diff --git a/src/lib/auth/__tests__/session-locale-reconcile.test.ts b/src/lib/auth/__tests__/session-locale-reconcile.test.ts
index 4766eaf1f..01b356880 100644
--- a/src/lib/auth/__tests__/session-locale-reconcile.test.ts
+++ b/src/lib/auth/__tests__/session-locale-reconcile.test.ts
@@ -66,14 +66,17 @@ vi.mock("next/headers", () => ({
 import { prisma } from "@/lib/db";
 import { getSession } from "../session";
 
+/** A cookie value in the shape `mintSessionSecret` produces. */
+const SECRET = `hls_${"a".repeat(64)}`;
+
 const THIRTY_DAYS_MS = 30 * 24 * 60 * 60 * 1000;
 
 function seedSession(userLocale: string | null) {
-  cookieState.sessionId = "hls_raw-secret";
+  cookieState.sessionId = SECRET;
   store.row = {
     id: "sess-1",
     userId: "user-1",
-    tokenHash: "hash:hls_raw-secret",
+    tokenHash: `hash:${SECRET}`,
     // Far enough out that the sliding-expiry refresh stays quiet.
     expiresAt: new Date(Date.now() + THIRTY_DAYS_MS),
     lastActiveAt: new Date(),
diff --git a/src/lib/auth/__tests__/session.test.ts b/src/lib/auth/__tests__/session.test.ts
index 91ec94f5f..3646fbda9 100644
--- a/src/lib/auth/__tests__/session.test.ts
+++ b/src/lib/auth/__tests__/session.test.ts
@@ -91,6 +91,11 @@ import {
 } from "../session";
 import { SessionStoreUnavailableError } from "@/lib/api-errors";
 
+/** A cookie value in the shape `mintSessionSecret` produces. */
+const SECRET = `hls_${"a".repeat(64)}`;
+const FORGED_SECRET = `hls_${"b".repeat(64)}`;
+const UNKNOWN_SECRET = `hls_${"c".repeat(64)}`;
+
 const THIRTY_DAYS_MS = 30 * 24 * 60 * 60 * 1000;
 
 function seed(row: Partial<SessionRow> & { id: string }): SessionRow {
@@ -155,21 +160,21 @@ describe("createSession", () => {
 
 describe("getSession — cookie resolution", () => {
   it("resolves a modern cookie by its hash, not by primary key", async () => {
-    const row = seed({ id: "sess_cuid_1", tokenHash: "hash:hls_abc" });
-    cookieState.sessionId = "hls_abc";
+    const row = seed({ id: "sess_cuid_1", tokenHash: `hash:${SECRET}` });
+    cookieState.sessionId = SECRET;
 
     const result = await getSession();
 
     expect(result?.session.id).toBe(row.id);
     // The lookup went through the hash column.
     expect(prisma.session.findUnique).toHaveBeenCalledWith(
-      expect.objectContaining({ where: { tokenHash: "hash:hls_abc" } }),
+      expect.objectContaining({ where: { tokenHash: `hash:${SECRET}` } }),
     );
   });
 
   it("refuses a modern-shaped cookie whose hash matches nothing", async () => {
     seed({ id: "sess_cuid_1", tokenHash: "hash:hls_real" });
-    cookieState.sessionId = "hls_forged";
+    cookieState.sessionId = FORGED_SECRET;
 
     await expect(getSession()).resolves.toBeNull();
   });
@@ -202,10 +207,10 @@ describe("getSession — sliding expiry", () => {
   it("extends a modern session and re-emits the same secret", async () => {
     seed({
       id: "sess_cuid_1",
-      tokenHash: "hash:hls_abc",
+      tokenHash: `hash:${SECRET}`,
       expiresAt: new Date(Date.now() + 2 * 24 * 60 * 60 * 1000),
     });
-    cookieState.sessionId = "hls_abc";
+    cookieState.sessionId = SECRET;
 
     await getSession();
 
@@ -215,7 +220,7 @@ describe("getSession — sliding expiry", () => {
     const cookieValue = cookieState.set.mock.calls.find(
       (c) => c[0] === "healthlog_session",
     )![1];
-    expect(cookieValue).toBe("hls_abc");
+    expect(cookieValue).toBe(SECRET);
   });
 
   it("withholds the extension from a legacy session so the id path drains", async () => {
@@ -270,7 +275,7 @@ describe("a session lookup that fails is not a missing session", () => {
   }
 
   it("getSession raises a 503 with a retryable code", async () => {
-    cookieState.sessionId = "hls_abc";
+    cookieState.sessionId = SECRET;
     failLookup();
     const err = await getSession().catch((e: unknown) => e);
     expect(err).toBeInstanceOf(SessionStoreUnavailableError);
@@ -284,26 +289,64 @@ describe("a session lookup that fails is not a missing session", () => {
 
   it("validateSessionFromCookieValue raises instead of answering null", async () => {
     failLookup();
-    await expect(
-      validateSessionFromCookieValue("hls_abc"),
-    ).rejects.toBeInstanceOf(SessionStoreUnavailableError);
+    await expect(validateSessionFromCookieValue(SECRET)).rejects.toBeInstanceOf(
+      SessionStoreUnavailableError,
+    );
   });
 
   it("validateSessionWithCreatedAt raises instead of answering null", async () => {
     failLookup();
-    await expect(
-      validateSessionWithCreatedAt("hls_abc"),
-    ).rejects.toBeInstanceOf(SessionStoreUnavailableError);
+    await expect(validateSessionWithCreatedAt(SECRET)).rejects.toBeInstanceOf(
+      SessionStoreUnavailableError,
+    );
+  });
+
+  it("raises for a pool that had no connection free (P2024)", async () => {
+    vi.mocked(prisma.session.findUnique).mockRejectedValueOnce(
+      Object.assign(new Error("connection pool exhausted"), {
+        code: "P2024",
+      }) as never,
+    );
+    await expect(validateSessionFromCookieValue(SECRET)).rejects.toBeInstanceOf(
+      SessionStoreUnavailableError,
+    );
+  });
+
+  it("answers null, not 503, when the query refused the value itself", async () => {
+    // Postgres 22021: a byte sequence it will not accept in text. Nothing is
+    // wrong with the store; a 503 here would let any anonymous caller produce
+    // server errors on demand.
+    vi.mocked(prisma.session.findUnique).mockRejectedValueOnce(
+      Object.assign(new Error("invalid byte sequence for encoding"), {
+        code: "P2010",
+        meta: { driverAdapterError: { cause: { originalCode: "22021" } } },
+      }) as never,
+    );
+    await expect(validateSessionFromCookieValue("sess_legacy")).resolves.toBe(
+      null,
+    );
+  });
+
+  it.each([
+    ["a NUL byte", "\u0000"],
+    ["a percent-encoded NUL", "%00"],
+    ["a short secret", "hls_abc"],
+    ["a secret with non-hex", `hls_${"z".repeat(64)}`],
+    ["an overlong legacy id", "c".repeat(65)],
+  ])("never asks the database about %s", async (_name, value) => {
+    vi.mocked(prisma.session.findUnique).mockClear();
+    await expect(validateSessionFromCookieValue(value)).resolves.toBe(null);
+    expect(prisma.session.findUnique).not.toHaveBeenCalled();
   });
 
   it("still answers null for a cookie that matches nothing", async () => {
-    await expect(validateSessionFromCookieValue("hls_nope")).resolves.toBe(
+    await expect(validateSessionFromCookieValue(UNKNOWN_SECRET)).resolves.toBe(
       null,
     );
   });
 
   it("logout still clears the cookie when the lookup fails", async () => {
-    cookieState.sessionId = "hls_abc";
+    cookieState.sessionId = SECRET;
     failLookup();
     await expect(destroySession()).resolves.toBeUndefined();
     expect(cookieState.delete).toHaveBeenCalledWith("healthlog_session");
@@ -314,8 +357,8 @@ describe("destroySession", () => {
   it("deletes the row the secret resolves to, not the cookie value", async () => {
     // The cookie is no longer the primary key, so a logout that deletes by
     // cookie value would no-op and leave the session alive.
-    seed({ id: "sess_cuid_1", tokenHash: "hash:hls_abc" });
-    cookieState.sessionId = "hls_abc";
+    seed({ id: "sess_cuid_1", tokenHash: `hash:${SECRET}` });
+    cookieState.sessionId = SECRET;
 
     await destroySession();
 
diff --git a/src/lib/auth/session.ts b/src/lib/auth/session.ts
index 5d2d53640..d779fa9c5 100644
--- a/src/lib/auth/session.ts
+++ b/src/lib/auth/session.ts
@@ -6,7 +6,7 @@ import { hashToken } from "@/lib/auth/hmac";
 import { ensureDbCompatibility } from "@/lib/db-compat";
 import { getEvent } from "@/lib/logging/context";
 import { shouldEmitSecureCookie } from "@/lib/auth/secure-cookie";
-import { isP2025 } from "@/lib/prisma-errors";
+import { isDatabaseUnavailable, isP2025 } from "@/lib/prisma-errors";
 import { SessionStoreUnavailableError } from "@/lib/api-errors";
 import { locales, type Locale } from "@/lib/i18n/config";
 import { LOCALE_COOKIE, setLocaleCookie } from "@/lib/i18n/locale-cookie";
@@ -41,6 +41,19 @@ function mintSessionSecret(): string {
   return `${SESSION_TOKEN_PREFIX}${randomBytes(SESSION_SECRET_BYTES).toString("hex")}`;
 }
 
+/** Exactly what `mintSessionSecret` produces. */
+const SESSION_SECRET_SHAPE = new RegExp(
+  `^${SESSION_TOKEN_PREFIX}[0-9a-f]{${SESSION_SECRET_BYTES * 2}}$`,
+);
+
+/**
+ * What a legacy cookie (a `Session.id`, a cuid) can look like. Deliberately a
+ * character class rather than the cuid grammar: it exists to keep a value that
+ * cannot be an id away from the query, where a NUL byte or invalid UTF-8 makes
+ * Postgres raise instead of finding nothing.
+ */
+const LEGACY_SESSION_ID_SHAPE = /^[A-Za-z0-9_-]{1,64}$/;
+
 /**
  * Resolve the session row a cookie value refers to, or null.
  *
@@ -74,6 +87,14 @@ function mintSessionSecret(): string {
  * login from the deploy onward is on a secret immediately.
  */
 async function findSessionByCookie(cookieValue: string) {
+  // A malformed value matches no session: it is answered as one, before the
+  // database is asked, so an anonymous caller cannot turn a crafted cookie into
+  // a query error.
+  if (cookieValue.startsWith(SESSION_TOKEN_PREFIX)) {
+    if (!SESSION_SECRET_SHAPE.test(cookieValue)) return null;
+  } else if (!LEGACY_SESSION_ID_SHAPE.test(cookieValue)) {
+    return null;
+  }
   try {
     if (cookieValue.startsWith(SESSION_TOKEN_PREFIX)) {
       return await prisma.session.findUnique({
@@ -94,7 +115,18 @@ async function findSessionByCookie(cookieValue: string) {
     // the callers, so a database restart answered 401 and every client
     // signed its person out. It is a 503 now, with a code a client can retry
     // on (`auth.session.unavailable`).
-    throw new SessionStoreUnavailableError(error);
+    //
+    // Only for a store that could not be reached. Any other query error says
+    // something about the value asked for, not about the store, and is
+    // answered like a value that matches nothing: a 503 there would let any
+    // anonymous caller produce server errors at will.
+    if (isDatabaseUnavailable(error)) {
+      throw new SessionStoreUnavailableError(error);
+    }
+    getEvent()?.addWarning(
+      `session lookup refused the value: ${error instanceof Error ? error.name : "unknown"}`,
+    );
+    return null;
   }
 }
 
diff --git a/src/lib/prisma-errors.ts b/src/lib/prisma-errors.ts
index 67745ad7a..d5612ea8e 100644
--- a/src/lib/prisma-errors.ts
+++ b/src/lib/prisma-errors.ts
@@ -13,6 +13,8 @@
  * unique-constraint code.
  */
 
+import { classifyRestoreFailure } from "@/lib/export/restore-failure-cause";
+
 /** Narrow a thrown Prisma error to a specific known-request code. */
 function hasPrismaCode(err: unknown, code: string): boolean {
   return (
@@ -37,3 +39,25 @@ export function isP2002(err: unknown): boolean {
 export function isP2025(err: unknown): boolean {
   return hasPrismaCode(err, "P2025");
 }
+
+/**
+ * Whether a query failed because the database could not be reached, as opposed
+ * to refusing what it was asked. The first is worth a retry and is a 503; the
+ * second is an answer about the input.
+ *
+ * Reads the same signals the restore classifier does (`connection` and
+ * `timeout`: a connection SQLSTATE, a server shutting down, a statement or
+ * idle timeout, Prisma's P1001 / P1002 / P1017 / P2028, a terminated
+ * connection), plus the two that only a pooled request meets: P2024, no
+ * connection free in the pool within its timeout, and P1008, an operation that
+ * timed out.
+ */
+export function isDatabaseUnavailable(err: unknown): boolean {
+  if (hasPrismaCode(err, "P2024") || hasPrismaCode(err, "P1008")) return true;
+  const { cause } = classifyRestoreFailure(err);
+  if (cause === "connection" || cause === "timeout") return true;
+  const message = err instanceof Error ? err.message : "";
+  return /ECONNREFUSED|ECONNRESET|ETIMEDOUT|Can't reach database server|timed out/i.test(
+    message,
+  );
+}