diff --git a/agentic/harness_optimizer/core.py b/agentic/harness_optimizer/core.py
index 8c9857602..b0a415cbc 100644
--- a/agentic/harness_optimizer/core.py
+++ b/agentic/harness_optimizer/core.py
@@ -7,6 +7,7 @@
 
 from __future__ import annotations
 
+import math
 from dataclasses import asdict, dataclass, field
 from enum import StrEnum
 
@@ -152,6 +153,12 @@ def __post_init__(self) -> None:
             raise AgenticError("run_report pass fields must be booleans", details={"variant_id": self.variant_id})
         if not isinstance(self.score, int | float) or isinstance(self.score, bool):
             raise AgenticError("run_report.score must be numeric", details={"variant_id": self.variant_id})
+        # decide_candidate's "score_not_improved" gate is a <= comparison, which
+        # is False for NaN on either side: a NaN score would fail that gate open.
+        # Floats only: an int is always finite, and math.isfinite(10**1000) raises
+        # OverflowError converting it.
+        if isinstance(self.score, float) and not math.isfinite(self.score):
+            raise AgenticError("run_report.score must be finite", details={"variant_id": self.variant_id})
 
     @property
     def has_critical_governance_finding(self) -> bool:
diff --git a/agentic/harness_optimizer/patching.py b/agentic/harness_optimizer/patching.py
index 4d42c8c05..2ce03e6aa 100644
--- a/agentic/harness_optimizer/patching.py
+++ b/agentic/harness_optimizer/patching.py
@@ -185,15 +185,24 @@ def apply_candidate_artifact(
     with _write_lock:  # serialize threads within this process
         _acquire_artifact_lock(lock_dir)  # serialize other processes too
         try:
-            previous = {}
+            version = 1
             if artifact_path.exists():
+                # Valid JSON of the wrong shape ([], null, {"version": "x"}) or a
+                # non-UTF-8 file is just as malformed as invalid JSON; without this
+                # it escaped as a raw AttributeError/ValueError, outside the typed
+                # AgenticError contract the CLI maps to its exit codes.
                 try:
                     previous = json.loads(artifact_path.read_text(encoding="utf-8"))
-                except json.JSONDecodeError as exc:
+                    if not isinstance(previous, dict):
+                        raise TypeError("artifact is not a JSON object")
+                    prior_version = previous.get("version", 0)
+                    if isinstance(prior_version, bool) or not isinstance(prior_version, int) or prior_version < 0:
+                        raise TypeError("artifact version is not a non-negative integer")
+                except (ValueError, TypeError, UnicodeDecodeError) as exc:
                     raise AgenticError(
                         "existing harness artifact is malformed", details={"path": str(artifact_path)}
                     ) from exc
-            version = int(previous.get("version", 0)) + 1
+                version = prior_version + 1
             record = {
                 "version": version,
                 "variant_id": proposal.variant_id,
diff --git a/agentic/harness_optimizer/scoring.py b/agentic/harness_optimizer/scoring.py
index 6798890c0..f0d16d89c 100644
--- a/agentic/harness_optimizer/scoring.py
+++ b/agentic/harness_optimizer/scoring.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import math
 from dataclasses import dataclass
 
 from agentic.harness_optimizer.core import CandidateDecision, RunReport
@@ -24,7 +25,10 @@ def __post_init__(self) -> None:
             raise AgenticError("case result passed must be a boolean", details={"case_id": self.case_id})
         if not isinstance(self.score, int | float) or isinstance(self.score, bool):
             raise AgenticError("case result score must be numeric", details={"case_id": self.case_id})
-        if self.score < 0.0 or self.score > 1.0:
+        # NaN compares False against everything, so it would slip past the
+        # range check below and poison score_cases()' average. Floats only:
+        # math.isfinite overflows on a huge int, which the range check rejects.
+        if (isinstance(self.score, float) and not math.isfinite(self.score)) or self.score < 0.0 or self.score > 1.0:
             raise AgenticError("case result score must be between 0 and 1", details={"case_id": self.case_id})
 
 
diff --git a/tests/test_agentic_harness_optimizer.py b/tests/test_agentic_harness_optimizer.py
index b0504a2bf..50029d861 100644
--- a/tests/test_agentic_harness_optimizer.py
+++ b/tests/test_agentic_harness_optimizer.py
@@ -19,6 +19,7 @@
     build_proposer_workspace,
     decide_candidate,
 )
+from agentic.harness_optimizer.scoring import CaseResult
 from agentic.harness_optimizer.patching import (
     _acquire_artifact_lock,
     _release_artifact_lock,
@@ -79,6 +80,27 @@ def test_candidate_rejects_no_improvement() -> None:
     assert "score_not_improved" in decision.rejected_gates
 
 
+@pytest.mark.parametrize("bad", [float("nan"), float("inf"), float("-inf")])
+def test_run_report_rejects_non_finite_score(bad: float) -> None:
+    # decide_candidate's score gate is `candidate <= baseline`, which is False
+    # whenever either side is NaN: a NaN report must never be constructible,
+    # or that gate fails open and the candidate is accepted.
+    with pytest.raises(AgenticError, match="finite"):
+        RunReport("candidate", train_passed=True, holdout_passed=True, score=bad)
+
+
+def test_run_report_accepts_a_huge_int_score_without_overflow() -> None:
+    # math.isfinite(10**1000) raises OverflowError; ints are always finite, so
+    # the finiteness check must not run on them (Codex review on this PR).
+    assert RunReport("candidate", train_passed=True, holdout_passed=True, score=10**1000).score == 10**1000
+
+
+@pytest.mark.parametrize("bad", [float("nan"), float("inf"), 10**1000])
+def test_case_result_rejects_non_finite_score(bad: float) -> None:
+    with pytest.raises(AgenticError, match="between 0 and 1"):
+        CaseResult("case-1", passed=True, score=bad)
+
+
 def test_candidate_rejects_unallowed_surface_and_visible_case_hardcoding() -> None:
     baseline = RunReport("baseline", train_passed=True, holdout_passed=True, score=0.40)
     candidate = RunReport(
diff --git a/tests/test_agentic_harness_phase679.py b/tests/test_agentic_harness_phase679.py
index cd5a520dc..fad0357c4 100644
--- a/tests/test_agentic_harness_phase679.py
+++ b/tests/test_agentic_harness_phase679.py
@@ -315,6 +315,39 @@ def test_apply_candidate_artifact_requires_all_human_gates(tmp_path: Path) -> No
     assert record["proposal_sha256"] == proposal.proposal_sha256
 
 
+@pytest.mark.parametrize("prior", ["[]", "null", '{"version": "x"}', '{"version": -1}', '{"version": true}'])
+def test_apply_candidate_artifact_maps_malformed_prior_artifact_to_agentic_error(tmp_path: Path, prior: str) -> None:
+    # Valid JSON of the wrong shape used to escape as AttributeError/ValueError,
+    # outside the typed-error contract the agentic CLI maps to exit codes.
+    workspace, cfg = _workspace(tmp_path)
+    workspace.proposal_path.write_text("# Proposal\n\nGeneral fix.", encoding="utf-8")
+    decision = decide_candidate(
+        baseline=RunReport("baseline", train_passed=True, holdout_passed=True, score=0.1),
+        candidate=RunReport(
+            "candidate", train_passed=True, holdout_passed=True, score=0.9, changed_surfaces=("planner",)
+        ),
+        allowed_surface_ids={"planner"},
+        proposal_present=True,
+    )
+    proposal = propose_candidate_application(
+        decision, Variant("candidate", ("planner",), "proposal.md", str(workspace.root)), workspace, cfg=cfg
+    )
+    config = _config(harness={"enabled": True})
+    config.mode = "write"
+    config.writes_enabled = True
+    config.harness_optimizer.output_dir = str(tmp_path / "output")
+    config.harness_optimizer.memory_dir = str(tmp_path / "memory")
+    artifact = tmp_path / "output" / "accepted" / f"{proposal.variant_id}.json"
+    artifact.parent.mkdir(parents=True)
+    artifact.write_text(prior, encoding="utf-8")
+
+    with pytest.raises(AgenticError, match="malformed"):
+        apply_candidate_artifact(proposal, config, reason="record fixture candidate", confirm=True, cfg=cfg)
+    # The malformed artifact is left as found, and the lock is released.
+    assert artifact.read_text(encoding="utf-8") == prior
+    assert not artifact.with_suffix(".json.lock.d").exists()
+
+
 def test_apply_candidate_artifact_blocked_when_lock_held(tmp_path: Path) -> None:
     # A held lock (another accept in progress) makes a concurrent apply refuse
     # rather than race the read-modify-write of the version counter.