diff --git a/.changeset/nanoclaw-and-harness-snapping.md b/.changeset/nanoclaw-and-harness-snapping.md
new file mode 100644
index 0000000..45d4498
--- /dev/null
+++ b/.changeset/nanoclaw-and-harness-snapping.md
@@ -0,0 +1,8 @@
+---
+"@tangle-network/agent-interface": minor
+---
+
+Make agent-interface the single source of truth for harness↔model snapping, and correct nanoclaw's capabilities.
+
+- `nanoclaw` is now treated as router-backed (runs any model via the Tangle router) instead of Anthropic-locked, and its reasoning ceiling is `none` (its runner sends no thinking flag) instead of `ultracode`.
+- Add `snapModelToHarness(harness, modelId, candidateIds)` and `snapHarnessToModel(harness, modelId)` so consumers (sandbox-ui, agent-app) import the catalog-aware snap logic instead of hand-rolling divergent copies.
diff --git a/packages/agent-interface/package.json b/packages/agent-interface/package.json
index 2525b0c..a415e4b 100644
--- a/packages/agent-interface/package.json
+++ b/packages/agent-interface/package.json
@@ -47,6 +47,8 @@
     "build": "tsc -p tsconfig.json",
     "check-types": "tsc --noEmit",
     "clean": "rm -rf dist",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "prepare": "pnpm run build"
   },
   "dependencies": {
@@ -54,6 +56,7 @@
   },
   "devDependencies": {
     "@types/node": "catalog:",
-    "typescript": "^6.0.3"
+    "typescript": "^6.0.3",
+    "vitest": "catalog:"
   }
 }
diff --git a/packages/agent-interface/src/harness-capabilities.test.ts b/packages/agent-interface/src/harness-capabilities.test.ts
new file mode 100644
index 0000000..d69f9e4
--- /dev/null
+++ b/packages/agent-interface/src/harness-capabilities.test.ts
@@ -0,0 +1,137 @@
+import { describe, expect, it } from "vitest";
+import {
+  harnessProviders,
+  harnessReasoningEfforts,
+  harnessSupportsModel,
+  modelProvider,
+  preferredHarnessForModel,
+  reasoningEffortsFor,
+  snapHarnessToModel,
+  snapModelToHarness,
+} from "./harness-capabilities.js";
+
+const CATALOG = [
+  "anthropic/claude-opus-4-6",
+  "anthropic/claude-sonnet-4-6",
+  "openai/gpt-5",
+  "openai/gpt-5-mini",
+  "moonshot/kimi-k2",
+  "zai/glm-4.7",
+];
+
+describe("modelProvider", () => {
+  it("extracts the provider prefix, or null for a bare id", () => {
+    expect(modelProvider("anthropic/claude-opus-4-6")).toBe("anthropic");
+    expect(modelProvider("openrouter/openai/gpt-5")).toBe("openrouter");
+    expect(modelProvider("gemini-2.5-flash-lite")).toBeNull();
+    expect(modelProvider("")).toBeNull();
+  });
+});
+
+describe("harness ↔ model compatibility", () => {
+  it("vendor-locked harnesses only accept their provider; router harnesses accept any", () => {
+    expect(harnessSupportsModel("claude-code", "anthropic/claude-sonnet-4-6")).toBe(true);
+    expect(harnessSupportsModel("claude-code", "openai/gpt-5")).toBe(false);
+    expect(harnessSupportsModel("codex", "openai/gpt-5")).toBe(true);
+    expect(harnessSupportsModel("codex", "anthropic/claude-sonnet-4-6")).toBe(false);
+    expect(harnessSupportsModel("kimi-code", "moonshot/kimi-k2")).toBe(true);
+    expect(harnessSupportsModel("opencode", "openai/gpt-5")).toBe(true);
+  });
+
+  it("aliases resolve to their base runner's lock", () => {
+    expect(harnessSupportsModel("claude", "openai/gpt-5")).toBe(false);
+    expect(harnessSupportsModel("kimi", "moonshot/kimi-k2")).toBe(true);
+  });
+
+  it("nanoclaw is router-backed — it runs any provider", () => {
+    expect(harnessProviders("nanoclaw")).toBeNull();
+    expect(harnessSupportsModel("nanoclaw", "openai/gpt-5")).toBe(true);
+    expect(harnessSupportsModel("nanoclaw", "anthropic/claude-sonnet-4-6")).toBe(true);
+  });
+
+  it("provider-less / sentinel ids are compatible everywhere", () => {
+    expect(harnessSupportsModel("claude-code", "default")).toBe(true);
+    expect(harnessSupportsModel("codex", "gemini-2.5-flash-lite")).toBe(true);
+  });
+
+  it("preferredHarnessForModel maps a vendor provider to its native harness", () => {
+    expect(preferredHarnessForModel("anthropic/claude-opus-4-6")).toBe("claude-code");
+    expect(preferredHarnessForModel("openai/gpt-5")).toBe("codex");
+    expect(preferredHarnessForModel("moonshot/kimi-k2")).toBe("kimi-code");
+    expect(preferredHarnessForModel("zai/glm-4.7")).toBeNull();
+    expect(preferredHarnessForModel("default")).toBeNull();
+  });
+});
+
+describe("snapModelToHarness", () => {
+  it("snaps an incompatible model to the harness's best catalog id (opus before sonnet)", () => {
+    expect(snapModelToHarness("claude-code", "openai/gpt-5", CATALOG)).toBe("anthropic/claude-opus-4-6");
+    expect(snapModelToHarness("codex", "anthropic/claude-sonnet-4-6", CATALOG)).toBe("openai/gpt-5");
+    expect(snapModelToHarness("kimi-code", "openai/gpt-5", CATALOG)).toBe("moonshot/kimi-k2");
+  });
+
+  it("prefers the standard-frontier gpt over a mini variant despite lexical order", () => {
+    expect(
+      snapModelToHarness("codex", "anthropic/claude-opus-4-6", ["openai/gpt-5-mini", "openai/gpt-5"]),
+    ).toBe("openai/gpt-5");
+  });
+
+  it("leaves an already-compatible model unchanged", () => {
+    expect(snapModelToHarness("claude-code", "anthropic/claude-sonnet-4-6", CATALOG)).toBe(
+      "anthropic/claude-sonnet-4-6",
+    );
+    expect(snapModelToHarness("opencode", "openai/gpt-5", CATALOG)).toBe("openai/gpt-5");
+    expect(snapModelToHarness("nanoclaw", "openai/gpt-5", CATALOG)).toBe("openai/gpt-5");
+  });
+
+  it("returns the original id when the catalog holds nothing compatible", () => {
+    expect(snapModelToHarness("claude-code", "openai/gpt-5", ["openai/gpt-5", "zai/glm-4.7"])).toBe(
+      "openai/gpt-5",
+    );
+  });
+});
+
+describe("snapHarnessToModel", () => {
+  it("adopts the model's native harness when the current one can't run it", () => {
+    expect(snapHarnessToModel("claude-code", "openai/gpt-5")).toBe("codex");
+    expect(snapHarnessToModel("codex", "anthropic/claude-opus-4-6")).toBe("claude-code");
+    expect(snapHarnessToModel("claude-code", "moonshot/kimi-k2")).toBe("kimi-code");
+  });
+
+  it("keeps the harness when it already runs the model", () => {
+    expect(snapHarnessToModel("claude-code", "anthropic/claude-opus-4-6")).toBe("claude-code");
+    expect(snapHarnessToModel("nanoclaw", "openai/gpt-5")).toBe("nanoclaw");
+  });
+
+  it("falls back to opencode for a provider with no native harness", () => {
+    expect(snapHarnessToModel("codex", "zai/glm-4.7")).toBe("opencode");
+  });
+});
+
+describe("reasoning effort support", () => {
+  it("clamps each harness to its native ceiling", () => {
+    expect(harnessReasoningEfforts("cli-base")).toEqual(["none"]);
+    expect(harnessReasoningEfforts("codex")).toEqual(["none", "minimal", "low", "medium", "high"]);
+    expect(harnessReasoningEfforts("kimi-code")).toEqual(["none", "minimal", "low", "medium", "high"]);
+    expect(harnessReasoningEfforts("claude-code")).toContain("ultracode");
+    expect(harnessReasoningEfforts("opencode")).toContain("ultracode");
+  });
+
+  it("nanoclaw expresses only `none` (its runner sends no thinking flag)", () => {
+    expect(harnessReasoningEfforts("nanoclaw")).toEqual(["none"]);
+  });
+
+  it("narrows by the model's own capability", () => {
+    expect(reasoningEffortsFor("claude-code", { supportsReasoning: false })).toEqual(["none"]);
+    expect(reasoningEffortsFor("claude-code", { maxEffort: "medium" })).toEqual([
+      "none",
+      "minimal",
+      "low",
+      "medium",
+    ]);
+    // model ceiling above the harness ceiling can't widen it
+    expect(reasoningEffortsFor("codex", { maxEffort: "ultracode" })).toEqual(
+      harnessReasoningEfforts("codex"),
+    );
+  });
+});
diff --git a/packages/agent-interface/src/harness-capabilities.ts b/packages/agent-interface/src/harness-capabilities.ts
index a8b8af2..75e6f8b 100644
--- a/packages/agent-interface/src/harness-capabilities.ts
+++ b/packages/agent-interface/src/harness-capabilities.ts
@@ -34,10 +34,13 @@ export const reasoningLadder: readonly ReasoningEffort[] = [
  * Provider prefixes a harness is vendor-locked to (canonical-id prefix, e.g. `anthropic`, `openai`).
  * A harness with no entry is router-backed: it runs any model. Keyed by the BASE runner — aliases
  * (`claude`/`claudish`/`kimi`) resolve through `canonicalizeHarness` first.
+ *
+ * `nanoclaw` is deliberately absent despite the "claw" name: its runner routes every provider through
+ * the Tangle router (canonical model id straight to the gateway), so it is router-backed like
+ * `opencode` — not Anthropic-locked.
  */
 const harnessProviderLock: Partial<Record<HarnessType, readonly string[]>> = {
   "claude-code": ["anthropic"],
-  nanoclaw: ["anthropic"],
   codex: ["openai"],
   "kimi-code": ["moonshot"],
 };
@@ -80,20 +83,82 @@ export function preferredHarnessForModel(modelId: string): HarnessType | null {
   return null;
 }
 
+// ── Harness ↔ model snapping (catalog-aware) ─────────────────────────────────
+
+/**
+ * Per-harness ranking patterns for {@link snapModelToHarness}, best first; within one pattern the
+ * highest version wins (numeric-aware). Only vendor-locked harnesses need an entry — a router-backed
+ * harness never snaps (it runs the model as-is). Keyed by the BASE runner (aliases canonicalized).
+ */
+const harnessPreferredModelPatterns: Partial<
+  Record<HarnessType, readonly RegExp[]>
+> = {
+  "claude-code": [
+    /^anthropic\/claude-opus-[\d.-]+$/,
+    /^anthropic\/claude-sonnet-[\d.-]+$/,
+    /^anthropic\//,
+  ],
+  codex: [/^openai\/gpt-\d+(\.\d+)?$/, /^openai\/gpt/, /^openai\//],
+  "kimi-code": [/^moonshot\//],
+};
+
+const numericDesc = new Intl.Collator(undefined, {
+  numeric: true,
+  sensitivity: "base",
+});
+
+/**
+ * Keep `modelId` when the harness can run it; otherwise return the harness's best compatible id from
+ * `candidateIds` (preferred patterns in order, highest version within a pattern). When nothing in the
+ * candidate list fits, the original id is returned unchanged so the caller sees the incompatibility
+ * instead of a silent wrong substitution. `candidateIds` are canonical ("provider/model") ids — the
+ * caller maps its own catalog shape down to ids, keeping this layer catalog-agnostic.
+ */
+export function snapModelToHarness(
+  harness: HarnessType,
+  modelId: string,
+  candidateIds: readonly string[],
+): string {
+  if (harnessSupportsModel(harness, modelId)) return modelId;
+  const patterns =
+    harnessPreferredModelPatterns[canonicalizeHarness(harness)] ?? [];
+  for (const pattern of patterns) {
+    const matches = candidateIds
+      .filter((id) => pattern.test(id))
+      .sort((a, b) => numericDesc.compare(b, a));
+    if (matches.length > 0) return matches[0]!;
+  }
+  return candidateIds.find((id) => harnessSupportsModel(harness, id)) ?? modelId;
+}
+
+/**
+ * Keep the harness when it can run `modelId`; otherwise return the model's native harness
+ * (anthropic → claude-code, openai → codex, moonshot → kimi-code), falling back to the router-backed
+ * `opencode` for everything else.
+ */
+export function snapHarnessToModel(
+  harness: HarnessType,
+  modelId: string,
+): HarnessType {
+  if (harnessSupportsModel(harness, modelId)) return harness;
+  return preferredHarnessForModel(modelId) ?? "opencode";
+}
+
 // ── Reasoning-effort support ──────────────────────────────────────────────────
 
 /**
  * The highest reasoning effort a harness's runtime can express (its native clamp ceiling). Grounded
  * in cli-bridge: codex's `model_reasoning_effort` caps at `high` (xhigh/ultracode clamp down); kimi's
  * `--thinking` is binary, so `high` is its "on"; claude-code carries the full range; `cli-base` has
- * no agent and thus no thinking. Router/model-driven harnesses default to the full range.
+ * no agent and thus no thinking; `nanoclaw`'s runner sends no thinking flag, so it expresses only
+ * `none`. Router/model-driven harnesses default to the full range.
  */
 const harnessReasoningCeiling: Partial<Record<HarnessType, ReasoningEffort>> = {
   "cli-base": "none",
   codex: "high",
   "kimi-code": "high",
   "claude-code": "ultracode",
-  nanoclaw: "ultracode",
+  nanoclaw: "none",
 };
 
 /** The reasoning efforts a harness can express, independent of model — `none` up to its ceiling. */
diff --git a/packages/agent-interface/tsconfig.json b/packages/agent-interface/tsconfig.json
index bc0dad3..9e28990 100644
--- a/packages/agent-interface/tsconfig.json
+++ b/packages/agent-interface/tsconfig.json
@@ -11,5 +11,6 @@
     "skipLibCheck": true,
     "types": ["node"]
   },
-  "include": ["src"]
+  "include": ["src"],
+  "exclude": ["src/**/*.test.ts"]
 }
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 837755b..9034a96 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -61,6 +61,9 @@ importers:
       typescript:
         specifier: ^6.0.3
         version: 6.0.3
+      vitest:
+        specifier: 'catalog:'
+        version: 4.1.9(@opentelemetry/api@1.9.1)(@types/node@25.6.0)(vite@8.1.0(@types/node@25.6.0)(yaml@2.9.0))
 
   packages/agent-provider-cli-bridge:
     dependencies: