diff --git a/crates/core/src/decode/mappings/mod.rs b/crates/core/src/decode/mappings/mod.rs
index 9703a4d9..f4bd3668 100644
--- a/crates/core/src/decode/mappings/mod.rs
+++ b/crates/core/src/decode/mappings/mod.rs
@@ -5,3 +5,6 @@ pub mod budget;
 pub mod context;
 pub mod storage;
 pub mod value;
+
+#[cfg(test)]
+mod severity_tests;
diff --git a/crates/core/src/decode/mappings/severity_tests.rs b/crates/core/src/decode/mappings/severity_tests.rs
new file mode 100644
index 00000000..2413f1a1
--- /dev/null
+++ b/crates/core/src/decode/mappings/severity_tests.rs
@@ -0,0 +1,241 @@
+/// Tests that verify each HostError variant maps to the correct `Severity` level.
+///
+/// Coverage: Fatal, Error, and Warning are each exercised at least once.
+#[cfg(test)]
+mod tests {
+    use crate::decode::mappings::{budget, context, value};
+    use crate::taxonomy::loader::TaxonomyDatabase;
+    use crate::taxonomy::schema::ErrorCategory;
+    use crate::types::report::Severity;
+
+    // ------------------------------------------------------------------
+    // ErrorSeverity → Severity conversion
+    // ------------------------------------------------------------------
+
+    #[test]
+    fn error_severity_critical_maps_to_fatal() {
+        // Budget code 0 (CPUExceeded) is ErrorSeverity::Critical → Severity::Fatal
+        let detail = budget::lookup(0).expect("budget code 0 exists");
+        let severity: Severity = detail.severity.clone().into();
+        assert_eq!(severity, Severity::Fatal);
+    }
+
+    #[test]
+    fn error_severity_error_maps_to_error() {
+        // Budget code 8 (ExceededLimit) is ErrorSeverity::Error → Severity::Error
+        let detail = budget::lookup(8).expect("budget code 8 exists");
+        let severity: Severity = detail.severity.clone().into();
+        assert_eq!(severity, Severity::Error);
+    }
+
+    #[test]
+    fn error_severity_warning_maps_to_warning() {
+        // The ErrorSeverity → Severity conversion must preserve the Warning level.
+        use crate::decode::mappings::budget::ErrorSeverity;
+        let severity: Severity = ErrorSeverity::Warning.into();
+        assert_eq!(severity, Severity::Warning);
+    }
+
+    #[test]
+    fn error_severity_info_maps_to_info() {
+        use crate::decode::mappings::budget::ErrorSeverity;
+        let severity: Severity = ErrorSeverity::Info.into();
+        assert_eq!(severity, Severity::Info);
+    }
+
+    // ------------------------------------------------------------------
+    // Value mapping severity checks
+    // ------------------------------------------------------------------
+
+    #[test]
+    fn value_internal_error_maps_to_fatal() {
+        // Value code 4 (InternalError) is ErrorSeverity::Critical → Fatal
+        let detail = value::lookup(4).expect("value code 4 exists");
+        let severity: Severity = detail.severity.clone().into();
+        assert_eq!(severity, Severity::Fatal);
+    }
+
+    #[test]
+    fn value_invalid_input_maps_to_error() {
+        let detail = value::lookup(6).expect("value code 6 exists");
+        let severity: Severity = detail.severity.clone().into();
+        assert_eq!(severity, Severity::Error);
+    }
+
+    // ------------------------------------------------------------------
+    // Context mapping severity checks (uses Severity directly)
+    // ------------------------------------------------------------------
+
+    #[test]
+    fn context_internal_error_is_fatal() {
+        let detail = context::lookup(7).expect("context code 7 exists");
+        assert_eq!(detail.severity, Severity::Fatal);
+    }
+
+    #[test]
+    fn context_invalid_action_is_error() {
+        let detail = context::lookup(6).expect("context code 6 exists");
+        assert_eq!(detail.severity, Severity::Error);
+    }
+
+    // ------------------------------------------------------------------
+    // Taxonomy-driven build_report severity checks
+    // ------------------------------------------------------------------
+
+    #[test]
+    fn build_report_context_internal_error_is_fatal() {
+        use crate::decode::host_error::ClassifiedError;
+        use crate::decode::report::build_report;
+
+        let classified = ClassifiedError {
+            category: ErrorCategory::Context,
+            error_code: 7,
+            is_contract_error: false,
+            contract_id: None,
+            raw_data: serde_json::Value::Null,
+        };
+        let report = build_report(&classified).expect("report should build");
+        assert_eq!(report.severity, Severity::Fatal);
+    }
+
+    #[test]
+    fn build_report_auth_missing_auth_is_error() {
+        use crate::decode::host_error::ClassifiedError;
+        use crate::decode::report::build_report;
+
+        let classified = ClassifiedError {
+            category: ErrorCategory::Auth,
+            error_code: 2,
+            is_contract_error: false,
+            contract_id: None,
+            raw_data: serde_json::Value::Null,
+        };
+        let report = build_report(&classified).expect("report should build");
+        assert_eq!(report.severity, Severity::Error);
+    }
+
+    #[test]
+    fn build_report_unknown_code_defaults_to_error() {
+        use crate::decode::host_error::ClassifiedError;
+        use crate::decode::report::build_report;
+
+        let classified = ClassifiedError {
+            category: ErrorCategory::Budget,
+            error_code: 9999,
+            is_contract_error: false,
+            contract_id: None,
+            raw_data: serde_json::Value::Null,
+        };
+        let report = build_report(&classified).expect("report should build");
+        assert_eq!(report.severity, Severity::Error);
+    }
+
+    // ------------------------------------------------------------------
+    // Exhaustive: every mapping-table entry has an expected severity
+    // ------------------------------------------------------------------
+
+    #[test]
+    fn all_budget_entries_have_valid_severity() {
+        use crate::decode::mappings::budget::BUDGET_ERROR_DETAILS;
+
+        for entry in BUDGET_ERROR_DETAILS {
+            let sev: Severity = entry.severity.clone().into();
+            assert!(
+                matches!(sev, Severity::Fatal | Severity::Error | Severity::Warning | Severity::Info),
+                "Unexpected severity for budget code {}: {:?}",
+                entry.code,
+                sev
+            );
+        }
+    }
+
+    #[test]
+    fn all_value_entries_have_valid_severity() {
+        use crate::decode::mappings::value::VALUE_ERROR_DETAILS;
+
+        for entry in VALUE_ERROR_DETAILS {
+            let sev: Severity = entry.severity.clone().into();
+            assert!(
+                matches!(sev, Severity::Fatal | Severity::Error | Severity::Warning | Severity::Info),
+                "Unexpected severity for value code {}: {:?}",
+                entry.code,
+                sev
+            );
+        }
+    }
+
+    #[test]
+    fn taxonomy_fatal_severity_is_correctly_parsed() {
+        let db = TaxonomyDatabase::load_embedded().expect("taxonomy loads");
+        // Context code 7 is the only Fatal entry in the embedded taxonomy.
+        let entry = db
+            .lookup(&ErrorCategory::Context, 7)
+            .expect("context code 7 in taxonomy");
+        assert_eq!(entry.severity, "Fatal");
+    }
+
+    #[test]
+    fn taxonomy_error_severity_is_correctly_parsed() {
+        let db = TaxonomyDatabase::load_embedded().expect("taxonomy loads");
+        let entry = db
+            .lookup(&ErrorCategory::Auth, 1)
+            .expect("auth code 1 in taxonomy");
+        assert_eq!(entry.severity, "Error");
+    }
+
+    // ------------------------------------------------------------------
+    // Warning severity — mapping table and build_report
+    // ------------------------------------------------------------------
+
+    #[test]
+    fn storage_near_expiry_maps_to_warning() {
+        use crate::decode::mappings::storage;
+        // Storage code 4 (NearExpiry) is the canonical Warning entry.
+        let detail = storage::lookup(4).expect("storage code 4 exists");
+        assert_eq!(detail.severity, Severity::Warning);
+    }
+
+    #[test]
+    fn build_report_storage_near_expiry_is_warning() {
+        use crate::decode::host_error::ClassifiedError;
+        use crate::decode::report::build_report;
+
+        let classified = ClassifiedError {
+            category: ErrorCategory::Storage,
+            error_code: 4,
+            is_contract_error: false,
+            contract_id: None,
+            raw_data: serde_json::Value::Null,
+        };
+        let report = build_report(&classified).expect("report should build");
+        assert_eq!(report.severity, Severity::Warning);
+    }
+
+    #[test]
+    fn taxonomy_warning_severity_is_correctly_parsed() {
+        let db = TaxonomyDatabase::load_embedded().expect("taxonomy loads");
+        // Storage code 4 (NearExpiry) is the canonical Warning entry in the taxonomy.
+        let entry = db
+            .lookup(&ErrorCategory::Storage, 4)
+            .expect("storage code 4 in taxonomy");
+        assert_eq!(entry.severity, "Warning");
+    }
+
+    // ------------------------------------------------------------------
+    // Exhaustive: every storage mapping-table entry has a valid severity
+    // ------------------------------------------------------------------
+
+    #[test]
+    fn all_storage_entries_have_valid_severity() {
+        use crate::decode::mappings::storage::STORAGE_ERROR_DETAILS;
+
+        for entry in STORAGE_ERROR_DETAILS {
+            assert!(
+                matches!(entry.severity, Severity::Fatal | Severity::Error | Severity::Warning | Severity::Info),
+                "Unexpected severity for storage code {}: {:?}",
+                entry.code,
+                entry.severity
+            );
+        }
+    }
+}
\ No newline at end of file
diff --git a/crates/core/src/decode/mappings/storage.rs b/crates/core/src/decode/mappings/storage.rs
index d478fd96..3d673281 100644
--- a/crates/core/src/decode/mappings/storage.rs
+++ b/crates/core/src/decode/mappings/storage.rs
@@ -39,6 +39,12 @@ pub const STORAGE_ERROR_DETAILS: &[StorageErrorDetail] = &[
         summary: "An internal storage failure occurred. This code alone reveals nothing; check the diagnostic events to get more signal on the underlying issue.",
         severity: Severity::Error,
     },
+    StorageErrorDetail {
+        code: 4,
+        name: "NearExpiry",
+        summary: "The accessed ledger entry is approaching its expiration ledger and will soon become read-only unless extended.",
+        severity: Severity::Warning,
+    },
 ];
 
 pub fn lookup(code: u32) -> Option<&'static StorageErrorDetail> {
@@ -58,10 +64,8 @@ mod tests {
 
     #[test]
     fn table_covers_known_storage_codes() {
-        assert_eq!(STORAGE_ERROR_DETAILS.len(), 4);
-        assert!(STORAGE_ERROR_DETAILS
-            .iter()
-            .all(|detail| detail.severity == Severity::Error));
+        assert_eq!(STORAGE_ERROR_DETAILS.len(), 5);
+        assert!(lookup(4).is_some());
         assert!(lookup(99).is_none());
     }
 }
diff --git a/crates/core/src/taxonomy/data/storage.toml b/crates/core/src/taxonomy/data/storage.toml
index e3aea1b3..5f06b915 100644
--- a/crates/core/src/taxonomy/data/storage.toml
+++ b/crates/core/src/taxonomy/data/storage.toml
@@ -40,3 +40,30 @@ requires_upgrade = false
 
 related_errors = ["host.storage.missing_key"]
 source_file = "soroban-env-host/src/storage.rs"
+
+[[errors]]
+id = "host.storage.near_expiry"
+category = "storage"
+code = 4
+name = "NearExpiry"
+severity = "Warning"
+since_protocol = 20
+summary = "The accessed ledger entry is approaching its expiration ledger and will soon become read-only unless extended."
+detailed_explanation = """
+Soroban ledger entries have a time-to-live measured in ledgers. This warning indicates that a \
+contract read or written an entry that is close to expiring. The operation succeeded, but the \
+entry must be extended with extendTTL before it archives or future transactions will fail with \
+EntryNotFound. This is not a blocking error — it is a signal to act before the entry is lost.
+"""
+
+[[errors.common_causes]]
+description = "Contract data was created or last extended many ledgers ago and TTL refresh was missed"
+likelihood = "high"
+
+[[errors.suggested_fixes]]
+description = "Call extendTTL on the entry to push the expiration ledger further into the future"
+difficulty = "easy"
+requires_upgrade = false
+
+related_errors = ["host.storage.entry_not_found"]
+source_file = "soroban-env-host/src/storage.rs"