diff --git a/src/features/claude-code-session-state/state.test.ts b/src/features/claude-code-session-state/state.test.ts
index 351211409..459dfa7a2 100644
--- a/src/features/claude-code-session-state/state.test.ts
+++ b/src/features/claude-code-session-state/state.test.ts
@@ -123,4 +123,40 @@ describe("claude-code-session-state", () => {
       expect(getSessionAgent(sessionID)).toBeUndefined()
     })
   })
+
+  describe("issue #893: custom agent switch reset", () => {
+    test("should preserve custom agent when default agent is sent on subsequent messages", () => {
+      // #given - user switches to custom agent "MyCustomAgent"
+      const sessionID = "test-session-custom"
+      const customAgent = "MyCustomAgent"
+      const defaultAgent = "Sisyphus"
+
+      // User switches to custom agent (via UI)
+      setSessionAgent(sessionID, customAgent)
+      expect(getSessionAgent(sessionID)).toBe(customAgent)
+
+      // #when - first message after switch sends default agent
+      // This simulates the bug: input.agent = "Sisyphus" on first message
+      // Using setSessionAgent (first-write wins) should preserve custom agent
+      setSessionAgent(sessionID, defaultAgent)
+
+      // #then - custom agent should be preserved, NOT overwritten
+      expect(getSessionAgent(sessionID)).toBe(customAgent)
+    })
+
+    test("should allow explicit agent update via updateSessionAgent", () => {
+      // #given - custom agent is set
+      const sessionID = "test-session-explicit"
+      const customAgent = "MyCustomAgent"
+      const newAgent = "AnotherAgent"
+
+      setSessionAgent(sessionID, customAgent)
+
+      // #when - explicit update (user intentionally switches)
+      updateSessionAgent(sessionID, newAgent)
+
+      // #then - should be updated
+      expect(getSessionAgent(sessionID)).toBe(newAgent)
+    })
+  })
 })
diff --git a/src/index.ts b/src/index.ts
index 4942fc46e..f155c19d7 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -314,7 +314,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
 
     "chat.message": async (input, output) => {
       if (input.agent) {
-        updateSessionAgent(input.sessionID, input.agent);
+        setSessionAgent(input.sessionID, input.agent);
       }
 
       const message = (output as { message: { variant?: string } }).message