diff --git a/diagnostics/claims_api.py b/diagnostics/claims_api.py
index 712a46b..c5d5c5c 100644
--- a/diagnostics/claims_api.py
+++ b/diagnostics/claims_api.py
@@ -10,7 +10,9 @@ The detail endpoint is the canonical /claims/{slug} backend per Ship's
 2026-04-29 brief. One round-trip, no N+1 cascade. Wikilinks resolved
 server-side via title→slug index built from a tree walk.
 """
+import asyncio
 import json
+import logging
 import re
 import sqlite3
 import time
@@ -19,6 +21,8 @@ from pathlib import Path
 import yaml
 from aiohttp import web
 
+logger = logging.getLogger("argus.claims")
+
 # Codex tree roots — claims live in three places (Sourcer Apr 26 fix scope)
 CODEX_BASE = Path("/opt/teleo-eval/workspaces/main")
 CLAIM_TREES = [CODEX_BASE / "domains", CODEX_BASE / "foundations", CODEX_BASE / "core"]
@@ -31,7 +35,14 @@ _list_cache = {"data": None, "ts": 0}
 _LIST_CACHE_TTL = 300  # 5 min — list view tolerates staleness
 
 _index_cache = {"by_title": None, "by_stem": None, "ts": 0}
-_INDEX_CACHE_TTL = 60  # 1 min — title→slug index for wikilink resolution
+_INDEX_CACHE_TTL = 300  # 5 min — title→slug index for wikilink resolution
+
+# Minimum normalized-stem length for prefix-fallback resolution.
+# Stems shorter than this are too generic to be unambiguous in the prefix
+# space (e.g. a "rio" stem would match any request starting with "rio").
+# Proper-prefix matching is much stronger than common-prefix at preventing
+# spurious hits, so this can be lower than the original common-prefix anchor.
+_PREFIX_ANCHOR_MIN = 16
 
 CORS_HEADERS = {"Access-Control-Allow-Origin": "*"}
 
@@ -71,12 +82,16 @@ def _normalize_for_match(s):
 _CODE_FENCE_WRAPPER_RE = re.compile(r"^\s*```(?:markdown|md)?\s*\n(.*?)\n```\s*$", re.DOTALL)
 
 
-def _split_frontmatter(text):
+def _split_frontmatter(text, filepath=None):
     """Return (frontmatter_dict, body_str) or (None, None) if not a claim file.
 
     Tolerates files wrapped in a top-level ```markdown ... ``` code fence —
     some agents have produced these (e.g. Montreal Protocol claim from Astra,
     2024-12-09). Unwrap once before frontmatter detection.
+
+    YAML parse failures are logged at WARNING with the file path (when
+    provided) so KB integrity drift surfaces in logs rather than silently
+    becoming 404s on the detail endpoint.
     """
     if not text:
         return None, None
@@ -92,7 +107,8 @@ def _split_frontmatter(text):
         return None, None
     try:
         fm = yaml.safe_load(text[3:end])
-    except Exception:
+    except yaml.YAMLError as e:
+        logger.warning("YAML parse failed in %s: %s", filepath or "<unknown>", e)
         return None, None
     if not isinstance(fm, dict):
         return None, None
@@ -106,7 +122,7 @@ def _read_claim_file(filepath):
         text = filepath.read_text(encoding="utf-8")
     except (OSError, UnicodeDecodeError):
         return None, None
-    return _split_frontmatter(text)
+    return _split_frontmatter(text, filepath)
 
 
 # ─── Tree walk + indexing ──────────────────────────────────────────────────
@@ -438,7 +454,12 @@ async def handle_claim_detail(request):
     One round-trip, all data resolved server-side. Wikilinks pre-resolved.
     """
     requested_slug = request.match_info["slug"]
-    by_title, by_stem = _build_indexes()
+    # Cold-cache rebuild walks ~1,900 files (~3.3s of sync I/O). Route through
+    # to_thread so the aiohttp event loop stays responsive while the index
+    # rebuilds — concurrent requests don't all stall behind one walk.
+    # Warm-cache cost is a dict access (microseconds), to_thread overhead
+    # negligible. Ganymede review 2026-05-11.
+    by_title, by_stem = await asyncio.to_thread(_build_indexes)
 
     # Resolution order: exact stem → title-normalized (handles description-derived
     # slugs from /api/activity-feed that are longer than on-disk file stems) →
@@ -454,21 +475,22 @@ async def handle_claim_detail(request):
             slug = resolved_stem
             rel_path = by_stem.get(resolved_stem)
     if not rel_path:
-        # Prefix fallback: walk stems sharing a common prefix with the request,
-        # pick longest match. Anchored at 32 chars to avoid spurious hits.
+        # Proper-prefix fallback: the requested slug should START WITH a known
+        # stem (covers activity-feed slugs longer than on-disk filenames). The
+        # earlier common-prefix variant of this was broken in two directions —
+        # served non-deterministic matches on same-prefix collisions, and
+        # missed legitimate matches under the 32-char anchor (e.g. stems
+        # shorter than 32 chars normalized). Ganymede review 2026-05-11.
         norm_req = _normalize_for_match(requested_slug)
         best_stem = None
         best_len = 0
         for stem in by_stem:
             norm_stem = _normalize_for_match(stem)
-            common = 0
-            for a, b in zip(norm_req, norm_stem):
-                if a != b:
-                    break
-                common += 1
-            if common >= 32 and common > best_len:
+            if len(norm_stem) < _PREFIX_ANCHOR_MIN:
+                continue  # too generic in prefix space
+            if norm_req.startswith(norm_stem) and len(norm_stem) > best_len:
                 best_stem = stem
-                best_len = common
+                best_len = len(norm_stem)
         if best_stem:
             slug = best_stem
             rel_path = by_stem.get(best_stem)