diff --git a/docs/guides/setup-preview.md b/docs/guides/setup-preview.md
new file mode 100644
index 0000000..27fafa7
--- /dev/null
+++ b/docs/guides/setup-preview.md
@@ -0,0 +1,11 @@
+# Setup preview contract
+
+`verdict setup --dry-run --json` produces a mutation-free `setup_plan`.
+Every proposed action includes its reason, security impact, postcondition, and
+exact undo description. The preview does not read configuration contents,
+probe optional tools, contact the network, or access credentials.
+
+An existing configuration is preserved as a read-only action. A missing
+configuration is presented as a consent-required creation action. Applying
+either action remains a separate transactional boundary; the preview itself
+does not authorize or perform mutations.
diff --git a/tests/test_setup_plan.py b/tests/test_setup_plan.py
new file mode 100644
index 0000000..68c133e
--- /dev/null
+++ b/tests/test_setup_plan.py
@@ -0,0 +1,39 @@
+"""Tests for auditable, mutation-free setup plans."""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+from verdict.setup_plan import build_setup_plan
+
+
+def test_setup_actions_explain_reason_security_postcondition_and_undo(
+    tmp_path: Path, monkeypatch
+) -> None:
+    monkeypatch.setenv("XDG_CONFIG_HOME", str(tmp_path / "config"))
+
+    report = build_setup_plan().to_dict()
+    action = report["actions"][0]
+
+    for field in ("reason", "security_impact", "postcondition", "undo"):
+        assert isinstance(action[field], str)
+        assert action[field]
+    assert report["mutation_free"] is True
+    assert not (tmp_path / "config").exists()
+
+
+def test_existing_config_preview_explains_noop_without_reading_contents(
+    tmp_path: Path, monkeypatch
+) -> None:
+    config_dir = tmp_path / "config" / "verdict"
+    config_dir.mkdir(parents=True)
+    config_path = config_dir / "verdict.yaml"
+    config_path.write_text("private: do-not-read\n", encoding="utf-8")
+    before = config_path.read_bytes()
+    monkeypatch.setenv("XDG_CONFIG_HOME", str(tmp_path / "config"))
+
+    action = build_setup_plan().to_dict()["actions"][0]
+
+    assert action["action_id"] == "preserve-config"
+    assert "byte-for-byte" in action["postcondition"]
+    assert config_path.read_bytes() == before
diff --git a/verdict/setup_plan.py b/verdict/setup_plan.py
index 3872040..aa27d32 100644
--- a/verdict/setup_plan.py
+++ b/verdict/setup_plan.py
@@ -25,6 +25,10 @@ class SetupAction:
     kind: str
     target: str
     description: str
+    reason: str
+    security_impact: str
+    postcondition: str
+    undo: str
     reversible: bool
     requires_consent: bool
 
@@ -36,6 +40,10 @@ def to_dict(self) -> dict[str, object]:
             "kind": self.kind,
             "target": self.target,
             "description": self.description,
+            "reason": self.reason,
+            "security_impact": self.security_impact,
+            "postcondition": self.postcondition,
+            "undo": self.undo,
             "reversible": self.reversible,
             "requires_consent": self.requires_consent,
         }
@@ -124,6 +132,10 @@ def build_setup_plan() -> SetupPlan:
                 kind="inspect",
                 target="config",
                 description="Preserve the existing Verdict configuration unchanged.",
+                reason="An existing configuration must not be overwritten implicitly.",
+                security_impact="No change; existing configuration and credential references remain untouched.",
+                postcondition="The existing configuration remains byte-for-byte unchanged.",
+                undo="No action required; this is a read-only inspection.",
                 reversible=True,
                 requires_consent=False,
             ),
@@ -135,6 +147,10 @@ def build_setup_plan() -> SetupPlan:
                 kind="write_config",
                 target="config",
                 description="Create a minimal Verdict configuration after explicit consent.",
+                reason="A missing configuration is required before an operator can apply setup.",
+                security_impact="Creates only local configuration; credentials remain environment/keyring references.",
+                postcondition="A validated minimal configuration exists at the planned target.",
+                undo="Remove the newly created configuration after confirming it was not modified.",
                 reversible=True,
                 requires_consent=True,
             ),