diff --git a/sonic.c b/sonic.c
index 8e2f696..0223189 100644
--- a/sonic.c
+++ b/sonic.c
@@ -461,12 +461,23 @@ void sonicSetNumChannels(sonicStream stream, int numChannels) {
   allocateStreamBuffers(stream, stream->sampleRate, numChannels);
 }
 
-/* Enlarge the output buffer if needed. */
-static int enlargeOutputBufferIfNeeded(sonicStream stream, int numSamples) {
+/* Enlarge the output buffer if needed. Not static: exercised directly by
+   the white-box overflow tests in tests/overflow_test.c. */
+int enlargeOutputBufferIfNeeded(sonicStream stream, int numSamples) {
   int outputBufferSize = stream->outputBufferSize;
+  int growth;
 
-  if (stream->numOutputSamples + numSamples > outputBufferSize) {
-    stream->outputBufferSize += (outputBufferSize >> 1) + numSamples;
+  if (numSamples < 0) {
+    return 0;
+  }
+  /* Written as a subtraction, not "numOutputSamples + numSamples", so the
+     comparison itself can't signed-overflow when numSamples is huge. */
+  if (stream->numOutputSamples > outputBufferSize - numSamples) {
+    growth = outputBufferSize >> 1;
+    if (numSamples > INT_MAX - outputBufferSize - growth) {
+      return 0;
+    }
+    stream->outputBufferSize = outputBufferSize + growth + numSamples;
     stream->outputBuffer = (short*)sonicRealloc(
         stream->outputBuffer, outputBufferSize, stream->outputBufferSize,
         sizeof(short) * stream->numChannels);
@@ -1040,10 +1051,13 @@ static int skipPitchPeriod(sonicStream stream, short* samples, float speed,
   return newSamples;
 }
 
-/* Insert a pitch period, and determine how much input to copy directly. */
-static int insertPitchPeriod(sonicStream stream, short* samples, float speed,
-                             int period) {
+/* Insert a pitch period, and determine how much input to copy directly. Not
+   static: exercised directly by the white-box overflow tests in
+   tests/overflow_test.c. */
+int insertPitchPeriod(sonicStream stream, short* samples, float speed,
+                      int period) {
   long newSamples;
+  long totalSamples;
   short* out;
   int numChannels = stream->numChannels;
 
@@ -1052,7 +1066,14 @@ static int insertPitchPeriod(sonicStream stream, short* samples, float speed,
   } else {
     newSamples = period;
   }
-  if (!enlargeOutputBufferIfNeeded(stream, period + newSamples)) {
+  /* period + newSamples is computed in long above, but
+     enlargeOutputBufferIfNeeded takes an int; reject here instead of
+     silently truncating a huge period into a small/negative int. */
+  totalSamples = (long)period + newSamples;
+  if (totalSamples < 0 || totalSamples > INT_MAX) {
+    return 0;
+  }
+  if (!enlargeOutputBufferIfNeeded(stream, (int)totalSamples)) {
     return 0;
   }
   out = stream->outputBuffer + stream->numOutputSamples * numChannels;
diff --git a/tests/Makefile b/tests/Makefile
index 9597190..028128f 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -16,7 +16,8 @@ CFLAGS=-Wall -Wno-unused-function -g -ansi -fPIC -pthread -I ..
 #CFLAGS += -Wall -Wno-unused-function -ansi -fPIC -pthread -I ..
 
 TEST_SRC = \
-input_clamping_test.c
+input_clamping_test.c \
+overflow_test.c
 
 CC=gcc
 
diff --git a/tests/overflow_test.c b/tests/overflow_test.c
new file mode 100644
index 0000000..f585cb8
--- /dev/null
+++ b/tests/overflow_test.c
@@ -0,0 +1,88 @@
+/* Sonic library
+   Copyright 2026
+   Bill Cox
+   This file is part of the Sonic Library.
+
+   This file is licensed under the Apache 2.0 license.
+*/
+
+#include "sonic.h"
+
+#include <limits.h>
+#include <stdio.h>
+
+#define SAMPLE_RATE 44100
+#define NUM_CHANNELS 1
+
+/* enlargeOutputBufferIfNeeded and insertPitchPeriod are not declared static
+   in sonic.c specifically so these white-box tests can call them directly,
+   without going through the public streaming API to reach the overflow
+   paths. They are not part of the public sonic.h contract. */
+int enlargeOutputBufferIfNeeded(sonicStream stream, int numSamples);
+int insertPitchPeriod(sonicStream stream, short* samples, float speed,
+                      int period);
+
+int sonicTestEnlargeOutputBufferRejectsOverflow(void) {
+    sonicStream stream;
+    int result;
+
+    stream = sonicCreateStream(SAMPLE_RATE, NUM_CHANNELS);
+    if (stream == NULL) {
+        fprintf(stderr, "sonicCreateStream failed\n");
+        return 0;
+    }
+    /* A request this large can't be satisfied; the guard must fail cleanly
+       instead of signed-overflowing the internal buffer size arithmetic. */
+    result = enlargeOutputBufferIfNeeded(stream, INT_MAX);
+    sonicDestroyStream(stream);
+    if (result != 0) {
+        fprintf(stderr,
+                "enlargeOutputBufferIfNeeded should reject an overflowing request\n");
+        return 0;
+    }
+    return 1;
+}
+
+int sonicTestEnlargeOutputBufferAcceptsNormalRequest(void) {
+    sonicStream stream;
+    int result;
+
+    stream = sonicCreateStream(SAMPLE_RATE, NUM_CHANNELS);
+    if (stream == NULL) {
+        fprintf(stderr, "sonicCreateStream failed\n");
+        return 0;
+    }
+    result = enlargeOutputBufferIfNeeded(stream, 128);
+    sonicDestroyStream(stream);
+    if (result != 1) {
+        fprintf(stderr,
+                "enlargeOutputBufferIfNeeded should accept a normal request\n");
+        return 0;
+    }
+    return 1;
+}
+
+int sonicTestInsertPitchPeriodRejectsOverflow(void) {
+    sonicStream stream;
+    short dummy[1];
+    int result;
+
+    stream = sonicCreateStream(SAMPLE_RATE, NUM_CHANNELS);
+    if (stream == NULL) {
+        fprintf(stderr, "sonicCreateStream failed\n");
+        return 0;
+    }
+    dummy[0] = 0;
+    /* period this large makes period + newSamples overflow INT_MAX before
+       the fix, which used to truncate silently at the
+       enlargeOutputBufferIfNeeded call boundary. The function must return
+       failure before touching samples, so a 1-element buffer is safe here. */
+    result = insertPitchPeriod(stream, dummy, 1.0f, INT_MAX);
+    sonicDestroyStream(stream);
+    if (result != 0) {
+        fprintf(stderr,
+                "insertPitchPeriod should reject an overflowing period\n");
+        return 0;
+    }
+    return 1;
+}
diff --git a/tests/runtests.c b/tests/runtests.c
index a7ef2e9..c87223d 100644
--- a/tests/runtests.c
+++ b/tests/runtests.c
@@ -16,6 +16,9 @@
 int main(int argc, char** argv) {
   assert(sonicTestInputClamping());
   assert(sonicTestInputsDontCrash());
+  assert(sonicTestEnlargeOutputBufferRejectsOverflow());
+  assert(sonicTestEnlargeOutputBufferAcceptsNormalRequest());
+  assert(sonicTestInsertPitchPeriodRejectsOverflow());
   printf("All tests passed.\n");
   return 0;
 }
diff --git a/tests/tests.h b/tests/tests.h
index 9f08d06..f34da4a 100644
--- a/tests/tests.h
+++ b/tests/tests.h
@@ -12,6 +12,9 @@ extern "C" {
 
 int sonicTestInputClamping(void);
 int sonicTestInputsDontCrash(void);
+int sonicTestEnlargeOutputBufferRejectsOverflow(void);
+int sonicTestEnlargeOutputBufferAcceptsNormalRequest(void);
+int sonicTestInsertPitchPeriodRejectsOverflow(void);
 
 #ifdef __cplusplus
 }