diff --git a/internal/runtime/bootstrap.go b/internal/runtime/bootstrap.go
index bb59d290b..942141f6e 100644
--- a/internal/runtime/bootstrap.go
+++ b/internal/runtime/bootstrap.go
@@ -1,5 +1,7 @@
 package runtime
 
+import "fmt"
+
 // BootstrapInput is the portable contract every runtime needs to provision
 // agent content into the sandbox. Implementations live outside this package
 // (runner adapter, tests).
@@ -16,3 +18,17 @@ type BootstrapInput interface {
 	SkillDirs() []string
 	PluginDirs() []string
 }
+
+// validateAgentNameMatch returns an error when requestedName and
+// definitionName are both non-empty and do not match. Both ClaudeRuntime
+// and PiRuntime call this shared helper so the mismatch message is defined
+// in one place.
+func validateAgentNameMatch(requestedName, definitionName string) error {
+	if requestedName == "" || definitionName == "" {
+		return nil
+	}
+	if definitionName != requestedName {
+		return fmt.Errorf("agent name mismatch: requested %q but definition declares %q", requestedName, definitionName)
+	}
+	return nil
+}
diff --git a/internal/runtime/claude.go b/internal/runtime/claude.go
index 6c43a691a..e23173ea8 100644
--- a/internal/runtime/claude.go
+++ b/internal/runtime/claude.go
@@ -47,6 +47,20 @@ func (r ClaudeRuntime) Bootstrap(input BootstrapInput) error {
 		return fmt.Errorf("agent path is required")
 	}
 
+	// Validate the frontmatter name: field against the requested agent name.
+	// Claude Code resolves --agent by frontmatter name, not filename, so a
+	// mismatch means the runtime silently falls back to the default agent,
+	// producing an unconstrained run (#6764).
+	if agentName := input.AgentName(); agentName != "" {
+		if data, readErr := os.ReadFile(agentPath); readErr == nil {
+			if def, parseErr := parsePiAgent(data); parseErr != nil {
+				fmt.Fprintf(os.Stderr, "Agent name validation: skipped for %s: %v\n", agentPath, parseErr)
+			} else if err := validateAgentNameMatch(agentName, def.Name); err != nil {
+				return err
+			}
+		}
+	}
+
 	sandboxName := input.SandboxName()
 	configDir := r.ConfigDir()
 
diff --git a/internal/runtime/claude_test.go b/internal/runtime/claude_test.go
index 2ab1f8734..42062bc8b 100644
--- a/internal/runtime/claude_test.go
+++ b/internal/runtime/claude_test.go
@@ -105,6 +105,92 @@ func TestAgentDestName(t *testing.T) {
 	}
 }
 
+func TestValidateAgentNameMatch(t *testing.T) {
+	tests := []struct {
+		name           string
+		requestedName  string
+		definitionName string
+		wantErr        string // empty means no error expected
+	}{
+		{
+			name:           "matching names pass",
+			requestedName:  "code",
+			definitionName: "code",
+			wantErr:        "",
+		},
+		{
+			name:           "mismatched names fail",
+			requestedName:  "coder",
+			definitionName: "code",
+			wantErr:        `agent name mismatch: requested "coder" but definition declares "code"`,
+		},
+		{
+			name:           "empty requested name skips validation",
+			requestedName:  "",
+			definitionName: "code",
+			wantErr:        "",
+		},
+		{
+			name:           "empty definition name skips validation",
+			requestedName:  "coder",
+			definitionName: "",
+			wantErr:        "",
+		},
+		{
+			name:           "both empty skips validation",
+			requestedName:  "",
+			definitionName: "",
+			wantErr:        "",
+		},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			err := validateAgentNameMatch(tc.requestedName, tc.definitionName)
+			if tc.wantErr == "" {
+				assert.NoError(t, err)
+			} else {
+				require.Error(t, err)
+				assert.Contains(t, err.Error(), tc.wantErr)
+			}
+		})
+	}
+}
+
+func TestBootstrap_AgentNameMismatch(t *testing.T) {
+	stubDir := t.TempDir()
+	require.NoError(t, os.WriteFile(filepath.Join(stubDir, "openshell"), []byte("#!/bin/sh\nexit 0\n"), 0o755))
+	t.Setenv("PATH", stubDir)
+
+	agentFile := filepath.Join(t.TempDir(), "agent.md")
+	require.NoError(t, os.WriteFile(agentFile, []byte("---\nname: code\n---\n# Code agent"), 0o644))
+
+	err := ClaudeRuntime{}.Bootstrap(bootstrapInput{
+		sandboxName: "test-sandbox",
+		agentPath:   agentFile,
+		agentName:   "coder",
+	})
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "agent name mismatch")
+	assert.Contains(t, err.Error(), `"coder"`)
+	assert.Contains(t, err.Error(), `"code"`)
+}
+
+func TestBootstrap_AgentNameMatch(t *testing.T) {
+	stubDir := t.TempDir()
+	require.NoError(t, os.WriteFile(filepath.Join(stubDir, "openshell"), []byte("#!/bin/sh\nexit 0\n"), 0o755))
+	t.Setenv("PATH", stubDir)
+
+	agentFile := filepath.Join(t.TempDir(), "agent.md")
+	require.NoError(t, os.WriteFile(agentFile, []byte("---\nname: code\n---\n# Code agent"), 0o644))
+
+	err := ClaudeRuntime{}.Bootstrap(bootstrapInput{
+		sandboxName: "test-sandbox",
+		agentPath:   agentFile,
+		agentName:   "code",
+	})
+	assert.NoError(t, err)
+}
+
 func TestBuildRunCommand_Basic(t *testing.T) {
 	cmd := testRunCommand("hello-world", "", "/sandbox/workspace/repo", nil, "")
 	assert.Contains(t, cmd, "cd /sandbox/workspace/repo")
diff --git a/internal/runtime/pi_bootstrap.go b/internal/runtime/pi_bootstrap.go
index 017767397..233da1f46 100644
--- a/internal/runtime/pi_bootstrap.go
+++ b/internal/runtime/pi_bootstrap.go
@@ -92,6 +92,12 @@ func (r PiRuntime) Bootstrap(input BootstrapInput) error {
 	if err != nil {
 		return err
 	}
+	// Fail fast when the harness-configured agent name does not match the
+	// definition's frontmatter name: — the runtime would silently fall back
+	// to the default agent, producing an unconstrained run (#6764).
+	if err := validateAgentNameMatch(input.AgentName(), def.Name); err != nil {
+		return err
+	}
 	agentName := input.AgentName()
 	if agentName == "" {
 		agentName = def.Name
diff --git a/internal/runtime/pi_bootstrap_test.go b/internal/runtime/pi_bootstrap_test.go
index b7192bd5b..31ab34a95 100644
--- a/internal/runtime/pi_bootstrap_test.go
+++ b/internal/runtime/pi_bootstrap_test.go
@@ -162,6 +162,39 @@ func TestPiRuntimeBootstrap_NoSecurityNoHooks(t *testing.T) {
 	assert.True(t, os.IsNotExist(err), "no hook extension without security config")
 }
 
+func TestPiRuntimeBootstrap_AgentNameMismatch(t *testing.T) {
+	work := t.TempDir()
+	logPath := filepath.Join(work, "openshell.log")
+	store := filepath.Join(work, "store")
+	fakeOpenshellPi(t, logPath, store, "/dev/null")
+
+	agentFile := writeAgentFile(t, "---\nname: code\n---\n# Code agent")
+	err := PiRuntime{}.Bootstrap(bootstrapInput{
+		sandboxName: "sb",
+		agentPath:   agentFile,
+		agentName:   "coder",
+	})
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "agent name mismatch")
+	assert.Contains(t, err.Error(), `"coder"`)
+	assert.Contains(t, err.Error(), `"code"`)
+}
+
+func TestPiRuntimeBootstrap_AgentNameMatch(t *testing.T) {
+	work := t.TempDir()
+	logPath := filepath.Join(work, "openshell.log")
+	store := filepath.Join(work, "store")
+	fakeOpenshellPi(t, logPath, store, "/dev/null")
+
+	agentFile := writeAgentFile(t, "---\nname: triage\n---\n# Triage agent")
+	err := PiRuntime{}.Bootstrap(bootstrapInput{
+		sandboxName: "sb",
+		agentPath:   agentFile,
+		agentName:   "triage",
+	})
+	assert.NoError(t, err)
+}
+
 func TestPiRuntimeBootstrap_PreflightFailure(t *testing.T) {
 	binDir := t.TempDir()
 	script := "#!/bin/sh\nfor last; do :; done\ncase \"$last\" in \"pi --version\") echo 'sh: pi: not found' >&2; exit 127 ;; esac\nexit 0\n"