fix(insight): fail open on NaN sampling_rate + on-change note

packages/aws-durable-execution-sdk-python-insight/README.md

@@ -55,6 +55,11 @@ and `top-level` vs `full-tree` operation detail all mirror the JS plugin.
 Behavior is validated cross-SDK by the `insight` conformance suite
 (`aws-durable-execution-conformance-tests-insight`).
 
+> **Note (`on-change` emission).** In `on-change` mode, exporter calls currently
+> run synchronously on the SDK checkpoint path, so a slow exporter can delay
+> workflow progress. Asynchronous scheduling/coalescing is deferred and tracked
+> in [issue #687](https://github.com/aws/aws-durable-execution-sdk-python/issues/687).
+
 ## Requirements
 
 - `aws-durable-execution-sdk-python` with the plugin invocation hooks that

packages/aws-durable-execution-sdk-python-insight/src/aws_durable_execution_sdk_python_insight/plugin.py

@@ -32,6 +32,7 @@
 
 import datetime
 import json
+import math
 import sys
 import threading
 from typing import Any, Callable
@@ -107,6 +108,13 @@ def _resolve_sampling_rate(rate: float | None) -> float:
         return 1.0
     if not isinstance(rate, (int, float)):
         return 1.0
+    if isinstance(rate, float) and math.isnan(rate):
+        # Fail open. NaN compares False to everything, so without this guard it
+        # would flow through _should_sample (rate >= 1 -> False, rate <= 0 ->
+        # False, x < NaN -> False) and silently sample OUT every execution,
+        # disabling all instrumentation. Coerce to full sampling instead, which
+        # matches the JS plugin's treatment of non-finite/invalid rates.
+        return 1.0
     if rate < 0 or rate > 1:
         return max(0.0, min(1.0, float(rate)))
     return float(rate)

packages/aws-durable-execution-sdk-python-insight/tests/test_plugin.py

@@ -38,6 +38,7 @@
     WorkflowInsightConfig,
     workflow_insight,
 )
+from aws_durable_execution_sdk_python_insight.plugin import _resolve_sampling_rate
 
 ARN = "arn:aws:lambda:us-west-2:123456789012:function:my-fn:$LATEST/durable-execution/exec-1/inv-1"
 ARN_B = "arn:aws:lambda:us-west-2:123456789012:function:my-fn:$LATEST/durable-execution/exec-2/inv-1"
@@ -196,6 +197,22 @@ def test_sampling_zero_emits_nothing():
     assert exporter.records == []
 
 
+def test_resolve_sampling_rate_nan_fails_open_to_one():
+    # NaN compares False to everything; without the guard this would sample OUT
+    # every execution. It must fail open to full sampling (JS parity).
+    assert _resolve_sampling_rate(float("nan")) == 1.0
+
+
+def test_nan_sampling_rate_emits_instead_of_silently_disabling():
+    exporter = CaptureExporter()
+    plugin = workflow_insight(
+        WorkflowInsightConfig(exporters=[exporter], sampling_rate=float("nan"))
+    )
+    _run(plugin, ops=[_step("greet")])
+    # A NaN rate must not disable instrumentation: the record is still emitted.
+    assert len(exporter.records) == 1
+
+
 def test_content_omit_input_output_without_drop_flags():
     exporter = CaptureExporter()
     plugin = workflow_insight(