diff --git a/pkg/agent/definition.go b/pkg/agent/definition.go
index 90a69eaa4..5b0e29137 100644
--- a/pkg/agent/definition.go
+++ b/pkg/agent/definition.go
@@ -45,6 +45,7 @@ type AgentPromptDefinition struct {
 	Body           string           `json:"body"`
 	RawFrontmatter string           `json:"raw_frontmatter,omitempty"`
 	Frontmatter    AgentFrontmatter `json:"frontmatter"`
+	FrontmatterErr string           `json:"frontmatter_error,omitempty"`
 }
 
 // SoulDefinition represents the resolved SOUL.md file linked to the agent.
@@ -146,19 +147,21 @@ func loadUserDefinition(workspace string) *UserDefinition {
 
 func parseAgentPromptDefinition(path, content string) AgentPromptDefinition {
 	frontmatter, body := splitAgentFrontmatter(content)
+	parsedFrontmatter, err := parseAgentFrontmatter(path, frontmatter)
 	return AgentPromptDefinition{
 		Path:           path,
 		Raw:            content,
 		Body:           body,
 		RawFrontmatter: frontmatter,
-		Frontmatter:    parseAgentFrontmatter(path, frontmatter),
+		Frontmatter:    parsedFrontmatter,
+		FrontmatterErr: errorString(err),
 	}
 }
 
-func parseAgentFrontmatter(path, frontmatter string) AgentFrontmatter {
+func parseAgentFrontmatter(path, frontmatter string) (AgentFrontmatter, error) {
 	frontmatter = strings.TrimSpace(frontmatter)
 	if frontmatter == "" {
-		return AgentFrontmatter{}
+		return AgentFrontmatter{}, nil
 	}
 
 	rawFields := make(map[string]any)
@@ -167,7 +170,7 @@ func parseAgentFrontmatter(path, frontmatter string) AgentFrontmatter {
 			"path":  path,
 			"error": err.Error(),
 		})
-		return AgentFrontmatter{}
+		return AgentFrontmatter{}, err
 	}
 
 	var typed struct {
@@ -184,7 +187,7 @@ func parseAgentFrontmatter(path, frontmatter string) AgentFrontmatter {
 			"path":  path,
 			"error": err.Error(),
 		})
-		return AgentFrontmatter{}
+		return AgentFrontmatter{}, err
 	}
 
 	return AgentFrontmatter{
@@ -196,7 +199,7 @@ func parseAgentFrontmatter(path, frontmatter string) AgentFrontmatter {
 		Skills:      append([]string(nil), typed.Skills...),
 		MCPServers:  append([]string(nil), typed.MCPServers...),
 		Fields:      rawFields,
-	}
+	}, nil
 }
 
 func splitAgentFrontmatter(content string) (frontmatter, body string) {
@@ -253,3 +256,10 @@ func fileExists(path string) bool {
 	_, err := os.Stat(path)
 	return err == nil
 }
+
+func errorString(err error) string {
+	if err == nil {
+		return ""
+	}
+	return err.Error()
+}
diff --git a/pkg/agent/instance_test.go b/pkg/agent/instance_test.go
index 869e5fbc7..3edac0724 100644
--- a/pkg/agent/instance_test.go
+++ b/pkg/agent/instance_test.go
@@ -330,3 +330,39 @@ Use frontmatter identity.
 		t.Fatal("expected slack MCP server to be blocked by frontmatter allowlist")
 	}
 }
+
+func TestNewAgentInstance_InvalidFrontmatterFailsClosedForToolsAndMCPServers(t *testing.T) {
+	workspace := setupWorkspace(t, map[string]string{
+		"AGENT.md": `---
+tools: [read_file
+mcpServers: [github]
+---
+# Agent
+`,
+	})
+	defer cleanupWorkspace(t, workspace)
+
+	cfg := &config.Config{
+		Agents: config.AgentsConfig{
+			Defaults: config.AgentDefaults{
+				Workspace: workspace,
+				ModelName: "default-model",
+			},
+		},
+		Tools: config.ToolsConfig{
+			ReadFile: config.ReadFileToolConfig{Enabled: true},
+		},
+	}
+
+	agent := NewAgentInstance(&config.AgentConfig{
+		ID:        "research",
+		Workspace: workspace,
+	}, &cfg.Agents.Defaults, cfg, &mockProvider{})
+
+	if _, ok := agent.Tools.Get("read_file"); ok {
+		t.Fatal("expected malformed frontmatter to fail closed and block read_file")
+	}
+	if agent.AllowsMCPServer("github") {
+		t.Fatal("expected malformed frontmatter to fail closed for MCP servers")
+	}
+}
diff --git a/pkg/agent/tool_allowlist.go b/pkg/agent/tool_allowlist.go
index de68352ad..f7434c188 100644
--- a/pkg/agent/tool_allowlist.go
+++ b/pkg/agent/tool_allowlist.go
@@ -6,6 +6,9 @@ import (
 )
 
 func resolveAgentToolAllowlist(definition AgentContextDefinition) []string {
+	if frontmatterParseFailed(definition) {
+		return []string{}
+	}
 	if definition.Agent == nil || definition.Agent.Frontmatter.Tools == nil {
 		return nil
 	}
@@ -28,6 +31,9 @@ func resolveAgentToolAllowlist(definition AgentContextDefinition) []string {
 }
 
 func resolveAgentMCPServerAllowlist(definition AgentContextDefinition) map[string]struct{} {
+	if frontmatterParseFailed(definition) {
+		return map[string]struct{}{}
+	}
 	if definition.Agent == nil || definition.Agent.Frontmatter.MCPServers == nil {
 		return nil
 	}
@@ -43,3 +49,13 @@ func resolveAgentMCPServerAllowlist(definition AgentContextDefinition) map[strin
 
 	return allowlist
 }
+
+func frontmatterParseFailed(definition AgentContextDefinition) bool {
+	if definition.Agent == nil {
+		return false
+	}
+	if strings.TrimSpace(definition.Agent.RawFrontmatter) == "" {
+		return false
+	}
+	return strings.TrimSpace(definition.Agent.FrontmatterErr) != ""
+}