diff --git a/.changeset/anchor-compaction-success-patterns.md b/.changeset/anchor-compaction-success-patterns.md
new file mode 100644
index 000000000..c4ed66e82
--- /dev/null
+++ b/.changeset/anchor-compaction-success-patterns.md
@@ -0,0 +1,5 @@
+---
+"@nanocollective/nanocoder": patch
+---
+
+Fixed default compaction collapsing a multi-line tool result into `Result: success` whenever it mentioned "completed successfully" or "no errors" anywhere, which dropped failure lines that followed. Only a short single-line result is treated as a success marker now. Closes #1633.
diff --git a/source/utils/message-compression.spec.ts b/source/utils/message-compression.spec.ts
index fdc87ac8f..e16056ec9 100644
--- a/source/utils/message-compression.spec.ts
+++ b/source/utils/message-compression.spec.ts
@@ -464,6 +464,41 @@ test('compressMessages detects success in tool results', t => {
 	t.true(toolMsg?.content?.includes('success') ?? false);
 });
 
+test('compressMessages keeps a log that mentions success but also reports failures', t => {
+	const tokenizer = createMockTokenizer();
+	const messages: Message[] = [
+		createToolMessage(
+			'execute_bash',
+			'Build completed successfully\nfailures: 3\nsee report.txt',
+		),
+		createUserMessage('Recent'),
+	];
+
+	const result = compressMessages(messages, tokenizer, {
+		mode: 'default',
+		keepRecentMessages: 1,
+	});
+
+	const toolMsg = result.compressedMessages[0];
+	t.false(toolMsg?.content?.includes('Result: success') ?? true);
+});
+
+test('compressMessages treats a single "completed successfully" line as success', t => {
+	const tokenizer = createMockTokenizer();
+	const messages: Message[] = [
+		createToolMessage('execute_bash', 'Build completed successfully'),
+		createUserMessage('Recent'),
+	];
+
+	const result = compressMessages(messages, tokenizer, {
+		mode: 'default',
+		keepRecentMessages: 1,
+	});
+
+	const toolMsg = result.compressedMessages[0];
+	t.true(toolMsg?.content?.includes('Result: success') ?? false);
+});
+
 // ==================== keepRecentMessages option ====================
 
 test('compressMessages respects custom keepRecentMessages value', t => {
@@ -603,3 +638,22 @@ test('compressMessages preserves display-only messages in output but excludes th
 	);
 	t.is(result.compressedTokenCount, expectedCompressedTokens);
 });
+
+test('compressMessages keeps the trailing detail of a one-line success', t => {
+	const tokenizer = createMockTokenizer();
+	const messages: Message[] = [
+		createToolMessage('execute_bash', 'Operation completed successfully in 5s'),
+		createUserMessage('Recent'),
+	];
+
+	const result = compressMessages(messages, tokenizer, {
+		mode: 'default',
+		keepRecentMessages: 1,
+	});
+
+	const toolMsg = result.compressedMessages[0];
+	t.is(
+		toolMsg?.content,
+		'Tool: execute_bash\nResult: Operation completed successfully in 5s',
+	);
+});
diff --git a/source/utils/message-compression.ts b/source/utils/message-compression.ts
index f769b42ab..3b42008ce 100644
--- a/source/utils/message-compression.ts
+++ b/source/utils/message-compression.ts
@@ -368,16 +368,23 @@ function extractErrorInfo(content: string): {
 	return null;
 }
 
-// Check if content indicates success
+// Check if content indicates success.
+// Only a short, single-line result counts: a multi-line log that happens to
+// contain "completed successfully" may also report failures further down,
+// so it must not be collapsed into "Result: success" (#1633).
 function isSuccess(content: string): boolean {
+	const text = content.trim();
+	if (text.includes('\n')) {
+		return false;
+	}
 	const successPatterns = [
 		/^success$/i,
 		/^ok$/i,
 		/^done$/i,
-		/completed successfully/i,
-		/no errors/i,
+		/^.{0,80}completed successfully\.?$/i,
+		/^no errors\.?$/i,
 	];
-	return successPatterns.some(pattern => pattern.test(content));
+	return successPatterns.some(pattern => pattern.test(text));
 }
 
 // Check for error in content