diff --git a/crates/starknet_transaction_prover/src/server/errors.rs b/crates/starknet_transaction_prover/src/server/errors.rs
index 8fde63ba67a..24b514e1cc2 100644
--- a/crates/starknet_transaction_prover/src/server/errors.rs
+++ b/crates/starknet_transaction_prover/src/server/errors.rs
@@ -20,6 +20,10 @@ use crate::errors::{
     VirtualSnosProverError,
 };
 
+#[cfg(test)]
+#[path = "errors_test.rs"]
+mod errors_test;
+
 /// Every JSON-RPC error the proving service itself defines, one variant per error documented in
 /// the proving-api OpenRPC spec. Each error's code and canonical message live only here; the
 /// constructor functions below are thin wrappers. The service can also return the standard
diff --git a/crates/starknet_transaction_prover/src/server/errors_test.rs b/crates/starknet_transaction_prover/src/server/errors_test.rs
new file mode 100644
index 00000000000..765fef5df9b
--- /dev/null
+++ b/crates/starknet_transaction_prover/src/server/errors_test.rs
@@ -0,0 +1,112 @@
+use jsonrpsee::types::error::ErrorCode::InternalError;
+use jsonrpsee::types::ErrorObjectOwned;
+use starknet_api::transaction::TransactionHash;
+use starknet_types_core::felt::Felt;
+
+use super::ServiceErrorCode;
+use crate::errors::{
+    ProofProviderError,
+    RunnerError,
+    VirtualBlockExecutorError,
+    VirtualSnosProverError,
+};
+
+fn reverted_transaction_error(reason: &str) -> ErrorObjectOwned {
+    let runner_error =
+        RunnerError::VirtualBlockExecutor(VirtualBlockExecutorError::TransactionReverted(
+            TransactionHash(Felt::ONE),
+            reason.to_string(),
+        ));
+    ErrorObjectOwned::from(VirtualSnosProverError::RunnerError(Box::new(runner_error)))
+}
+
+fn upstream_rpc_error(code: i64, data: Option<serde_json::Value>) -> ErrorObjectOwned {
+    let runner_error = RunnerError::ProofProvider(ProofProviderError::UpstreamRpcError {
+        code,
+        message: "upstream says no".to_string(),
+        data,
+    });
+    ErrorObjectOwned::from(VirtualSnosProverError::RunnerError(Box::new(runner_error)))
+}
+
+/// Starknet application errors carry non-negative codes and are meaningful to the caller, so they
+/// are forwarded verbatim rather than collapsed into -32603.
+#[test]
+fn upstream_application_error_is_forwarded_verbatim() {
+    let data = serde_json::json!({ "expected_nonce": "0x2" });
+
+    let error = upstream_rpc_error(41, Some(data.clone()));
+
+    assert_eq!(error.code(), 41);
+    assert_eq!(error.message(), "upstream says no");
+    assert_eq!(
+        error.data().map(|raw| serde_json::from_str::<serde_json::Value>(raw.get()).unwrap()),
+        Some(data)
+    );
+}
+
+/// Negative codes are JSON-RPC transport/infrastructure failures rather than anything the caller
+/// can act on, so the code collapses to -32603. The upstream message is not hidden — it moves
+/// into `data`.
+#[test]
+fn upstream_infrastructure_error_is_hidden_behind_internal_error() {
+    let error = upstream_rpc_error(-32000, None);
+
+    assert_eq!(error.code(), InternalError.code());
+    assert_ne!(error.message(), "upstream says no");
+}
+
+/// A code outside `i32` cannot be a valid JSON-RPC code; it must not wrap around into an
+/// unrelated one. `u32::MAX + 42` truncates to a *positive* `i32` (41), so a wrapping cast would
+/// forward it as a real application error instead of rejecting it.
+#[test]
+fn upstream_code_wider_than_i32_falls_back_to_internal_error() {
+    let error = upstream_rpc_error(i64::from(u32::MAX) + 42, None);
+
+    assert_eq!(error.code(), InternalError.code());
+}
+
+#[test]
+fn out_of_gas_revert_keeps_the_upstream_reason_and_appends_the_hint() {
+    let reason = "Transaction execution has failed: Out of gas";
+
+    let error = reverted_transaction_error(reason);
+
+    assert_eq!(error.code(), ServiceErrorCode::InvalidTransactionInput.code());
+    let data = error.data().expect("out-of-gas errors carry the reason and hint in data");
+    let data: String = serde_json::from_str(data.get()).unwrap();
+    assert!(data.starts_with(reason), "expected the upstream reason first, got: {data}");
+    assert!(data.contains("starknet_estimateFee"), "expected the hint, got: {data}");
+}
+
+/// A revert that is not an out-of-gas one has no actionable hint to add, so it stays an internal
+/// error rather than being mislabelled as invalid input.
+#[test]
+fn non_out_of_gas_revert_is_an_internal_error() {
+    let error = reverted_transaction_error("Transaction execution has failed: assertion failed");
+
+    assert_eq!(error.code(), InternalError.code());
+}
+
+/// A pending block is reported as "block not found" rather than a validation failure, because the
+/// prover only proves closed blocks.
+#[test]
+fn pending_block_validation_error_maps_to_block_not_found() {
+    let error = ErrorObjectOwned::from(VirtualSnosProverError::ValidationError(
+        "Pending block".to_string(),
+    ));
+
+    assert_eq!(error.code(), ServiceErrorCode::BlockNotFound.code());
+}
+
+#[test]
+fn other_validation_error_maps_to_account_validation_failed() {
+    let error = ErrorObjectOwned::from(VirtualSnosProverError::ValidationError(
+        "nonce mismatch".to_string(),
+    ));
+
+    assert_eq!(error.code(), ServiceErrorCode::AccountValidationFailed.code());
+    let data: String = serde_json::from_str(error.data().expect("data carries the reason").get())
+        .expect("data is a JSON string");
+    assert_eq!(data, "nonce mismatch");
+}