fix: verify file edit and return confirmation snippet

environment/filesystem.go

@@ -63,14 +63,20 @@ type FileEditRequest struct {
 	MatchID     string
 }
 
-func (env *Environment) FileEdit(ctx context.Context, req FileEditRequest) 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, req FileEditRequest) (*FileEditResult, error) {
 	if err := env.validateNotSubmoduleFile(req.TargetFile); err != nil {
-		return err
+		return nil, err
 	}
 
 	contents, err := env.container().File(req.TargetFile).Contents(ctx)
 	if err != nil {
-		return err
+		return nil, err
 	}
 
 	matches := []int{}
@@ -86,7 +92,7 @@ func (env *Environment) FileEdit(ctx context.Context, req FileEditRequest) error
 	}
 
 	if len(matches) == 0 {
-		return fmt.Errorf("search text not found in file %s", req.TargetFile)
+		return nil, fmt.Errorf("search text not found in file %s", req.TargetFile)
 	}
 
 	if len(matches) > 1 && req.MatchID == "" {
@@ -97,7 +103,7 @@ func (env *Environment) FileEdit(ctx context.Context, req FileEditRequest) error
 			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",
 			req.TargetFile, strings.Join(matchDescriptions, "\n\n"))
 	}
 
@@ -115,7 +121,7 @@ func (env *Environment) FileEdit(ctx context.Context, req FileEditRequest) error
 			}
 		}
 		if !found {
-			return fmt.Errorf("match ID %s not found", req.MatchID)
+			return nil, fmt.Errorf("match ID %s not found", req.MatchID)
 		}
 	}
 
@@ -125,10 +131,28 @@ func (env *Environment) FileEdit(ctx context.Context, req FileEditRequest) error
 	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(req.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", req.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", req.TargetFile)
-	return nil
+	return &FileEditResult{Snippet: snippet}, nil
 }
 
 func (env *Environment) FileDelete(ctx context.Context, explanation, targetFile string) error {

mcpserver/tools.go

@@ -788,21 +788,22 @@ func createEnvironmentFileEditTool(singleTenant bool) *Tool {
 				return nil, err
 			}
 
-			if err := env.FileEdit(ctx, environment.FileEditRequest{
+			result, err := env.FileEdit(ctx, environment.FileEditRequest{
 				Explanation: request.GetString("explanation", ""),
 				TargetFile:  targetFile,
 				Search:      search,
 				Replace:     replace,
 				MatchID:     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
 		},
 	}
 }