diff --git a/docs/src/pages/agents/approvals.mdx b/docs/src/pages/agents/approvals.mdx
index 0c24dc24..cdf53a56 100644
--- a/docs/src/pages/agents/approvals.mdx
+++ b/docs/src/pages/agents/approvals.mdx
@@ -82,4 +82,6 @@ db.CancelPendingTurnRequests(ctx, sessionID, promptRunID, "run cancelled")
 
 `api.RequireToolPolicySupport` is checked before a run's first model call. A per-tool policy of `deny` or `allow` is refused outright on a runtime that cannot carry an allow/deny list at all — running without it would grant the agent more than the spec allows. A per-tool policy of `ask` is refused unconditionally: no transport exposes a per-tool prompt to the *runtime's own* declarative tool filter, so it would resolve to "allowed" on every runtime that does support allow/deny lists.
 
+One entry is exempt: an `allow` that names another agent's built-in (Claude's `Read: allow` on a codex run) is skipped, because an allow only pre-approves a tool the agent already has and constrains nothing on an agent without it. That lets one spec carry a Claude-style allowlist across backends. The exemption is deliberately narrow — a `deny` or `ask` on a foreign name is still refused, since Claude's `Bash` is codex's `shell` under another name and dropping the deny would hand the agent the very tool the spec forbade; and a name no agent declares is never treated as foreign, so a stale vocabulary table fails loud instead of waving a new built-in through.
+
 That check is about the runtime's declarative filter, not about whether a broker exists — an `ask` decision made per call, live, is a different mechanism: it is what `approval.Broker.CanUseTool` implements as the `PermissionFunc` itself, and it is how the caller-tool path (aichat) honours a per-tool `ask` policy today. A streaming provider run's declared `permissions.tools` still cannot say `ask` — only `allow`/`deny` — regardless of whether a broker is attached.
diff --git a/pkg/api/permissions.go b/pkg/api/permissions.go
index bbb08751..e8be2990 100644
--- a/pkg/api/permissions.go
+++ b/pkg/api/permissions.go
@@ -92,16 +92,26 @@ func (t Tools) toolsWithPolicy(want ToolPolicy) []string {
 // proceeding as if the policy had been applied.
 //
 // Allow-lists are checked too: on a runtime with no tool filter, an allowlist is
-// equally unenforced. `ask` is refused everywhere — no transport has a per-tool
-// prompt, so it would resolve to "allowed" on the runtimes that advertise tool
-// policy support. `auto` constrains nothing, so it needs no runtime support.
+// equally unenforced. The one entry that may be dropped is an allow naming
+// another agent's built-in (a Claude `Read: allow` on a codex run): an allow only
+// pre-approves a tool the agent already has, so on an agent without that tool it
+// constrains nothing, and a portable spec can carry both vocabularies. A deny or
+// ask is never dropped on that basis — `Bash` is codex's `shell` under another
+// name, and dropping the deny would hand the agent the very tool the spec
+// forbade. `ask` is refused everywhere: no transport has a per-tool prompt, so it
+// would resolve to "allowed" on the runtimes that advertise tool policy support.
+// `auto` constrains nothing, so it needs no runtime support.
 func RequireToolPolicySupport(p *ModelProvider, mode RuntimeMode, permissions Permissions) error {
 	if asked := permissions.Tools.toolsWithPolicy(ToolPolicyAsk); len(asked) > 0 {
 		return fmt.Errorf(
 			"per-tool policy \"ask\" (%s) is not enforceable on any runtime: transports carry allow/deny tool lists only, so the tool would run unprompted; use allow or deny",
 			strings.Join(asked, ", "))
 	}
-	enforced := append(permissions.Tools.AllowList(), permissions.Tools.DenyList()...)
+	vocabulary := PermissionCapabilitiesFor(RuntimeOf(p, mode)).Tools
+	allowed := slices.DeleteFunc(permissions.Tools.AllowList(), func(tool string) bool {
+		return isForeignBuiltin(vocabulary, tool)
+	})
+	enforced := append(allowed, permissions.Tools.DenyList()...)
 	if len(enforced) == 0 || registry.SupportsToolPolicy(p, mode) {
 		return nil
 	}
@@ -111,6 +121,27 @@ func RequireToolPolicySupport(p *ModelProvider, mode RuntimeMode, permissions Pe
 		registry.RuntimeOf(p, mode), strings.Join(enforced, ", "), registry.RuntimesList(registry.ToolPolicyRuntimes()))
 }
 
+// isForeignBuiltin reports whether tool is positively identified as another
+// agent's built-in and absent from the selected runtime's vocabulary. It is the
+// only basis on which an allow entry may be skipped; see RequireToolPolicySupport.
+//
+// A name no agent declares stays enforceable: the vocabularies are hand-kept,
+// and a stale table must fail loud on a newly added built-in rather than wave it
+// through. A runtime with no vocabulary at all (the API modes) owns every name.
+func isForeignBuiltin(vocabulary []string, tool string) bool {
+	if len(vocabulary) == 0 || slices.Contains(vocabulary, tool) {
+		return false
+	}
+	for _, tools := range agentTools {
+		if slices.ContainsFunc(tools, func(candidate AgentTool) bool {
+			return candidate.Name == tool
+		}) {
+			return true
+		}
+	}
+	return false
+}
+
 // Validate checks the mode, presets, tool policies, and resource modes are
 // recognised.
 func (p Permissions) Validate() error {
diff --git a/pkg/api/runtime_profiles.go b/pkg/api/runtime_profiles.go
index 10bff344..6688ade6 100644
--- a/pkg/api/runtime_profiles.go
+++ b/pkg/api/runtime_profiles.go
@@ -259,7 +259,12 @@ func validateResolvedPermissions(spec Spec) error {
 	if posture := spec.Permissions.Mode; posture != "" && !caps.ModeSupport(posture).Honoured() {
 		return fmt.Errorf("permissions.mode %q is not available for %s", posture, runtime)
 	}
-	for _, policy := range spec.Permissions.Tools {
+	for tool, policy := range spec.Permissions.Tools {
+		// Same rule as RequireToolPolicySupport: only an allow for another
+		// agent's built-in is inert here; a deny or ask must still be honoured.
+		if policy == ToolPolicyAllow && isForeignBuiltin(caps.Tools, tool) {
+			continue
+		}
 		if err := requireResolvedToolPolicy(caps, runtime, ProvenanceAgent, policy); err != nil {
 			return err
 		}
diff --git a/pkg/api/runtime_profiles_ginkgo_test.go b/pkg/api/runtime_profiles_ginkgo_test.go
index d077e427..fceb3c52 100644
--- a/pkg/api/runtime_profiles_ginkgo_test.go
+++ b/pkg/api/runtime_profiles_ginkgo_test.go
@@ -124,6 +124,36 @@ var _ = Describe("Runtime profiles", func() {
 		Expect(err).To(MatchError(ContainSubstring("is not available for openai agent")))
 	})
 
+	// Profile validation shares RequireToolPolicySupport's one relaxation: an
+	// allow for a tool the runtime does not have (Claude's Read on codex) is
+	// inert, but a deny on a foreign name still describes a codex capability and
+	// must be refused like any other agent-tool policy codex cannot carry.
+	It("skips a foreign allow but refuses a foreign deny for the resolved runtime", func() {
+		profile := func(tools api.Tools) api.RuntimeProfile {
+			return api.RuntimeProfile{
+				ID: "codex", Name: "Codex", Spec: api.Spec{
+					Model:       api.Model{Name: "gpt-5", Mode: api.ModeAgent},
+					Permissions: api.Permissions{Tools: tools},
+				},
+			}
+		}
+
+		_, err := api.ResolveRuntimeProfile(api.RuntimeProfileResolveRequest{
+			Profile: profile(api.Tools{"Read": api.ToolPolicyAllow, "Edit": api.ToolPolicyAllow}),
+		})
+		Expect(err).NotTo(HaveOccurred())
+
+		_, err = api.ResolveRuntimeProfile(api.RuntimeProfileResolveRequest{
+			Profile: profile(api.Tools{"Bash": api.ToolPolicyDeny}),
+		})
+		Expect(err).To(MatchError(ContainSubstring(`agent-tool policy "deny" is not available for openai agent`)))
+
+		_, err = api.ResolveRuntimeProfile(api.RuntimeProfileResolveRequest{
+			Profile: profile(api.Tools{"shell": api.ToolPolicyAllow}),
+		})
+		Expect(err).To(MatchError(ContainSubstring(`agent-tool policy "allow" is not available for openai agent`)))
+	})
+
 	// The posture is independent of isolation: a run with no sandbox at all must
 	// still carry the mode it asked for, which folding it into SandboxRef broke.
 	It("keeps a permission mode when no sandbox is configured", func() {
diff --git a/pkg/api/tool_policy_support_test.go b/pkg/api/tool_policy_support_test.go
index eab93621..99d04ebf 100644
--- a/pkg/api/tool_policy_support_test.go
+++ b/pkg/api/tool_policy_support_test.go
@@ -139,11 +139,74 @@ func TestToolsAllowDenyLists(t *testing.T) {
 // the same backends: where there is no tool filter, an allowlist is equally
 // unenforced, and silently ignoring it grants more than the spec allowed.
 func TestRequireToolPolicySupport_AllowListToo(t *testing.T) {
-	policy := Permissions{Tools: Tools{"Read": ToolPolicyAllow}}
-	if err := RequireToolPolicySupport(OpenAI, ModeCLI, policy); err == nil {
+	codexPolicy := Permissions{Tools: Tools{"shell": ToolPolicyAllow}}
+	err := RequireToolPolicySupport(OpenAI, ModeCLI, codexPolicy)
+	if err == nil {
 		t.Fatal("codex-cli silently drops an allow-list; want a loud refusal")
 	}
-	if err := RequireToolPolicySupport(Anthropic, ModeCLI, policy); err != nil {
+	if !strings.Contains(err.Error(), "shell") {
+		t.Errorf("error %q does not name the offending tool", err)
+	}
+	claudePolicy := Permissions{Tools: Tools{"Read": ToolPolicyAllow}}
+	if err := RequireToolPolicySupport(Anthropic, ModeCLI, claudePolicy); err != nil {
 		t.Fatalf("claude-cli must carry an allow-list, got %v", err)
 	}
 }
+
+// TestRequireToolPolicySupport_ForeignAllowIsInert pins the one relaxation of the
+// guard: an allow naming another agent's built-in constrains nothing on a runtime
+// that has no such tool, so a portable Claude-style allowlist (issue #110) must
+// not abort a codex run. The check is per entry, so a mixed map keeps failing on
+// the entries the runtime does own.
+func TestRequireToolPolicySupport_ForeignAllowIsInert(t *testing.T) {
+	claudeAllow := Tools{}
+	for _, tool := range []string{"Bash", "Edit", "Glob", "Grep", "Read", "Write"} {
+		claudeAllow[tool] = ToolPolicyAllow
+	}
+	for _, runtime := range []Runtime{RuntimeOf(OpenAI, ModeAgent), RuntimeOf(OpenAI, ModeCLI), RuntimeOf(Google, ModeCLI)} {
+		p, _ := runtime.ModelProvider()
+		if err := RequireToolPolicySupport(p, runtime.Mode, Permissions{Tools: claudeAllow}); err != nil {
+			t.Errorf("%s refused an allowlist of tools it does not have: %v", runtime, err)
+		}
+	}
+
+	mixed := Permissions{Tools: Tools{"Read": ToolPolicyAllow, "shell": ToolPolicyAllow}}
+	err := RequireToolPolicySupport(OpenAI, ModeAgent, mixed)
+	if err == nil {
+		t.Fatal("codex-agent dropped an allow for its own shell alongside a foreign one")
+	}
+	if !strings.Contains(err.Error(), "shell") || strings.Contains(err.Error(), "Read") {
+		t.Errorf("error %q should name shell and only shell", err)
+	}
+}
+
+// TestRequireToolPolicySupport_ForeignDenyStaysLoud pins the boundary of that
+// relaxation. Claude's Bash is codex's shell under another name, so a deny or ask
+// on a foreign name still describes a capability the runtime has; dropping it
+// would hand the agent the very tool the spec forbade. Both must keep failing.
+func TestRequireToolPolicySupport_ForeignDenyStaysLoud(t *testing.T) {
+	for _, policy := range []ToolPolicy{ToolPolicyDeny, ToolPolicyAsk} {
+		err := RequireToolPolicySupport(OpenAI, ModeAgent, Permissions{Tools: Tools{"Bash": policy}})
+		if err == nil {
+			t.Errorf("codex-agent silently dropped a foreign %s; want a loud refusal", policy)
+			continue
+		}
+		if !strings.Contains(err.Error(), "Bash") {
+			t.Errorf("%s: error %q does not name the offending tool", policy, err)
+		}
+	}
+}
+
+// TestRequireToolPolicySupport_UnknownAllowStaysLoud pins that only a name some
+// agent positively declares is treated as foreign. The vocabularies are hand-kept,
+// so a built-in they have not caught up with must fail closed, not be waved
+// through as if it belonged to some other agent. A runtime with no vocabulary at
+// all (the API modes) owns every name for the same reason.
+func TestRequireToolPolicySupport_UnknownAllowStaysLoud(t *testing.T) {
+	if err := RequireToolPolicySupport(OpenAI, ModeAgent, Permissions{Tools: Tools{"NotATool": ToolPolicyAllow}}); err == nil {
+		t.Error("codex-agent dropped an allow for a name no agent declares")
+	}
+	if err := RequireToolPolicySupport(OpenAI, ModeAPI, Permissions{Tools: Tools{"Read": ToolPolicyAllow}}); err == nil {
+		t.Error("openai api has no vocabulary and cannot call any name foreign")
+	}
+}