HTTP API: server, credential and the list of chats (closes #4)

The bot now serves an HTTP API on PORT (default 8080) beside the chat
client. Every request needs the credential read at startup from the
file named by API_TOKEN_FILE, sent as a bearer token; without one,
every request is refused. Responses carry the security headers, bodies
are capped at 64 KiB and each request's work at 10 seconds.

GET /api/v1/chats lists the bot's contacts from the chat client's
/_contacts command, ordered by id, and marks the contacts who deleted
their chat with the bot, which the chat client keeps listing. bot.Run
starts the API after set-up and stops it within 5 seconds; a listener
failure ends the bot as a chat client failure does.

Model: opus-5-5

AGENTS.md

@@ -1,6 +1,6 @@
 ---
 title: Agent Guidance
-last_modified: 2026-09-28
+last_modified: 2026-09-29
 ---
 
 This file is the single source of guidance for any automated agent
@@ -119,6 +119,7 @@ Do not weaken them.
 
 ```
 cmd/simplexcalc/    main(), a single call into internal/cli
+internal/api/       the HTTP API: its credential, headers and endpoints
 internal/bot/       startup, address setup, and the reply to a message
 internal/calc/      the arithmetic: go/parser and go/constant
 internal/cli/       cobra command tree: run and version

Dockerfile

@@ -119,4 +119,8 @@ USER simplexcalc
 
 ENV DATA_DIR=/var/lib/simplexcalc
 
+# The API, on its default PORT. The chat client's WebSocket on 5225 is
+# not exposed: it has no authentication.
+EXPOSE 8080
+
 CMD ["/app/simplexcalc", "run"]

README.md

@@ -11,18 +11,28 @@ else gets a short explanation instead of a result.
 
 ## Getting Started
 
-Build the image and run the bot, with its SimpleX profile on a named
+Build the image, write a random API credential onto a named volume, and
-volume:
+run the bot with its SimpleX profile on the same volume:
 
 ```sh
 git clone git@git.eeqj.de:clawbot/simplexcalc.git
 cd simplexcalc
 make docker
+docker run --rm -v simplexcalc-data:/var/lib/simplexcalc simplexcalc \
+    sh -c 'umask 077 && od -An -N32 -tx1 /dev/urandom | tr -d " \n" \
+        >/var/lib/simplexcalc/api-token'
 docker run -d --name simplexcalc --restart unless-stopped \
-    -v simplexcalc-data:/var/lib/simplexcalc simplexcalc
+    -v simplexcalc-data:/var/lib/simplexcalc \
+    -p 127.0.0.1:8080:8080 -e API_TOKEN_FILE=/var/lib/simplexcalc/api-token \
+    simplexcalc
 docker logs simplexcalc 2>&1 | grep '"msg":"ready"'
 ```
 
+The credential is 32 random bytes written as 64 hexadecimal characters
+to `/var/lib/simplexcalc/api-token`, readable only by the bot's user.
+`-p 127.0.0.1:8080:8080` makes the API reachable from this host only;
+see [API](#api).
+
 The `ready` log line carries the bot's contact address: `address` is the
 short link to share, and `full_address` is the same address in the long
 form that older SimpleX clients need. Open the link in any SimpleX Chat
@@ -67,6 +77,60 @@ variables that are absent.
   `/var/lib/simplexcalc`.
 - `DEBUG` — `true` or `false`, default `false`. `true` logs every event
   the chat client sends.
+- `PORT` — the TCP port the API listens on, on all interfaces: a whole
+  number from 1 to 65535, default `8080`.
+- `API_TOKEN_FILE` — path of a file holding the API credential, at least
+  32 characters not counting the whitespace around them. The file is
+  read once, at startup; one that cannot be read, or holds a shorter
+  credential, aborts startup. Absent, the API still listens but refuses
+  every request, and startup logs a warning saying so.
+
+## API
+
+An HTTP API beside the chat client lets another program read the bot's
+chats. It speaks JSON on `PORT`.
+
+**Authentication.** Every request carries the credential from
+`API_TOKEN_FILE`:
+
+```
+Authorization: Bearer {credential}
+```
+
+A request without it, or with a wrong one, gets `401` with
+`WWW-Authenticate: Bearer` and `{"error":"unauthorized"}`. No path is
+exempt. Without `API_TOKEN_FILE`, every request gets that answer. The
+file is read at startup, so a new credential takes a restart.
+
+**Errors** are JSON with a short explanation, such as
+`{"error":"not found"}`; what went wrong inside goes to the bot's log.
+
+### `GET /api/v1/chats`
+
+The bot's chats, ordered by `id`. The bot talks to people only one to
+one, so each chat is one of its contacts.
+
+```sh
+TOKEN=$(docker exec simplexcalc cat /var/lib/simplexcalc/api-token)
+curl -H "Authorization: Bearer $TOKEN" http://127.0.0.1:8080/api/v1/chats
+```
+
+```json
+{
+    "chats": [
+        { "id": 3, "display_name": "tester", "contact_deleted": false },
+        { "id": 4, "display_name": "tester", "contact_deleted": true }
+    ]
+}
+```
+
+- `id` — the chat's number, which is its contact's number in the chat
+  client.
+- `display_name` — the name the contact gave themselves. Nothing makes
+  it unique.
+- `contact_deleted` — `true` once the contact has deleted their chat
+  with the bot. The chat stays in the list, but nothing more reaches
+  them.
 
 ## Entrypoints
 
@@ -142,8 +206,8 @@ container.
 - **One process tree, one container.** `simplexcalc run` starts the
   SimpleX Chat command-line client, `simplex-chat`, as a child process,
   with its database under `$DATA_DIR/simplex` and its WebSocket API on
-  `127.0.0.1:5225`. The API has no authentication, which is why it is
+  `127.0.0.1:5225`. That WebSocket has no authentication, which is why
-  never exposed outside the container.
+  it is never exposed outside the container.
 - **The protocol** (`internal/simplex`) is JSON over that WebSocket: a
   command carries a correlation id, its response carries the same id,
   and anything without one is an event. Only the fields the bot reads
@@ -154,6 +218,16 @@ container.
   accept every contact request and to greet each new contact. The first
   start creates the profile itself, a bot profile named `calc`. The
   address is logged in the `ready` line.
+- **The API** (`internal/api`): an HTTP server, routed with chi, that
+  starts once set-up is done; if it cannot listen, the bot exits with an
+  error, as when the chat client fails. Every request must carry the
+  credential, compared in constant time; every response carries headers
+  that forbid framing, content sniffing, caching and referrers; a
+  request body is capped at 64 KiB and a request's work at 10 seconds.
+  Handlers call the chat client on the request's own goroutine, never on
+  the one that delivers events, which also delivers the chat client's
+  answers. When the bot stops, requests in progress get 5 seconds to
+  finish.
 - **Replies**: for each text message a contact sends in a direct chat,
   the bot sends back the result, as a reply quoting the message. Group
   messages, files and the bot's own messages are ignored.

docs/TODO.md

@@ -27,6 +27,9 @@ with no deprecation warning.
 
 # Completed Steps
 
+- 2026-09-29 Added the HTTP API in `internal/api`: the server on `PORT`,
+  the bearer credential from `API_TOKEN_FILE`, security headers and
+  limits, and `GET /api/v1/chats`
 - 2026-09-28 Moved the command tree and the `run` and `version` commands
   from `cmd/simplexcalc/` into `internal/cli`; `cmd/simplexcalc/main.go`
   is now a single call to `cli.Main`

go.mod

@@ -3,6 +3,7 @@ module sneak.berlin/go/simplexcalc
 go 1.25.0
 
 require (
+	github.com/go-chi/chi/v5 v5.3.2
 	github.com/gorilla/websocket v1.5.3
 	github.com/joho/godotenv v1.5.1
 	github.com/spf13/cobra v1.10.2

go.sum

@@ -5,6 +5,8 @@ github.com/frankban/quicktest v1.14.6 h1:7Xjx+VpznH+oBnejlPUj8oUpdxnVs4f8XU8WnHk
 github.com/frankban/quicktest v1.14.6/go.mod h1:4ptaffx2x8+WTWXmUCuVU6aPUX1/Mz7zb5vbUoiM6w0=
 github.com/fsnotify/fsnotify v1.9.0 h1:2Ml+OJNzbYCTzsxtv8vKSFD9PbJjmhYF14k/jKC7S9k=
 github.com/fsnotify/fsnotify v1.9.0/go.mod h1:8jBTzvmWwFyi3Pb8djgCCO5IBqzKJ/Jwo8TRcHyHii0=
+github.com/go-chi/chi/v5 v5.3.2 h1:5YQkICvTCSZ25hoRsyJazN0scjzKGiu4VAUc7H1o1nY=
+github.com/go-chi/chi/v5 v5.3.2/go.mod h1:R+tYY2hNuVUUjxoPtqUdgBqevM9s9njzkTLutVsOCto=
 github.com/go-viper/mapstructure/v2 v2.4.0 h1:EBsztssimR/CONLSZZ04E8qAkxNYq4Qp9LvH92wZUgs=
 github.com/go-viper/mapstructure/v2 v2.4.0/go.mod h1:oJDH3BJKyqBA2TXFhDsKDGDTlndYOZ6rGS0BRZIxGhM=
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=

internal/api/api.go

@@ -0,0 +1,180 @@
+// Package api is the bot's HTTP API, through which another program
+// reads the bot's chats.
+//
+// Every request must carry the credential, as "Authorization: Bearer
+// {credential}"; with no credential configured, every request is
+// refused. No path is exempt.
+//
+// Handlers call the chat client on the request's own goroutine. Doing
+// so from the chat client's event handler would wait forever, since
+// that goroutine also delivers the responses (see simplex.EventHandler).
+package api
+
+import (
+	"context"
+	"crypto/subtle"
+	"encoding/json"
+	"log/slog"
+	"net/http"
+	"strconv"
+	"strings"
+	"time"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/go-chi/chi/v5/middleware"
+	"sneak.berlin/go/simplexcalc/internal/simplex"
+)
+
+const (
+	// maxBodyBytes caps a request body.
+	maxBodyBytes = 64 << 10
+
+	// requestTimeout bounds the work behind one request, which is
+	// mostly waiting for the chat client.
+	requestTimeout = 10 * time.Second
+
+	// Limits on clients that send or read slowly. writeTimeout outlasts
+	// requestTimeout, so a handler that ran out of time can still
+	// answer.
+	readHeaderTimeout = 5 * time.Second
+	readTimeout       = 10 * time.Second
+	writeTimeout      = requestTimeout + 5*time.Second
+	idleTimeout       = 60 * time.Second
+)
+
+// ChatClient is the part of the chat client the API uses.
+// *simplex.Client provides it.
+type ChatClient interface {
+	// Contacts returns the contacts of the user userID.
+	Contacts(ctx context.Context, userID int64) ([]simplex.Contact, error)
+}
+
+// Params configures New.
+type Params struct {
+	Log *slog.Logger
+
+	// Client is the chat client, and UserID the bot's user profile in
+	// it.
+	Client ChatClient
+	UserID int64
+
+	// Port is the TCP port to listen on, on all interfaces.
+	Port int
+
+	// Token is the credential every request must carry. Empty refuses
+	// every request.
+	Token string
+}
+
+// New returns the API's server. The caller starts it with
+// ListenAndServe and stops it with Shutdown.
+func New(p Params) *http.Server {
+	if p.Token == "" {
+		p.Log.Warn("API_TOKEN_FILE is not set, so the API refuses every request")
+	}
+
+	h := &handlers{log: p.Log, client: p.Client, userID: p.UserID, token: p.Token}
+
+	router := chi.NewRouter()
+	router.Use(securityHeaders, h.authenticate,
+		middleware.RequestSize(maxBodyBytes), withTimeout)
+	router.NotFound(func(w http.ResponseWriter, _ *http.Request) {
+		h.respondError(w, http.StatusNotFound, "not found")
+	})
+	router.MethodNotAllowed(func(w http.ResponseWriter, _ *http.Request) {
+		h.respondError(w, http.StatusMethodNotAllowed, "method not allowed")
+	})
+	router.Route("/api/v1", func(r chi.Router) {
+		r.Get("/chats", h.handleChats())
+	})
+
+	return &http.Server{
+		Addr:              ":" + strconv.Itoa(p.Port),
+		Handler:           router,
+		ReadHeaderTimeout: readHeaderTimeout,
+		ReadTimeout:       readTimeout,
+		WriteTimeout:      writeTimeout,
+		IdleTimeout:       idleTimeout,
+		// net/http's own messages, such as a handler's panic, go to the
+		// same JSON log as everything else.
+		ErrorLog: slog.NewLogLogger(p.Log.Handler(), slog.LevelError),
+	}
+}
+
+// handlers holds what the handlers share.
+type handlers struct {
+	log    *slog.Logger
+	client ChatClient
+	userID int64
+	token  string
+}
+
+// authenticate lets a request through only if it carries the
+// credential. With no credential it refuses everything, and must:
+// ConstantTimeCompare finds two empty strings equal, so an empty bearer
+// would get in.
+func (h *handlers) authenticate(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		scheme, credential, _ := strings.Cut(r.Header.Get("Authorization"), " ")
+
+		if h.token == "" || !strings.EqualFold(scheme, "Bearer") ||
+			subtle.ConstantTimeCompare([]byte(credential), []byte(h.token)) != 1 {
+			w.Header().Set("WWW-Authenticate", "Bearer")
+			h.respondError(w, http.StatusUnauthorized, "unauthorized")
+
+			return
+		}
+
+		next.ServeHTTP(w, r)
+	})
+}
+
+// respond sends v as the JSON body of a response with status.
+func (h *handlers) respond(w http.ResponseWriter, status int, v any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+
+	err := json.NewEncoder(w).Encode(v)
+	if err != nil {
+		h.log.Warn("sending a response", "error", err)
+	}
+}
+
+// respondError sends status with a chosen sentence. An error's own text
+// never goes to the client, since it can describe the machine; it goes
+// to the log.
+func (h *handlers) respondError(w http.ResponseWriter, status int, sentence string) {
+	h.respond(w, status, struct {
+		Error string `json:"error"`
+	}{sentence})
+}
+
+// securityHeaders go on every response. The API returns JSON to
+// programs, so a browser may not frame, sniff, cache or refer from it,
+// and must reach it over HTTPS.
+func securityHeaders(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		header := w.Header()
+		header.Set("X-Content-Type-Options", "nosniff")
+		header.Set("Content-Security-Policy",
+			"default-src 'none'; frame-ancestors 'none'")
+		header.Set("X-Frame-Options", "DENY")
+		header.Set("Referrer-Policy", "no-referrer")
+		header.Set("Strict-Transport-Security",
+			"max-age=31536000; includeSubDomains")
+		header.Set("Cache-Control", "no-store")
+
+		next.ServeHTTP(w, r)
+	})
+}
+
+// withTimeout ends each request's context after requestTimeout, so a
+// handler waiting on the chat client gives up and answers.
+func withTimeout(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		ctx, cancel := context.WithTimeout(r.Context(), requestTimeout)
+		defer cancel()
+
+		next.ServeHTTP(w, r.WithContext(ctx))
+	})
+}

internal/api/api_test.go

@@ -0,0 +1,214 @@
+package api_test
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/simplexcalc/internal/api"
+	"sneak.berlin/go/simplexcalc/internal/simplex"
+)
+
+const (
+	// credential is what the API under test is configured with.
+	credential = "a-credential-for-these-tests" //nolint:gosec // G101: invented for tests
+	bearer     = "Bearer " + credential
+
+	chatsPath    = "/api/v1/chats"
+	unauthorized = `{"error":"unauthorized"}` + "\n"
+)
+
+var errChat = errors.New("sqlite: database is locked at /var/lib/simplexcalc")
+
+// fakeClient stands in for the chat client. It answers with contacts,
+// or with err, and remembers what it was asked.
+type fakeClient struct {
+	contacts []simplex.Contact
+	err      error
+
+	userID      int64
+	hadDeadline bool
+}
+
+func (f *fakeClient) Contacts(
+	ctx context.Context, userID int64,
+) ([]simplex.Contact, error) {
+	f.userID = userID
+	_, f.hadDeadline = ctx.Deadline()
+
+	return f.contacts, f.err
+}
+
+func newAPI(token string, client api.ChatClient) *http.Server {
+	return api.New(api.Params{
+		Log:    slog.New(slog.DiscardHandler),
+		Client: client,
+		UserID: 1,
+		Port:   8080,
+		Token:  token,
+	})
+}
+
+// request sends srv one request, with the Authorization header auth
+// unless that is empty.
+func request(
+	t *testing.T, srv *http.Server, method, path, auth string,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	req := httptest.NewRequestWithContext(t.Context(), method, path, nil)
+	if auth != "" {
+		req.Header.Set("Authorization", auth)
+	}
+
+	rec := httptest.NewRecorder()
+	srv.Handler.ServeHTTP(rec, req)
+
+	return rec
+}
+
+// TestCredential: only the configured credential, sent as a bearer,
+// gets in. With none configured, nothing does, an empty bearer
+// included.
+func TestCredential(t *testing.T) {
+	t.Parallel()
+
+	for name, tc := range map[string]struct {
+		token, auth string
+		in          bool
+	}{
+		"right":                         {credential, bearer, true},
+		"right, scheme in lower case":   {credential, "bearer " + credential, true},
+		"wrong":                         {credential, strings.ToUpper(bearer), false},
+		"missing":                       {credential, "", false},
+		"another scheme":                {credential, "Basic " + credential, false},
+		"no scheme":                     {credential, credential, false},
+		"empty bearer":                  {credential, "Bearer ", false},
+		"none configured":               {"", bearer, false},
+		"none configured, empty bearer": {"", "Bearer ", false},
+	} {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			rec := request(t, newAPI(tc.token, &fakeClient{}),
+				http.MethodGet, chatsPath, tc.auth)
+
+			if tc.in {
+				if rec.Code != http.StatusOK {
+					t.Errorf("status = %d, want 200", rec.Code)
+				}
+
+				return
+			}
+
+			if rec.Code != http.StatusUnauthorized {
+				t.Fatalf("status = %d, want 401", rec.Code)
+			}
+
+			if got := rec.Header().Get("WWW-Authenticate"); got != "Bearer" {
+				t.Errorf("WWW-Authenticate = %q, want Bearer", got)
+			}
+
+			if rec.Body.String() != unauthorized {
+				t.Errorf("body = %q, want %q", rec.Body.String(), unauthorized)
+			}
+		})
+	}
+}
+
+// TestNoCredentialWarns: an API without a credential says at startup
+// that it refuses every request.
+func TestNoCredentialWarns(t *testing.T) {
+	t.Parallel()
+
+	var logged bytes.Buffer
+
+	api.New(api.Params{
+		Log:    slog.New(slog.NewJSONHandler(&logged, nil)),
+		Client: &fakeClient{},
+		Port:   8080,
+	})
+
+	if !strings.Contains(logged.String(), `"level":"WARN"`) ||
+		!strings.Contains(logged.String(), "API_TOKEN_FILE is not set") {
+		t.Errorf("log = %q, want a warning that API_TOKEN_FILE is not set",
+			logged.String())
+	}
+}
+
+// TestNoPathIsExempt: an unknown path or method needs the credential
+// like everything else, and then gets a JSON error.
+func TestNoPathIsExempt(t *testing.T) {
+	t.Parallel()
+
+	srv := newAPI(credential, &fakeClient{})
+
+	for _, tc := range []struct {
+		method, path, auth string
+		want               int
+		body               string
+	}{
+		{http.MethodGet, "/", "", http.StatusUnauthorized, unauthorized},
+		{
+			http.MethodGet, "/.well-known/healthcheck", "",
+			http.StatusUnauthorized, unauthorized,
+		},
+		{
+			http.MethodGet, "/api/v1/nothing", bearer,
+			http.StatusNotFound, `{"error":"not found"}` + "\n",
+		},
+		{
+			http.MethodPost, chatsPath, bearer,
+			http.StatusMethodNotAllowed, `{"error":"method not allowed"}` + "\n",
+		},
+	} {
+		rec := request(t, srv, tc.method, tc.path, tc.auth)
+		if rec.Code != tc.want || rec.Body.String() != tc.body {
+			t.Errorf("%s %s: %d %q, want %d %q", tc.method, tc.path,
+				rec.Code, rec.Body.String(), tc.want, tc.body)
+		}
+	}
+}
+
+// TestHeaders: every response, whatever its status, carries the
+// security headers, and none lets another origin in.
+func TestHeaders(t *testing.T) {
+	t.Parallel()
+
+	want := map[string]string{
+		"X-Content-Type-Options":    "nosniff",
+		"Content-Security-Policy":   "default-src 'none'; frame-ancestors 'none'",
+		"X-Frame-Options":           "DENY",
+		"Referrer-Policy":           "no-referrer",
+		"Strict-Transport-Security": "max-age=31536000; includeSubDomains",
+		"Cache-Control":             "no-store",
+	}
+
+	for _, tc := range []struct {
+		client     *fakeClient
+		path, auth string
+	}{
+		{&fakeClient{}, chatsPath, bearer},
+		{&fakeClient{}, chatsPath, ""},
+		{&fakeClient{}, "/nothing", bearer},
+		{&fakeClient{err: errChat}, chatsPath, bearer},
+	} {
+		rec := request(t, newAPI(credential, tc.client),
+			http.MethodGet, tc.path, tc.auth)
+
+		for key, value := range want {
+			if got := rec.Header().Get(key); got != value {
+				t.Errorf("%d response: %s = %q, want %q", rec.Code, key, got, value)
+			}
+		}
+
+		if got := rec.Header().Get("Access-Control-Allow-Origin"); got != "" {
+			t.Errorf("%d response: Access-Control-Allow-Origin = %q", rec.Code, got)
+		}
+	}
+}

internal/api/chats.go

@@ -0,0 +1,46 @@
+package api
+
+import (
+	"cmp"
+	"net/http"
+	"slices"
+)
+
+// handleChats lists the bot's chats, ordered by id. The bot talks to
+// people only one to one, so its chats are its contacts, and a chat's
+// id is its contact's.
+func (h *handlers) handleChats() http.HandlerFunc {
+	type chat struct {
+		ID             int64  `json:"id"`
+		DisplayName    string `json:"display_name"`
+		ContactDeleted bool   `json:"contact_deleted"`
+	}
+
+	type response struct {
+		Chats []chat `json:"chats"`
+	}
+
+	return func(w http.ResponseWriter, r *http.Request) {
+		contacts, err := h.client.Contacts(r.Context(), h.userID)
+		if err != nil {
+			h.log.Error("listing the chats", "error", err)
+			h.respondError(w, http.StatusInternalServerError,
+				"the chats could not be read")
+
+			return
+		}
+
+		chats := make([]chat, 0, len(contacts))
+		for _, c := range contacts {
+			chats = append(chats, chat{
+				ID:             c.ContactID,
+				DisplayName:    c.Profile.DisplayName,
+				ContactDeleted: c.Deleted(),
+			})
+		}
+
+		slices.SortFunc(chats, func(a, b chat) int { return cmp.Compare(a.ID, b.ID) })
+
+		h.respond(w, http.StatusOK, response{Chats: chats})
+	}
+}

internal/api/chats_test.go

@@ -0,0 +1,64 @@
+package api_test
+
+import (
+	"net/http"
+	"testing"
+
+	"sneak.berlin/go/simplexcalc/internal/simplex"
+)
+
+// TestChats: the chats are the bot's contacts, ordered by id, deleted
+// ones marked, and the chat client is asked for the bot's user with a
+// deadline.
+func TestChats(t *testing.T) {
+	t.Parallel()
+
+	client := &fakeClient{contacts: []simplex.Contact{
+		{ContactID: 3, Profile: simplex.Profile{DisplayName: "bob"}, Status: "deleted"},
+		{ContactID: 2, Profile: simplex.Profile{DisplayName: "alice"}, Status: "active"},
+	}}
+
+	rec := request(t, newAPI(credential, client), http.MethodGet, chatsPath, bearer)
+
+	want := `{"chats":[{"id":2,"display_name":"alice","contact_deleted":false},` +
+		`{"id":3,"display_name":"bob","contact_deleted":true}]}` + "\n"
+	if rec.Code != http.StatusOK || rec.Body.String() != want {
+		t.Errorf("response = %d %q, want 200 %q", rec.Code, rec.Body.String(), want)
+	}
+
+	if got := rec.Header().Get("Content-Type"); got != "application/json" {
+		t.Errorf("Content-Type = %q, want application/json", got)
+	}
+
+	if client.userID != 1 || !client.hadDeadline {
+		t.Errorf("the chat client was asked for user %d, deadline %v; "+
+			"want user 1 with a deadline", client.userID, client.hadDeadline)
+	}
+}
+
+// TestNoChats: no contacts is an empty list, not null.
+func TestNoChats(t *testing.T) {
+	t.Parallel()
+
+	rec := request(t, newAPI(credential, &fakeClient{}),
+		http.MethodGet, chatsPath, bearer)
+
+	want := `{"chats":[]}` + "\n"
+	if rec.Code != http.StatusOK || rec.Body.String() != want {
+		t.Errorf("response = %d %q, want 200 %q", rec.Code, rec.Body.String(), want)
+	}
+}
+
+// TestChatsFailure: when the chat client fails, the response says so
+// in a chosen sentence, never in the error's own text.
+func TestChatsFailure(t *testing.T) {
+	t.Parallel()
+
+	rec := request(t, newAPI(credential, &fakeClient{err: errChat}),
+		http.MethodGet, chatsPath, bearer)
+
+	want := `{"error":"the chats could not be read"}` + "\n"
+	if rec.Code != http.StatusInternalServerError || rec.Body.String() != want {
+		t.Errorf("response = %d %q, want 500 %q", rec.Code, rec.Body.String(), want)
+	}
+}

internal/bot/bot.go

@@ -8,12 +8,15 @@ import (
 	"errors"
 	"fmt"
 	"log/slog"
+	"net/http"
 	"os"
 	"path/filepath"
 	"strconv"
 	"time"
 
+	"sneak.berlin/go/simplexcalc/internal/api"
 	"sneak.berlin/go/simplexcalc/internal/calc"
+	"sneak.berlin/go/simplexcalc/internal/config"
 	"sneak.berlin/go/simplexcalc/internal/simplex"
 )
 
@@ -42,17 +45,22 @@ const (
 	// retryInterval paces the connection attempts.
 	retryInterval = 250 * time.Millisecond
 
+	// apiStopTimeout is how long API requests in progress get to finish
+	// when the bot stops, before their connections are closed.
+	apiStopTimeout = 5 * time.Second
+
 	dataDirMode = 0o700
 )
 
 var errExited = errors.New("simplex-chat exited")
 
-// Run starts the chat client with its database in dataDir, connects to
+// Run starts the chat client with its database in cfg.DataDir, connects
-// it, sets up the bot's address, and answers messages until ctx is
+// to it, sets up the bot's address, then answers messages and serves the
-// cancelled — which is a clean stop and returns nil — or until the chat
+// API until ctx is cancelled — which is a clean stop and returns nil —
-// client or the connection to it fails, which returns the error.
+// or until the chat client, the connection to it or the API's listener
-func Run(ctx context.Context, log *slog.Logger, dataDir string) error {
+// fails, which returns the error.
-	err := os.MkdirAll(dataDir, dataDirMode)
+func Run(ctx context.Context, log *slog.Logger, cfg *config.Config) error {
+	err := os.MkdirAll(cfg.DataDir, dataDirMode)
 	if err != nil {
 		return fmt.Errorf("creating data directory: %w", err)
 	}
@@ -61,7 +69,7 @@ func Run(ctx context.Context, log *slog.Logger, dataDir string) error {
 	// Run return only once it has exited, whatever path Run takes.
 	cliCtx, stopCLI := context.WithCancel(ctx)
 
-	cli, err := simplex.StartCLI(cliCtx, log, filepath.Join(dataDir, "simplex"),
+	cli, err := simplex.StartCLI(cliCtx, log, filepath.Join(cfg.DataDir, "simplex"),
 		DisplayName, chatPort)
 	if err != nil {
 		stopCLI()
@@ -81,11 +89,31 @@ func Run(ctx context.Context, log *slog.Logger, dataDir string) error {
 
 	defer func() { _ = client.Close() }()
 
-	err = setUp(ctx, log, client)
+	user, err := setUp(ctx, log, client)
 	if err != nil {
 		return err
 	}
 
+	srv := api.New(api.Params{
+		Log:    log,
+		Client: client,
+		UserID: user.UserID,
+		Port:   cfg.Port,
+		Token:  cfg.APIToken,
+	})
+
+	served := make(chan error, 1)
+
+	go func() {
+		served <- srv.ListenAndServe()
+	}()
+
+	// Deferred last, so it runs first: requests in progress finish while
+	// the chat client is still there to answer them.
+	defer stopAPI(ctx, log, srv)
+
+	log.Info("starting the API", "port", cfg.Port)
+
 	select {
 	case <-ctx.Done():
 		return nil
@@ -93,6 +121,23 @@ func Run(ctx context.Context, log *slog.Logger, dataDir string) error {
 		return fmt.Errorf("%w: %w", simplex.ErrClosed, client.Err())
 	case <-cli.Done():
 		return fmt.Errorf("%w: %w", errExited, cli.Err())
+	case err := <-served:
+		return fmt.Errorf("serving the API: %w", err)
+	}
+}
+
+// stopAPI stops the API server, giving requests in progress up to
+// apiStopTimeout to finish before closing their connections.
+func stopAPI(ctx context.Context, log *slog.Logger, srv *http.Server) {
+	// ctx is usually cancelled by now: that is why the bot is stopping.
+	ctx, cancel := context.WithTimeout(context.WithoutCancel(ctx), apiStopTimeout)
+	defer cancel()
+
+	err := srv.Shutdown(ctx)
+	if err != nil {
+		log.Warn("stopping the API", "error", err)
+
+		_ = srv.Close()
 	}
 }
 
@@ -125,19 +170,22 @@ func connect(
 // setUp gives the bot a long-term contact address, creating it on the
 // first start, and sets it to accept every contact request and to greet
 // each new contact. The settings are written on every start, so an
-// address whose settings were changed by hand is put right.
+// address whose settings were changed by hand is put right. It returns
-func setUp(ctx context.Context, log *slog.Logger, client *simplex.Client) error {
+// the bot's user profile.
+func setUp(
+	ctx context.Context, log *slog.Logger, client *simplex.Client,
+) (simplex.User, error) {
 	ctx, cancel := context.WithTimeout(ctx, setupTimeout)
 	defer cancel()
 
 	user, err := client.ActiveUser(ctx)
 	if err != nil {
-		return fmt.Errorf("reading the bot's profile: %w", err)
+		return user, fmt.Errorf("reading the bot's profile: %w", err)
 	}
 
 	link, ok, err := client.Address(ctx, user.UserID)
 	if err != nil {
-		return fmt.Errorf("reading the bot's address: %w", err)
+		return user, fmt.Errorf("reading the bot's address: %w", err)
 	}
 
 	if !ok {
@@ -145,7 +193,7 @@ func setUp(ctx context.Context, log *slog.Logger, client *simplex.Client) error
 
 		link, err = client.CreateAddress(ctx, user.UserID)
 		if err != nil {
-			return fmt.Errorf("creating the bot's address: %w", err)
+			return user, fmt.Errorf("creating the bot's address: %w", err)
 		}
 	}
 
@@ -154,7 +202,7 @@ func setUp(ctx context.Context, log *slog.Logger, client *simplex.Client) error
 		AutoReply:  &simplex.MsgContent{Type: "text", Text: Welcome},
 	})
 	if err != nil {
-		return fmt.Errorf("setting the bot's address to accept everyone: %w", err)
+		return user, fmt.Errorf("setting the bot's address to accept everyone: %w", err)
 	}
 
 	log.Info("ready",
@@ -163,7 +211,7 @@ func setUp(ctx context.Context, log *slog.Logger, client *simplex.Client) error
 		"full_address", link.FullLink,
 	)
 
-	return nil
+	return user, nil
 }
 
 // handle answers each text message a contact sends.

internal/cli/run.go

@@ -45,7 +45,7 @@ func run(ctx context.Context, version string) error {
 	ctx, stop := signal.NotifyContext(ctx, syscall.SIGINT, syscall.SIGTERM)
 	defer stop()
 
-	err = bot.Run(ctx, log, cfg.DataDir)
+	err = bot.Run(ctx, log, cfg)
 	if err != nil {
 		log.Error("stopped", "error", err)
 

internal/config/config.go

@@ -16,8 +16,10 @@ package config
 import (
 	"errors"
 	"fmt"
+	"os"
 	"strconv"
 	"strings"
+	"unicode/utf8"
 
 	"github.com/spf13/viper"
 
@@ -34,12 +36,23 @@ import (
 // Environment variable names. Bare names, no prefix: this matches the
 // other services and keeps a compose file readable.
 const (
-	EnvDataDir = "DATA_DIR"
+	EnvDataDir      = "DATA_DIR"
-	EnvDebug   = "DEBUG"
+	EnvDebug        = "DEBUG"
+	EnvPort         = "PORT"
+	EnvAPITokenFile = "API_TOKEN_FILE" //nolint:gosec // G101: a name, not a credential
 )
 
-// DefaultDataDir applies when DATA_DIR is absent.
+// Defaults, for the variables that are absent.
-const DefaultDataDir = "./data"
+const (
+	DefaultDataDir = "./data"
+	DefaultPort    = 8080
+)
+
+// MinAPITokenLength is the fewest characters the API credential may
+// have, not counting whitespace around it.
+const MinAPITokenLength = 32
+
+const maxPort = 65535
 
 // ErrInvalidConfig is the sentinel every configuration failure wraps,
 // so callers can distinguish "the operator got it wrong" from "the
@@ -55,6 +68,14 @@ type Config struct {
 	// address and its contacts. Losing it loses the address.
 	DataDir string
 	Debug   bool
+
+	// Port is the API's TCP port.
+	Port int
+
+	// APIToken is the credential every API request must carry, read
+	// from the file named by API_TOKEN_FILE. Empty when that is absent,
+	// and then the API refuses every request. Never log it.
+	APIToken string
 }
 
 // loader parses one environment into a Config, accumulating every
@@ -109,6 +130,53 @@ func (l *loader) boolean(key string, def bool) bool {
 	return b
 }
 
+// port accepts a whole number from 1 to 65535 and refuses everything
+// else.
+func (l *loader) port(key string, def int) int {
+	s, ok := l.raw(key)
+	if !ok {
+		return def
+	}
+
+	n, err := strconv.Atoi(s)
+	if err != nil || n < 1 || n > maxPort {
+		l.fail(key, s, "not a port (use a whole number from 1 to 65535)")
+
+		return def
+	}
+
+	return n
+}
+
+// tokenFile returns the credential held in the file named by key, with
+// the whitespace around it trimmed, or "" when key is absent. A file
+// that cannot be read, or holds fewer than MinAPITokenLength
+// characters, is a failure; the message names the file, never what it
+// holds.
+func (l *loader) tokenFile(key string) string {
+	path, ok := l.raw(key)
+	if !ok {
+		return ""
+	}
+
+	b, err := os.ReadFile(path) //nolint:gosec // G304: the operator names the file.
+	if err != nil {
+		l.fail(key, path, "unreadable: "+err.Error())
+
+		return ""
+	}
+
+	token := strings.TrimSpace(string(b))
+	if utf8.RuneCountInString(token) < MinAPITokenLength {
+		l.fail(key, path, fmt.Sprintf("a file holding fewer than %d characters",
+			MinAPITokenLength))
+
+		return ""
+	}
+
+	return token
+}
+
 // New parses and validates the environment. An error here aborts
 // startup before the chat client is launched, so there is no partially
 // configured running state to reason about.
@@ -125,8 +193,10 @@ func load(v *viper.Viper) (*Config, error) {
 	l := &loader{v: v}
 
 	c := &Config{
-		DataDir: l.str(EnvDataDir, DefaultDataDir),
+		DataDir:  l.str(EnvDataDir, DefaultDataDir),
-		Debug:   l.boolean(EnvDebug, false),
+		Debug:    l.boolean(EnvDebug, false),
+		Port:     l.port(EnvPort, DefaultPort),
+		APIToken: l.tokenFile(EnvAPITokenFile),
 	}
 
 	if len(l.errs) > 0 {

internal/config/config_test.go

@@ -2,6 +2,9 @@ package config_test
 
 import (
 	"errors"
+	"os"
+	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/spf13/viper"
@@ -26,8 +29,11 @@ func TestAbsentValuesTakeDefaults(t *testing.T) {
 	t.Parallel()
 
 	for name, kv := range map[string]map[string]string{
-		"unset":           nil,
+		"unset": nil,
-		"whitespace only": {config.EnvDataDir: "  ", config.EnvDebug: " "},
+		"whitespace only": {
+			config.EnvDataDir: "  ", config.EnvDebug: " ",
+			config.EnvPort: " ", config.EnvAPITokenFile: "\t",
+		},
 	} {
 		t.Run(name, func(t *testing.T) {
 			t.Parallel()
@@ -44,6 +50,14 @@ func TestAbsentValuesTakeDefaults(t *testing.T) {
 			if c.Debug {
 				t.Error("Debug must default off")
 			}
+
+			if c.Port != config.DefaultPort {
+				t.Errorf("Port = %d, want %d", c.Port, config.DefaultPort)
+			}
+
+			if c.APIToken != "" {
+				t.Error("APIToken must default to none")
+			}
 		})
 	}
 }
@@ -94,3 +108,79 @@ func TestValidValuesAreUsed(t *testing.T) {
 		t.Error("Debug = false, want true")
 	}
 }
+
+// TestPort: PORT is a whole number from 1 to 65535, and anything else
+// aborts.
+func TestPort(t *testing.T) {
+	t.Parallel()
+
+	for raw, want := range map[string]int{"1": 1, "8081": 8081, "65535": 65535} {
+		c, err := config.Load(env(map[string]string{config.EnvPort: raw}))
+		if err != nil {
+			t.Errorf("PORT=%q was rejected: %v", raw, err)
+
+			continue
+		}
+
+		if c.Port != want {
+			t.Errorf("PORT=%q: Port = %d, want %d", raw, c.Port, want)
+		}
+	}
+
+	for _, raw := range []string{"0", "65536", "-1", "80.5", "8080x", "http"} {
+		_, err := config.Load(env(map[string]string{config.EnvPort: raw}))
+		if !errors.Is(err, config.ErrInvalidConfig) {
+			t.Errorf("PORT=%q: error = %v, want ErrInvalidConfig", raw, err)
+		}
+	}
+}
+
+// TestAPITokenFile: the credential is the file's content without the
+// whitespace around it. A file that cannot be read, or holds too short
+// a credential, aborts, and the error never shows what the file holds.
+func TestAPITokenFile(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	token := strings.Repeat("k", config.MinAPITokenLength)
+	short := strings.Repeat("s", config.MinAPITokenLength-1)
+
+	for name, content := range map[string]string{
+		"good":  " " + token + "\n",
+		"short": "\n" + short + "     \n",
+	} {
+		err := os.WriteFile(filepath.Join(dir, name), []byte(content), 0o600)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	load := func(name string) (*config.Config, error) {
+		return config.Load(env(map[string]string{
+			config.EnvAPITokenFile: filepath.Join(dir, name),
+		}))
+	}
+
+	c, err := load("good")
+	if err != nil {
+		t.Fatalf("a good file was rejected: %v", err)
+	}
+
+	if c.APIToken != token {
+		t.Errorf("APIToken = %q, want %q", c.APIToken, token)
+	}
+
+	_, err = load("missing")
+	if !errors.Is(err, config.ErrInvalidConfig) {
+		t.Errorf("a missing file: error = %v, want ErrInvalidConfig", err)
+	}
+
+	_, err = load("short")
+	if !errors.Is(err, config.ErrInvalidConfig) {
+		t.Fatalf("a short credential: error = %v, want ErrInvalidConfig", err)
+	}
+
+	if strings.Contains(err.Error(), short) {
+		t.Errorf("the error shows the file's content: %v", err)
+	}
+}

internal/simplex/client.go

@@ -168,6 +168,18 @@ func (c *Client) SetAddressSettings(
 	return c.command(ctx, cmd, TypeUserContactLinkUpdated, nil)
 }
 
+// Contacts returns the user's contacts: everyone it has a direct chat
+// with.
+func (c *Client) Contacts(ctx context.Context, userID int64) ([]Contact, error) {
+	var r struct {
+		Contacts []Contact `json:"contacts"`
+	}
+
+	err := c.command(ctx, cmdListContacts(userID), TypeContactsList, &r)
+
+	return r.Contacts, err
+}
+
 // SendText sends a text message to a contact, as a reply to the message
 // quotedItemID (0 for none). It does not wait for the chat client to
 // accept it; a failure is logged when the client's answer arrives.

internal/simplex/client_test.go

@@ -7,6 +7,7 @@ import (
 	"log/slog"
 	"net/http"
 	"net/http/httptest"
+	"slices"
 	"strings"
 	"sync"
 	"testing"
@@ -43,6 +44,19 @@ const (
 
 	contactConnected = `{"type":"contactConnected","user":{"userId":1},
 		"contact":{"contactId":3,"localDisplayName":"alice"}}`
+
+	// Two contacts with the same display name, which the chat client
+	// tells apart by the local name it gives the second. The second has
+	// deleted its chat with the bot, and is still listed.
+	contactsList = `{"type":"contactsList","user":{"userId":1},"contacts":[
+		{"contactId":3,"localDisplayName":"tester","profile":{"profileId":3,
+		"displayName":"tester","fullName":"","localAlias":""},
+		"activeConn":{"connId":2,"connStatus":{"type":"ready"}},
+		"contactUsed":true,"contactStatus":"active","chatDeleted":false},
+		{"contactId":4,"localDisplayName":"tester_1","profile":{"profileId":4,
+		"displayName":"tester","fullName":"","localAlias":""},
+		"activeConn":{"connId":3,"connStatus":{"type":"deleted"}},
+		"contactUsed":true,"contactStatus":"deleted","chatDeleted":false}]}`
 )
 
 // fakeChat stands in for the chat client's API. It answers each command
@@ -220,6 +234,38 @@ func TestAddressSetup(t *testing.T) {
 	}
 }
 
+// TestContacts: the contacts of the given user come back with their
+// ids, display names and whether they are deleted.
+func TestContacts(t *testing.T) {
+	t.Parallel()
+
+	f, url := newFakeChat(t, map[string]string{"/_contacts": contactsList})
+	c, ctx := dial(t, url, nil)
+
+	contacts, err := c.Contacts(ctx, 1)
+	if err != nil {
+		t.Fatalf("Contacts: %v", err)
+	}
+
+	if got := f.next(t); got != "/_contacts 1" {
+		t.Errorf("command = %s, want /_contacts 1", got)
+	}
+
+	tester := simplex.Profile{DisplayName: "tester"}
+
+	want := []simplex.Contact{
+		{ContactID: 3, Profile: tester, Status: "active"},
+		{ContactID: 4, Profile: tester, Status: "deleted"},
+	}
+	if !slices.Equal(contacts, want) {
+		t.Fatalf("Contacts = %+v\nwant       %+v", contacts, want)
+	}
+
+	if contacts[0].Deleted() || !contacts[1].Deleted() {
+		t.Error("Deleted must be false for contact 3 and true for contact 4")
+	}
+}
+
 // TestRefusedCommand: a command the chat client refuses is an error
 // that names the reason.
 func TestRefusedCommand(t *testing.T) {

internal/simplex/protocol.go

@@ -14,6 +14,7 @@ const (
 	TypeUserContactLink        = "userContactLink"
 	TypeUserContactLinkCreated = "userContactLinkCreated"
 	TypeUserContactLinkUpdated = "userContactLinkUpdated"
+	TypeContactsList           = "contactsList"
 	TypeNewChatItems           = "newChatItems"
 	TypeContactConnected       = "contactConnected"
 	TypeChatCmdError           = "chatCmdError"
@@ -46,10 +47,14 @@ func (e Event) Decode(v any) error {
 type (
 	// User is the chat client's local user profile: the bot itself.
 	User struct {
-		UserID  int64 `json:"userId"`
+		UserID  int64   `json:"userId"`
-		Profile struct {
+		Profile Profile `json:"profile"`
-			DisplayName string `json:"displayName"`
+	}
-		} `json:"profile"`
+
+	// Profile is how the bot or a contact presents itself. Nothing
+	// makes a display name unique.
+	Profile struct {
+		DisplayName string `json:"displayName"`
 	}
 
 	// ConnLink is a SimpleX link. The short form is what people share;
@@ -61,7 +66,9 @@ type (
 
 	// Contact is a person connected to the bot.
 	Contact struct {
-		ContactID int64 `json:"contactId"`
+		ContactID int64   `json:"contactId"`
+		Profile   Profile `json:"profile"`
+		Status    string  `json:"contactStatus"`
 	}
 
 	// NewChatItems is the record of a newChatItems event: messages
@@ -142,6 +149,13 @@ type (
 	}
 )
 
+// Deleted reports whether the contact is gone, as it is once the person
+// deletes their chat with the bot. The chat client still lists such a
+// contact, with its chat, but nothing more reaches them.
+func (c Contact) Deleted() bool {
+	return c.Status != "active"
+}
+
 // Message is a text message a contact sent to the bot.
 type Message struct {
 	ContactID int64
@@ -190,6 +204,10 @@ func cmdSetAddressSettings(userID int64, s AddressSettings) (string, error) {
 	return "/_address_settings " + strconv.FormatInt(userID, 10) + " " + string(b), nil
 }
 
+func cmdListContacts(userID int64) string {
+	return "/_contacts " + strconv.FormatInt(userID, 10)
+}
+
 func cmdSendText(contactID, quotedItemID int64, text string) (string, error) {
 	b, err := json.Marshal([]composedMessage{{
 		QuotedItemID: quotedItemID,