diff --git a/changelog.d/fixes/8029-promptql-issuer-substring.md b/changelog.d/fixes/8029-promptql-issuer-substring.md
new file mode 100644
index 0000000000..9b523c0391
--- /dev/null
+++ b/changelog.d/fixes/8029-promptql-issuer-substring.md
@@ -0,0 +1 @@
+- fix(sse): replace spoofable `.includes()` PromptQL issuer check with hostname comparison (#8029)
diff --git a/open-sse/services/promptql/jwt.ts b/open-sse/services/promptql/jwt.ts
index a82df3176b..fb827ab4b1 100644
--- a/open-sse/services/promptql/jwt.ts
+++ b/open-sse/services/promptql/jwt.ts
@@ -102,13 +102,31 @@ export function isPlaygroundPromptQlToken(token: string): boolean {
   return false;
 }
 
+/**
+ * Hosts trusted as PromptQL DDN/lux token issuers. A bare `String.includes()` against
+ * `iss` is spoofable (`https://auth.pro.hasura.io.evil.com/...` also "contains" the
+ * trusted substring) — CodeQL js/incomplete-url-substring-sanitization, issue #8029.
+ * Always parse `iss` as a URL and compare the hostname instead.
+ */
+const TRUSTED_DDN_ISSUER_HOSTS = ["auth.pro.hasura.io", "auth.pro.ql.app"];
+
+/** True when `iss` parses as a URL whose hostname is (or is a subdomain of) a trusted host. */
+export function issuerHostIsTrusted(iss: string): boolean {
+  try {
+    const host = new URL(iss).hostname.toLowerCase();
+    return TRUSTED_DDN_ISSUER_HOSTS.some((h) => host === h || host.endsWith(`.${h}`));
+  } catch {
+    return false;
+  }
+}
+
 /** DDN/lux project JWT (iss auth.pro.hasura.io) — credits yes, playground chat no. */
 export function isDdnProjectPromptQlToken(token: string): boolean {
   if (!token || isPlaygroundPromptQlToken(token)) return false;
   const payload = decodeJwtPayload(token);
   if (!payload) return false;
-  const iss = readStr(payload.iss).toLowerCase();
-  if (iss.includes("auth.pro.hasura.io") || iss.includes("auth.pro.ql.app")) return true;
+  const iss = readStr(payload.iss);
+  if (issuerHostIsTrusted(iss)) return true;
   // aud is a project UUID and no hasura claims → treat as DDN
   const aud = payload.aud;
   if (typeof aud === "string" && looksLikeUuid(aud)) return true;
diff --git a/open-sse/services/usage/promptql.ts b/open-sse/services/usage/promptql.ts
index 29c215b7c1..4a92755d74 100644
--- a/open-sse/services/usage/promptql.ts
+++ b/open-sse/services/usage/promptql.ts
@@ -23,6 +23,7 @@ import {
   looksLikeUuid,
   extractProjectIdFromToken,
   decodeJwtPayload,
+  issuerHostIsTrusted,
 } from "../promptql/jwt.ts";
 
 // Re-exported for backward compatibility — external/test consumers previously
@@ -121,9 +122,9 @@ function collectCreditsTokens(
 function isLikelyDdnToken(token: string): boolean {
   const json = decodeJwtPayload(token);
   if (!json) return false;
-  const iss = typeof json.iss === "string" ? json.iss.toLowerCase() : "";
-  if (iss.includes("auth.pro.hasura.io") || iss.includes("auth.pro.ql.app")) return true;
-  if (iss === "enrich-token" || iss.includes("enrich-token")) return false;
+  const iss = typeof json.iss === "string" ? json.iss : "";
+  if (issuerHostIsTrusted(iss)) return true;
+  if (iss.toLowerCase().includes("enrich-token")) return false;
   const aud = json.aud;
   if (typeof aud === "string" && looksLikeUuid(aud)) return true;
   return false;
diff --git a/tests/unit/bug-8029-promptql-issuer-substring.test.ts b/tests/unit/bug-8029-promptql-issuer-substring.test.ts
new file mode 100644
index 0000000000..c3f8cb13b8
--- /dev/null
+++ b/tests/unit/bug-8029-promptql-issuer-substring.test.ts
@@ -0,0 +1,105 @@
+// Regression test for #8029 — PromptQL issuer checks used a bare `String.includes()`
+// against the JWT `iss` claim, which is an incomplete-url-substring-sanitization bug
+// (CodeQL js/incomplete-url-substring-sanitization). A spoofed issuer such as
+// `https://auth.pro.hasura.io.evil.com/ddn/token` satisfied `.includes("auth.pro.hasura.io")`
+// even though its actual host is `auth.pro.hasura.io.evil.com`, not `auth.pro.hasura.io`.
+//
+// Two call sites shared the same flawed predicate (copy-pasted):
+//   - open-sse/services/promptql/jwt.ts::isDdnProjectPromptQlToken()
+//   - open-sse/services/usage/promptql.ts::isLikelyDdnToken()
+//
+// The fix replaces both with a shared `issuerHostIsTrusted()` helper that parses the
+// issuer with `new URL()` and compares the hostname (exact match or a `.`-delimited
+// subdomain of a trusted host), never a raw substring check.
+
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+
+const jwtMod = await import("../../open-sse/services/promptql/jwt.ts");
+const usageMod = await import("../../open-sse/services/usage/promptql.ts");
+
+function makeFakeJwt(claims: Record<string, unknown>): string {
+  const header = Buffer.from(JSON.stringify({ alg: "none", typ: "JWT" })).toString("base64url");
+  const payload = Buffer.from(JSON.stringify(claims)).toString("base64url");
+  return `${header}.${payload}.sig`;
+}
+
+const NON_UUID_AUD = "not-a-project-uuid";
+
+const spoofedDdnJwt = makeFakeJwt({
+  iss: "https://auth.pro.hasura.io.evil.com/ddn/token",
+  aud: NON_UUID_AUD,
+  exp: Math.floor(Date.now() / 1000) + 3600,
+});
+
+const spoofedQlAppJwt = makeFakeJwt({
+  iss: "https://auth.pro.ql.app.evil.com/ddn/token",
+  aud: NON_UUID_AUD,
+  exp: Math.floor(Date.now() / 1000) + 3600,
+});
+
+const legitimateDdnJwt = makeFakeJwt({
+  iss: "https://auth.pro.hasura.io/ddn/token",
+  aud: NON_UUID_AUD,
+  exp: Math.floor(Date.now() / 1000) + 3600,
+});
+
+const legitimateSubdomainJwt = makeFakeJwt({
+  iss: "https://eu.auth.pro.hasura.io/ddn/token",
+  aud: NON_UUID_AUD,
+  exp: Math.floor(Date.now() / 1000) + 3600,
+});
+
+const garbageIssuerJwt = makeFakeJwt({
+  iss: "not a url at all",
+  aud: NON_UUID_AUD,
+  exp: Math.floor(Date.now() / 1000) + 3600,
+});
+
+describe("BUG #8029 — PromptQL issuer host check (js/incomplete-url-substring-sanitization)", () => {
+  it("jwt.ts::isDdnProjectPromptQlToken rejects a spoofed host that merely CONTAINS the trusted substring", () => {
+    assert.equal(jwtMod.isDdnProjectPromptQlToken(spoofedDdnJwt), false);
+    assert.equal(jwtMod.isDdnProjectPromptQlToken(spoofedQlAppJwt), false);
+  });
+
+  it("jwt.ts::isDdnProjectPromptQlToken still accepts the real trusted host and its subdomains", () => {
+    assert.equal(jwtMod.isDdnProjectPromptQlToken(legitimateDdnJwt), true);
+    assert.equal(jwtMod.isDdnProjectPromptQlToken(legitimateSubdomainJwt), true);
+  });
+
+  it("jwt.ts::isDdnProjectPromptQlToken rejects a non-URL issuer instead of throwing", () => {
+    assert.equal(jwtMod.isDdnProjectPromptQlToken(garbageIssuerJwt), false);
+  });
+
+  it("jwt.ts::issuerHostIsTrusted is exported and shared", () => {
+    assert.equal(typeof jwtMod.issuerHostIsTrusted, "function");
+    assert.equal(jwtMod.issuerHostIsTrusted("https://auth.pro.hasura.io.evil.com/x"), false);
+    assert.equal(jwtMod.issuerHostIsTrusted("https://auth.pro.hasura.io/x"), true);
+    assert.equal(jwtMod.issuerHostIsTrusted("https://eu.auth.pro.hasura.io/x"), true);
+    assert.equal(jwtMod.issuerHostIsTrusted("https://auth.pro.ql.app/x"), true);
+    assert.equal(jwtMod.issuerHostIsTrusted("garbage"), false);
+  });
+
+  it("usage/promptql.ts collectCreditsTokens sorts a spoofed-issuer token as non-DDN (via getPromptQlUsage token ordering)", async () => {
+    // isLikelyDdnToken is not exported directly; exercise it indirectly through
+    // getPromptQlUsage's "onlyEnrich" fallback message, which flips to the DDN-required
+    // message only when at least one token is classified as DDN-shaped.
+    const originalFetch = globalThis.fetch;
+    globalThis.fetch = (async () =>
+      new Response(JSON.stringify({ errors: [{ message: "access-denied" }] }), {
+        status: 200,
+      })) as typeof fetch;
+    try {
+      const result = await usageMod.getPromptQlUsage(spoofedDdnJwt, {
+        projectId: "01a0fe61-baf4-4e31-9311-8cc0bb3eba91",
+      });
+      // A spoofed-host token must NOT be treated as DDN-shaped, so the fallback
+      // "onlyEnrich" branch (which requires isLikelyDdnToken to be false for every
+      // token) is taken and the DDN-specific instructional message is returned.
+      assert.ok("message" in result);
+      assert.match(String(result.message), /DDN\/project JWT/);
+    } finally {
+      globalThis.fetch = originalFetch;
+    }
+  });
+});