diff --git a/codex-rs/Cargo.lock b/codex-rs/Cargo.lock
index 2320afeecb..8e27a1655b 100644
--- a/codex-rs/Cargo.lock
+++ b/codex-rs/Cargo.lock
@@ -1281,6 +1281,7 @@ dependencies = [
  "serde_json",
  "shlex",
  "starlark",
+ "tempfile",
  "thiserror 2.0.17",
 ]
 
diff --git a/codex-rs/execpolicy/Cargo.toml b/codex-rs/execpolicy/Cargo.toml
index 77278bb118..890c23faa7 100644
--- a/codex-rs/execpolicy/Cargo.toml
+++ b/codex-rs/execpolicy/Cargo.toml
@@ -28,3 +28,4 @@ thiserror = { workspace = true }
 
 [dev-dependencies]
 pretty_assertions = { workspace = true }
+tempfile = { workspace = true }
diff --git a/codex-rs/execpolicy/src/amend.rs b/codex-rs/execpolicy/src/amend.rs
new file mode 100644
index 0000000000..36a02cebf9
--- /dev/null
+++ b/codex-rs/execpolicy/src/amend.rs
@@ -0,0 +1,226 @@
+use std::fs::OpenOptions;
+use std::io::Read;
+use std::io::Seek;
+use std::io::SeekFrom;
+use std::io::Write;
+use std::path::Path;
+use std::path::PathBuf;
+
+use serde_json;
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum AmendError {
+    #[error("prefix rule requires at least one token")]
+    EmptyPrefix,
+    #[error("policy path has no parent: {path}")]
+    MissingParent { path: PathBuf },
+    #[error("failed to create policy directory {dir}: {source}")]
+    CreatePolicyDir {
+        dir: PathBuf,
+        source: std::io::Error,
+    },
+    #[error("failed to format prefix tokens: {source}")]
+    SerializePrefix { source: serde_json::Error },
+    #[error("failed to open policy file {path}: {source}")]
+    OpenPolicyFile {
+        path: PathBuf,
+        source: std::io::Error,
+    },
+    #[error("failed to write to policy file {path}: {source}")]
+    WritePolicyFile {
+        path: PathBuf,
+        source: std::io::Error,
+    },
+    #[error("failed to lock policy file {path}: {source}")]
+    LockPolicyFile {
+        path: PathBuf,
+        source: std::io::Error,
+    },
+    #[error("failed to seek policy file {path}: {source}")]
+    SeekPolicyFile {
+        path: PathBuf,
+        source: std::io::Error,
+    },
+    #[error("failed to read policy file {path}: {source}")]
+    ReadPolicyFile {
+        path: PathBuf,
+        source: std::io::Error,
+    },
+    #[error("failed to read metadata for policy file {path}: {source}")]
+    PolicyMetadata {
+        path: PathBuf,
+        source: std::io::Error,
+    },
+}
+
+/// Note this thread uses advisory file locking and performs blocking I/O, so it should be used with
+/// [`tokio::task::spawn_blocking`] when called from an async context.
+pub fn blocking_append_allow_prefix_rule(
+    policy_path: &Path,
+    prefix: &[String],
+) -> Result<(), AmendError> {
+    if prefix.is_empty() {
+        return Err(AmendError::EmptyPrefix);
+    }
+
+    let tokens = prefix
+        .iter()
+        .map(serde_json::to_string)
+        .collect::<Result<Vec<_>, _>>()
+        .map_err(|source| AmendError::SerializePrefix { source })?;
+    let pattern = format!("[{}]", tokens.join(", "));
+    let rule = format!(r#"prefix_rule(pattern={pattern}, decision="allow")"#);
+
+    let dir = policy_path
+        .parent()
+        .ok_or_else(|| AmendError::MissingParent {
+            path: policy_path.to_path_buf(),
+        })?;
+    match std::fs::create_dir(dir) {
+        Ok(()) => {}
+        Err(ref source) if source.kind() == std::io::ErrorKind::AlreadyExists => {}
+        Err(source) => {
+            return Err(AmendError::CreatePolicyDir {
+                dir: dir.to_path_buf(),
+                source,
+            });
+        }
+    }
+    append_locked_line(policy_path, &rule)
+}
+
+fn append_locked_line(policy_path: &Path, line: &str) -> Result<(), AmendError> {
+    let mut file = OpenOptions::new()
+        .create(true)
+        .read(true)
+        .append(true)
+        .open(policy_path)
+        .map_err(|source| AmendError::OpenPolicyFile {
+            path: policy_path.to_path_buf(),
+            source,
+        })?;
+    file.lock().map_err(|source| AmendError::LockPolicyFile {
+        path: policy_path.to_path_buf(),
+        source,
+    })?;
+
+    let len = file
+        .metadata()
+        .map_err(|source| AmendError::PolicyMetadata {
+            path: policy_path.to_path_buf(),
+            source,
+        })?
+        .len();
+
+    // Ensure file ends in a newline before appending.
+    if len > 0 {
+        file.seek(SeekFrom::End(-1))
+            .map_err(|source| AmendError::SeekPolicyFile {
+                path: policy_path.to_path_buf(),
+                source,
+            })?;
+        let mut last = [0; 1];
+        file.read_exact(&mut last)
+            .map_err(|source| AmendError::ReadPolicyFile {
+                path: policy_path.to_path_buf(),
+                source,
+            })?;
+
+        if last[0] != b'\n' {
+            file.write_all(b"\n")
+                .map_err(|source| AmendError::WritePolicyFile {
+                    path: policy_path.to_path_buf(),
+                    source,
+                })?;
+        }
+    }
+
+    file.write_all(format!("{line}\n").as_bytes())
+        .map_err(|source| AmendError::WritePolicyFile {
+            path: policy_path.to_path_buf(),
+            source,
+        })?;
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+    use tempfile::tempdir;
+
+    #[test]
+    fn appends_rule_and_creates_directories() {
+        let tmp = tempdir().expect("create temp dir");
+        let policy_path = tmp.path().join("policy").join("default.codexpolicy");
+
+        blocking_append_allow_prefix_rule(
+            &policy_path,
+            &[String::from("echo"), String::from("Hello, world!")],
+        )
+        .expect("append rule");
+
+        let contents =
+            std::fs::read_to_string(&policy_path).expect("default.codexpolicy should exist");
+        assert_eq!(
+            contents,
+            r#"prefix_rule(pattern=["echo", "Hello, world!"], decision="allow")
+"#
+        );
+    }
+
+    #[test]
+    fn appends_rule_without_duplicate_newline() {
+        let tmp = tempdir().expect("create temp dir");
+        let policy_path = tmp.path().join("policy").join("default.codexpolicy");
+        std::fs::create_dir_all(policy_path.parent().unwrap()).expect("create policy dir");
+        std::fs::write(
+            &policy_path,
+            r#"prefix_rule(pattern=["ls"], decision="allow")
+"#,
+        )
+        .expect("write seed rule");
+
+        blocking_append_allow_prefix_rule(
+            &policy_path,
+            &[String::from("echo"), String::from("Hello, world!")],
+        )
+        .expect("append rule");
+
+        let contents = std::fs::read_to_string(&policy_path).expect("read policy");
+        assert_eq!(
+            contents,
+            r#"prefix_rule(pattern=["ls"], decision="allow")
+prefix_rule(pattern=["echo", "Hello, world!"], decision="allow")
+"#
+        );
+    }
+
+    #[test]
+    fn inserts_newline_when_missing_before_append() {
+        let tmp = tempdir().expect("create temp dir");
+        let policy_path = tmp.path().join("policy").join("default.codexpolicy");
+        std::fs::create_dir_all(policy_path.parent().unwrap()).expect("create policy dir");
+        std::fs::write(
+            &policy_path,
+            r#"prefix_rule(pattern=["ls"], decision="allow")"#,
+        )
+        .expect("write seed rule without newline");
+
+        blocking_append_allow_prefix_rule(
+            &policy_path,
+            &[String::from("echo"), String::from("Hello, world!")],
+        )
+        .expect("append rule");
+
+        let contents = std::fs::read_to_string(&policy_path).expect("read policy");
+        assert_eq!(
+            contents,
+            r#"prefix_rule(pattern=["ls"], decision="allow")
+prefix_rule(pattern=["echo", "Hello, world!"], decision="allow")
+"#
+        );
+    }
+}
diff --git a/codex-rs/execpolicy/src/lib.rs b/codex-rs/execpolicy/src/lib.rs
index b459caea16..31062f1cb6 100644
--- a/codex-rs/execpolicy/src/lib.rs
+++ b/codex-rs/execpolicy/src/lib.rs
@@ -1,3 +1,4 @@
+pub mod amend;
 pub mod decision;
 pub mod error;
 pub mod execpolicycheck;
@@ -5,6 +6,8 @@ pub mod parser;
 pub mod policy;
 pub mod rule;
 
+pub use amend::AmendError;
+pub use amend::blocking_append_allow_prefix_rule;
 pub use decision::Decision;
 pub use error::Error;
 pub use error::Result;
diff --git a/codex-rs/execpolicy/src/policy.rs b/codex-rs/execpolicy/src/policy.rs
index e048fce1ff..10858c9fad 100644
--- a/codex-rs/execpolicy/src/policy.rs
+++ b/codex-rs/execpolicy/src/policy.rs
@@ -1,9 +1,15 @@
 use crate::decision::Decision;
+use crate::error::Error;
+use crate::error::Result;
+use crate::rule::PatternToken;
+use crate::rule::PrefixPattern;
+use crate::rule::PrefixRule;
 use crate::rule::RuleMatch;
 use crate::rule::RuleRef;
 use multimap::MultiMap;
 use serde::Deserialize;
 use serde::Serialize;
+use std::sync::Arc;
 
 #[derive(Clone, Debug)]
 pub struct Policy {
@@ -23,6 +29,27 @@ impl Policy {
         &self.rules_by_program
     }
 
+    pub fn add_prefix_rule(&mut self, prefix: &[String], decision: Decision) -> Result<()> {
+        let (first_token, rest) = prefix
+            .split_first()
+            .ok_or_else(|| Error::InvalidPattern("prefix cannot be empty".to_string()))?;
+
+        let rule: RuleRef = Arc::new(PrefixRule {
+            pattern: PrefixPattern {
+                first: Arc::from(first_token.as_str()),
+                rest: rest
+                    .iter()
+                    .map(|token| PatternToken::Single(token.clone()))
+                    .collect::<Vec<_>>()
+                    .into(),
+            },
+            decision,
+        });
+
+        self.rules_by_program.insert(first_token.clone(), rule);
+        Ok(())
+    }
+
     pub fn check(&self, cmd: &[String]) -> Evaluation {
         let rules = match cmd.first() {
             Some(first) => match self.rules_by_program.get_vec(first) {
diff --git a/codex-rs/execpolicy/tests/basic.rs b/codex-rs/execpolicy/tests/basic.rs
index e4189caa21..9a7ec58b1e 100644
--- a/codex-rs/execpolicy/tests/basic.rs
+++ b/codex-rs/execpolicy/tests/basic.rs
@@ -1,8 +1,12 @@
 use std::any::Any;
 use std::sync::Arc;
 
+use anyhow::Context;
+use anyhow::Result;
 use codex_execpolicy::Decision;
+use codex_execpolicy::Error;
 use codex_execpolicy::Evaluation;
+use codex_execpolicy::Policy;
 use codex_execpolicy::PolicyParser;
 use codex_execpolicy::RuleMatch;
 use codex_execpolicy::RuleRef;
@@ -35,16 +39,14 @@ fn rule_snapshots(rules: &[RuleRef]) -> Vec<RuleSnapshot> {
 }
 
 #[test]
-fn basic_match() {
+fn basic_match() -> Result<()> {
     let policy_src = r#"
 prefix_rule(
     pattern = ["git", "status"],
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser
-        .parse("test.codexpolicy", policy_src)
-        .expect("parse policy");
+    parser.parse("test.codexpolicy", policy_src)?;
     let policy = parser.build();
     let cmd = tokens(&["git", "status"]);
     let evaluation = policy.check(&cmd);
@@ -58,10 +60,54 @@ prefix_rule(
         },
         evaluation
     );
+    Ok(())
 }
 
 #[test]
-fn parses_multiple_policy_files() {
+fn add_prefix_rule_extends_policy() -> Result<()> {
+    let mut policy = Policy::empty();
+    policy.add_prefix_rule(&tokens(&["ls", "-l"]), Decision::Prompt)?;
+
+    let rules = rule_snapshots(policy.rules().get_vec("ls").context("missing ls rules")?);
+    assert_eq!(
+        vec![RuleSnapshot::Prefix(PrefixRule {
+            pattern: PrefixPattern {
+                first: Arc::from("ls"),
+                rest: vec![PatternToken::Single(String::from("-l"))].into(),
+            },
+            decision: Decision::Prompt,
+        })],
+        rules
+    );
+
+    let evaluation = policy.check(&tokens(&["ls", "-l", "/tmp"]));
+    assert_eq!(
+        Evaluation::Match {
+            decision: Decision::Prompt,
+            matched_rules: vec![RuleMatch::PrefixRuleMatch {
+                matched_prefix: tokens(&["ls", "-l"]),
+                decision: Decision::Prompt,
+            }],
+        },
+        evaluation
+    );
+    Ok(())
+}
+
+#[test]
+fn add_prefix_rule_rejects_empty_prefix() -> Result<()> {
+    let mut policy = Policy::empty();
+    let result = policy.add_prefix_rule(&[], Decision::Allow);
+
+    match result.unwrap_err() {
+        Error::InvalidPattern(message) => assert_eq!(message, "prefix cannot be empty"),
+        other => panic!("expected InvalidPattern(..), got {other:?}"),
+    }
+    Ok(())
+}
+
+#[test]
+fn parses_multiple_policy_files() -> Result<()> {
     let first_policy = r#"
 prefix_rule(
     pattern = ["git"],
@@ -75,15 +121,11 @@ prefix_rule(
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser
-        .parse("first.codexpolicy", first_policy)
-        .expect("parse policy");
-    parser
-        .parse("second.codexpolicy", second_policy)
-        .expect("parse policy");
+    parser.parse("first.codexpolicy", first_policy)?;
+    parser.parse("second.codexpolicy", second_policy)?;
     let policy = parser.build();
 
-    let git_rules = rule_snapshots(policy.rules().get_vec("git").expect("git rules"));
+    let git_rules = rule_snapshots(policy.rules().get_vec("git").context("missing git rules")?);
     assert_eq!(
         vec![
             RuleSnapshot::Prefix(PrefixRule {
@@ -133,23 +175,27 @@ prefix_rule(
         },
         commit_eval
     );
+    Ok(())
 }
 
 #[test]
-fn only_first_token_alias_expands_to_multiple_rules() {
+fn only_first_token_alias_expands_to_multiple_rules() -> Result<()> {
     let policy_src = r#"
 prefix_rule(
     pattern = [["bash", "sh"], ["-c", "-l"]],
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser
-        .parse("test.codexpolicy", policy_src)
-        .expect("parse policy");
+    parser.parse("test.codexpolicy", policy_src)?;
     let policy = parser.build();
 
-    let bash_rules = rule_snapshots(policy.rules().get_vec("bash").expect("bash rules"));
-    let sh_rules = rule_snapshots(policy.rules().get_vec("sh").expect("sh rules"));
+    let bash_rules = rule_snapshots(
+        policy
+            .rules()
+            .get_vec("bash")
+            .context("missing bash rules")?,
+    );
+    let sh_rules = rule_snapshots(policy.rules().get_vec("sh").context("missing sh rules")?);
     assert_eq!(
         vec![RuleSnapshot::Prefix(PrefixRule {
             pattern: PrefixPattern {
@@ -194,22 +240,21 @@ prefix_rule(
         },
         sh_eval
     );
+    Ok(())
 }
 
 #[test]
-fn tail_aliases_are_not_cartesian_expanded() {
+fn tail_aliases_are_not_cartesian_expanded() -> Result<()> {
     let policy_src = r#"
 prefix_rule(
     pattern = ["npm", ["i", "install"], ["--legacy-peer-deps", "--no-save"]],
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser
-        .parse("test.codexpolicy", policy_src)
-        .expect("parse policy");
+    parser.parse("test.codexpolicy", policy_src)?;
     let policy = parser.build();
 
-    let rules = rule_snapshots(policy.rules().get_vec("npm").expect("npm rules"));
+    let rules = rule_snapshots(policy.rules().get_vec("npm").context("missing npm rules")?);
     assert_eq!(
         vec![RuleSnapshot::Prefix(PrefixRule {
             pattern: PrefixPattern {
@@ -251,10 +296,11 @@ prefix_rule(
         },
         npm_install
     );
+    Ok(())
 }
 
 #[test]
-fn match_and_not_match_examples_are_enforced() {
+fn match_and_not_match_examples_are_enforced() -> Result<()> {
     let policy_src = r#"
 prefix_rule(
     pattern = ["git", "status"],
@@ -266,9 +312,7 @@ prefix_rule(
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser
-        .parse("test.codexpolicy", policy_src)
-        .expect("parse policy");
+    parser.parse("test.codexpolicy", policy_src)?;
     let policy = parser.build();
     let match_eval = policy.check(&tokens(&["git", "status"]));
     assert_eq!(
@@ -289,10 +333,11 @@ prefix_rule(
         "status",
     ]));
     assert_eq!(Evaluation::NoMatch {}, no_match_eval);
+    Ok(())
 }
 
 #[test]
-fn strictest_decision_wins_across_matches() {
+fn strictest_decision_wins_across_matches() -> Result<()> {
     let policy_src = r#"
 prefix_rule(
     pattern = ["git"],
@@ -304,9 +349,7 @@ prefix_rule(
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser
-        .parse("test.codexpolicy", policy_src)
-        .expect("parse policy");
+    parser.parse("test.codexpolicy", policy_src)?;
     let policy = parser.build();
 
     let commit = policy.check(&tokens(&["git", "commit", "-m", "hi"]));
@@ -326,10 +369,11 @@ prefix_rule(
         },
         commit
     );
+    Ok(())
 }
 
 #[test]
-fn strictest_decision_across_multiple_commands() {
+fn strictest_decision_across_multiple_commands() -> Result<()> {
     let policy_src = r#"
 prefix_rule(
     pattern = ["git"],
@@ -341,9 +385,7 @@ prefix_rule(
 )
     "#;
     let mut parser = PolicyParser::new();
-    parser
-        .parse("test.codexpolicy", policy_src)
-        .expect("parse policy");
+    parser.parse("test.codexpolicy", policy_src)?;
     let policy = parser.build();
 
     let commands = vec![
@@ -372,4 +414,5 @@ prefix_rule(
         },
         evaluation
     );
+    Ok(())
 }