diff --git a/internal/normalize/normalize.go b/internal/normalize/normalize.go
index d176d7c..48465a1 100644
--- a/internal/normalize/normalize.go
+++ b/internal/normalize/normalize.go
@@ -14,6 +14,10 @@ func Run(ctx context.Context, cfg config.NormalizeConfig) error {
 		return err
 	}
 
+	if _, err := ParseFile(cfg.InputFile); err != nil {
+		return err
+	}
+
 	// TODO: Implement transcript normalization transformation.
 	return fmt.Errorf("normalize command is not implemented yet")
 }
diff --git a/internal/normalize/parse.go b/internal/normalize/parse.go
new file mode 100644
index 0000000..6f130d3
--- /dev/null
+++ b/internal/normalize/parse.go
@@ -0,0 +1,197 @@
+package normalize
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+)
+
+// InputShape identifies which top-level input shape was parsed.
+type InputShape string
+
+const (
+	ShapeObjectWithSegments InputShape = "object_with_segments"
+	ShapeBareSegmentsArray  InputShape = "bare_segments_array"
+)
+
+// ParsedTranscript is the validated normalize input model.
+type ParsedTranscript struct {
+	Shape    InputShape
+	Segments []InputSegment
+}
+
+// InputSegment is a validated segment from normalize input.
+type InputSegment struct {
+	InputIndex         int
+	OriginalID         *int
+	Start              float64
+	End                float64
+	Speaker            string
+	Text               string
+	Categories         []string
+	Source             string
+	SourceSegmentIndex *int
+	SourceRef          string
+	DerivedFrom        []string
+	OverlapGroupID     *int
+}
+
+type inputSegmentPayload struct {
+	ID                 *int     `json:"id"`
+	Start              *float64 `json:"start"`
+	End                *float64 `json:"end"`
+	Speaker            *string  `json:"speaker"`
+	Text               *string  `json:"text"`
+	Categories         []string `json:"categories"`
+	Source             string   `json:"source"`
+	SourceSegmentIndex *int     `json:"source_segment_index"`
+	SourceRef          string   `json:"source_ref"`
+	DerivedFrom        []string `json:"derived_from"`
+	OverlapGroupID     *int     `json:"overlap_group_id"`
+}
+
+// ParseFile parses normalize input JSON from file path.
+func ParseFile(path string) (ParsedTranscript, error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return ParsedTranscript{}, err
+	}
+	defer file.Close()
+
+	return ParseReader(file)
+}
+
+// ParseReader parses normalize input JSON from a reader.
+func ParseReader(reader io.Reader) (ParsedTranscript, error) {
+	var raw json.RawMessage
+	decoder := json.NewDecoder(reader)
+	decoder.UseNumber()
+	if err := decoder.Decode(&raw); err != nil {
+		return ParsedTranscript{}, fmt.Errorf("decode normalize input JSON: %w", err)
+	}
+	if err := ensureSingleValue(decoder); err != nil {
+		return ParsedTranscript{}, err
+	}
+
+	trimmed := bytes.TrimSpace(raw)
+	if len(trimmed) == 0 {
+		return ParsedTranscript{}, fmt.Errorf("normalize input is empty")
+	}
+
+	switch trimmed[0] {
+	case '{':
+		return parseObjectShape(trimmed)
+	case '[':
+		segments, err := parseSegmentsArray(trimmed)
+		if err != nil {
+			return ParsedTranscript{}, err
+		}
+		return ParsedTranscript{
+			Shape:    ShapeBareSegmentsArray,
+			Segments: segments,
+		}, nil
+	default:
+		return ParsedTranscript{}, fmt.Errorf("normalize input must be a top-level object with \"segments\" or a top-level segment array")
+	}
+}
+
+func ensureSingleValue(decoder *json.Decoder) error {
+	var extra json.RawMessage
+	err := decoder.Decode(&extra)
+	if err == io.EOF {
+		return nil
+	}
+	if err == nil {
+		return fmt.Errorf("normalize input must contain exactly one top-level JSON value")
+	}
+	return fmt.Errorf("decode normalize input JSON: %w", err)
+}
+
+func parseObjectShape(raw []byte) (ParsedTranscript, error) {
+	var object map[string]json.RawMessage
+	if err := json.Unmarshal(raw, &object); err != nil {
+		return ParsedTranscript{}, fmt.Errorf("decode normalize object input: %w", err)
+	}
+
+	segmentsRaw, exists := object["segments"]
+	if !exists {
+		return ParsedTranscript{}, fmt.Errorf("normalize object input must contain a \"segments\" field")
+	}
+
+	segments, err := parseSegmentsArray(segmentsRaw)
+	if err != nil {
+		return ParsedTranscript{}, err
+	}
+
+	return ParsedTranscript{
+		Shape:    ShapeObjectWithSegments,
+		Segments: segments,
+	}, nil
+}
+
+func parseSegmentsArray(raw []byte) ([]InputSegment, error) {
+	var segmentValues []json.RawMessage
+	if err := json.Unmarshal(raw, &segmentValues); err != nil {
+		return nil, fmt.Errorf("normalize input \"segments\" must be an array")
+	}
+
+	segments := make([]InputSegment, len(segmentValues))
+	for index, segmentRaw := range segmentValues {
+		segment, err := parseSegment(index, segmentRaw)
+		if err != nil {
+			return nil, err
+		}
+		segments[index] = segment
+	}
+	return segments, nil
+}
+
+func parseSegment(index int, raw []byte) (InputSegment, error) {
+	var payload inputSegmentPayload
+	if err := json.Unmarshal(raw, &payload); err != nil {
+		return InputSegment{}, fmt.Errorf("segment %d: invalid segment object: %w", index, err)
+	}
+
+	if payload.Start == nil {
+		return InputSegment{}, fmt.Errorf("segment %d is missing required field \"start\"", index)
+	}
+	if payload.End == nil {
+		return InputSegment{}, fmt.Errorf("segment %d is missing required field \"end\"", index)
+	}
+	if payload.Speaker == nil {
+		return InputSegment{}, fmt.Errorf("segment %d is missing required field \"speaker\"", index)
+	}
+	if payload.Text == nil {
+		return InputSegment{}, fmt.Errorf("segment %d is missing required field \"text\"", index)
+	}
+
+	if *payload.Start < 0 {
+		return InputSegment{}, fmt.Errorf("segment %d has start %v; start must be >= 0", index, *payload.Start)
+	}
+	if *payload.End < *payload.Start {
+		return InputSegment{}, fmt.Errorf("segment %d has end %v before start %v", index, *payload.End, *payload.Start)
+	}
+
+	speaker := strings.TrimSpace(*payload.Speaker)
+	if speaker == "" {
+		return InputSegment{}, fmt.Errorf("segment %d has empty \"speaker\"; speaker must be non-empty", index)
+	}
+
+	return InputSegment{
+		InputIndex:         index,
+		OriginalID:         payload.ID,
+		Start:              *payload.Start,
+		End:                *payload.End,
+		Speaker:            speaker,
+		Text:               *payload.Text,
+		Categories:         append([]string(nil), payload.Categories...),
+		Source:             payload.Source,
+		SourceSegmentIndex: payload.SourceSegmentIndex,
+		SourceRef:          payload.SourceRef,
+		DerivedFrom:        append([]string(nil), payload.DerivedFrom...),
+		OverlapGroupID:     payload.OverlapGroupID,
+	}, nil
+}
diff --git a/internal/normalize/parse_test.go b/internal/normalize/parse_test.go
new file mode 100644
index 0000000..3a3bb1a
--- /dev/null
+++ b/internal/normalize/parse_test.go
@@ -0,0 +1,181 @@
+package normalize
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestParseReaderObjectWithSegmentsParses(t *testing.T) {
+	input := `{
+		"segments": [
+			{"start": 1.0, "end": 2.0, "speaker": " Alice ", "text": "hello", "id": 100}
+		]
+	}`
+
+	parsed, err := ParseReader(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("parse failed: %v", err)
+	}
+	if parsed.Shape != ShapeObjectWithSegments {
+		t.Fatalf("shape = %q, want %q", parsed.Shape, ShapeObjectWithSegments)
+	}
+	if len(parsed.Segments) != 1 {
+		t.Fatalf("segment count = %d, want 1", len(parsed.Segments))
+	}
+	segment := parsed.Segments[0]
+	if segment.Speaker != "Alice" {
+		t.Fatalf("speaker = %q, want %q", segment.Speaker, "Alice")
+	}
+	if segment.OriginalID == nil || *segment.OriginalID != 100 {
+		t.Fatalf("original id = %v, want 100", segment.OriginalID)
+	}
+}
+
+func TestParseReaderBareSegmentArrayParses(t *testing.T) {
+	input := `[
+		{"start": 1.0, "end": 2.0, "speaker": "Alice", "text": "hello"},
+		{"start": 3.0, "end": 4.0, "speaker": "Bob", "text": "world"}
+	]`
+
+	parsed, err := ParseReader(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("parse failed: %v", err)
+	}
+	if parsed.Shape != ShapeBareSegmentsArray {
+		t.Fatalf("shape = %q, want %q", parsed.Shape, ShapeBareSegmentsArray)
+	}
+	if len(parsed.Segments) != 2 {
+		t.Fatalf("segment count = %d, want 2", len(parsed.Segments))
+	}
+}
+
+func TestParseReaderInvalidJSONFails(t *testing.T) {
+	_, err := ParseReader(strings.NewReader(`{"segments":`))
+	if err == nil {
+		t.Fatal("expected parse error")
+	}
+	if !strings.Contains(err.Error(), "decode normalize input JSON") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestParseReaderObjectMissingSegmentsFails(t *testing.T) {
+	_, err := ParseReader(strings.NewReader(`{"items":[]}`))
+	if err == nil {
+		t.Fatal("expected missing segments error")
+	}
+	if !strings.Contains(err.Error(), "must contain a \"segments\" field") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestParseReaderSegmentsNotArrayFails(t *testing.T) {
+	_, err := ParseReader(strings.NewReader(`{"segments": {}}`))
+	if err == nil {
+		t.Fatal("expected segments not array error")
+	}
+	if !strings.Contains(err.Error(), "\"segments\" must be an array") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestParseReaderTopLevelScalarShapesFail(t *testing.T) {
+	tests := []string{`"text"`, `42`, `null`}
+	for _, input := range tests {
+		_, err := ParseReader(strings.NewReader(input))
+		if err == nil {
+			t.Fatalf("expected top-level shape error for %s", input)
+		}
+		if !strings.Contains(err.Error(), "top-level object") {
+			t.Fatalf("unexpected error for %s: %v", input, err)
+		}
+	}
+}
+
+func TestParseReaderMissingStartFails(t *testing.T) {
+	_, err := ParseReader(strings.NewReader(`[{"end":2,"speaker":"A","text":"t"}]`))
+	assertContains(t, err, `missing required field "start"`)
+}
+
+func TestParseReaderMissingEndFails(t *testing.T) {
+	_, err := ParseReader(strings.NewReader(`[{"start":1,"speaker":"A","text":"t"}]`))
+	assertContains(t, err, `missing required field "end"`)
+}
+
+func TestParseReaderMissingSpeakerFails(t *testing.T) {
+	_, err := ParseReader(strings.NewReader(`[{"start":1,"end":2,"text":"t"}]`))
+	assertContains(t, err, `missing required field "speaker"`)
+}
+
+func TestParseReaderEmptySpeakerFails(t *testing.T) {
+	_, err := ParseReader(strings.NewReader(`[{"start":1,"end":2,"speaker":"   ","text":"t"}]`))
+	assertContains(t, err, `speaker must be non-empty`)
+}
+
+func TestParseReaderMissingTextFails(t *testing.T) {
+	_, err := ParseReader(strings.NewReader(`[{"start":1,"end":2,"speaker":"A"}]`))
+	assertContains(t, err, `missing required field "text"`)
+}
+
+func TestParseReaderEndBeforeStartFails(t *testing.T) {
+	_, err := ParseReader(strings.NewReader(`[{"start":3,"end":2,"speaker":"A","text":"t"}]`))
+	assertContains(t, err, "before start")
+}
+
+func TestParseReaderNegativeStartFails(t *testing.T) {
+	_, err := ParseReader(strings.NewReader(`[{"start":-1,"end":2,"speaker":"A","text":"t"}]`))
+	assertContains(t, err, "start must be >= 0")
+}
+
+func TestParseReaderEmptySegmentsArrayAccepted(t *testing.T) {
+	parsed, err := ParseReader(strings.NewReader(`{"segments":[]}`))
+	if err != nil {
+		t.Fatalf("parse failed: %v", err)
+	}
+	if len(parsed.Segments) != 0 {
+		t.Fatalf("segment count = %d, want 0", len(parsed.Segments))
+	}
+}
+
+func TestParseReaderCategoriesPreservedWhenValid(t *testing.T) {
+	parsed, err := ParseReader(strings.NewReader(`[{"start":1,"end":2,"speaker":"A","text":"t","categories":["filler","backchannel"]}]`))
+	if err != nil {
+		t.Fatalf("parse failed: %v", err)
+	}
+	if len(parsed.Segments) != 1 {
+		t.Fatalf("segment count = %d, want 1", len(parsed.Segments))
+	}
+	if len(parsed.Segments[0].Categories) != 2 {
+		t.Fatalf("categories length = %d, want 2", len(parsed.Segments[0].Categories))
+	}
+	if parsed.Segments[0].Categories[0] != "filler" || parsed.Segments[0].Categories[1] != "backchannel" {
+		t.Fatalf("categories = %v", parsed.Segments[0].Categories)
+	}
+}
+
+func TestParseReaderOriginalInputIndexPreserved(t *testing.T) {
+	input := `[
+		{"start":1,"end":2,"speaker":"A","text":"one"},
+		{"start":2,"end":3,"speaker":"B","text":"two"},
+		{"start":3,"end":4,"speaker":"C","text":"three"}
+	]`
+	parsed, err := ParseReader(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("parse failed: %v", err)
+	}
+	for index, segment := range parsed.Segments {
+		if segment.InputIndex != index {
+			t.Fatalf("segment %d input index = %d, want %d", index, segment.InputIndex, index)
+		}
+	}
+}
+
+func assertContains(t *testing.T, err error, fragment string) {
+	t.Helper()
+	if err == nil {
+		t.Fatalf("expected error containing %q", fragment)
+	}
+	if !strings.Contains(err.Error(), fragment) {
+		t.Fatalf("error = %q, want substring %q", err.Error(), fragment)
+	}
+}