diff --git a/internal/test/test.go b/internal/test/test.go
index 2bdebe4a7..12108360b 100644
--- a/internal/test/test.go
+++ b/internal/test/test.go
@@ -214,10 +214,6 @@ func testCode(
 		return network.Host, true
 	}
 
-	runner := cdcTests.NewTestRunner().
-		WithLogger(logger).
-		WithNetworkResolver(resolveNetworkFromState)
-
 	// Configure fork mode if requested
 	var effectiveForkHost string
 
@@ -257,7 +253,6 @@ func testCode(
 			ForkHeight: flags.ForkHeight,
 		}
 		forkCfg = &cfg
-		runner = runner.WithFork(cfg)
 
 		// Map chain ID to a sensible network label if not provided explicitly
 		if strings.TrimSpace(flags.Fork) == "" {
@@ -270,9 +265,6 @@ func testCode(
 		}
 	}
 
-	// Apply the network label on the base runner now that it is known
-	runner = runner.WithNetworkLabel(networkLabel)
-
 	var coverageReport *runtime.CoverageReport
 	if flags.Cover {
 		coverageReport = state.CreateCoverageReport("testing")
@@ -287,16 +279,13 @@ func testCode(
 				},
 			)
 		}
-		runner = runner.WithCoverageReport(coverageReport)
 	}
 
 	var seed int64
 	if flags.Seed > 0 {
 		seed = flags.Seed
-		runner = runner.WithRandomSeed(seed)
 	} else if flags.Random {
 		seed = int64(rand.Intn(150000))
-		runner = runner.WithRandomSeed(seed)
 	}
 
 	testResults := make(map[string]cdcTests.Results, 0)
@@ -305,7 +294,12 @@ func testCode(
 		// Set current test file for network resolution tracking
 		currentTestFile = scriptPath
 
-		fileRunner := runner.
+		// Create a new test runner per file to ensure complete isolation.
+		// Each file gets its own runner with its own backend state.
+		fileRunner := cdcTests.NewTestRunner().
+			WithLogger(logger).
+			WithNetworkResolver(resolveNetworkFromState).
+			WithNetworkLabel(networkLabel).
 			WithImportResolver(importResolver(scriptPath, state)).
 			WithFileResolver(fileResolver(scriptPath, state)).
 			WithContractAddressResolver(func(network string, contractName string) (common.Address, error) {
@@ -328,11 +322,15 @@ func testCode(
 				return common.Address{}, fmt.Errorf("no address for contract %s on network %s", contractName, network)
 			})
 
-		// Ensure the file runner has the correct network label and fork config
-		fileRunner = fileRunner.WithNetworkLabel(networkLabel)
 		if forkCfg != nil {
 			fileRunner = fileRunner.WithFork(*forkCfg)
 		}
+		if coverageReport != nil {
+			fileRunner = fileRunner.WithCoverageReport(coverageReport)
+		}
+		if seed > 0 {
+			fileRunner = fileRunner.WithRandomSeed(seed)
+		}
 
 		if flags.Name != "" {
 			testFunctions, err := fileRunner.GetTests(string(code))