diff --git a/internal/server/source_url_internal_test.go b/internal/server/source_url_internal_test.go
new file mode 100644
index 0000000..05dd2a3
--- /dev/null
+++ b/internal/server/source_url_internal_test.go
@@ -0,0 +1,330 @@
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"regexp"
+	"slices"
+	"strings"
+	"sync"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/session"
+)
+
+// generateFormPattern extracts the CSRF token rendered into the generator
+// page's form.
+var generateFormPattern = regexp.MustCompile(
+	`(?s)action="/generate".*?name="gorilla\.csrf\.Token" value="([^"]+)"`)
+
+// generatedURLPattern extracts the path of the encrypted URL the generator
+// page shows.
+var generatedURLPattern = regexp.MustCompile(`value="[^"]*(/v1/e/[^"]+)"`)
+
+// recordingUpstream is a test upstream that answers every request with a PNG
+// and records the request target, the path and query as sent, of each request
+// it receives.
+type recordingUpstream struct {
+	*httptest.Server
+
+	mu       sync.Mutex
+	received []string
+}
+
+// startRecordingUpstream starts a recordingUpstream, which is closed when the
+// test ends.
+func startRecordingUpstream(t *testing.T) *recordingUpstream {
+	t.Helper()
+
+	source := encodeTestPNG(t, 64, 48)
+	upstream := &recordingUpstream{}
+
+	upstream.Server = httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			upstream.mu.Lock()
+			upstream.received = append(upstream.received, r.RequestURI)
+			upstream.mu.Unlock()
+
+			w.Header().Set("Content-Type", "image/png")
+			_, _ = w.Write(source)
+		}))
+	t.Cleanup(upstream.Close)
+
+	return upstream
+}
+
+// requestTargets returns the request targets the upstream has received, in
+// the order it received them.
+func (u *recordingUpstream) requestTargets() []string {
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
+	return slices.Clone(u.received)
+}
+
+// pageOrigin is the origin of the login and generator pages below, opened
+// over HTTPS; httptest gives every request the host example.com.
+const pageOrigin = "https://example.com"
+
+// openRoot sends GET / to s over HTTPS, with cookie when it is not nil, and
+// returns the page: the login page, or the generator page for a login session.
+func openRoot(
+	t *testing.T, s *Server, cookie *http.Cookie,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet,
+		pageOrigin+"/", nil)
+	if cookie != nil {
+		req.AddCookie(cookie)
+	}
+
+	page := httptest.NewRecorder()
+	s.ServeHTTP(page, req)
+
+	return page
+}
+
+// submitForm posts form to path on s as a browser submits it from page over
+// HTTPS: with the CSRF token tokenPattern finds in page, the cookies page set,
+// cookie when it is not nil, and a Referer naming the page, which pixa needs
+// while debug is off, as it is in startImageProxy's config.
+func submitForm(
+	t *testing.T, s *Server, page *httptest.ResponseRecorder,
+	tokenPattern *regexp.Regexp, path string, form url.Values,
+	cookie *http.Cookie,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	match := tokenPattern.FindStringSubmatch(page.Body.String())
+	if match == nil {
+		t.Fatalf("the page has no form for %s with a CSRF token", path)
+	}
+
+	form.Set("gorilla.csrf.Token", match[1])
+
+	req := httptest.NewRequestWithContext(t.Context(), http.MethodPost,
+		pageOrigin+path, strings.NewReader(form.Encode()))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.Header.Set("Referer", pageOrigin+"/")
+
+	for _, c := range page.Result().Cookies() {
+		req.AddCookie(c)
+	}
+
+	if cookie != nil {
+		req.AddCookie(cookie)
+	}
+
+	rec := httptest.NewRecorder()
+	s.ServeHTTP(rec, req)
+
+	return rec
+}
+
+// postGenerate logs in to s with the signing key, submits the generator page's
+// form with sourceURL, and returns the response.
+func postGenerate(
+	t *testing.T, s *Server, sourceURL string,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	login := submitForm(t, s, openRoot(t, s, nil), csrfFieldPattern, "/",
+		url.Values{"key": {testSigningKey}}, nil)
+
+	var sessionCookie *http.Cookie
+
+	for _, c := range login.Result().Cookies() {
+		if c.Name == session.CookieName {
+			sessionCookie = c
+		}
+	}
+
+	if sessionCookie == nil {
+		t.Fatalf("logging in answered %d and set no session cookie", login.Code)
+	}
+
+	rec := submitForm(t, s, openRoot(t, s, sessionCookie), generateFormPattern,
+		"/generate", url.Values{"url": {sourceURL}}, sessionCookie)
+	t.Logf("POST /generate with url %s: %d", sourceURL, rec.Code)
+
+	return rec
+}
+
+// TestImageURLFetchesItsSourcePathAndQuery requests image URLs whose source
+// query holds a space and whose source path holds a "%", each written
+// percent-encoded, and checks the one request the upstream receives: the
+// source path percent-encoded again, and the source query with the space
+// written %20.
+func TestImageURLFetchesItsSourcePathAndQuery(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		// pathAndQuery is the source path and query as the image URL writes
+		// them
+		pathAndQuery string
+		// wantRequest is the path and query the upstream must receive
+		wantRequest string
+	}{
+		{"space in the query", "/photo.png%3Fname=my%20cat",
+			"/photo.png?name=my%20cat"},
+		{"percent sign in the path", "/100%25.png", "/100%25.png"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			upstream := startRecordingUpstream(t)
+			s, _, _ := startImageProxy(t, upstream.Server)
+
+			getImage(t, s, "/v1/image/"+upstreamHost+tt.pathAndQuery+"/orig.png")
+
+			got := upstream.requestTargets()
+			t.Logf("upstream received %q", got)
+
+			if !slices.Equal(got, []string{tt.wantRequest}) {
+				t.Errorf("upstream received %q, want [%q]", got, tt.wantRequest)
+			}
+		})
+	}
+}
+
+// TestSourcePathsHoldingSpaceAndPercent20AreFetchedApart requests twice each
+// of the source paths /my cat.png and /my%20cat.png, which the image URL
+// writes my%20cat.png and my%2520cat.png. Each is fetched once, as a request
+// of its own, and its second request is served from the cache.
+func TestSourcePathsHoldingSpaceAndPercent20AreFetchedApart(t *testing.T) {
+	t.Parallel()
+
+	upstream := startRecordingUpstream(t)
+	s, _, _ := startImageProxy(t, upstream.Server)
+
+	imageURLs := []string{
+		"/v1/image/" + upstreamHost + "/my%20cat.png/orig.png",
+		"/v1/image/" + upstreamHost + "/my%2520cat.png/orig.png",
+	}
+
+	for _, imageURL := range imageURLs {
+		getImage(t, s, imageURL)
+	}
+
+	for _, imageURL := range imageURLs {
+		cacheStatus := getImage(t, s, imageURL).Header().Get("X-Pixa-Cache")
+		if cacheStatus != "HIT" {
+			t.Errorf("second GET %s: X-Pixa-Cache = %q, want HIT",
+				imageURL, cacheStatus)
+		}
+	}
+
+	got := upstream.requestTargets()
+	t.Logf("upstream received %q", got)
+
+	want := []string{"/my%20cat.png", "/my%2520cat.png"}
+	if !slices.Equal(got, want) {
+		t.Errorf("upstream received %q, want %q", got, want)
+	}
+}
+
+// TestSourceQueryWrittenTwoWaysIsOneUpstreamRequest requests the source query
+// name=my%20cat written as the README says, name=my%2520cat, and with the
+// space written %20, name=my%20cat. Both are one source query: the upstream
+// receives one request, and the second image URL is served from the cache.
+func TestSourceQueryWrittenTwoWaysIsOneUpstreamRequest(t *testing.T) {
+	t.Parallel()
+
+	upstream := startRecordingUpstream(t)
+	s, _, _ := startImageProxy(t, upstream.Server)
+
+	getImage(t, s, "/v1/image/"+upstreamHost+"/photo.png%3Fname=my%2520cat/orig.png")
+
+	second := getImage(t, s,
+		"/v1/image/"+upstreamHost+"/photo.png%3Fname=my%20cat/orig.png")
+	if got := second.Header().Get("X-Pixa-Cache"); got != "HIT" {
+		t.Errorf("second X-Pixa-Cache = %q, want HIT", got)
+	}
+
+	got := upstream.requestTargets()
+	t.Logf("upstream received %q", got)
+
+	want := []string{"/photo.png?name=my%20cat"}
+	if !slices.Equal(got, want) {
+		t.Errorf("upstream received %q, want %q", got, want)
+	}
+}
+
+// TestGeneratedURLFetchesTheSourceURLAsEntered makes an encrypted URL on the
+// generator page for source URLs holding %20, %25 and %23 in the path or the
+// query, requests it, and checks that the upstream receives the path and
+// query exactly as entered.
+func TestGeneratedURLFetchesTheSourceURLAsEntered(t *testing.T) {
+	t.Parallel()
+
+	// The names hold no "%": each test's state directory is named after it
+	// and goes into the database URL, which decodes percent-escapes
+	tests := []struct {
+		name         string
+		pathAndQuery string
+	}{
+		{"space in the path", "/my%20cat.png"},
+		{"percent sign in the path", "/100%25.png"},
+		{"number sign in the path", "/a%23b.png"},
+		{"all three in the query", "/photo.png?name=my%20cat%25%23"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			upstream := startRecordingUpstream(t)
+			s, _, _ := startImageProxy(t, upstream.Server)
+
+			rec := postGenerate(t, s, "https://"+upstreamHost+tt.pathAndQuery)
+
+			match := generatedURLPattern.FindStringSubmatch(rec.Body.String())
+			if rec.Code != http.StatusOK || match == nil {
+				t.Fatalf("POST /generate: %d with no encrypted URL; body %s",
+					rec.Code, rec.Body.String())
+			}
+
+			getImage(t, s, match[1])
+
+			got := upstream.requestTargets()
+			t.Logf("upstream received %q", got)
+
+			if !slices.Equal(got, []string{tt.pathAndQuery}) {
+				t.Errorf("upstream received %q, want [%q]", got, tt.pathAndQuery)
+			}
+		})
+	}
+}
+
+// TestGeneratorRefusesSourceURLItWouldFetchAnotherWay submits a source URL
+// whose path holds %2F. pixa keeps the path decoded and would fetch it with a
+// "/" there, which is another URL, so the generator page answers 400, shows
+// the URL pixa would fetch, and makes no encrypted URL.
+func TestGeneratorRefusesSourceURLItWouldFetchAnotherWay(t *testing.T) {
+	t.Parallel()
+
+	s, _, _ := startImageProxy(t, startRecordingUpstream(t).Server)
+
+	rec := postGenerate(t, s, "https://"+upstreamHost+"/a%2Fb.png")
+	body := rec.Body.String()
+
+	if rec.Code != http.StatusBadRequest {
+		t.Errorf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+	}
+
+	if strings.Contains(body, "/v1/e/") {
+		t.Error("the page shows an encrypted URL")
+	}
+
+	// The test config sets allow_http, so pixa would fetch over plain HTTP
+	fetchURL := "http://" + upstreamHost + "/a/b.png"
+	if !strings.Contains(body, fetchURL) {
+		t.Errorf("the page does not show the URL pixa would fetch, %s; body %s",
+			fetchURL, body)
+	}
+}