diff --git a/crates/uv-requirements/src/pyproject.rs b/crates/uv-requirements/src/pyproject.rs
index ca92cc5d5..5904f486c 100644
--- a/crates/uv-requirements/src/pyproject.rs
+++ b/crates/uv-requirements/src/pyproject.rs
@@ -27,6 +27,7 @@ use uv_configuration::PreviewMode;
 use uv_fs::Simplified;
 use uv_git::GitReference;
 use uv_normalize::{ExtraName, PackageName};
+use uv_warnings::warn_user_once;
 
 use crate::ExtrasSpecification;
 
@@ -280,6 +281,7 @@ impl Pep621Metadata {
         let requirements = lower_requirements(
             &project.dependencies.unwrap_or_default(),
             &project.optional_dependencies.unwrap_or_default(),
+            &project.name,
             project_dir,
             &project_sources.unwrap_or_default(),
             workspace_sources,
@@ -318,6 +320,7 @@ impl Pep621Metadata {
 pub(crate) fn lower_requirements(
     dependencies: &[String],
     optional_dependencies: &IndexMap<ExtraName, Vec<String>>,
+    project_name: &PackageName,
     project_dir: &Path,
     project_sources: &HashMap<PackageName, Source>,
     workspace_sources: &HashMap<PackageName, Source>,
@@ -331,6 +334,7 @@ pub(crate) fn lower_requirements(
             let name = requirement.name.clone();
             lower_requirement(
                 requirement,
+                project_name,
                 project_dir,
                 project_sources,
                 workspace_sources,
@@ -350,6 +354,7 @@ pub(crate) fn lower_requirements(
                     let name = requirement.name.clone();
                     lower_requirement(
                         requirement,
+                        project_name,
                         project_dir,
                         project_sources,
                         workspace_sources,
@@ -371,6 +376,7 @@ pub(crate) fn lower_requirements(
 /// Combine `project.dependencies` or `project.optional-dependencies` with `tool.uv.sources`.
 pub(crate) fn lower_requirement(
     requirement: pep508_rs::Requirement,
+    project_name: &PackageName,
     project_dir: &Path,
     project_sources: &HashMap<PackageName, Source>,
     workspace_sources: &HashMap<PackageName, Source>,
@@ -395,7 +401,15 @@ pub(crate) fn lower_requirement(
     }
 
     let Some(source) = source else {
+        let has_sources = !project_sources.is_empty() || !workspace_sources.is_empty();
         // Support recursive editable inclusions.
+        if has_sources && requirement.version_or_url.is_none() && &requirement.name != project_name
+        {
+            warn_user_once!(
+                "Missing version constraint (e.g., a lower bound) for `{}`",
+                requirement.name
+            );
+        }
         return Ok(Requirement::from_pep508(requirement)?);
     };
 
@@ -476,10 +490,16 @@ pub(crate) fn lower_requirement(
             path_source(path, project_dir, editable)?
         }
         Source::Registry { index } => match requirement.version_or_url {
-            None => RequirementSource::Registry {
-                specifier: VersionSpecifiers::empty(),
-                index: Some(index),
-            },
+            None => {
+                warn_user_once!(
+                    "Missing version constraint (e.g., a lower bound) for `{}`",
+                    requirement.name
+                );
+                RequirementSource::Registry {
+                    specifier: VersionSpecifiers::empty(),
+                    index: Some(index),
+                }
+            }
             Some(VersionOrUrl::VersionSpecifier(version)) => RequirementSource::Registry {
                 specifier: version,
                 index: Some(index),
@@ -630,8 +650,8 @@ mod test {
     use anyhow::Context;
     use indoc::indoc;
     use insta::assert_snapshot;
-    use uv_configuration::PreviewMode;
 
+    use uv_configuration::PreviewMode;
     use uv_fs::Simplified;
 
     use crate::{ExtrasSpecification, RequirementsSpecification};
@@ -756,6 +776,20 @@ mod test {
         "###);
     }
 
+    #[test]
+    fn missing_constraint() {
+        let input = indoc! {r#"
+            [project]
+            name = "foo"
+            version = "0.0.0"
+            dependencies = [
+              "tqdm",
+            ]
+        "#};
+
+        assert!(from_source(input, "pyproject.toml", &ExtrasSpecification::None).is_ok());
+    }
+
     #[test]
     fn invalid_syntax() {
         let input = indoc! {r#"
diff --git a/crates/uv/tests/pip_compile.rs b/crates/uv/tests/pip_compile.rs
index 0f02606ff..a88400452 100644
--- a/crates/uv/tests/pip_compile.rs
+++ b/crates/uv/tests/pip_compile.rs
@@ -8653,6 +8653,85 @@ fn git_source_missing_tag() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn warn_missing_constraint() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "foo"
+        version = "0.0.0"
+        dependencies = [
+          "tqdm",
+          "anyio==4.3.0",
+        ]
+
+        [tool.uv.sources]
+        anyio = { url = "https://files.pythonhosted.org/packages/14/fd/2f20c40b45e4fb4324834aea24bd4afdf1143390242c0b33774da0e2e34f/anyio-4.3.0-py3-none-any.whl" }
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.compile()
+            .arg("--preview")
+            .arg("pyproject.toml"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z --preview pyproject.toml
+    anyio @ https://files.pythonhosted.org/packages/14/fd/2f20c40b45e4fb4324834aea24bd4afdf1143390242c0b33774da0e2e34f/anyio-4.3.0-py3-none-any.whl
+    idna==3.6
+        # via anyio
+    sniffio==1.3.1
+        # via anyio
+    tqdm==4.66.2
+
+    ----- stderr -----
+    warning: Missing version constraint (e.g., a lower bound) for `tqdm`
+    Resolved 4 packages in [TIME]
+    "###);
+
+    Ok(())
+}
+
+/// Ensure that this behavior is constraint to preview mode.
+#[test]
+fn dont_warn_missing_constraint_without_sources() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! {r#"
+        [project]
+        name = "foo"
+        version = "0.0.0"
+        dependencies = [
+          "tqdm",
+          "anyio==4.3.0",
+        ]
+    "#})?;
+
+    uv_snapshot!(context.filters(), context.compile()
+            .arg("--preview")
+            .arg("pyproject.toml"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    # This file was autogenerated by uv via the following command:
+    #    uv pip compile --cache-dir [CACHE_DIR] --exclude-newer 2024-03-25T00:00:00Z --preview pyproject.toml
+    anyio==4.3.0
+    idna==3.6
+        # via anyio
+    sniffio==1.3.1
+        # via anyio
+    tqdm==4.66.2
+
+    ----- stderr -----
+    Resolved 4 packages in [TIME]
+    "###);
+
+    Ok(())
+}
+
 #[test]
 fn tool_uv_sources() -> Result<()> {
     let context = TestContext::new("3.12");