fix: redact single encoded forms and retain partial decode failures safely

internal/logging/redact.go

@@ -203,6 +203,9 @@ func redactErrorJSON(s string) string {
 
 func cleanString(s string) string {
 	s = privatePEMInText.ReplaceAllString(s, redacted)
+	if form, ok := redactForm(s); ok {
+		s = form
+	}
 	// Scalar messages/errors and HTTP text share the same labelled-content
 	// protection, rather than protecting conversational blocks only in bodies.
 	s = redactConversationLines(redactMarkup(s))

internal/logging/text.go

@@ -88,26 +88,31 @@ func voidMarkupElement(name string) bool {
 // A form can be identified without guessing semantic values. Untouched pairs
 // preserve their original encoding and order, including duplicate field names.
 func redactForm(body string) (string, bool) {
-	if strings.ContainsAny(body, "\r\n<>") || !strings.Contains(body, "&") || !strings.Contains(body, "=") {
+	if strings.ContainsAny(body, "\r\n<>") || !strings.Contains(body, "=") {
 		return body, false
 	}
 	parts := strings.Split(body, "&")
 	for i, part := range parts {
-		key, value, ok := strings.Cut(part, "=")
-		if !ok {
-			return body, false
-		}
+		key, value, hasValue := strings.Cut(part, "=")
 		decodedKey, err := url.QueryUnescape(key)
 		if err != nil {
-			return body, false
+			// A failed field decode must not restore earlier sensitive pairs or
+			// expose the unclassifiable field. Keep the failure explicit in the copy.
+			parts[i] = url.QueryEscape("[REDACTED_INVALID_FORM_KEY]")
+			continue
 		}
 		if sensitive(decodedKey) {
 			parts[i] = key + "=" + url.QueryEscape(redacted)
 			continue
 		}
+		// Bare flags are valid empty-value form fields, not a parse failure.
+		if !hasValue {
+			continue
+		}
 		decoded, err := url.QueryUnescape(value)
 		if err != nil {
-			return body, false
+			parts[i] = key + "=" + url.QueryEscape("[REDACTED_INVALID_FORM_VALUE]")
+			continue
 		}
 		safe := cleanString(redactConversationLines(decoded))
 		if safe != decoded {

internal/logging/text_security_test.go

@@ -61,6 +61,61 @@ func TestContentFieldClassificationAndEveryLoggingPathAgree(t *testing.T) {
 	}
 }
 
+func TestPercentEncodedSingleAndMixedFormsKeepWireBytesAndHideSecrets(t *testing.T) {
+	for _, body := range []string{
+		"%74oken=private-access-token",
+		"%6dessages=private-user-turn+private-assistant-turn",
+		"task_id=4&%74oken=private-access-token&status=ready",
+		"task_id=4&%74oken=private-access-token&broken%zz=x&status=ready",
+		"flag&%6dessages=private-user-turn+private-assistant-turn&status=ready",
+		"%74oken=private-access-token&field=x%zz&status=ready",
+		"broken%zz=private-access-token&status=ready",
+	} {
+		for _, debug := range []bool{false, true} {
+			out := installTestLogger(t, debug)
+			ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				got, err := io.ReadAll(r.Body)
+				if err != nil || string(got) != body {
+					t.Error("wire request changed", err)
+				}
+				w.Header().Set("Content-Type", "application/x-www-form-urlencoded")
+				_, _ = io.WriteString(w, body)
+			}))
+			req, err := http.NewRequest(http.MethodPost, ts.URL, strings.NewReader(body))
+			if err != nil {
+				t.Fatal(err)
+			}
+			req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+			response, err := HTTPClient(ts.Client(), "test.form", "upstream").Do(req)
+			if err != nil {
+				ts.Close()
+				t.Fatal(err)
+			}
+			got, err := io.ReadAll(response.Body)
+			_ = response.Body.Close()
+			ts.Close()
+			if err != nil || string(got) != body {
+				t.Fatal("wire response changed", err)
+			}
+			slog.Error("form diagnostic", "error", errors.New(body), "detail", body)
+			text := out.String()
+			for _, secret := range []string{"private-access-token", "private-user-turn", "private-assistant-turn"} {
+				if strings.Contains(text, secret) {
+					t.Fatalf("form %q debug %v leak: %s", body, debug, text)
+				}
+			}
+			for _, keep := range []string{"task_id=4", "status=ready"} {
+				if strings.Contains(body, keep) && !strings.Contains(text, keep) {
+					t.Fatal("diagnostic lost", text)
+				}
+			}
+			if strings.Contains(body, "%zz") && !strings.Contains(text, "INVALID_FORM") {
+				t.Fatal("malformed encoding must be explicit", text)
+			}
+		}
+	}
+}
+
 func TestQuotedContentLabelsEstablishOwnershipOnEveryLoggingPath(t *testing.T) {
 	for _, label := range []string{`"messages"`, `'conversation'`, "`history`", `“prompt”`, `‘messages’`, `\"messages\"`, `"m\u0065ssages"`, `"m\qssages"`} {
 		value := "upstream failed: " + label + ":\nrole: user\nmessage: private-user-turn\nrole: assistant\nmessage: private-assistant-turn\nstatus=retry_pending\ntask_id=4\nattempts=2"