fix: gradual fidelity degradation with recency guard and better stubs

Root cause of retry loops: mass degradation stubbed 85% of context at
once, confusing the model into infinite retries.

Fixes:
- Cap degradations at 20 per turn (gradual compression)
- Protect objects accessed within last 10 turns from degradation
- Estimate L1/L2 token counts (30%/10% of L0) so FM pressure tracks
  correctly after degradation
- Improved stubs: '[compressed 3.4KB -> stub] first 200 chars...'
- Re-enabled _apply_fidelity

src/mnemosyne/fidelity.py

@@ -164,8 +164,16 @@ def make_object(
         created_at_turn=created_at_turn,
         last_accessed_turn=created_at_turn,
         token_count_l0=_estimate_tokens(content_full),
-        token_count_l1=_estimate_tokens(summary_detailed) if summary_detailed else None,
+        token_count_l1=(
-        token_count_l2=_estimate_tokens(summary_compact) if summary_compact else None,
+            _estimate_tokens(summary_detailed)
+            if summary_detailed
+            else max(10, _estimate_tokens(content_full) // 3)  # ~30% of L0
+        ),
+        token_count_l2=(
+            _estimate_tokens(summary_compact)
+            if summary_compact
+            else max(5, _estimate_tokens(content_full) // 10)  # ~10% of L0
+        ),
         token_count_l3=_estimate_tokens(stub) if stub else 25,
     )
     return obj
@@ -310,9 +318,22 @@ class FidelityManager:
                     obj.pinned = False
                     obj.pin_until_turn = None
 
-    def _degradable(self, obj: SemanticObject) -> bool:
+    # Objects accessed within the last N turns are protected from degradation.
-        """Check if an object can be degraded (not pinned, not already L4)."""
+    _RECENCY_GUARD_TURNS = 10
-        return not obj.pinned and obj.current_fidelity < FidelityLevel.L4
+    # Maximum number of objects to degrade per turn (prevents mass stubbing).
+    _MAX_DEGRADES_PER_TURN = 20
+
+    def _degradable(self, obj: SemanticObject, current_turn: int = 0) -> bool:
+        """Check if an object can be degraded.
+
+        Protected if: pinned, already L4, or accessed within the last
+        ``_RECENCY_GUARD_TURNS`` turns.
+        """
+        if obj.pinned or obj.current_fidelity >= FidelityLevel.L4:
+            return False
+        if current_turn > 0 and (current_turn - obj.last_accessed_turn) < self._RECENCY_GUARD_TURNS:
+            return False
+        return True
 
     def _degrade_caution(
         self,
@@ -323,11 +344,13 @@ class FidelityManager:
         candidates = [
             o
             for o in self._objects.values()
-            if o.current_fidelity == FidelityLevel.L0 and self._degradable(o)
+            if o.current_fidelity == FidelityLevel.L0 and self._degradable(o, current_turn)
         ]
         candidates = self._sorted_by_age(candidates)
 
         for obj in candidates:
+            if len(transitions) >= self._MAX_DEGRADES_PER_TURN:
+                break
             old = obj.current_fidelity
             new = self._degrade_one(obj)
             if new is not None:
@@ -344,16 +367,19 @@ class FidelityManager:
     ) -> list[tuple[str, FidelityLevel, FidelityLevel]]:
         """Warning zone: degrade L0→L1, L1→L2, oldest L2→L3."""
         transitions: list[tuple[str, FidelityLevel, FidelityLevel]] = []
+        cap = self._MAX_DEGRADES_PER_TURN
 
         # First pass: L0 → L1
         l0_objs = self._sorted_by_age(
             [
                 o
                 for o in self._objects.values()
-                if o.current_fidelity == FidelityLevel.L0 and self._degradable(o)
+                if o.current_fidelity == FidelityLevel.L0 and self._degradable(o, current_turn)
             ]
         )
         for obj in l0_objs:
+            if len(transitions) >= cap:
+                return transitions
             old = obj.current_fidelity
             new = self._degrade_one(obj)
             if new is not None:
@@ -366,10 +392,12 @@ class FidelityManager:
             [
                 o
                 for o in self._objects.values()
-                if o.current_fidelity == FidelityLevel.L1 and self._degradable(o)
+                if o.current_fidelity == FidelityLevel.L1 and self._degradable(o, current_turn)
             ]
         )
         for obj in l1_objs:
+            if len(transitions) >= cap:
+                return transitions
             old = obj.current_fidelity
             new = self._degrade_one(obj)
             if new is not None:
@@ -382,7 +410,7 @@ class FidelityManager:
             [
                 o
                 for o in self._objects.values()
-                if o.current_fidelity == FidelityLevel.L2 and self._degradable(o)
+                if o.current_fidelity == FidelityLevel.L2 and self._degradable(o, current_turn)
             ]
         )
         for obj in l2_objs:
@@ -408,7 +436,7 @@ class FidelityManager:
                 [
                     o
                     for o in self._objects.values()
-                    if o.current_fidelity == target_level and self._degradable(o)
+                    if o.current_fidelity == target_level and self._degradable(o, current_turn)
                 ]
             )
             for obj in candidates:
@@ -426,7 +454,7 @@ class FidelityManager:
             [
                 o
                 for o in self._objects.values()
-                if o.current_fidelity == FidelityLevel.L3 and self._degradable(o)
+                if o.current_fidelity == FidelityLevel.L3 and self._degradable(o, current_turn)
             ]
         )
         for obj in l3_objs:
@@ -455,7 +483,7 @@ class FidelityManager:
             [
                 o
                 for o in self._objects.values()
-                if o.current_fidelity < FidelityLevel.L4 and self._degradable(o)
+                if o.current_fidelity < FidelityLevel.L4 and self._degradable(o, current_turn)
             ]
         )
         for obj in all_objs:

src/mnemosyne/gateway.py

@@ -1209,11 +1209,17 @@ def _block_text(block: dict) -> str:
 
 
 def _auto_stub(text: str) -> str:
-    """Generate a simple stub from the first line of content."""
+    """Generate a compact stub preserving the first ~200 chars of content.
-    first_line = text.split("\n", 1)[0].strip()
+
-    if len(first_line) > 120:
+    Keeps enough context for the model to understand what the content was
-        first_line = first_line[:117] + "..."
+    without needing the full text.  The stub format includes a size hint
-    return f"[evicted content: {first_line}]"
+    so the model knows how much was removed.
+    """
+    size_kb = len(text.encode("utf-8", errors="replace")) / 1024
+    preview = text[:200].replace("\n", " ").strip()
+    if len(text) > 200:
+        preview = preview[:197] + "..."
+    return f"[compressed {size_kb:.1f}KB → stub] {preview}"
 
 
 import logging as _logging
@@ -2183,9 +2189,7 @@ def create_app(
         #     )
 
         # 4b. Apply fidelity-based content replacement
-        # DISABLED: L1/L2 stub replacement causes response issues that trigger
+        _apply_fidelity(payload, session)
-        # rapid retries in opencode. Re-enable once stub format is validated.
-        # _apply_fidelity(payload, session)
 
         # Place cache_control markers at optimal positions
         _place_cache_controls(payload)