diff --git a/README.md b/README.md
index 2997b45..e443a72 100644
--- a/README.md
+++ b/README.md
@@ -14,7 +14,7 @@ Export your personal nutrition, biometric, and food-log data from [Cronometer](h
 - **Date selection** — `--today`, `--days N`, or `--start YYYY-MM-DD --end YYYY-MM-DD` on every subcommand
 - **Single static binary** — no Python or Node runtime; drop it in `~/bin/` and go
 - **Credentials via env** — `CRONOMETER_USERNAME` / `CRONOMETER_PASSWORD`, no config file needed
-- **Built for agents** — designed to be called as a terminal tool by LLMs (Claude, hermes-agent, etc.)
+- **Built for agents** — designed to be called as a terminal tool by LLMs (Claude, hermes-agent, etc.); run `crono-export prime` for a one-screen orientation (I/O contract, subcommands, jq recipes)
 
 ## Quick Start
 
diff --git a/cmd/prime.go b/cmd/prime.go
new file mode 100644
index 0000000..cf533f0
--- /dev/null
+++ b/cmd/prime.go
@@ -0,0 +1,103 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/spf13/cobra"
+)
+
+const primeText = `crono-export — primer for LLM agents
+=====================================
+
+WHAT IT IS
+  A CLI that reads your personal Cronometer data (per-food log, daily totals,
+  weight/biometrics, exercises, notes) and prints it as JSON on stdout.
+
+I/O CONTRACT
+  - Output: pretty-printed JSON ARRAY on stdout. THIS IS THE ONLY MODE.
+    There is no --json / --format flag; JSON is always the format.
+  - Errors: human-readable text on stderr. You do NOT need '2>&1'.
+  - Empty result '[]' = success with zero rows in the window, not an error.
+  - Exit code: 0 success, non-zero on auth or network failure.
+  - Filter with jq. Don't pipe to python — output is already JSON.
+
+AUTH
+  Set both env vars before invoking. No config file or token cache; the CLI
+  logs in on every run.
+    CRONOMETER_USERNAME   your Cronometer email
+    CRONOMETER_PASSWORD   your Cronometer password
+
+DATE FLAGS  (every export subcommand accepts these)
+  --today                              just today (LOCAL calendar date)
+  --days N                             last N days, ending today
+  --start YYYY-MM-DD --end YYYY-MM-DD  explicit inclusive window
+  (no flag)                            last 7 days, ending today
+
+SUBCOMMANDS
+
+  servings   — per-food log: one row per food eaten, full nutrient breakdown.
+    Typed numbers. Keys (subset):
+      RecordedTime, Group, FoodName, QuantityValue, QuantityUnits,
+      EnergyKcal, ProteinG, CarbsG, FiberG, FatG, SodiumMg, CalciumMg,
+      IronMg, B12Mg, VitaminDUI, Omega3G, Omega6G, ... (60+ nutrients).
+
+  nutrition  — daily totals: one row per day across all foods logged that day.
+    String-keyed (raw CSV columns, ALL VALUES ARE STRINGS — cast in jq).
+    Keys (subset):
+      "Date", "Energy (kcal)", "Protein (g)", "Carbs (g)", "Fat (g)",
+      "Fiber (g)", "Sodium (mg)", "Iron (mg)", "Calcium (mg)",
+      "B12 (Cobalamin) (µg)", "Cholesterol (mg)", "Completed", ...
+
+  biometrics — weight, body fat, blood pressure, custom metrics.
+    Typed. Keys: RecordedTime, Metric, Unit, Amount.
+
+  exercises  — logged cardio / strength / custom activities.
+    Typed. Keys: RecordedTime, Exercise, Minutes, CaloriesBurned, Group.
+
+  notes      — user-entered notes per day. String-keyed (raw CSV).
+
+EXAMPLES
+
+  # Today's macros, as numbers
+  crono-export nutrition --today | jq '.[] | {
+    date:    .Date,
+    kcal:    (."Energy (kcal)" | tonumber),
+    protein: (."Protein (g)"   | tonumber),
+    carbs:   (."Carbs (g)"     | tonumber),
+    fat:     (."Fat (g)"       | tonumber)
+  }'
+
+  # 7-day protein total (servings is typed — no tonumber needed)
+  crono-export servings --days 7 | jq '[.[] | .ProteinG] | add'
+
+  # All foods from today's breakfast
+  crono-export servings --today | jq '[.[] | select(.Group == "Breakfast") | .FoodName]'
+
+  # Latest weight reading in a 30-day window
+  crono-export biometrics --days 30 | jq 'map(select(.Metric == "Weight")) | sort_by(.RecordedTime) | last'
+
+GOTCHAS
+  - "Today" is your LOCAL calendar day, not UTC.
+  - 'nutrition' and 'notes' values are STRINGS (raw CSV) — cast with
+    'jq tonumber' when doing math. 'servings', 'biometrics', 'exercises'
+    are already typed numbers.
+  - Cronometer logs by calendar day; nothing here is real-time. The same
+    --today call moments apart returns the same data.
+`
+
+var primeCmd = &cobra.Command{
+	Use:   "prime",
+	Short: "Print an LLM-targeted primer (I/O contract, subcommands, jq recipes)",
+	Long: `Print a one-screen primer aimed at LLM agents calling this CLI as a tool.
+Covers the output contract (JSON-on-stdout, no --json flag), auth env vars,
+the subcommands and what their rows look like, the shared date flags, and a
+few jq recipes for common questions.`,
+	RunE: func(cmd *cobra.Command, _ []string) error {
+		_, err := fmt.Fprint(cmd.OutOrStdout(), primeText)
+		return err
+	},
+}
+
+func init() {
+	rootCmd.AddCommand(primeCmd)
+}
diff --git a/cmd/root.go b/cmd/root.go
index 93bc471..06947ff 100644
--- a/cmd/root.go
+++ b/cmd/root.go
@@ -20,7 +20,10 @@ Credentials must be supplied via environment variables:
   CRONOMETER_PASSWORD  your Cronometer password
 
 Designed for use by personal LLM agents and scripts that want structured
-nutrition data — for example, an LLM-driven bariatric or fitness coach.`,
+nutrition data — for example, an LLM-driven bariatric or fitness coach.
+
+LLM agents: run 'crono-export prime' for a one-screen orientation
+(I/O contract, subcommands, date flags, jq recipes).`,
 	SilenceUsage:  true,
 	SilenceErrors: true,
 }