diff --git a/.changeset/brave-sandboxes-intercept.md b/.changeset/brave-sandboxes-intercept.md
new file mode 100644
index 000000000..568714f8f
--- /dev/null
+++ b/.changeset/brave-sandboxes-intercept.md
@@ -0,0 +1,5 @@
+---
+"eve": patch
+---
+
+Broker microsandbox header credentials through its managed secret API so placeholders are replaced at the verified network boundary.
diff --git a/packages/eve/src/execution/sandbox/bindings/microsandbox-network.ts b/packages/eve/src/execution/sandbox/bindings/microsandbox-network.ts
index 1e17c2f10..ce60fb75a 100644
--- a/packages/eve/src/execution/sandbox/bindings/microsandbox-network.ts
+++ b/packages/eve/src/execution/sandbox/bindings/microsandbox-network.ts
@@ -24,10 +24,6 @@ interface MicrosandboxSecretBuilderShape {
 interface MicrosandboxNetworkBuilderShape {
   enabled(enabled: boolean): this;
   policyJson(json: string): this;
-  secret(
-    configure: (secret: MicrosandboxSecretBuilderShape) => MicrosandboxSecretBuilderShape,
-  ): this;
-  trustHostCAs(enabled: boolean): this;
 }
 
 interface MicrosandboxTransformHeaderRule {
@@ -81,48 +77,45 @@ export function applyMicrosandboxNetwork(
     return builder;
   }
 
-  return builder.network((network: MicrosandboxNetworkBuilderShape) => {
+  let next = builder.network((network: MicrosandboxNetworkBuilderShape) => {
     let next = network.enabled(true);
     if (networkPlan.policy !== null) {
       next = next.policyJson(serializeMicrosandboxNetworkPolicyJson(networkPlan.policy));
     }
-    if (networkPlan.transformHeaderRules.length === 0) {
-      return next;
-    }
+    return next;
+  });
 
-    next = next.trustHostCAs(true);
-    for (const rule of networkPlan.transformHeaderRules) {
-      for (const [headerName, headerValue] of Object.entries(rule.headers)) {
-        const placeholder = rule.placeholderHeaders[headerName];
-        if (placeholder === undefined) {
-          continue;
-        }
-        const secretEnvName = createSecretEnvName(rule.domain, headerName, headerValue);
-        next = next.secret((secret) => {
-          let configured = secret
-            .env(secretEnvName)
-            .value(headerValue)
-            .placeholder(placeholder)
-            .injectHeaders(true)
-            .injectBasicAuth(true)
-            .injectQuery(false)
-            .injectBody(false)
-            .requireTlsIdentity(true);
-
-          if (rule.domain === "*") {
-            configured = configured.allowAnyHostDangerous(true);
-          } else if (rule.domain.startsWith("*.")) {
-            configured = configured.allowHostPattern(rule.domain);
-          } else {
-            configured = configured.allowHost(rule.domain);
-          }
-
-          return configured;
-        });
+  for (const rule of networkPlan.transformHeaderRules) {
+    for (const [headerName, headerValue] of Object.entries(rule.headers)) {
+      const placeholder = rule.placeholderHeaders[headerName];
+      if (placeholder === undefined) {
+        continue;
       }
+      const secretEnvName = createSecretEnvName(rule.domain, headerName, headerValue);
+      next = next.secret((secret: MicrosandboxSecretBuilderShape) => {
+        let configured = secret
+          .env(secretEnvName)
+          .value(headerValue)
+          .placeholder(placeholder)
+          .injectHeaders(true)
+          .injectBasicAuth(true)
+          .injectQuery(false)
+          .injectBody(false)
+          .requireTlsIdentity(true);
+
+        if (rule.domain === "*") {
+          configured = configured.allowAnyHostDangerous(true);
+        } else if (rule.domain.startsWith("*.")) {
+          configured = configured.allowHostPattern(rule.domain);
+        } else {
+          configured = configured.allowHost(rule.domain);
+        }
+
+        return configured;
+      });
     }
-    return next;
-  });
+  }
+  return next;
 }
 
 export function serializeMicrosandboxNetworkPolicyJson(policy: MicrosandboxNetworkPolicy): string {
diff --git a/packages/eve/src/execution/sandbox/bindings/microsandbox-runtime.test.ts b/packages/eve/src/execution/sandbox/bindings/microsandbox-runtime.test.ts
index cb72e887d..652f600b8 100644
--- a/packages/eve/src/execution/sandbox/bindings/microsandbox-runtime.test.ts
+++ b/packages/eve/src/execution/sandbox/bindings/microsandbox-runtime.test.ts
@@ -614,6 +614,10 @@ function createMockSandboxBuilder(create: (fromSnapshot: string) => unknown) {
       configure(createMockNetworkBuilder());
       return builder;
     },
+    secret(configure: (secret: unknown) => unknown) {
+      configure(createMockSecretBuilder());
+      return builder;
+    },
     pullPolicy() {
       return builder;
     },
@@ -649,13 +653,6 @@ function createMockNetworkBuilder() {
     policyJson() {
       return builder;
     },
-    secret(configure: (secret: unknown) => unknown) {
-      configure(createMockSecretBuilder());
-      return builder;
-    },
-    trustHostCAs() {
-      return builder;
-    },
   };
   return builder;
 }
diff --git a/packages/eve/src/execution/sandbox/bindings/microsandbox.test.ts b/packages/eve/src/execution/sandbox/bindings/microsandbox.test.ts
index 4f8aae65f..8649668b3 100644
--- a/packages/eve/src/execution/sandbox/bindings/microsandbox.test.ts
+++ b/packages/eve/src/execution/sandbox/bindings/microsandbox.test.ts
@@ -2,6 +2,7 @@ import { beforeEach, describe, expect, it, vi } from "vitest";
 
 import { createMicrosandboxSandboxBackend } from "#execution/sandbox/bindings/microsandbox.js";
 import {
+  applyMicrosandboxNetwork,
   createMicrosandboxNetworkPlan,
   createTransformBrokerEnvironment,
   serializeMicrosandboxNetworkPolicyJson,
@@ -225,3 +226,73 @@ describe.skipIf(onWindows)("createMicrosandboxNetworkPlan", () => {
     });
   });
 });
+
+describe("applyMicrosandboxNetwork", () => {
+  it("registers brokered header transforms through sandbox secrets", () => {
+    const secret = createFluentMock([
+      "allowAnyHostDangerous",
+      "allowHost",
+      "allowHostPattern",
+      "env",
+      "injectBasicAuth",
+      "injectBody",
+      "injectHeaders",
+      "injectQuery",
+      "placeholder",
+      "requireTlsIdentity",
+      "value",
+    ]);
+    const network = {
+      enabled: vi.fn(() => network),
+      policyJson: vi.fn(() => network),
+    };
+    const sandbox = {
+      disableNetwork: vi.fn(),
+      network: vi.fn((configure: (builder: typeof network) => unknown) => {
+        configure(network);
+        return sandbox;
+      }),
+      secret: vi.fn((configure: (builder: typeof secret) => unknown) => {
+        configure(secret);
+        return sandbox;
+      }),
+    };
+
+    applyMicrosandboxNetwork(sandbox as never, {
+      allow: {
+        "api.example.com": [{ transform: [{ headers: { authorization: "Bearer real-secret" } }] }],
+      },
+    });
+
+    expect(sandbox.secret).toHaveBeenCalledOnce();
+    expect(secret.requireTlsIdentity).toHaveBeenCalledWith(true);
+    expect(secret.allowHost).toHaveBeenCalledWith("api.example.com");
+  });
+
+  it("does not register sandbox secrets without brokered header transforms", () => {
+    const network = {
+      enabled: vi.fn(() => network),
+      policyJson: vi.fn(() => network),
+    };
+    const sandbox = {
+      disableNetwork: vi.fn(),
+      network: vi.fn((configure: (builder: typeof network) => unknown) => {
+        configure(network);
+        return sandbox;
+      }),
+      secret: vi.fn(),
+    };
+
+    applyMicrosandboxNetwork(sandbox as never, { allow: ["api.example.com"] });
+
+    expect(sandbox.secret).not.toHaveBeenCalled();
+  });
+});
+
+function createFluentMock<const Method extends string>(methods: readonly Method[]) {
+  const builder = {} as Record<Method, ReturnType<typeof vi.fn>>;
+  for (const method of methods) {
+    builder[method] = vi.fn(() => builder);
+  }
+  return builder;
+}