diff --git a/CHANGELOG.md b/CHANGELOG.md
index e8f8d7c6d..33d4a2e64 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,6 +27,15 @@ When releasing, please add the new-release-boilerplate to docs/pallas/CHANGELOG.
       that directly accesses shards accordingly. The rank of the per-shard-shape
       now matches that of the global shape which is the same behavior as jit.
       This avoids costly reshapes when passing results from pmap into jit.
+  * `jax.experimental.host_callback` has been deprecated since March 2024, with
+    JAX version 0.4.26. Now we set the default value of the
+    `--jax_host_callback_legacy` configuration value to `True`, which means that
+    if your code uses `jax.experimental.host_callback` APIs, those API calls
+    will be implemented in terms of the new `jax.experimental.io_callback` API.
+    If this breaks your code, for a very limited time, you can set the
+    `--jax_host_callback_legacy` to `True`. Soon we will remove that
+    configuration option, so you should instead transition to using the
+    new JAX callback APIs. See {jax-issue}`#20385` for a discussion.
 
 * Deprecations
   * In {func}`jax.numpy.trim_zeros`, non-arraylike arguments or arraylike
diff --git a/jax/experimental/host_callback.py b/jax/experimental/host_callback.py
index 49162809a..bc5477ebc 100644
--- a/jax/experimental/host_callback.py
+++ b/jax/experimental/host_callback.py
@@ -568,10 +568,10 @@ _HOST_CALLBACK_OUTFEED = config.bool_flag(
 )
 _HOST_CALLBACK_LEGACY = config.bool_flag(
     'jax_host_callback_legacy',
-    config.bool_env('JAX_HOST_CALLBACK_LEGACY', True),
+    config.bool_env('JAX_HOST_CALLBACK_LEGACY', False),
     help=(
         'Use old implementation of host_callback, documented in the module docstring.'
-        'If False, use the jax.experimental.io_callback implementation. '
+        'If False, use the new jax.experimental.io_callback implementation. '
         'See https://github.com/jax-ml/jax/issues/20385.'
     )
 )
diff --git a/tests/host_callback_test.py b/tests/host_callback_test.py
index 244fe6ef4..5e624ef9a 100644
--- a/tests/host_callback_test.py
+++ b/tests/host_callback_test.py
@@ -349,6 +349,7 @@ class HostCallbackTapTest(jtu.JaxTestCase):
       ( 6.00 9.00 )""")
 
   def test_tap_eval_exception(self):
+    self.supported_only_in_legacy_mode()
     if not hcb._HOST_CALLBACK_OUTFEED.value:
       raise SkipTest("TODO: implement error handling for customcall")
 
@@ -852,6 +853,7 @@ class HostCallbackTapTest(jtu.JaxTestCase):
     self.assertEqual(100, count)
 
   def test_tap_jit_tap_exception(self):
+    self.supported_only_in_legacy_mode()
     if not hcb._HOST_CALLBACK_OUTFEED.value:
       raise SkipTest("TODO: implement error handling for customcall")
     # Simulate a tap error