feat(error): add structured exit codes for scriptable error handling (#428)

* feat(error): add structured exit codes for scriptable error handling

Replace the hardcoded `std::process::exit(1)` with a type-specific exit
code derived from the GwsError variant:

  0 — success
  1 — API error      (GwsError::Api)
  2 — auth error     (GwsError::Auth)
  3 — validation     (GwsError::Validation)
  4 — discovery      (GwsError::Discovery)
  5 — internal       (GwsError::Other)

This allows shell scripts to branch on failure type without parsing the
JSON error output:

  gws drive files list ...
  case $? in
    1) echo "API error — check your params" ;;
    2) echo "Auth error — run: gws auth login" ;;
    3) echo "Bad arguments" ;;
  esac

Changes:
- Add GwsError::exit_code() mapping variants to codes
- Update main() to call std::process::exit(err.exit_code())
- Document exit codes in gws --help (print_usage)
- Document exit codes in README under new Exit Codes section
- Add 6 unit tests including a regression guard asserting all codes are distinct

* refactor(error): replace magic exit-code numbers with named constants

Add EXIT_CODE_API/AUTH/VALIDATION/DISCOVERY/OTHER associated constants
on GwsError so callers and tests reference symbolic names rather than
bare integers. Update exit_code() match arms and all tests accordingly.
The distinctness test now validates the constants array directly.

Addresses code-review feedback requesting named constants.

* refactor(error): centralize exit code help text via EXIT_CODE_DOCUMENTATION

Add a module-level EXIT_CODE_DOCUMENTATION constant — a static slice of
(code, description) pairs built from the EXIT_CODE_* constants. Replace
the hardcoded println! block in print_usage() with a loop over this slice
so the help output is always in sync with the defined constants and cannot
drift out of date.

Addresses code-review feedback requesting a single source of truth.

.changeset/structured-exit-codes.md

@@ -0,0 +1,18 @@
+---
+"@googleworkspace/cli": minor
+---
+
+Add structured exit codes for scriptable error handling
+
+`gws` now exits with a type-specific code instead of always using `1`:
+
+| Code | Meaning |
+|------|---------|
+| `0` | Success |
+| `1` | API error — Google returned a 4xx/5xx response |
+| `2` | Auth error — credentials missing, expired, or invalid |
+| `3` | Validation error — bad arguments, unknown service, invalid flag |
+| `4` | Discovery error — could not fetch the API schema document |
+| `5` | Internal error — unexpected failure |
+
+Exit codes are documented in `gws --help` and in the README.

README.md

@@ -33,6 +33,7 @@ npm install -g @googleworkspace/cli
 - [AI Agent Skills](#ai-agent-skills)
 - [Advanced Usage](#advanced-usage)
 - [Environment Variables](#environment-variables)
+- [Exit Codes](#exit-codes)
 - [Architecture](#architecture)
 - [Troubleshooting](#troubleshooting)
 - [Development](#development)
@@ -317,6 +318,27 @@ All variables are optional. See [`.env.example`](.env.example) for a copy-paste
 
 Environment variables can also be set in a `.env` file (loaded via [dotenvy](https://crates.io/crates/dotenvy)).
 
+## Exit Codes
+
+`gws` uses structured exit codes so scripts can branch on the failure type without parsing error output.
+
+| Code | Meaning | Example cause |
+|------|---------|---------------|
+| `0` | Success | Command completed normally |
+| `1` | API error | Google returned a 4xx/5xx response |
+| `2` | Auth error | Credentials missing, expired, or invalid |
+| `3` | Validation error | Bad arguments, unknown service, invalid flag |
+| `4` | Discovery error | Could not fetch the API schema document |
+| `5` | Internal error | Unexpected failure |
+
+```bash
+gws drive files list --params '{"fileId": "bad"}'
+echo $?   # 1 — API error
+
+gws unknown-service files list
+echo $?   # 3 — validation error (unknown service)
+```
+
 ## Architecture
 
 `gws` uses a **two-phase parsing** strategy:

src/error.rs

@@ -39,7 +39,51 @@ pub enum GwsError {
     Other(#[from] anyhow::Error),
 }
 
+/// Human-readable exit code table, keyed by (code, description).
+///
+/// Used by `print_usage()` so the help text stays in sync with the
+/// constants defined below without requiring manual updates in two places.
+pub const EXIT_CODE_DOCUMENTATION: &[(i32, &str)] = &[
+    (0, "Success"),
+    (GwsError::EXIT_CODE_API, "API error  — Google returned an error response"),
+    (GwsError::EXIT_CODE_AUTH, "Auth error — credentials missing or invalid"),
+    (GwsError::EXIT_CODE_VALIDATION, "Validation — bad arguments or input"),
+    (GwsError::EXIT_CODE_DISCOVERY, "Discovery  — could not fetch API schema"),
+    (GwsError::EXIT_CODE_OTHER, "Internal   — unexpected failure"),
+];
+
 impl GwsError {
+    /// Exit code for [`GwsError::Api`] variants.
+    pub const EXIT_CODE_API: i32 = 1;
+    /// Exit code for [`GwsError::Auth`] variants.
+    pub const EXIT_CODE_AUTH: i32 = 2;
+    /// Exit code for [`GwsError::Validation`] variants.
+    pub const EXIT_CODE_VALIDATION: i32 = 3;
+    /// Exit code for [`GwsError::Discovery`] variants.
+    pub const EXIT_CODE_DISCOVERY: i32 = 4;
+    /// Exit code for [`GwsError::Other`] variants.
+    pub const EXIT_CODE_OTHER: i32 = 5;
+
+    /// Map each error variant to a stable, documented exit code.
+    ///
+    /// | Code | Meaning                                      |
+    /// |------|----------------------------------------------|
+    /// |  0   | Success (never returned here)                |
+    /// |  1   | API error — Google returned an error response |
+    /// |  2   | Auth error — credentials missing or invalid  |
+    /// |  3   | Validation error — bad arguments or input    |
+    /// |  4   | Discovery error — could not fetch API schema |
+    /// |  5   | Internal error — unexpected failure          |
+    pub fn exit_code(&self) -> i32 {
+        match self {
+            GwsError::Api { .. } => Self::EXIT_CODE_API,
+            GwsError::Auth(_) => Self::EXIT_CODE_AUTH,
+            GwsError::Validation(_) => Self::EXIT_CODE_VALIDATION,
+            GwsError::Discovery(_) => Self::EXIT_CODE_DISCOVERY,
+            GwsError::Other(_) => Self::EXIT_CODE_OTHER,
+        }
+    }
+
     pub fn to_json(&self) -> serde_json::Value {
         match self {
             GwsError::Api {
@@ -126,6 +170,63 @@ pub fn print_error_json(err: &GwsError) {
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_exit_code_api() {
+        let err = GwsError::Api {
+            code: 404,
+            message: "Not Found".to_string(),
+            reason: "notFound".to_string(),
+            enable_url: None,
+        };
+        assert_eq!(err.exit_code(), GwsError::EXIT_CODE_API);
+    }
+
+    #[test]
+    fn test_exit_code_auth() {
+        assert_eq!(
+            GwsError::Auth("bad token".to_string()).exit_code(),
+            GwsError::EXIT_CODE_AUTH
+        );
+    }
+
+    #[test]
+    fn test_exit_code_validation() {
+        assert_eq!(
+            GwsError::Validation("missing arg".to_string()).exit_code(),
+            GwsError::EXIT_CODE_VALIDATION
+        );
+    }
+
+    #[test]
+    fn test_exit_code_discovery() {
+        assert_eq!(
+            GwsError::Discovery("fetch failed".to_string()).exit_code(),
+            GwsError::EXIT_CODE_DISCOVERY
+        );
+    }
+
+    #[test]
+    fn test_exit_code_other() {
+        assert_eq!(
+            GwsError::Other(anyhow::anyhow!("oops")).exit_code(),
+            GwsError::EXIT_CODE_OTHER
+        );
+    }
+
+    #[test]
+    fn test_exit_codes_are_distinct() {
+        // Ensure all named constants are unique (regression guard).
+        let codes = [
+            GwsError::EXIT_CODE_API,
+            GwsError::EXIT_CODE_AUTH,
+            GwsError::EXIT_CODE_VALIDATION,
+            GwsError::EXIT_CODE_DISCOVERY,
+            GwsError::EXIT_CODE_OTHER,
+        ];
+        let unique: std::collections::HashSet<i32> = codes.iter().copied().collect();
+        assert_eq!(unique.len(), codes.len(), "exit codes must be distinct: {codes:?}");
+    }
+
     #[test]
     fn test_error_to_json_api() {
         let err = GwsError::Api {

src/main.rs

@@ -49,7 +49,7 @@ async fn main() {
 
     if let Err(err) = run().await {
         print_error_json(&err);
-        std::process::exit(1);
+        std::process::exit(err.exit_code());
     }
 }
 
@@ -460,6 +460,11 @@ fn print_usage() {
         "    GOOGLE_WORKSPACE_PROJECT_ID              Override the GCP project ID for quota and billing"
     );
     println!();
+    println!("EXIT CODES:");
+    for (code, description) in crate::error::EXIT_CODE_DOCUMENTATION {
+        println!("    {:<5}{}", code, description);
+    }
+    println!();
     println!("COMMUNITY:");
     println!("    Star the repo: https://github.com/googleworkspace/cli");
     println!("    Report bugs / request features: https://github.com/googleworkspace/cli/issues");