Initial CLI: list + watch for Epic free games (cobra, epic storefront API)

.gitignore

@@ -0,0 +1,5 @@
+/bin/
+/cheapo
+*.test
+*.out
+data/

AGENTS.md

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+# AGENTS.md — guidelines for coding agents
+
+## Project
+
+**cheapo** — CLI in Go (Cobra) to fetch and observe current free game offers.
+Later planned: exposing the tools as an MCP server (see README.md, section
+"Roadmap / MCP").
+
+## Reference project
+
+**free-games-claimer** — https://github.com/DoSpamu/free-games-claimer (fork of
+https://github.com/vogler/free-games-claimer). How it gets the free-games
+info:
+
+- The fork (DoSpamu) is only an overlay (runner `run.js`, Discord notify,
+  logger) on the upstream Docker image. The actual logic lives upstream in
+  `vogler/free-games-claimer` (`epic-games.js`, `prime-gaming.js`, `gog.js`,
+  `steam-games.js`, `aliexpress.js`).
+- Upstream uses **Playwright (Firefox)** with a persistent session: it opens
+  the store page "Free Games", detects games via the locator
+  `a:has(span:text-is("Free Now"))`, follows the links and claims them in
+  the browser (login per cookie session, 2FA via OTP key, captcha handling).
+- **Important (comment in `epic-games.js`, ~line 151):** the page is built
+  from the JSON of the public backend:
+  `https://store-site-backend-static.ak.epicgames.com/freeGamesPromotions`
+  (filter `data.Catalog.searchStore.elements` on active `promotions.promotionalOffers`
+  with 100 % discount; slug preferably from `catalogNs.mappings[0].pageSlug`, since
+  `urlSlug` is sometimes wrong).
+- **Our approach (Go):** we query this endpoint **directly** — no browser,
+  no login needed, as long as we only want to *detect* (list + watch). The
+  actual *claiming* of the games (like in the reference project) is deliberate
+  **not** part of this project.
+
+## Toolchain & linting
+
+- Go ≥ 1.27 (`go version`).
+- **Formatting:** `gofmt -w .` (mandatory before every commit; CI checks `gofmt -l .` = empty).
+- **Linting/analysis:** `gopls` (install: `go install golang.org/x/tools/gopls@latest`).
+  Use in editors as LSP; additionally `go vet ./...` before every commit.
+- Build & test:
+  ```bash
+  go build ./... && go vet ./... && go test ./...
+  gofmt -l .   # must be empty
+  ```
+
+## Conventions
+
+- Module path: `gogs.netdome.biz/paul/cheapo`.
+- CLI framework: **Cobra** (`github.com/spf13/cobra`), commands in `cmd/cheapo/main.go`.
+- Internal code under `internal/` (epic: store client, watch: polling/eventing).
+- Wrap errors with context: `fmt.Errorf("epic: ...: %w", err)`.
+- User-facing strings in German (CLI output, help texts, error messages),
+  code comments in English.
+- New store platforms (Prime, GOG, …) follow the `epic` package pattern
+  (client + `GetFreeGames(ctx)`), so `watch` can stay generic.

LICENSE

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+cheapo - free games lister
+Copyright (C) 2026 Paul Klumpp
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU Affero General Public License as published
+by the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Affero General Public License for more details.
+
+You should have received a copy of the GNU Affero General Public License
+along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+======================================================================
+
+                    GNU AFFERO GENERAL PUBLIC LICENSE
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+conveyed by you (or copies made from those copies), or (b) primarily
+for and in connection with specific products or compilations that
+contain the covered work, unless you entered into that arrangement,
+or that patent license was granted, prior to 28 March 2007.
+
+  Nothing in this License shall be construed as excluding or limiting
+any implied license or other defenses to infringement that may
+otherwise be available to you under applicable patent law.
+
+  12. No Surrender of Others' Freedom.
+
+  If conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot convey a
+covered work so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you may
+not convey it at all.  For example, if you agree to terms that obligate you
+to collect a royalty for further conveying from those to whom you convey
+the Program, the only way you could satisfy both those terms and this
+License would be to refrain entirely from conveying the Program.
+
+  13. Remote Network Interaction; Use with the GNU General Public License.
+
+  Notwithstanding any other provision of this License, if you modify the
+Program, your modified version must prominently offer all users
+interacting with it remotely through a computer network (if your version
+supports such interaction) an opportunity to receive the Corresponding
+Source of your version by providing access to the Corresponding Source
+from a network server at no charge, through some standard or customary
+means of facilitating copying of software.  This Corresponding Source
+shall include the Corresponding Source for any work covered by version 3
+of the GNU General Public License that is incorporated pursuant to the
+following paragraph.
+
+  Notwithstanding any other provision of this License, you have
+permission to link or combine any covered work with a work licensed
+under version 3 of the GNU General Public License into a single
+combined work, and to convey the resulting work.  The terms of this
+License will continue to apply to the part which is the covered work,
+but the work with which it is combined will remain governed by version
+3 of the GNU General Public License.
+
+  14. Revised Versions of this License.
+
+  The Free Software Foundation may publish revised and/or new versions of
+the GNU Affero General Public License from time to time.  Such new versions
+will be similar in spirit to the present version, but may differ in detail to
+address new problems or concerns.
+
+  Each version is given a distinguishing version number.  If the
+Program specifies that a certain numbered version of the GNU Affero General
+Public License "or any later version" applies to it, you have the
+option of following the terms and conditions either of that numbered
+version or of any later version published by the Free Software
+Foundation.  If the Program does not specify a version number of the
+GNU Affero General Public License, you may choose any version ever published
+by the Free Software Foundation.
+
+  If the Program specifies that a proxy can decide which future
+versions of the GNU Affero General Public License can be used, that proxy's
+public statement of acceptance of a version permanently authorizes you
+to choose that version for the Program.
+
+  Later license versions may give you additional or different
+permissions.  However, no additional obligations are imposed on any
+author or copyright holder as a result of your choosing to follow a
+later version.
+
+  15. Disclaimer of Warranty.
+
+  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
+APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
+HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
+OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
+IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. Limitation of Liability.
+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU Affero General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU Affero General Public License for more details.
+
+    You should have received a copy of the GNU Affero General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.

README.md

@@ -0,0 +1,81 @@
+# cheapo
+
+A CLI that finds all those free games from all those game stores. And, if you want, posts them to a Chatto channel.
+
+Free games CLI in Go — lists current free game offers (currently: Epic Games
+Store) and can use `watch` to wait for **new** free games.
+
+Inspired by / reference: [free-games-claimer](https://github.com/DoSpamu/free-games-claimer)
+(a fork of [vogler/free-games-claimer](https://github.com/vogler/free-games-claimer)).
+Unlike the reference project, we need **no browser and no login**:
+we read the public storefront backend endpoint directly
+(`freeGamesPromotions`, see `AGENTS.md`). Claiming the games is not part
+of this tool — it only detects and reports them.
+
+## Install / build
+
+```bash
+go build -o bin/cheapo ./cmd/cheapo
+```
+
+Prerequisites: Go ≥ 1.27. For linting: `gopls` (`go install golang.org/x/tools/gopls@latest`), formatting with `gofmt` (details: `AGENTS.md`).
+
+## Usage
+
+```bash
+# Games free right now (Germany, German)
+cheapo list
+
+# As JSON (e.g. for further processing / bot)
+cheapo list --json
+
+# Other region / locale
+cheapo list --country AT --locale en-US
+
+# Wait for new free games (polling every 15 min, baseline = current state)
+cheapo watch
+
+# Different interval, single check
+cheapo watch --interval 5m
+cheapo watch --once
+```
+
+Example output:
+
+```
+🎮 Monument Valley
+   By: Netflix, Inc.
+   Instead of: 6.99 €
+   Free until: 02.11.2026 17:00 (CET)
+   https://store.epicgames.com/en-US/p/monument-valley-1d99d3
+```
+
+`cheapo watch` is suitable as a long runner for a systemd service or cron/timer;
+every newly detected game is reported on stderr as soon as a poll finds it.
+
+## Roadmap / MCP (later)
+
+Planned next step: expose the tools (`list`, `watch`) as an **MCP server**,
+so they can be used e.g. by Hermes or other MCP clients. Planned:
+
+- Transport: stdio (classic) — one binary `cheapo mcp`
+- Tools: `list_free_games` (immediately) and `check_now` (single poll, for cron/eventing)
+- The watch logic (`internal/watch`) stays reusable; the MCP server only
+  wraps it.
+- **Not implemented yet** — deliberately a second step after the CLI stabilizes.
+
+## Architecture
+
+```
+cmd/cheapo/       Cobra CLI (list, watch)
+internal/epic/    Client for the Epic storefront endpoint (freeGamesPromotions)
+internal/watch/   Polling/eventing: reports only newly added games
+```
+
+## Reference: how the original project works
+
+The reference project (vogler/free-games-claimer) automates a real browser
+with Playwright: login (session cookies, 2FA, captcha handling), detecting
+the "Free Now" tiles on the store page and clicking through the purchase
+process. The free games **info** comes from the same backend JSON that we
+query directly — details and sources in [AGENTS.md](AGENTS.md).

cmd/cheapo/main.go

@@ -0,0 +1,134 @@
+// Command cheapo lists and watches free game offers (currently: Epic Games Store).
+//
+// Data is read from the public storefront backend, no browser and no login
+// needed. Reference implementation: vogler/free-games-claimer.
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"github.com/spf13/cobra"
+
+	"gogs.netdome.biz/paul/cheapo/internal/epic"
+	"gogs.netdome.biz/paul/cheapo/internal/watch"
+)
+
+var (
+	flagCountry string
+	flagLocale  string
+	flagJSON    bool
+)
+
+func main() {
+	root := &cobra.Command{
+		Use:   "cheapo",
+		Short: "cheapo — free games lister (Go edition)",
+		Long: `cheapo lists and watches free game offers (currently: Epic Games Store).
+
+Data is read from the public storefront backend, no browser and no login needed.
+Reference: https://github.com/vogler/free-games-claimer`,
+		SilenceUsage: true,
+	}
+
+	root.PersistentFlags().StringVar(&flagCountry, "country", "DE", "ISO country code for store region")
+	root.PersistentFlags().StringVar(&flagLocale, "locale", "de", "store locale (e.g. de, en-US)")
+	root.PersistentFlags().BoolVar(&flagJSON, "json", false, "output as JSON")
+
+	root.AddCommand(listCmd(), watchCmd())
+
+	if err := root.Execute(); err != nil {
+		os.Exit(1)
+	}
+}
+
+func newClient() *epic.Client {
+	c := epic.NewClient()
+	c.Country = flagCountry
+	c.Locale = flagLocale
+	return c
+}
+
+func printGames(cmd *cobra.Command, games []epic.Game) error {
+	if flagJSON {
+		enc := json.NewEncoder(os.Stdout)
+		enc.SetIndent("", "  ")
+		return enc.Encode(games)
+	}
+	if len(games) == 0 {
+		fmt.Fprintln(cmd.OutOrStdout(), "No free games on offer right now.")
+		return nil
+	}
+	out := cmd.OutOrStdout()
+	for _, g := range games {
+		fmt.Fprintf(out, "🎮 %s\n", g.Title)
+		if g.Seller != "" {
+			fmt.Fprintf(out, "   By: %s\n", g.Seller)
+		}
+		if g.PriceFmt != "" {
+			fmt.Fprintf(out, "   Instead of: %s\n", g.PriceFmt)
+		}
+		fmt.Fprintf(out, "   Free until: %s\n", g.EndDate.Local().Format("02.01.2006 15:04 (MST)"))
+		fmt.Fprintf(out, "   %s\n", g.URL)
+		fmt.Fprintln(out)
+	}
+	return nil
+}
+
+func listCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:     "list",
+		Aliases: []string{"ls"},
+		Short:   "List games currently free",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			games, err := newClient().GetFreeGames(cmd.Context())
+			if err != nil {
+				return err
+			}
+			return printGames(cmd, games)
+		},
+	}
+}
+
+func watchCmd() *cobra.Command {
+	var (
+		interval time.Duration
+		once     bool
+	)
+	cmd := &cobra.Command{
+		Use:   "watch",
+		Short: "Wait for new free games (polling)",
+		Long: `Polls the store and reports every game that becomes newly free.
+The first pass records the current state as the baseline (no reports).
+Exit with Ctrl+C.`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			ctx, stop := signal.NotifyContext(cmd.Context(), os.Interrupt, syscall.SIGTERM)
+			defer stop()
+
+			c := newClient()
+			if once {
+				games, err := c.GetFreeGames(ctx)
+				if err != nil {
+					return err
+				}
+				return printGames(cmd, games)
+			}
+
+			w := watch.NewWatcher(c, interval)
+			return w.Run(ctx, false, watch.NotifierFunc(func(games []epic.Game) {
+				fmt.Fprintf(os.Stderr, "\n🔔 New free games (%s):\n", time.Now().Format("15:04:05"))
+				for _, g := range games {
+					fmt.Fprintf(os.Stderr, "  🎮 %s — until %s — %s\n",
+						g.Title, g.EndDate.Local().Format("02.01."), g.URL)
+				}
+			}))
+		},
+	}
+	cmd.Flags().DurationVar(&interval, "interval", 15*time.Minute, "Polling interval")
+	cmd.Flags().BoolVar(&once, "once", false, "Check once and exit")
+	return cmd
+}

go.mod

@@ -0,0 +1,9 @@
+module gogs.netdome.biz/paul/cheapo
+
+go 1.27.1
+
+require (
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/spf13/cobra v1.10.2 // indirect
+	github.com/spf13/pflag v1.0.9 // indirect
+)

go.sum

@@ -0,0 +1,10 @@
+github.com/cpuguy83/go-md2man/v2 v2.0.6/go.mod h1:oOW0eioCTA6cOiMLiUPZOpcVxMig6NIQQ7OS05n1F4g=
+github.com/inconshreveable/mousetrap v1.1.0 h1:wN+x4NVGpMsO7ErUn/mUI3vEoE6Jt13X2s0bqwp9tc8=
+github.com/inconshreveable/mousetrap v1.1.0/go.mod h1:vpF70FUmC8bwa3OWnCshd2FqLfsEA9PFc4w1p2J65bw=
+github.com/russross/blackfriday/v2 v2.1.0/go.mod h1:+Rmxgy9KzJVeS9/2gXHxylqXiyQDYRxCVz55jmeOWTM=
+github.com/spf13/cobra v1.10.2 h1:DMTTonx5m65Ic0GOoRY2c16WCbHxOOw6xxezuLaBpcU=
+github.com/spf13/cobra v1.10.2/go.mod h1:7C1pvHqHw5A4vrJfjNwvOdzYu0Gml16OCs2GRiTUUS4=
+github.com/spf13/pflag v1.0.9 h1:9exaQaMOCwffKiiiYk6/BndUBv+iRViNW+4lEMi0PvY=
+github.com/spf13/pflag v1.0.9/go.mod h1:McXfInJRrz4CZXVZOBLb0bTZqETkiAhM9Iw0y3An2Bg=
+go.yaml.in/yaml/v3 v3.0.4/go.mod h1:DhzuOOF2ATzADvBadXxruRBLzYTpT36CKvDb3+aBEFg=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=

internal/epic/client.go

@@ -0,0 +1,200 @@
+// Package epic implements a client for the Epic Games Store's public
+// storefront backend to discover free games.
+//
+// Reference: vogler/free-games-claimer (epic-games.js) scrapes the rendered
+// "Free Games" page; it also notes (epic-games.js, line ~151) that the page is
+// built from the JSON served by the storefront backend. We query that JSON
+// directly, so no browser is needed.
+package epic
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/url"
+	"sort"
+	"time"
+)
+
+const (
+	// promotionsURL is the public backend endpoint that powers the
+	// "Free Games" page of the Epic Games Store.
+	promotionsURL = "https://store-site-backend-static.ak.epicgames.com/freeGamesPromotions"
+
+	defaultCountry = "DE"
+	defaultLocale  = "de"
+
+	userAgent = "Mozilla/5.0 (X11; Linux x86_64; rv:127.0) Gecko/20100101 Firefox/127.0"
+)
+
+// Game describes one free game offer.
+type Game struct {
+	Title       string    `json:"title"`
+	ID          string    `json:"id"`
+	Slug        string    `json:"slug"` // store page slug, e.g. "monument-valley-02"
+	URL         string    `json:"url"`  // full store page URL
+	Store       string    `json:"store"`
+	Description string    `json:"description,omitempty"`
+	Seller      string    `json:"seller,omitempty"`
+	StartDate   time.Time `json:"start_date"`
+	EndDate     time.Time `json:"end_date"`                  // end of the free period
+	PriceFmt    string    `json:"price_formatted,omitempty"` // original price, formatted
+}
+
+// Client queries the Epic storefront backend.
+type Client struct {
+	HTTP    *http.Client
+	BaseURL string // override for tests
+	Country string // ISO 3166 country code, e.g. "DE"
+	Locale  string // e.g. "de", "en-US"
+}
+
+// NewClient returns a client with defaults for Germany.
+func NewClient() *Client {
+	return &Client{
+		HTTP:    &http.Client{Timeout: 30 * time.Second},
+		BaseURL: promotionsURL,
+		Country: defaultCountry,
+		Locale:  defaultLocale,
+	}
+}
+
+type promotionsResponse struct {
+	Data struct {
+		Catalog struct {
+			SearchStore struct {
+				Elements []struct {
+					ID          string `json:"id"`
+					Title       string `json:"title"`
+					Description string `json:"description"`
+					URLSlug     string `json:"urlSlug"`
+					CatalogNs   struct {
+						Mappings []struct {
+							PageSlug string `json:"pageSlug"`
+						} `json:"mappings"`
+					} `json:"catalogNs"`
+					Seller struct {
+						Name string `json:"displayName"`
+					} `json:"seller"`
+					Price struct {
+						TotalPrice struct {
+							FmtPrice struct {
+								OriginalPrice string `json:"originalPrice"`
+							} `json:"fmtPrice"`
+						} `json:"totalPrice"`
+					} `json:"price"`
+					Promotions *struct {
+						PromotionalOffers []struct {
+							PromotionalOffers []struct {
+								StartDate time.Time `json:"startDate"`
+								EndDate   time.Time `json:"endDate"`
+								Discount  struct {
+									DiscountPrice int `json:"discountPrice"`
+								} `json:"discountSetting"`
+							} `json:"promotionalOffers"`
+						} `json:"promotionalOffers"`
+					} `json:"promotions"`
+				} `json:"elements"`
+			} `json:"searchStore"`
+		} `json:"Catalog"`
+	} `json:"data"`
+}
+
+// GetFreeGames returns the games that are currently free (100 % discount,
+// active promotion window), sorted by end date (soonest first).
+func (c *Client) GetFreeGames(ctx context.Context) ([]Game, error) {
+	u, err := url.Parse(c.BaseURL)
+	if err != nil {
+		return nil, fmt.Errorf("epic: bad base URL: %w", err)
+	}
+	q := u.Query()
+	q.Set("locale", c.Locale)
+	q.Set("country", c.Country)
+	q.Set("allowCountries", c.Country)
+	u.RawQuery = q.Encode()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u.String(), nil)
+	if err != nil {
+		return nil, fmt.Errorf("epic: %w", err)
+	}
+	req.Header.Set("User-Agent", userAgent)
+	req.Header.Set("Accept", "application/json")
+
+	resp, err := c.HTTP.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("epic: request failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, fmt.Errorf("epic: unexpected status %s", resp.Status)
+	}
+
+	var pr promotionsResponse
+	if err := json.NewDecoder(resp.Body).Decode(&pr); err != nil {
+		return nil, fmt.Errorf("epic: decode response: %w", err)
+	}
+
+	now := time.Now()
+	var games []Game
+	for _, el := range pr.Data.Catalog.SearchStore.Elements {
+		if el.Promotions == nil {
+			continue
+		}
+		for _, offerGroup := range el.Promotions.PromotionalOffers {
+			for _, offer := range offerGroup.PromotionalOffers {
+				// Free = 100 % discount, and the window must be active right now.
+				if offer.Discount.DiscountPrice != 0 {
+					continue
+				}
+				if now.Before(offer.StartDate) || now.After(offer.EndDate) {
+					continue
+				}
+				slug := el.URLSlug
+				if len(el.CatalogNs.Mappings) > 0 && el.CatalogNs.Mappings[0].PageSlug != "" {
+					// urlSlug is sometimes a wrong ID (see vogler/free-games-claimer,
+					// epic-games.js comment on spirit-of-the-north): prefer the
+					// catalog mapping's page slug.
+					slug = el.CatalogNs.Mappings[0].PageSlug
+				}
+				g := Game{
+					Title:       el.Title,
+					ID:          el.ID,
+					Slug:        slug,
+					URL:         "https://store.epicgames.com/en-US/p/" + slug,
+					Store:       "epic",
+					Description: el.Description,
+					Seller:      el.Seller.Name,
+					StartDate:   offer.StartDate,
+					EndDate:     offer.EndDate,
+					PriceFmt:    el.Price.TotalPrice.FmtPrice.OriginalPrice,
+				}
+				// A game can appear with several promotion windows (e.g. extended
+				// giveaway); keep the one with the latest end date.
+				if i := indexOfID(games, g.ID); i >= 0 {
+					if g.EndDate.After(games[i].EndDate) {
+						games[i] = g
+					}
+					continue
+				}
+				games = append(games, g)
+			}
+		}
+	}
+
+	sort.Slice(games, func(i, j int) bool {
+		return games[i].EndDate.Before(games[j].EndDate)
+	})
+	return games, nil
+}
+
+// indexOfID returns the index of the game with the given ID, or -1.
+func indexOfID(games []Game, id string) int {
+	for i, g := range games {
+		if g.ID == id {
+			return i
+		}
+	}
+	return -1
+}

internal/watch/watch.go

@@ -0,0 +1,76 @@
+// Package watch implements waiting/polling for new free game offers.
+package watch
+
+import (
+	"context"
+	"log"
+	"time"
+
+	"gogs.netdome.biz/paul/cheapo/internal/epic"
+)
+
+// Notifier is called for every game that newly became free.
+type Notifier interface {
+	Notify(games []epic.Game)
+}
+
+// NotifierFunc adapts a function to Notifier.
+type NotifierFunc func(games []epic.Game)
+
+func (f NotifierFunc) Notify(games []epic.Game) { f(games) }
+
+// Watcher polls the client and reports games that were not seen before.
+type Watcher struct {
+	Client   *epic.Client
+	Interval time.Duration
+
+	seen map[string]bool
+}
+
+// NewWatcher creates a watcher. Interval defaults to 15m.
+func NewWatcher(c *epic.Client, interval time.Duration) *Watcher {
+	if interval <= 0 {
+		interval = 15 * time.Minute
+	}
+	return &Watcher{Client: c, Interval: interval, seen: map[string]bool{}}
+}
+
+// Run polls until ctx is done. On the first pass it reports all current free
+// games as "new" unless baselineOnly is true (useful to seed the seen-set
+// without notifying).
+func (w *Watcher) Run(ctx context.Context, baselineOnly bool, n Notifier) error {
+	t := time.NewTicker(w.Interval)
+	defer t.Stop()
+
+	first := true
+	for {
+		games, err := w.Client.GetFreeGames(ctx)
+		if err != nil {
+			return err
+		}
+
+		var fresh []epic.Game
+		for _, g := range games {
+			if !w.seen[g.ID] {
+				fresh = append(fresh, g)
+			}
+		}
+		for _, g := range games {
+			w.seen[g.ID] = true
+		}
+
+		if len(fresh) > 0 && !(first && baselineOnly) {
+			n.Notify(fresh)
+		}
+		if first {
+			log.Printf("watcher: baseline set with %d current free games, polling every %s", len(games), w.Interval)
+			first = false
+		}
+
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case <-t.C:
+		}
+	}
+}