diff --git a/changelog.d/fixes/9505-atu-effort-beta-allowlist.md b/changelog.d/fixes/9505-atu-effort-beta-allowlist.md
new file mode 100644
index 0000000000..e580e1cb6b
--- /dev/null
+++ b/changelog.d/fixes/9505-atu-effort-beta-allowlist.md
@@ -0,0 +1 @@
+- fix(sse): stop force-injecting advanced-tool-use beta via the effort-2025-11-24 gate; forward client-negotiated effort through the allowlist (#9505)
diff --git a/open-sse/config/anthropicHeaders.ts b/open-sse/config/anthropicHeaders.ts
index 2edc489d1e..cf6710c4fe 100644
--- a/open-sse/config/anthropicHeaders.ts
+++ b/open-sse/config/anthropicHeaders.ts
@@ -57,6 +57,11 @@ export const FORWARDABLE_CLIENT_BETAS = Object.freeze([
   "context-1m-2025-08-07",
   "code-execution-2025-08-25",
   "skills-2025-10-02",
+  // effort-2025-11-24 is a client-negotiated beta (Claude Code sends it on every
+  // request). selectBetaFlags no longer force-adds it as a side-effect of the ATU
+  // gate (#9505), so a client that sent it must keep it through the merge —
+  // otherwise its effort negotiation is silently dropped.
+  "effort-2025-11-24",
 ]);
 
 /**
diff --git a/open-sse/executors/claudeIdentity.ts b/open-sse/executors/claudeIdentity.ts
index c9544c6743..8fed8fa597 100644
--- a/open-sse/executors/claudeIdentity.ts
+++ b/open-sse/executors/claudeIdentity.ts
@@ -357,10 +357,12 @@ export function selectBetaFlags(
   // betas it actually asked for. Opaque clients (clientBetaSet === null) keep them all.
   const allowThinking =
     clientBetaSet === null || clientBetaSet.has("interleaved-thinking-2025-05-14");
+  // effort-2025-11-24 must NOT imply advanced-tool-use-2025-11-20 (#9505): Claude
+  // Code sends effort on every request and never sends ATU, so treating effort as
+  // a proxy for ATU force-injects the heavy-agent pair the client never negotiated —
+  // the same class of mutation #3415 closed. Opaque clients keep the full set.
   const allowHeavy =
-    clientBetaSet === null ||
-    clientBetaSet.has("advanced-tool-use-2025-11-20") ||
-    clientBetaSet.has("effort-2025-11-24");
+    clientBetaSet === null || clientBetaSet.has("advanced-tool-use-2025-11-20");
   const hasSystem =
     !!b.system &&
     (typeof b.system === "string" || (Array.isArray(b.system) && b.system.length > 0));
diff --git a/tests/unit/claude-atu-effort-leak-9505.test.ts b/tests/unit/claude-atu-effort-leak-9505.test.ts
new file mode 100644
index 0000000000..f180f23217
--- /dev/null
+++ b/tests/unit/claude-atu-effort-leak-9505.test.ts
@@ -0,0 +1,74 @@
+// Regression guard for #9505 — forced advanced-tool-use beta must not survive
+// the effort-2025-11-24 gate; client-negotiated effort must survive the merge.
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { selectBetaFlags } = await import(
+  "../../open-sse/executors/claudeIdentity.ts"
+);
+const { mergeClientAnthropicBeta } = await import(
+  "../../open-sse/config/anthropicHeaders.ts"
+);
+
+function fullAgentBody(model: string) {
+  return {
+    model,
+    system: "You are a coding agent.",
+    tools: [
+      { name: "read_file", description: "x", input_schema: { type: "object" } },
+    ],
+  };
+}
+
+function pipeline(model: string, clientBeta: string | null) {
+  return mergeClientAnthropicBeta(
+    selectBetaFlags(fullAgentBody(model), null, clientBeta),
+    clientBeta
+  );
+}
+
+test("#9505 client sends effort but NOT advanced-tool-use -> ATU must NOT be forced", () => {
+  const out = pipeline("claude-opus-5", "claude-code-20250219,effort-2025-11-24");
+  assert.ok(
+    !out.split(",").includes("advanced-tool-use-2025-11-20"),
+    "must NOT force ATU when client only sent effort"
+  );
+});
+
+test("#9505 client sends effort only -> effort still survives the merge", () => {
+  const out = pipeline("claude-opus-5", "claude-code-20250219,effort-2025-11-24");
+  assert.ok(
+    out.split(",").includes("effort-2025-11-24"),
+    "client-sent effort must survive the allowlist merge"
+  );
+});
+
+test("#9505 client sends advanced-tool-use explicitly -> ATU preserved", () => {
+  const out = pipeline(
+    "claude-opus-5",
+    "claude-code-20250219,advanced-tool-use-2025-11-20"
+  );
+  assert.ok(
+    out.split(",").includes("advanced-tool-use-2025-11-20"),
+    "must keep ATU when client requested it"
+  );
+  assert.ok(
+    out.split(",").includes("effort-2025-11-24"),
+    "ATU+effort pair stays together when ATU requested"
+  );
+});
+
+test("#9505 client sends BOTH effort and ATU -> both preserved", () => {
+  const out = pipeline(
+    "claude-sonnet-5",
+    "claude-code-20250219,advanced-tool-use-2025-11-20,effort-2025-11-24"
+  );
+  assert.ok(out.split(",").includes("advanced-tool-use-2025-11-20"));
+  assert.ok(out.split(",").includes("effort-2025-11-24"));
+});
+
+test("#9505 opaque client (no clientBeta) still gets full heavy-agent set", () => {
+  const flags = selectBetaFlags(fullAgentBody("claude-opus-5"));
+  assert.ok(flags.includes("advanced-tool-use-2025-11-20"));
+  assert.ok(flags.includes("effort-2025-11-24"));
+});