diff --git a/codex/synthesis.py b/codex/synthesis.py
index dcc1586..6bfb68c 100644
--- a/codex/synthesis.py
+++ b/codex/synthesis.py
@@ -248,9 +248,23 @@ def _grounded_ratio(claims: list[Claim]) -> float:
     return n_grounded / len(claims)
 
 
-def _make_lead_id(seq: int) -> str:
-    """Format a sequential lead id like ``L-0001``."""
-    return f"L-{seq:04d}"
+_KIND_PREFIX: dict[LeadKind, str] = {
+    "connection": "C",
+    "gap": "G",
+    "improvement": "I",
+    "conjecture": "X",
+}
+
+
+def _make_lead_id(seq: int, kind: LeadKind) -> str:
+    """Format a per-kind sequential lead id like ``L-C-0001``.
+
+    Each synthesis stage numbers its leads from 1, so without a kind prefix a
+    connection, a gap, and an improvement all produce ``L-0001`` and clobber
+    one another in the shared ``grounded/`` directory (audit C-11). The kind
+    prefix namespaces the ids so they stay distinct.
+    """
+    return f"L-{_KIND_PREFIX[kind]}-{seq:04d}"
 
 
 def _safe_llm_generate(llm: LLMClient, prompt: str, model: str) -> str:
@@ -296,7 +310,7 @@ def _finalise_grounded_lead(
         return None
     marked = _mark_ungrounded(body, claims)
     return Lead(
-        id=_make_lead_id(seq),
+        id=_make_lead_id(seq, kind),
         kind=kind,
         title=title,
         body=marked,
@@ -428,7 +442,7 @@ def find_gaps(
             # No coverage at all → record the gap directly (LLM not strictly needed).
             leads.append(
                 Lead(
-                    id=_make_lead_id(seq),
+                    id=_make_lead_id(seq, "gap"),
                     kind="gap",
                     title=f"Gap: corpus has no material on '{topic}'",
                     body=(
@@ -481,7 +495,7 @@ def find_gaps(
         for bibkey in sorted(missing):
             leads.append(
                 Lead(
-                    id=_make_lead_id(seq),
+                    id=_make_lead_id(seq, "gap"),
                     kind="gap",
                     title=f"Gap: code cites '{bibkey}' but corpus does not contain it",
                     body=(
@@ -658,7 +672,7 @@ def propose_conjectures(
             continue
         leads.append(
             Lead(
-                id=_make_lead_id(seq),
+                id=_make_lead_id(seq, "conjecture"),
                 kind="conjecture",
                 title=title,
                 body=body,
diff --git a/tests/synthesis/test_grounded.py b/tests/synthesis/test_grounded.py
index 17e6fb8..7d27ba7 100644
--- a/tests/synthesis/test_grounded.py
+++ b/tests/synthesis/test_grounded.py
@@ -76,7 +76,7 @@ def test_finalise_grounded_lead_succeeds_when_grounded(
     )
     assert lead is not None
     assert lead.kind == "connection"
-    assert lead.id == "L-0001"
+    assert lead.id == "L-C-0001"  # kind-prefixed id (audit C-11)
     assert lead.confidence >= 0.5
     assert lead.status == "unverified"
     assert lead.suggested_validation == ""
diff --git a/tests/synthesis/test_model.py b/tests/synthesis/test_model.py
index e45c18d..b2c991a 100644
--- a/tests/synthesis/test_model.py
+++ b/tests/synthesis/test_model.py
@@ -92,9 +92,21 @@ def test_lead_empty_provenance_raises() -> None:
 
 
 def test_lead_id_format_helper() -> None:
-    """The internal _make_lead_id helper produces zero-padded L-XXXX strings."""
+    """_make_lead_id produces a kind-prefixed, zero-padded L-<K>-XXXX string."""
     from codex.synthesis import _make_lead_id
 
-    assert _make_lead_id(1) == "L-0001"
-    assert _make_lead_id(42) == "L-0042"
-    assert _make_lead_id(9999) == "L-9999"
+    assert _make_lead_id(1, "connection") == "L-C-0001"
+    assert _make_lead_id(42, "gap") == "L-G-0042"
+    assert _make_lead_id(9999, "improvement") == "L-I-9999"
+    assert _make_lead_id(1, "conjecture") == "L-X-0001"
+
+
+def test_lead_id_namespaced_per_kind_no_collision() -> None:
+    """Regression for audit C-11: each stage numbers from 1, so without a kind
+    prefix connection/gap/improvement all collide on L-0001 and overwrite one
+    another in grounded/. The prefix must keep same-seq ids distinct.
+    """
+    from codex.synthesis import _make_lead_id
+
+    ids = {_make_lead_id(1, k) for k in ("connection", "gap", "improvement", "conjecture")}
+    assert len(ids) == 4, f"same-seq ids must stay distinct across kinds, got {ids}"