fix: prune expired model cooldowns from session state

src/plugin.ts

@@ -276,6 +276,12 @@ function isEnabled(config: Config, state: SessionState | undefined): boolean {
   return state?.enabledOverride ?? config.enabled
 }
 
+function pruneExpiredCooldowns(state: SessionState, now: number): void {
+  for (const [model, until] of state.failedUntil) {
+    if (until <= now) state.failedUntil.delete(model)
+  }
+}
+
 function nextModel(config: Config, state: SessionState): string | undefined {
   const now = Date.now()
   return config.fallbackModels.find((model) => {
@@ -441,10 +447,13 @@ export function createModelFallbackPlugin(input: RuntimeInput, options: Options
       if (state.awaitingModel && eventModel && eventModel !== state.awaitingModel) return
       state.awaitingModel = undefined
 
+      const now = Date.now()
+      pruneExpiredCooldowns(state, now)
+
       const failedModel = eventModel ?? state.currentModel
       if (failedModel) {
         state.currentModel = failedModel
-        state.failedUntil.set(failedModel, Date.now() + config.cooldownMs)
+        state.failedUntil.set(failedModel, now + config.cooldownMs)
       }
 
       const model = nextModel(config, state)

test/index.test.ts

@@ -492,6 +492,32 @@ describe("model fallback plugin", () => {
     expect(prompts).toHaveLength(2)
   })
 
+  test("#given a model cooldown has expired #when a later error fires #then the model is reusable and stale cooldowns are pruned", async () => {
+    const { runtime, prompts } = createRuntime()
+    const plugin = createModelFallbackPlugin(runtime, {
+      fallback_models: ["openai/gpt-5.1-codex", "anthropic/claude-sonnet"],
+      max_attempts: 10,
+      cooldown_ms: 1,
+    })
+    await emitSessionCreated(plugin)
+
+    // opus fails -> switch to codex
+    await emitSessionError(plugin, { providerID: "anthropic", modelID: "claude-opus" })
+    expect(prompts[0]?.body.model).toEqual({ providerID: "openai", modelID: "gpt-5.1-codex" })
+
+    // codex fails -> switch to sonnet
+    await emitSessionError(plugin, { providerID: "openai", modelID: "gpt-5.1-codex" })
+    expect(prompts[1]?.body.model).toEqual({ providerID: "anthropic", modelID: "claude-sonnet" })
+
+    // wait for the 1ms cooldowns to expire so codex is selectable again
+    await new Promise((resolve) => setTimeout(resolve, 5))
+
+    // sonnet fails -> codex cooldown expired, so it is reused
+    await emitSessionError(plugin, { providerID: "anthropic", modelID: "claude-sonnet" })
+    expect(prompts).toHaveLength(3)
+    expect(prompts[2]?.body.model).toEqual({ providerID: "openai", modelID: "gpt-5.1-codex" })
+  })
+
   test("#given first fallback is also unavailable #when retrying a failed model #then it skips to the next fallback", async () => {
     const { runtime, prompts } = createRuntime()
     const plugin = createModelFallbackPlugin(runtime, {