diff --git a/CHANGELOG.md b/CHANGELOG.md
index ec20c2e..9dc80e3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,8 @@ and versions are tracked in the repo-root `VERSION` file.
 
 ### Fixed
 
+- Reject non-finite numbers in JSON and NDJSON output so emitted records remain
+  standards-compliant and failed NDJSON writes do not leave partial records.
 - Preserve explicit application identities losslessly while using
   collision-resistant, path-safe runtime namespace components.
 - Give `BatteriesIncludedConfigLoader.cli_name` a documented identity role by
diff --git a/docs/json-contracts.md b/docs/json-contracts.md
index 625f98c..a69db7c 100644
--- a/docs/json-contracts.md
+++ b/docs/json-contracts.md
@@ -53,6 +53,11 @@ contains the numeric `exit_code` and captured command stdout. A command's
 human output is represented as a JSON string, so it cannot introduce prose or
 ANSI escapes as a second stdout record.
 
+All JSON and NDJSON emitters use strict JSON serialization and reject
+non-finite numeric values (`NaN`, positive infinity, and negative infinity).
+An invalid NDJSON record is fully serialized before it is written, so it does
+not leave a partial line in the output stream.
+
 `run_id` is the lifecycle run identifier when startup reached a runtime
 context, otherwise it is `null`. Unexpected failures intentionally expose only
 the generic message `Unexpected internal error.`; diagnostics stay in logs.
diff --git a/lib/python/base_cli/inspection.py b/lib/python/base_cli/inspection.py
index 4bbd229..7a1485d 100644
--- a/lib/python/base_cli/inspection.py
+++ b/lib/python/base_cli/inspection.py
@@ -1,9 +1,10 @@
 from __future__ import annotations
 
-import json
 from collections.abc import Mapping
 from typing import Any, Literal
 
+from .json_contracts import dumps_strict_json
+
 InspectionStatus = Literal["ok", "warn", "error"]
 
 
@@ -35,7 +36,7 @@ def render_inspection_json(
 ) -> str:
     """Serialize the stable inspection envelope with Python's JSON encoder."""
     return (
-        json.dumps(
+        dumps_strict_json(
             inspection_envelope(command=command, status=status, data=data, error=error),
             ensure_ascii=False,
             indent=2,
diff --git a/lib/python/base_cli/json_contracts.py b/lib/python/base_cli/json_contracts.py
index 804c59f..1632015 100644
--- a/lib/python/base_cli/json_contracts.py
+++ b/lib/python/base_cli/json_contracts.py
@@ -43,6 +43,7 @@
     "error_envelope",
     "success_envelope",
     "dumps_envelope",
+    "dumps_strict_json",
     "redact_json_value",
 ]
 
@@ -91,7 +92,7 @@ def dumps_envelope(envelope: Mapping[str, Any]) -> str:
     """Serialize an envelope as one compact, newline-terminated JSON record."""
 
     return (
-        json.dumps(
+        dumps_strict_json(
             redact_json_value(dict(envelope)),
             ensure_ascii=False,
             separators=(",", ":"),
@@ -100,6 +101,13 @@ def dumps_envelope(envelope: Mapping[str, Any]) -> str:
     )
 
 
+def dumps_strict_json(value: Any, **kwargs: Any) -> str:
+    """Serialize JSON while rejecting non-finite numeric values."""
+
+    kwargs["allow_nan"] = False
+    return json.dumps(value, **kwargs)
+
+
 def redact_json_value(value: Any, *, _key: str | None = None) -> Any:
     """Recursively redact secret-looking JSON keys and text values."""
 
@@ -140,7 +148,7 @@ def format(self, record: LogRecord) -> str:
             payload["details"] = {
                 "exception_type": record.exc_info[0].__name__,
             }
-        return json.dumps(payload, ensure_ascii=False, separators=(",", ":"))
+        return dumps_strict_json(payload, ensure_ascii=False, separators=(",", ":"))
 
 
 def _timestamp(value: float) -> str:
diff --git a/lib/python/base_cli/output.py b/lib/python/base_cli/output.py
index a8d7fba..6411b25 100644
--- a/lib/python/base_cli/output.py
+++ b/lib/python/base_cli/output.py
@@ -15,6 +15,7 @@
 
 from ._dependencies import require_yaml
 from .integrations import try_render_rich_table
+from .json_contracts import dumps_strict_json
 
 PUBLIC_OUTPUT_FORMATS = ("text", "csv", "tsv", "yaml", "json", "ndjson")
 NDJSON_SCHEMA = "base-cli.record"
@@ -62,7 +63,10 @@ def write(self, record: StructuredRecord) -> None:
             "schema": self.schema,
             "record": dict(record),
         }
-        self.stream.write(json.dumps(payload, separators=(",", ":")))
+        # Serialize the complete record before touching the sink: with strict
+        # JSON, a nested NaN/Infinity must not leave a partial NDJSON line.
+        encoded = json.dumps(payload, separators=(",", ":"), allow_nan=False)
+        self.stream.write(encoded)
         self.stream.write("\n")
         self.stream.flush()
 
@@ -135,9 +139,11 @@ def render_records(
     resolved = resolve_output_format(requested_format, stream=target)
 
     if resolved in ("csv", "tsv"):
+        record_list = [dict(record) for record in records]
+        _validate_delimited_records(record_list, columns)
         delimiter = "," if resolved == "csv" else "\t"
         writer = csv.writer(target, delimiter=delimiter, lineterminator="\n")
-        for record in records:
+        for record in record_list:
             writer.writerow([_delimited_value(record.get(key)) for _header, key in columns])
         return resolved
 
@@ -149,7 +155,7 @@ def render_records(
 
     record_list = [dict(record) for record in records]
     if resolved == "json":
-        target.write(json.dumps(record_list, separators=(",", ":")))
+        target.write(dumps_strict_json(record_list, separators=(",", ":")))
         target.write("\n")
         return resolved
 
@@ -195,7 +201,7 @@ def render_document(
     if resolved == "text":
         return resolved
     if resolved == "json":
-        target.write(json.dumps(dict(document), indent=2))
+        target.write(dumps_strict_json(dict(document), indent=2))
         target.write("\n")
         return resolved
     if resolved == "yaml":
@@ -249,10 +255,23 @@ def _cell_value(value: Any) -> str:
     if isinstance(value, bool):
         return "true" if value else "false"
     if isinstance(value, (Mapping, list, tuple)):
-        return json.dumps(value, separators=(",", ":"))
+        return dumps_strict_json(value, separators=(",", ":"))
     return str(value)
 
 
+def _validate_delimited_records(
+    records: Sequence[Mapping[str, Any]],
+    columns: Sequence[tuple[str, str]],
+) -> None:
+    """Validate nested cell values before a delimited stream is touched."""
+
+    for record in records:
+        for _header, key in columns:
+            value = record.get(key)
+            if isinstance(value, (Mapping, list, tuple)):
+                dumps_strict_json(value, separators=(",", ":"))
+
+
 def _delimited_value(value: Any) -> str:
     """Return a safe scalar for redirected CSV/TSV output.
 
diff --git a/tests/test_json_contracts.py b/tests/test_json_contracts.py
index 388475a..8e2e07d 100644
--- a/tests/test_json_contracts.py
+++ b/tests/test_json_contracts.py
@@ -52,6 +52,12 @@ def test_envelopes_have_stable_fields_and_recursive_redaction(self) -> None:
         self.assertEqual(failure["message"], "authorization=[REDACTED]")
         self.assertEqual(json.loads(base_cli.dumps_envelope(failure)), failure)
 
+    def test_json_contract_emitters_reject_nested_non_finite_values(self) -> None:
+        invalid = {"nested": [{"value": float("inf")}]}
+        envelope = base_cli.success_envelope(run_id=None, details=invalid)
+        with self.assertRaises(ValueError):
+            base_cli.dumps_envelope(envelope)
+
     def test_inline_secret_redaction_keeps_delimiters_inside_values(self) -> None:
         for value in ("abc,def", "abc;def"):
             with self.subTest(value=value):
diff --git a/tests/test_output.py b/tests/test_output.py
index a4630a6..15bf6a3 100644
--- a/tests/test_output.py
+++ b/tests/test_output.py
@@ -65,6 +65,32 @@ def isatty(self) -> bool:
 
 
 class OutputTest(unittest.TestCase):
+    def test_json_emitters_reject_non_finite_values_without_partial_output(self) -> None:
+        invalid_record = {"nested": [{"value": float("nan")}]}
+        emitters = (
+            lambda stream: render_records((invalid_record,), requested_format="json", columns=(), stream=stream),
+            lambda stream: render_document(invalid_record, requested_format="json", stream=stream),
+            lambda stream: NdjsonWriter(stream).write(invalid_record),
+        )
+
+        for emit in emitters:
+            with self.subTest(emit=emit):
+                stream = io.StringIO()
+                with self.assertRaises(ValueError):
+                    emit(stream)
+                self.assertEqual(stream.getvalue(), "")
+
+    def test_delimited_emitters_validate_nested_values_before_writing(self) -> None:
+        records = ({"name": "valid"}, {"name": {"value": float("nan")}})
+        for requested_format in ("csv", "tsv"):
+            with self.subTest(format=requested_format):
+                stream = io.StringIO()
+                with self.assertRaises(ValueError):
+                    render_records(
+                        records, requested_format=requested_format, columns=(("NAME", "name"),), stream=stream
+                    )
+                self.assertEqual(stream.getvalue(), "")
+
     def test_tsv_consumes_one_pass_iterable_without_materializing(self) -> None:
         consumed = False