diff --git a/crates/pdftract-core/src/output/multi.rs b/crates/pdftract-core/src/output/multi.rs
new file mode 100644
index 0000000..e5566bc
--- /dev/null
+++ b/crates/pdftract-core/src/output/multi.rs
@@ -0,0 +1,251 @@
+//! Multi-output specification types.
+//!
+//! This module defines the types for specifying multiple output formats
+//! and destinations for a single extraction pass.
+
+use std::path::PathBuf;
+
+/// Output format type.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum Format {
+    /// Full JSON document format.
+    Json,
+    /// CommonMark Markdown format.
+    Markdown,
+    /// Plain text format.
+    Text,
+    /// Newline-delimited JSON streaming format.
+    Ndjson,
+}
+
+impl Format {
+    /// Returns the file extension for this format.
+    pub fn extension(&self) -> &'static str {
+        match self {
+            Format::Json => "json",
+            Format::Markdown => "md",
+            Format::Text => "txt",
+            Format::Ndjson => "ndjson",
+        }
+    }
+
+    /// Returns the content-type MIME for this format.
+    pub fn content_type(&self) -> &'static str {
+        match self {
+            Format::Json => "application/json",
+            Format::Markdown => "text/markdown; charset=UTF-8",
+            Format::Text => "text/plain; charset=UTF-8",
+            Format::Ndjson => "application/x-ndjson",
+        }
+    }
+
+    /// Parses a format name string.
+    ///
+    /// Accepts: "json", "markdown" (or "md"), "text", "ndjson"
+    pub fn from_name(name: &str) -> Option<Self> {
+        match name.to_lowercase().as_str() {
+            "json" => Some(Format::Json),
+            "markdown" | "md" => Some(Format::Markdown),
+            "text" | "txt" => Some(Format::Text),
+            "ndjson" => Some(Format::Ndjson),
+            _ => None,
+        }
+    }
+
+    /// Returns true if this format can be combined with others in multi-output.
+    ///
+    /// NDJSON is mutually exclusive with other formats because it streams
+    /// page-by-page rather than emitting a whole document.
+    pub fn is_combinable(&self) -> bool {
+        !matches!(self, Format::Ndjson)
+    }
+}
+
+/// Output destination.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Destination {
+    /// Write to a file at the given path.
+    File(PathBuf),
+    /// Write to stdout.
+    Stdout,
+}
+
+impl Destination {
+    /// Returns true if this destination is stdout.
+    pub fn is_stdout(&self) -> bool {
+        matches!(self, Destination::Stdout)
+    }
+}
+
+/// A single output specification combining format and destination.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct OutputSpec {
+    /// The output format.
+    pub format: Format,
+    /// The output destination.
+    pub dest: Destination,
+}
+
+impl OutputSpec {
+    /// Creates a new OutputSpec for writing to a file.
+    pub fn file(format: Format, path: PathBuf) -> Self {
+        OutputSpec {
+            format,
+            dest: Destination::File(path),
+        }
+    }
+
+    /// Creates a new OutputSpec for writing to stdout.
+    pub fn stdout(format: Format) -> Self {
+        OutputSpec {
+            format,
+            dest: Destination::Stdout,
+        }
+    }
+
+    /// Returns true if this spec writes to stdout.
+    pub fn is_stdout(&self) -> bool {
+        self.dest.is_stdout()
+    }
+
+    /// Returns the file path if this spec writes to a file.
+    pub fn file_path(&self) -> Option<&PathBuf> {
+        match &self.dest {
+            Destination::File(path) => Some(path),
+            Destination::Stdout => None,
+        }
+    }
+
+    /// Creates an OutputSpec from a base path and format by appending the format's extension.
+    pub fn from_base(base: PathBuf, format: Format) -> Self {
+        let mut path = base.clone();
+        let ext = format.extension();
+        if !path.as_os_str().to_str().unwrap_or("").ends_with(&format!(".{}", ext)) {
+            path.set_extension(ext);
+        }
+        OutputSpec::file(format, path)
+    }
+}
+
+/// Validation errors for multi-output specifications.
+#[derive(Debug, Clone, thiserror::Error)]
+pub enum ValidationError {
+    /// Duplicate format flag specified.
+    #[error("duplicate --{0} flag (each format may be specified at most once)")]
+    DuplicateFormat(String),
+
+    /// At most one output may go to stdout.
+    #[error("at most one output may use '-' for stdout")]
+    MultipleStdout,
+
+    /// NDJSON cannot be combined with other formats.
+    #[error("--ndjson cannot be combined with other output formats")]
+    NdjsonExclusive,
+
+    /// --format requires -o to specify the output base path.
+    #[error("--format requires -o to specify the output base path")]
+    FormatRequiresOutput,
+
+    /// Invalid format name in --format list.
+    #[error("invalid format '{0}' in --format list (valid: json, markdown, text, ndjson)")]
+    InvalidFormat(String),
+}
+
+/// Validates a list of output specifications.
+///
+/// # Errors
+///
+/// Returns `Err` if any validation rule is violated:
+/// - More than one spec writes to stdout
+/// - NDJSON is combined with other formats
+pub fn validate_outputs(specs: &[OutputSpec]) -> Result<(), ValidationError> {
+    // Check for multiple stdout destinations
+    let stdout_count = specs.iter().filter(|s| s.is_stdout()).count();
+    if stdout_count > 1 {
+        return Err(ValidationError::MultipleStdout);
+    }
+
+    // Check for NDJSON combined with other formats
+    let has_ndjson = specs.iter().any(|s| s.format == Format::Ndjson);
+    if has_ndjson && specs.len() > 1 {
+        return Err(ValidationError::NdjsonExclusive);
+    }
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_format_extension() {
+        assert_eq!(Format::Json.extension(), "json");
+        assert_eq!(Format::Markdown.extension(), "md");
+        assert_eq!(Format::Text.extension(), "txt");
+        assert_eq!(Format::Ndjson.extension(), "ndjson");
+    }
+
+    #[test]
+    fn test_format_from_name() {
+        assert_eq!(Format::from_name("json"), Some(Format::Json));
+        assert_eq!(Format::from_name("JSON"), Some(Format::Json));
+        assert_eq!(Format::from_name("markdown"), Some(Format::Markdown));
+        assert_eq!(Format::from_name("md"), Some(Format::Markdown));
+        assert_eq!(Format::from_name("text"), Some(Format::Text));
+        assert_eq!(Format::from_name("ndjson"), Some(Format::Ndjson));
+        assert_eq!(Format::from_name("invalid"), None);
+    }
+
+    #[test]
+    fn test_output_spec_from_base() {
+        let spec = OutputSpec::from_base(PathBuf::from("out"), Format::Json);
+        assert_eq!(spec.file_path(), Some(&PathBuf::from("out.json")));
+
+        let spec = OutputSpec::from_base(PathBuf::from("out"), Format::Markdown);
+        assert_eq!(spec.file_path(), Some(&PathBuf::from("out.md")));
+
+        let spec = OutputSpec::from_base(PathBuf::from("out.json"), Format::Json);
+        // Already has .json extension, shouldn't double it
+        assert_eq!(spec.file_path(), Some(&PathBuf::from("out.json")));
+    }
+
+    #[test]
+    fn test_validate_outputs_accepts_single() {
+        let specs = vec![OutputSpec::stdout(Format::Json)];
+        assert!(validate_outputs(&specs).is_ok());
+    }
+
+    #[test]
+    fn test_validate_outputs_accepts_multiple_files() {
+        let specs = vec![
+            OutputSpec::file(Format::Json, PathBuf::from("a.json")),
+            OutputSpec::file(Format::Markdown, PathBuf::from("b.md")),
+        ];
+        assert!(validate_outputs(&specs).is_ok());
+    }
+
+    #[test]
+    fn test_validate_outputs_rejects_multiple_stdout() {
+        let specs = vec![
+            OutputSpec::stdout(Format::Json),
+            OutputSpec::stdout(Format::Markdown),
+        ];
+        assert_eq!(
+            validate_outputs(&specs),
+            Err(ValidationError::MultipleStdout)
+        );
+    }
+
+    #[test]
+    fn test_validate_outputs_rejects_ndjson_combined() {
+        let specs = vec![
+            OutputSpec::file(Format::Ndjson, PathBuf::from("out.ndjson")),
+            OutputSpec::file(Format::Markdown, PathBuf::from("out.md")),
+        ];
+        assert_eq!(
+            validate_outputs(&specs),
+            Err(ValidationError::NdjsonExclusive)
+        );
+    }
+}