Repository: astral-sh/uv (2026-04-12T13:02:02Z)

Issue:
uv sync ignores `--output-format` with --check flag
### Summary

The docs for uv sync indicate that there is an `--output-format` flag that can be used with `uv sync` https://docs.astral.sh/uv/reference/cli/#uv-sync

however, this doesn't work when the environment is out of date. 

**Without every syncing**

```
flask-hello ⚡ uv sync --output-format=json --check
Using CPython 3.12.12
Resolved 9 packages in 2ms
Would download 7 packages
Would install 7 packages
 + blinker==1.9.0
 + click==8.3.1
 + flask==3.1.3
 + itsdangerous==2.2.0
 + jinja2==3.1.6
 + markupsafe==3.0.3
 + werkzeug==3.1.6
The environment is outdated; run `uv sync` to update the environment
```

**uv sync --output-format=json**

notice it works correctly here:

```
flask-hello ⚡ uv sync --output-format=json
Using CPython 3.12.12
Creating virtual environment at: .venv
Resolved 9 packages in 7ms
Prepared 7 packages in 511ms
Installed 7 packages in 11ms
 + blinker==1.9.0
 + click==8.3.1
 + flask==3.1.3
 + itsdangerous==2.2.0
 + jinja2==3.1.6
 + markupsafe==3.0.3
 + werkzeug==3.1.6
{
  "schema": {
    "version": "preview"
  },
  "target": "project",
  "project": {
    "path": "/Users/josiahparry/github/rust/rico-root/test-apps/python/flask-hello",
    "workspace": {
      "path": "/Users/josiahparry/github/rust/rico-root/test-apps/python/flask-hello"
    }
  },
  "sync": {
    "environment": {
      "path": "/Users/josiahparry/github/rust/rico-root/test-apps/python/flask-hello/.venv",
      "python": {
        "path": "/Users/josiahparry/github/rust/rico-root/test-apps/python/flask-hello/.venv/bin/python",
        "version": "3.12.12",
        "implementation": "cpython"
      }
    },
    "action": "create",
    "changes": [
      {
        "name": "blinker",
        "version": "1.9.0",
        "action": "installed"
      },
      {
        "name": "click",
        "version": "8.3.1",
        "action": "installed"
      },
      {
        "name": "flask",
        "version": "3.1.3",
        "action": "installed"
      },
      {
        "name": "itsdangerous",
        "version": "2.2.0",
        "action": "installed"
      },
      {
        "name": "jinja2",
        "version": "3.1.6",
        "action": "installed"
      },
      {
        "name": "markupsafe",
        "version": "3.0.3",
        "action": "installed"
      },
      {
        "name": "werkzeug",
        "version": "3.1.6",
        "action": "installed"
      }
    ]
  },
  "lock": {
    "path": "/Users/josiahparry/github/rust/rico-root/test-apps/python/flask-hello/uv.lock",
    "action": "check"
  },
  "dry_run": false
}
```

**After successfully syncing** 

Now that the it has correctly synced the `--output-format=json --check` flag works:

```
flask-hello ⚡ uv sync --output-format=json --check
Resolved 9 packages in 2ms
Checked 7 packages in 1ms
Would make no changes
{
  "schema": {
    "version": "preview"
  },
  "target": "project",
  "project": {
    "path": "/Users/josiahparry/github/rust/rico-root/test-apps/python/flask-hello",
    "workspace": {
      "path": "/Users/josiahparry/github/rust/rico-root/test-apps/python/flask-hello"
    }
  },
  "sync": {
    "environment": {
      "path": "/Users/josiahparry/github/rust/rico-root/test-apps/python/flask-hello/.venv",
      "python": {
        "path": "/Users/josiahparry/github/rust/rico-root/test-apps/python/flask-hello/.venv/bin/python3",
        "version": "3.12.12",
        "implementation": "cpython"
      }
    },
    "action": "check",
    "changes": []
  },
  "lock": {
    "path": "/Users/josiahparry/github/rust/rico-root/test-apps/python/flask-hello/uv.lock",
    "action": "check"
  },
  "dry_run": true
}
```

## repro info: 

`pyproject.toml`

```toml
# pyproject.toml
[project]
name = "flask-hello"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = ">=3.12"
dependencies = [
    "flask>=3.1.3",
]
```

`app.py`

```py
import os

from flask import Flask, jsonify

app = Flask(__name__)


@app.route("/")
def root():
    return jsonify(message="Hello from Flask!")


@app.route("/health")
def health():
    return jsonify(status="ok")


if __name__ == "__main__":
    host = os.environ.get("FLASK_RUN_HOST", "0.0.0.0")
    port = int(os.environ.get("FLASK_RUN_PORT", "5000"))
    app.run(host=host, port=port)
```

<details> <summary>uv.lock </summary> 

```toml
version = 1
revision = 3
requires-python = ">=3.12"

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name = "werkzeug"
version = "3.1.6"
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```
</details>


### Platform

macOS 15.0.1 (24A348)

### Version

0.11.2

### Python version

3.12.12


Discussion:

Patch:
diff --git a/crates/uv/src/commands/diagnostics.rs b/crates/uv/src/commands/diagnostics.rs
index 95eb7e8f56f61..7032f4d56d485 100644
--- a/crates/uv/src/commands/diagnostics.rs
+++ b/crates/uv/src/commands/diagnostics.rs
@@ -136,7 +136,7 @@ impl OperationDiagnostic {
                 system_certs_hint(err);
                 None
             }
-            pip::operations::Error::OutdatedEnvironment => {
+            err @ pip::operations::Error::OutdatedEnvironment(..) => {
                 anstream::eprintln!("{}", err);
                 None
             }
diff --git a/crates/uv/src/commands/pip/operations.rs b/crates/uv/src/commands/pip/operations.rs
index d3bd8c4c9f9ec..493b7b5e723a5 100644
--- a/crates/uv/src/commands/pip/operations.rs
+++ b/crates/uv/src/commands/pip/operations.rs
@@ -1046,7 +1046,7 @@ fn report_dry_run(
     logger.on_complete(&changelog, printer, dry_run)?;
 
     if matches!(dry_run, DryRun::Check) {
-        return Err(Error::OutdatedEnvironment);
+        return Err(Error::OutdatedEnvironment(Box::new(changelog)));
     }
 
     Ok(changelog)
@@ -1124,5 +1124,5 @@ pub(crate) enum Error {
     Anyhow(#[from] anyhow::Error),
 
     #[error("The environment is outdated; run `{}` to update the environment", "uv sync".cyan())]
-    OutdatedEnvironment,
+    OutdatedEnvironment(Box<Changelog>),
 }
diff --git a/crates/uv/src/commands/project/sync.rs b/crates/uv/src/commands/project/sync.rs
index 6ed886deb735b..040db9425f20d 100644
--- a/crates/uv/src/commands/project/sync.rs
+++ b/crates/uv/src/commands/project/sync.rs
@@ -285,25 +285,33 @@ pub(crate) async fn sync(
             .await
             {
                 Ok(EnvironmentUpdate { changelog, .. }) => {
-                    // Generate a report for the script without a lockfile
-                    let report = Report {
-                        schema: SchemaReport::default(),
-                        target: TargetName::from(&target),
-                        project: None,
-                        script: Some(ScriptReport::from(script)),
-                        sync: SyncReport {
-                            changes: PackageChangesReport::from_changelog(&changelog),
-                            ..sync_report
-                        },
-                        lock: None,
-                        dry_run: dry_run.enabled(),
-                    };
-                    if let Some(output) = report.format(output_format) {
-                        writeln!(printer.stdout_important(), "{output}")?;
-                    }
+                    write_sync_report(
+                        &target,
+                        &environment,
+                        &changelog,
+                        None,
+                        dry_run,
+                        output_format,
+                        printer,
+                    )?;
                     return Ok(ExitStatus::Success);
                 }
-                // TODO(zanieb): We should respect `--output-format json` for the error case
+                Err(ProjectError::Operation(operations::Error::OutdatedEnvironment(changelog))) => {
+                    write_sync_report(
+                        &target,
+                        &environment,
+                        &changelog,
+                        None,
+                        dry_run,
+                        output_format,
+                        printer,
+                    )?;
+                    return diagnostics::OperationDiagnostic::with_system_certs(
+                        client_builder.system_certs(),
+                    )
+                    .report(operations::Error::OutdatedEnvironment(changelog))
+                    .map_or(Ok(ExitStatus::Failure), |err| Err(err.into()));
+                }
                 Err(ProjectError::Operation(err)) => {
                     return diagnostics::OperationDiagnostic::with_system_certs(
                         client_builder.system_certs(),
@@ -431,6 +439,22 @@ pub(crate) async fn sync(
     .await
     {
         Ok(changelog) => changelog,
+        Err(ProjectError::Operation(operations::Error::OutdatedEnvironment(changelog))) => {
+            write_sync_report(
+                &target,
+                &environment,
+                &changelog,
+                Some(lock_report),
+                dry_run,
+                output_format,
+                printer,
+            )?;
+            return diagnostics::OperationDiagnostic::with_system_certs(
+                client_builder.system_certs(),
+            )
+            .report(operations::Error::OutdatedEnvironment(changelog))
+            .map_or(Ok(ExitStatus::Failure), |err| Err(err.into()));
+        }
         Err(ProjectError::Operation(err)) => {
             return diagnostics::OperationDiagnostic::with_system_certs(
                 client_builder.system_certs(),
@@ -441,22 +465,15 @@ pub(crate) async fn sync(
         Err(err) => return Err(err.into()),
     };
 
-    let report = Report {
-        schema: SchemaReport::default(),
-        target: TargetName::from(&target),
-        project: target.project().map(ProjectReport::from),
-        script: target.script().map(ScriptReport::from),
-        sync: SyncReport {
-            changes: PackageChangesReport::from_changelog(&changelog),
-            ..sync_report
-        },
-        lock: Some(lock_report),
-        dry_run: dry_run.enabled(),
-    };
-
-    if let Some(output) = report.format(output_format) {
-        writeln!(printer.stdout_important(), "{output}")?;
-    }
+    write_sync_report(
+        &target,
+        &environment,
+        &changelog,
+        Some(lock_report),
+        dry_run,
+        output_format,
+        printer,
+    )?;
 
     match outcome {
         Outcome::Success(..) => Ok(ExitStatus::Success),
@@ -1445,3 +1462,35 @@ impl Report {
         }
     }
 }
+
+fn write_sync_report(
+    target: &SyncTarget,
+    environment: &SyncEnvironment,
+    changelog: &Changelog,
+    lock: Option<LockReport>,
+    dry_run: DryRun,
+    output_format: SyncFormat,
+    printer: Printer,
+) -> Result<()> {
+    let report = Report {
+        schema: SchemaReport::default(),
+        target: TargetName::from(target),
+        project: target.project().map(ProjectReport::from),
+        script: target.script().map(ScriptReport::from),
+        sync: SyncReport {
+            environment: EnvironmentReport::from(environment),
+            action: SyncAction::from(environment),
+            changes: PackageChangesReport::from_changelog(changelog),
+            dry_run: dry_run.enabled(),
+            target: TargetName::from(target),
+        },
+        lock,
+        dry_run: dry_run.enabled(),
+    };
+
+    if let Some(output) = report.format(output_format) {
+        writeln!(printer.stdout_important(), "{output}")?;
+    }
+
+    Ok(())
+}


Test patch:
diff --git a/crates/uv/tests/it/sync.rs b/crates/uv/tests/it/sync.rs
index 934415893dfb4..ef4b0f3417803 100644
--- a/crates/uv/tests/it/sync.rs
+++ b/crates/uv/tests/it/sync.rs
@@ -600,6 +600,76 @@ fn sync_json() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn sync_json_check_outdated_environment() -> Result<()> {
+    let context = uv_test::test_context!("3.12")
+        .with_filtered_python_names()
+        .with_filtered_virtualenv_bin();
+
+    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 = ["iniconfig"]
+        "#,
+    )?;
+
+    uv_snapshot!(context.filters(), context.sync()
+        .arg("--check")
+        .arg("--output-format").arg("json"), @r#"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    {
+      "schema": {
+        "version": "preview"
+      },
+      "target": "project",
+      "project": {
+        "path": "[TEMP_DIR]/",
+        "workspace": {
+          "path": "[TEMP_DIR]/"
+        }
+      },
+      "sync": {
+        "environment": {
+          "path": "[VENV]/",
+          "python": {
+            "path": "[VENV]/[BIN]/[PYTHON]",
+            "version": "3.12.[X]",
+            "implementation": "cpython"
+          }
+        },
+        "action": "check",
+        "changes": [
+          {
+            "name": "iniconfig",
+            "version": "2.0.0",
+            "action": "installed"
+          }
+        ]
+      },
+      "lock": {
+        "path": "[TEMP_DIR]/uv.lock",
+        "action": "create"
+      },
+      "dry_run": true
+    }
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    Would download 1 package
+    Would install 1 package
+     + iniconfig==2.0.0
+    The environment is outdated; run `uv sync` to update the environment
+    "#);
+
+    Ok(())
+}
+
 /// Test --dry json output
 #[test]
 fn sync_dry_json() -> Result<()> {