diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 0544a90db3..4f95dee1d0 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -70,11 +70,8 @@ import {
 import { createRequestLogger } from "../utils/requestLogger.ts";
 import { createPreparedRequestLogger, runWithCapture } from "../utils/providerRequestLogging.ts";
 import { applyResponsesPreviousResponseIdPolicy } from "../utils/responsesStatePolicy.ts";
-import {
-  getModelTargetFormat,
-  PROVIDER_ID_TO_ALIAS,
-  splitClaudeEffortSuffix,
-} from "../config/providerModels.ts";
+import { getModelTargetFormat, PROVIDER_ID_TO_ALIAS } from "../config/providerModels.ts";
+import { applyClaudeEffortVariant } from "./chatCore/claudeEffortVariant.ts";
 import { DEFAULT_THINKING_CLAUDE_SIGNATURE } from "../config/defaultThinkingSignature.ts";
 import {
   getStripTypesForProviderModel,
@@ -954,30 +951,18 @@ export async function handleChatCore({
   // it into Claude thinking/effort config. An explicit client-supplied effort always
   // wins; native Claude passthrough is left untouched (it carries its own `thinking`),
   // and non-thinking base models are cleaned up later by normalizeThinkingForModel().
-  if (
-    (provider === "claude" || isClaudeCodeCompatibleProvider(provider)) &&
-    typeof effectiveModel === "string"
-  ) {
-    const { baseModel, effort } = splitClaudeEffortSuffix(effectiveModel);
-    if (effort) {
-      effectiveModel = baseModel;
-      if (body && typeof body === "object" && !Array.isArray(body)) {
-        const claudeBody = body as Record<string, unknown>;
-        claudeBody.model = baseModel;
-        if (sourceFormat !== FORMATS.CLAUDE) {
-          const explicitEffort =
-            claudeBody.reasoning_effort ??
-            (claudeBody.reasoning as Record<string, unknown> | undefined)?.effort ??
-            (claudeBody.output_config as Record<string, unknown> | undefined)?.effort;
-          if (explicitEffort === undefined || explicitEffort === null || explicitEffort === "") {
-            claudeBody.reasoning_effort = effort;
-          }
-        }
-      }
-      log?.info?.(
-        "PARAMS",
-        `Claude effort variant: stripped "-${effort}" → ${baseModel} (reasoning_effort=${effort})`
-      );
+  // Extracted to chatCore/claudeEffortVariant.ts (#3501); mutates body in place and returns the
+  // stripped model + an optional log line, keeping behaviour byte-identical.
+  {
+    const effortVariant = applyClaudeEffortVariant({
+      provider,
+      effectiveModel,
+      body,
+      sourceFormat,
+    });
+    effectiveModel = effortVariant.effectiveModel;
+    if (effortVariant.log) {
+      log?.info?.("PARAMS", effortVariant.log);
     }
   }
 
diff --git a/open-sse/handlers/chatCore/claudeEffortVariant.ts b/open-sse/handlers/chatCore/claudeEffortVariant.ts
new file mode 100644
index 0000000000..e2a2a8e188
--- /dev/null
+++ b/open-sse/handlers/chatCore/claudeEffortVariant.ts
@@ -0,0 +1,62 @@
+/**
+ * chatCore Claude effort-variant normalizer (Quality Gate v2 / Fase 9 — chatCore god-file
+ * decomposition, #3501).
+ *
+ * The Claude / Claude-Code model picker (e.g. VS Code's "Effort" slider) advertises
+ * claude-...-{low,medium,high,xhigh,max}. Anthropic has no such model, so the suffixed id 404s
+ * upstream. This strips it back to the real base id and surfaces the level as reasoning_effort so
+ * the OpenAI→Claude translator / Claude-Code bridge can turn it into Claude thinking/effort config.
+ * An explicit client-supplied effort always wins; native Claude passthrough (sourceFormat === claude)
+ * is left untouched (it carries its own `thinking`). The body is mutated in place (model +
+ * reasoning_effort), byte-identical to the previous inline block; the new effectiveModel and an
+ * optional log line are returned for the handler to apply.
+ */
+
+import { splitClaudeEffortSuffix } from "../../config/providerModels.ts";
+import { isClaudeCodeCompatibleProvider } from "../../services/claudeCodeCompatible.ts";
+import { FORMATS } from "../../translator/formats.ts";
+
+/**
+ * True when the client already supplied an explicit reasoning effort (top-level reasoning_effort,
+ * reasoning.effort, or output_config.effort) — in which case the stripped suffix must not overwrite
+ * it. A blank string counts as "not supplied".
+ */
+function hasExplicitClaudeEffort(claudeBody: Record<string, unknown>): boolean {
+  const explicitEffort =
+    claudeBody.reasoning_effort ??
+    (claudeBody.reasoning as Record<string, unknown> | undefined)?.effort ??
+    (claudeBody.output_config as Record<string, unknown> | undefined)?.effort;
+  return !(explicitEffort === undefined || explicitEffort === null || explicitEffort === "");
+}
+
+export function applyClaudeEffortVariant(opts: {
+  provider: string | null | undefined;
+  effectiveModel: string;
+  /** Mutated in place (model + reasoning_effort) when an effort suffix is stripped. */
+  body: unknown;
+  sourceFormat: string;
+}): { effectiveModel: string; log: string | null } {
+  const { provider, body, sourceFormat } = opts;
+  let effectiveModel = opts.effectiveModel;
+  let log: string | null = null;
+
+  if (
+    (provider === "claude" || isClaudeCodeCompatibleProvider(provider)) &&
+    typeof effectiveModel === "string"
+  ) {
+    const { baseModel, effort } = splitClaudeEffortSuffix(effectiveModel);
+    if (effort) {
+      effectiveModel = baseModel;
+      if (body && typeof body === "object" && !Array.isArray(body)) {
+        const claudeBody = body as Record<string, unknown>;
+        claudeBody.model = baseModel;
+        if (sourceFormat !== FORMATS.CLAUDE && !hasExplicitClaudeEffort(claudeBody)) {
+          claudeBody.reasoning_effort = effort;
+        }
+      }
+      log = `Claude effort variant: stripped "-${effort}" → ${baseModel} (reasoning_effort=${effort})`;
+    }
+  }
+
+  return { effectiveModel, log };
+}
diff --git a/tests/unit/chatcore-claude-effort-variant.test.ts b/tests/unit/chatcore-claude-effort-variant.test.ts
new file mode 100644
index 0000000000..03bf40a4c1
--- /dev/null
+++ b/tests/unit/chatcore-claude-effort-variant.test.ts
@@ -0,0 +1,107 @@
+// tests/unit/chatcore-claude-effort-variant.test.ts
+// Characterization of applyClaudeEffortVariant — the Claude effort-suffix normalization extracted
+// from handleChatCore (chatCore god-file decomposition, #3501). The VS Code "Effort" slider
+// advertises claude-...-{low,medium,high,xhigh,max}; Anthropic has no such model, so the suffix is
+// stripped to the base id and surfaced as reasoning_effort. Locks: the provider gate (claude /
+// claude-code-compatible only), the in-place body mutation (model + reasoning_effort), the
+// sourceFormat==="claude" skip, the explicit-effort-wins rule, and the returned effectiveModel/log.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+import { applyClaudeEffortVariant } from "../../open-sse/handlers/chatCore/claudeEffortVariant.ts";
+import { FORMATS } from "../../open-sse/translator/formats.ts";
+
+test("claude provider + effort suffix → strips to base, mutates body model + reasoning_effort, returns log", () => {
+  const body: Record<string, unknown> = { model: "claude-sonnet-4-high", messages: [] };
+  const r = applyClaudeEffortVariant({
+    provider: "claude",
+    effectiveModel: "claude-sonnet-4-high",
+    body,
+    sourceFormat: FORMATS.OPENAI,
+  });
+  assert.equal(r.effectiveModel, "claude-sonnet-4");
+  assert.equal(body.model, "claude-sonnet-4");
+  assert.equal(body.reasoning_effort, "high");
+  assert.match(String(r.log), /stripped "-high" → claude-sonnet-4 \(reasoning_effort=high\)/);
+});
+
+test("claude-code-compatible provider triggers the same stripping", () => {
+  const body: Record<string, unknown> = { model: "claude-opus-4-xhigh", messages: [] };
+  const r = applyClaudeEffortVariant({
+    provider: "anthropic-compatible-cc-default",
+    effectiveModel: "claude-opus-4-xhigh",
+    body,
+    sourceFormat: FORMATS.OPENAI,
+  });
+  assert.equal(r.effectiveModel, "claude-opus-4");
+  assert.equal(body.model, "claude-opus-4");
+  assert.equal(body.reasoning_effort, "xhigh");
+});
+
+test("sourceFormat 'claude' strips the model but does NOT inject reasoning_effort", () => {
+  const body: Record<string, unknown> = { model: "claude-sonnet-4-medium", messages: [] };
+  const r = applyClaudeEffortVariant({
+    provider: "claude",
+    effectiveModel: "claude-sonnet-4-medium",
+    body,
+    sourceFormat: FORMATS.CLAUDE,
+  });
+  assert.equal(r.effectiveModel, "claude-sonnet-4");
+  assert.equal(body.model, "claude-sonnet-4");
+  assert.equal(body.reasoning_effort, undefined);
+});
+
+test("an explicit client reasoning_effort wins (not overwritten)", () => {
+  const body: Record<string, unknown> = { model: "claude-sonnet-4-low", reasoning_effort: "high", messages: [] };
+  const r = applyClaudeEffortVariant({
+    provider: "claude",
+    effectiveModel: "claude-sonnet-4-low",
+    body,
+    sourceFormat: FORMATS.OPENAI,
+  });
+  assert.equal(r.effectiveModel, "claude-sonnet-4");
+  assert.equal(body.reasoning_effort, "high"); // unchanged
+});
+
+test("explicit effort nested under reasoning.effort also wins", () => {
+  const body: Record<string, unknown> = {
+    model: "claude-sonnet-4-low",
+    reasoning: { effort: "medium" },
+    messages: [],
+  };
+  const r = applyClaudeEffortVariant({
+    provider: "claude",
+    effectiveModel: "claude-sonnet-4-low",
+    body,
+    sourceFormat: FORMATS.OPENAI,
+  });
+  assert.equal(body.reasoning_effort, undefined); // explicit reasoning.effort present → no injection
+  assert.equal(r.effectiveModel, "claude-sonnet-4");
+});
+
+test("no effort suffix → no change, no log", () => {
+  const body: Record<string, unknown> = { model: "claude-sonnet-4", messages: [] };
+  const r = applyClaudeEffortVariant({
+    provider: "claude",
+    effectiveModel: "claude-sonnet-4",
+    body,
+    sourceFormat: FORMATS.OPENAI,
+  });
+  assert.equal(r.effectiveModel, "claude-sonnet-4");
+  assert.equal(body.model, "claude-sonnet-4");
+  assert.equal(body.reasoning_effort, undefined);
+  assert.equal(r.log, null);
+});
+
+test("non-claude provider is a no-op even with an effort suffix", () => {
+  const body: Record<string, unknown> = { model: "gpt-5-high", messages: [] };
+  const r = applyClaudeEffortVariant({
+    provider: "openai",
+    effectiveModel: "gpt-5-high",
+    body,
+    sourceFormat: FORMATS.OPENAI,
+  });
+  assert.equal(r.effectiveModel, "gpt-5-high");
+  assert.equal(body.model, "gpt-5-high");
+  assert.equal(body.reasoning_effort, undefined);
+  assert.equal(r.log, null);
+});