diff --git a/tests/unit/capacity-adapter.test.js b/tests/unit/capacity-adapter.test.js
new file mode 100644
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+++ b/tests/unit/capacity-adapter.test.js
@@ -0,0 +1,180 @@
+import { describe, it, expect, vi } from "vitest";
+
+// Mock capabilities: claude-sonnet has vision + small context to exercise stripping;
+// everything else is text-only. This isolates tests from the real registry.
+vi.mock("../../open-sse/providers/capabilities.js", () => ({
+  getCapabilitiesForModel: (provider, model) => {
+    const base = model.includes("/") ? model.split("/").pop() : model;
+    return {
+      vision: base.includes("claude-sonnet"),
+      pdf: false,
+      audioInput: false,
+      videoInput: false,
+      contextWindow: base.includes("claude-sonnet") ? 2000 : 128000,
+    };
+  },
+}));
+
+import {
+  getCapacityAdapterModels,
+  augmentModelsWithCapacityAdapter,
+  stripHistoryForContext,
+  withCapacityAdapterStripping,
+} from "../../open-sse/services/capacityAdapter.js";
+
+const VISION_MODEL = "anthropic/claude-sonnet-4.6";
+const TEXT_MODEL = "deepseek/deepseek-chat";
+
+const settingsWith = (cap, entry) => ({
+  capacityAdapter: {
+    vision: { enabled: false, roundRobin: false, models: [] },
+    pdf: { enabled: false, roundRobin: false, models: [] },
+    audioInput: { enabled: false, roundRobin: false, models: [] },
+    videoInput: { enabled: false, roundRobin: false, models: [] },
+    [cap]: entry,
+  },
+});
+
+describe("getCapacityAdapterModels", () => {
+  it("flattens enabled pools in priority order, deduped across pools", () => {
+    const settings = {
+      capacityAdapter: {
+        vision: { enabled: true, roundRobin: false, models: ["a/x", "b/y"] },
+        pdf: { enabled: true, roundRobin: false, models: ["a/x", "c/z"] },
+        audioInput: { enabled: false, roundRobin: false, models: ["d/w"] },
+        videoInput: { enabled: false, roundRobin: false, models: [] },
+      },
+    };
+    expect(getCapacityAdapterModels(settings)).toEqual(["a/x", "b/y", "c/z"]);
+  });
+
+  it("returns [] when no settings", () => {
+    expect(getCapacityAdapterModels(undefined)).toEqual([]);
+    expect(getCapacityAdapterModels({})).toEqual([]);
+  });
+});
+
+describe("augmentModelsWithCapacityAdapter", () => {
+  it("prepends a capable adapter model (priority) when no original model satisfies vision", () => {
+    const settings = settingsWith("vision", [{ model: VISION_MODEL, enabled: true }]);
+    const out = augmentModelsWithCapacityAdapter([TEXT_MODEL], new Set(["vision"]), settings);
+    expect(out).toEqual([VISION_MODEL, TEXT_MODEL]);
+  });
+
+  it("leaves models untouched when an original model already satisfies the cap", () => {
+    const settings = settingsWith("vision", [{ model: VISION_MODEL, enabled: true }]);
+    const out = augmentModelsWithCapacityAdapter([VISION_MODEL, TEXT_MODEL], new Set(["vision"]), settings);
+    expect(out).toEqual([VISION_MODEL, TEXT_MODEL]);
+  });
+
+  it("skips disabled adapter entries", () => {
+    const settings = settingsWith("vision", { enabled: false, roundRobin: false, models: [VISION_MODEL] });
+    const out = augmentModelsWithCapacityAdapter([TEXT_MODEL], new Set(["vision"]), settings);
+    expect(out).toEqual([TEXT_MODEL]);
+  });
+
+  it("skips adapter models that don't satisfy the required cap", () => {
+    // adapter pool only has a text model -> useless for vision
+    const settings = settingsWith("vision", [{ model: TEXT_MODEL, enabled: true }]);
+    const out = augmentModelsWithCapacityAdapter([TEXT_MODEL], new Set(["vision"]), settings);
+    expect(out).toEqual([TEXT_MODEL]);
+  });
+
+  it("no required capabilities -> unchanged", () => {
+    const settings = settingsWith("vision", [{ model: VISION_MODEL, enabled: true }]);
+    const out = augmentModelsWithCapacityAdapter([TEXT_MODEL], new Set(), settings);
+    expect(out).toEqual([TEXT_MODEL]);
+  });
+
+  it("filters to hard caps only (ignores soft caps like search)", () => {
+    const settings = settingsWith("vision", [{ model: VISION_MODEL, enabled: true }]);
+    const out = augmentModelsWithCapacityAdapter([TEXT_MODEL], new Set(["search"]), settings);
+    expect(out).toEqual([TEXT_MODEL]);
+  });
+});
+
+describe("stripHistoryForContext", () => {
+  const big = "x".repeat(10000);
+
+  it("keeps system + trailing user turn, drops oldest when over budget", () => {
+    const body = {
+      messages: [
+        { role: "system", content: "sys" },
+        { role: "user", content: big },
+        { role: "assistant", content: big },
+        { role: "user", content: big },
+        { role: "assistant", content: big },
+        { role: "user", content: [{ type: "image_url", image_url: { url: "data:x" } }] },
+      ],
+    };
+    // Tiny budget forces dropping. ~4000 chars budget.
+    const out = stripHistoryForContext(body, 2000);
+    // System + the last (image) user turn must always survive.
+    expect(out.messages[0]).toMatchObject({ role: "system" });
+    expect(out.messages.at(-1)).toMatchObject({ role: "user" });
+    expect(out.messages.length).toBeLessThan(body.messages.length);
+  });
+
+  it("returns body unchanged when already within budget", () => {
+    const body = { messages: [
+      { role: "user", content: "hi" },
+      { role: "assistant", content: "hey" },
+      { role: "user", content: "again" },
+    ] };
+    expect(stripHistoryForContext(body, 200000)).toBe(body);
+  });
+
+  it("no-op on unsupported shape", () => {
+    const body = { foo: 1 };
+    expect(stripHistoryForContext(body, 1000)).toBe(body);
+  });
+
+  it("handles gemini contents/parts shape", () => {
+    const body = {
+      contents: [
+        { role: "user", parts: [{ text: big }] },
+        { role: "model", parts: [{ text: big }] },
+        { role: "user", parts: [{ inlineData: { mimeType: "image/png", data: "x" } }] },
+      ],
+    };
+    const out = stripHistoryForContext(body, 2000);
+    expect(out.contents.at(-1).role).toBe("user");
+    expect(out.contents.length).toBeLessThan(body.contents.length);
+  });
+});
+
+describe("withCapacityAdapterStripping", () => {
+  it("strips history only for adapter models", async () => {
+    const seen = [];
+    const inner = vi.fn(async (body, modelStr) => {
+      seen.push({ model: modelStr, len: body.messages?.length });
+      return { ok: true };
+    });
+    const wrapped = withCapacityAdapterStripping(inner, [VISION_MODEL]);
+
+    const big = "x".repeat(10000);
+    const body = {
+      messages: [
+        { role: "user", content: big },
+        { role: "assistant", content: big },
+        { role: "user", content: big },
+        { role: "assistant", content: big },
+        { role: "user", content: [{ type: "image_url", image_url: { url: "x" } }] },
+      ],
+    };
+
+    // Adapter model -> history stripped (fewer messages)
+    await wrapped(body, VISION_MODEL);
+    expect(seen[0].len).toBeLessThan(body.messages.length);
+
+    // Non-adapter model -> body untouched (same length)
+    await wrapped(body, TEXT_MODEL);
+    expect(seen[1].len).toBe(body.messages.length);
+  });
+
+  it("passthrough when adapterModels empty", async () => {
+    const inner = vi.fn(async () => ({ ok: true }));
+    const wrapped = withCapacityAdapterStripping(inner, []);
+    expect(wrapped).toBe(inner);
+  });
+});