diff --git a/tests/test_output_format.py b/tests/test_output_format.py
new file mode 100644
index 0000000..56cab11
--- /dev/null
+++ b/tests/test_output_format.py
@@ -0,0 +1,52 @@
+"""Tests for the JSON output-formatting helpers."""
+
+import math
+
+import numpy as np
+
+from mcltspice.output_format import compact_list, downsample_indices, round_sig
+
+
+class TestRoundSig:
+    def test_significant_figures(self):
+        assert round_sig(123456.789, 3) == 123000.0
+        assert round_sig(0.00123456, 3) == 0.00123
+
+    def test_zero_and_nonfinite_pass_through(self):
+        assert round_sig(0.0) == 0.0
+        assert math.isinf(round_sig(float("inf")))
+        assert math.isnan(round_sig(float("nan")))
+
+
+class TestCompactList:
+    def test_decimal_places(self):
+        assert compact_list([1.23456, 2.98765], decimal_places=2) == [1.23, 2.99]
+
+    def test_sig_figs(self):
+        assert compact_list([123456.0], sig_figs=3) == [123000.0]
+
+    def test_no_rounding_returns_floats(self):
+        out = compact_list([1, 2, 3])
+        assert out == [1.0, 2.0, 3.0]
+        assert all(isinstance(v, float) for v in out)
+
+
+class TestDownsampleIndices:
+    def test_no_downsample_when_under_limit(self):
+        idx = downsample_indices(50, 100)
+        assert np.array_equal(idx, np.arange(50))
+
+    def test_stride_for_transient(self):
+        idx = downsample_indices(1000, 100)
+        assert len(idx) <= 100
+        # plain stride: evenly spaced
+        assert idx[0] == 0
+        assert idx[1] - idx[0] == idx[2] - idx[1]
+
+    def test_peak_preserving_keeps_a_narrow_spike(self):
+        # A flat magnitude with one sharp spike: peak-preserving must keep it.
+        n = 1000
+        mag = np.ones(n, dtype=complex)
+        mag[497] = 1000.0  # narrow resonance peak
+        idx = downsample_indices(n, 50, signals=[mag])
+        assert 497 in idx  # spike survived downsampling
diff --git a/tests/test_waveform_query.py b/tests/test_waveform_query.py
new file mode 100644
index 0000000..7ecc0e7
--- /dev/null
+++ b/tests/test_waveform_query.py
@@ -0,0 +1,77 @@
+"""Tests for extract_waveform / analyze_signal -- RawFile queries sans simulator."""
+
+import numpy as np
+
+from mcltspice.raw_parser import RawFile, Variable
+from mcltspice.waveform_query import analyze_signal, extract_waveform
+
+
+def _ac_raw() -> RawFile:
+    freq = np.logspace(0, 6, 300)
+    hout = 1.0 / (1.0 + 1j * (freq / 1000.0))
+    data = np.array([freq.astype(complex), hout])
+    return RawFile(
+        title="", date="", plotname="AC Analysis", flags=["complex"],
+        variables=[Variable(0, "frequency", "frequency"), Variable(1, "V(out)", "voltage")],
+        points=len(freq), data=data,
+    )
+
+
+def _tran_raw() -> RawFile:
+    t = np.linspace(0, 0.01, 800)
+    v = 2.0 * np.sin(2 * np.pi * 1000 * t)
+    return RawFile(
+        title="", date="", plotname="Transient Analysis", flags=["real"],
+        variables=[Variable(0, "time", "time"), Variable(1, "V(out)", "voltage")],
+        points=len(t), data=np.array([t, v]),
+    )
+
+
+class TestExtractWaveform:
+    def test_transient_values(self):
+        r = extract_waveform(_tran_raw(), ["V(out)"], max_points=100)
+        assert r["x_axis_name"] == "time"
+        assert r["returned_points"] <= 100
+        assert "values" in r["signals"]["V(out)"]
+
+    def test_ac_gives_magnitude_and_phase(self):
+        r = extract_waveform(_ac_raw(), ["V(out)"])
+        assert r["x_axis_name"] == "frequency"
+        sig = r["signals"]["V(out)"]
+        assert "magnitude_db" in sig and "phase_degrees" in sig
+
+    def test_x_range_filter(self):
+        full = extract_waveform(_tran_raw(), ["V(out)"], max_points=10000)["returned_points"]
+        clipped = extract_waveform(
+            _tran_raw(), ["V(out)"], max_points=10000, x_min=0.002, x_max=0.004
+        )["returned_points"]
+        assert clipped < full
+
+    def test_empty_range_error(self):
+        r = extract_waveform(_tran_raw(), ["V(out)"], x_min=1e12)
+        assert "error" in r and "range" in r["error"]
+
+    def test_run_on_non_stepped_error(self):
+        r = extract_waveform(_tran_raw(), ["V(out)"], run=2)
+        assert "error" in r and "stepped" in r["error"]
+
+
+class TestAnalyzeSignal:
+    def test_rms_and_peak_to_peak(self):
+        r = analyze_signal(_tran_raw(), "V(out)", ["rms", "peak_to_peak"])
+        assert r["signal"] == "V(out)"
+        assert abs(r["rms"] - 2.0 / np.sqrt(2)) < 0.05   # RMS of a 2 V amplitude sine
+        assert abs(r["peak_to_peak"]["peak_to_peak"] - 4.0) < 0.05
+
+    def test_fft_and_thd_present(self):
+        r = analyze_signal(_tran_raw(), "V(out)", ["fft", "thd"])
+        assert "fft" in r and "thd" in r
+
+    def test_missing_signal_error(self):
+        r = analyze_signal(_tran_raw(), "V(ghost)", ["rms"])
+        assert "error" in r and "V(ghost)" in r["error"]
+
+    def test_complex_signal_uses_magnitude(self):
+        # AC (complex) signal: analysis should run on |signal| without raising
+        r = analyze_signal(_ac_raw(), "V(out)", ["rms"])
+        assert "rms" in r and r["rms"] >= 0