diff --git a/changelog.d/fixes/10286-gemini-3-5-flash-thinking.md b/changelog.d/fixes/10286-gemini-3-5-flash-thinking.md
new file mode 100644
index 0000000000..30a3c44bcb
--- /dev/null
+++ b/changelog.d/fixes/10286-gemini-3-5-flash-thinking.md
@@ -0,0 +1 @@
+- fix(sse): mark gemini-3.5-flash as thinking-capable so reasoning_effort is no longer rejected with a spurious 400 (#10286)
diff --git a/src/shared/constants/modelSpecs.ts b/src/shared/constants/modelSpecs.ts
index d15fe50425..7d38a776f5 100644
--- a/src/shared/constants/modelSpecs.ts
+++ b/src/shared/constants/modelSpecs.ts
@@ -226,8 +226,16 @@ export const MODEL_SPECS: Record<string, ModelSpec> = {
   },
 
   // ── Gemini 3.5 Flash ─────────────────────────────────────────────
+  // #10286: the base Google AI Studio model DOES support reasoning (it has
+  // an effort-tier alias gemini-3.5-flash-high) — override the shared spec's
+  // supportsThinking:false here only. Do NOT flip GEMINI_35_FLASH_MODEL_SPEC
+  // itself: it is also spread into the Antigravity flash-tier aliases
+  // (gemini-3.5-flash-low/-extra-low, gemini-3-flash-agent, gemini-3.6-flash-*)
+  // which reject client-supplied thinking params because the model id itself
+  // selects the reasoning tier upstream.
   "gemini-3.5-flash": {
     ...GEMINI_35_FLASH_MODEL_SPEC,
+    supportsThinking: true,
     aliases: ["gemini-3.5-flash-high"],
   },
 
diff --git a/tests/unit/gemini-3-5-flash-thinking.test.ts b/tests/unit/gemini-3-5-flash-thinking.test.ts
new file mode 100644
index 0000000000..80636a0b03
--- /dev/null
+++ b/tests/unit/gemini-3-5-flash-thinking.test.ts
@@ -0,0 +1,76 @@
+// Regression test for #10286: gemini-3.5-flash was incorrectly marked
+// supportsThinking:false, causing a spurious pre-provider HTTP 400 for any
+// request with reasoning_effort set, even though the base Google AI Studio
+// model supports reasoning (it has an effort-tier alias gemini-3.5-flash-high).
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+
+const TEST_DATA_DIR = fs.mkdtempSync(path.join(os.tmpdir(), "omniroute-repro-10286-"));
+process.env.DATA_DIR = TEST_DATA_DIR;
+process.env.API_KEY_SECRET = process.env.API_KEY_SECRET || "test-repro-10286-secret";
+
+const caps = await import("../../src/lib/modelCapabilities.ts");
+const core = await import("../../src/lib/db/core.ts");
+const rulesDb = await import("../../src/lib/db/reasoningRoutingRules.ts");
+const policy = await import("../../src/lib/reasoningRouting/policy.ts");
+
+async function resetStorage() {
+  core.resetDbInstance();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+  fs.mkdirSync(TEST_DATA_DIR, { recursive: true });
+  rulesDb.invalidateReasoningRoutingRuleCache();
+}
+
+function ruleInput(patch: Record<string, unknown> = {}) {
+  return {
+    name: "Enable thinking on gemini-3.5-flash",
+    description: "",
+    scope: "global",
+    apiKeyId: null,
+    comboId: null,
+    connectionId: null,
+    modelPattern: "gemini-3.5-flash",
+    sourceEffort: "any",
+    requestTags: [],
+    tagMatchMode: "any",
+    effortMode: "inherit",
+    targetEffort: null,
+    targetKind: "keep",
+    targetModel: null,
+    targetComboId: null,
+    budgetAction: "preserve",
+    budgetTokens: null,
+    priority: 0,
+    enabled: true,
+    ...patch,
+  };
+}
+
+test.beforeEach(resetStorage);
+test.after(async () => {
+  await resetStorage();
+  fs.rmSync(TEST_DATA_DIR, { recursive: true, force: true });
+});
+
+test("gemini-3.5-flash (AI Studio provider) resolves as thinking-capable", () => {
+  const resolved = caps.getResolvedModelCapabilities({
+    provider: "gemini",
+    model: "gemini-3.5-flash",
+  });
+  assert.equal(resolved.supportsThinking, true);
+});
+
+test("reasoning_effort 'high' on gemini-3.5-flash is NOT rejected by routing policy", async () => {
+  await rulesDb.createReasoningRoutingRule(ruleInput());
+  const decision = await policy.resolveReasoningRoutingRule({
+    sourceModel: "gemini/gemini-3.5-flash",
+    sourceModelAliases: ["gemini-3.5-flash"],
+    sourceEffort: "high",
+    hasReasoningSignal: true,
+  });
+  assert.ok(decision, "a matching rule must produce a decision");
+  assert.equal(decision.capability, "supported");
+});