diff --git a/actions/setup/js/generate_git_patch.cjs b/actions/setup/js/generate_git_patch.cjs
index d485f8ecc59..73752c28356 100644
--- a/actions/setup/js/generate_git_patch.cjs
+++ b/actions/setup/js/generate_git_patch.cjs
@@ -6,6 +6,7 @@ const path = require("path");
 
 const { getErrorMessage } = require("./error_helpers.cjs");
 const { execGitSync } = require("./git_helpers.cjs");
+const { ERR_SYSTEM } = require("./error_codes.cjs");
 
 /**
  * Debug logging helper - logs to stderr when DEBUG env var matches
@@ -163,7 +164,7 @@ async function generateGitPatch(branchName, baseBranch, options = {}) {
             } else {
               // No remote refs available - fall through to Strategy 2
               debugLog(`Strategy 1 (full): No remote refs available, falling through to Strategy 2`);
-              throw new Error("No remote refs available for merge-base calculation");
+              throw new Error(`${ERR_SYSTEM}: No remote refs available for merge-base calculation`);
             }
           }
         }
diff --git a/actions/setup/js/generate_git_patch.test.cjs b/actions/setup/js/generate_git_patch.test.cjs
index 01f9b2af5c5..1dd690a024f 100644
--- a/actions/setup/js/generate_git_patch.test.cjs
+++ b/actions/setup/js/generate_git_patch.test.cjs
@@ -324,6 +324,44 @@ describe("sanitizeBranchNameForPatch", () => {
   });
 });
 
+describe("generateGitPatch - standardized error codes", () => {
+  let originalEnv;
+
+  beforeEach(() => {
+    originalEnv = {
+      GITHUB_SHA: process.env.GITHUB_SHA,
+      GITHUB_WORKSPACE: process.env.GITHUB_WORKSPACE,
+    };
+  });
+
+  afterEach(() => {
+    Object.keys(originalEnv).forEach(key => {
+      if (originalEnv[key] !== undefined) {
+        process.env[key] = originalEnv[key];
+      } else {
+        delete process.env[key];
+      }
+    });
+  });
+
+  it("should fail gracefully and return a non-empty error string when no commits can be found", async () => {
+    const { generateGitPatch } = await import("./generate_git_patch.cjs");
+
+    process.env.GITHUB_WORKSPACE = "/tmp/nonexistent-repo";
+    process.env.GITHUB_SHA = "abc123";
+
+    const result = await generateGitPatch("feature-branch", "main");
+
+    expect(result.success).toBe(false);
+    expect(result).toHaveProperty("error");
+    // Note: E005 is used as an internal control-flow signal in Strategy 1 (full mode)
+    // and is caught before reaching the final return value. The conformance check
+    // validates the E005 code at source level via check-safe-outputs-conformance.sh.
+    expect(typeof result.error).toBe("string");
+    expect(result.error.length).toBeGreaterThan(0);
+  });
+});
+
 describe("getPatchPath", () => {
   it("should return correct path format", async () => {
     const { getPatchPath } = await import("./generate_git_patch.cjs");