diff --git a/.github/scripts/issue-review/common.mjs b/.github/scripts/issue-review/common.mjs
index 4de3f27..f318993 100644
--- a/.github/scripts/issue-review/common.mjs
+++ b/.github/scripts/issue-review/common.mjs
@@ -1,7 +1,7 @@
 import crypto from "node:crypto";
 
 export const ISSUE_REVIEW_SCHEMA_VERSION = 4;
-export const PREFIXED_TITLE = /^[a-z][a-z0-9-]*(?:\/[a-z][a-z0-9-]*)*: \S.*$/;
+export const PREFIXED_TITLE = /^[a-z][a-z0-9_-]*(?:\/[a-z][a-z0-9_-]*)*: \S.*$/;
 
 export function sha256(value) {
   return crypto.createHash("sha256").update(value).digest("hex");
diff --git a/.github/scripts/pr-readiness/test.mjs b/.github/scripts/pr-readiness/test.mjs
index 81679db..ba8109b 100644
--- a/.github/scripts/pr-readiness/test.mjs
+++ b/.github/scripts/pr-readiness/test.mjs
@@ -108,12 +108,25 @@ assert.deepEqual(
   analyzePullRequest(manyLinkedIssuesInput).deterministic_blockers,
   [],
 );
+assert.ok(!blockerCodes(analyzePullRequest({
+  ...input,
+  title: "h106/zero_esp: add the Zero ESP Main App package",
+})).includes("invalid-title"));
 assert.ok(blockerCodes(analyzePullRequest({
   ...manyLinkedIssuesInput,
   linked_issue_count: manyLinkedIssues.length + 1,
 })).includes("too-many-closing-issues"));
-assert.ok(analyzePullRequest({ ...input, title: "Bad title" })
-  .deterministic_blockers.some((item) => item.code === "invalid-title"));
+for (const title of [
+  "Bad title",
+  "H106/zero_esp: add the Zero ESP Main App package",
+  "h106/_zero_esp: add the Zero ESP Main App package",
+  "h106//zero_esp: add the Zero ESP Main App package",
+  "h106/zero esp: add the Zero ESP Main App package",
+  "h106/zero_esp: ",
+]) {
+  assert.ok(analyzePullRequest({ ...input, title })
+    .deterministic_blockers.some((item) => item.code === "invalid-title"));
+}
 assert.ok(analyzePullRequest({ ...input, body: "" })
   .deterministic_blockers.some((item) => item.code === "missing-body"));
 assert.ok(analyzePullRequest({ ...input, body_truncated: true })
diff --git a/README.md b/README.md
index dfb87a6..1c47f28 100644
--- a/README.md
+++ b/README.md
@@ -128,7 +128,9 @@ default-branch `AGENTS.md` hierarchy and the Issue-review documents it
 designates. The workflow itself enforces only cross-project integrity and PR
 linkage requirements:
 
-- PR titles use lowercase `prefix: Subject` form.
+- PR titles use lowercase `prefix: Subject` form. Each slash-separated prefix
+  segment starts with a lowercase letter and may then contain lowercase letters,
+  digits, hyphens, or underscores.
 - The PR body describes the delivered result and validation.
 - The PR has at least one same-repository native closing Issue from GraphQL
   `closingIssuesReferences`; text-only references do not count.