fix: verify file edit and return confirmation snippet

environment/filesystem.go

@@ -54,15 +54,21 @@ func (env *Environment) FileWrite(ctx context.Context, explanation, targetFile,
 	return nil
 }
 
-func (env *Environment) FileEdit(ctx context.Context, explanation, targetFile, search, replace, matchID string) error {
+// FileEditResult contains information about a successful file edit
+type FileEditResult struct {
+	// Snippet is a short context snippet around the replaced text
+	Snippet string
+}
+
+func (env *Environment) FileEdit(ctx context.Context, explanation, targetFile, search, replace, matchID string) (*FileEditResult, error) {
 	// Check if the file is within a submodule
 	if err := env.validateNotSubmoduleFile(targetFile); err != nil {
-		return err
+		return nil, err
 	}
 
 	contents, err := env.container().File(targetFile).Contents(ctx)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	// Find all matches of the search text
@@ -79,7 +85,7 @@ func (env *Environment) FileEdit(ctx context.Context, explanation, targetFile, s
 	}
 
 	if len(matches) == 0 {
-		return fmt.Errorf("search text not found in file %s", targetFile)
+		return nil, fmt.Errorf("search text not found in file %s", targetFile)
 	}
 
 	// If there are multiple matches and no matchID is provided, return an error with all matches
@@ -95,7 +101,7 @@ func (env *Environment) FileEdit(ctx context.Context, explanation, targetFile, s
 			matchDescriptions = append(matchDescriptions, fmt.Sprintf("Match %d (ID: %s):\n%s", i+1, id, context))
 		}
 
-		return fmt.Errorf("multiple matches found for search text in %s. Please specify which_match parameter with one of the following IDs:\n\n%s",
+		return nil, fmt.Errorf("multiple matches found for search text in %s. Please specify which_match parameter with one of the following IDs:\n\n%s",
 			targetFile, strings.Join(matchDescriptions, "\n\n"))
 	}
 
@@ -115,7 +121,7 @@ func (env *Environment) FileEdit(ctx context.Context, explanation, targetFile, s
 			}
 		}
 		if !found {
-			return fmt.Errorf("match ID %s not found", matchID)
+			return nil, fmt.Errorf("match ID %s not found", matchID)
 		}
 	}
 
@@ -128,10 +134,28 @@ func (env *Environment) FileEdit(ctx context.Context, explanation, targetFile, s
 	ctr := env.container()
 	err = env.apply(ctx, ctr.WithDirectory(".", ctr.Directory(".").WithPatch(patch)))
 	if err != nil {
-		return fmt.Errorf("failed applying file edit, skipping git propagation: %w", err)
+		return nil, fmt.Errorf("failed applying file edit, skipping git propagation: %w", err)
+	}
+
+	// Verify the edit was actually applied by reading the file back
+	verifiedContents, err := env.container().File(targetFile).Contents(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("failed to verify edit was applied: %w", err)
 	}
+
+	if verifiedContents != newContents {
+		return nil, fmt.Errorf("edit verification failed: file contents of %s after applying the patch do not match expected result", targetFile)
+	}
+
+	// Generate a context snippet around the replacement for confirmation
+	replaceIndex := targetMatchIndex
+	if replaceIndex >= len(verifiedContents) {
+		replaceIndex = max(0, len(verifiedContents)-1)
+	}
+	snippet := getMatchContext(verifiedContents, replaceIndex)
+
 	env.Notes.Add("Edit %s", targetFile)
-	return nil
+	return &FileEditResult{Snippet: snippet}, nil
 }
 
 func (env *Environment) FileDelete(ctx context.Context, explanation, targetFile string) error {

mcpserver/tools.go

@@ -732,21 +732,22 @@ func createEnvironmentFileEditTool(singleTenant bool) *Tool {
 				return nil, err
 			}
 
-			if err := env.FileEdit(ctx,
+			result, err := env.FileEdit(ctx,
 				request.GetString("explanation", ""),
 				targetFile,
 				search,
 				replace,
 				request.GetString("which_match", ""),
-			); err != nil {
-				return mcp.NewToolResultErrorFromErr("failed to write file", err), nil
+			)
+			if err != nil {
+				return mcp.NewToolResultErrorFromErr("failed to edit file", err), nil
 			}
 
 			if err := repo.UpdateFile(ctx, env, targetFile, request.GetString("explanation", "")); err != nil {
 				return mcp.NewToolResultErrorFromErr("unable to update the environment", err), nil
 			}
 
-			return mcp.NewToolResultText(fmt.Sprintf("file %s edited successfully and committed to container-use/%s remote ref", targetFile, env.ID)), nil
+			return mcp.NewToolResultText(fmt.Sprintf("file %s edited successfully and committed to container-use/%s remote ref\n\nVerified edit - snippet around replacement:\n%s", targetFile, env.ID, result.Snippet)), nil
 		},
 	}
 }