fix(conflict): reject resolver commentary instead of writing it to the file

internal/engine/conflict_extract_test.go

@@ -33,3 +33,34 @@ func TestExtractResolvedFileContent(t *testing.T) {
 		t.Fatalf("fenced-only extraction failed: %q", g)
 	}
 }
+
+func TestLooksLikeResolverChatter(t *testing.T) {
+	// Prose-only replies that destroy files when written verbatim (no fence to
+	// extract, so extraction returns the prose itself).
+	chatter := []string{
+		"Conflict resolved. Kept both sides:\n\nNo selector collisions — separate class namespaces, all functionality retained.",
+		"Resolved file content:",
+		"Permission denied by harness. Cannot write the file myself. Resolved content below:",
+		"Working tree is `master` (no `src/`), so I can't write the file here.",
+		"Both sides merged — HEAD tests + branch tests. Write blocked on permission, so resolved content below:",
+		"Want me to apply this on branch nxd/abc and run the tests?",
+	}
+	for _, c := range chatter {
+		if !looksLikeResolverChatter(c) {
+			t.Errorf("expected chatter to be flagged:\n%q", c)
+		}
+	}
+
+	// Real merged source must NOT be flagged.
+	code := []string{
+		"package main\n\nfunc main() {}\n",
+		"import { useState } from 'react';\nexport const App = () => null;",
+		".ui-button { color: red; }\n.app-layout { display: flex; }",
+		"{\n  \"name\": \"frontend\",\n  \"private\": true\n}",
+	}
+	for _, c := range code {
+		if looksLikeResolverChatter(c) {
+			t.Errorf("real source wrongly flagged as chatter:\n%q", c)
+		}
+	}
+}

internal/engine/conflict_resolver.go

@@ -362,6 +362,12 @@ File: %s
 		return "", fmt.Errorf("LLM output still contains conflict markers")
 	}
 
+	// Reject conversational commentary. When the model returns prose with no
+	// fenced block, writing it would destroy the file; failing here escalates.
+	if looksLikeResolverChatter(resolved) {
+		return "", fmt.Errorf("LLM returned commentary, not file content")
+	}
+
 	return resolved, nil
 }
 
@@ -437,6 +443,10 @@ CRITICAL OUTPUT RULES:
 		return "", fmt.Errorf("tech lead output still contains conflict markers")
 	}
 
+	if looksLikeResolverChatter(resolved) {
+		return "", fmt.Errorf("tech lead returned commentary, not file content")
+	}
+
 	return resolved, nil
 }
 
@@ -528,6 +538,56 @@ func extractResolvedFileContent(resp string) string {
 	return strings.TrimSpace(stripCodeFences(resp))
 }
 
+// resolverChatterMarkers are phrases that appear in a conflict-resolution model's
+// CONVERSATIONAL reply but should never appear in actual merged source. When the
+// model ignores the "return only file content" instruction and emits prose with
+// NO fenced block (so extractResolvedFileContent has nothing to extract and
+// returns the prose itself), writing that result DESTROYS the file. These markers
+// are specific enough that real code/comments don't trip them.
+var resolverChatterMarkers = []string{
+	"conflict resolved",
+	"resolved content",
+	"resolved file content",
+	"resolved content below",
+	"resolved content:",
+	"kept both sides",
+	"both sides merged",
+	"kept head's",
+	"kept incoming",
+	"write blocked on permission",
+	"blocked on permission",
+	"permission denied by harness",
+	"cannot write the file",
+	"can't write the file",
+	"i cannot write",
+	"i can't write",
+	"can't write it here",
+	"want me to apply",
+	"want me to run",
+	"grant write to apply",
+	"working tree is",
+	"returning the resolved content",
+	"apply it to",
+	"apply this on branch",
+	"all functionality retained",
+	"no selector collisions",
+	"separate class namespaces",
+}
+
+// looksLikeResolverChatter reports whether s is conflict-resolver commentary
+// rather than merged file content. It is intentionally conservative: a single
+// high-confidence marker is enough, because these phrases do not occur in valid
+// source files, and writing chatter to disk corrupts the file irrecoverably.
+func looksLikeResolverChatter(s string) bool {
+	lower := strings.ToLower(s)
+	for _, m := range resolverChatterMarkers {
+		if strings.Contains(lower, m) {
+			return true
+		}
+	}
+	return false
+}
+
 func stripCodeFences(s string) string {
 	s = strings.TrimSpace(s)
 	if strings.HasPrefix(s, "```") {

internal/engine/conflict_resolver_test.go

@@ -137,7 +137,7 @@ func TestResolveFile_PromptContainsNoFenceInstruction(t *testing.T) {
 	// The ReplayClient returns a fixed response without recording the prompt,
 	// so we verify the prompt by calling resolveFile with a controlled input
 	// and checking there are no conflict markers in the output.
-	client := llm.NewReplayClient(llm.CompletionResponse{Content: "resolved content"})
+	client := llm.NewReplayClient(llm.CompletionResponse{Content: "package main\n\nfunc main() {}\n"})
 
 	cr := &ConflictResolver{
 		llmClient: client,