diff --git a/tests/test_profile.py b/tests/test_profile.py
new file mode 100644
index 0000000..bddf5b1
--- /dev/null
+++ b/tests/test_profile.py
@@ -0,0 +1,348 @@
+"""Unit tests for `_fpforge/profile.py`.
+
+Covers `_validate_pin_key`, `_apply_pins_to_raw`, and `generate_profile`.
+Test cases derived via ECP/BVA/error guessing.
+"""
+from dataclasses import FrozenInstanceError
+
+import pytest
+
+from invisible_playwright._fpforge import generate_profile
+from invisible_playwright._fpforge.profile import (
+    Profile,
+    _PIN_GROUPS,
+    _PIN_TO_RAW,
+    _apply_pins_to_raw,
+    _validate_pin_key,
+)
+
+
+# ─────────────────────────────────────────────────────────────────────
+#  _validate_pin_key
+# ─────────────────────────────────────────────────────────────────────
+
+@pytest.mark.unit
+def test_validate_pin_key_top_level_fonts():
+    """VK1 — `fonts` is a known top-level key."""
+    _validate_pin_key("fonts")
+
+
+@pytest.mark.unit
+def test_validate_pin_key_top_level_dark_theme():
+    """VK2 — `dark_theme` is a known top-level key."""
+    _validate_pin_key("dark_theme")
+
+
+@pytest.mark.unit
+def test_validate_pin_key_dotted_screen_width():
+    """VK3 — valid dotted path `screen.width`."""
+    _validate_pin_key("screen.width")
+
+
+@pytest.mark.unit
+def test_validate_pin_key_dotted_gpu_renderer():
+    """VK4 — valid dotted path `gpu.renderer`."""
+    _validate_pin_key("gpu.renderer")
+
+
+@pytest.mark.unit
+def test_validate_pin_key_dotted_webgl_msaa_samples():
+    """VK5 — valid dotted path `webgl.msaa_samples`."""
+    _validate_pin_key("webgl.msaa_samples")
+
+
+@pytest.mark.unit
+def test_validate_pin_key_no_dot_not_top_level_raises():
+    """VK6 — bare key not in top-level set raises with hint."""
+    with pytest.raises(ValueError, match="group.field"):
+        _validate_pin_key("bogus")
+
+
+@pytest.mark.unit
+def test_validate_pin_key_unknown_group_raises():
+    """VK7 — unknown group prefix."""
+    with pytest.raises(ValueError, match="unknown group"):
+        _validate_pin_key("network.port")
+
+
+@pytest.mark.unit
+def test_validate_pin_key_unknown_field_in_valid_group_raises():
+    """VK8 — known group, unknown field."""
+    with pytest.raises(ValueError, match="unknown field"):
+        _validate_pin_key("screen.brightness")
+
+
+@pytest.mark.unit
+def test_validate_pin_key_empty_string_raises():
+    """VK9 — empty key fails the dotted-form check."""
+    with pytest.raises(ValueError):
+        _validate_pin_key("")
+
+
+@pytest.mark.unit
+@pytest.mark.parametrize("group,fields", sorted(_PIN_GROUPS.items()))
+def test_validate_pin_key_all_groups_first_field(group, fields):
+    """VK10 — every defined group accepts its sorted-first field."""
+    first = sorted(fields)[0]
+    _validate_pin_key(f"{group}.{first}")
+
+
+# ─────────────────────────────────────────────────────────────────────
+#  _apply_pins_to_raw
+# ─────────────────────────────────────────────────────────────────────
+
+def _raw_baseline():
+    """A minimal raw dict for pin tests — only the keys we care about."""
+    return {
+        "screen_w": 1920,
+        "screen_h": 1080,
+        "webgl_vendor": "Google Inc. (Intel)",
+        "webgl_renderer": "ANGLE (Intel)",
+        "font_whitelist": "arial,calibri",
+        "dark_theme": 0,
+    }
+
+
+@pytest.mark.unit
+def test_apply_pins_to_raw_screen_width():
+    """AP1 — `screen.width` rewrites `screen_w` in raw."""
+    out = _apply_pins_to_raw(_raw_baseline(), {"screen.width": 2560})
+    assert out["screen_w"] == 2560
+
+
+@pytest.mark.unit
+def test_apply_pins_to_raw_fonts_list():
+    """AP2 — list pin joined into comma-separated whitelist."""
+    out = _apply_pins_to_raw(_raw_baseline(), {"fonts": ["Arial", "Verdana"]})
+    assert out["font_whitelist"] == "Arial,Verdana"
+
+
+@pytest.mark.unit
+def test_apply_pins_to_raw_fonts_tuple():
+    """AP3 — tuple pin is also accepted."""
+    out = _apply_pins_to_raw(_raw_baseline(), {"fonts": ("Arial",)})
+    assert out["font_whitelist"] == "Arial"
+
+
+@pytest.mark.unit
+def test_apply_pins_to_raw_fonts_string_raises():
+    """AP4 — bare string is not a list/tuple, must raise."""
+    with pytest.raises(TypeError, match="list/tuple"):
+        _apply_pins_to_raw(_raw_baseline(), {"fonts": "Arial"})
+
+
+@pytest.mark.unit
+def test_apply_pins_to_raw_fonts_int_raises():
+    """AP5 — int is also rejected."""
+    with pytest.raises(TypeError):
+        _apply_pins_to_raw(_raw_baseline(), {"fonts": 42})
+
+
+@pytest.mark.unit
+def test_apply_pins_to_raw_multiple_pins():
+    """AP6 — multiple pins all land in raw."""
+    pin = {"gpu.vendor": "X", "gpu.renderer": "Y"}
+    out = _apply_pins_to_raw(_raw_baseline(), pin)
+    assert out["webgl_vendor"] == "X"
+    assert out["webgl_renderer"] == "Y"
+
+
+@pytest.mark.unit
+def test_apply_pins_to_raw_returns_copy_not_mutation():
+    """AP7 — input dict is not mutated."""
+    raw = _raw_baseline()
+    snapshot = dict(raw)
+    _apply_pins_to_raw(raw, {"screen.width": 9999})
+    assert raw == snapshot
+
+
+@pytest.mark.unit
+def test_apply_pins_to_raw_unknown_key_silent():
+    """AP8 — key not in `_PIN_TO_RAW` (and not 'fonts') is ignored.
+
+    Validation happens upstream in `generate_profile`; the inner helper
+    guards defensively but does not raise.
+    """
+    raw = _raw_baseline()
+    out = _apply_pins_to_raw(raw, {"some.unknown": 123})
+    # No change to known fields
+    assert out["screen_w"] == raw["screen_w"]
+    # No new key added
+    assert "some.unknown" not in out
+
+
+# ─────────────────────────────────────────────────────────────────────
+#  generate_profile
+# ─────────────────────────────────────────────────────────────────────
+
+@pytest.mark.unit
+def test_generate_profile_happy_path():
+    """GP1 — returns a fully populated Profile."""
+    p = generate_profile(seed=42)
+    assert isinstance(p, Profile)
+    assert p.seed == 42
+    assert p.gpu.vendor
+    assert p.gpu.renderer
+    assert p.gpu.class_tier in _PIN_GROUPS["gpu"].union({"low_end", "mid_range",
+        "high_end", "integrated_old", "integrated_modern", "workstation"})
+    assert p.screen.width > 0
+    assert p.screen.height > 0
+    assert p.hardware.concurrency > 0
+    assert p.audio.sample_rate > 0
+
+
+@pytest.mark.unit
+def test_generate_profile_deterministic():
+    """GP2 — same seed → identical Profile (equality on frozen dataclass)."""
+    a = generate_profile(seed=42)
+    b = generate_profile(seed=42)
+    assert a == b
+
+
+@pytest.mark.unit
+def test_generate_profile_seed_float_coerced():
+    """GP3 — float seed is coerced to int (truncated)."""
+    a = generate_profile(seed=42.7)
+    b = generate_profile(seed=42)
+    assert a == b
+
+
+@pytest.mark.unit
+def test_generate_profile_seed_string_coerced():
+    """GP4 — numeric string seed works via int() coercion."""
+    a = generate_profile(seed="42")
+    b = generate_profile(seed=42)
+    assert a == b
+
+
+@pytest.mark.unit
+def test_generate_profile_no_pin_samples_freely():
+    """GP5 — no pin: every field is sampler-derived (sanity: 2 seeds differ)."""
+    a = generate_profile(seed=1)
+    b = generate_profile(seed=2)
+    assert a != b
+
+
+@pytest.mark.unit
+def test_generate_profile_pin_overrides_screen_width():
+    """GP6 — pinned width visible on the Profile dataclass."""
+    p = generate_profile(seed=42, pin={"screen.width": 9999})
+    assert p.screen.width == 9999
+
+
+@pytest.mark.unit
+def test_generate_profile_pin_visible_in_prefs_dict():
+    """GP7 — pinned values flow through to to_prefs_dict()."""
+    p = generate_profile(seed=42, pin={"screen.width": 9999})
+    assert p.to_prefs_dict()["screen_w"] == 9999
+
+
+@pytest.mark.unit
+def test_generate_profile_invalid_pin_raises():
+    """GP8 — bad pin key surfaces ValueError from validation."""
+    with pytest.raises(ValueError):
+        generate_profile(seed=42, pin={"bogus": 1})
+
+
+@pytest.mark.unit
+def test_generate_profile_empty_pin_equals_no_pin():
+    """GP9 — empty pin dict is a no-op."""
+    a = generate_profile(seed=42, pin={})
+    b = generate_profile(seed=42)
+    assert a == b
+
+
+@pytest.mark.unit
+def test_generate_profile_is_frozen():
+    """GP10 — Profile dataclass is immutable."""
+    p = generate_profile(seed=42)
+    with pytest.raises(FrozenInstanceError):
+        p.seed = 99  # type: ignore[misc]
+
+
+@pytest.mark.unit
+def test_generate_profile_fonts_is_list_of_strings():
+    """GP11 — fonts is a non-empty list of stripped strings."""
+    p = generate_profile(seed=42)
+    assert isinstance(p.fonts, list)
+    assert len(p.fonts) > 0
+    assert all(isinstance(f, str) and f.strip() == f for f in p.fonts)
+
+
+@pytest.mark.unit
+def test_generate_profile_to_prefs_dict_flat_and_matches_raw():
+    """GP12 — to_prefs_dict() returns a flat dict containing core sampler keys."""
+    p = generate_profile(seed=42)
+    d = p.to_prefs_dict()
+    assert isinstance(d, dict)
+    for key in ("screen_w", "screen_h", "webgl_vendor", "webgl_renderer",
+                "hw_concurrency", "stealth_seed"):
+        assert key in d
+
+
+@pytest.mark.unit
+def test_generate_profile_seed_zero():
+    """GP13 — seed=0 is a valid lowest-value boundary."""
+    p = generate_profile(seed=0)
+    assert p.seed == 0
+
+
+@pytest.mark.unit
+def test_generate_profile_seed_max_int31():
+    """GP14 — seed at int31 upper bound works."""
+    seed = (1 << 31) - 1
+    p = generate_profile(seed=seed)
+    assert p.seed == seed
+
+
+@pytest.mark.unit
+def test_generate_profile_dark_theme_is_bool():
+    """GP15 — dark_theme is coerced to bool on the dataclass."""
+    p = generate_profile(seed=42)
+    assert isinstance(p.dark_theme, bool)
+
+
+# ─────────────────────────────────────────────────────────────────────
+#  Additional pin coverage (recheck pass)
+# ─────────────────────────────────────────────────────────────────────
+
+@pytest.mark.unit
+def test_generate_profile_pin_dark_theme_true():
+    """Pinning dark_theme=True flows through coercion to bool."""
+    p = generate_profile(seed=42, pin={"dark_theme": True})
+    assert p.dark_theme is True
+
+
+@pytest.mark.unit
+def test_generate_profile_pin_dark_theme_false():
+    p = generate_profile(seed=42, pin={"dark_theme": False})
+    assert p.dark_theme is False
+
+
+@pytest.mark.unit
+def test_generate_profile_pin_fonts_list_visible_on_profile():
+    """fonts pin: list → joined raw string → split back to list on Profile."""
+    p = generate_profile(seed=42, pin={"fonts": ["Arial", "Verdana"]})
+    assert p.fonts == ["Arial", "Verdana"]
+
+
+@pytest.mark.unit
+def test_generate_profile_pin_gpu_renderer_propagates():
+    p = generate_profile(seed=42, pin={"gpu.renderer": "FORCED_RENDERER"})
+    assert p.gpu.renderer == "FORCED_RENDERER"
+    assert p.to_prefs_dict()["webgl_renderer"] == "FORCED_RENDERER"
+
+
+@pytest.mark.unit
+def test_generate_profile_pin_to_raw_keymap_complete():
+    """Every dotted pin key (besides 'fonts') has a `_PIN_TO_RAW` mapping.
+
+    Guards against silently-ignored pins if someone adds a key to `_PIN_GROUPS`
+    but forgets the raw-key mapping.
+    """
+    dotted = {f"{group}.{field}" for group, fields in _PIN_GROUPS.items()
+              for field in fields}
+    # 'dark_theme' is top-level and present in _PIN_TO_RAW; 'fonts' is handled
+    # specially and intentionally absent.
+    missing = dotted - set(_PIN_TO_RAW.keys())
+    assert missing == set(), f"pin keys without raw mapping: {sorted(missing)}"