diff --git a/CHANGELOG.md b/CHANGELOG.md
index fcd6c8109a..7bec27a1a1 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -48,6 +48,9 @@ and this project adheres to
   [PR#4551](https://github.com/OpenFn/lightning/pull/4551)
 - Fix AI assistant authorization for support users on projects with support
   access enabled [#4571](https://github.com/OpenFn/lightning/issues/4571)
+- REST API controllers now return 400 instead of 500 for malformed UUID
+  parameters. Extracts a shared `validate_uuid/1` helper used across all API
+  controllers. [#4588](https://github.com/OpenFn/lightning/issues/4588)
 
 ## [2.16.0] - 2026-03-24
 
diff --git a/lib/lightning_web/controllers/api/ai_assistant_controller.ex b/lib/lightning_web/controllers/api/ai_assistant_controller.ex
index b6bd6ad130..57b0ea0c1f 100644
--- a/lib/lightning_web/controllers/api/ai_assistant_controller.ex
+++ b/lib/lightning_web/controllers/api/ai_assistant_controller.ex
@@ -9,6 +9,7 @@ defmodule LightningWeb.API.AiAssistantController do
   alias Lightning.Policies.Permissions
   alias Lightning.Projects
   alias Lightning.Workflows
+  alias LightningWeb.API.Helpers
   alias LightningWeb.Channels.AiAssistantJSON
 
   action_fallback LightningWeb.FallbackController
@@ -75,7 +76,9 @@ defmodule LightningWeb.API.AiAssistantController do
   end
 
   defp get_resource("job_code", %{"job_id" => job_id}) do
-    {:ok, job_id}
+    with :ok <- Helpers.validate_uuid(job_id) do
+      {:ok, job_id}
+    end
   end
 
   defp get_resource("job_code", _params) do
@@ -83,9 +86,11 @@ defmodule LightningWeb.API.AiAssistantController do
   end
 
   defp get_resource("workflow_template", %{"project_id" => project_id}) do
-    case Projects.get_project(project_id) do
-      nil -> {:error, :not_found}
-      project -> {:ok, project}
+    with :ok <- Helpers.validate_uuid(project_id) do
+      case Projects.get_project(project_id) do
+        nil -> {:error, :not_found}
+        project -> {:ok, project}
+      end
     end
   end
 
diff --git a/lib/lightning_web/controllers/api/credential_controller.ex b/lib/lightning_web/controllers/api/credential_controller.ex
index c8a12b3baf..9204dd0ee2 100644
--- a/lib/lightning_web/controllers/api/credential_controller.ex
+++ b/lib/lightning_web/controllers/api/credential_controller.ex
@@ -25,6 +25,7 @@ defmodule LightningWeb.API.CredentialController do
   alias Lightning.Policies.Permissions
   alias Lightning.Policies.ProjectUsers
   alias Lightning.Projects
+  alias LightningWeb.API.Helpers
 
   action_fallback LightningWeb.FallbackController
 
@@ -61,7 +62,8 @@ defmodule LightningWeb.API.CredentialController do
   def index(conn, %{"project_id" => project_id}) do
     current_user = conn.assigns.current_resource
 
-    with project when not is_nil(project) <- Projects.get_project(project_id),
+    with :ok <- Helpers.validate_uuid(project_id),
+         project when not is_nil(project) <- Projects.get_project(project_id),
          :ok <-
            ProjectUsers
            |> Permissions.can(
@@ -166,15 +168,15 @@ defmodule LightningWeb.API.CredentialController do
   def delete(conn, %{"id" => id}) do
     current_user = conn.assigns.current_resource
 
-    with :ok <- validate_uuid(id),
+    with :ok <- Helpers.validate_uuid(id),
          credential when not is_nil(credential) <-
            Credentials.get_credential(id),
          :ok <- validate_credential_ownership(credential, current_user),
          {:ok, _} <- Credentials.delete_credential(credential) do
       send_resp(conn, :no_content, "")
     else
-      {:error, :invalid_uuid} ->
-        {:error, :not_found}
+      {:error, :bad_request} ->
+        {:error, :bad_request}
 
       nil ->
         {:error, :not_found}
@@ -187,13 +189,6 @@ defmodule LightningWeb.API.CredentialController do
     end
   end
 
-  defp validate_uuid(id) do
-    case Ecto.UUID.dump(to_string(id)) do
-      {:ok, _bin} -> :ok
-      :error -> {:error, :invalid_uuid}
-    end
-  end
-
   defp validate_credential_ownership(credential, current_user) do
     if credential.user_id == current_user.id do
       :ok
diff --git a/lib/lightning_web/controllers/api/helpers.ex b/lib/lightning_web/controllers/api/helpers.ex
index 0c21dae60e..629007576b 100644
--- a/lib/lightning_web/controllers/api/helpers.ex
+++ b/lib/lightning_web/controllers/api/helpers.ex
@@ -51,4 +51,20 @@ defmodule LightningWeb.API.Helpers do
     }
     |> URI.to_string()
   end
+
+  @doc """
+  Validates that the given value is a well-formed UUID.
+
+  Returns `:ok` on success or `{:error, :bad_request}` when the value
+  cannot be parsed as a UUID. Use this in API controllers before passing
+  an ID to the database layer, which would raise `Ecto.Query.CastError`
+  for invalid values.
+  """
+  @spec validate_uuid(any()) :: :ok | {:error, :bad_request}
+  def validate_uuid(id) do
+    case Ecto.UUID.dump(to_string(id)) do
+      {:ok, _bin} -> :ok
+      :error -> {:error, :bad_request}
+    end
+  end
 end
diff --git a/lib/lightning_web/controllers/api/job_controller.ex b/lib/lightning_web/controllers/api/job_controller.ex
index a417753ecc..c749ae518a 100644
--- a/lib/lightning_web/controllers/api/job_controller.ex
+++ b/lib/lightning_web/controllers/api/job_controller.ex
@@ -24,6 +24,7 @@ defmodule LightningWeb.API.JobController do
   alias Lightning.Policies.Permissions
   alias Lightning.Policies.ProjectUsers
   alias Lightning.Workflows
+  alias LightningWeb.API.Helpers
 
   action_fallback LightningWeb.FallbackController
 
@@ -64,7 +65,8 @@ defmodule LightningWeb.API.JobController do
   def index(conn, %{"project_id" => project_id} = params) do
     pagination_attrs = Map.take(params, ["page_size", "page"])
 
-    with project <- Lightning.Projects.get_project(project_id),
+    with :ok <- Helpers.validate_uuid(project_id),
+         project <- Lightning.Projects.get_project(project_id),
          :ok <-
            ProjectUsers
            |> Permissions.can(
@@ -115,7 +117,8 @@ defmodule LightningWeb.API.JobController do
   """
   @spec show(Plug.Conn.t(), map()) :: Plug.Conn.t()
   def show(conn, %{"id" => id}) do
-    with job <- Jobs.get_job!(id),
+    with :ok <- Helpers.validate_uuid(id),
+         job <- Jobs.get_job!(id),
          job_with_project <- Lightning.Repo.preload(job, workflow: :project),
          :ok <-
            ProjectUsers
diff --git a/lib/lightning_web/controllers/api/project_controller.ex b/lib/lightning_web/controllers/api/project_controller.ex
index 888d20f7b4..c5813f2cda 100644
--- a/lib/lightning_web/controllers/api/project_controller.ex
+++ b/lib/lightning_web/controllers/api/project_controller.ex
@@ -20,6 +20,7 @@ defmodule LightningWeb.API.ProjectController do
   alias Lightning.Policies.Permissions
   alias Lightning.Policies.ProjectUsers
   alias Lightning.Projects
+  alias LightningWeb.API.Helpers
 
   action_fallback LightningWeb.FallbackController
 
@@ -78,7 +79,8 @@ defmodule LightningWeb.API.ProjectController do
   """
   @spec show(Plug.Conn.t(), map()) :: Plug.Conn.t()
   def show(conn, %{"id" => id}) do
-    with project <- Projects.get_project(id),
+    with :ok <- Helpers.validate_uuid(id),
+         project <- Projects.get_project(id),
          :ok <-
            ProjectUsers
            |> Permissions.can(
diff --git a/lib/lightning_web/controllers/api/provisioning_controller.ex b/lib/lightning_web/controllers/api/provisioning_controller.ex
index d993d4b3ac..199f638ffc 100644
--- a/lib/lightning_web/controllers/api/provisioning_controller.ex
+++ b/lib/lightning_web/controllers/api/provisioning_controller.ex
@@ -20,6 +20,7 @@ defmodule LightningWeb.API.ProvisioningController do
   alias Lightning.Projects.Provisioner
   alias Lightning.Workflows
   alias Lightning.WorkflowVersions
+  alias LightningWeb.API.Helpers
 
   action_fallback(LightningWeb.FallbackController)
 
@@ -129,7 +130,8 @@ defmodule LightningWeb.API.ProvisioningController do
   """
   @spec show(Plug.Conn.t(), map()) :: Plug.Conn.t()
   def show(conn, params) do
-    with project = %Project{} <-
+    with :ok <- Helpers.validate_uuid(params["id"]),
+         project = %Project{} <-
            Projects.get_project(params["id"]) || {:error, :not_found},
          :ok <-
            Permissions.can(
@@ -186,7 +188,8 @@ defmodule LightningWeb.API.ProvisioningController do
   """
   @spec show_yaml(Plug.Conn.t(), map()) :: Plug.Conn.t()
   def show_yaml(conn, %{"id" => id} = params) do
-    with %Projects.Project{} = project <-
+    with :ok <- Helpers.validate_uuid(id),
+         %Projects.Project{} = project <-
            Projects.get_project(id) || {:error, :not_found},
          :ok <-
            Permissions.can(
diff --git a/lib/lightning_web/controllers/api/run_controller.ex b/lib/lightning_web/controllers/api/run_controller.ex
index b04ef94a60..6bce70ea39 100644
--- a/lib/lightning_web/controllers/api/run_controller.ex
+++ b/lib/lightning_web/controllers/api/run_controller.ex
@@ -25,6 +25,7 @@ defmodule LightningWeb.API.RunController do
   alias Lightning.Policies.Permissions
   alias Lightning.Policies.ProjectUsers
   alias Lightning.Runs
+  alias LightningWeb.API.Helpers
 
   action_fallback LightningWeb.FallbackController
 
@@ -72,7 +73,8 @@ defmodule LightningWeb.API.RunController do
   def index(conn, %{"project_id" => project_id} = params) do
     pagination_attrs = Map.take(params, ["page_size", "page"])
 
-    with :ok <-
+    with :ok <- Helpers.validate_uuid(project_id),
+         :ok <-
            Invocation.Query.validate_datetime_params(params, [
              "inserted_after",
              "inserted_before",
@@ -140,7 +142,8 @@ defmodule LightningWeb.API.RunController do
   """
   @spec show(Plug.Conn.t(), map()) :: Plug.Conn.t()
   def show(conn, %{"id" => id}) do
-    with run <- Runs.get(id, include: [work_order: [workflow: :project]]),
+    with :ok <- Helpers.validate_uuid(id),
+         run <- Runs.get(id, include: [work_order: [workflow: :project]]),
          :ok <-
            ProjectUsers
            |> Permissions.can(
diff --git a/lib/lightning_web/controllers/api/work_orders_controller.ex b/lib/lightning_web/controllers/api/work_orders_controller.ex
index 2d3ea6fb89..124c671fe2 100644
--- a/lib/lightning_web/controllers/api/work_orders_controller.ex
+++ b/lib/lightning_web/controllers/api/work_orders_controller.ex
@@ -25,6 +25,7 @@ defmodule LightningWeb.API.WorkOrdersController do
   alias Lightning.Policies.Permissions
   alias Lightning.Policies.ProjectUsers
   alias Lightning.WorkOrders
+  alias LightningWeb.API.Helpers
 
   action_fallback LightningWeb.FallbackController
 
@@ -72,7 +73,8 @@ defmodule LightningWeb.API.WorkOrdersController do
   def index(conn, %{"project_id" => project_id} = params) do
     pagination_attrs = Map.take(params, ["page_size", "page"])
 
-    with :ok <-
+    with :ok <- Helpers.validate_uuid(project_id),
+         :ok <-
            Invocation.Query.validate_datetime_params(params, [
              "inserted_after",
              "inserted_before",
@@ -140,7 +142,8 @@ defmodule LightningWeb.API.WorkOrdersController do
   """
   @spec show(Plug.Conn.t(), map()) :: Plug.Conn.t()
   def show(conn, %{"id" => id}) do
-    with work_order <-
+    with :ok <- Helpers.validate_uuid(id),
+         work_order <-
            WorkOrders.get(id, include: [workflow: :project, runs: []]),
          :ok <-
            ProjectUsers
diff --git a/lib/lightning_web/controllers/api/workflows_controller.ex b/lib/lightning_web/controllers/api/workflows_controller.ex
index 4ff62b7b4b..0b8386d530 100644
--- a/lib/lightning_web/controllers/api/workflows_controller.ex
+++ b/lib/lightning_web/controllers/api/workflows_controller.ex
@@ -44,6 +44,7 @@ defmodule LightningWeb.API.WorkflowsController do
   alias Lightning.Workflows.Edge
   alias Lightning.Workflows.Presence
   alias Lightning.Workflows.Workflow
+  alias LightningWeb.API.Helpers
   alias LightningWeb.ChangesetJSON
 
   action_fallback LightningWeb.FallbackController
@@ -451,10 +452,10 @@ defmodule LightningWeb.API.WorkflowsController do
     end
   end
 
-  defp validate_uuid(project_id) do
-    case Ecto.UUID.dump(to_string(project_id)) do
-      {:ok, _bin} -> :ok
-      :error -> {:error, :invalid_id, project_id}
+  defp validate_uuid(id) do
+    case Helpers.validate_uuid(id) do
+      :ok -> :ok
+      {:error, :bad_request} -> {:error, :invalid_id, id}
     end
   end
 
diff --git a/test/lightning_web/controllers/api/credential_controller_test.exs b/test/lightning_web/controllers/api/credential_controller_test.exs
index d2b41c5aed..22c73f4c82 100644
--- a/test/lightning_web/controllers/api/credential_controller_test.exs
+++ b/test/lightning_web/controllers/api/credential_controller_test.exs
@@ -242,6 +242,11 @@ defmodule LightningWeb.API.CredentialControllerTest do
       assert json_response(conn, 404) == %{"error" => "Not Found"}
     end
 
+    test "returns 400 for malformed project_id", %{conn: conn, user: _user} do
+      conn = get(conn, ~p"/api/projects/not-a-uuid/credentials")
+      assert json_response(conn, 400) == %{"error" => "Bad Request"}
+    end
+
     test "returns empty list when project has no credentials", %{
       conn: conn,
       user: user
@@ -822,7 +827,7 @@ defmodule LightningWeb.API.CredentialControllerTest do
 
     test "handles invalid UUID format", %{conn: conn, user: _user} do
       conn = delete(conn, ~p"/api/credentials/invalid-uuid")
-      assert json_response(conn, 404) == %{"error" => "Not Found"}
+      assert json_response(conn, 400) == %{"error" => "Bad Request"}
     end
   end
 end
diff --git a/test/lightning_web/controllers/api/job_controller_test.exs b/test/lightning_web/controllers/api/job_controller_test.exs
index 50ae7e60a2..b55eabb18c 100644
--- a/test/lightning_web/controllers/api/job_controller_test.exs
+++ b/test/lightning_web/controllers/api/job_controller_test.exs
@@ -89,6 +89,20 @@ defmodule LightningWeb.API.JobControllerTest do
                "type" => "jobs"
              }
     end
+
+    test "returns 400 for malformed id", %{conn: conn} do
+      conn = get(conn, ~p"/api/jobs/not-a-uuid")
+      assert json_response(conn, 400) == %{"error" => "Bad Request"}
+    end
+  end
+
+  describe "index with invalid project_id" do
+    setup [:assign_bearer_for_api]
+
+    test "returns 400 for malformed project_id", %{conn: conn} do
+      conn = get(conn, ~p"/api/projects/not-a-uuid/jobs")
+      assert json_response(conn, 400) == %{"error" => "Bad Request"}
+    end
   end
 
   defp create_job(%{project: project}) do
diff --git a/test/lightning_web/controllers/api/project_controller_test.exs b/test/lightning_web/controllers/api/project_controller_test.exs
index a38394797d..88dd6dbba7 100644
--- a/test/lightning_web/controllers/api/project_controller_test.exs
+++ b/test/lightning_web/controllers/api/project_controller_test.exs
@@ -86,6 +86,11 @@ defmodule LightningWeb.API.ProjectControllerTest do
       assert json_response(conn, 401) == %{"error" => "Unauthorized"}
     end
 
+    test "returns 400 for malformed id", %{conn: conn} do
+      conn = get(conn, ~p"/api/projects/not-a-uuid")
+      assert json_response(conn, 400) == %{"error" => "Bad Request"}
+    end
+
     test "shows the project", %{conn: conn, project: project} do
       conn = get(conn, Routes.api_project_path(conn, :show, project))
       response = json_response(conn, 200)
diff --git a/test/lightning_web/controllers/api/provisioning_controller_test.exs b/test/lightning_web/controllers/api/provisioning_controller_test.exs
index 5587cb4446..1051ba877e 100644
--- a/test/lightning_web/controllers/api/provisioning_controller_test.exs
+++ b/test/lightning_web/controllers/api/provisioning_controller_test.exs
@@ -1362,4 +1362,18 @@ defmodule LightningWeb.API.ProvisioningControllerTest do
       end)
     end)
   end
+
+  describe "malformed UUID params" do
+    setup [:assign_bearer_for_api]
+
+    test "returns 400 for malformed id in show", %{conn: conn} do
+      conn = get(conn, ~p"/api/provision/not-a-uuid")
+      assert json_response(conn, 400) == %{"error" => "Bad Request"}
+    end
+
+    test "returns 400 for malformed id in show_yaml", %{conn: conn} do
+      conn = get(conn, ~p"/api/provision/yaml?id=not-a-uuid")
+      assert json_response(conn, 400) == %{"error" => "Bad Request"}
+    end
+  end
 end
diff --git a/test/lightning_web/controllers/api/run_controller_test.exs b/test/lightning_web/controllers/api/run_controller_test.exs
index 2beb038b45..d83a7cba55 100644
--- a/test/lightning_web/controllers/api/run_controller_test.exs
+++ b/test/lightning_web/controllers/api/run_controller_test.exs
@@ -688,4 +688,18 @@ defmodule LightningWeb.API.RunControllerTest do
       assert error_message =~ "123456"
     end
   end
+
+  describe "malformed UUID params" do
+    setup [:assign_bearer_for_api]
+
+    test "returns 400 for malformed project_id in index", %{conn: conn} do
+      conn = get(conn, ~p"/api/projects/not-a-uuid/runs")
+      assert json_response(conn, 400) == %{"error" => "Bad Request"}
+    end
+
+    test "returns 400 for malformed id in show", %{conn: conn} do
+      conn = get(conn, ~p"/api/runs/not-a-uuid")
+      assert json_response(conn, 400) == %{"error" => "Bad Request"}
+    end
+  end
 end
diff --git a/test/lightning_web/controllers/api/work_orders_controller_test.exs b/test/lightning_web/controllers/api/work_orders_controller_test.exs
index b9886e0316..3748001502 100644
--- a/test/lightning_web/controllers/api/work_orders_controller_test.exs
+++ b/test/lightning_web/controllers/api/work_orders_controller_test.exs
@@ -446,4 +446,18 @@ defmodule LightningWeb.API.WorkOrdersControllerTest do
       assert error_message =~ "123456"
     end
   end
+
+  describe "malformed UUID params" do
+    setup [:assign_bearer_for_api]
+
+    test "returns 400 for malformed project_id in index", %{conn: conn} do
+      conn = get(conn, ~p"/api/projects/not-a-uuid/work_orders")
+      assert json_response(conn, 400) == %{"error" => "Bad Request"}
+    end
+
+    test "returns 400 for malformed id in show", %{conn: conn} do
+      conn = get(conn, ~p"/api/work_orders/not-a-uuid")
+      assert json_response(conn, 400) == %{"error" => "Bad Request"}
+    end
+  end
 end