fix(agent-switch): clear fallback markers on session.error

processedFallbackMessages was only cleaned up on session.deleted, not session.error. This could leak memory for errored sessions. Mirrors the existing session.deleted cleanup pattern.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/hooks/agent-switch/hook.test.ts

@@ -139,6 +139,54 @@ describe("agent-switch hook", () => {
     expect(getPendingSwitch("ses-4")).toBeUndefined()
   })
 
+  test("clears pending switch on session.error with info.id", async () => {
+    const ctx = {
+      client: {
+        session: {
+          promptAsync: async () => {},
+          messages: async () => ({ data: [] }),
+          message: async () => ({ data: { parts: [] } }),
+        },
+      },
+    } as any
+
+    setPendingSwitch("ses-10", "atlas", "fix this")
+    const hook = createAgentSwitchHook(ctx)
+
+    await hook.event({
+      event: {
+        type: "session.error",
+        properties: { info: { id: "ses-10" } },
+      },
+    })
+
+    expect(getPendingSwitch("ses-10")).toBeUndefined()
+  })
+
+  test("clears pending switch on session.error with sessionID property", async () => {
+    const ctx = {
+      client: {
+        session: {
+          promptAsync: async () => {},
+          messages: async () => ({ data: [] }),
+          message: async () => ({ data: { parts: [] } }),
+        },
+      },
+    } as any
+
+    setPendingSwitch("ses-11", "atlas", "fix this")
+    const hook = createAgentSwitchHook(ctx)
+
+    await hook.event({
+      event: {
+        type: "session.error",
+        properties: { sessionID: "ses-11" },
+      },
+    })
+
+    expect(getPendingSwitch("ses-11")).toBeUndefined()
+  })
+
   test("recovers missing switch_agent tool call from Athena handoff text", async () => {
     const promptAsyncCalls: Array<Record<string, unknown>> = []
     let switched = false

src/hooks/agent-switch/hook.ts

@@ -56,6 +56,21 @@ export function createAgentSwitchHook(ctx: PluginInput) {
         return
       }
 
+      if (input.event.type === "session.error") {
+        const props = input.event.properties as Record<string, unknown> | undefined
+        const info = props?.info as Record<string, unknown> | undefined
+        const erroredSessionID = info?.id ?? props?.sessionID
+        if (typeof erroredSessionID === "string") {
+          clearPendingSwitchRuntime(erroredSessionID)
+          for (const key of Array.from(processedFallbackMessages)) {
+            if (key.startsWith(`${erroredSessionID}:`)) {
+              processedFallbackMessages.delete(key)
+            }
+          }
+        }
+        return
+      }
+
       if (input.event.type === "message.updated") {
         const props = input.event.properties as Record<string, unknown> | undefined
         const info = props?.info as Record<string, unknown> | undefined