diff --git a/codex-rs/Cargo.lock b/codex-rs/Cargo.lock
index 17bab1e24b..adead3ac98 100644
--- a/codex-rs/Cargo.lock
+++ b/codex-rs/Cargo.lock
@@ -3846,6 +3846,7 @@ dependencies = [
  "pretty_assertions",
  "serde",
  "serde_json",
+ "toml 0.9.11+spec-1.1.0",
 ]
 
 [[package]]
diff --git a/codex-rs/schema-evolution/Cargo.toml b/codex-rs/schema-evolution/Cargo.toml
index 3dc8e0247d..dd6011ca0e 100644
--- a/codex-rs/schema-evolution/Cargo.toml
+++ b/codex-rs/schema-evolution/Cargo.toml
@@ -16,6 +16,7 @@ workspace = true
 anyhow = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
+toml = { workspace = true }
 
 [dev-dependencies]
 pretty_assertions = { workspace = true }
diff --git a/codex-rs/schema-evolution/src/known_breakage.rs b/codex-rs/schema-evolution/src/known_breakage.rs
new file mode 100644
index 0000000000..d8d35cf36b
--- /dev/null
+++ b/codex-rs/schema-evolution/src/known_breakage.rs
@@ -0,0 +1,276 @@
+use crate::ApiSchema;
+use crate::SchemaBreakage;
+use crate::ViolationKind;
+use crate::find_request_narrowing;
+use anyhow::Context;
+use anyhow::Result;
+use anyhow::bail;
+use serde::Deserialize;
+use serde::Serialize;
+use serde_json::Value;
+
+const KNOWN_BREAKAGE_LOG_VERSION: u32 = 1;
+
+#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
+#[serde(deny_unknown_fields)]
+pub struct KnownBreakage {
+    pub id: u64,
+    pub kind: ViolationKind,
+    pub method: String,
+    pub path: String,
+    pub before_json: String,
+    pub after_json: String,
+    pub justification: String,
+}
+
+#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
+#[serde(deny_unknown_fields)]
+pub struct KnownBreakageLog {
+    version: u32,
+    #[serde(default)]
+    breakages: Vec<KnownBreakage>,
+}
+
+#[derive(Serialize)]
+struct KnownBreakageTemplate<'a> {
+    breakages: &'a [KnownBreakage],
+}
+
+impl KnownBreakage {
+    fn from_breakage(id: u64, breakage: &SchemaBreakage) -> Self {
+        Self {
+            id,
+            kind: breakage.kind.clone(),
+            method: breakage.method.clone(),
+            path: breakage.path.clone(),
+            before_json: canonical_json(&breakage.before),
+            after_json: canonical_json(&breakage.after),
+            justification: String::new(),
+        }
+    }
+
+    fn matches(&self, breakage: &SchemaBreakage) -> bool {
+        self.kind == breakage.kind
+            && self.method == breakage.method
+            && self.path == breakage.path
+            && self.before_json == canonical_json(&breakage.before)
+            && self.after_json == canonical_json(&breakage.after)
+    }
+}
+
+impl KnownBreakageLog {
+    /// Parses one versioned, append-only known-breakage log.
+    pub fn parse(contents: &str, label: &str) -> Result<Self> {
+        toml::from_str(contents).with_context(|| format!("parse known-breakage log {label}"))
+    }
+}
+
+/// Checks request-schema narrowing and verifies that each breakage was newly recorded.
+///
+/// The earlier log must be an exact prefix of the current log. This makes the log an
+/// append-only history and prevents an older entry from acknowledging a new change.
+pub fn check_request_narrowing(
+    before: &ApiSchema,
+    after: &ApiSchema,
+    before_log: &KnownBreakageLog,
+    after_log: &KnownBreakageLog,
+) -> Result<Vec<SchemaBreakage>> {
+    let breakages = find_request_narrowing(before, after)?;
+    let problems = known_breakage_problems(before_log, after_log, &breakages);
+    if !problems.is_empty() {
+        bail!(failure_report(
+            &problems, &breakages, before_log, after_log
+        )?);
+    }
+    Ok(breakages)
+}
+
+fn known_breakage_problems(
+    before_log: &KnownBreakageLog,
+    after_log: &KnownBreakageLog,
+    breakages: &[SchemaBreakage],
+) -> Vec<String> {
+    let mut problems = validate_log("before", before_log);
+    problems.extend(validate_log("after", after_log));
+
+    for (index, previous) in before_log.breakages.iter().enumerate() {
+        match after_log.breakages.get(index) {
+            Some(current) if current == previous => {}
+            Some(_) => problems.push(format!(
+                "known breakage {} was edited or reordered; existing entries are append-only",
+                previous.id
+            )),
+            None => problems.push(format!(
+                "known breakage {} was deleted; existing entries are append-only",
+                previous.id
+            )),
+        }
+    }
+
+    let appended = after_log
+        .breakages
+        .get(before_log.breakages.len()..)
+        .unwrap_or_default();
+    for breakage in breakages {
+        match appended
+            .iter()
+            .filter(|entry| entry.matches(breakage))
+            .count()
+        {
+            0 => problems.push(format!(
+                "missing a new known-breakage entry for {} {:?} at {}",
+                breakage.method, breakage.kind, breakage.path
+            )),
+            1 => {}
+            _ => problems.push(format!(
+                "multiple new known-breakage entries match {} at {}",
+                breakage.method, breakage.path
+            )),
+        }
+    }
+    for entry in appended {
+        if !breakages.iter().any(|breakage| entry.matches(breakage)) {
+            problems.push(format!(
+                "new known breakage {} does not match a detected request-schema breakage",
+                entry.id
+            ));
+        }
+    }
+    problems
+}
+
+fn validate_log(label: &str, log: &KnownBreakageLog) -> Vec<String> {
+    let mut problems = Vec::new();
+    if log.version != KNOWN_BREAKAGE_LOG_VERSION {
+        problems.push(format!(
+            "{label} known-breakage log must use version {KNOWN_BREAKAGE_LOG_VERSION}"
+        ));
+    }
+    for (index, entry) in log.breakages.iter().enumerate() {
+        let expected_id = index as u64 + 1;
+        if entry.id != expected_id {
+            problems.push(format!(
+                "{label} known-breakage entry {} must have id {expected_id}",
+                entry.id
+            ));
+        }
+        if entry.justification.trim().is_empty() {
+            problems.push(format!(
+                "{label} known breakage {} needs a justification",
+                entry.id
+            ));
+        }
+        if entry.method.trim().is_empty() {
+            problems.push(format!(
+                "{label} known breakage {} needs a method",
+                entry.id
+            ));
+        }
+        if entry.path.trim().is_empty() {
+            problems.push(format!("{label} known breakage {} needs a path", entry.id));
+        }
+        validate_snapshot(
+            label,
+            entry,
+            "before_json",
+            &entry.before_json,
+            &mut problems,
+        );
+        validate_snapshot(label, entry, "after_json", &entry.after_json, &mut problems);
+    }
+    problems
+}
+
+fn validate_snapshot(
+    label: &str,
+    entry: &KnownBreakage,
+    field: &str,
+    snapshot: &str,
+    problems: &mut Vec<String>,
+) {
+    if serde_json::from_str::<Value>(snapshot).is_err() {
+        problems.push(format!(
+            "{label} known breakage {} has invalid JSON in {field}",
+            entry.id
+        ));
+    }
+}
+
+fn failure_report(
+    problems: &[String],
+    breakages: &[SchemaBreakage],
+    before_log: &KnownBreakageLog,
+    after_log: &KnownBreakageLog,
+) -> Result<String> {
+    let mut report = String::from("request schema compatibility lint failed:\n");
+    for problem in problems {
+        report.push_str(&format!("- {problem}\n"));
+    }
+    for breakage in breakages {
+        report.push_str(&format!(
+            "- {} {:?} at {}: {} -> {}\n",
+            breakage.method, breakage.kind, breakage.path, breakage.before, breakage.after
+        ));
+    }
+
+    let missing = if log_can_be_extended(before_log, after_log, breakages) {
+        let appended = &after_log.breakages[before_log.breakages.len()..];
+        breakages
+            .iter()
+            .filter(|breakage| !appended.iter().any(|entry| entry.matches(breakage)))
+            .enumerate()
+            .map(|(index, breakage)| {
+                KnownBreakage::from_breakage(
+                    after_log.breakages.len() as u64 + index as u64 + 1,
+                    breakage,
+                )
+            })
+            .collect::<Vec<_>>()
+    } else {
+        Vec::new()
+    };
+    if !missing.is_empty() {
+        report.push_str(&format!(
+            "\nappend one entry per breakage to the known-breakage log:\n{}\n",
+            toml::to_string_pretty(&KnownBreakageTemplate {
+                breakages: &missing
+            })?
+        ));
+    }
+    Ok(report)
+}
+
+fn log_can_be_extended(
+    before_log: &KnownBreakageLog,
+    after_log: &KnownBreakageLog,
+    breakages: &[SchemaBreakage],
+) -> bool {
+    if !validate_log("before", before_log).is_empty()
+        || !validate_log("after", after_log).is_empty()
+        || !after_log.breakages.starts_with(&before_log.breakages)
+    {
+        return false;
+    }
+    let appended = &after_log.breakages[before_log.breakages.len()..];
+    appended.iter().all(|entry| {
+        breakages
+            .iter()
+            .filter(|breakage| entry.matches(breakage))
+            .count()
+            == 1
+    }) && breakages.iter().all(|breakage| {
+        appended
+            .iter()
+            .filter(|entry| entry.matches(breakage))
+            .count()
+            <= 1
+    })
+}
+
+fn canonical_json(value: &Value) -> String {
+    serde_json::to_string(value).unwrap_or_default()
+}
+
+#[cfg(test)]
+#[path = "known_breakage_tests.rs"]
+mod tests;
diff --git a/codex-rs/schema-evolution/src/known_breakage_tests.rs b/codex-rs/schema-evolution/src/known_breakage_tests.rs
new file mode 100644
index 0000000000..587806a49e
--- /dev/null
+++ b/codex-rs/schema-evolution/src/known_breakage_tests.rs
@@ -0,0 +1,184 @@
+use super::*;
+use crate::test_support::request_schema;
+use pretty_assertions::assert_eq;
+use serde_json::json;
+
+#[test]
+fn only_new_exact_entries_acknowledge_breakages() -> Result<()> {
+    let (before, after, breakage) = type_narrowing()?;
+    let before_log = log(Vec::new());
+    let after_log = log(vec![known(/*id*/ 1, &breakage)]);
+
+    assert_eq!(
+        check_request_narrowing(&before, &after, &before_log, &after_log)?,
+        vec![breakage]
+    );
+    Ok(())
+}
+
+#[test]
+fn existing_entries_form_a_complete_sequential_prefix() {
+    let first = old_breakage(/*id*/ 1, "test/old");
+    let before = log(vec![first.clone()]);
+    let after = log(Vec::new());
+
+    assert_eq!(
+        known_breakage_problems(&before, &after, &[]),
+        vec!["known breakage 1 was deleted; existing entries are append-only".to_string()]
+    );
+
+    let out_of_sequence = log(vec![KnownBreakage { id: 2, ..first }]);
+    assert_eq!(
+        validate_log("after", &out_of_sequence),
+        vec!["after known-breakage entry 2 must have id 1".to_string()]
+    );
+}
+
+#[test]
+fn historical_entries_cannot_acknowledge_a_new_diff_and_cannot_be_edited() -> Result<()> {
+    let (_, _, breakage) = type_narrowing()?;
+    let historical = known(/*id*/ 1, &breakage);
+    let before = log(vec![historical.clone()]);
+
+    assert_eq!(
+        known_breakage_problems(
+            &before,
+            &log(vec![historical.clone()]),
+            std::slice::from_ref(&breakage),
+        ),
+        vec![
+            "missing a new known-breakage entry for test/method TypeNarrowed at params".to_string()
+        ]
+    );
+
+    let edited = KnownBreakage {
+        justification: "a different justification".to_string(),
+        ..historical
+    };
+    assert_eq!(
+        known_breakage_problems(&before, &log(vec![edited]), &[]),
+        vec![
+            "known breakage 1 was edited or reordered; existing entries are append-only"
+                .to_string()
+        ]
+    );
+    Ok(())
+}
+
+#[test]
+fn stale_mismatched_and_duplicate_new_entries_are_rejected() -> Result<()> {
+    let (_, _, breakage) = type_narrowing()?;
+    let stale = KnownBreakage {
+        method: "test/other".to_string(),
+        ..known(/*id*/ 1, &breakage)
+    };
+    assert_eq!(
+        known_breakage_problems(
+            &log(Vec::new()),
+            &log(vec![stale]),
+            std::slice::from_ref(&breakage),
+        ),
+        vec![
+            "missing a new known-breakage entry for test/method TypeNarrowed at params".to_string(),
+            "new known breakage 1 does not match a detected request-schema breakage".to_string(),
+        ]
+    );
+
+    assert_eq!(
+        known_breakage_problems(
+            &log(Vec::new()),
+            &log(vec![known(/*id*/ 1, &breakage), known(/*id*/ 2, &breakage),]),
+            &[breakage],
+        ),
+        vec!["multiple new known-breakage entries match test/method at params".to_string()]
+    );
+    Ok(())
+}
+
+#[test]
+fn log_metadata_and_json_snapshots_are_validated() {
+    let invalid = KnownBreakageLog {
+        version: 2,
+        breakages: vec![KnownBreakage {
+            id: 2,
+            kind: ViolationKind::MethodRemoved,
+            method: String::new(),
+            path: String::new(),
+            before_json: "not json".to_string(),
+            after_json: "also not json".to_string(),
+            justification: String::new(),
+        }],
+    };
+
+    assert_eq!(
+        validate_log("after", &invalid),
+        vec![
+            "after known-breakage log must use version 1".to_string(),
+            "after known-breakage entry 2 must have id 1".to_string(),
+            "after known breakage 2 needs a justification".to_string(),
+            "after known breakage 2 needs a method".to_string(),
+            "after known breakage 2 needs a path".to_string(),
+            "after known breakage 2 has invalid JSON in before_json".to_string(),
+            "after known breakage 2 has invalid JSON in after_json".to_string(),
+        ]
+    );
+}
+
+#[test]
+fn templates_are_numbered_after_a_valid_prefix_only() -> Result<()> {
+    let (_, _, breakage) = type_narrowing()?;
+    let before = log(vec![old_breakage(/*id*/ 1, "test/old")]);
+    let valid_after = before.clone();
+    let valid_report = failure_report(
+        &["missing".to_string()],
+        std::slice::from_ref(&breakage),
+        &before,
+        &valid_after,
+    )?;
+    assert!(valid_report.contains("[[breakages]]\nid = 2"));
+
+    let invalid_after = log(vec![
+        old_breakage(/*id*/ 1, "test/old"),
+        old_breakage(/*id*/ 2, "test/stale"),
+    ]);
+    let invalid_report =
+        failure_report(&["stale".to_string()], &[breakage], &before, &invalid_after)?;
+    assert!(!invalid_report.contains("[[breakages]]"));
+    Ok(())
+}
+
+fn type_narrowing() -> Result<(ApiSchema, ApiSchema, SchemaBreakage)> {
+    let before = ApiSchema::parse(&request_schema(json!({ "type": ["null", "string"] })))?;
+    let after = ApiSchema::parse(&request_schema(json!({ "type": "string" })))?;
+    let breakage = find_request_narrowing(&before, &after)?
+        .into_iter()
+        .next()
+        .context("expected a type narrowing")?;
+    Ok((before, after, breakage))
+}
+
+fn known(id: u64, breakage: &SchemaBreakage) -> KnownBreakage {
+    KnownBreakage {
+        justification: "documents the accepted wire-format break".to_string(),
+        ..KnownBreakage::from_breakage(id, breakage)
+    }
+}
+
+fn old_breakage(id: u64, method: &str) -> KnownBreakage {
+    KnownBreakage {
+        id,
+        kind: ViolationKind::MethodRemoved,
+        method: method.to_string(),
+        path: "request".to_string(),
+        before_json: "true".to_string(),
+        after_json: "false".to_string(),
+        justification: "documents the accepted wire-format break".to_string(),
+    }
+}
+
+fn log(breakages: Vec<KnownBreakage>) -> KnownBreakageLog {
+    KnownBreakageLog {
+        version: KNOWN_BREAKAGE_LOG_VERSION,
+        breakages,
+    }
+}
diff --git a/codex-rs/schema-evolution/src/lib.rs b/codex-rs/schema-evolution/src/lib.rs
index a8ef15f421..323f3e3857 100644
--- a/codex-rs/schema-evolution/src/lib.rs
+++ b/codex-rs/schema-evolution/src/lib.rs
@@ -1,4 +1,5 @@
 mod compare;
+mod known_breakage;
 mod model;
 mod parse;
 mod violation;
@@ -7,6 +8,9 @@ mod violation;
 mod test_support;
 
 use compare::compare_api_schemas;
+pub use known_breakage::KnownBreakage;
+pub use known_breakage::KnownBreakageLog;
+pub use known_breakage::check_request_narrowing;
 pub(crate) use model::AdditionalItems;
 pub(crate) use model::AdditionalProperties;
 pub use model::ApiSchema;