diff --git a/apps/e2e/tests/backend/endpoints/api/v1/auth/saml/discover.test.ts b/apps/e2e/tests/backend/endpoints/api/v1/auth/saml/discover.test.ts
new file mode 100644
index 000000000..a6dcf2897
--- /dev/null
+++ b/apps/e2e/tests/backend/endpoints/api/v1/auth/saml/discover.test.ts
@@ -0,0 +1,97 @@
+import { it } from "../../../../../../helpers";
+import { Project, niceBackendFetch } from "../../../../../backend-helpers";
+
+/**
+ * Tests for GET /auth/saml/discover.
+ *
+ * Test integrity: drives the API only — no imports from
+ * apps/backend/src/saml/. Project config is set via the standard config
+ * override endpoint (no special test-only mutator), so the discovery
+ * lookup runs through the same code path real customers would hit.
+ *
+ * Connection isolation note: each `it` block creates its own project via
+ * Project.createAndSwitch, so connections don't leak across tests.
+ */
+
+async function createProjectWithSamlConnection(slug: string, domain: string) {
+  const { projectId } = await Project.createAndSwitch();
+  // Push the SAML connection at the environment level — that's where the
+  // IdP-side fields live. The discovery endpoint reads from the rendered
+  // organization config which folds in env overrides.
+  await Project.updateConfig({
+    [`auth.saml.connections.${slug}.displayName`]: `${slug} SSO`,
+    [`auth.saml.connections.${slug}.allowSignIn`]: true,
+    [`auth.saml.connections.${slug}.domain`]: domain,
+    [`auth.saml.connections.${slug}.idpEntityId`]: `https://idp.${domain}/saml/metadata`,
+    [`auth.saml.connections.${slug}.idpSsoUrl`]: `https://idp.${domain}/saml/sso`,
+    [`auth.saml.connections.${slug}.idpCertificate`]: "MIICertificatePlaceholderForDiscoveryTest=",
+  });
+  return { projectId };
+}
+
+it("returns the matching connection for a known email domain", async ({ expect }) => {
+  const { projectId } = await createProjectWithSamlConnection("acme", "acme.test");
+
+  const response = await niceBackendFetch(
+    `/api/v1/auth/saml/discover?email=alice@acme.test&project_id=${projectId}`,
+    { method: "GET" },
+  );
+
+  expect(response).toMatchInlineSnapshot(`
+    NiceResponse {
+      "status": 200,
+      "body": {
+        "connection_id": "acme",
+        "display_name": "acme SSO",
+      },
+      "headers": Headers { <some fields may have been hidden> },
+    }
+  `);
+});
+
+it("returns 404 when no connection matches the email's domain", async ({ expect }) => {
+  const { projectId } = await createProjectWithSamlConnection("acme", "acme.test");
+
+  const response = await niceBackendFetch(
+    `/api/v1/auth/saml/discover?email=stranger@unknown.test&project_id=${projectId}`,
+    { method: "GET" },
+  );
+
+  expect(response.status).toBe(404);
+});
+
+it("matches the connection case-insensitively on the email domain", async ({ expect }) => {
+  const { projectId } = await createProjectWithSamlConnection("acme", "acme.test");
+
+  const response = await niceBackendFetch(
+    `/api/v1/auth/saml/discover?email=ALICE@ACME.TEST&project_id=${projectId}`,
+    { method: "GET" },
+  );
+
+  expect(response.status).toBe(200);
+  expect(response.body).toEqual({ connection_id: "acme", display_name: "acme SSO" });
+});
+
+it("returns 404 for an unknown project_id", async ({ expect }) => {
+  await createProjectWithSamlConnection("acme", "acme.test");
+
+  const response = await niceBackendFetch(
+    `/api/v1/auth/saml/discover?email=alice@acme.test&project_id=00000000-0000-0000-0000-000000000000`,
+    { method: "GET" },
+  );
+
+  expect(response.status).toBe(404);
+});
+
+it("isolates connections across projects (B's connection is not visible from A)", async ({ expect }) => {
+  // Project A has acme; project B has globex. Querying A for globex's domain must miss.
+  const { projectId: projectA } = await createProjectWithSamlConnection("acme", "acme.test");
+  await createProjectWithSamlConnection("globex", "globex.test");
+
+  const response = await niceBackendFetch(
+    `/api/v1/auth/saml/discover?email=bob@globex.test&project_id=${projectA}`,
+    { method: "GET" },
+  );
+
+  expect(response.status).toBe(404);
+});
diff --git a/apps/e2e/tests/backend/endpoints/api/v1/auth/saml/login.test.ts b/apps/e2e/tests/backend/endpoints/api/v1/auth/saml/login.test.ts
new file mode 100644
index 000000000..a3b3d3228
--- /dev/null
+++ b/apps/e2e/tests/backend/endpoints/api/v1/auth/saml/login.test.ts
@@ -0,0 +1,115 @@
+import { it } from "../../../../../../helpers";
+import { InternalApiKey, Project, backendContext, niceBackendFetch } from "../../../../../backend-helpers";
+
+/**
+ * Tests for GET /auth/saml/login/[connection_id].
+ *
+ * Verifies the SDK-facing endpoint that begins SP-initiated SSO. We don't
+ * follow the redirect into the IdP here — that's the round-trip test
+ * (deferred). These tests check:
+ * - URL is built correctly and points at the configured IdP SSO URL
+ * - SAMLRequest + RelayState query params are present
+ * - CSRF cookie is set in browser-redirect mode
+ * - JSON-mode response returns the location instead of redirecting
+ * - Error paths: invalid client, unknown connection, sign-in disabled
+ */
+
+async function setupProjectWithSamlConnection(slug: string, idpHost: string) {
+  await Project.createAndSwitch();
+  await InternalApiKey.createAndSetProjectKeys();
+  await Project.updateConfig({
+    [`auth.saml.connections.${slug}.displayName`]: `${slug} SSO`,
+    [`auth.saml.connections.${slug}.allowSignIn`]: true,
+    [`auth.saml.connections.${slug}.idpEntityId`]: `https://${idpHost}/saml/metadata`,
+    [`auth.saml.connections.${slug}.idpSsoUrl`]: `https://${idpHost}/saml/sso`,
+    [`auth.saml.connections.${slug}.idpCertificate`]: "MIICertificatePlaceholderForLoginTest=",
+  });
+}
+
+function loginQuery() {
+  const projectKeys = backendContext.value.projectKeys;
+  if (projectKeys === "no-project") throw new Error("No project keys");
+  const branchId = backendContext.value.currentBranchId;
+  return {
+    client_id: !branchId ? projectKeys.projectId : `${projectKeys.projectId}#${branchId}`,
+    client_secret: projectKeys.publishableClientKey ?? "",
+    redirect_uri: "http://localhost:8101/handler/oauth-callback",
+    scope: "legacy",
+    state: "this-is-some-state",
+    grant_type: "authorization_code",
+    code_challenge: "some-code-challenge",
+    code_challenge_method: "S256",
+    response_type: "code",
+  };
+}
+
+it("returns the IdP SSO URL with SAMLRequest in JSON-mode", async ({ expect }) => {
+  await setupProjectWithSamlConnection("acme", "idp.acme.test");
+
+  const response = await niceBackendFetch("/api/v1/auth/saml/login/acme", {
+    method: "GET",
+    query: { ...loginQuery(), stack_response_mode: "json" },
+  });
+
+  expect(response.status).toBe(200);
+  expect(typeof (response.body as { location?: unknown }).location).toBe("string");
+  const location = new URL((response.body as { location: string }).location);
+  expect(location.host).toBe("idp.acme.test");
+  expect(location.pathname).toBe("/saml/sso");
+  expect(location.searchParams.get("SAMLRequest")).toBeTruthy();
+  expect(location.searchParams.get("RelayState")).toBe("this-is-some-state");
+});
+
+it("redirects + sets CSRF cookie in browser-redirect mode", async ({ expect }) => {
+  await setupProjectWithSamlConnection("acme", "idp.acme.test");
+
+  const response = await niceBackendFetch("/api/v1/auth/saml/login/acme", {
+    method: "GET",
+    redirect: "manual",
+    query: loginQuery(),
+  });
+
+  expect(response.status).toBe(307);
+  const location = response.headers.get("location");
+  expect(location).toBeTruthy();
+  expect(new URL(location!).host).toBe("idp.acme.test");
+  // CSRF cookie keyed to the AuthnRequest ID — snapshot serializer strips
+  // the suffix via the keyedCookieNamePrefixes registration.
+  expect(response.headers.get("set-cookie")).toMatch(/^stack-saml-inner-[^;]+=true;/);
+});
+
+it("returns 404 for an unknown connection ID", async ({ expect }) => {
+  await setupProjectWithSamlConnection("acme", "idp.acme.test");
+
+  const response = await niceBackendFetch("/api/v1/auth/saml/login/does-not-exist", {
+    method: "GET",
+    query: loginQuery(),
+  });
+
+  expect(response.status).toBe(404);
+});
+
+it("returns 403 when allowSignIn is false on the connection", async ({ expect }) => {
+  await setupProjectWithSamlConnection("acme", "idp.acme.test");
+  await Project.updateConfig({ "auth.saml.connections.acme.allowSignIn": false });
+
+  const response = await niceBackendFetch("/api/v1/auth/saml/login/acme", {
+    method: "GET",
+    query: loginQuery(),
+  });
+
+  expect(response.status).toBe(403);
+});
+
+it("rejects an invalid client_id", async ({ expect }) => {
+  await setupProjectWithSamlConnection("acme", "idp.acme.test");
+
+  const response = await niceBackendFetch("/api/v1/auth/saml/login/acme", {
+    method: "GET",
+    query: { ...loginQuery(), client_id: "00000000-0000-0000-0000-000000000000" },
+  });
+
+  // Tenancy lookup fails → InvalidOAuthClientIdOrSecret known error.
+  expect(response.status).not.toBe(200);
+  expect(response.status).not.toBe(307);
+});
diff --git a/apps/e2e/tests/backend/endpoints/api/v1/auth/saml/metadata.test.ts b/apps/e2e/tests/backend/endpoints/api/v1/auth/saml/metadata.test.ts
new file mode 100644
index 000000000..dbfcec020
--- /dev/null
+++ b/apps/e2e/tests/backend/endpoints/api/v1/auth/saml/metadata.test.ts
@@ -0,0 +1,72 @@
+import { it } from "../../../../../../helpers";
+import { Project, niceBackendFetch } from "../../../../../backend-helpers";
+
+/**
+ * Tests for GET /auth/saml/metadata/[connection_id].
+ *
+ * The IdP admin fetches this URL to wire up the SP side. We assert the
+ * returned XML contains entityID + AssertionConsumerService URLs that
+ * match what the IdP would actually need.
+ */
+
+async function setupSamlConnection(slug: string) {
+  const { projectId } = await Project.createAndSwitch();
+  await Project.updateConfig({
+    [`auth.saml.connections.${slug}.displayName`]: `${slug} SSO`,
+    [`auth.saml.connections.${slug}.allowSignIn`]: true,
+    [`auth.saml.connections.${slug}.idpEntityId`]: `https://idp.${slug}.test/saml/metadata`,
+    [`auth.saml.connections.${slug}.idpSsoUrl`]: `https://idp.${slug}.test/saml/sso`,
+    [`auth.saml.connections.${slug}.idpCertificate`]: "MIICertificatePlaceholderForMetadataTest=",
+  });
+  return { projectId };
+}
+
+it("returns SP metadata XML with entityID and ACS URL embedded", async ({ expect }) => {
+  const { projectId } = await setupSamlConnection("acme");
+
+  const response = await niceBackendFetch(
+    `/api/v1/auth/saml/metadata/acme?project_id=${projectId}`,
+    { method: "GET" },
+  );
+
+  expect(response.status).toBe(200);
+  expect(typeof response.body).toBe("string");
+  const xml = response.body as string;
+  // entityID should reference our metadata URL.
+  expect(xml).toContain('entityID="');
+  expect(xml).toContain("/api/v1/auth/saml/metadata/acme");
+  // AssertionConsumerService should point at the ACS endpoint.
+  expect(xml).toContain("/api/v1/auth/saml/acs/acme");
+  // Must declare the HTTP-POST binding for the IdP to know where to POST.
+  expect(xml).toContain("urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST");
+});
+
+it("returns 404 for an unknown connection ID", async ({ expect }) => {
+  const { projectId } = await setupSamlConnection("acme");
+
+  const response = await niceBackendFetch(
+    `/api/v1/auth/saml/metadata/does-not-exist?project_id=${projectId}`,
+    { method: "GET" },
+  );
+
+  expect(response.status).toBe(404);
+});
+
+it("returns 404 when the connection exists but has no IdP cert configured", async ({ expect }) => {
+  const { projectId } = await Project.createAndSwitch();
+  // Create a connection at branch level but skip the env-level IdP fields.
+  await Project.updateConfig({
+    "auth.saml.connections.partial.displayName": "Partial",
+    "auth.saml.connections.partial.allowSignIn": true,
+    // No idpEntityId / idpSsoUrl / idpCertificate.
+  });
+
+  const response = await niceBackendFetch(
+    `/api/v1/auth/saml/metadata/partial?project_id=${projectId}`,
+    { method: "GET" },
+  );
+
+  // Without the IdP fields, the SP metadata wouldn't be useful (the IdP
+  // and SP need to know each other's cert for trust). Return 404.
+  expect(response.status).toBe(404);
+});