diff --git a/crates/aura-registry/src/lib.rs b/crates/aura-registry/src/lib.rs
index 6fa1562..784e09e 100644
--- a/crates/aura-registry/src/lib.rs
+++ b/crates/aura-registry/src/lib.rs
@@ -173,6 +173,95 @@ impl Registry {
         }
     }
 
+    /// The label sidecar path (#317) — a sibling of `blueprint_identity_index.jsonl`,
+    /// the same "fixed-name sidecar, per-directory isolation" discipline (#191).
+    fn blueprint_names_path(&self) -> PathBuf {
+        self.path.with_file_name("blueprint_names.jsonl")
+    }
+
+    /// Raw, ungated append: one label line, unconditionally, under the
+    /// registry's write mutex (#276). No shape or content-id check —
+    /// [`Registry::put_blueprint_label`] is the gated public entry; this
+    /// exists so tests can hand-append a stale line, mirroring
+    /// `append_identity_index`.
+    fn append_blueprint_label_line(&self, name: &str, content_id: &str) -> Result<(), RegistryError> {
+        let _guard = self.append_write_lock.lock().unwrap_or_else(std::sync::PoisonError::into_inner);
+        let path = self.blueprint_names_path();
+        if let Some(parent) = path.parent().filter(|p| !p.as_os_str().is_empty()) {
+            fs::create_dir_all(parent)?;
+        }
+        let line = serde_json::to_string(&BlueprintNameLine {
+            name: name.to_string(),
+            content_id: content_id.to_string(),
+        })
+        .expect("two plain strings serialize");
+        let mut file = fs::OpenOptions::new().create(true).append(true).open(&path)?;
+        writeln!(file, "{line}")?;
+        Ok(())
+    }
+
+    /// Label a stored blueprint (#317): appends `{"name":..,"content_id":..}`
+    /// to `blueprint_names.jsonl`, a sidecar beside the identity index. Gated
+    /// on a deterministic label shape (nonempty after trim, no whitespace, no
+    /// control characters — `RegistryError::BadLabel`, C29 discipline: shape
+    /// only, never content judgement) and on `content_id` resolving via
+    /// [`Registry::get_blueprint`] (`RegistryError::UnknownBlueprint`
+    /// otherwise) — both checked BEFORE the write lock is taken. Re-labelling
+    /// an existing name appends a new line (a repoint); resolution is
+    /// latest-wins (see [`Registry::resolve_blueprint_label`]).
+    pub fn put_blueprint_label(&self, name: &str, content_id: &str) -> Result<(), RegistryError> {
+        gate_label_shape(name)?;
+        if self.get_blueprint(content_id)?.is_none() {
+            return Err(RegistryError::UnknownBlueprint(content_id.to_string()));
+        }
+        self.append_blueprint_label_line(name, content_id)
+    }
+
+    /// Resolve a label to the content id of the LATEST line naming it, skipping
+    /// any line whose content id no longer resolves via `get_blueprint`
+    /// (self-repair spirit, #191) — so "latest" always means the latest VALID
+    /// entry, even when a later append points at a since-vanished id. Missing
+    /// sidecar file -> `None` (treat-as-empty, like `load_identity_index`).
+    pub fn resolve_blueprint_label(&self, name: &str) -> Option<String> {
+        self.latest_blueprint_labels().remove(name)
+    }
+
+    /// Every registered label, latest-wins-deduped and name-sorted — the
+    /// discovery surface's data source (`graph introspect --registered`).
+    /// Dangling entries (content id no longer resolves) are skipped.
+    pub fn blueprint_labels(&self) -> Vec<(String, String)> {
+        self.latest_blueprint_labels().into_iter().collect()
+    }
+
+    /// Every content id currently stored in the blueprint store — the
+    /// candidate list a `use` ref's content-id-PREFIX resolution (#302
+    /// semantics, #317) filters against, mirroring `list_process_ids`/
+    /// `list_campaign_ids`. `blueprints_dir()` and `doc_dir("blueprints")`
+    /// name the same directory, so `list_doc_ids`'s filename-stem walk
+    /// applies unchanged.
+    pub fn list_blueprint_ids(&self) -> Result<Vec<String>, RegistryError> {
+        self.list_doc_ids("blueprints")
+    }
+
+    /// Shared latest-wins-and-dangling-skipping read of `blueprint_names.jsonl`
+    /// into a name -> content_id map (`BTreeMap` gives the name-sorted order
+    /// `blueprint_labels` promises for free). A garbage line (torn write,
+    /// manual edit) is skipped, like every other JSONL sidecar read in this
+    /// module.
+    fn latest_blueprint_labels(&self) -> std::collections::BTreeMap<String, String> {
+        let Ok(text) = fs::read_to_string(self.blueprint_names_path()) else {
+            return std::collections::BTreeMap::new();
+        };
+        let mut map = std::collections::BTreeMap::new();
+        for raw in text.lines() {
+            let Ok(line) = serde_json::from_str::<BlueprintNameLine>(raw) else { continue };
+            if self.get_blueprint(&line.content_id).ok().flatten().is_some() {
+                map.insert(line.name, line.content_id);
+            }
+        }
+        map
+    }
+
     /// The content-addressed document-store dir for one document kind —
     /// a sibling of the runs store, like `blueprints/`.
     fn doc_dir(&self, kind: &str) -> PathBuf {
@@ -273,10 +362,42 @@ impl Registry {
     }
 }
 
+/// One label->content mapping, one JSON object per line in
+/// `blueprint_names.jsonl` (#317) — the `IdentityIndexLine` sidecar-line
+/// shape, mirrored for the label store.
+#[derive(serde::Serialize, serde::Deserialize)]
+struct BlueprintNameLine {
+    name: String,
+    content_id: String,
+}
+
+/// Deterministic label-shape gate for `put_blueprint_label` (#317): nonempty
+/// after trim, no whitespace, no control characters. String shape only —
+/// never content judgement (the C29 discipline `doc_gate` also follows).
+fn gate_label_shape(name: &str) -> Result<(), RegistryError> {
+    if name.trim().is_empty() {
+        return Err(RegistryError::BadLabel { name: name.to_string(), rule: "must be nonempty" });
+    }
+    if name.chars().any(char::is_whitespace) {
+        return Err(RegistryError::BadLabel { name: name.to_string(), rule: "must not contain whitespace" });
+    }
+    if name.chars().any(|c| c.is_control()) {
+        return Err(RegistryError::BadLabel {
+            name: name.to_string(),
+            rule: "must not contain control characters",
+        });
+    }
+    Ok(())
+}
+
 /// C29 walk (#316): the root and every named nested composite must carry a
 /// gate-passing doc. `doc: None` refuses exactly like `Some("")` — both are
-/// the Empty fault ("carries no doc").
-fn gate_composite_docs(c: &CompositeData) -> Result<(), RegistryError> {
+/// the Empty fault ("carries no doc"). `pub` (#317): the `use` construction
+/// seam (`aura-cli`) re-runs this SAME walk over a freshly-fetched composite
+/// before it enters a new composition (fail fast on a doc-less fetched
+/// entry) — the one walk, two call sites (register's write-path gate, use's
+/// read-path gate), never a second implementation.
+pub fn gate_composite_docs(c: &CompositeData) -> Result<(), RegistryError> {
     let fault = match c.doc.as_deref() {
         None => Some(aura_core::DocGateFault::Empty),
         Some(d) => aura_core::doc_gate(&c.name, d).err(),
@@ -938,6 +1059,14 @@ pub enum RegistryError {
     /// the store without a gate-passing doc. Registered artifacts are never
     /// retroactively invalidated — the gate guards the write path only.
     UndescribedComposite { name: String, fault: aura_core::DocGateFault },
+    /// Label sidecar (#317): a `put_blueprint_label` name failing the
+    /// deterministic label-shape gate (nonempty after trim, no whitespace, no
+    /// control characters). By-identifier, naming the violated rule.
+    BadLabel { name: String, rule: &'static str },
+    /// Label sidecar (#317): a `put_blueprint_label` content id that does not
+    /// resolve via [`Registry::get_blueprint`] — the label sidecar never
+    /// labels a blueprint the store does not have.
+    UnknownBlueprint(String),
 }
 
 impl fmt::Display for RegistryError {
@@ -978,6 +1107,12 @@ impl fmt::Display for RegistryError {
                      its name — a registered composite describes itself (C29)"
                 ),
             },
+            RegistryError::BadLabel { name, rule } => {
+                write!(f, "blueprint label {name:?}: {rule}")
+            }
+            RegistryError::UnknownBlueprint(id) => {
+                write!(f, "blueprint: no stored blueprint with content id \"{id}\"")
+            }
         }
     }
 }
@@ -1172,6 +1307,134 @@ mod tests {
         );
     }
 
+    /// A described fixture blueprint, ready for `put_blueprint`'s C29 gate —
+    /// `put_blueprint_label`'s content-id verification routes through
+    /// `get_blueprint`, so its target must first be a gate-passing entry.
+    fn described_fixture_blueprint(name: &str) -> String {
+        format!(
+            r#"{{"format_version":1,"blueprint":{{"name":"{name}","doc":"a described fixture blueprint","nodes":[]}}}}"#
+        )
+    }
+
+    /// Property (#317): a label round-trips to the blueprint it names, and
+    /// re-labelling the SAME name repoints it — resolution and the discovery
+    /// listing both answer with the latest target, never the first.
+    #[test]
+    fn blueprint_label_round_trips_latest_wins() {
+        let reg = Registry::open(temp_family_dir("label_round_trip"));
+        let a = described_fixture_blueprint("a");
+        let b = described_fixture_blueprint("b");
+        reg.put_blueprint("ha", &a).expect("put a");
+        reg.put_blueprint("hb", &b).expect("put b");
+
+        reg.put_blueprint_label("x", "ha").expect("label x -> a");
+        assert_eq!(reg.resolve_blueprint_label("x"), Some("ha".to_string()));
+
+        reg.put_blueprint_label("x", "hb").expect("re-label x -> b");
+        assert_eq!(reg.resolve_blueprint_label("x"), Some("hb".to_string()), "latest label wins");
+        assert_eq!(
+            reg.blueprint_labels(),
+            vec![("x".to_string(), "hb".to_string())],
+            "the discovery listing dedups to the latest target",
+        );
+    }
+
+    /// Property (#317): the label-shape gate is deterministic string shape
+    /// only (nonempty after trim, no whitespace, no control characters) —
+    /// each bad shape refuses with `BadLabel` naming a (non-empty) rule.
+    #[test]
+    fn blueprint_label_refuses_bad_shapes() {
+        let reg = Registry::open(temp_family_dir("label_bad_shapes"));
+        let bp = described_fixture_blueprint("s");
+        reg.put_blueprint("h1", &bp).expect("put");
+
+        for bad in ["", "  ", "a b", "a\tb"] {
+            match reg.put_blueprint_label(bad, "h1") {
+                Err(RegistryError::BadLabel { name, rule }) => {
+                    assert_eq!(name, bad);
+                    assert!(!rule.is_empty(), "BadLabel must name the violated rule for {bad:?}");
+                }
+                other => panic!("expected BadLabel for {bad:?}, got {other:?}"),
+            }
+        }
+    }
+
+    /// Property (#317): a label may only ever point at a blueprint the store
+    /// actually has — labelling an unregistered content id refuses
+    /// by-identifier, never silently writing a dangling line.
+    #[test]
+    fn blueprint_label_refuses_unknown_content_id() {
+        let reg = Registry::open(temp_family_dir("label_unknown_id"));
+        match reg.put_blueprint_label("x", "never-written") {
+            Err(RegistryError::UnknownBlueprint(id)) => assert_eq!(id, "never-written"),
+            other => panic!("expected UnknownBlueprint, got {other:?}"),
+        }
+        assert_eq!(reg.resolve_blueprint_label("x"), None, "the refused label must not have written");
+    }
+
+    /// Property (#317, #191 spirit): a dangling LATEST line (content id no
+    /// longer resolves) is skipped by resolution, which falls back to the
+    /// earlier still-valid line for the same name — latest-wins means
+    /// latest-VALID-wins.
+    #[test]
+    fn blueprint_label_resolution_skips_a_dangling_entry() {
+        let reg = Registry::open(temp_family_dir("label_dangling"));
+        let bp = described_fixture_blueprint("s");
+        reg.put_blueprint("h1", &bp).expect("put");
+        reg.put_blueprint_label("x", "h1").expect("label x -> h1");
+
+        // hand-append a dangling line after the valid one, mirroring
+        // `append_identity_index`'s stale-line test shape: the content id it
+        // names was never registered.
+        reg.append_blueprint_label_line("x", "never-written").expect("append stale line");
+
+        assert_eq!(
+            reg.resolve_blueprint_label("x"),
+            Some("h1".to_string()),
+            "the dangling latest line is skipped; the earlier valid line for \"x\" wins",
+        );
+        assert_eq!(reg.blueprint_labels(), vec![("x".to_string(), "h1".to_string())]);
+    }
+
+    /// Property (#317, #276 discipline): N threads appending labels through
+    /// one shared `&Registry` must leave `blueprint_names.jsonl` well-formed —
+    /// every successful append surviving as its own intact, resolvable line,
+    /// none torn or merged (mirrors `concurrent_appends_keep_the_family_
+    /// store_well_formed`).
+    #[test]
+    fn concurrent_label_appends_keep_the_sidecar_well_formed() {
+        let reg = Registry::open(temp_family_dir("concurrent_label"));
+        let bp = described_fixture_blueprint("s");
+        reg.put_blueprint("h1", &bp).expect("put");
+        let n_threads = 8usize;
+        let iters = 50usize;
+
+        let total_ok: usize = std::thread::scope(|scope| {
+            let handles: Vec<_> = (0..n_threads)
+                .map(|tid| {
+                    let reg = &reg;
+                    scope.spawn(move || {
+                        let mut ok = 0usize;
+                        for iter in 0..iters {
+                            let name = format!("t{tid}-l{iter}");
+                            if reg.put_blueprint_label(&name, "h1").is_ok() {
+                                ok += 1;
+                            }
+                        }
+                        ok
+                    })
+                })
+                .collect();
+            handles.into_iter().map(|h| h.join().expect("worker thread joined")).sum()
+        });
+
+        assert_eq!(
+            reg.blueprint_labels().len(),
+            total_ok,
+            "every successful label append must survive as its own intact, resolvable entry",
+        );
+    }
+
     #[test]
     fn document_stores_round_trip_by_content_id() {
         let reg = Registry::open(temp_family_dir("document_store_round_trip"));