diff --git a/CLAUDE.md b/CLAUDE.md
index e22e1ac..4f952e0 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -357,6 +357,12 @@ Non-obvious, load-bearing design points:
     leaving a host-configurable default unspecified. `WithUntrackedFiles(...)` replaces the default
     rather than joining it, so the vector never carries two `--untracked-files` values.
 
+15. **`Log()` passes `--no-show-signature`.** With `log.showSignature=true`, which is common among
+    people who sign commits, git prints each signed commit's verification to standard output ahead
+    of the record even under a custom `--format`, and the parser reads it as part of the commit id.
+    `Commit()` reuses the `Log()` vector for its read-back, so without the flag a signed commit that
+    git did make is reported to the caller as a failure, and a retry makes a duplicate.
+
 **Hosting layer.** `GitProvider` is an abstract base with two implementations: `GitHubProvider` over
 Octokit, and `AzureDevOpsProvider` over a raw `HttpClient` — Azure DevOps has no client library this
 library uses (see the dependency note below). Both go through the same shape: every request-issuing
diff --git a/GitIntegration.Test/Builders/GitCommitBuilderTests.cs b/GitIntegration.Test/Builders/GitCommitBuilderTests.cs
index 7360cf3..0496962 100644
--- a/GitIntegration.Test/Builders/GitCommitBuilderTests.cs
+++ b/GitIntegration.Test/Builders/GitCommitBuilderTests.cs
@@ -153,6 +153,21 @@ public async Task TheReadBackUsesThePinnedLogFormatAndAsksForOneCommitAsync()
 		CollectionAssert.Contains(readBack, "-z");
 	}
 
+	[TestMethod]
+	public async Task TheReadBackSuppressesSignatureOutputAsync()
+	{
+		// With log.showSignature=true git prints the new commit's signature check ahead of the record,
+		// so a signed commit that git made would be reported to the caller as a failure.
+		ScriptedGitProcessRunner runner = new ScriptedGitProcessRunner()
+			.Then(standardOutput: "[main 9429d20] subject here\n")
+			.Then(standardOutput: ReadBack);
+		GitCommitBuilder builder = new(runner, TestPaths.Root, Message);
+
+		_ = await builder.ExecuteAsync(TestContext.CancellationTokenSource.Token).ConfigureAwait(false);
+
+		CollectionAssert.Contains(runner.Invocations[1].ToArray(), "--no-show-signature");
+	}
+
 	[TestMethod]
 	public async Task ThrowsNothingToCommitWhenTheTreeIsCleanAsync()
 	{
diff --git a/GitIntegration.Test/Builders/GitLogBuilderTests.cs b/GitIntegration.Test/Builders/GitLogBuilderTests.cs
index 7ca2661..d3cd7d9 100644
--- a/GitIntegration.Test/Builders/GitLogBuilderTests.cs
+++ b/GitIntegration.Test/Builders/GitLogBuilderTests.cs
@@ -29,6 +29,7 @@ public void BuildsTheDefaultLogVector()
 			"-c", "core.quotepath=false",
 			"-c", "color.ui=false",
 			"log",
+			"--no-show-signature",
 			"-z",
 			ExpectedFormat,
 		];
diff --git a/GitIntegration.Test/Integration/GitRoundTripTests.cs b/GitIntegration.Test/Integration/GitRoundTripTests.cs
index 5492edd..5366994 100644
--- a/GitIntegration.Test/Integration/GitRoundTripTests.cs
+++ b/GitIntegration.Test/Integration/GitRoundTripTests.cs
@@ -111,6 +111,58 @@ public async Task AddAndCommitProduceAReadableCommitAsync()
 		Assert.AreEqual(40, commit.Sha.WeakString.Length);
 	}
 
+	[TestMethod]
+	public async Task LogAndCommitIgnoreTheHostsShowSignatureSettingAsync()
+	{
+		// log.showSignature is common among people who sign commits, and with it git prints each signed
+		// commit's verification ahead of the record, custom --format or not. The signature here is
+		// forged rather than made, so the test needs no key: git still hands it to gpg and prints what
+		// gpg says about it, which is all it takes to put text where the parser expects a commit id.
+		CancellationToken cancellationToken = TestContext.CancellationTokenSource.Token;
+		await IntegrationGitFixture.RequireGitAsync(cancellationToken).ConfigureAwait(false);
+
+		using TemporaryRepository temporary = new();
+		GitRepository repository = await InitializeAsync(temporary, cancellationToken).ConfigureAwait(false);
+
+		temporary.WriteFile("a.txt", "one\n");
+		_ = await repository.Add().All().ExecuteAsync(cancellationToken).ConfigureAwait(false);
+		_ = await repository.Commit("c1".As<GitCommitMessage>()).ExecuteAsync(cancellationToken).ConfigureAwait(false);
+
+		IGitProcessRunner runner = repository.ProcessRunner!;
+		string unsigned = await new GitTextBuilder(runner, repository.LocalPath, "cat-file", "commit", "HEAD")
+			.ExecuteAsync(cancellationToken).ConfigureAwait(false);
+
+		// The header block ends at the first blank line; gpgsig goes last in it, as git writes it.
+		int headerEnd = unsigned.IndexOf("\n\n", StringComparison.Ordinal);
+		string signed = unsigned[..headerEnd] +
+			"\ngpgsig -----BEGIN PGP SIGNATURE-----\n \n iQEzBAABCAAdFiEEAAAAAAAAAAAAAAAAAAAAAAAAAAAFAmYAAAAACgkQAAAAAAAAAAAA\n" +
+			" =AAAA\n -----END PGP SIGNATURE-----" +
+			unsigned[headerEnd..] + "\n";
+
+		// Written inside .git so the forged object never shows up as working-tree content.
+		temporary.WriteFile(".git/forged-commit", signed);
+		string forgedSha = await new GitTextBuilder(
+			runner, repository.LocalPath, "hash-object", "-t", "commit", "-w", ".git/forged-commit")
+			.ExecuteAsync(cancellationToken).ConfigureAwait(false);
+		_ = await new GitTextBuilder(runner, repository.LocalPath, "update-ref", "HEAD", forgedSha)
+			.ExecuteAsync(cancellationToken).ConfigureAwait(false);
+		_ = await new GitTextBuilder(runner, repository.LocalPath, "config", "log.showSignature", "true")
+			.ExecuteAsync(cancellationToken).ConfigureAwait(false);
+
+		IReadOnlyList<GitCommit> history = await repository.Log().ExecuteAsync(cancellationToken).ConfigureAwait(false);
+
+		Assert.AreEqual(1, history.Count);
+		Assert.AreEqual(forgedSha, history[0].Sha.WeakString);
+
+		temporary.WriteFile("b.txt", "two\n");
+		_ = await repository.Add().All().ExecuteAsync(cancellationToken).ConfigureAwait(false);
+		GitCommit second = await repository.Commit("c2".As<GitCommitMessage>())
+			.ExecuteAsync(cancellationToken).ConfigureAwait(false);
+
+		Assert.AreEqual("c2", second.Subject);
+		Assert.AreEqual(forgedSha, second.ParentShas.Single().WeakString);
+	}
+
 	[TestMethod]
 	public async Task CommittingWithNothingStagedThrowsTheDedicatedExceptionAsync()
 	{
diff --git a/GitIntegration/Builders/GitLogBuilder.cs b/GitIntegration/Builders/GitLogBuilder.cs
index bc1305a..eb7bcbc 100644
--- a/GitIntegration/Builders/GitLogBuilder.cs
+++ b/GitIntegration/Builders/GitLogBuilder.cs
@@ -159,6 +159,11 @@ protected override void AppendVerbArguments(ICollection<string> arguments)
 		Ensure.NotNull(arguments);
 
 		arguments.Add("log");
+
+		// log.showSignature makes git print each commit's signature check to standard output ahead of
+		// the record, custom --format or not, which the parser would read as part of the commit id.
+		// Commit() reuses this vector for its read-back, so the same flag keeps it working too.
+		arguments.Add("--no-show-signature");
 		arguments.Add("-z");
 		arguments.Add("--format=" + GitOutputFormats.LogFormat);