fix(exec): improve stdin prompt decoding (#9151)

Fixes #8733.

- Read prompt from stdin as raw bytes and decode more helpfully.
- Strip UTF-8 BOM; decode UTF-16LE/UTF-16BE when a BOM is present.
- For other non-UTF8 input, fail with an actionable message (offset +
iconv hint).

Tests: `cargo test -p codex-exec`.

codex-rs/exec/src/lib.rs

@@ -557,6 +557,79 @@ fn load_output_schema(path: Option<PathBuf>) -> Option<Value> {
     }
 }
 
+#[derive(Debug, Clone, PartialEq, Eq)]
+enum PromptDecodeError {
+    InvalidUtf8 { valid_up_to: usize },
+    InvalidUtf16 { encoding: &'static str },
+    UnsupportedBom { encoding: &'static str },
+}
+
+impl std::fmt::Display for PromptDecodeError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            PromptDecodeError::InvalidUtf8 { valid_up_to } => write!(
+                f,
+                "input is not valid UTF-8 (invalid byte at offset {valid_up_to}). Convert it to UTF-8 and retry (e.g., `iconv -f <ENC> -t UTF-8 prompt.txt`)."
+            ),
+            PromptDecodeError::InvalidUtf16 { encoding } => write!(
+                f,
+                "input looked like {encoding} but could not be decoded. Convert it to UTF-8 and retry."
+            ),
+            PromptDecodeError::UnsupportedBom { encoding } => write!(
+                f,
+                "input appears to be {encoding}. Convert it to UTF-8 and retry."
+            ),
+        }
+    }
+}
+
+fn decode_prompt_bytes(input: &[u8]) -> Result<String, PromptDecodeError> {
+    let input = input.strip_prefix(&[0xEF, 0xBB, 0xBF]).unwrap_or(input);
+
+    if input.starts_with(&[0xFF, 0xFE, 0x00, 0x00]) {
+        return Err(PromptDecodeError::UnsupportedBom {
+            encoding: "UTF-32LE",
+        });
+    }
+
+    if input.starts_with(&[0x00, 0x00, 0xFE, 0xFF]) {
+        return Err(PromptDecodeError::UnsupportedBom {
+            encoding: "UTF-32BE",
+        });
+    }
+
+    if let Some(rest) = input.strip_prefix(&[0xFF, 0xFE]) {
+        return decode_utf16(rest, "UTF-16LE", u16::from_le_bytes);
+    }
+
+    if let Some(rest) = input.strip_prefix(&[0xFE, 0xFF]) {
+        return decode_utf16(rest, "UTF-16BE", u16::from_be_bytes);
+    }
+
+    std::str::from_utf8(input)
+        .map(str::to_string)
+        .map_err(|e| PromptDecodeError::InvalidUtf8 {
+            valid_up_to: e.valid_up_to(),
+        })
+}
+
+fn decode_utf16(
+    input: &[u8],
+    encoding: &'static str,
+    decode_unit: fn([u8; 2]) -> u16,
+) -> Result<String, PromptDecodeError> {
+    if !input.len().is_multiple_of(2) {
+        return Err(PromptDecodeError::InvalidUtf16 { encoding });
+    }
+
+    let units: Vec<u16> = input
+        .chunks_exact(2)
+        .map(|chunk| decode_unit([chunk[0], chunk[1]]))
+        .collect();
+
+    String::from_utf16(&units).map_err(|_| PromptDecodeError::InvalidUtf16 { encoding })
+}
+
 fn resolve_prompt(prompt_arg: Option<String>) -> String {
     match prompt_arg {
         Some(p) if p != "-" => p,
@@ -573,11 +646,22 @@ fn resolve_prompt(prompt_arg: Option<String>) -> String {
             if !force_stdin {
                 eprintln!("Reading prompt from stdin...");
             }
-            let mut buffer = String::new();
-            if let Err(e) = std::io::stdin().read_to_string(&mut buffer) {
+
+            let mut bytes = Vec::new();
+            if let Err(e) = std::io::stdin().read_to_end(&mut bytes) {
                 eprintln!("Failed to read prompt from stdin: {e}");
                 std::process::exit(1);
-            } else if buffer.trim().is_empty() {
+            }
+
+            let buffer = match decode_prompt_bytes(&bytes) {
+                Ok(s) => s,
+                Err(e) => {
+                    eprintln!("Failed to read prompt from stdin: {e}");
+                    std::process::exit(1);
+                }
+            };
+
+            if buffer.trim().is_empty() {
                 eprintln!("No prompt provided via stdin.");
                 std::process::exit(1);
             }
@@ -682,4 +766,79 @@ mod tests {
 
         assert_eq!(request, expected);
     }
+
+    #[test]
+    fn decode_prompt_bytes_strips_utf8_bom() {
+        let input = [0xEF, 0xBB, 0xBF, b'h', b'i', b'\n'];
+
+        let out = decode_prompt_bytes(&input).expect("decode utf-8 with BOM");
+
+        assert_eq!(out, "hi\n");
+    }
+
+    #[test]
+    fn decode_prompt_bytes_decodes_utf16le_bom() {
+        // UTF-16LE BOM + "hi\n"
+        let input = [0xFF, 0xFE, b'h', 0x00, b'i', 0x00, b'\n', 0x00];
+
+        let out = decode_prompt_bytes(&input).expect("decode utf-16le with BOM");
+
+        assert_eq!(out, "hi\n");
+    }
+
+    #[test]
+    fn decode_prompt_bytes_decodes_utf16be_bom() {
+        // UTF-16BE BOM + "hi\n"
+        let input = [0xFE, 0xFF, 0x00, b'h', 0x00, b'i', 0x00, b'\n'];
+
+        let out = decode_prompt_bytes(&input).expect("decode utf-16be with BOM");
+
+        assert_eq!(out, "hi\n");
+    }
+
+    #[test]
+    fn decode_prompt_bytes_rejects_utf32le_bom() {
+        // UTF-32LE BOM + "hi\n"
+        let input = [
+            0xFF, 0xFE, 0x00, 0x00, b'h', 0x00, 0x00, 0x00, b'i', 0x00, 0x00, 0x00, b'\n', 0x00,
+            0x00, 0x00,
+        ];
+
+        let err = decode_prompt_bytes(&input).expect_err("utf-32le should be rejected");
+
+        assert_eq!(
+            err,
+            PromptDecodeError::UnsupportedBom {
+                encoding: "UTF-32LE"
+            }
+        );
+    }
+
+    #[test]
+    fn decode_prompt_bytes_rejects_utf32be_bom() {
+        // UTF-32BE BOM + "hi\n"
+        let input = [
+            0x00, 0x00, 0xFE, 0xFF, 0x00, 0x00, 0x00, b'h', 0x00, 0x00, 0x00, b'i', 0x00, 0x00,
+            0x00, b'\n',
+        ];
+
+        let err = decode_prompt_bytes(&input).expect_err("utf-32be should be rejected");
+
+        assert_eq!(
+            err,
+            PromptDecodeError::UnsupportedBom {
+                encoding: "UTF-32BE"
+            }
+        );
+    }
+
+    #[test]
+    fn decode_prompt_bytes_rejects_invalid_utf8() {
+        // Invalid UTF-8 sequence: 0xC3 0x28
+        let input = [0xC3, 0x28];
+
+        let err = decode_prompt_bytes(&input).expect_err("invalid utf-8 should fail");
+
+        assert_eq!(err, PromptDecodeError::InvalidUtf8 { valid_up_to: 0 });
+    }
 }