diff --git a/pkg/constants/engine_constants.go b/pkg/constants/engine_constants.go
index 1e0603d35af..49c38c2c422 100644
--- a/pkg/constants/engine_constants.go
+++ b/pkg/constants/engine_constants.go
@@ -195,6 +195,11 @@ const (
 	// isolated in the AWF API proxy sidecar.
 	CopilotBYOKDummyAPIKey = "dummy-byok-key-for-offline-mode"
 
+	// CopilotBYOKDefaultModel is the explicit fallback model for Copilot BYOK mode.
+	// BYOK providers require a non-empty model, so this value is used when the
+	// corresponding GH_AW_MODEL_*_COPILOT variable is unset.
+	CopilotBYOKDefaultModel = "claude-sonnet-4.6"
+
 	// ClaudeCLIModelEnvVar is the native environment variable name supported by the Claude Code CLI
 	// for selecting the model. Setting this env var is equivalent to passing --model to the CLI.
 	ClaudeCLIModelEnvVar = "ANTHROPIC_MODEL"
diff --git a/pkg/workflow/copilot_engine_execution.go b/pkg/workflow/copilot_engine_execution.go
index c142223b741..9f5556c54df 100644
--- a/pkg/workflow/copilot_engine_execution.go
+++ b/pkg/workflow/copilot_engine_execution.go
@@ -392,12 +392,17 @@ touch %s
 	// When model is explicitly configured, use its value directly.
 	// When model is not configured, map the GitHub org variable to COPILOT_MODEL so users can set
 	// a default via GitHub Actions variables without requiring per-workflow frontmatter changes.
+	// In byok-copilot mode, use a non-empty fallback because BYOK providers require an explicit model.
 	if modelConfigured {
 		copilotExecLog.Printf("Setting %s env var for model: %s", constants.CopilotCLIModelEnvVar, workflowData.EngineConfig.Model)
 		env[constants.CopilotCLIModelEnvVar] = workflowData.EngineConfig.Model
 	} else {
-		// No model configured - map org variable to native COPILOT_MODEL env var
-		env[constants.CopilotCLIModelEnvVar] = fmt.Sprintf("${{ vars.%s || '' }}", modelEnvVar)
+		if isFeatureEnabled(constants.ByokCopilotFeatureFlag, workflowData) {
+			env[constants.CopilotCLIModelEnvVar] = fmt.Sprintf("${{ vars.%s || '%s' }}", modelEnvVar, constants.CopilotBYOKDefaultModel)
+		} else {
+			// No model configured - map org variable to native COPILOT_MODEL env var
+			env[constants.CopilotCLIModelEnvVar] = fmt.Sprintf("${{ vars.%s || '' }}", modelEnvVar)
+		}
 	}
 
 	// Add custom environment variables from engine config
diff --git a/pkg/workflow/model_env_vars_test.go b/pkg/workflow/model_env_vars_test.go
index 8532dc0aabf..49aa9bc8350 100644
--- a/pkg/workflow/model_env_vars_test.go
+++ b/pkg/workflow/model_env_vars_test.go
@@ -218,16 +218,29 @@ func TestCopilotFallbackModelMapsToNativeEnvVar(t *testing.T) {
 		name           string
 		safeOutputs    *SafeOutputsConfig
 		expectedOrgVar string
+		features       map[string]any
+		expectedTail   string
 	}{
 		{
 			name:           "Agent job maps GH_AW_MODEL_AGENT_COPILOT to COPILOT_MODEL",
 			safeOutputs:    &SafeOutputsConfig{},
 			expectedOrgVar: constants.EnvVarModelAgentCopilot,
+			expectedTail:   "''",
+		},
+		{
+			name:        "Agent job with byok-copilot uses non-empty COPILOT_MODEL fallback",
+			safeOutputs: &SafeOutputsConfig{},
+			features: map[string]any{
+				string(constants.ByokCopilotFeatureFlag): true,
+			},
+			expectedOrgVar: constants.EnvVarModelAgentCopilot,
+			expectedTail:   "'" + constants.CopilotBYOKDefaultModel + "'",
 		},
 		{
 			name:           "Detection job maps GH_AW_MODEL_DETECTION_COPILOT to COPILOT_MODEL",
 			safeOutputs:    nil,
 			expectedOrgVar: constants.EnvVarModelDetectionCopilot,
+			expectedTail:   "''",
 		},
 	}
 
@@ -240,6 +253,7 @@ func TestCopilotFallbackModelMapsToNativeEnvVar(t *testing.T) {
 					"bash": []any{"echo"},
 				},
 				SafeOutputs: tt.safeOutputs,
+				Features:    tt.features,
 			}
 
 			engine, err := GetGlobalEngineRegistry().GetEngine("copilot")
@@ -259,7 +273,7 @@ func TestCopilotFallbackModelMapsToNativeEnvVar(t *testing.T) {
 			stepsContent := stepsStr.String()
 
 			// The model must be passed via COPILOT_MODEL env var pointing to the org variable
-			expectedEnvLine := constants.CopilotCLIModelEnvVar + ": ${{ vars." + tt.expectedOrgVar + " || '' }}"
+			expectedEnvLine := constants.CopilotCLIModelEnvVar + ": ${{ vars." + tt.expectedOrgVar + " || " + tt.expectedTail + " }}"
 			if !strings.Contains(stepsContent, expectedEnvLine) {
 				t.Errorf("Expected env line '%s' not found in steps:\n%s", expectedEnvLine, stepsContent)
 			}