feat: add support for -c/--config to override individual config items

codex-rs/common/Cargo.toml

@@ -9,6 +9,7 @@ workspace = true
 [dependencies]
 clap = { version = "4", features = ["derive", "wrap_help"], optional = true }
 codex-core = { path = "../core" }
+serde_json = { version = "1" }
 
 [features]
 # Separate feature so that `clap` is not a mandatory dependency.

codex-rs/common/src/config_override.rs

@@ -0,0 +1,130 @@
+//! Support for `-c key=value` overrides shared across Codex CLI tools.
+//!
+//! This module provides a [`CliConfigOverrides`] struct that can be embedded
+//! into a `clap`-derived CLI struct using `#[clap(flatten)]`. Each occurrence
+//! of `-c key=value` (or `--config key=value`) will be collected as a raw
+//! string. Helper methods are provided to convert the raw strings into
+//! key/value pairs as well as to apply them onto a mutable
+//! `serde_json::Value` representing the configuration tree.
+
+use clap::ArgAction;
+use clap::Parser;
+use serde_json::Value;
+
+/// CLI option that captures arbitrary configuration overrides specified as
+/// `-c key=value`. It intentionally keeps both halves **unparsed** so that the
+/// calling code can decide how to interpret the right-hand side.
+#[derive(Parser, Debug, Default, Clone)]
+pub struct CliConfigOverrides {
+    /// Override a configuration value that would otherwise be loaded from
+    /// `~/.codex/config.toml`. Use a dotted path (`foo.bar.baz`) to override
+    /// nested values. The `value` portion is parsed as JSON. If it fails to
+    /// parse as JSON, the raw string is used as a literal.
+    ///
+    /// Examples:
+    ///   - `-c model="o4-mini"`
+    ///   - `-c sandbox-permissions=[\"disk-write-cwd\"]`
+    ///   - `-c history.max-lines=0`
+    #[arg(
+        short = 'c',
+        long = "config",
+        value_name = "key=value",
+        action = ArgAction::Append,
+    )]
+    pub raw_overrides: Vec<String>,
+}
+
+impl CliConfigOverrides {
+    /// Parse the raw strings captured from the CLI into a list of `(path,
+    /// value)` tuples where `value` is a `serde_json::Value`.
+    pub fn parse_overrides(&self) -> Result<Vec<(String, Value)>, String> {
+        self.raw_overrides
+            .iter()
+            .map(|s| {
+                // Only split on the *first* '=' so values are free to contain
+                // the character.
+                let mut parts = s.splitn(2, '=');
+                let key = match parts.next() {
+                    Some(k) => k.trim(),
+                    None => return Err("Override missing key".to_string()),
+                };
+                let value_str = parts
+                    .next()
+                    .ok_or_else(|| format!("Invalid override (missing '='): {s}"))?
+                    .trim();
+
+                if key.is_empty() {
+                    return Err(format!("Empty key in override: {s}"));
+                }
+
+                // Attempt to parse as JSON. If that fails, treat it as a raw
+                // string. This allows convenient usage such as
+                // `-c model=o4-mini` without the quotes.
+                let value: Value = match serde_json::from_str(value_str) {
+                    Ok(v) => v,
+                    Err(_) => Value::String(value_str.to_string()),
+                };
+
+                Ok((key.to_string(), value))
+            })
+            .collect()
+    }
+
+    /// Apply all parsed overrides onto `target`. Intermediate objects will be
+    /// created as necessary. Values located at the destination path will be
+    /// replaced.
+    pub fn apply_on_value(&self, target: &mut Value) -> Result<(), String> {
+        let overrides = self.parse_overrides()?;
+        for (path, value) in overrides {
+            apply_single_override(target, &path, value);
+        }
+        Ok(())
+    }
+}
+
+/// Apply a single override onto `root`, creating intermediate objects as
+/// necessary.
+fn apply_single_override(root: &mut Value, path: &str, value: Value) {
+    use serde_json::Map;
+
+    let parts: Vec<&str> = path.split('.').collect();
+    let mut current = root;
+
+    for (i, part) in parts.iter().enumerate() {
+        let is_last = i == parts.len() - 1;
+
+        if is_last {
+            // Replace value at leaf.
+            if let Value::Object(obj) = current {
+                obj.insert(part.to_string(), value);
+            } else {
+                // Replace non-object with object containing the leaf.
+                *current = Value::Object({
+                    let mut m = Map::new();
+                    m.insert(part.to_string(), value);
+                    m
+                });
+            }
+            return;
+        }
+
+        // Traverse or create intermediate object.
+        match current {
+            Value::Object(obj) => {
+                current = obj
+                    .entry(part.to_string())
+                    .or_insert_with(|| Value::Object(Map::new()));
+            }
+            _ => {
+                // Non-object encountered, replace with object so we can
+                // continue traversal.
+                *current = Value::Object(Map::new());
+                if let Value::Object(obj) = current {
+                    current = obj
+                        .entry((*part).to_string())
+                        .or_insert_with(|| Value::Object(Map::new()));
+                }
+            }
+        }
+    }
+}

codex-rs/common/src/lib.rs

@@ -8,3 +8,9 @@ pub mod elapsed;
 pub use approval_mode_cli_arg::ApprovalModeCliArg;
 #[cfg(feature = "cli")]
 pub use approval_mode_cli_arg::SandboxPermissionOption;
+
+#[cfg(feature = "cli")]
+mod config_override;
+
+#[cfg(feature = "cli")]
+pub use config_override::CliConfigOverrides;