diff --git a/changelog.d/features/6879-default-reasoning-effort.md b/changelog.d/features/6879-default-reasoning-effort.md
new file mode 100644
index 0000000000..ff62a749d6
--- /dev/null
+++ b/changelog.d/features/6879-default-reasoning-effort.md
@@ -0,0 +1 @@
+- feat(sse): per-model default `reasoning_effort` (`ModelSpec.defaultReasoningEffort`, injected only when the request carries no reasoning field) and make `no-think/` express `reasoning_effort:"none"` instead of deleting the field on the OpenAI path, so thinks-by-default models actually stop thinking (#6879)
diff --git a/open-sse/handlers/chatCore.ts b/open-sse/handlers/chatCore.ts
index 53c4218cf6..83a3eb7152 100644
--- a/open-sse/handlers/chatCore.ts
+++ b/open-sse/handlers/chatCore.ts
@@ -109,6 +109,7 @@ import { resolveModelAlias } from "../services/modelDeprecation.ts";
 import { normalizeMimoThinking } from "../services/mimoThinking.ts";
 import { normalizeClaudeAdaptiveThinking } from "../services/claudeAdaptiveThinking.ts";
 import { normalizeClaudeHaikuConstraints } from "../services/claudeHaikuConstraints.ts";
+import { applyDefaultReasoningEffort } from "../services/defaultReasoningEffort.ts";
 import { echoModelInObject } from "../services/responseModelEcho.ts";
 import {
   stripGpt5SamplingWhenReasoning,
@@ -2044,6 +2045,14 @@ export async function handleChatCore({
     // model substitution. Mirrors upstream 9router 401d93bd5. See
     // services/claudeHaikuConstraints.ts.
     translatedBody = normalizeClaudeHaikuConstraints(translatedBody, finalModelToUpstream);
+    // #6879: per-model default reasoning_effort, injected only when the request
+    // carries no reasoning field of any shape — an explicit client/combo-leg value
+    // always wins. Scoped to the OpenAI Chat Completions dispatch shape (the shape
+    // `reasoning_effort` is native to); unset ModelSpec.defaultReasoningEffort is a
+    // no-op. See open-sse/services/defaultReasoningEffort.ts.
+    if (targetFormat === FORMATS.OPENAI) {
+      translatedBody = applyDefaultReasoningEffort(translatedBody, finalModelToUpstream);
+    }
   }
 
   // Xiaomi MiMo controls reasoning ONLY via `thinking:{type:"enabled"|"disabled"}` and
diff --git a/open-sse/services/defaultReasoningEffort.ts b/open-sse/services/defaultReasoningEffort.ts
new file mode 100644
index 0000000000..75ab833bbb
--- /dev/null
+++ b/open-sse/services/defaultReasoningEffort.ts
@@ -0,0 +1,42 @@
+// Per-model default reasoning effort (#6879, "Ask 1"). Many models think by
+// default with no client-visible way to turn it off (measured:
+// gemini-flash-lite-latest burns ~277 reasoning tokens on a plain request with
+// no reasoning params). ModelSpec.defaultReasoningEffort lets an operator
+// configure a strip-by-default (or steer-by-default) value fleet-wide without
+// patching every client.
+//
+// Semantics: applied ONLY when the request carries no reasoning field of any
+// shape (`reasoning_effort`, `reasoning`, `thinking`) — an explicit client
+// value, including one forwarded verbatim through a combo leg, always wins
+// and this is a no-op. Models without a configured default are untouched
+// (regression-safe). Wired at the OpenAI-format dispatch chokepoint in
+// chatCore.ts, after model resolution, so the *upstream* model's default is
+// used even when a combo/route substituted it.
+import { getModelSpec } from "@/shared/constants/modelSpecs.ts";
+
+/** True when `body` already expresses a reasoning-effort choice, in any known shape. */
+function hasExplicitReasoningField(body: Record<string, unknown>): boolean {
+  return (
+    body.reasoning_effort !== undefined ||
+    body.reasoning !== undefined ||
+    body.thinking !== undefined
+  );
+}
+
+/**
+ * Inject the resolved model's `defaultReasoningEffort` as `reasoning_effort` when the
+ * request has no reasoning field. Returns `body` unchanged (same reference) when there
+ * is nothing to inject, so callers can chain it without extra guards.
+ */
+export function applyDefaultReasoningEffort<T extends Record<string, unknown>>(
+  body: T,
+  modelId: string
+): T {
+  if (!body || typeof body !== "object") return body;
+  if (hasExplicitReasoningField(body)) return body;
+
+  const defaultEffort = getModelSpec(modelId)?.defaultReasoningEffort;
+  if (!defaultEffort) return body;
+
+  return { ...body, reasoning_effort: defaultEffort };
+}
diff --git a/open-sse/utils/noThinkingAlias.ts b/open-sse/utils/noThinkingAlias.ts
index c0dfcfbb6b..f83cefc69b 100644
--- a/open-sse/utils/noThinkingAlias.ts
+++ b/open-sse/utils/noThinkingAlias.ts
@@ -10,9 +10,17 @@
  *
  * When such an id arrives on a request we strip the prefix back to the real
  * `<provider>/<model>` and suppress reasoning (`thinking:{type:"disabled"}` for the
- * Claude/Messages path; drop `reasoning`/`reasoning_effort` for the OpenAI path).
+ * Claude/Messages path; `reasoning_effort:"none"` for the OpenAI path — #6879: a
+ * thinks-by-default OpenAI-shape model left with no reasoning field at all keeps
+ * thinking with its provider default, so the alias must express "none" rather than
+ * merely deleting the field. The `reasoning` object is still dropped, since a
+ * Responses-shaped client's `reasoning:{...}` cannot itself express "none" and the
+ * translator promotes `reasoning_effort` into it downstream when absent).
  * The existing `normalizeThinkingForModel()` still runs downstream, so models that
- * reject `disabled` are handled exactly as before.
+ * reject `disabled` are handled exactly as before, and the per-lane
+ * unsupported-param strip (open-sse/translator/paramSupport.ts) still removes
+ * `reasoning_effort` for lanes known to reject it, falling back to today's
+ * delete-only behavior for those.
  *
  * Catalog visibility is gated (see `shouldExposeNoThinkingAlias`): we only advertise
  * the variant for Claude-family models that actually support thinking AND honor
@@ -61,8 +69,15 @@ export function applyNoThinkingAlias(
   body.model = realModel;
   if (opts.claudeFormat === true) {
     body.thinking = { type: "disabled" };
+    delete body.reasoning_effort;
+  } else {
+    // #6879: express "none" instead of deleting, so a thinks-by-default model
+    // actually stops thinking instead of falling back to its provider default.
+    // Lanes that reject reasoning_effort are still cleaned up downstream by the
+    // per-lane unsupported-param strip (paramSupport.ts), which removes it just
+    // like it would have been removed here — same end state, correct on more lanes.
+    body.reasoning_effort = "none";
   }
-  delete body.reasoning_effort;
   delete body.reasoning;
   return { applied: true, realModel };
 }
diff --git a/src/shared/constants/modelSpecs.ts b/src/shared/constants/modelSpecs.ts
index e70783b4ef..6d3ab0c767 100644
--- a/src/shared/constants/modelSpecs.ts
+++ b/src/shared/constants/modelSpecs.ts
@@ -30,6 +30,16 @@ export interface ModelSpec {
   // Claude-family thinking-capable models that honor `disabled`. Set `true` to force the
   // variant on for any other model, or `false` to suppress it. See open-sse/utils/noThinkingAlias.ts.
   noThinkingAlias?: boolean;
+  // Per-model default reasoning effort (#6879). When the incoming request carries no
+  // `reasoning_effort` / `reasoning` / `thinking` field of any shape, the resolved
+  // upstream model's `defaultReasoningEffort` is injected as `reasoning_effort` on the
+  // OpenAI-format dispatch path before the request leaves the gateway. An explicit
+  // client value — including one forwarded verbatim through a combo leg — always wins;
+  // this is a no-op for it. Unset preserves current behavior (no injection). Lets an
+  // operator strip-by-default a thinks-by-default model (measured: gemini-flash-lite
+  // burns ~277 reasoning tokens on a plain request; `reasoning_effort:"none"` → 0)
+  // without patching every client. See open-sse/services/defaultReasoningEffort.ts.
+  defaultReasoningEffort?: "none" | "low" | "medium" | "high";
 }
 
 const BEDROCK_CLAUDE_ALIASES = (...modelIds: string[]) => [
diff --git a/tests/unit/default-reasoning-effort-6879.test.ts b/tests/unit/default-reasoning-effort-6879.test.ts
new file mode 100644
index 0000000000..1cff3f7e54
--- /dev/null
+++ b/tests/unit/default-reasoning-effort-6879.test.ts
@@ -0,0 +1,139 @@
+/**
+ * #6879 — per-model default `reasoning_effort` + `no-think/` expresses "none" on the
+ * OpenAI path (instead of deleting the field).
+ *
+ * Ask 1: `ModelSpec.defaultReasoningEffort` is injected as `reasoning_effort` only
+ * when the request carries no reasoning field of any shape (`reasoning_effort`,
+ * `reasoning`, `thinking`); an explicit client value always wins.
+ *
+ * Ask 2: `applyNoThinkingAlias` sets `reasoning_effort:"none"` on the OpenAI path
+ * (instead of deleting the field), so a thinks-by-default model actually stops
+ * thinking rather than falling back to its provider default. The Claude/Messages
+ * path is unchanged (`thinking:{type:"disabled"}`). Lanes known to reject
+ * `reasoning_effort` still end up with the field removed via the existing
+ * per-lane unsupported-param strip (open-sse/translator/paramSupport.ts) — same
+ * end state as today's delete-only behavior, just correct on more lanes.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+import { applyDefaultReasoningEffort } from "../../open-sse/services/defaultReasoningEffort.ts";
+import { applyNoThinkingAlias } from "../../open-sse/utils/noThinkingAlias.ts";
+import { stripUnsupportedParams } from "../../open-sse/translator/paramSupport.ts";
+import { MODEL_SPECS } from "../../src/shared/constants/modelSpecs.ts";
+
+const FIXTURE_MODEL_ID = "__test_6879_default_reasoning_effort_model__";
+
+test.before(() => {
+  MODEL_SPECS[FIXTURE_MODEL_ID] = { defaultReasoningEffort: "none" };
+});
+
+test.after(() => {
+  delete MODEL_SPECS[FIXTURE_MODEL_ID];
+});
+
+// ---------------------------------------------------------------------------
+// Ask 1: applyDefaultReasoningEffort
+// ---------------------------------------------------------------------------
+
+test("applyDefaultReasoningEffort: injects the model's default when no reasoning field is present", () => {
+  const body = { model: FIXTURE_MODEL_ID, messages: [] };
+  const result = applyDefaultReasoningEffort(body, FIXTURE_MODEL_ID);
+  assert.equal(result.reasoning_effort, "none");
+});
+
+test("applyDefaultReasoningEffort: an explicit reasoning_effort always wins over the model default", () => {
+  const body = { model: FIXTURE_MODEL_ID, messages: [], reasoning_effort: "high" };
+  const result = applyDefaultReasoningEffort(body, FIXTURE_MODEL_ID);
+  assert.equal(result.reasoning_effort, "high");
+});
+
+test("applyDefaultReasoningEffort: an explicit reasoning object always wins (Responses shape)", () => {
+  const body = { model: FIXTURE_MODEL_ID, messages: [], reasoning: { effort: "medium" } };
+  const result = applyDefaultReasoningEffort(body, FIXTURE_MODEL_ID);
+  assert.deepEqual(result.reasoning, { effort: "medium" });
+  assert.equal("reasoning_effort" in result, false);
+});
+
+test("applyDefaultReasoningEffort: an explicit thinking block always wins (Claude legacy shape)", () => {
+  const body = { model: FIXTURE_MODEL_ID, messages: [], thinking: { type: "enabled" } };
+  const result = applyDefaultReasoningEffort(body, FIXTURE_MODEL_ID);
+  assert.deepEqual(result.thinking, { type: "enabled" });
+  assert.equal("reasoning_effort" in result, false);
+});
+
+test("applyDefaultReasoningEffort: no injection when the model has no configured default (regression)", () => {
+  const body = { model: "gpt-5.4-sol", messages: [] };
+  const result = applyDefaultReasoningEffort(body, "gpt-5.4-sol");
+  assert.equal("reasoning_effort" in result, false);
+  assert.equal(result, body); // same reference — no allocation when nothing to inject
+});
+
+// ---------------------------------------------------------------------------
+// Ask 2: applyNoThinkingAlias on the OpenAI path expresses "none"
+// ---------------------------------------------------------------------------
+
+test("applyNoThinkingAlias: OpenAI path sets reasoning_effort:none instead of deleting the field", () => {
+  const body: Record<string, unknown> = {
+    model: "no-think/gemini/gemini-flash-lite-latest",
+    messages: [],
+  };
+  const result = applyNoThinkingAlias(body, { claudeFormat: false });
+  assert.equal(result.applied, true);
+  assert.equal(result.realModel, "gemini/gemini-flash-lite-latest");
+  assert.equal(body.model, "gemini/gemini-flash-lite-latest");
+  assert.equal(body.reasoning_effort, "none");
+  assert.equal("reasoning" in body, false);
+});
+
+test("applyNoThinkingAlias: OpenAI path overrides a client-supplied reasoning_effort with none (alias always wins)", () => {
+  const body: Record<string, unknown> = {
+    model: "no-think/gemini/gemini-flash-lite-latest",
+    messages: [],
+    reasoning_effort: "high",
+  };
+  const result = applyNoThinkingAlias(body, { claudeFormat: false });
+  assert.equal(result.applied, true);
+  assert.equal(body.reasoning_effort, "none");
+});
+
+test("applyNoThinkingAlias: Claude/Messages path is unchanged (thinking:disabled, no reasoning_effort field)", () => {
+  const body: Record<string, unknown> = {
+    model: "no-think/claude/claude-opus-4-6",
+    messages: [],
+    reasoning_effort: "high",
+  };
+  const result = applyNoThinkingAlias(body, { claudeFormat: true });
+  assert.equal(result.applied, true);
+  assert.deepEqual(body.thinking, { type: "disabled" });
+  assert.equal("reasoning_effort" in body, false);
+  assert.equal("reasoning" in body, false);
+});
+
+test("applyNoThinkingAlias: non-alias model is untouched (regression)", () => {
+  const body: Record<string, unknown> = { model: "gemini/gemini-flash-lite-latest", messages: [] };
+  const result = applyNoThinkingAlias(body, { claudeFormat: false });
+  assert.equal(result.applied, false);
+  assert.equal("reasoning_effort" in body, false);
+});
+
+// ---------------------------------------------------------------------------
+// Ask 2, fallback: a lane that rejects reasoning_effort still ends up with the
+// field removed, via the pre-existing per-lane unsupported-param strip — same
+// end state as today's delete-only behavior, correct on more lanes.
+// ---------------------------------------------------------------------------
+
+test("a lane known to reject reasoning_effort still drops it downstream (delete-fallback preserved)", () => {
+  const body: Record<string, unknown> = {
+    model: "no-think/github/claude-3-5-sonnet",
+    messages: [],
+  };
+  const alias = applyNoThinkingAlias(body, { claudeFormat: false });
+  assert.equal(alias.applied, true);
+  assert.equal(body.reasoning_effort, "none"); // set by the alias itself
+
+  // github/claude (non-4.6) is a known-unsupported lane for both thinking and
+  // reasoning_effort (paramSupport.ts STRIP_RULES) — the field ends up removed
+  // before dispatch, exactly like the old delete-only behavior.
+  const stripped = stripUnsupportedParams("github", "claude-3-5-sonnet", body);
+  assert.equal("reasoning_effort" in stripped, false);
+});