diff --git a/src/reasoning-effort.ts b/src/reasoning-effort.ts
index ff168f4b7d..81b35b7af5 100644
--- a/src/reasoning-effort.ts
+++ b/src/reasoning-effort.ts
@@ -14,6 +14,16 @@ export const CODEX_REASONING_LEVELS: { effort: string; description: string }[] =
 const CODEX_REASONING_ORDER = CODEX_REASONING_LEVELS.map(l => l.effort);
 const CODEX_REASONING_SET = new Set(CODEX_REASONING_ORDER);
 
+/**
+ * Sentinel wire value in reasoningEffortMap / modelReasoningEffortMap to explicitly
+ * omit the reasoning_effort field from the upstream wire request (issue #2356).
+ */
+export const REASONING_EFFORT_OMIT_SENTINEL = "__omit__";
+
+export function isReasoningEffortOmitted(wireEffort: string | undefined): boolean {
+  return wireEffort === REASONING_EFFORT_OMIT_SENTINEL;
+}
+
 /** True when `effort` is a member of the Codex reasoning ladder (low..ultra). */
 export function isCodexReasoningEffort(effort: string): boolean {
   return CODEX_REASONING_SET.has(effort);
@@ -170,7 +180,10 @@ export function mapReasoningEffort(provider: OcxProviderConfig, modelId: string,
   const boundary = requested === "ultra" ? "max" : requested;
 
   const wireMap = reasoningEffortMapFor(provider, modelId);
-  if (wireMap && Object.prototype.hasOwnProperty.call(wireMap, boundary)) return wireMap[boundary];
+  if (wireMap && Object.prototype.hasOwnProperty.call(wireMap, boundary)) {
+    const mapped = wireMap[boundary];
+    return mapped === REASONING_EFFORT_OMIT_SENTINEL ? undefined : mapped;
+  }
 
   const supported = configuredReasoningEfforts(provider, modelId);
   const codexEffort = supported !== undefined ? clampToSupportedCodexEffort(boundary, supported) : requestToCodexEffort(boundary);
@@ -178,6 +191,10 @@ export function mapReasoningEffort(provider: OcxProviderConfig, modelId: string,
 
   // Belt for the odd config where the supported ladder is ultra-only and the clamp lands on it.
   const wire = codexEffort === "ultra" ? "max" : codexEffort;
-  if (wireMap && Object.prototype.hasOwnProperty.call(wireMap, wire)) return wireMap[wire];
+  if (wireMap && Object.prototype.hasOwnProperty.call(wireMap, wire)) {
+    const mapped = wireMap[wire];
+    return mapped === REASONING_EFFORT_OMIT_SENTINEL ? undefined : mapped;
+  }
+  if (wire === REASONING_EFFORT_OMIT_SENTINEL) return undefined;
   return wire;
 }
diff --git a/tests/reasoning-effort.test.ts b/tests/reasoning-effort.test.ts
index 269da1a81a..cf34b8070b 100644
--- a/tests/reasoning-effort.test.ts
+++ b/tests/reasoning-effort.test.ts
@@ -973,4 +973,66 @@ describe("stale reasoning-ladder self-heal", () => {
     };
     expect(configuredReasoningEfforts(prov, "model")).toEqual([]);
   });
+
+  test("per-effort omission sentinel (__omit__) drops reasoning_effort from the wire (#2356)", () => {
+    const ollamaProv: OcxProviderConfig = {
+      adapter: "openai-chat",
+      baseUrl: "http://localhost:11434/v1",
+      modelReasoningEfforts: {
+        "qwen3.8-uncensored:27b-q4": ["low", "medium", "high", "xhigh", "max"],
+      },
+      modelReasoningEffortMap: {
+        "qwen3.8-uncensored:27b-q4": {
+          low: "low",
+          medium: "medium",
+          high: "__omit__",
+          xhigh: "__omit__",
+          max: "__omit__",
+        },
+      },
+    };
+
+    // High/xhigh/max/ultra map to undefined (omitted on wire)
+    expect(mapReasoningEffort(ollamaProv, "qwen3.8-uncensored:27b-q4", "high")).toBeUndefined();
+    expect(mapReasoningEffort(ollamaProv, "qwen3.8-uncensored:27b-q4", "xhigh")).toBeUndefined();
+    expect(mapReasoningEffort(ollamaProv, "qwen3.8-uncensored:27b-q4", "max")).toBeUndefined();
+    expect(mapReasoningEffort(ollamaProv, "qwen3.8-uncensored:27b-q4", "ultra")).toBeUndefined();
+
+    // Low and medium map to explicit wire values
+    expect(mapReasoningEffort(ollamaProv, "qwen3.8-uncensored:27b-q4", "low")).toBe("low");
+    expect(mapReasoningEffort(ollamaProv, "qwen3.8-uncensored:27b-q4", "medium")).toBe("medium");
+
+    const fallbackProv: OcxProviderConfig = {
+      ...ollamaProv,
+      modelReasoningEfforts: {
+        "qwen3.8-uncensored:27b-q4": ["low", "high"],
+      },
+      modelReasoningEffortMap: {
+        "qwen3.8-uncensored:27b-q4": { high: "__omit__" },
+      },
+    };
+    expect(mapReasoningEffort(fallbackProv, "qwen3.8-uncensored:27b-q4", "xhigh")).toBeUndefined();
+
+    // Verify in openai-chat adapter buildRequest: field is completely omitted when mapped to __omit__
+    const adapter = createOpenAIChatAdapter(ollamaProv);
+    const reqMax = adapter.buildRequest({
+      modelId: "qwen3.8-uncensored:27b-q4",
+      stream: false,
+      context: { messages: [{ role: "user", content: "deep thinking" }] },
+      options: { reasoning: "max" },
+    } as OcxParsedRequest);
+    const bodyMax = JSON.parse(reqMax.body as string);
+    expect(bodyMax.reasoning_effort).toBeUndefined();
+    expect(bodyMax).not.toHaveProperty("reasoning_effort");
+
+    // Field is present when mapped to a real string
+    const reqLow = adapter.buildRequest({
+      modelId: "qwen3.8-uncensored:27b-q4",
+      stream: false,
+      context: { messages: [{ role: "user", content: "fast turn" }] },
+      options: { reasoning: "low" },
+    } as OcxParsedRequest);
+    const bodyLow = JSON.parse(reqLow.body as string);
+    expect(bodyLow.reasoning_effort).toBe("low");
+  });
 });