diff --git a/scripts/lib/workflow-bootstrap.mjs b/scripts/lib/workflow-bootstrap.mjs
index a87d7760..05c0d38d 100644
--- a/scripts/lib/workflow-bootstrap.mjs
+++ b/scripts/lib/workflow-bootstrap.mjs
@@ -26,6 +26,13 @@ function normalizeHead(headSha) {
   return value;
 }
 
+function normalizeBase(baseSha) {
+  if (baseSha === null || baseSha === undefined || baseSha === "") return null;
+  const value = String(baseSha).trim().toLowerCase();
+  if (!HEAD_RE.test(value)) throw new Error("workflow_bootstrap_base_invalid");
+  return value;
+}
+
 function stateRoot(override) {
   return resolve(
     override || process.env.GITHUB_DELIVERY_STATE_DIR || join(homedir(), ".github-delivery"),
@@ -67,6 +74,7 @@ function assertCheckpointIdentity(snapshot, { repo, headSha }) {
 export function bootstrapLocalPrWorkflow({ repo, headSha, baseSha = null, stateDir } = {}) {
   const normalizedRepo = normalizeRepo(repo);
   const normalizedHead = normalizeHead(headSha);
+  const normalizedBase = normalizeBase(baseSha);
   const checkpointPath = localPrWorkflowCheckpointPath({
     repo: normalizedRepo,
     headSha: normalizedHead,
@@ -76,7 +84,7 @@ export function bootstrapLocalPrWorkflow({ repo, headSha, baseSha = null, stateD
   const initialSnapshot = createDeliveryWorkflowController({
     workflow: profile.workflow,
     repo: normalizedRepo,
-    baseSha: baseSha ? String(baseSha) : null,
+    baseSha: normalizedBase,
     headSha: normalizedHead,
     graph: profile.graph,
     startPhase: profile.startPhase,
@@ -101,9 +109,9 @@ export function bootstrapLocalPrWorkflow({ repo, headSha, baseSha = null, stateD
     headSha: normalizedHead,
   });
   if (
-    baseSha &&
+    normalizedBase &&
     storedSnapshot.baseSha &&
-    String(storedSnapshot.baseSha).toLowerCase() !== String(baseSha).toLowerCase()
+    String(storedSnapshot.baseSha).toLowerCase() !== normalizedBase
   ) {
     throw new Error("workflow_bootstrap_checkpoint_base_mismatch");
   }
diff --git a/tests/unit/workflow-state-grounding.test.mjs b/tests/unit/workflow-state-grounding.test.mjs
index f4c000d6..c3dedff3 100644
--- a/tests/unit/workflow-state-grounding.test.mjs
+++ b/tests/unit/workflow-state-grounding.test.mjs
@@ -124,6 +124,23 @@ test("show re-derives controller state instead of trusting raw checkpoint guidan
   }
 });
 
+test("local workflow bootstrap rejects abbreviated base SHA before checkpoint comparison", () => {
+  const stateDir = mkdtempSync(join(tmpdir(), "github-delivery-bootstrap-base-"));
+  try {
+    assert.throws(
+      () => bootstrapLocalPrWorkflow({
+        repo: "acme/widgets",
+        headSha: HEAD,
+        baseSha: "a".repeat(9),
+        stateDir,
+      }),
+      /workflow_bootstrap_base_invalid/,
+    );
+  } finally {
+    rmSync(stateDir, { recursive: true, force: true });
+  }
+});
+
 test("local workflow bootstrap upgrades reused raw checkpoint guidance", () => {
   const stateDir = mkdtempSync(join(tmpdir(), "github-delivery-bootstrap-grounding-"));
   try {