Reject non-PEP 751 filenames in `uv pip compile` and `uv export` (#13119)

We shouldn't let users create files that won't work in subsequent
commands.

Closes https://github.com/astral-sh/uv/issues/13117.

crates/uv/src/commands/pip/compile.rs

@@ -1,5 +1,6 @@
 use std::collections::{BTreeMap, BTreeSet};
 use std::env;
+use std::ffi::OsStr;
 use std::path::{Path, PathBuf};
 use std::str::FromStr;
 use std::sync::Arc;
@@ -33,7 +34,7 @@ use uv_python::{
 };
 use uv_requirements::upgrade::{read_pylock_toml_requirements, LockedRequirements};
 use uv_requirements::{
-    upgrade::read_requirements_txt, RequirementsSource, RequirementsSpecification,
+    is_pylock_toml, upgrade::read_requirements_txt, RequirementsSource, RequirementsSpecification,
 };
 use uv_resolver::{
     AnnotationStyle, DependencyMode, DisplayResolutionGraph, ExcludeNewer, FlatIndex, ForkStrategy,
@@ -133,6 +134,20 @@ pub(crate) async fn pip_compile(
         }
     });
 
+    // If the user is exporting to PEP 751, ensure the filename matches the specification.
+    if matches!(format, ExportFormat::PylockToml) {
+        if let Some(file_name) = output_file
+            .and_then(Path::file_name)
+            .and_then(OsStr::to_str)
+        {
+            if !is_pylock_toml(file_name) {
+                return Err(anyhow!(
+                    "Expected the output filename to start with `pylock.` and end with `.toml` (e.g., `pylock.toml`, `pylock.dev.toml`); `{file_name}` won't be recognized as a `pylock.toml` file in subsequent commands",
+                ));
+            }
+        }
+    }
+
     // Respect `UV_PYTHON`
     if python.is_none() && python_version.is_none() {
         if let Ok(request) = std::env::var("UV_PYTHON") {

crates/uv/src/commands/project/export.rs

@@ -2,7 +2,7 @@ use std::env;
 use std::ffi::OsStr;
 use std::path::{Path, PathBuf};
 
-use anyhow::{Context, Result};
+use anyhow::{anyhow, Context, Result};
 use itertools::Itertools;
 use owo_colors::OwoColorize;
 
@@ -282,6 +282,21 @@ pub(crate) async fn export(
         }
     });
 
+    // If the user is exporting to PEP 751, ensure the filename matches the specification.
+    if matches!(format, ExportFormat::PylockToml) {
+        if let Some(file_name) = output_file
+            .as_deref()
+            .and_then(Path::file_name)
+            .and_then(OsStr::to_str)
+        {
+            if !is_pylock_toml(file_name) {
+                return Err(anyhow!(
+                    "Expected the output filename to start with `pylock.` and end with `.toml` (e.g., `pylock.toml`, `pylock.dev.toml`); `{file_name}` won't be recognized as a `pylock.toml` file in subsequent commands",
+                ));
+            }
+        }
+    }
+
     // Generate the export.
     match format {
         ExportFormat::RequirementsTxt => {

crates/uv/tests/it/export.rs

@@ -4066,3 +4066,37 @@ fn pep_751_infer_output_format() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn pep_751_filename() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["anyio==3.7.0"]
+
+        [build-system]
+        requires = ["setuptools>=42"]
+        build-backend = "setuptools.build_meta"
+        "#,
+    )?;
+
+    context.lock().assert().success();
+
+    uv_snapshot!(context.filters(), context.export().arg("--format").arg("pylock.toml").arg("-o").arg("test.toml"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 4 packages in [TIME]
+    error: Expected the output filename to start with `pylock.` and end with `.toml` (e.g., `pylock.toml`, `pylock.dev.toml`); `test.toml` won't be recognized as a `pylock.toml` file in subsequent commands
+    ");
+
+    Ok(())
+}

crates/uv/tests/it/pip_compile.rs

@@ -16262,6 +16262,32 @@ fn compile_invalid_output_file() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn pep_751_filename() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let requirements_txt = context.temp_dir.child("requirements.txt");
+    requirements_txt.write_str("iniconfig")?;
+
+    uv_snapshot!(context.filters(), context
+        .pip_compile()
+        .arg("requirements.txt")
+        .arg("--universal")
+        .arg("--format")
+        .arg("pylock.toml")
+        .arg("-o")
+        .arg("test.toml"), @r"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Expected the output filename to start with `pylock.` and end with `.toml` (e.g., `pylock.toml`, `pylock.dev.toml`); `test.toml` won't be recognized as a `pylock.toml` file in subsequent commands
+    ");
+
+    Ok(())
+}
+
 #[test]
 fn pep_751_compile_registry_wheel() -> Result<()> {
     let context = TestContext::new("3.12");