diff --git a/script/Build.sol b/script/Build.sol
index 1ff8d5d..dcb1f6c 100644
--- a/script/Build.sol
+++ b/script/Build.sol
@@ -59,28 +59,34 @@ contract Build is BuildScript, RegistryDeploySuites {
         return names;
     }
 
+    /// The directory every generated lib is written into.
+    ///
+    /// Overridable for the reason `BuildScript.recordRoot` is: a hook that can
+    /// only be pointed at the committed tree can only be RUN by overwriting
+    /// files the rest of the suite compiles and reads, and forge runs test
+    /// contracts in parallel. Only the per-contract libs and the aggregate go
+    /// here — `regenerateSnapshots` has no equivalent, because `LibFs` confines
+    /// every snapshot it writes to `src/generated/`.
+    /// @return The lib directory.
+    function libDir() internal view virtual returns (string memory) {
+        return LibRainDeploySnapshot.LIB_DIR;
+    }
+
     /// @inheritdoc BuildScript
     /// @dev Every alias lib, every released-suites lib and the aggregate over
     /// them.
     function regenerateLibs() internal override {
         GeneratedContract[] memory contracts = generatedContracts();
+        string memory dir = libDir();
         for (uint256 i = 0; i < contracts.length; i++) {
             LibRainDeploySnapshot.writeAliasLib(
-                vm,
-                LibRainDeploySnapshot.LIB_DIR,
-                contracts[i].contractName,
-                contracts[i].constantPrefix,
-                LibRainDeploySnapshot.CANDIDATE
+                vm, dir, contracts[i].contractName, contracts[i].constantPrefix, LibRainDeploySnapshot.CANDIDATE
             );
             LibRainDeploySnapshot.writeReleasedSuitesLib(
-                vm,
-                LibRainDeploySnapshot.LIB_DIR,
-                recordRoot(),
-                contracts[i].contractName,
-                contracts[i].candidate.snapshot
+                vm, dir, recordRoot(), contracts[i].contractName, contracts[i].candidate.snapshot
             );
         }
-        LibRainDeploySnapshot.writeReleasedSuitesAggregate(vm, LibRainDeploySnapshot.LIB_DIR, snapshotContractNames());
+        LibRainDeploySnapshot.writeReleasedSuitesAggregate(vm, dir, snapshotContractNames());
     }
 
     /// @inheritdoc BuildScript
diff --git a/test/concrete/BuildHarness.sol b/test/concrete/BuildHarness.sol
index 4061a81..b03780b 100644
--- a/test/concrete/BuildHarness.sol
+++ b/test/concrete/BuildHarness.sol
@@ -5,15 +5,50 @@ pragma solidity =0.8.25;
 import {Build, GeneratedContract} from "../../script/Build.sol";
 import {DeployCandidate} from "../../src/abstract/RainDeploySuitesBase.sol";
 
+/// Thrown when a harness left pointed at `Build`'s own lib directory is asked
+/// to run the lib half of a build. That write lands on the committed libs the
+/// rest of the suite compiles and reads, which forge runs in parallel with it.
+error BuildHarnessWouldWriteTheCommittedLibs();
+
 /// @title BuildHarness
 /// @notice An external seam onto the two internal declarations `BuildTest`
-/// compares, so a test can hold both at once.
+/// compares, so a test can hold both at once, and onto `regenerateLibs()`
+/// pointed somewhere nothing compiles, so a test can RUN it.
 ///
 /// A harness rather than a change to `Build`: `generatedContracts()` is the
 /// script's own declaration and has no caller outside it, and widening it to
 /// `public` to be testable would put a second entry point on a script whose
 /// whole surface is `run()` and `cutRelease()`.
 contract BuildHarness is Build {
+    /// The lib directory the hooks write into. Empty defers to `Build`'s own,
+    /// which is what makes the default assertable and the write refusable.
+    string internal sLibDir;
+
+    /// @param libDirectory The directory `regenerateLibs` writes into, or empty
+    /// for `Build`'s own.
+    constructor(string memory libDirectory) {
+        sLibDir = libDirectory;
+    }
+
+    /// @inheritdoc Build
+    function libDir() internal view override returns (string memory) {
+        return bytes(sLibDir).length > 0 ? sLibDir : super.libDir();
+    }
+
+    function externalLibDir() external view returns (string memory) {
+        return libDir();
+    }
+
+    /// Runs the lib half of a build, into the fixture directory this harness
+    /// was handed. The directory has to exist already, as it does for every
+    /// writer `regenerateLibs` calls.
+    function externalRegenerateLibs() external {
+        if (bytes(sLibDir).length == 0) {
+            revert BuildHarnessWouldWriteTheCommittedLibs();
+        }
+        regenerateLibs();
+    }
+
     /// The generator's list.
     /// @return The generated contracts.
     function externalGeneratedContracts() external pure returns (GeneratedContract[] memory) {
diff --git a/test/concrete/BuildRecordRootHarness.sol b/test/concrete/BuildRecordRootHarness.sol
index d18ebf7..efa7f3b 100644
--- a/test/concrete/BuildRecordRootHarness.sol
+++ b/test/concrete/BuildRecordRootHarness.sol
@@ -17,7 +17,7 @@ import {BuildHarness} from "./BuildHarness.sol";
 contract BuildRecordRootHarness is BuildHarness {
     string internal sRoot;
 
-    constructor(string memory root) {
+    constructor(string memory root) BuildHarness("") {
         sRoot = root;
     }
 
diff --git a/test/script/Build.t.sol b/test/script/Build.t.sol
index 2be9841..03cd555 100644
--- a/test/script/Build.t.sol
+++ b/test/script/Build.t.sol
@@ -8,7 +8,7 @@ import {DeployCandidate} from "../../src/abstract/RainDeploySuitesBase.sol";
 import {LibCodeGen} from "rain-sol-codegen-0.1.37/src/lib/LibCodeGen.sol";
 import {LibRainDeploySnapshot} from "../../src/lib/LibRainDeploySnapshot.sol";
 import {LibReleasedSuitesAggregate} from "../lib/LibReleasedSuitesAggregate.sol";
-import {BuildHarness} from "../concrete/BuildHarness.sol";
+import {BuildHarness, BuildHarnessWouldWriteTheCommittedLibs} from "../concrete/BuildHarness.sol";
 import {LibMemoryKV, MemoryKV, MemoryKVKey, MemoryKVVal} from "rain-lib-memkv-0.1.5/src/lib/LibMemoryKV.sol";
 
 /// @title BuildTest
@@ -38,10 +38,10 @@ import {LibMemoryKV, MemoryKV, MemoryKVKey, MemoryKVVal} from "rain-lib-memkv-0.
 /// contract's pins are written into.
 ///
 /// Deliberately nothing here calls `run()` or `cutRelease()`. Both rewrite the
-/// committed `src/generated/` snapshots and `src/lib/` libs that other test
-/// contracts read, and forge runs test contracts in parallel — a contract
-/// rewriting what another one is reading is a race, not a check. Nothing below
-/// writes anything.
+/// committed `src/generated/` snapshots that other test contracts read, and
+/// forge runs test contracts in parallel — a contract rewriting what another
+/// one is reading is a race, not a check. `regenerateLibs()` is run, and only
+/// into a fixture directory nothing compiles; the one write below is that.
 contract BuildTest is Test {
     using LibMemoryKV for MemoryKV;
 
@@ -49,7 +49,7 @@ contract BuildTest is Test {
     BuildHarness internal sBuild;
 
     function setUp() external {
-        sBuild = new BuildHarness();
+        sBuild = new BuildHarness("");
     }
 
     /// PROPERTY: the generator's list and the deploy declaration are the SAME
@@ -312,4 +312,115 @@ contract BuildTest is Test {
             );
         }
     }
+
+    /// PROPERTY: a build with nothing overridden writes its libs into the
+    /// directory the committed ones are in.
+    ///
+    /// Asserted against the path as text rather than against
+    /// `LibRainDeploySnapshot.LIB_DIR`, which is the constant the default
+    /// returns: a default pointed at a directory nothing compiles leaves every
+    /// committed lib stale forever while the regeneration reports success, and
+    /// the test below cannot see it, because that test overrides this.
+    function testTheDefaultLibDirIsWhereTheCommittedLibsAre() external view {
+        assertEq(sBuild.externalLibDir(), "src/lib");
+    }
+
+    /// Where `regenerateLibs()` is driven.
+    ///
+    /// Outside `src/` and `test/`, which is everything `fs_permissions`
+    /// otherwise grants and both of which are compiled: a generated lib imports
+    /// `../generated/`, `../abstract/` and `./Lib<Contract>Released.sol`, which
+    /// resolve from `src/lib` and nowhere else, so a copy under either root
+    /// fails the build for every suite — including the copy a failing test
+    /// leaves behind. `foundry.toml` grants this root for exactly that, and
+    /// nothing compiles it.
+    string constant LIBS_FIXTURE_DIR = "fixture-lib/build-regenerate-libs";
+
+    /// PROPERTY: `regenerateLibs()` RUN emits exactly the committed libs — one
+    /// alias lib and one released lib per generated contract, one aggregate,
+    /// and nothing else.
+    ///
+    /// Every assertion above this one is output-anchored: it compares a
+    /// committed file against the emitters, so it sees drift only AFTER
+    /// somebody re-runs the generator and commits what came out. The hook that
+    /// decides which emitter is called, with which arguments, how many times,
+    /// was executed by nothing at all — a loop bound that stopped one contract
+    /// short, or a `constantPrefix` taken from `contracts[0]` on every pass,
+    /// was invisible until the next release cut it into the record. Both hooks
+    /// took a `revert()` as their first statement with the whole suite still
+    /// green.
+    ///
+    /// So this runs it, and the oracle is the committed tree rather than the
+    /// emitters: a regeneration of a clean checkout is a no-op, so every file
+    /// it writes MUST be byte-identical to the file already there. That is
+    /// independent of the emitters in the way the pins above are not — they say
+    /// the committed files are what the emitters produce, and this says the
+    /// hook asks the emitters for those files.
+    ///
+    /// The count is asserted as well as the contents, because a loop that
+    /// stopped short writes nothing wrong — it writes nothing at all — and a
+    /// file the hook wrote that the repo does not commit is a generated file
+    /// nothing regenerates.
+    ///
+    /// `regenerateSnapshots()` has no counterpart here and can have none:
+    /// `LibFs` confines every snapshot write to `src/generated/<tag>/`, the
+    /// record `frozenSnapshotPaths` walks, so there is no directory to drive it
+    /// into that is not read by the suites running beside this one.
+    ///
+    /// Read, then removed, then asserted: forge-std assertions revert, so a
+    /// removal after them removes in every case except a failure, which is the
+    /// only case that leaves a directory behind. `vm.isFile` before each read
+    /// for the same reason — a missing file is what the loop-bound failure
+    /// looks like, and a cheatcode revert there would strand the fixture.
+    function testRegenerateLibsEmitsExactlyTheCommittedLibs() external {
+        GeneratedContract[] memory generated = sBuild.externalGeneratedContracts();
+        BuildHarness harness = new BuildHarness(LIBS_FIXTURE_DIR);
+
+        //forge-lint: disable-next-line(unsafe-cheatcode)
+        vm.createDir(LIBS_FIXTURE_DIR, true);
+        harness.externalRegenerateLibs();
+
+        string[] memory names = new string[](generated.length * 2 + 1);
+        for (uint256 i = 0; i < generated.length; i++) {
+            names[i * 2] = string.concat("Lib", generated[i].contractName, "Deploy.sol");
+            names[i * 2 + 1] =
+                string.concat(LibRainDeploySnapshot.releasedLibraryName(generated[i].contractName), ".sol");
+        }
+        names[names.length - 1] = string.concat(LibRainDeploySnapshot.RELEASED_SUITES_LIBRARY, ".sol");
+
+        uint256 written = vm.readDir(LIBS_FIXTURE_DIR).length;
+        bool[] memory emitted = new bool[](names.length);
+        string[] memory bodies = new string[](names.length);
+        for (uint256 i = 0; i < names.length; i++) {
+            string memory path = string.concat(LIBS_FIXTURE_DIR, "/", names[i]);
+            emitted[i] = vm.isFile(path);
+            bodies[i] = emitted[i] ? vm.readFile(path) : "";
+        }
+
+        //forge-lint: disable-next-line(unsafe-cheatcode)
+        vm.removeDir(LIBS_FIXTURE_DIR, true);
+
+        assertEq(written, names.length, "regenerateLibs emitted a different number of files than the repo commits");
+        for (uint256 i = 0; i < names.length; i++) {
+            assertTrue(emitted[i], string.concat("regenerateLibs emitted no ", names[i]));
+            assertEq(
+                bodies[i],
+                vm.readFile(string.concat("src/lib/", names[i])),
+                string.concat("regenerateLibs emitted something other than the committed ", names[i])
+            );
+        }
+    }
+
+    /// A harness left pointed at `Build`'s own lib directory MUST refuse to run
+    /// the lib half of a build.
+    ///
+    /// `setUp` builds one, because the default is what
+    /// `testTheDefaultLibDirIsWhereTheCommittedLibsAre` reads, and it is shared
+    /// with every other test in this contract. A call that went through would
+    /// rewrite `src/lib/` while the suites that compile and read those files
+    /// are running.
+    function testRegenerateLibsRefusesToWriteTheCommittedLibs() external {
+        vm.expectRevert(BuildHarnessWouldWriteTheCommittedLibs.selector);
+        sBuild.externalRegenerateLibs();
+    }
 }