diff --git a/base-action/src/install-plugins.ts b/base-action/src/install-plugins.ts
index 0eb12e744..4b404d439 100644
--- a/base-action/src/install-plugins.ts
+++ b/base-action/src/install-plugins.ts
@@ -194,11 +194,61 @@ async function addMarketplace(
 ): Promise<void> {
   console.log(`Adding marketplace: ${marketplace}`);
 
-  return executeClaudeCommand(
-    claudeExecutable,
-    ["plugin", "marketplace", "add", marketplace],
-    `Failed to add marketplace '${marketplace}'`,
-  );
+  return new Promise((resolve, reject) => {
+    const outputChunks: Buffer[] = [];
+    const childProcess: ChildProcess = spawn(
+      claudeExecutable,
+      ["plugin", "marketplace", "add", marketplace],
+      { stdio: ["inherit", "pipe", "pipe"] },
+    );
+
+    // Mirror output to the parent process so it remains visible in logs.
+    childProcess.stdout?.on("data", (chunk: Buffer) => {
+      outputChunks.push(chunk);
+      process.stdout.write(chunk);
+    });
+    childProcess.stderr?.on("data", (chunk: Buffer) => {
+      outputChunks.push(chunk);
+      process.stderr.write(chunk);
+    });
+
+    childProcess.on("close", (code: number | null) => {
+      if (code === 0) {
+        resolve();
+        return;
+      }
+      // Non-ephemeral runners retain ~/.claude state across runs. Treat
+      // "already installed" as success so the action is idempotent on
+      // persistent runners without requiring a manual cleanup step.
+      const output = Buffer.concat(outputChunks).toString();
+      if (output.includes("already installed")) {
+        console.log(
+          `Marketplace '${marketplace}' is already installed, skipping`,
+        );
+        resolve();
+        return;
+      }
+      if (code === null) {
+        reject(
+          new Error(
+            `Failed to add marketplace '${marketplace}': process terminated by signal`,
+          ),
+        );
+      } else {
+        reject(
+          new Error(
+            `Failed to add marketplace '${marketplace}' (exit code: ${code})`,
+          ),
+        );
+      }
+    });
+
+    childProcess.on("error", (err: Error) => {
+      reject(
+        new Error(`Failed to add marketplace '${marketplace}': ${err.message}`),
+      );
+    });
+  });
 }
 
 /**
diff --git a/base-action/test/install-plugins.test.ts b/base-action/test/install-plugins.test.ts
index 7b0ab28ba..e4095a2f7 100644
--- a/base-action/test/install-plugins.test.ts
+++ b/base-action/test/install-plugins.test.ts
@@ -17,14 +17,25 @@ describe("installPlugins", () => {
   function createMockSpawn(
     exitCode: number | null = 0,
     shouldError: boolean = false,
+    output: string = "",
   ) {
+    const mockStream = {
+      on: mock((event: string, handler: Function) => {
+        if (event === "data" && output) {
+          setTimeout(() => handler(Buffer.from(output)), 0);
+        }
+        return mockStream;
+      }),
+    };
+
     const mockProcess = {
+      stdout: mockStream,
+      stderr: mockStream,
       on: mock((event: string, handler: Function) => {
         if (event === "close" && !shouldError) {
-          // Simulate successful close
-          setTimeout(() => handler(exitCode), 0);
+          // Delay past any data events so output is captured before close fires.
+          setTimeout(() => handler(exitCode), 10);
         } else if (event === "error" && shouldError) {
-          // Simulate error
           setTimeout(() => handler(new Error("spawn error")), 0);
         }
         return mockProcess;
@@ -370,7 +381,7 @@ describe("installPlugins", () => {
         "add",
         "https://github.com/user/marketplace.git",
       ],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
     // Second call: install plugin
     expect(spy).toHaveBeenNthCalledWith(
@@ -394,13 +405,13 @@ describe("installPlugins", () => {
       1,
       "claude",
       ["plugin", "marketplace", "add", "https://github.com/user/m1.git"],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
     expect(spy).toHaveBeenNthCalledWith(
       2,
       "claude",
       ["plugin", "marketplace", "add", "https://github.com/user/m2.git"],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
     // Third call: install plugin
     expect(spy).toHaveBeenNthCalledWith(
@@ -429,7 +440,7 @@ describe("installPlugins", () => {
         "add",
         "https://github.com/user/marketplace.git",
       ],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
     // Next calls: install plugins
     expect(spy).toHaveBeenNthCalledWith(
@@ -460,7 +471,7 @@ describe("installPlugins", () => {
         "add",
         "https://github.com/user/marketplace.git",
       ],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
   });
 
@@ -491,13 +502,13 @@ describe("installPlugins", () => {
         "add",
         "https://github.com/user/marketplace.git",
       ],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
     expect(spy).toHaveBeenNthCalledWith(
       2,
       "claude",
       ["plugin", "marketplace", "add", "https://github.com/user/m2.git"],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
   });
 
@@ -587,7 +598,7 @@ describe("installPlugins", () => {
         "add",
         "https://github.com/user/marketplace.git",
       ],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
     expect(spy).toHaveBeenNthCalledWith(
       2,
@@ -607,7 +618,7 @@ describe("installPlugins", () => {
       1,
       "claude",
       ["plugin", "marketplace", "add", "./my-local-marketplace"],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
     expect(spy).toHaveBeenNthCalledWith(
       2,
@@ -626,7 +637,7 @@ describe("installPlugins", () => {
       1,
       "claude",
       ["plugin", "marketplace", "add", "/home/user/my-marketplace"],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
   });
 
@@ -639,7 +650,7 @@ describe("installPlugins", () => {
       1,
       "claude",
       ["plugin", "marketplace", "add", "C:\\Users\\user\\marketplace"],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
   });
 
@@ -655,13 +666,13 @@ describe("installPlugins", () => {
       1,
       "claude",
       ["plugin", "marketplace", "add", "./local-marketplace"],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
     expect(spy).toHaveBeenNthCalledWith(
       2,
       "claude",
       ["plugin", "marketplace", "add", "https://github.com/user/remote.git"],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
   });
 
@@ -674,7 +685,7 @@ describe("installPlugins", () => {
       1,
       "claude",
       ["plugin", "marketplace", "add", "../shared-plugins/marketplace"],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
   });
 
@@ -687,7 +698,7 @@ describe("installPlugins", () => {
       1,
       "claude",
       ["plugin", "marketplace", "add", "./plugins/my-org/my-marketplace"],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
     );
   });
 
@@ -700,7 +711,33 @@ describe("installPlugins", () => {
       1,
       "claude",
       ["plugin", "marketplace", "add", "./my.plugin.marketplace"],
-      { stdio: "inherit" },
+      { stdio: ["inherit", "pipe", "pipe"] },
+    );
+  });
+
+  test("should treat 'already installed' marketplace as success (idempotent on persistent runners)", async () => {
+    // Simulate the CLI exiting 1 with an "already installed" message —
+    // the output text is what matters, not which stream it arrives on.
+    const spy = createMockSpawn(
+      1,
+      false,
+      "Marketplace 'claude-code-plugins' is already installed.",
+    );
+    await expect(
+      installPlugins(
+        "https://github.com/anthropics/claude-code.git",
+        undefined,
+      ),
+    ).resolves.toBeUndefined();
+    expect(spy).toHaveBeenCalledTimes(1);
+  });
+
+  test("should still throw for non-'already-installed' marketplace failures", async () => {
+    createMockSpawn(1, false, "Network error: could not reach remote");
+    await expect(
+      installPlugins("https://github.com/user/marketplace.git", undefined),
+    ).rejects.toThrow(
+      "Failed to add marketplace 'https://github.com/user/marketplace.git' (exit code: 1)",
     );
   });
 });