diff --git a/internal/config/load_test.go b/internal/config/load_test.go
new file mode 100644
index 0000000..8dadfac
--- /dev/null
+++ b/internal/config/load_test.go
@@ -0,0 +1,169 @@
+package config
+
+import (
+	"log/slog"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+)
+
+// initDir makes a temp working directory holding a .env file (if env is not
+// nil) and a config.yaml (if config is not empty), and returns its path.
+func initDir(t *testing.T, env []string, config string) string {
+	t.Helper()
+	resetViper(t)
+	dir := t.TempDir()
+	t.Chdir(dir)
+	if env != nil {
+		// godotenv sets variables without cleaning up after the test.
+		for _, kv := range env {
+			key, _, _ := strings.Cut(kv, "=")
+			t.Cleanup(func() { os.Unsetenv(key) })
+		}
+		writeFile(t, filepath.Join(dir, ".env"), strings.Join(env, "\n")+"\n")
+	}
+	if config != "" {
+		writeFile(t, filepath.Join(dir, "config.yaml"), config)
+	}
+	return dir
+}
+
+func writeFile(t *testing.T, path, contents string) {
+	t.Helper()
+	if err := os.WriteFile(path, []byte(contents), 0o644); err != nil {
+		t.Fatal(err)
+	}
+}
+
+const testConfigYAML = `logging:
+  level: info
+sources:
+  fts_mock:
+    enabled: true
+    interval: 30s
+`
+
+func TestInitLoadsEnvAndConfigFile(t *testing.T) {
+	initDir(t, []string{"JOBSEARCH_TEST_ENV=from-dotenv"}, testConfigYAML)
+	captureLogs(t, slog.LevelError)
+
+	var conf JobSearchConfig
+	if err := Init("", &conf); err != nil {
+		t.Fatal(err)
+	}
+	if got := os.Getenv("JOBSEARCH_TEST_ENV"); got != "from-dotenv" {
+		t.Errorf("JOBSEARCH_TEST_ENV = %q; want it loaded from .env", got)
+	}
+	if conf.Logging.Level != "info" {
+		t.Errorf("Logging.Level = %q; want info", conf.Logging.Level)
+	}
+	if !conf.Sources.FTSMock.Enabled || conf.Sources.FTSMock.Interval != 30*time.Second {
+		t.Errorf("FTSMock = %+v; want enabled every 30s", conf.Sources.FTSMock)
+	}
+	// Keys missing from the file keep their defaults.
+	if conf.Sources.FindATender.Interval != 10*time.Minute {
+		t.Errorf("FindATender.Interval = %v; want default 10m", conf.Sources.FindATender.Interval)
+	}
+}
+
+func TestInitExplicitConfigFile(t *testing.T) {
+	dir := initDir(t, []string{}, "logging:\n  level: info\n")
+	other := filepath.Join(dir, "other.yaml")
+	writeFile(t, other, "logging:\n  level: error\n")
+	captureLogs(t, slog.LevelError)
+
+	var conf JobSearchConfig
+	if err := Init(other, &conf); err != nil {
+		t.Fatal(err)
+	}
+	if conf.Logging.Level != "error" {
+		t.Errorf("Logging.Level = %q; want error from %s", conf.Logging.Level, other)
+	}
+}
+
+func TestInitNoConfigFile(t *testing.T) {
+	initDir(t, []string{}, "")
+	logs := captureLogs(t, slog.LevelWarn)
+
+	var conf JobSearchConfig
+	if err := Init("", &conf); err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(logs.String(), "No config file found") {
+		t.Errorf("no warning logged: %s", logs)
+	}
+	if conf.Logging.Level != "warn" {
+		t.Errorf("Logging.Level = %q; want default warn", conf.Logging.Level)
+	}
+}
+
+// The -debug and -quiet levels set before Init win over the config file.
+func TestInitKeepsCommandLineLevel(t *testing.T) {
+	for _, level := range []string{"debug", "error"} {
+		t.Run(level, func(t *testing.T) {
+			initDir(t, []string{}, "logging:\n  level: info\n")
+			logs := captureLogs(t, slog.LevelDebug)
+
+			conf := JobSearchConfig{Logging: JobSearchConfigLogging{Level: level}}
+			if err := Init("", &conf); err != nil {
+				t.Fatal(err)
+			}
+			if conf.Logging.Level != level {
+				t.Errorf("Logging.Level = %q; want %s kept", conf.Logging.Level, level)
+			}
+			// At debug, the environment and loaded config are logged.
+			gotDebug := strings.Contains(logs.String(), "Loaded config:")
+			if gotDebug != (level == "debug") {
+				t.Errorf("debug output logged = %v at level %s: %s", gotDebug, level, logs)
+			}
+		})
+	}
+}
+
+func TestInitErrors(t *testing.T) {
+	t.Run("no .env file", func(t *testing.T) {
+		initDir(t, nil, testConfigYAML)
+		var conf JobSearchConfig
+		if err := Init("", &conf); err == nil || !strings.Contains(err.Error(), ".env") {
+			t.Fatalf("err = %v; want an invalid .env file error", err)
+		}
+	})
+	t.Run("invalid config file", func(t *testing.T) {
+		initDir(t, []string{}, "logging: [unclosed\n")
+		captureLogs(t, slog.LevelError)
+		var conf JobSearchConfig
+		if err := Init("", &conf); err == nil {
+			t.Fatal("expected an error for invalid YAML")
+		}
+	})
+	t.Run("missing explicit config file", func(t *testing.T) {
+		dir := initDir(t, []string{}, "")
+		captureLogs(t, slog.LevelError)
+		var conf JobSearchConfig
+		if err := Init(filepath.Join(dir, "missing.yaml"), &conf); err == nil {
+			t.Fatal("expected an error for a missing config file")
+		}
+	})
+}
+
+// Environment variables override the config file, with nested keys joined
+// by underscores.
+func TestInitEnvOverride(t *testing.T) {
+	initDir(t, []string{}, testConfigYAML)
+	captureLogs(t, slog.LevelError)
+	t.Setenv("JOBSEARCH_LOGGING_LEVEL", "error")
+	t.Setenv("JOBSEARCH_SOURCES_FTS_MOCK_INTERVAL", "45s")
+
+	var conf JobSearchConfig
+	if err := Init("", &conf); err != nil {
+		t.Fatal(err)
+	}
+	if conf.Logging.Level != "error" {
+		t.Errorf("Logging.Level = %q; want error from the environment", conf.Logging.Level)
+	}
+	if conf.Sources.FTSMock.Interval != 45*time.Second {
+		t.Errorf("FTSMock.Interval = %v; want 45s from the environment", conf.Sources.FTSMock.Interval)
+	}
+}
diff --git a/internal/config/modify_test.go b/internal/config/modify_test.go
new file mode 100644
index 0000000..abba1f7
--- /dev/null
+++ b/internal/config/modify_test.go
@@ -0,0 +1,163 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/spf13/viper"
+)
+
+// useConfigFile points Viper at a new YAML config file holding contents.
+func useConfigFile(t *testing.T, contents string) string {
+	t.Helper()
+	resetViper(t)
+	path := filepath.Join(t.TempDir(), "config.yaml")
+	if err := os.WriteFile(path, []byte(contents), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	viper.SetConfigFile(path)
+	if err := viper.ReadInConfig(); err != nil {
+		t.Fatal(err)
+	}
+	return path
+}
+
+// readConfigFile reads path with a fresh Viper.
+func readConfigFile(t *testing.T, path string) *viper.Viper {
+	t.Helper()
+	v := viper.New()
+	v.SetConfigFile(path)
+	if err := v.ReadInConfig(); err != nil {
+		t.Fatal(err)
+	}
+	return v
+}
+
+func TestStoreValue(t *testing.T) {
+	tests := []struct {
+		key, value string
+		want       any
+		kind       reflect.Kind
+	}{
+		{"logging.level", "debug", "debug", reflect.String},
+		{"sources.sleeper.enabled", "true", true, reflect.Bool},
+		{"sources.fts_mock.api_root", "http://mock", "http://mock", reflect.String},
+		{"sources.fts_mock.interval", "30s", "30s", reflect.String},
+	}
+	for _, tt := range tests {
+		t.Run(tt.key, func(t *testing.T) {
+			path := useConfigFile(t, "logging:\n  level: warn\n")
+			conf := defaultConfig(t)
+
+			got, kind, err := StoreValue(&conf, tt.key, tt.value)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if got != tt.want || kind != tt.kind {
+				t.Errorf("got %#v <%s>; want %#v <%s>", got, kind, tt.want, tt.kind)
+			}
+			if v := readConfigFile(t, path).Get(tt.key); v != tt.want {
+				t.Errorf("file has %s = %#v; want %#v", tt.key, v, tt.want)
+			}
+		})
+	}
+}
+
+func TestStoreValueKeepsOtherKeys(t *testing.T) {
+	path := useConfigFile(t, "logging:\n  level: warn\n  file: /tmp/js.log\n")
+	conf := defaultConfig(t)
+	if _, _, err := StoreValue(&conf, "logging.level", "error"); err != nil {
+		t.Fatal(err)
+	}
+	v := readConfigFile(t, path)
+	if got := v.GetString("logging.file"); got != "/tmp/js.log" {
+		t.Errorf("logging.file = %q; want it kept", got)
+	}
+}
+
+func TestStoreValueErrors(t *testing.T) {
+	tests := []struct {
+		name, key, value, wantErr string
+	}{
+		{"unknown key", "nope", "x", ""},
+		{"section key", "sources", "x", ""},
+		{"bad bool", "sources.sleeper.enabled", "maybe", "expected <bool>"},
+		{"invalid log level", "logging.level", "shouty", "invalid log level"},
+		{"bad duration", "sources.fts_mock.interval", "eventually", "duration"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			path := useConfigFile(t, "logging:\n  level: warn\n")
+			before, _ := os.ReadFile(path)
+			conf := defaultConfig(t)
+
+			_, _, err := StoreValue(&conf, tt.key, tt.value)
+			if err == nil {
+				t.Fatal("expected an error")
+			}
+			if !strings.Contains(err.Error(), tt.wantErr) {
+				t.Errorf("err = %v; want it to mention %q", err, tt.wantErr)
+			}
+			if after, _ := os.ReadFile(path); string(after) != string(before) {
+				t.Errorf("config file changed:\n%s", after)
+			}
+		})
+	}
+}
+
+func TestStoreValueNoConfigFile(t *testing.T) {
+	resetViper(t)
+	conf := defaultConfig(t)
+	if _, _, err := StoreValue(&conf, "logging.level", "debug"); err == nil {
+		t.Fatal("expected an error with no config file to write")
+	}
+}
+
+func TestPurgeValue(t *testing.T) {
+	path := useConfigFile(t, "logging:\n  level: debug\n  file: /tmp/js.log\n")
+	conf := defaultConfig(t)
+
+	if err := PurgeValue(&conf, "logging.file"); err != nil {
+		t.Fatal(err)
+	}
+	v := readConfigFile(t, path)
+	if v.IsSet("logging.file") {
+		t.Errorf("logging.file still set: %v", v.Get("logging.file"))
+	}
+	if got := v.GetString("logging.level"); got != "debug" {
+		t.Errorf("logging.level = %q; want it kept", got)
+	}
+}
+
+func TestPurgeValueErrors(t *testing.T) {
+	path := useConfigFile(t, "logging:\n  level: debug\n")
+	before, _ := os.ReadFile(path)
+	conf := defaultConfig(t)
+
+	if err := PurgeValue(&conf, "nope"); err == nil {
+		t.Error("expected an error for an unknown key")
+	}
+	if after, _ := os.ReadFile(path); string(after) != string(before) {
+		t.Errorf("config file changed:\n%s", after)
+	}
+
+	resetViper(t)
+	if err := PurgeValue(&conf, "logging.level"); err == nil {
+		t.Error("expected an error with no config file to write")
+	}
+}
+
+func TestStoreValueUpdatesConf(t *testing.T) {
+	useConfigFile(t, "logging:\n  level: warn\n")
+	conf := defaultConfig(t)
+	if _, _, err := StoreValue(&conf, "sources.fts_mock.interval", "30s"); err != nil {
+		t.Fatal(err)
+	}
+	if conf.Sources.FTSMock.Interval != 30*time.Second {
+		t.Errorf("FTSMock.Interval = %v; want 30s", conf.Sources.FTSMock.Interval)
+	}
+}
diff --git a/internal/config/section.go b/internal/config/section.go
index 68ef5cf..d3122d6 100644
--- a/internal/config/section.go
+++ b/internal/config/section.go
@@ -142,6 +142,8 @@ func mapOfValue(section reflect.Value) map[string]any {
 		fv := section.FieldByIndex(f.index)
 		if f.section {
 			m[f.key] = mapOfValue(fv)
+		} else if f.typ == durationType {
+			m[f.key] = fv.Interface().(time.Duration).String()
 		} else {
 			m[f.key] = fv.Interface()
 		}
diff --git a/internal/config/section_test.go b/internal/config/section_test.go
new file mode 100644
index 0000000..0850398
--- /dev/null
+++ b/internal/config/section_test.go
@@ -0,0 +1,297 @@
+package config
+
+import (
+	"errors"
+	"reflect"
+	"strings"
+	"testing"
+	"time"
+)
+
+// Test sections exercising every field shape section.go supports.
+type testInner struct {
+	Name string `cfg:"name" default:"inner"`
+}
+
+type testEmbedded struct {
+	On bool `cfg:"on" default:"true"`
+}
+
+type testSection struct {
+	testEmbedded
+	Str      string        `cfg:"str" default:"hello"`
+	Int      int           `cfg:"int" default:"-42"`
+	Int8     int8          `cfg:"int8"`
+	Uint     uint          `cfg:"uint" default:"7"`
+	Uint16   uint16        `cfg:"uint16"`
+	Float    float64       `cfg:"float" default:"1.5"`
+	Float32  float32       `cfg:"float32"`
+	Dur      time.Duration `cfg:"dur" default:"90s"`
+	When     time.Time     `cfg:"when"`
+	Slice    []string      `cfg:"slice"`
+	Inner    testInner     `cfg:"inner"`
+	NoTag    string
+	Skipped  string `cfg:"-"`
+	Empty    string `cfg:""`
+	unexport string `cfg:"unexported"`
+}
+
+type testValidated struct {
+	Value string `cfg:"value"`
+}
+
+var errRejected = errors.New("rejected")
+
+func (c *testValidated) validate(key string, value any) error {
+	if key == "value" && value.(string) == "bad" {
+		return errRejected
+	}
+	return nil
+}
+
+func TestKeysOf(t *testing.T) {
+	got := keysOf(&testSection{})
+	want := []string{"on", "str", "int", "int8", "uint", "uint16", "float",
+		"float32", "dur", "when", "slice", "inner"}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("keysOf = %v; want %v", got, want)
+	}
+}
+
+func TestFieldsOfSkipsUntagged(t *testing.T) {
+	for _, f := range fieldsOf(reflect.TypeOf(testSection{})) {
+		switch f.key {
+		case "", "-", "unexported":
+			t.Errorf("fieldsOf included field with key %q", f.key)
+		}
+		if f.key == "when" && f.section {
+			t.Error("time.Time field treated as a section")
+		}
+		if f.key == "inner" && !f.section {
+			t.Error("struct field not treated as a section")
+		}
+	}
+}
+
+func TestMapOf(t *testing.T) {
+	var s testSection
+	if err := applyDefaults(&s); err != nil {
+		t.Fatalf("applyDefaults: %v", err)
+	}
+	m := mapOf(s)
+	if got := m["str"]; got != "hello" {
+		t.Errorf("m[str] = %#v", got)
+	}
+	if got := m["on"]; got != true {
+		t.Errorf("m[on] (promoted) = %#v", got)
+	}
+	inner, ok := m["inner"].(map[string]any)
+	if !ok {
+		t.Fatalf("m[inner] = %#v; want nested map", m["inner"])
+	}
+	if inner["name"] != "inner" {
+		t.Errorf("m[inner][name] = %#v", inner["name"])
+	}
+	if _, ok := m["inner.name"]; ok {
+		t.Error("mapOf produced a flattened key")
+	}
+	// Pointer and value give the same result.
+	if !reflect.DeepEqual(m, mapOf(&s)) {
+		t.Error("mapOf(&s) != mapOf(s)")
+	}
+}
+
+func TestFlattenOfPrefix(t *testing.T) {
+	m := flattenOf("p", testSection{})
+	for _, k := range []string{"p.str", "p.on", "p.inner.name"} {
+		if _, ok := m[k]; !ok {
+			t.Errorf("flattenOf missing %q in %v", k, m)
+		}
+	}
+}
+
+func TestStructValuePanics(t *testing.T) {
+	defer func() {
+		if recover() == nil {
+			t.Error("structValue(int) did not panic")
+		}
+	}()
+	structValue(42)
+}
+
+func TestApplyDefaults(t *testing.T) {
+	var s testSection
+	if err := applyDefaults(&s); err != nil {
+		t.Fatalf("applyDefaults: %v", err)
+	}
+	want := testSection{
+		testEmbedded: testEmbedded{On: true},
+		Str:          "hello",
+		Int:          -42,
+		Uint:         7,
+		Float:        1.5,
+		Dur:          90 * time.Second,
+		Inner:        testInner{Name: "inner"},
+	}
+	if !reflect.DeepEqual(s, want) {
+		t.Errorf("applyDefaults = %+v; want %+v", s, want)
+	}
+}
+
+func TestApplyDefaultsInvalid(t *testing.T) {
+	var bad struct {
+		N int `cfg:"n" default:"nope"`
+	}
+	err := applyDefaults(&bad)
+	if err == nil || !strings.Contains(err.Error(), "invalid default for n") {
+		t.Errorf("applyDefaults = %v; want invalid default error", err)
+	}
+
+	var badNested struct {
+		Sub struct {
+			D time.Duration `cfg:"d" default:"forever"`
+		} `cfg:"sub"`
+	}
+	err = applyDefaults(&badNested)
+	if err == nil || !strings.Contains(err.Error(), "sub.d") {
+		t.Errorf("applyDefaults nested = %v; want error naming sub.d", err)
+	}
+}
+
+func TestTypeOf(t *testing.T) {
+	tests := []struct {
+		key  string
+		want reflect.Kind
+	}{
+		{"str", reflect.String},
+		{"int", reflect.Int},
+		{"uint", reflect.Uint},
+		{"float", reflect.Float64},
+		{"on", reflect.Bool},
+		{"dur", reflect.String}, // durations are read as strings
+		{"when", reflect.Struct},
+		{"slice", reflect.Slice},
+		{"inner.name", reflect.String},
+	}
+	for _, tt := range tests {
+		got, err := typeOf(testSection{}, tt.key)
+		if err != nil || got != tt.want {
+			t.Errorf("typeOf(%q) = %v, %v; want %v", tt.key, got, err, tt.want)
+		}
+	}
+}
+
+func TestUnknownKeys(t *testing.T) {
+	keys := []string{
+		"missing",
+		"inner",         // section without sub-key
+		"str.sub",       // value with a sub-key
+		"inner.missing", // unknown nested key
+		"NoTag",         // untagged field
+		"unexported",
+		"",
+	}
+	var s testSection
+	for _, key := range keys {
+		if k, err := typeOf(s, key); !errors.Is(err, errUnknownKey) || k != reflect.Invalid {
+			t.Errorf("typeOf(%q) = %v, %v; want Invalid, errUnknownKey", key, k, err)
+		}
+		if v, err := valueOf(s, key); !errors.Is(err, errUnknownKey) || v != nil {
+			t.Errorf("valueOf(%q) = %v, %v; want nil, errUnknownKey", key, v, err)
+		}
+		if err := setValue(&s, key, "x"); !errors.Is(err, errUnknownKey) {
+			t.Errorf("setValue(%q) = %v; want errUnknownKey", key, err)
+		}
+	}
+}
+
+func TestSetValue(t *testing.T) {
+	tests := []struct {
+		key   string
+		value any
+		want  any
+	}{
+		// strings are parsed
+		{"str", "world", "world"},
+		{"on", "false", false},
+		{"int", "123", 123},
+		{"int8", "-128", int8(-128)},
+		{"uint", "9", uint(9)},
+		{"uint16", "65535", uint16(65535)},
+		{"float", "2.25", 2.25},
+		{"float32", "0.5", float32(0.5)},
+		{"dur", "1h30m", 90 * time.Minute},
+		{"inner.name", "nested", "nested"},
+		// values of the field's type are used as-is
+		{"int", 5, 5},
+		{"on", true, true},
+		{"dur", 3 * time.Second, 3 * time.Second},
+		{"slice", []string{"a", "b"}, []string{"a", "b"}},
+		{"when", time.Unix(0, 0).UTC(), time.Unix(0, 0).UTC()},
+	}
+	for _, tt := range tests {
+		var s testSection
+		if err := setValue(&s, tt.key, tt.value); err != nil {
+			t.Errorf("setValue(%q, %#v): %v", tt.key, tt.value, err)
+			continue
+		}
+		got, err := valueOf(s, tt.key)
+		if err != nil || !reflect.DeepEqual(got, tt.want) {
+			t.Errorf("after setValue(%q, %#v): valueOf = %#v, %v; want %#v",
+				tt.key, tt.value, got, err, tt.want)
+		}
+	}
+}
+
+func TestSetValueErrors(t *testing.T) {
+	tests := []struct {
+		key   string
+		value any
+		want  string
+	}{
+		{"int", "abc", "unable to convert"},
+		{"int8", "200", "unable to convert"}, // out of range
+		{"uint", "-1", "unable to convert"},
+		{"float", "x", "unable to convert"},
+		{"on", "maybe", "unable to convert"},
+		{"dur", "soon", "unable to convert to duration"},
+		{"slice", "a,b", "unsupported type string"},
+		{"when", "2024-01-01", "unsupported type string"},
+		{"int", 1.5, "unsupported type float64"},
+		{"int", int64(1), "unsupported type int64"},
+		{"str", nil, "unsupported type <nil>"},
+	}
+	for _, tt := range tests {
+		var s testSection
+		before := s
+		err := setValue(&s, tt.key, tt.value)
+		if err == nil || !strings.Contains(err.Error(), tt.want) {
+			t.Errorf("setValue(%q, %#v) = %v; want error containing %q",
+				tt.key, tt.value, err, tt.want)
+		}
+		if !reflect.DeepEqual(s, before) {
+			t.Errorf("setValue(%q, %#v) modified the section on error", tt.key, tt.value)
+		}
+	}
+}
+
+func TestSetValueValidator(t *testing.T) {
+	var s testValidated
+	if err := setValue(&s, "value", "good"); err != nil || s.Value != "good" {
+		t.Errorf("setValue(good) = %v, Value = %q", err, s.Value)
+	}
+	if err := setValue(&s, "value", "bad"); !errors.Is(err, errRejected) {
+		t.Errorf("setValue(bad) = %v; want errRejected", err)
+	}
+	if s.Value != "good" {
+		t.Errorf("rejected value was stored: %q", s.Value)
+	}
+
+	// The validator of the section holding the field is used for nested keys.
+	var outer struct {
+		V testValidated `cfg:"v"`
+	}
+	if err := setValue(&outer, "v.value", "bad"); !errors.Is(err, errRejected) {
+		t.Errorf("nested setValue(bad) = %v; want errRejected", err)
+	}
+}