diff --git a/docs/architecture.md b/docs/architecture.md
index 8800227..f61dccb 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -179,6 +179,11 @@ reservations, then opaque account ID.
 
 Anonymous traffic is balanced but never assigned using inferred identity.
 
+When no candidate is eligible, the portable core returns a typed `NoEligibleAccountsError` carrying
+the complete `CandidateExplanation` array produced during selection, including each opaque account
+ID and its rejection reason. AgentOS and other consumers should render those explanations rather
+than reimplementing eligibility policy.
+
 ## Generation-safe credential lifecycle
 
 Every subscription credential has a monotonically increasing router generation.
diff --git a/packages/core/test/selection.test.ts b/packages/core/test/selection.test.ts
index b71ebfb..e559113 100644
--- a/packages/core/test/selection.test.ts
+++ b/packages/core/test/selection.test.ts
@@ -228,16 +228,40 @@ describe("selectAccount", () => {
     Effect.gen(function* () {
       const failure = yield* Effect.flip(
         selectAccount({
-          candidates: [candidate("unknown", { usage: false })],
+          candidates: [
+            candidate("reauth", { requiresReauthentication: true }),
+            candidate("blocked", {
+              block: AccountBlock.make({
+                kind: "quota",
+                retryAt: now + hour
+              })
+            }),
+            candidate("unknown", { usage: false }),
+            candidate("short-exhausted", { shortUsed: 91 }),
+            candidate("weekly-exhausted", { weeklyUsed: 98 }),
+            candidate("too-old", { observedAt: now - day - 1 }),
+            candidate("expired-reset", { weeklyResetAt: now })
+          ],
           config: defaultRoutingConfig,
           now
         })
       )
 
       expect(failure).toBeInstanceOf(NoEligibleAccountsError)
-      expect(failure.explanations).toHaveLength(1)
-      expect(failure.explanations[0]?.accountId).toBe("unknown")
-      expect(failure.explanations[0]?.rejection.valueOrUndefined).toBe("usage_unknown")
+      expect(
+        failure.explanations.map((explanation) => [
+          explanation.accountId,
+          Option.getOrUndefined(explanation.rejection)
+        ])
+      ).toEqual([
+        [AccountId.make("reauth"), "reauthentication_required"],
+        [AccountId.make("blocked"), "active_block"],
+        [AccountId.make("unknown"), "usage_unknown"],
+        [AccountId.make("short-exhausted"), "short_headroom"],
+        [AccountId.make("weekly-exhausted"), "weekly_headroom"],
+        [AccountId.make("too-old"), "usage_too_old"],
+        [AccountId.make("expired-reset"), "weekly_reset_elapsed"]
+      ])
     })
   )
 })