diff --git a/zenith/src/zenith_harness/config.py b/zenith/src/zenith_harness/config.py
index 22adcc0..45a96a9 100644
--- a/zenith/src/zenith_harness/config.py
+++ b/zenith/src/zenith_harness/config.py
@@ -168,18 +168,44 @@ def resolved_worker_acp_command(self) -> str | None:
 
     @property
     def resolved_validator_acp_command(self) -> str | None:
+        """Explicit override, else inherit; provider default only on a
+        provider switch.
+
+        Mirrors `ProviderSelection.resolved_validation_worker_acp_command`
+        (providers.py): a custom worker command must cascade to a validator
+        on the *same* provider. The provider default is a fallback for a
+        *different* validator provider, not a shadow over the inherited
+        command — `default_worker_acp_command` is always truthy, so putting
+        it ahead of inheritance in a plain or-chain silently discards the
+        user's custom command (model flags, wrapper scripts, mocks).
+        """
+        if self.validator_acp_command:
+            return self.validator_acp_command
+        if self.validator_provider.name != self.worker_provider.name:
+            return (
+                self.validator_provider.default_worker_acp_command
+                or self.resolved_worker_acp_command
+            )
         return (
-            self.validator_acp_command
+            self.resolved_worker_acp_command
             or self.validator_provider.default_worker_acp_command
-            or self.resolved_worker_acp_command
         )
 
     @property
     def resolved_terminal_reviewer_acp_command(self) -> str | None:
+        """Same cascade as the validator, one level up: inherit the
+        validator's resolved command unless the reviewer switches provider.
+        """
+        if self.terminal_reviewer_acp_command:
+            return self.terminal_reviewer_acp_command
+        if self.terminal_reviewer_provider.name != self.validator_provider.name:
+            return (
+                self.terminal_reviewer_provider.default_worker_acp_command
+                or self.resolved_validator_acp_command
+            )
         return (
-            self.terminal_reviewer_acp_command
+            self.resolved_validator_acp_command
             or self.terminal_reviewer_provider.default_worker_acp_command
-            or self.resolved_validator_acp_command
         )
 
     @property
diff --git a/zenith/tests/test_config.py b/zenith/tests/test_config.py
index 83c9c49..d4fab9a 100644
--- a/zenith/tests/test_config.py
+++ b/zenith/tests/test_config.py
@@ -6,6 +6,31 @@
 import pytest
 
 from zenith_harness.config import HarnessConfig
+from zenith_harness.providers import ProviderSelection, get_provider
+
+_PROVIDER_ENV_KEYS = (
+    "ZENITH_ORCHESTRATOR_PROVIDER",
+    "ZENITH_WORKER_PROVIDER",
+    "ZENITH_WORKER_ACP_COMMAND",
+    "ZENITH_VALIDATOR_PROVIDER",
+    "ZENITH_VALIDATOR_ACP_COMMAND",
+    "ZENITH_TERMINAL_REVIEWER_PROVIDER",
+    "ZENITH_TERMINAL_REVIEWER_ACP_COMMAND",
+)
+
+
+def _apply_selection_env(
+    monkeypatch, harness_home: Path, selection: ProviderSelection
+) -> HarnessConfig:
+    """Write selection.env() to a clean environment and re-discover from it."""
+    monkeypatch.setenv("ZENITH_HOME", str(harness_home))
+    monkeypatch.delenv("ZENITH_PROJECT_BUCKET_DIR", raising=False)
+    for key in _PROVIDER_ENV_KEYS:
+        monkeypatch.delenv(key, raising=False)
+    for key, value in selection.env().items():
+        monkeypatch.setenv(key, value)
+    return HarnessConfig.discover()
+
 
 _EFFORT_ENV_VARS = (
     "ZENITH_WORKER_REASONING_EFFORT",
@@ -140,3 +165,119 @@ def test_for_role_reasoning_effort_explicit_override_wins(
     assert config.for_role("validator").worker_reasoning_effort == "low"
     # terminal_reviewer falls back to the validator setting first.
     assert config.for_role("terminal_reviewer").worker_reasoning_effort == "low"
+
+
+def test_validator_inherits_custom_worker_command_same_provider(
+    monkeypatch,
+    harness_home: Path,
+) -> None:
+    """A custom worker command cascades to a same-provider validator.
+
+    Regression: the or-chain preferred the provider default (always
+    truthy) over inheritance, so validators silently ran the stock
+    adapter while workers ran the custom command (model flags, wrapper
+    scripts, mocks).
+    """
+    selection = ProviderSelection(
+        orchestrator=get_provider("claude"),
+        worker=get_provider("claude"),
+        worker_acp_command="claude-agent-acp --model custom",
+    )
+
+    config = _apply_selection_env(monkeypatch, harness_home, selection)
+
+    assert (
+        config.resolved_validator_acp_command
+        == "claude-agent-acp --model custom"
+    )
+    assert (
+        config.resolved_terminal_reviewer_acp_command
+        == "claude-agent-acp --model custom"
+    )
+    # for_role is the dispatch-time consumer of the cascade.
+    assert (
+        config.for_role("validator").worker_acp_command
+        == "claude-agent-acp --model custom"
+    )
+    assert (
+        config.for_role("terminal_reviewer").worker_acp_command
+        == "claude-agent-acp --model custom"
+    )
+
+
+def test_validator_provider_switch_uses_provider_default(
+    monkeypatch,
+    harness_home: Path,
+) -> None:
+    """A different validator provider must NOT inherit the worker command."""
+    selection = ProviderSelection(
+        orchestrator=get_provider("claude"),
+        worker=get_provider("claude"),
+        worker_acp_command="claude-agent-acp --model custom",
+        validation_worker=get_provider("codex"),
+    )
+
+    config = _apply_selection_env(monkeypatch, harness_home, selection)
+
+    assert config.resolved_validator_acp_command == "codex-acp"
+    # Reviewer cascades from the validator (same provider as validator).
+    assert config.resolved_terminal_reviewer_acp_command == "codex-acp"
+
+
+def test_explicit_validator_command_beats_inheritance(
+    monkeypatch,
+    harness_home: Path,
+) -> None:
+    selection = ProviderSelection(
+        orchestrator=get_provider("claude"),
+        worker=get_provider("claude"),
+        worker_acp_command="claude-agent-acp --model custom",
+        validation_worker=get_provider("claude"),
+        validation_worker_acp_command="claude-agent-acp --model validator",
+    )
+
+    config = _apply_selection_env(monkeypatch, harness_home, selection)
+
+    assert (
+        config.resolved_validator_acp_command
+        == "claude-agent-acp --model validator"
+    )
+    # Reviewer inherits the validator's explicit command, not the worker's.
+    assert (
+        config.resolved_terminal_reviewer_acp_command
+        == "claude-agent-acp --model validator"
+    )
+
+
+def test_config_resolution_matches_provider_selection(
+    monkeypatch,
+    harness_home: Path,
+) -> None:
+    """config.py and providers.py implement the same cascade — the read
+    side of env() must resolve identically to the write side.
+    """
+    cases = [
+        ProviderSelection(
+            orchestrator=get_provider("claude"),
+            worker=get_provider("claude"),
+            worker_acp_command="claude-agent-acp --model custom",
+        ),
+        ProviderSelection(
+            orchestrator=get_provider("claude"),
+            worker=get_provider("claude"),
+            worker_acp_command="claude-agent-acp --model custom",
+            validation_worker=get_provider("codex"),
+        ),
+        ProviderSelection(
+            orchestrator=get_provider("claude"),
+            worker=get_provider("codex"),
+            worker_acp_command='codex-acp -c model="custom"',
+            validation_worker=get_provider("claude"),
+        ),
+    ]
+    for selection in cases:
+        config = _apply_selection_env(monkeypatch, harness_home, selection)
+        assert (
+            config.resolved_validator_acp_command
+            == selection.resolved_validation_worker_acp_command
+        ), selection