fix: only write AI model prices that changed (#27923)

Previously, the AI Gateway price seeder rewrote every row of
`ai_model_prices` on each server start, because `ON CONFLICT` fires on a
key conflict rather than on a value difference. `updated_at` therefore
recorded when the server last restarted rather than when a price last
changed.

Guard the `DO UPDATE` branch so a conflicting row is only rewritten when
one of its four prices differs. The comparison uses `IS DISTINCT FROM`
rather than `<>` because the price columns are nullable, and `<>` yields
NULL when either side is NULL, which would skip the update and leave a
stale price in place.

Related to
https://linear.app/codercom/issue/AIGOV-567/experimental-cli-command-to-set-prices-for-unpriced-ai-models

> [!NOTE]
> Initially generated by Claude Opus 5, modified and reviewed by
@ssncferreira

coderd/aibridge/prices/prices_test.go

@@ -1,6 +1,7 @@
 package prices_test
 
 import (
+	"fmt"
 	"testing"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -58,6 +59,7 @@ func TestSeedFromBytes(t *testing.T) {
 		require.Equal(t, int64(25_000_000), opus.OutputPrice.Int64)
 		require.Equal(t, int64(500_000), opus.CacheReadPrice.Int64)
 		require.Equal(t, int64(6_250_000), opus.CacheWritePrice.Int64)
+		require.Equal(t, opus.CreatedAt, opus.UpdatedAt)
 
 		// Spot-check a row where the seed has a NULL price (OpenAI does not
 		// publish a cache_write_price). The column should land as SQL NULL.
@@ -90,11 +92,11 @@ func TestSeedFromBytes(t *testing.T) {
 		})
 		require.NoError(t, err)
 
-		// Prices must be identical across runs and CreatedAt must be
-		// preserved (only updated_at moves on a no-op upsert).
+		// A re-seed that changes nothing must not touch the row at all.
 		require.Equal(t, first.InputPrice, second.InputPrice)
 		require.Equal(t, first.OutputPrice, second.OutputPrice)
 		require.Equal(t, first.CreatedAt, second.CreatedAt)
+		require.Equal(t, first.UpdatedAt, second.UpdatedAt)
 	})
 
 	t.Run("OverwritesExistingPrices", func(t *testing.T) {
@@ -114,6 +116,10 @@ func TestSeedFromBytes(t *testing.T) {
 			"cache_read_price": 3,
 			"cache_write_price": 4
 		}]`)))
+		before, err := db.GetAIModelPriceByProviderModel(ctx, database.GetAIModelPriceByProviderModelParams{
+			Provider: "openai", Model: "gpt-4o",
+		})
+		require.NoError(t, err)
 
 		require.NoError(t, prices.SeedFromBytes(ctx, db, []byte(testSeedJSON)))
 
@@ -126,6 +132,8 @@ func TestSeedFromBytes(t *testing.T) {
 		require.Equal(t, int64(1_250_000), got.CacheReadPrice.Int64)
 		require.False(t, got.CacheWritePrice.Valid)
 		require.Zero(t, got.CacheWritePrice.Int64)
+		require.Equal(t, before.CreatedAt, got.CreatedAt)
+		require.True(t, got.UpdatedAt.After(before.UpdatedAt))
 	})
 
 	t.Run("LeavesOrphanRowsUntouched", func(t *testing.T) {
@@ -174,6 +182,101 @@ func TestSeedFromBytes(t *testing.T) {
 		require.True(t, got.InputPrice.Valid)
 		require.Equal(t, int64(2_500_000), got.InputPrice.Int64)
 	})
+
+	// Every price column counts toward the comparison, and a NULL on either
+	// side counts as a difference.
+	t.Run("UpdatedAtTracksPriceChanges", func(t *testing.T) {
+		t.Parallel()
+
+		key := database.GetAIModelPriceByProviderModelParams{Provider: "openai", Model: "gpt-4o"}
+		seed := func(priceFields string) []byte {
+			return fmt.Appendf(nil, `[{"provider": %q, "model": %q, %s}]`, key.Provider, key.Model, priceFields)
+		}
+
+		tests := []struct {
+			name             string
+			initial, updated string
+		}{
+			{
+				name:    "InputPriceChanged",
+				initial: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+				updated: `"input_price": 111, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+			},
+			{
+				name:    "InputPriceSetFromNull",
+				initial: `"input_price": null, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+				updated: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+			},
+			{
+				name:    "InputPriceClearedToNull",
+				initial: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+				updated: `"input_price": null, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+			},
+			{
+				name:    "OutputPriceChanged",
+				initial: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+				updated: `"input_price": 100, "output_price": 222, "cache_read_price": 300, "cache_write_price": 400`,
+			},
+			{
+				name:    "OutputPriceSetFromNull",
+				initial: `"input_price": 100, "output_price": null, "cache_read_price": 300, "cache_write_price": 400`,
+				updated: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+			},
+			{
+				name:    "OutputPriceClearedToNull",
+				initial: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+				updated: `"input_price": 100, "output_price": null, "cache_read_price": 300, "cache_write_price": 400`,
+			},
+			{
+				name:    "CacheReadPriceChanged",
+				initial: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+				updated: `"input_price": 100, "output_price": 200, "cache_read_price": 333, "cache_write_price": 400`,
+			},
+			{
+				name:    "CacheReadPriceSetFromNull",
+				initial: `"input_price": 100, "output_price": 200, "cache_read_price": null, "cache_write_price": 400`,
+				updated: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+			},
+			{
+				name:    "CacheReadPriceClearedToNull",
+				initial: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+				updated: `"input_price": 100, "output_price": 200, "cache_read_price": null, "cache_write_price": 400`,
+			},
+			{
+				name:    "CacheWritePriceChanged",
+				initial: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+				updated: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 444`,
+			},
+			{
+				name:    "CacheWritePriceSetFromNull",
+				initial: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": null`,
+				updated: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+			},
+			{
+				name:    "CacheWritePriceClearedToNull",
+				initial: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": 400`,
+				updated: `"input_price": 100, "output_price": 200, "cache_read_price": 300, "cache_write_price": null`,
+			},
+		}
+
+		for _, tt := range tests {
+			t.Run(tt.name, func(t *testing.T) {
+				t.Parallel()
+				ctx := testutil.Context(t, testutil.WaitShort)
+				db, _ := dbtestutil.NewDB(t)
+
+				require.NoError(t, prices.SeedFromBytes(ctx, db, seed(tt.initial)))
+				before, err := db.GetAIModelPriceByProviderModel(ctx, key)
+				require.NoError(t, err)
+
+				require.NoError(t, prices.SeedFromBytes(ctx, db, seed(tt.updated)))
+				after, err := db.GetAIModelPriceByProviderModel(ctx, key)
+				require.NoError(t, err)
+
+				require.True(t, after.UpdatedAt.After(before.UpdatedAt), "updated_at should advance when a price changes")
+			})
+		}
+	})
 }
 
 // TestSeed exercises the real embedded prices.json so we catch a corrupted,

coderd/database/querier.go

@@ -1611,6 +1611,9 @@ type sqlcQuerier interface {
 	// Upsert a batch of (provider, model) rows from a JSON array. Each element
 	// must have provider, model, and the four price fields; null prices are
 	// written as SQL NULL.
+	// A conflicting row is only rewritten when a price differs, so updated_at
+	// records when a price last changed. Prices are nullable and a NULL on
+	// either side counts as a difference.
 	UpsertAIModelPrices(ctx context.Context, seed json.RawMessage) error
 	// Returns true if a new rows was inserted, false otherwise.
 	UpsertAISeatState(ctx context.Context, arg UpsertAISeatStateParams) (bool, error)

coderd/database/queries.sql.go

@@ -3459,11 +3459,25 @@ ON CONFLICT (provider, model) DO UPDATE SET
 	cache_read_price  = EXCLUDED.cache_read_price,
 	cache_write_price = EXCLUDED.cache_write_price,
 	updated_at        = NOW()
+WHERE (
+	ai_model_prices.input_price,
+	ai_model_prices.output_price,
+	ai_model_prices.cache_read_price,
+	ai_model_prices.cache_write_price
+) IS DISTINCT FROM (
+	EXCLUDED.input_price,
+	EXCLUDED.output_price,
+	EXCLUDED.cache_read_price,
+	EXCLUDED.cache_write_price
+)
 `
 
 // Upsert a batch of (provider, model) rows from a JSON array. Each element
 // must have provider, model, and the four price fields; null prices are
 // written as SQL NULL.
+// A conflicting row is only rewritten when a price differs, so updated_at
+// records when a price last changed. Prices are nullable and a NULL on
+// either side counts as a difference.
 func (q *sqlQuerier) UpsertAIModelPrices(ctx context.Context, seed json.RawMessage) error {
 	_, err := q.db.ExecContext(ctx, upsertAIModelPrices, seed)
 	return err

coderd/database/queries/aicostcontrol.sql

@@ -2,6 +2,9 @@
 -- Upsert a batch of (provider, model) rows from a JSON array. Each element
 -- must have provider, model, and the four price fields; null prices are
 -- written as SQL NULL.
+-- A conflicting row is only rewritten when a price differs, so updated_at
+-- records when a price last changed. Prices are nullable and a NULL on
+-- either side counts as a difference.
 INSERT INTO ai_model_prices (
 	provider, model, input_price, output_price, cache_read_price, cache_write_price
 )
@@ -18,7 +21,18 @@ ON CONFLICT (provider, model) DO UPDATE SET
 	output_price      = EXCLUDED.output_price,
 	cache_read_price  = EXCLUDED.cache_read_price,
 	cache_write_price = EXCLUDED.cache_write_price,
-	updated_at        = NOW();
+	updated_at        = NOW()
+WHERE (
+	ai_model_prices.input_price,
+	ai_model_prices.output_price,
+	ai_model_prices.cache_read_price,
+	ai_model_prices.cache_write_price
+) IS DISTINCT FROM (
+	EXCLUDED.input_price,
+	EXCLUDED.output_price,
+	EXCLUDED.cache_read_price,
+	EXCLUDED.cache_write_price
+);
 
 -- name: GetAIModelPriceByProviderModel :one
 SELECT *