diff --git a/crates/uv-configuration/src/dev.rs b/crates/uv-configuration/src/dev.rs
index a8980c060..80ae4f064 100644
--- a/crates/uv-configuration/src/dev.rs
+++ b/crates/uv-configuration/src/dev.rs
@@ -346,6 +346,18 @@ impl DevGroupsManifest {
         self.spec.prod()
     }
 
+    /// Returns `true` if the group was enabled by default.
+    pub fn default(&self, group: &GroupName) -> bool {
+        if self.spec.contains(group) {
+            // If the group was explicitly requested, then it wasn't enabled by default.
+            false
+        } else {
+            // If the group was enabled, but wasn't explicitly requested, then it was enabled by
+            // default.
+            self.contains(group)
+        }
+    }
+
     /// Returns `true` if the group is included in the manifest.
     pub fn contains(&self, group: &GroupName) -> bool {
         if self.spec.contains(group) {
diff --git a/crates/uv/src/commands/project/mod.rs b/crates/uv/src/commands/project/mod.rs
index 4538b89b0..5dce4614b 100644
--- a/crates/uv/src/commands/project/mod.rs
+++ b/crates/uv/src/commands/project/mod.rs
@@ -81,26 +81,8 @@ pub(crate) enum ProjectError {
     #[error("The current Python platform is not compatible with the lockfile's supported environments: {0}")]
     LockedPlatformIncompatibility(String),
 
-    #[error(
-        "{} are incompatible with the declared conflicts: {{{}}}",
-        _1.iter().map(|conflict| {
-            match conflict {
-                ConflictPackage::Extra(ref extra) => format!("extra `{extra}`"),
-                ConflictPackage::Group(ref group) => format!("group `{group}`"),
-            }
-        }).collect::<Vec<String>>().join(", "),
-        _0
-            .iter()
-            .map(|item| {
-                match item.conflict() {
-                    ConflictPackage::Extra(ref extra) => format!("`{}[{}]`", item.package(), extra),
-                    ConflictPackage::Group(ref group) => format!("`{}:{}`", item.package(), group),
-                }
-            })
-            .collect::<Vec<String>>()
-            .join(", "),
-    )]
-    ConflictIncompatibility(ConflictSet, Vec<ConflictPackage>),
+    #[error(transparent)]
+    Conflict(#[from] ConflictError),
 
     #[error("The requested interpreter resolved to Python {0}, which is incompatible with the project's Python requirement: `{1}`")]
     RequestedPythonProjectIncompatibility(Version, RequiresPython),
@@ -227,6 +209,43 @@ pub(crate) enum ProjectError {
     Anyhow(#[from] anyhow::Error),
 }
 
+#[derive(Debug)]
+pub(crate) struct ConflictError {
+    /// The set from which the conflict was derived.
+    pub(crate) set: ConflictSet,
+    /// The items from the set that were enabled, and thus create the conflict.
+    pub(crate) conflicts: Vec<ConflictPackage>,
+    /// The manifest of enabled dependency groups.
+    pub(crate) dev: DevGroupsManifest,
+}
+
+impl std::fmt::Display for ConflictError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f,
+            "{} are incompatible with the declared conflicts: {{{}}}",
+            self.conflicts
+                .iter()
+                .map(|conflict| match conflict {
+                    ConflictPackage::Extra(ref extra) => format!("extra `{extra}`"),
+                    ConflictPackage::Group(ref group) if self.dev.default(group) =>
+                        format!("group `{group}` (enabled by default)"),
+                    ConflictPackage::Group(ref group) => format!("group `{group}`"),
+                })
+                .join(", "),
+            self.set
+                .iter()
+                .map(|item| match item.conflict() {
+                    ConflictPackage::Extra(ref extra) => format!("`{}[{}]`", item.package(), extra),
+                    ConflictPackage::Group(ref group) => format!("`{}:{}`", item.package(), group),
+                })
+                .join(", ")
+        )
+    }
+}
+
+impl std::error::Error for ConflictError {}
+
 /// Compute the `Requires-Python` bound for the [`Workspace`].
 ///
 /// For a [`Workspace`] with multiple packages, the `Requires-Python` bound is the union of the
@@ -1662,10 +1681,11 @@ pub(crate) fn detect_conflicts(
             }
         }
         if conflicts.len() >= 2 {
-            return Err(ProjectError::ConflictIncompatibility(
-                set.clone(),
+            return Err(ProjectError::Conflict(ConflictError {
+                set: set.clone(),
                 conflicts,
-            ));
+                dev: dev.clone(),
+            }));
         }
     }
     Ok(())
diff --git a/crates/uv/tests/it/lock.rs b/crates/uv/tests/it/lock.rs
index c4a98331f..5f9ea2ac2 100644
--- a/crates/uv/tests/it/lock.rs
+++ b/crates/uv/tests/it/lock.rs
@@ -3765,6 +3765,197 @@ fn lock_conflicting_group_basic() -> Result<()> {
     Ok(())
 }
 
+/// This tests a case of specifying conflicting groups, where some of the conflicts are enabled by
+/// default.
+#[test]
+fn lock_conflicting_group_default() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    // Tell uv about the conflicting groups, which forces it to resolve each in
+    // their own fork.
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        description = "Add your description here"
+        requires-python = ">=3.12"
+
+        [tool.uv]
+        conflicts = [
+            [
+              { group = "project1" },
+              { group = "project2" },
+            ],
+        ]
+        default-groups = ["project1"]
+
+        [dependency-groups]
+        project1 = ["sortedcontainers==2.3.0"]
+        project2 = ["sortedcontainers==2.4.0"]
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+        "#,
+    )?;
+
+    uv_snapshot!(context.filters(), context.lock(), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    "###);
+
+    let lock = context.read("uv.lock");
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            lock, @r###"
+        version = 1
+        requires-python = ">=3.12"
+        resolution-markers = [
+        ]
+        conflicts = [[
+            { package = "project", group = "project1" },
+            { package = "project", group = "project2" },
+        ]]
+
+        [options]
+        exclude-newer = "2024-03-25T00:00:00Z"
+
+        [[package]]
+        name = "project"
+        version = "0.1.0"
+        source = { editable = "." }
+
+        [package.dev-dependencies]
+        project1 = [
+            { name = "sortedcontainers", version = "2.3.0", source = { registry = "https://pypi.org/simple" } },
+        ]
+        project2 = [
+            { name = "sortedcontainers", version = "2.4.0", source = { registry = "https://pypi.org/simple" } },
+        ]
+
+        [package.metadata]
+
+        [package.metadata.requires-dev]
+        project1 = [{ name = "sortedcontainers", specifier = "==2.3.0" }]
+        project2 = [{ name = "sortedcontainers", specifier = "==2.4.0" }]
+
+        [[package]]
+        name = "sortedcontainers"
+        version = "2.3.0"
+        source = { registry = "https://pypi.org/simple" }
+        resolution-markers = [
+        ]
+        sdist = { url = "https://files.pythonhosted.org/packages/14/10/6a9481890bae97da9edd6e737c9c3dec6aea3fc2fa53b0934037b35c89ea/sortedcontainers-2.3.0.tar.gz", hash = "sha256:59cc937650cf60d677c16775597c89a960658a09cf7c1a668f86e1e4464b10a1", size = 30509 }
+        wheels = [
+            { url = "https://files.pythonhosted.org/packages/20/4d/a7046ae1a1a4cc4e9bbed194c387086f06b25038be596543d026946330c9/sortedcontainers-2.3.0-py2.py3-none-any.whl", hash = "sha256:37257a32add0a3ee490bb170b599e93095eed89a55da91fa9f48753ea12fd73f", size = 29479 },
+        ]
+
+        [[package]]
+        name = "sortedcontainers"
+        version = "2.4.0"
+        source = { registry = "https://pypi.org/simple" }
+        resolution-markers = [
+        ]
+        sdist = { url = "https://files.pythonhosted.org/packages/e8/c4/ba2f8066cceb6f23394729afe52f3bf7adec04bf9ed2c820b39e19299111/sortedcontainers-2.4.0.tar.gz", hash = "sha256:25caa5a06cc30b6b83d11423433f65d1f9d76c4c6a0c90e3379eaa43b9bfdb88", size = 30594 }
+        wheels = [
+            { url = "https://files.pythonhosted.org/packages/32/46/9cb0e58b2deb7f82b84065f37f3bffeb12413f947f9388e4cac22c4621ce/sortedcontainers-2.4.0-py2.py3-none-any.whl", hash = "sha256:a163dcaede0f1c021485e957a39245190e74249897e2ae4b2aa38595db237ee0", size = 29575 },
+        ]
+        "###
+        );
+    });
+
+    // Re-run with `--locked`.
+    uv_snapshot!(context.filters(), context.lock().arg("--locked"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 3 packages in [TIME]
+    "###);
+
+    // Install from the lockfile, which should include the `project1` group by default.
+    uv_snapshot!(context.filters(), context.sync().arg("--frozen"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Prepared 2 packages in [TIME]
+    Installed 2 packages in [TIME]
+     + project==0.1.0 (from file://[TEMP_DIR]/)
+     + sortedcontainers==2.3.0
+    "###);
+
+    // Another install, but with one of the groups enabled.
+    uv_snapshot!(context.filters(), context.sync().arg("--frozen").arg("--group=project1"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Audited 2 packages in [TIME]
+    "###);
+
+    // Another install, but with the other group enabled. This should error, since `project1` is
+    // enabled by default.
+    uv_snapshot!(context.filters(), context.sync().arg("--frozen").arg("--group=project2"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: group `project1` (enabled by default), group `project2` are incompatible with the declared conflicts: {`project:project1`, `project:project2`}
+    "###);
+
+    // If the group is explicitly requested, we should still fail, but shouldn't mark it as
+    // "enabled by default".
+    uv_snapshot!(context.filters(), context.sync().arg("--frozen").arg("--group=project1").arg("--group=project2"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: group `project1`, group `project2` are incompatible with the declared conflicts: {`project:project1`, `project:project2`}
+    "###);
+
+    // If we install via `--all-groups`, we should also avoid marking the group as "enabled by
+    // default".
+    uv_snapshot!(context.filters(), context.sync().arg("--frozen").arg("--all-groups"), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: group `project1`, group `project2` are incompatible with the declared conflicts: {`project:project1`, `project:project2`}
+    "###);
+
+    // Disabling the default group should succeed.
+    uv_snapshot!(context.filters(), context.sync().arg("--frozen").arg("--no-group=project1").arg("--group=project2"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Prepared 1 package in [TIME]
+    Uninstalled 1 package in [TIME]
+    Installed 1 package in [TIME]
+     - sortedcontainers==2.3.0
+     + sortedcontainers==2.4.0
+    "###);
+
+    Ok(())
+}
+
 /// This tests a case where we declare an extra and a group as conflicting.
 #[test]
 fn lock_conflicting_mixed() -> Result<()> {