diff --git a/src/hooks/anthropic-effort/hook.ts b/src/hooks/anthropic-effort/hook.ts
index 76fba5245..fb5a9a209 100644
--- a/src/hooks/anthropic-effort/hook.ts
+++ b/src/hooks/anthropic-effort/hook.ts
@@ -1,6 +1,7 @@
 import { log, normalizeModelID } from "../../shared"
 
 const OPUS_PATTERN = /claude-.*opus/i
+const EFFORT_UNSUPPORTED_PATTERN = /claude-.*haiku/i
 const INTERNAL_SKIP_AGENTS = new Set(["title", "summary", "compaction"])
 
 function isClaudeProvider(providerID: string, modelID: string): boolean {
@@ -14,6 +15,11 @@ function isOpusModel(modelID: string): boolean {
   return OPUS_PATTERN.test(normalized)
 }
 
+function isEffortUnsupportedModel(modelID: string): boolean {
+  const normalized = normalizeModelID(modelID)
+  return EFFORT_UNSUPPORTED_PATTERN.test(normalized)
+}
+
 function shouldSkipForInternalAgent(agentName: string | undefined): boolean {
   if (!agentName) return false
   return INTERNAL_SKIP_AGENTS.has(agentName.trim().toLowerCase())
@@ -60,6 +66,7 @@ export function createAnthropicEffortHook() {
       if (!isClaudeProvider(model.providerID, model.modelID)) return
       if (shouldSkipForInternalAgent(agent?.name)) return
       if (output.options.effort !== undefined) return
+      if (isEffortUnsupportedModel(model.modelID)) return
 
       const opus = isOpusModel(model.modelID)
       const clamped = clampVariant(message.variant, opus)
diff --git a/src/hooks/anthropic-effort/index.test.ts b/src/hooks/anthropic-effort/index.test.ts
index 056ff0a28..5ca923cd2 100644
--- a/src/hooks/anthropic-effort/index.test.ts
+++ b/src/hooks/anthropic-effort/index.test.ts
@@ -147,6 +147,30 @@ describe("createAnthropicEffortHook", () => {
 
       expect(output.options.effort).toBeUndefined()
     })
+
+    describe("#given haiku models (effort unsupported)", () => {
+      const haikuModels = [
+        "claude-haiku-4-5",
+        "claude-haiku-4.6",
+        "claude-haiku",
+        "claude-haiku-20240307",
+      ]
+
+      for (const modelID of haikuModels) {
+        it(`skips effort injection for ${modelID}`, async () => {
+          // given
+          const hook = createAnthropicEffortHook()
+          const { input, output } = createMockParams({ modelID })
+
+          // when
+          await hook["chat.params"](input, output)
+
+          // then
+          expect(output.options.effort).toBeUndefined()
+          expect(input.message.variant).toBe("max")
+        })
+      }
+    })
   })
 
   describe("existing options", () => {