diff --git a/internal/engine/doc.go b/internal/engine/doc.go
index 4aa55d2..0b99e5d 100644
--- a/internal/engine/doc.go
+++ b/internal/engine/doc.go
@@ -1,2 +1,4 @@
 // Package engine orchestrates workflow execution, steps, and interpolation.
+//
+// [InterpolateString] and [InterpolateWalk] implement ${input.*} and ${steps.*} dot paths only (§13.1 MVP).
 package engine
diff --git a/internal/engine/interpolation.go b/internal/engine/interpolation.go
new file mode 100644
index 0000000..c6af68c
--- /dev/null
+++ b/internal/engine/interpolation.go
@@ -0,0 +1,179 @@
+package engine
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+// StepResult is the MVP step result shape (design doc §13.2).
+type StepResult struct {
+	Output any
+	Meta   map[string]any
+}
+
+// Context holds values for ${input.*} and ${steps.*} interpolation (§13.1).
+type Context struct {
+	Input map[string]any
+	Steps map[string]StepResult
+}
+
+var tokenRE = regexp.MustCompile(`\$\{([^}]*)\}`)
+
+// InterpolateString replaces every ${...} token in s using dot-path lookup only (§13.1 MVP).
+// Resolved values are embedded as strings: scalars and JSON for objects/arrays.
+func InterpolateString(s string, ctx Context) (string, error) {
+	var errs []error
+	out := tokenRE.ReplaceAllStringFunc(s, func(full string) string {
+		m := tokenRE.FindStringSubmatch(full)
+		if m == nil {
+			errs = append(errs, fmt.Errorf("interpolation: malformed token %q", full))
+			return full
+		}
+		path := strings.TrimSpace(m[1])
+		if path == "" {
+			errs = append(errs, errors.New("interpolation: empty placeholder"))
+			return full
+		}
+		val, err := resolvePath(ctx, path)
+		if err != nil {
+			errs = append(errs, err)
+			return full
+		}
+		str, err := valueToString(val)
+		if err != nil {
+			errs = append(errs, err)
+			return full
+		}
+		return str
+	})
+	if len(errs) > 0 {
+		return out, errors.Join(errs...)
+	}
+	return out, nil
+}
+
+// InterpolateWalk walks v recursively: it interpolates string leaves and descends into
+// map[string]any and []any. Other JSON-like types are left unchanged.
+func InterpolateWalk(v any, ctx Context) (any, error) {
+	switch t := v.(type) {
+	case string:
+		return InterpolateString(t, ctx)
+	case map[string]any:
+		out := make(map[string]any, len(t))
+		for k, val := range t {
+			iv, err := InterpolateWalk(val, ctx)
+			if err != nil {
+				return nil, err
+			}
+			out[k] = iv
+		}
+		return out, nil
+	case []any:
+		out := make([]any, len(t))
+		for i := range t {
+			iv, err := InterpolateWalk(t[i], ctx)
+			if err != nil {
+				return nil, err
+			}
+			out[i] = iv
+		}
+		return out, nil
+	default:
+		return v, nil
+	}
+}
+
+func resolvePath(ctx Context, path string) (any, error) {
+	parts := splitPath(path)
+	if len(parts) < 2 {
+		return nil, fmt.Errorf("interpolation: path %q must use input.<field>... or steps.<id>.output|meta...", path)
+	}
+	switch parts[0] {
+	case "input":
+		if ctx.Input == nil {
+			return nil, fmt.Errorf("interpolation: no input in context for path %q", path)
+		}
+		return walkAny(ctx.Input, parts[1:], path)
+	case "steps":
+		if len(parts) < 3 {
+			return nil, fmt.Errorf("interpolation: path %q must be steps.<step_id>.output|meta...", path)
+		}
+		stepID := parts[1]
+		if ctx.Steps == nil {
+			return nil, fmt.Errorf("interpolation: unknown step %q", stepID)
+		}
+		sr, ok := ctx.Steps[stepID]
+		if !ok {
+			return nil, fmt.Errorf("interpolation: unknown step %q", stepID)
+		}
+		switch parts[2] {
+		case "output":
+			return walkAny(sr.Output, parts[3:], path)
+		case "meta":
+			if sr.Meta == nil {
+				return nil, fmt.Errorf("interpolation: step %q has no meta", stepID)
+			}
+			return walkAny(sr.Meta, parts[3:], path)
+		default:
+			return nil, fmt.Errorf("interpolation: steps.%s must use .output or .meta, not %q", stepID, parts[2])
+		}
+	default:
+		return nil, fmt.Errorf("interpolation: path must start with input or steps, not %q", parts[0])
+	}
+}
+
+func splitPath(path string) []string {
+	var parts []string
+	for _, p := range strings.Split(path, ".") {
+		p = strings.TrimSpace(p)
+		if p != "" {
+			parts = append(parts, p)
+		}
+	}
+	return parts
+}
+
+func walkAny(v any, parts []string, fullPath string) (any, error) {
+	if len(parts) == 0 {
+		return v, nil
+	}
+	m, ok := v.(map[string]any)
+	if !ok {
+		return nil, fmt.Errorf("interpolation: cannot resolve %q: need map at %q, got %T", fullPath, parts[0], v)
+	}
+	next, ok := m[parts[0]]
+	if !ok {
+		return nil, fmt.Errorf("interpolation: undefined path %q (missing %q)", fullPath, parts[0])
+	}
+	return walkAny(next, parts[1:], fullPath)
+}
+
+func valueToString(v any) (string, error) {
+	if v == nil {
+		return "", nil
+	}
+	switch x := v.(type) {
+	case string:
+		return x, nil
+	case bool:
+		return strconv.FormatBool(x), nil
+	case int:
+		return strconv.Itoa(x), nil
+	case int64:
+		return strconv.FormatInt(x, 10), nil
+	case float64:
+		return strconv.FormatFloat(x, 'g', -1, 64), nil
+	case json.Number:
+		return x.String(), nil
+	default:
+		b, err := json.Marshal(x)
+		if err != nil {
+			return "", fmt.Errorf("interpolation: encode value: %w", err)
+		}
+		return string(b), nil
+	}
+}
diff --git a/internal/engine/interpolation_test.go b/internal/engine/interpolation_test.go
new file mode 100644
index 0000000..d4978c4
--- /dev/null
+++ b/internal/engine/interpolation_test.go
@@ -0,0 +1,175 @@
+package engine
+
+import (
+	"strings"
+	"testing"
+)
+
+// Examples aligned with design doc §7.4 / §13.1 (repo, number, step outputs).
+func TestInterpolateString_inputRepoAndNumber(t *testing.T) {
+	ctx := Context{
+		Input: map[string]any{
+			"repo":   "acme/api",
+			"number": float64(42),
+		},
+	}
+
+	got, err := InterpolateString(`repo=${input.repo} number=${input.number}`, ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := "repo=acme/api number=42"
+	if got != want {
+		t.Fatalf("got %q want %q", got, want)
+	}
+}
+
+func TestInterpolateString_stepsFetchPROutput_wholeObject(t *testing.T) {
+	ctx := Context{
+		Steps: map[string]StepResult{
+			"fetch_pr": {
+				Output: map[string]any{
+					"title": "Fix bug",
+					"id":    float64(99),
+				},
+				Meta: map[string]any{"durationMs": float64(1200), "costUsd": 0.02},
+			},
+		},
+	}
+
+	got, err := InterpolateString(`body=${steps.fetch_pr.output}`, ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.HasPrefix(got, "body=") {
+		t.Fatalf("got %q", got)
+	}
+	if !strings.Contains(got, "Fix bug") || !strings.Contains(got, `"id":99`) {
+		t.Fatalf("expected JSON with PR fields, got %q", got)
+	}
+}
+
+func TestInterpolateString_stepsReviewOutputSummary_nested(t *testing.T) {
+	ctx := Context{
+		Steps: map[string]StepResult{
+			"review": {
+				Output: map[string]any{
+					"summary": "LGTM",
+					"findings": []any{
+						map[string]any{"id": "f1"},
+					},
+				},
+				Meta: map[string]any{"durationMs": float64(800)},
+			},
+		},
+	}
+
+	got, err := InterpolateString(`${steps.review.output.summary}`, ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "LGTM" {
+		t.Fatalf("got %q", got)
+	}
+}
+
+func TestInterpolateString_stepsMetaDuration(t *testing.T) {
+	ctx := Context{
+		Steps: map[string]StepResult{
+			"fetch_pr": {
+				Output: map[string]any{},
+				Meta:   map[string]any{"durationMs": float64(1200), "costUsd": 0.02},
+			},
+		},
+	}
+	got, err := InterpolateString(`ms=${steps.fetch_pr.meta.durationMs}`, ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "ms=1200" {
+		t.Fatalf("got %q", got)
+	}
+}
+
+func TestInterpolateString_unknownPlaceholder_validationFriendly(t *testing.T) {
+	ctx := Context{Input: map[string]any{"repo": "x"}}
+	_, err := InterpolateString(`${input.unknown_key}`, ctx)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if !strings.Contains(err.Error(), "undefined path") || !strings.Contains(err.Error(), "unknown_key") {
+		t.Fatalf("expected undefined path detail, got: %v", err)
+	}
+}
+
+func TestInterpolateString_unknownStep(t *testing.T) {
+	ctx := Context{Steps: map[string]StepResult{}}
+	_, err := InterpolateString(`${steps.missing.output}`, ctx)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if !strings.Contains(err.Error(), "unknown step") {
+		t.Fatalf("got %v", err)
+	}
+}
+
+func TestInterpolateString_typeMismatch_cannotDrill(t *testing.T) {
+	ctx := Context{
+		Steps: map[string]StepResult{
+			"fetch_pr": {
+				Output: "not-a-map",
+				Meta:   map[string]any{},
+			},
+		},
+	}
+	_, err := InterpolateString(`${steps.fetch_pr.output.title}`, ctx)
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if !strings.Contains(err.Error(), "cannot resolve") {
+		t.Fatalf("got %v", err)
+	}
+}
+
+func TestInterpolateWalk_mapNested(t *testing.T) {
+	ctx := Context{
+		Input: map[string]any{"repo": "acme/api", "number": float64(7)},
+	}
+	in := map[string]any{
+		"repo":   "${input.repo}",
+		"nested": map[string]any{"n": "${input.number}"},
+	}
+	got, err := InterpolateWalk(in, ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	m := got.(map[string]any)
+	if m["repo"] != "acme/api" {
+		t.Fatalf("repo %v", m["repo"])
+	}
+	n := m["nested"].(map[string]any)
+	if n["n"] != "7" {
+		t.Fatalf("n %v", n["n"])
+	}
+}
+
+func TestInterpolateString_emptyPlaceholder(t *testing.T) {
+	_, err := InterpolateString(`x=${}`, Context{})
+	if err == nil {
+		t.Fatal("expected error")
+	}
+	if !strings.Contains(err.Error(), "empty placeholder") {
+		t.Fatalf("got %v", err)
+	}
+}
+
+func TestInterpolateString_whitespaceInsideToken(t *testing.T) {
+	ctx := Context{Input: map[string]any{"repo": "z"}}
+	got, err := InterpolateString(`${  input.repo  }`, ctx)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "z" {
+		t.Fatalf("got %q", got)
+	}
+}