diff --git a/src/benchflow/diagnostics.py b/src/benchflow/diagnostics.py
index f00c41cc..17022a19 100644
--- a/src/benchflow/diagnostics.py
+++ b/src/benchflow/diagnostics.py
@@ -93,8 +93,8 @@ class Diagnostic:
     summary_description: ClassVar[str] = ""
 
     def to_dict(self) -> dict[str, Any]:
-        """Serialize for result.json — drops Nones is the caller's choice."""
-        return asdict(self)
+        """Serialize for result.json, omitting unpopulated optional fields."""
+        return {k: v for k, v in asdict(self).items() if v is not None}
 
     def format_issue(self, task_name: str) -> str:
         """Render the per-task line check_results emits for this diagnostic."""
@@ -237,24 +237,7 @@ def to_dict(self) -> dict[str, Any]:
         the same way so result.json stays compact and tests that compare
         explicit dicts continue to work.
         """
-        raw = asdict(self)
-        out: dict[str, Any] = {}
-        for k, v in raw.items():
-            if v is None and k in {
-                "stderr_snippet",
-                "process_pid",
-                "sandbox_reachable",
-                "sandbox_probe_rc",
-                "sandbox_probe_stdout",
-                "sandbox_probe_error",
-                "sandbox_probe_error_type",
-                "sandbox_probe_traceback",
-            }:
-                continue
-            if k == "raw_message" and not v:
-                continue
-            out[k] = v
-        return out
+        return {k: v for k, v in super().to_dict().items() if k != "raw_message" or v}
 
 
 @dataclass
diff --git a/src/benchflow/sandbox/daytona_dind.py b/src/benchflow/sandbox/daytona_dind.py
index d8fd7edc..11c44bbd 100644
--- a/src/benchflow/sandbox/daytona_dind.py
+++ b/src/benchflow/sandbox/daytona_dind.py
@@ -96,6 +96,15 @@ def _raise_if_startup_provider_error(env: DaytonaSandbox, exc: BaseException) ->
     ) from exc
 
 
+def _startup_config_error(env: DaytonaSandbox, message: str) -> SandboxStartupError:
+    return SandboxStartupError(
+        message,
+        sandbox_state="config_error",
+        attempts=0,
+        build_timeout_sec=env.task_env_config.build_timeout_sec,
+    )
+
+
 def _with_long_exec_heartbeat(command: str, timeout_sec: int | None) -> str:
     if (
         timeout_sec is None
@@ -346,8 +355,12 @@ async def start(self, force_build: bool) -> None:
 
         env._client_manager = await _sdk.DaytonaClientManager.get_instance()
 
-        dind_image: str = env._kwargs.get("dind_image", "docker:28.3.3-dind")
-        dind_snapshot: str | None = env._kwargs.get("dind_snapshot")
+        dind_image = env._kwargs.get("dind_image", "docker:28.3.3-dind")
+        if not isinstance(dind_image, str):
+            raise _startup_config_error(env, "dind_image must be a string")
+        dind_snapshot = env._kwargs.get("dind_snapshot")
+        if dind_snapshot is not None and not isinstance(dind_snapshot, str):
+            raise _startup_config_error(env, "dind_snapshot must be a string")
 
         params: _SandboxParams
         if dind_snapshot:
diff --git a/tests/test_acp.py b/tests/test_acp.py
index 403246a9..dfdf3e3e 100644
--- a/tests/test_acp.py
+++ b/tests/test_acp.py
@@ -1766,6 +1766,64 @@ def test_diagnostic_reason_constants_share_scoring_source(self) -> None:
             DIAGNOSTIC_REASON_TRANSPORT_CLOSED,
         )
 
+    def test_diagnostic_serialization_drops_none_fields_uniformly(self) -> None:
+        """Guards the fix from PR #895 for issue #531: diagnostic info dicts use one None-policy."""
+        from benchflow.diagnostics import (
+            AgentPromptTimeoutDiagnostic,
+            IdleTimeoutDiagnostic,
+            ProviderApiErrorDiagnostic,
+            SandboxStartupDiagnostic,
+            SuspectedApiErrorDiagnostic,
+            TransportClosedDiagnostic,
+            VerifierTimeoutDiagnostic,
+        )
+
+        expected_keys_by_cls = {
+            IdleTimeoutDiagnostic: {
+                "reason",
+                "idle_timeout_sec",
+                "idle_duration_sec",
+                "wall_clock_elapsed_sec",
+                "n_tool_calls",
+                "n_message_chunks",
+                "n_thought_chunks",
+                "last_activity_at",
+            },
+            AgentPromptTimeoutDiagnostic: {
+                "reason",
+                "timeout_sec",
+                "n_tool_calls",
+                "pending_tool_call_ids",
+                "terminal_event_recorded",
+                "terminal_trajectory_complete",
+            },
+            SandboxStartupDiagnostic: {"reason", "attempts", "raw_message"},
+            TransportClosedDiagnostic: {"reason", "transport_diagnosis"},
+            VerifierTimeoutDiagnostic: {
+                "timeout_budget_sec",
+                "elapsed_sec",
+                "task_name",
+            },
+            ProviderApiErrorDiagnostic: {
+                "subcategory",
+                "transient",
+                "total_requests",
+                "failed_requests",
+                "fingerprint",
+            },
+            SuspectedApiErrorDiagnostic: {
+                "total_tokens",
+                "n_tool_calls",
+                "total_requests",
+                "failed_requests",
+            },
+        }
+
+        for diag_cls, expected_keys in expected_keys_by_cls.items():
+            payload = diag_cls().to_dict()
+            assert set(payload) == expected_keys
+            assert all(value is not None for value in payload.values())
+
     def test_summary_warning_uses_registry_metadata(self) -> None:
         """Summary warning text comes from the registry's
         ``summary_description``, not a per-call f-string in evaluation.py."""
diff --git a/tests/test_daytona_dind_compose_up.py b/tests/test_daytona_dind_compose_up.py
index 96c92e57..0b0e032e 100644
--- a/tests/test_daytona_dind_compose_up.py
+++ b/tests/test_daytona_dind_compose_up.py
@@ -149,6 +149,20 @@ def test_daytona_sdk_error_during_dind_start_is_structured_startup_error():
     assert "Daytona DinD startup failed" in str(raised.value)
 
 
+def test_daytona_dind_type_guard_uses_structured_startup_error():
+    """Guards the follow-up fix from PR #895 for invalid DinD config values."""
+    from benchflow.sandbox.daytona_dind import _startup_config_error
+
+    env = SimpleNamespace(
+        task_env_config=SimpleNamespace(build_timeout_sec=1800),
+    )
+    err = _startup_config_error(env, "dind_image must be a string")
+    assert isinstance(err, SandboxStartupError)
+    assert err.diagnostic.sandbox_state == "config_error"
+    assert err.diagnostic.build_timeout_sec == 1800
+    assert "dind_image must be a string" in err.diagnostic.raw_message
+
+
 def test_long_exec_heartbeat_only_wraps_long_commands():
     command = "python preprocess.py"
     assert _with_long_exec_heartbeat(command, 599) == command