diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3bdbb9a..0eda6cd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,12 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.0.4] - 2026-06-13
+
+### Fixed
+
+- `*_and_log/*` and `log/*` now declare `unique_constraint/3` on the audit_log pkey (using configured table name) so duplicate PK inserts add a constraint error to the changeset instead of raising `Ecto.ConstraintError`. Best-effort audit logging behavior is unchanged. Closes OPS-4578.
+
 ## [1.0.3] - 2026-05-28
 
 ### Fixed
diff --git a/lib/ecto_trail/ecto_trail.ex b/lib/ecto_trail/ecto_trail.ex
index 74d7075..7e95380 100644
--- a/lib/ecto_trail/ecto_trail.ex
+++ b/lib/ecto_trail/ecto_trail.ex
@@ -55,6 +55,8 @@ defmodule EctoTrail do
   @default_max_params 65_000
   @changelog_fields [:actor_id, :resource, :resource_id, :changeset, :change_type]
   @not_loaded_pattern "Ecto.Association.NotLoaded"
+  @table_name Application.compile_env(:ecto_trail, :table_name, "audit_log")
+  @pkey_constraint_name "#{@table_name}_pkey"
 
   defmacro __using__(_) do
     quote do
@@ -572,6 +574,8 @@ defmodule EctoTrail do
   defp map_custom_ecto_type(value), do: value
 
   defp changelog_changeset(attrs) do
-    Changeset.cast(%Changelog{}, attrs, @changelog_fields)
+    %Changelog{}
+    |> Changeset.cast(attrs, @changelog_fields)
+    |> Changeset.unique_constraint(:id, name: @pkey_constraint_name)
   end
 end
diff --git a/mix.exs b/mix.exs
index c895a54..e08c811 100644
--- a/mix.exs
+++ b/mix.exs
@@ -1,7 +1,7 @@
 defmodule EctoTrail.Mixfile do
   use Mix.Project
 
-  @version "1.0.3"
+  @version "1.0.4"
 
   def project do
     [
diff --git a/test/unit/ecto_trail_test.exs b/test/unit/ecto_trail_test.exs
index 1a44768..3e8c4fe 100644
--- a/test/unit/ecto_trail_test.exs
+++ b/test/unit/ecto_trail_test.exs
@@ -198,6 +198,50 @@ defmodule EctoTrailTest do
       assert [%{name: "name"}] = TestRepo.all(Resource)
       assert [] == TestRepo.all(Changelog)
     end
+
+    test "does not raise constraint error and succeeds when audit_log pkey collides (best-effort audit)", %{
+      schema: schema
+    } do
+      # Force a collision on the next audit_log PK insert by pre-inserting a row at a high id
+      # and resetting the sequence so the generated insert attempts to reuse it.
+      table = Application.get_env(:ecto_trail, :table_name, "audit_log")
+      seq_name = "#{table}_id_seq"
+
+      alias EctoTrail.Changelog
+
+      high_id = 2_000_000
+      now = DateTime.utc_now() |> DateTime.truncate(:second)
+
+      sentinel = %Changelog{
+        id: high_id,
+        actor_id: "seed",
+        resource: "resources",
+        resource_id: "0",
+        changeset: %{},
+        change_type: :insert,
+        inserted_at: now
+      }
+
+      TestRepo.insert!(sentinel)
+
+      # Set sequence so the *next* value produced is `high_id`; the audit insert in update_and_log will collide on pkey.
+      Ecto.Adapters.SQL.query!(TestRepo, "SELECT setval($1, $2, true)", [seq_name, high_id - 1])
+
+      result =
+        schema
+        |> Changeset.change(%{name: "collision-test"})
+        |> TestRepo.update_and_log("constraint-actor")
+
+      # The update must succeed; the audit log attempt will hit unique violation on pkey,
+      # be turned into a changeset error (thanks to unique_constraint/3), logged, and swallowed per best-effort contract.
+      assert {:ok, %Resource{name: "collision-test"}} = result
+
+      # Sentinel row is still the only one for that seed; colliding insert did not create a second row.
+      assert TestRepo.aggregate(
+               from(c in Changelog, where: c.resource_id == "0" and c.actor_id == "seed"),
+               :count
+             ) == 1
+    end
   end
 
   describe "upsert_and_log/3" do