diff --git a/packages/opencode/src/mcp/index.ts b/packages/opencode/src/mcp/index.ts
index 66843aedc1..a0a3297755 100644
--- a/packages/opencode/src/mcp/index.ts
+++ b/packages/opencode/src/mcp/index.ts
@@ -795,6 +795,11 @@ export namespace MCP {
     // The SDK has already added the state parameter to the authorization URL
     // We just need to open the browser
     log.info("opening browser for oauth", { mcpName, url: authorizationUrl, state: oauthState })
+
+    // Register the callback BEFORE opening the browser to avoid race condition
+    // when the IdP has an active SSO session and redirects immediately
+    const callbackPromise = McpOAuthCallback.waitForCallback(oauthState)
+
     try {
       const subprocess = await open(authorizationUrl)
       // The open package spawns a detached process and returns immediately.
@@ -822,8 +827,8 @@ export namespace MCP {
       Bus.publish(BrowserOpenFailed, { mcpName, url: authorizationUrl })
     }
 
-    // Wait for callback using the OAuth state parameter
-    const code = await McpOAuthCallback.waitForCallback(oauthState)
+    // Wait for callback using the already-registered promise
+    const code = await callbackPromise
 
     // Validate and clear the state
     const storedState = await McpAuth.getOAuthState(mcpName)