diff --git a/packages/server/src/server/agent/plugin-provider.test.ts b/packages/server/src/server/agent/plugin-provider.test.ts
index ae8ed805d7..aa0aa1a924 100644
--- a/packages/server/src/server/agent/plugin-provider.test.ts
+++ b/packages/server/src/server/agent/plugin-provider.test.ts
@@ -761,6 +761,72 @@ describe("PluginAgentClientRegistry", () => {
     await expect.poll(harness.closeCount).toBe(1);
     expect(harness.inputs.map((input) => input.type)).toContain("session.close");
   });
+
+  test("delivers a supported steer to the active turn", async () => {
+    const harness = createProviderHarness({
+      capabilities: [...CAPABILITIES, "prompt.steer"],
+      handleInput: async (input, emit) => {
+        if (input.type !== "session.prompt") return false;
+        emit({
+          type: "session.prompt_result",
+          sessionId: input.sessionId,
+          clientMessageId: input.prompt.clientMessageId,
+          result: { type: "steer", turnId: "turn-1" },
+        });
+        return true;
+      },
+    });
+    const registry = new PluginAgentClientRegistry(createTestLogger());
+    registry.replace([harness.registration]);
+    const client = registry.clients()[harness.registration.id]!;
+    const session = await client.createSession({ provider: harness.registration.id, cwd: "/tmp" });
+    try {
+      await expect(
+        session.steerActiveTurn!("follow up", {
+          expectedTurnId: "turn-1",
+          clientMessageId: "steer-1",
+          clearPendingPermissions: true,
+        }),
+      ).resolves.toEqual({ status: "accepted" });
+      expect(harness.inputs.find((input) => input.type === "session.prompt")).toMatchObject({
+        prompt: {
+          delivery: "steer",
+          clientMessageId: "steer-1",
+          input: { type: "message", content: [{ type: "text", text: "follow up" }] },
+          clearPendingPermissions: true,
+        },
+      });
+    } finally {
+      await session.close();
+      registry.replace([]);
+    }
+  });
+
+  test("returns unavailable when steering a provider without prompt.steer capability", async () => {
+    const harness = createProviderHarness({
+      capabilities: [
+        "prompt.message",
+        "session.configure",
+        "session.persistence",
+        "session.subsession",
+        "permission",
+      ],
+    });
+    const registry = new PluginAgentClientRegistry(createTestLogger());
+    registry.replace([harness.registration]);
+    const session = await registry.clients()[harness.registration.id]!.createSession({
+      provider: harness.registration.id,
+      cwd: "/workspace",
+    });
+    try {
+      const result = await session.steerActiveTurn!("hello", { expectedTurnId: "turn-1" });
+      expect(result).toEqual({ status: "unavailable" });
+      expect(harness.inputs.filter((input) => input.type === "session.prompt")).toHaveLength(0);
+    } finally {
+      await session.close();
+      registry.replace([]);
+    }
+  });
 });
 
 type RequestKind = "session.open" | "catalog" | "session.configure" | "session.prompt";
diff --git a/packages/server/src/server/agent/plugin-provider.ts b/packages/server/src/server/agent/plugin-provider.ts
index e575ff0d45..db5d7651a2 100644
--- a/packages/server/src/server/agent/plugin-provider.ts
+++ b/packages/server/src/server/agent/plugin-provider.ts
@@ -1175,6 +1175,9 @@ class PluginAgentSession implements AgentSession {
     prompt: AgentPromptInput,
     options: SteerActiveTurnOptions,
   ): Promise<SteerResult> {
+    if (!this.bridge.negotiatedCapabilities.includes("prompt.steer")) {
+      return { status: "unavailable" };
+    }
     const result = await this.bridge.prompt({
       clientMessageId: options.clientMessageId ?? randomUUID(),
       delivery: "steer",