diff --git a/src/oauth.ts b/src/oauth.ts
index bbcca01..07994e5 100644
--- a/src/oauth.ts
+++ b/src/oauth.ts
@@ -429,6 +429,22 @@ export async function completeOAuthFlow(
   return { sessionId, session };
 }
 
+/**
+ * Does this bearer token have the shape of a session ID we issued?
+ *
+ * Session IDs are `randomBytes(32).toString("hex")` - 64 lowercase hex chars,
+ * no dashes. Digital Samba developer keys are UUIDs, so the two never collide.
+ *
+ * The HTTP transport uses this to tell "expired OAuth session, tell the client
+ * to re-authenticate" apart from "legacy developer key, pass it through". Get
+ * it wrong and an expired session is sent to the API as if it were a key, so
+ * the client sees the API's "Unauthenticated" rather than a 401 and never
+ * learns to re-authorise.
+ */
+export function isOAuthSessionId(token: string): boolean {
+  return /^[0-9a-f]{64}$/.test(token);
+}
+
 /**
  * Get session by ID
  */
diff --git a/src/transports/http.ts b/src/transports/http.ts
index db9ff27..61e8332 100644
--- a/src/transports/http.ts
+++ b/src/transports/http.ts
@@ -43,6 +43,7 @@ import {
   exchangeAuthorizationCode,
   exchangeCodeForTokens,
   getRegisteredClientCount,
+  isOAuthSessionId,
 } from "../oauth.js";
 
 export interface HttpTransportConfig {
@@ -132,18 +133,25 @@ function authMiddleware(requireAuth: boolean) {
       let sessionId: string | null = null;
       if (token.startsWith("oauth:")) {
         sessionId = token.substring(6);
-      } else {
-        // Try to use token directly as session ID (Claude Desktop DCR flow)
-        const directSession = await getAccessTokenFromSession(token);
-        if (directSession) {
-          sessionId = token;
-        }
+      } else if (isOAuthSessionId(token)) {
+        // Shaped like a session ID we issued, so treat it as one even when the
+        // session is gone. Falling through to the developer-key branch here
+        // would send a dead session ID to the API as if it were a key: the
+        // client gets a confusing "Unauthenticated" from the API instead of a
+        // 401, never learns to re-authenticate, and can never recover.
+        sessionId = token;
       }
 
       if (sessionId) {
         const accessToken = await getAccessTokenFromSession(sessionId);
 
         if (!accessToken) {
+          // Point the client at the OAuth metadata so it can re-authorize
+          // itself rather than needing the connector removed and re-added.
+          res.setHeader(
+            "WWW-Authenticate",
+            'Bearer realm="mcp", error="invalid_token", error_description="The OAuth session has expired", resource_metadata="/.well-known/oauth-protected-resource"',
+          );
           res.status(401).json({
             jsonrpc: "2.0",
             error: {
diff --git a/tests/unit/oauth-session-id-shape.test.ts b/tests/unit/oauth-session-id-shape.test.ts
new file mode 100644
index 0000000..23032ef
--- /dev/null
+++ b/tests/unit/oauth-session-id-shape.test.ts
@@ -0,0 +1,65 @@
+/**
+ * Unit tests for isOAuthSessionId (src/oauth.ts)
+ *
+ * This predicate decides whether an unrecognised bearer token is treated as an
+ * expired OAuth session (401 + WWW-Authenticate, so the client re-authorises)
+ * or as a legacy direct developer key (passed to the API as-is).
+ *
+ * Getting it wrong in either direction is user-visible:
+ *  - too narrow: expired sessions are sent to the API as developer keys, the
+ *    client sees "Unauthenticated" instead of a 401, and can never recover.
+ *  - too broad: legacy developer keys are rejected as expired sessions.
+ *
+ * @module tests/unit/oauth-session-id-shape
+ */
+
+import { randomBytes } from "node:crypto";
+import { isOAuthSessionId } from "../../src/oauth.js";
+
+describe("isOAuthSessionId", () => {
+  it("accepts session IDs generated the way oauth.ts generates them", () => {
+    for (let i = 0; i < 20; i++) {
+      const sessionId = randomBytes(32).toString("hex");
+      expect(isOAuthSessionId(sessionId)).toBe(true);
+    }
+  });
+
+  it("rejects Digital Samba developer keys, which are UUIDs", () => {
+    // Real-world shape, from docs/digital-samba-api.md
+    expect(isOAuthSessionId("fc16892a-556b-4c2d-a522-82e6b3e884cc")).toBe(
+      false,
+    );
+    expect(isOAuthSessionId("57670ebd-0de2-4f92-8bce-661bec142dde")).toBe(
+      false,
+    );
+  });
+
+  it("rejects tokens of the wrong length", () => {
+    expect(isOAuthSessionId("a".repeat(63))).toBe(false);
+    expect(isOAuthSessionId("a".repeat(65))).toBe(false);
+    expect(isOAuthSessionId(randomBytes(16).toString("hex"))).toBe(false);
+  });
+
+  it("rejects non-hex characters", () => {
+    // 64 chars but 'g' is not hex
+    expect(isOAuthSessionId("g".repeat(64))).toBe(false);
+    expect(isOAuthSessionId(`${"a".repeat(63)}Z`)).toBe(false);
+  });
+
+  it("rejects uppercase hex, which we never emit", () => {
+    const upper = randomBytes(32).toString("hex").toUpperCase();
+    expect(isOAuthSessionId(upper)).toBe(false);
+  });
+
+  it("rejects empty and whitespace-padded tokens", () => {
+    expect(isOAuthSessionId("")).toBe(false);
+    expect(isOAuthSessionId(` ${"a".repeat(64)}`)).toBe(false);
+    expect(isOAuthSessionId(`${"a".repeat(64)}\n`)).toBe(false);
+  });
+
+  it("rejects the oauth: prefixed form, which is handled separately", () => {
+    expect(isOAuthSessionId(`oauth:${randomBytes(32).toString("hex")}`)).toBe(
+      false,
+    );
+  });
+});