diff --git a/README.md b/README.md
index 021126a..56815a5 100644
--- a/README.md
+++ b/README.md
@@ -14,38 +14,50 @@ That's it for most users -- CANDy's default toolchain is fully bundled:
 
 - **Clustering**: [MMseqs2](https://github.com/soedinglab/MMseqs2) -- auto-downloaded and cached on first use (no conda needed). On Linux/macOS this just works. On **Windows**, MMseqs2's clustering workflows internally need a POSIX shell; the official Windows build handles this itself by installing a small helper (`busybox`) the first time it runs, which may ask for administrator permission **once** -- never again after that. (This mirrors upstream: MMseqs2's own docs list WSL as the recommended Windows path and this static build as the fallback for anyone who can't use WSL.)
 - **MSA**: [FAMSA](https://github.com/refresh-bio/FAMSA) via [`pyfamsa`](https://github.com/althonos/pyfamsa) -- a real pip dependency, runs in-process, no download needed.
-- **Phylogenetics**: [VeryFastTree](https://github.com/citiususc/veryfasttree) via [`veryfasttree`](https://github.com/citiususc/veryfasttree-python) -- also a real pip dependency, no download needed.
+- **Phylogenetics**: [VeryFastTree](https://github.com/citiususc/veryfasttree) via [`veryfasttree`](https://github.com/citiususc/veryfasttree-python) -- also a real pip dependency, no download needed. **Except on Apple Silicon Macs**: `veryfasttree` has no `macOS arm64` wheel at all (as of 4.0.4.1) and its from-source build fails on stock macOS (an upstream OpenMP-detection bug). CANDy detects this automatically and defaults `--tree-tool` to [FastTree](http://www.microbesonline.org/fasttree/) instead there -- see below.
 
 ### Apple Silicon (M1/M2/M3/M4) setup
 
-FAMSA and VeryFastTree ship native SIMD code. If your Python is x86_64 running under Rosetta 2 translation instead of native `arm64`, that's a known cause of a silent `illegal hardware instruction` crash during `--tree`, and can also make other native calls (e.g. the Gemini curation backend) unreliable. CANDy detects this at startup and logs a warning, but it's worth fixing properly rather than working around it -- **pip can't do this for you**: by the time `pip install` runs, the interpreter architecture is already fixed, so no package (including this one) can retroactively fix it. Two ways to get a correct native setup:
+There are two distinct, independent issues here -- you may hit either, both, or neither depending on your setup:
 
-**Option A -- [`uv`](https://docs.astral.sh/uv/) (recommended, one-time setup):** `uv` manages isolated Python installs and always defaults to the native architecture.
+**1. `--tree` crashes with `illegal hardware instruction`, no traceback.** This means your Python itself is x86_64 running under Rosetta 2 translation instead of native `arm64` -- FAMSA ships native SIMD code, and Rosetta's emulation of some CPU instructions is a known cause of exactly this crash (it can also make other native calls, e.g. Gemini curation, unreliable). CANDy logs a warning about this at startup if detected, but **pip can't fix it for you**: by the time `pip install` runs, the interpreter architecture is already fixed.
+
+**Step 0, always do this first:** confirm your *terminal itself* is native, not just your hardware:
 
 ```bash
-# only run the first line if you don't already have uv
-curl -LsSf https://astral.sh/uv/install.sh | sh   
-uv tool install candy-cazyme
+arch   # must print "arm64", not "i386"
 ```
 
-`uv tool install` gives you a `candy` command backed by its own isolated, native-arm64 Python -- no venv/PATH management needed. (Skip straight to Option B if `arch` below still reports `i386` after this -- see the Terminal note.)
+If it prints `i386`, your terminal app (Terminal/iTerm) is launching under Rosetta -- and anything installed from it, including tools like `uv` that are supposed to auto-detect the native architecture, will get fooled into installing x86_64 builds too. Fix this first: quit the app, Finder > select it > `Cmd+I` > uncheck "Open using Rosetta" > relaunch, then re-run `arch` to confirm.
 
-**Option B -- Homebrew, manually:**
+Once `arch` says `arm64`, two ways to get a correct native Python:
+
+**Option A -- [`uv`](https://docs.astral.sh/uv/) (recommended):** `uv` manages isolated Python installs and defaults to the native architecture -- *once its own install wasn't done under a translated shell* (see Step 0).
+
+```bash
+curl -LsSf https://astral.sh/uv/install.sh | sh   # only if you don't already have uv
+uv python install 3.12
+uv tool install --python 3.12 "candy-cazyme[gemini]"
+```
+
+If you already ran `uv tool install` before fixing Step 0, it will have cached an x86_64 environment -- force it to redo the install natively:
 
 ```bash
-# 1. Confirm your shell itself is running natively (should print "arm64"):
-arch
+uv tool uninstall candy-cazyme
+uv tool install --force --python 3.12 "candy-cazyme[gemini]"
+```
+
+This gives you a `candy` command backed by its own isolated, native-arm64 Python -- no venv/PATH management needed.
 
-# 2. If it printed "i386", your terminal app is running under Rosetta --
-#    quit it, then in Finder: select the app (Terminal/iTerm) > Cmd+I >
-#    uncheck "Open using Rosetta" > relaunch it, and re-run `arch`.
+**Option B -- Homebrew, manually:**
 
-# 3. Install (or confirm) Homebrew at the Apple Silicon prefix, /opt/homebrew
-#    (a pre-existing Homebrew at /usr/local is the Intel-only one):
+```bash
+# Install (or confirm) Homebrew at the Apple Silicon prefix, /opt/homebrew
+# (a pre-existing Homebrew at /usr/local is the Intel-only one):
 /opt/homebrew/bin/brew --version || arch -arm64 /bin/bash -c \
   "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
 
-# 4. Install Python from that prefix and use it explicitly:
+# Install Python from that prefix and use it explicitly:
 /opt/homebrew/bin/brew install python@3.12
 /opt/homebrew/bin/python3.12 -m venv .venv
 source .venv/bin/activate
@@ -58,13 +70,17 @@ Either way, verify before running a real job:
 python3 -c "import platform; print(platform.machine())"   # should print "arm64", not "x86_64"
 ```
 
-If you'd rather use the original CD-HIT/MAFFT/FastTree tools instead (e.g. to reproduce results bit-for-bit against the published notebook), a conda environment with those is still provided:
+**2. `--tree` fails to install or build `veryfasttree` (e.g. a CMake/OpenMP compiler error).** This is unrelated to Rosetta -- `veryfasttree` simply has no `macOS arm64` wheel at all, for any Python version, so it always falls back to a from-source build there, and that build fails on stock macOS due to an upstream bug (`find_package(OpenMP)` fails, since Apple's Clang has no OpenMP support out of the box, and `veryfasttree`'s CMake fallback for that case is itself broken). **You shouldn't normally hit this**: CANDy detects a Mac without a working native `veryfasttree` build and automatically defaults `--tree-tool` to `fasttree` instead, which *does* have a real `arm64` conda-forge/bioconda build (no compiling anything). That needs the bundled conda environment (a real, one-time dependency for this one platform):
 
 ```bash
-conda env create -f environment.yml   # only needed for CD-HIT as a clustering alternative to MMseqs2
+conda env create -f environment.yml
 conda activate candy
+candy GH173 --email you@example.com --tree   # --tree-tool defaults to fasttree here automatically
 ```
-and pass `--clustering-software cd-hit` / build a `PipelineConfig` with `alignment_tool="mafft"`, `tree_tool="fasttree"`.
+
+If you'd rather force `veryfasttree` anyway (e.g. you've solved the OpenMP build issue yourself), pass `--tree-tool veryfasttree` explicitly.
+
+If you'd rather use the original CD-HIT/MAFFT/FastTree tools instead (e.g. to reproduce results bit-for-bit against the published notebook), `environment.yml` provides CD-HIT and FastTree (`conda env create -f environment.yml && conda activate candy`, then `--clustering-software cd-hit --tree-tool fasttree`); MAFFT isn't included there (no `osx-arm64` build -- see above) and needs a separate install, e.g. `brew install mafft` on Intel Mac/Linux, then `--alignment-tool mafft`.
 
 To also enable automated Gemini-based domain-name curation, see [Domain-name curation](#domain-name-curation) below.
 
@@ -87,6 +103,10 @@ candy GH173 --email you@example.com --db-preference PFAM,SMART
 
 # Analyse your own FASTA file instead
 candy my_sequences.fasta --tree
+
+# Pick a specific MSA/phylogenetics backend explicitly (see Installation for
+# when you'd want to -- e.g. --tree-tool fasttree needs the conda environment)
+candy GH173 --email you@example.com --tree --alignment-tool mafft --tree-tool fasttree
 ```
 
 `--email` falls back to the `CANDY_EMAIL` environment variable, then an interactive prompt, so `export CANDY_EMAIL=you@example.com` once and just run `candy GH173` from then on. Run `candy --help` for the full list of options.
diff --git a/environment.yml b/environment.yml
index 337341c..92348a3 100644
--- a/environment.yml
+++ b/environment.yml
@@ -1,10 +1,20 @@
 # Optional. CANDy's default toolchain (MMseqs2 for clustering, FAMSA for MSA,
 # VeryFastTree for phylogenetics) ships via `pip install candy-cazyme` alone --
 # MSA/phylogenetics are bundled Python packages, and MMseqs2 is auto-downloaded
-# on first use. This environment is only needed if you specifically want
-# CD-HIT as the clustering backend instead of MMseqs2: CD-HIT has no official
-# Windows build and no pip-installable bindings, so (unlike MMseqs2) it can't
-# be auto-downloaded and must come from conda/bioconda.
+# on first use. This environment provides the non-default alternatives that
+# have no pip-installable bindings:
+#   - cd-hit:   clustering alternative to MMseqs2 (--clustering-software cd-hit).
+#               No official Windows build and no pip bindings at all.
+#   - fasttree: phylogenetics alternative to VeryFastTree (--tree-tool fasttree).
+#               This is also CANDy's *default* on a Mac where VeryFastTree has
+#               no working native build (Apple Silicon, or an x86_64 Python
+#               under Rosetta translation) -- see the README's Apple Silicon
+#               setup section.
+# MAFFT (the alternative to FAMSA, --alignment-tool mafft) is deliberately
+# NOT included here: bioconda has no osx-arm64 build for it, which would
+# break `conda env create` on Apple Silicon entirely -- exactly the platform
+# that needs this file for fasttree. Install MAFFT separately if you want it
+# (e.g. `brew install mafft` on Intel Mac/Linux, or see mafft.cbrc.jp).
 name: candy
 channels:
   - bioconda
@@ -13,5 +23,6 @@ dependencies:
   - python>=3.10
   - pip
   - cd-hit
+  - fasttree
   - pip:
       - -e .
diff --git a/src/candy/alignment/mafft.py b/src/candy/alignment/mafft.py
index ccd931b..9a8a340 100644
--- a/src/candy/alignment/mafft.py
+++ b/src/candy/alignment/mafft.py
@@ -19,7 +19,14 @@ class MafftAligner:
     name = "mafft"
 
     def align(self, input_fasta: Path, output_fasta: Path) -> Path:
-        binary = require_binary("mafft")
+        binary = require_binary(
+            "mafft",
+            hint=(
+                "Install it separately and ensure it's on PATH (e.g. `brew install mafft` on "
+                "Intel Mac/Linux, or see https://mafft.cbrc.jp/alignment/software/) -- it isn't "
+                "in CANDy's conda environment.yml, which has no macOS arm64 build for it."
+            ),
+        )
         logger.info("Aligning %s with MAFFT.", input_fasta)
         run_tool([binary, str(input_fasta)], stdout_path=output_fasta)
         return output_fasta
diff --git a/src/candy/cli.py b/src/candy/cli.py
index b5a124c..8d640d8 100644
--- a/src/candy/cli.py
+++ b/src/candy/cli.py
@@ -81,6 +81,18 @@ def main(
         + ", ".join(DEFAULT_DATABASE_PREFERENCE) + ".",
     ),
     build_tree: bool = typer.Option(False, "--tree/--no-tree", help="Run MSA + phylogenetics + iTOL export."),
+    alignment_tool: Optional[str] = typer.Option(
+        None,
+        help="MSA backend: 'famsa' (default; bundled, no setup needed) or 'mafft' (must be installed "
+        "separately and be on PATH -- not available via CANDy's conda environment.yml, which has no "
+        "macOS arm64 build for it; see README).",
+    ),
+    tree_tool: Optional[str] = typer.Option(
+        None,
+        help="Phylogenetics backend: 'veryfasttree' or 'fasttree'. Defaults to 'veryfasttree', except "
+        "on a Mac where it has no working native build (Apple Silicon, or Rosetta translation), where "
+        "it defaults to 'fasttree' instead (needs the conda environment; see README).",
+    ),
     curation_backend: str = typer.Option(
         "manual", help="Domain-name curation backend: 'manual' or 'gemini'."
     ),
@@ -145,6 +157,12 @@ def main(
     else:
         database_preference = list(DEFAULT_DATABASE_PREFERENCE)
 
+    tool_kwargs = {}
+    if alignment_tool:
+        tool_kwargs["alignment_tool"] = alignment_tool
+    if tree_tool:
+        tool_kwargs["tree_tool"] = tree_tool
+
     config = PipelineConfig(
         input=pipeline_input,
         jobname=_sanitize_jobname(jobname or default_jobname),
@@ -158,6 +176,7 @@ def main(
         curation=CurationConfig(backend=curation_backend, api_key=curation_api_key, model=curation_model),
         blast_identity_threshold=blast_identity,
         build_tree=build_tree,
+        **tool_kwargs,
     )
 
     try:
diff --git a/src/candy/config.py b/src/candy/config.py
index 6dae5b3..ee5a030 100644
--- a/src/candy/config.py
+++ b/src/candy/config.py
@@ -15,6 +15,19 @@
 from enum import Enum
 from pathlib import Path
 
+from candy.platform_utils import is_macos_without_native_veryfasttree_wheel
+
+
+def default_tree_tool() -> str:
+    """'fasttree' on a Mac where veryfasttree has no reliable wheel, else 'veryfasttree'.
+
+    fasttree itself needs the conda environment (see README), but that's a
+    real, one-time, clearly-reported dependency -- preferable to defaulting
+    everyone on the affected Macs into a from-source build that's known to
+    fail, or a translated binary that's known to crash.
+    """
+    return "fasttree" if is_macos_without_native_veryfasttree_wheel() else "veryfasttree"
+
 
 class Taxonomy(str, Enum):
     ALL = "All"
@@ -167,7 +180,7 @@ class PipelineConfig:
 
     build_tree: bool = False
     alignment_tool: str = "famsa"
-    tree_tool: str = "veryfasttree"
+    tree_tool: str = field(default_factory=default_tree_tool)
 
     def __post_init__(self) -> None:
         self.output_dir = Path(self.output_dir)
diff --git a/src/candy/pipeline.py b/src/candy/pipeline.py
index 8b6791f..a889335 100644
--- a/src/candy/pipeline.py
+++ b/src/candy/pipeline.py
@@ -17,8 +17,6 @@
 from __future__ import annotations
 
 import logging
-import platform
-import subprocess
 from dataclasses import dataclass
 from pathlib import Path
 
@@ -31,6 +29,7 @@
 from candy.config import CAZyFamilyInput, CustomFastaInput, PipelineConfig, Taxonomy
 from candy.curation import get_curation_backend
 from candy.phylogenetics import get_tree_builder
+from candy.platform_utils import is_apple_silicon_under_rosetta
 
 logger = logging.getLogger(__name__)
 
@@ -54,7 +53,7 @@
 
 
 def _warn_if_running_under_rosetta() -> None:
-    """Detect x86_64 Python running via Rosetta 2 translation on Apple Silicon.
+    """Warn about x86_64 Python running via Rosetta 2 translation on Apple Silicon.
 
     Surfaced by a real run: FAMSA's alignment step crashed with a SIGILL and
     no Python traceback at all (just the shell's own "illegal hardware
@@ -64,18 +63,7 @@ def _warn_if_running_under_rosetta() -> None:
     Rosetta. Warning about this *before* the (potentially long) clustering
     and curation stages run is much more useful than the crash itself.
     """
-    if platform.system() != "Darwin" or platform.machine() != "x86_64":
-        return
-    try:
-        translated = subprocess.run(
-            ["sysctl", "-n", "sysctl.proc_translated"],
-            capture_output=True,
-            text=True,
-            timeout=5,
-        )
-    except (OSError, subprocess.SubprocessError):
-        return
-    if translated.stdout.strip() == "1":
+    if is_apple_silicon_under_rosetta():
         logger.warning(_ROSETTA_WARNING)
 
 
diff --git a/src/candy/platform_utils.py b/src/candy/platform_utils.py
new file mode 100644
index 0000000..ff0018b
--- /dev/null
+++ b/src/candy/platform_utils.py
@@ -0,0 +1,47 @@
+"""OS/CPU-architecture detection helpers.
+
+Split out on its own so both ``pipeline.py`` (which warns about a translated
+interpreter) and ``config.py`` (which picks a platform-appropriate default
+tree-building backend) can share the same detection logic without a
+circular import between those two modules.
+"""
+
+from __future__ import annotations
+
+import platform
+import subprocess
+
+
+def is_apple_silicon_under_rosetta() -> bool:
+    """True if this is x86_64 Python running via Rosetta 2 translation on Apple Silicon."""
+    if platform.system() != "Darwin" or platform.machine() != "x86_64":
+        return False
+    try:
+        translated = subprocess.run(
+            ["sysctl", "-n", "sysctl.proc_translated"],
+            capture_output=True,
+            text=True,
+            timeout=5,
+        )
+    except (OSError, subprocess.SubprocessError):
+        return False
+    return translated.stdout.strip() == "1"
+
+
+def is_macos_without_native_veryfasttree_wheel() -> bool:
+    """True if ``veryfasttree`` has no reliable prebuilt wheel on this Mac.
+
+    ``veryfasttree`` (as of 4.0.4.1) only publishes a macOS wheel for
+    x86_64 -- there is no ``arm64`` build at all. That makes this true on
+    two distinct occasions: genuine Apple Silicon hardware (no wheel
+    exists, so installing it falls back to compiling from source, which
+    fails on stock macOS due to an upstream OpenMP-detection bug in
+    veryfasttree's own CMakeLists.txt), and x86_64 Python running under
+    Rosetta 2 translation on Apple Silicon (a wheel *does* exist and
+    installs fine, but running translated native SIMD code is a known
+    cause of a silent 'illegal hardware instruction' crash -- the same
+    class of bug already seen with FAMSA on a translated interpreter).
+    """
+    if platform.system() != "Darwin":
+        return False
+    return platform.machine() == "arm64" or is_apple_silicon_under_rosetta()
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 0995a94..ff09ccf 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -255,3 +255,53 @@ def fake_run_pipeline(config):
 
     assert result.exit_code == 0, result.output
     assert captured["config"].domain_cleaning.database_preference == DEFAULT_DATABASE_PREFERENCE
+
+
+def test_cli_tree_tool_and_alignment_tool_default_to_platform_detection(tmp_path, monkeypatch):
+    # Not passing --tree-tool/--alignment-tool should leave PipelineConfig's
+    # own platform-aware default_factory in charge, not a fixed CLI default.
+    captured = {}
+
+    def fake_run_pipeline(config):
+        captured["config"] = config
+        return _fake_pipeline_result(tmp_path)
+
+    monkeypatch.setattr("candy.cli.run_pipeline", fake_run_pipeline)
+    monkeypatch.setattr("candy.config.is_macos_without_native_veryfasttree_wheel", lambda: True)
+
+    result = runner.invoke(app, ["GH173", "--email", "you@example.com"])
+
+    assert result.exit_code == 0, result.output
+    assert captured["config"].tree_tool == "fasttree"
+    assert captured["config"].alignment_tool == "famsa"
+
+
+def test_cli_explicit_tree_tool_overrides_platform_default(tmp_path, monkeypatch):
+    captured = {}
+
+    def fake_run_pipeline(config):
+        captured["config"] = config
+        return _fake_pipeline_result(tmp_path)
+
+    monkeypatch.setattr("candy.cli.run_pipeline", fake_run_pipeline)
+    monkeypatch.setattr("candy.config.is_macos_without_native_veryfasttree_wheel", lambda: True)
+
+    result = runner.invoke(app, ["GH173", "--email", "you@example.com", "--tree-tool", "veryfasttree"])
+
+    assert result.exit_code == 0, result.output
+    assert captured["config"].tree_tool == "veryfasttree"
+
+
+def test_cli_explicit_alignment_tool_is_passed_through(tmp_path, monkeypatch):
+    captured = {}
+
+    def fake_run_pipeline(config):
+        captured["config"] = config
+        return _fake_pipeline_result(tmp_path)
+
+    monkeypatch.setattr("candy.cli.run_pipeline", fake_run_pipeline)
+
+    result = runner.invoke(app, ["GH173", "--email", "you@example.com", "--alignment-tool", "mafft"])
+
+    assert result.exit_code == 0, result.output
+    assert captured["config"].alignment_tool == "mafft"
diff --git a/tests/test_config.py b/tests/test_config.py
index fedcee6..3a8d653 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -1,6 +1,15 @@
+from unittest.mock import patch
+
 import pytest
 
-from candy.config import DEFAULT_DATABASE_PREFERENCE, reorder_database_preference
+from candy.config import (
+    DEFAULT_DATABASE_PREFERENCE,
+    CAZyFamilyInput,
+    PipelineConfig,
+    Taxonomy,
+    default_tree_tool,
+    reorder_database_preference,
+)
 
 
 def test_reorder_moves_named_databases_to_front_in_given_order():
@@ -35,3 +44,37 @@ def test_reorder_deduplicates_repeated_names():
 def test_reorder_raises_on_unknown_database_name():
     with pytest.raises(ValueError, match="Unknown database name"):
         reorder_database_preference(["NOTAREALDB"])
+
+
+def test_default_tree_tool_is_fasttree_when_veryfasttree_has_no_wheel():
+    with patch("candy.config.is_macos_without_native_veryfasttree_wheel", return_value=True):
+        assert default_tree_tool() == "fasttree"
+
+
+def test_default_tree_tool_is_veryfasttree_otherwise():
+    with patch("candy.config.is_macos_without_native_veryfasttree_wheel", return_value=False):
+        assert default_tree_tool() == "veryfasttree"
+
+
+def test_pipeline_config_tree_tool_defaults_via_platform_detection():
+    # PipelineConfig() should pick up the platform-aware default automatically,
+    # not just a fixed literal -- Python API users benefit without needing to
+    # know about this quirk at all.
+    with patch("candy.config.is_macos_without_native_veryfasttree_wheel", return_value=True):
+        config = PipelineConfig(
+            input=CAZyFamilyInput(enzyme_class="GH", family_number=5, email="a@b.com", taxonomy=Taxonomy.ALL),
+            jobname="job",
+            output_dir=".",
+        )
+    assert config.tree_tool == "fasttree"
+
+
+def test_pipeline_config_tree_tool_explicit_value_overrides_default():
+    with patch("candy.config.is_macos_without_native_veryfasttree_wheel", return_value=True):
+        config = PipelineConfig(
+            input=CAZyFamilyInput(enzyme_class="GH", family_number=5, email="a@b.com", taxonomy=Taxonomy.ALL),
+            jobname="job",
+            output_dir=".",
+            tree_tool="veryfasttree",
+        )
+    assert config.tree_tool == "veryfasttree"
diff --git a/tests/test_pipeline.py b/tests/test_pipeline.py
index 897c483..32ffa22 100644
--- a/tests/test_pipeline.py
+++ b/tests/test_pipeline.py
@@ -8,7 +8,6 @@
 """
 
 import xml.etree.ElementTree as ET
-from types import SimpleNamespace
 from unittest.mock import patch
 
 import pandas as pd
@@ -17,68 +16,22 @@
 from candy.pipeline import _warn_if_running_under_rosetta, run_pipeline
 
 
-def test_warn_if_running_under_rosetta_warns_when_translated(caplog):
-    with patch("candy.pipeline.platform.system", return_value="Darwin"), patch(
-        "candy.pipeline.platform.machine", return_value="x86_64"
-    ), patch(
-        "candy.pipeline.subprocess.run",
-        return_value=SimpleNamespace(stdout="1\n"),
-    ):
+def test_warn_if_running_under_rosetta_warns_when_detected(caplog):
+    with patch("candy.pipeline.is_apple_silicon_under_rosetta", return_value=True):
         with caplog.at_level("WARNING"):
             _warn_if_running_under_rosetta()
 
     assert any("Rosetta" in record.message for record in caplog.records)
 
 
-def test_warn_if_running_under_rosetta_silent_when_native_arm64(caplog):
-    with patch("candy.pipeline.platform.system", return_value="Darwin"), patch(
-        "candy.pipeline.platform.machine", return_value="arm64"
-    ), patch("candy.pipeline.subprocess.run") as mock_run:
-        with caplog.at_level("WARNING"):
-            _warn_if_running_under_rosetta()
-
-    mock_run.assert_not_called()
-    assert caplog.records == []
-
-
-def test_warn_if_running_under_rosetta_silent_on_genuine_intel_mac(caplog):
-    # On a real Intel Mac, `sysctl -n sysctl.proc_translated` reports "0"
-    # (the key exists but the process isn't translated).
-    with patch("candy.pipeline.platform.system", return_value="Darwin"), patch(
-        "candy.pipeline.platform.machine", return_value="x86_64"
-    ), patch(
-        "candy.pipeline.subprocess.run",
-        return_value=SimpleNamespace(stdout="0\n"),
-    ):
+def test_warn_if_running_under_rosetta_silent_when_not_detected(caplog):
+    with patch("candy.pipeline.is_apple_silicon_under_rosetta", return_value=False):
         with caplog.at_level("WARNING"):
             _warn_if_running_under_rosetta()
 
     assert caplog.records == []
 
 
-def test_warn_if_running_under_rosetta_silent_on_non_macos(caplog):
-    with patch("candy.pipeline.platform.system", return_value="Windows"), patch(
-        "candy.pipeline.subprocess.run"
-    ) as mock_run:
-        with caplog.at_level("WARNING"):
-            _warn_if_running_under_rosetta()
-
-    mock_run.assert_not_called()
-    assert caplog.records == []
-
-
-def test_warn_if_running_under_rosetta_tolerates_missing_sysctl(caplog):
-    # Defensive: sysctl.proc_translated not existing (or sysctl missing
-    # entirely) must never crash the pipeline over a best-effort warning.
-    with patch("candy.pipeline.platform.system", return_value="Darwin"), patch(
-        "candy.pipeline.platform.machine", return_value="x86_64"
-    ), patch("candy.pipeline.subprocess.run", side_effect=OSError("no such command")):
-        with caplog.at_level("WARNING"):
-            _warn_if_running_under_rosetta()  # must not raise
-
-    assert caplog.records == []
-
-
 class StubCurationBackend:
     name = "stub"
 
diff --git a/tests/test_platform_utils.py b/tests/test_platform_utils.py
new file mode 100644
index 0000000..5f890b3
--- /dev/null
+++ b/tests/test_platform_utils.py
@@ -0,0 +1,82 @@
+from types import SimpleNamespace
+from unittest.mock import patch
+
+from candy.platform_utils import is_apple_silicon_under_rosetta, is_macos_without_native_veryfasttree_wheel
+
+
+def test_is_apple_silicon_under_rosetta_true_when_translated():
+    with patch("candy.platform_utils.platform.system", return_value="Darwin"), patch(
+        "candy.platform_utils.platform.machine", return_value="x86_64"
+    ), patch(
+        "candy.platform_utils.subprocess.run",
+        return_value=SimpleNamespace(stdout="1\n"),
+    ):
+        assert is_apple_silicon_under_rosetta() is True
+
+
+def test_is_apple_silicon_under_rosetta_false_on_native_arm64():
+    with patch("candy.platform_utils.platform.system", return_value="Darwin"), patch(
+        "candy.platform_utils.platform.machine", return_value="arm64"
+    ), patch("candy.platform_utils.subprocess.run") as mock_run:
+        assert is_apple_silicon_under_rosetta() is False
+
+    mock_run.assert_not_called()
+
+
+def test_is_apple_silicon_under_rosetta_false_on_genuine_intel_mac():
+    # On a real Intel Mac, `sysctl -n sysctl.proc_translated` reports "0"
+    # (the key exists but the process isn't translated).
+    with patch("candy.platform_utils.platform.system", return_value="Darwin"), patch(
+        "candy.platform_utils.platform.machine", return_value="x86_64"
+    ), patch(
+        "candy.platform_utils.subprocess.run",
+        return_value=SimpleNamespace(stdout="0\n"),
+    ):
+        assert is_apple_silicon_under_rosetta() is False
+
+
+def test_is_apple_silicon_under_rosetta_false_on_non_macos():
+    with patch("candy.platform_utils.platform.system", return_value="Windows"), patch(
+        "candy.platform_utils.subprocess.run"
+    ) as mock_run:
+        assert is_apple_silicon_under_rosetta() is False
+
+    mock_run.assert_not_called()
+
+
+def test_is_apple_silicon_under_rosetta_tolerates_missing_sysctl():
+    # Defensive: sysctl.proc_translated not existing (or sysctl missing
+    # entirely) must never crash the caller over a best-effort check.
+    with patch("candy.platform_utils.platform.system", return_value="Darwin"), patch(
+        "candy.platform_utils.platform.machine", return_value="x86_64"
+    ), patch("candy.platform_utils.subprocess.run", side_effect=OSError("no such command")):
+        assert is_apple_silicon_under_rosetta() is False  # must not raise
+
+
+def test_is_macos_without_native_veryfasttree_wheel_true_on_native_arm64():
+    # No macOS arm64 wheel exists for veryfasttree at all (as of 4.0.4.1).
+    with patch("candy.platform_utils.platform.system", return_value="Darwin"), patch(
+        "candy.platform_utils.platform.machine", return_value="arm64"
+    ):
+        assert is_macos_without_native_veryfasttree_wheel() is True
+
+
+def test_is_macos_without_native_veryfasttree_wheel_true_under_rosetta():
+    # A wheel exists for macOS x86_64 and installs fine, but running it
+    # translated is a known SIGILL risk -- same class of bug as FAMSA.
+    with patch("candy.platform_utils.platform.system", return_value="Darwin"), patch(
+        "candy.platform_utils.platform.machine", return_value="x86_64"
+    ), patch("candy.platform_utils.is_apple_silicon_under_rosetta", return_value=True):
+        assert is_macos_without_native_veryfasttree_wheel() is True
+
+
+def test_is_macos_without_native_veryfasttree_wheel_false_on_genuine_intel_mac():
+    with patch("candy.platform_utils.platform.system", return_value="Darwin"), patch(
+        "candy.platform_utils.platform.machine", return_value="x86_64"
+    ), patch("candy.platform_utils.is_apple_silicon_under_rosetta", return_value=False):
+        assert is_macos_without_native_veryfasttree_wheel() is False
+
+
+def test_is_macos_without_native_veryfasttree_wheel_false_on_non_macos():
+    with patch("candy.platform_utils.platform.system", return_value="Linux"):
+        assert is_macos_without_native_veryfasttree_wheel() is False