diff --git a/src/components/ChatPopup.tsx b/src/components/ChatPopup.tsx
index 8a5988a0..5c39c51a 100644
--- a/src/components/ChatPopup.tsx
+++ b/src/components/ChatPopup.tsx
@@ -18,8 +18,11 @@ import {
   type ThinkingLevel,
   type ProviderReasoningConfig,
   THINKING_LEVEL_LABELS,
+  SAFE_THINKING_LEVEL,
   getProviderReasoningConfig,
   readPersistedThinkingLevel,
+  resolveWireThinkingLevel,
+  parseUnsupportedThinkingLevelError,
   PERSIST_KEY_PREFIX,
 } from '@/lib/chat-reasoning'
 
@@ -289,11 +292,13 @@ function ChatPopup({ isOpen, onClose, onOpenFull, onOpenSettingsSection, onThink
   const [errorMsg, setErrorMsg] = useState('')
   const [chatModelState, setChatModelState] = useState<ChatModelState | null>(null)
   const [switchingModel, setSwitchingModel] = useState(false)
-  // Initialised to a generic placeholder; the real value is snapped to
-  // the active provider's persisted choice (or that provider's default)
-  // by the [headerProvider] effect below as soon as chatModelState
-  // resolves.
-  const [thinkingLevel, setThinkingLevel] = useState<ThinkingLevel>('high')
+  // Initialised to the always-safe level, not a speculative `high`: the real
+  // value is snapped to the active provider's persisted choice (or that
+  // provider's default) by the [headerProvider] effect below as soon as
+  // chatModelState resolves. Starting at `off` means that if the socket
+  // connects before the catalog does, the first wire push can't offer a
+  // reasoning level a local (off-only) model would reject.
+  const [thinkingLevel, setThinkingLevel] = useState<ThinkingLevel>(SAFE_THINKING_LEVEL)
   const fileInputRef = useRef<HTMLInputElement>(null)
   const [attachments, setAttachments] = useState<{ name: string; path: string; type: string }[]>([])
 
@@ -665,20 +670,48 @@ function ChatPopup({ isOpen, onClose, onOpenFull, onOpenSettingsSection, onThink
     if (status !== 'connected') return
     const key = sessionKeyRef.current
     if (!key) return
-    const wireValue: string = effectiveThinkingLevel
+    // Never push a level the ACTIVE model doesn't support. `resolveWireThinkingLevel`
+    // clamps to the provider's config (so a stale `high` carried over from a
+    // reasoning-capable model is folded to the local model's `off`) and returns
+    // null while the provider is still unknown (catalog loading) so we hold the
+    // push rather than sending a speculative value the gateway would reject.
+    const wireLevel = resolveWireThinkingLevel(headerProvider, thinkingLevel)
+    if (wireLevel === null) return
+    const wireValue: string = wireLevel
     if (wireValue === lastSentThinkingRef.current) return
     lastSentThinkingRef.current = wireValue
     void wsRequest('sessions.patch', { key, thinkingLevel: wireValue }).catch((err: unknown) => {
       // Reset so a reconnect or next user change retries.
       lastSentThinkingRef.current = undefined
+      // The gateway itself tells us the level to fall back to when a model
+      // exposes no (or a narrower) reasoning control — e.g. local Gemma:
+      //   thinkingLevel "high" is not supported for llamacpp/... (use off)
+      // Honour that silently: snap to the suggested level and re-push it,
+      // surfacing a plain note rather than a red failure banner (a residual
+      // race, an external model change, or an API client can still reach here).
+      const message = err instanceof Error ? err.message : 'unknown error'
+      const suggested = parseUnsupportedThinkingLevelError(message)
+      if (suggested !== null) {
+        if (headerProvider) {
+          try { window.localStorage?.setItem(`${PERSIST_KEY_PREFIX}:${headerProvider}`, suggested) } catch { /* localStorage unavailable */ }
+        }
+        setThinkingLevel(prev => (prev === suggested ? prev : suggested))
+        setMessages(msgs => [...msgs, {
+          role: 'system',
+          text: `Reasoning effort isn't available for this model — using ${THINKING_LEVEL_LABELS[suggested] ?? suggested}.`,
+          timestamp: Date.now(),
+          variant: 'success',
+        }])
+        return
+      }
       setMessages(msgs => [...msgs, {
         role: 'system',
-        text: `Failed to change effort: ${err instanceof Error ? err.message : 'unknown error'}`,
+        text: `Failed to change effort: ${message}`,
         timestamp: Date.now(),
         variant: 'error',
       }])
     })
-  }, [status, effectiveThinkingLevel, wsRequest])
+  }, [status, headerProvider, thinkingLevel, wsRequest])
 
   // Snap thinkingLevel to the active provider's persisted choice (or its
   // default) whenever the active provider changes. Without this the
diff --git a/src/lib/chat-reasoning.ts b/src/lib/chat-reasoning.ts
index 01d09a33..52f711ee 100644
--- a/src/lib/chat-reasoning.ts
+++ b/src/lib/chat-reasoning.ts
@@ -28,6 +28,10 @@ export const THINKING_LEVEL_LABELS: Record<ThinkingLevel, string> = {
   adaptive: "Adaptive",
 };
 
+export function isThinkingLevel(value: unknown): value is ThinkingLevel {
+  return typeof value === "string" && value in THINKING_LEVEL_LABELS;
+}
+
 // Per-provider effort levels and defaults.
 //
 // Product decision (2026-07-23): the reasoning-effort picker is UNIFORM across
@@ -82,6 +86,56 @@ export function getProviderReasoningConfig(
   return REASONING_BY_PROVIDER[provider] ?? FALLBACK_REASONING_CONFIG;
 }
 
+// The single level that is always safe to send: every provider config includes
+// `off`, and the gateway accepts it for models that expose no reasoning control
+// at all (local llama.cpp/Gemma). Used as the wire value whenever the active
+// provider is still unknown, so the picker can never push a speculative `high`
+// at a model that would reject it.
+export const SAFE_THINKING_LEVEL: ThinkingLevel = "off";
+
+// Clamp a desired level to what the active provider actually supports, so the
+// value pushed to the gateway is never one the model will reject.
+//
+// This is the last line of defence behind the picker's own gating: the picker
+// only *offers* supported levels, but a level can still go stale across a model
+// switch (a `high` chosen on DeepSeek carried into a local Gemma session before
+// the header state has caught up). Routing every wire push through here means
+// such a stale value is silently folded to the new provider's default (`off`
+// for local Gemma) instead of reaching the gateway and erroring.
+//
+// When the provider is not yet known (`null` — the model catalog is still
+// loading), returns `null` so the caller can hold the push until it knows what
+// the active model supports, rather than guessing with the permissive fallback.
+export function resolveWireThinkingLevel(
+  provider: string | null | undefined,
+  desired: ThinkingLevel,
+): ThinkingLevel | null {
+  if (!provider) return null;
+  const cfg = getProviderReasoningConfig(provider);
+  return cfg.levels.includes(desired) ? desired : cfg.default;
+}
+
+// The gateway rejects an unsupported effort with a message that also names the
+// level it WILL accept, e.g.:
+//
+//   thinkingLevel "high" is not supported for llamacpp/gemma4-e2b-it-q4_0 (use off)
+//
+// That parenthetical is the backend telling us the model's real capability.
+// Parse it so a client that still races into this error can silently retry with
+// the level the backend itself asked for, instead of surfacing a red banner.
+// Returns null when the message isn't this specific rejection (any other
+// failure should keep its normal error handling).
+export function parseUnsupportedThinkingLevelError(
+  message: string | null | undefined,
+): ThinkingLevel | null {
+  if (typeof message !== "string") return null;
+  if (!/thinkinglevel/i.test(message) || !/not supported/i.test(message)) return null;
+  const match = /\(\s*use\s+([a-z-]+)\s*\)/i.exec(message);
+  if (!match) return null;
+  const suggested = match[1].toLowerCase();
+  return isThinkingLevel(suggested) ? suggested : null;
+}
+
 export const PERSIST_KEY_PREFIX = "clawbox:chat:thinkingLevel";
 
 export function readPersistedThinkingLevel(
diff --git a/src/lib/openclaw-config.ts b/src/lib/openclaw-config.ts
index bda39a1b..8792e6b5 100644
--- a/src/lib/openclaw-config.ts
+++ b/src/lib/openclaw-config.ts
@@ -5,6 +5,7 @@ import { execFile, spawn } from "child_process";
 import { promisify } from "util";
 import { getLlamaCppProxyBaseUrl } from "@/lib/llamacpp";
 import { readEdition } from "@/lib/edition-source";
+import { getProviderReasoningConfig, isThinkingLevel } from "@/lib/chat-reasoning";
 
 const exec = promisify(execFile);
 
@@ -389,6 +390,20 @@ export async function applyModelOverrideToAllAgentSessions(
       session.authProfileOverrideSource = source;
       session.modelProvider = update.provider;
       session.model = update.modelId;
+      // Normalise the sticky reasoning-effort override to the new model's
+      // capability. `thinkingLevel` is a per-session sticky the gateway keeps
+      // (set via `sessions.patch`); repointing the session to a model that
+      // can't honour the old level would otherwise leave e.g. a DeepSeek
+      // `high` on a local llama.cpp Gemma session, and the gateway rejects the
+      // next turn with `thinkingLevel "high" is not supported for llamacpp/…
+      // (use off)`. Only rewrite when the existing level is actually
+      // unsupported, so a compatible level (e.g. cloud→cloud) is left intact.
+      if (isThinkingLevel(session.thinkingLevel)) {
+        const reasoning = getProviderReasoningConfig(update.provider);
+        if (!reasoning.levels.includes(session.thinkingLevel)) {
+          session.thinkingLevel = reasoning.default;
+        }
+      }
       touchedInFile += 1;
     }
 
diff --git a/src/tests/unit/apply-model-override.test.ts b/src/tests/unit/apply-model-override.test.ts
index b5390242..3a2e10a8 100644
--- a/src/tests/unit/apply-model-override.test.ts
+++ b/src/tests/unit/apply-model-override.test.ts
@@ -120,6 +120,74 @@ describe("applyModelOverrideToAllAgentSessions — skipUserTagged", () => {
     expect(after.already_matching.modelOverrideSource).toBe("user");
   });
 
+  it("normalises a stale reasoning effort the new model can't honour (deepseek high -> local Gemma off)", async () => {
+    // The production bug: a session ran DeepSeek with a sticky
+    // `thinkingLevel: "high"`, then the chat picker repointed it to the
+    // local llama.cpp Gemma model (which supports `off` only). Without
+    // normalising the sticky value here, the gateway rejects the next turn
+    // with `thinkingLevel "high" is not supported for llamacpp/… (use off)`.
+    await seedSessions("main", {
+      chat: {
+        modelOverride: "deepseek-v4-flash",
+        modelProvider: "deepseek",
+        providerOverride: "deepseek",
+        modelOverrideSource: "user",
+        thinkingLevel: "high",
+      },
+    });
+
+    await applyModelOverrideToAllAgentSessions(
+      { provider: "llamacpp", modelId: "gemma4-e2b-it-q4_0" },
+      { agentsDir },
+    );
+
+    const after = await readSessions("main");
+    expect(after.chat.model).toBe("gemma4-e2b-it-q4_0");
+    expect(after.chat.modelProvider).toBe("llamacpp");
+    // The stale high is folded down to the local model's only level.
+    expect(after.chat.thinkingLevel).toBe("off");
+  });
+
+  it("leaves a reasoning effort the new provider still supports untouched (cloud -> cloud)", async () => {
+    await seedSessions("main", {
+      chat: {
+        modelOverride: "deepseek-v4-flash",
+        modelProvider: "deepseek",
+        providerOverride: "deepseek",
+        modelOverrideSource: "user",
+        thinkingLevel: "high",
+      },
+    });
+
+    await applyModelOverrideToAllAgentSessions(
+      { provider: "anthropic", modelId: "claude-sonnet-4-6" },
+      { agentsDir },
+    );
+
+    const after = await readSessions("main");
+    // anthropic honours `high`, so the sticky effort is preserved.
+    expect(after.chat.thinkingLevel).toBe("high");
+  });
+
+  it("does not inject a reasoning effort into sessions that never had one", async () => {
+    await seedSessions("main", {
+      chat: {
+        modelOverride: "deepseek-v4-flash",
+        modelProvider: "deepseek",
+        modelOverrideSource: "auto",
+        // no thinkingLevel field
+      },
+    });
+
+    await applyModelOverrideToAllAgentSessions(
+      { provider: "llamacpp", modelId: "gemma4-e2b-it-q4_0" },
+      { agentsDir },
+    );
+
+    const after = await readSessions("main");
+    expect("thinkingLevel" in after.chat).toBe(false);
+  });
+
   it("ignores sessions files that fail to parse instead of bailing the sweep", async () => {
     await seedSessions("main", {
       ok: { modelOverride: "old", modelOverrideSource: "auto" },
diff --git a/src/tests/unit/chat-reasoning.test.ts b/src/tests/unit/chat-reasoning.test.ts
index 7dccc5db..d360d811 100644
--- a/src/tests/unit/chat-reasoning.test.ts
+++ b/src/tests/unit/chat-reasoning.test.ts
@@ -2,9 +2,14 @@ import { describe, expect, it } from "vitest";
 import {
   getProviderReasoningConfig,
   readPersistedThinkingLevel,
+  resolveWireThinkingLevel,
+  parseUnsupportedThinkingLevelError,
+  isThinkingLevel,
   REASONING_BY_PROVIDER,
   FALLBACK_REASONING_CONFIG,
   THINKING_LEVEL_LABELS,
+  SAFE_THINKING_LEVEL,
+  type ThinkingLevel,
 } from "@/lib/chat-reasoning";
 
 describe("chat-reasoning", () => {
@@ -64,4 +69,108 @@ describe("chat-reasoning", () => {
       expect(readPersistedThinkingLevel(null, cfg)).toBe(cfg.default);
     });
   });
+
+  describe("SAFE_THINKING_LEVEL", () => {
+    it("is `off` and is a member of every provider config's levels", () => {
+      expect(SAFE_THINKING_LEVEL).toBe("off");
+      for (const cfg of Object.values(REASONING_BY_PROVIDER)) {
+        expect(cfg.levels).toContain(SAFE_THINKING_LEVEL);
+      }
+      expect(FALLBACK_REASONING_CONFIG.levels).toContain(SAFE_THINKING_LEVEL);
+    });
+  });
+
+  describe("resolveWireThinkingLevel", () => {
+    it("passes a supported level through unchanged", () => {
+      expect(resolveWireThinkingLevel("anthropic", "high")).toBe("high");
+      expect(resolveWireThinkingLevel("clawai", "off")).toBe("off");
+    });
+
+    it("clamps an unsupported level to the provider default", () => {
+      // Local Gemma (llamacpp) supports `off` only.
+      expect(resolveWireThinkingLevel("llamacpp", "high")).toBe("off");
+      expect(resolveWireThinkingLevel("llamacpp", "medium")).toBe("off");
+    });
+
+    it("holds (returns null) while the active provider is still unknown", () => {
+      // Catalog still loading: the caller must not push a speculative value.
+      expect(resolveWireThinkingLevel(null, "high")).toBeNull();
+      expect(resolveWireThinkingLevel(undefined, "high")).toBeNull();
+      expect(resolveWireThinkingLevel("", "high")).toBeNull();
+    });
+
+    // The exact production bug: a `high` chosen while a reasoning-capable
+    // remote model (DeepSeek → normalized `clawai`) was active, then the user
+    // switches the picker to the local llama.cpp Gemma model. The stale `high`
+    // must never reach the gateway for the local model.
+    it("folds a stale `high` to `off` across a remote→local model switch", () => {
+      // Before the switch: clawai honours `high`.
+      expect(resolveWireThinkingLevel("clawai", "high")).toBe("high");
+
+      // After the switch to local Gemma, both the state-snap path and the
+      // wire-clamp path independently yield `off` — so even if the header
+      // state lags a render behind, the wire value is already safe.
+      const localCfg = getProviderReasoningConfig("llamacpp");
+      const snapped = readPersistedThinkingLevel("llamacpp", localCfg); // no persisted choice
+      expect(snapped).toBe("off");
+      expect(resolveWireThinkingLevel("llamacpp", "high")).toBe("off");
+    });
+
+    it("never returns a level the resolved provider rejects", () => {
+      const providers = [
+        "openai", "codex", "anthropic", "google",
+        "deepseek", "clawai", "openrouter", "llamacpp",
+      ];
+      const desired: ThinkingLevel[] = ["off", "minimal", "low", "medium", "high", "xhigh", "max", "adaptive"];
+      for (const provider of providers) {
+        const cfg = getProviderReasoningConfig(provider);
+        for (const level of desired) {
+          const resolved = resolveWireThinkingLevel(provider, level);
+          expect(resolved).not.toBeNull();
+          expect(cfg.levels).toContain(resolved as ThinkingLevel);
+        }
+      }
+    });
+  });
+
+  describe("parseUnsupportedThinkingLevelError", () => {
+    it("parses the gateway's `(use off)` hint from the real rejection message", () => {
+      const msg = 'thinkingLevel "high" is not supported for llamacpp/gemma4-e2b-it-q4_0 (use off)';
+      expect(parseUnsupportedThinkingLevelError(msg)).toBe("off");
+    });
+
+    it("honours whatever level the gateway suggests, not just off", () => {
+      const msg = 'thinkingLevel "xhigh" is not supported for some/model (use low)';
+      expect(parseUnsupportedThinkingLevelError(msg)).toBe("low");
+    });
+
+    it("is tolerant of casing and spacing around the hint", () => {
+      expect(
+        parseUnsupportedThinkingLevelError('ThinkingLevel "high" is NOT SUPPORTED for x/y ( use  off )'),
+      ).toBe("off");
+    });
+
+    it("returns null for unrelated errors so they keep normal handling", () => {
+      expect(parseUnsupportedThinkingLevelError("Request timeout")).toBeNull();
+      expect(parseUnsupportedThinkingLevelError("session not found")).toBeNull();
+      // Right shape but an unknown suggested level → don't invent one.
+      expect(
+        parseUnsupportedThinkingLevelError('thinkingLevel "high" is not supported for x/y (use bogus)'),
+      ).toBeNull();
+      expect(parseUnsupportedThinkingLevelError(null)).toBeNull();
+      expect(parseUnsupportedThinkingLevelError(undefined)).toBeNull();
+    });
+  });
+
+  describe("isThinkingLevel", () => {
+    it("accepts every labelled level and rejects anything else", () => {
+      for (const level of Object.keys(THINKING_LEVEL_LABELS)) {
+        expect(isThinkingLevel(level)).toBe(true);
+      }
+      expect(isThinkingLevel("ultra")).toBe(false);
+      expect(isThinkingLevel("")).toBe(false);
+      expect(isThinkingLevel(null)).toBe(false);
+      expect(isThinkingLevel(3)).toBe(false);
+    });
+  });
 });