
+            inst(ItemKind::Hook, "notify-on-stop", &c),
+            inst(ItemKind::Plugin, "example-plugin", &c),
+        ];
+        let marketplaces = vec![market("m1aaaaaaaa")];
+        let p = build_payload(&PayloadInput {
+            installs: &installs,
+            marketplaces: &marketplaces,
+            data_root: data.path(),
+        })
+        .unwrap();
+
+        assert!(p.skipped.is_empty(), "{:?}", p.skipped);
+        let files = unpack(&p.tar);
+        let slug = marketplace_slug("Team Tools", "m1aaaaaaaa");
+        for path in [
+            "agents/code-reviewer.md".to_string(),
+            "skills/example-skill/SKILL.md".to_string(),
+            "commands/example-command.md".to_string(),
+            "hooks/notify-on-stop/hook.json".to_string(),
+            "hooks/notify-on-stop/notify.sh".to_string(),
+            format!("plugins/{slug}/.claude-plugin/marketplace.json"),
+            format!("plugins/{slug}/example-plugin/.claude-plugin/plugin.json"),
+            format!("plugins/{slug}/example-plugin/skills/hello/SKILL.md"),
+            "manifest.json".to_string(),
+        ] {
+            assert!(
+                files.contains_key(&path),
+                "missing {path}; have {:?}",
+                files.keys().collect::<Vec<_>>()
+            );
+        }
+        assert_eq!(files["hooks/notify-on-stop/notify.sh"].mode & 0o777, 0o755);
+        assert_eq!(files["agents/code-reviewer.md"].mode & 0o777, 0o644);
+    }
+
+    #[test]
+    fn manifest_and_generated_catalog_match_the_contract() {
+        let Some(fx) = GitFixture::new() else { return };
+        let c = fx.with_all_kinds();
+        let data = tempfile::tempdir().unwrap();
+        cache(&fx, data.path());
+        let installs = vec![
+            inst(ItemKind::Hook, "notify-on-stop", &c),
+            inst(ItemKind::Plugin, "example-plugin", &c),
+        ];
+        let marketplaces = vec![market("m1aaaaaaaa")];
+        let p = build_payload(&PayloadInput {
+            installs: &installs,
+            marketplaces: &marketplaces,
+            data_root: data.path(),
+        })
+        .unwrap();
+        let slug = marketplace_slug("Team Tools", "m1aaaaaaaa");
+
+        let files = unpack(&p.tar);
+        let manifest: Value = serde_json::from_slice(&files["manifest.json"].data).unwrap();
+        assert_eq!(manifest, p.manifest);
+        assert_eq!(manifest["version"], 1);
+        let hook = &manifest["items"][0];
+        assert_eq!(hook["kind"], "hook");
+        assert_eq!(hook["dir"], "hooks/notify-on-stop");
+        assert_eq!(
+            hook["settings"]["Stop"][0]["hooks"][0]["command"],
+            "/home/claude/.claude/triple-c/hooks/notify-on-stop/notify.sh"
+        );
+        let plugin = &manifest["items"][1];
+        assert_eq!(plugin["kind"], "plugin");
+        assert_eq!(plugin["slug"], slug.as_str());
+        assert_eq!(
+            manifest["plugin_marketplaces"],
+            json!([{ "slug": slug, "dir": format!("plugins/{slug}"), "plugins": ["example-plugin"] }])
+        );
+
+        let catalog: Value = serde_json::from_slice(
+            &files[&format!("plugins/{slug}/.claude-plugin/marketplace.json")].data,
+        )
+        .unwrap();
+        assert_eq!(catalog["name"], format!("triple-c-{slug}"));
+        assert_eq!(catalog["owner"]["name"], "Triple-C");
+        assert_eq!(catalog["plugins"][0]["name"], "example-plugin");
+        assert_eq!(catalog["plugins"][0]["source"], "./example-plugin");
+    }
+
+    #[test]
+    fn items_that_cannot_be_built_are_skipped_not_fatal() {
+        let Some(fx) = GitFixture::new() else { return };
+        let c = fx.with_all_kinds();
+        let data = tempfile::tempdir().unwrap();
+        cache(&fx, data.path());
+        let mut gone = inst(ItemKind::Agent, "code-reviewer", &c);
+        gone.marketplace_id = "removed".into();
+        let installs = vec![
+            gone,
+            inst(ItemKind::Agent, "code-reviewer", &"0".repeat(40)),
+            inst(ItemKind::Agent, "does-not-exist", &c),
+            inst(ItemKind::Command, "example-command", &c),
+        ];
+        let marketplaces = vec![market("m1aaaaaaaa")];
+        let p = build_payload(&PayloadInput {
+            installs: &installs,
+            marketplaces: &marketplaces,
+            data_root: data.path(),
+        })
+        .unwrap();
+
+        let skipped: Vec<&str> = p.skipped.iter().map(|s| s.item.as_str()).collect();
+        assert_eq!(
+            skipped,
+            vec![
+                "agent:code-reviewer",
+                "agent:code-reviewer",
+                "agent:does-not-exist"
+            ]
+        );
+        assert!(
+            p.skipped[0].reason.contains("marketplace"),
+            "{}",
+            p.skipped[0].reason
+        );
+        assert!(
+            p.skipped[1].reason.contains("cache"),
+            "{}",
+            p.skipped[1].reason
+        );
+        assert_eq!(p.manifest["items"].as_array().unwrap().len(), 1);
+    }
+
+    #[test]
+    fn a_second_marketplace_cannot_shadow_an_installed_name() {
+        let Some(fx) = GitFixture::new() else { return };
+        let c = fx.with_all_kinds();
+        let data = tempfile::tempdir().unwrap();
+        cache(&fx, data.path());
+        let other = git::cache_path(data.path(), "m2bbbbbbbb");
+        git::fetch(&other, &fx.url(), None, None).unwrap();
+        let mut second = inst(ItemKind::Agent, "code-reviewer", &c);
+        second.marketplace_id = "m2bbbbbbbb".into();
+        let installs = vec![inst(ItemKind::Agent, "code-reviewer", &c), second];
+        let marketplaces = vec![market("m1aaaaaaaa"), market("m2bbbbbbbb")];
+        let p = build_payload(&PayloadInput {
+            installs: &installs,
+            marketplaces: &marketplaces,
+            data_root: data.path(),
+        })
+        .unwrap();
+        assert_eq!(p.manifest["items"].as_array().unwrap().len(), 1);
+        assert_eq!(p.skipped.len(), 1);
+        assert!(p.skipped[0].reason.contains("another marketplace"));
+    }
+
+    #[test]
+    fn an_empty_install_set_still_yields_a_manifest() {
+        let data = tempfile::tempdir().unwrap();
+        let p = build_payload(&PayloadInput {
+            installs: &[],
+            marketplaces: &[],
+            data_root: data.path(),
+        })
+        .unwrap();
+        assert_eq!(
+            p.manifest,
+            json!({ "version": 1, "items": [], "plugin_marketplaces": [] })
+        );
+        assert!(unpack(&p.tar).contains_key("manifest.json"));
+    }
+}