diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5e4a7005d..44a2e6021 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,7 @@ Status: unreleased.
 ### Changes
 - Doctor: warn on gateway exposure without auth. (#2016) Thanks @Alex-Alaniz.
 - Docs: add Vercel AI Gateway to providers sidebar. (#1901) Thanks @jerilynzheng.
+- Agents: expand cron tool description with full schema docs. (#1988) Thanks @tomascupr.
 
 ## 2026.1.24-3
 
diff --git a/src/agents/tools/cron-tool.ts b/src/agents/tools/cron-tool.ts
index a1d218dd7..739b3ada3 100644
--- a/src/agents/tools/cron-tool.ts
+++ b/src/agents/tools/cron-tool.ts
@@ -133,8 +133,50 @@ export function createCronTool(opts?: CronToolOptions): AnyAgentTool {
   return {
     label: "Cron",
     name: "cron",
-    description:
-      "Manage Gateway cron jobs (status/list/add/update/remove/run/runs) and send wake events. Use `jobId` as the canonical identifier; `id` is accepted for compatibility. Use `contextMessages` (0-10) to add previous messages as context to the job text.",
+    description: `Manage Gateway cron jobs (status/list/add/update/remove/run/runs) and send wake events.
+
+ACTIONS:
+- status: Check cron scheduler status
+- list: List jobs (use includeDisabled:true to include disabled)
+- add: Create job (requires job object, see schema below)
+- update: Modify job (requires jobId + patch object)
+- remove: Delete job (requires jobId)
+- run: Trigger job immediately (requires jobId)
+- runs: Get job run history (requires jobId)
+- wake: Send wake event (requires text, optional mode)
+
+JOB SCHEMA (for add action):
+{
+  "name": "string (optional)",
+  "schedule": { ... },      // Required: when to run
+  "payload": { ... },       // Required: what to execute
+  "sessionTarget": "main" | "isolated",  // Required
+  "enabled": true | false   // Optional, default true
+}
+
+SCHEDULE TYPES (schedule.kind):
+- "at": One-shot at absolute time
+  { "kind": "at", "atMs": <unix-ms-timestamp> }
+- "every": Recurring interval
+  { "kind": "every", "everyMs": <interval-ms>, "anchorMs": <optional-start-ms> }
+- "cron": Cron expression
+  { "kind": "cron", "expr": "<cron-expression>", "tz": "<optional-timezone>" }
+
+PAYLOAD TYPES (payload.kind):
+- "systemEvent": Injects text as system event into session
+  { "kind": "systemEvent", "text": "<message>" }
+- "agentTurn": Runs agent with message (isolated sessions only)
+  { "kind": "agentTurn", "message": "<prompt>", "model": "<optional>", "thinking": "<optional>", "timeoutSeconds": <optional>, "deliver": <optional-bool>, "channel": "<optional>", "to": "<optional>", "bestEffortDeliver": <optional-bool> }
+
+CRITICAL CONSTRAINTS:
+- sessionTarget="main" REQUIRES payload.kind="systemEvent"
+- sessionTarget="isolated" REQUIRES payload.kind="agentTurn"
+
+WAKE MODES (for wake action):
+- "next-heartbeat" (default): Wake on next heartbeat
+- "now": Wake immediately
+
+Use jobId as the canonical identifier; id is accepted for compatibility. Use contextMessages (0-10) to add previous messages as context to the job text.`,
     parameters: CronToolSchema,
     execute: async (_toolCallId, args) => {
       const params = args as Record<string, unknown>;