diff --git a/apps/api/tests/module-http-routes.test.ts b/apps/api/tests/module-http-routes.test.ts
index 2b95461..73a5334 100644
--- a/apps/api/tests/module-http-routes.test.ts
+++ b/apps/api/tests/module-http-routes.test.ts
@@ -5,6 +5,7 @@ import {
   type ModuleRouteHandler,
   type ModuleRouteHandlerContext,
   type ModuleRuntimeServices,
+  moduleRoutesOverlap,
   parsePermissionId,
   type PermissionId,
   type RegisteredModuleHttpRoute,
@@ -1454,3 +1455,153 @@ describe("module routes and the rest of the API", () => {
     expect(response.status).toBe(200);
   });
 });
+
+/**
+ * Pass 4: the decoded-parameter boundary and the routing ambiguity that made it dangerous.
+ *
+ * These drive the pinned Hono matcher with real requests rather than asserting what a mock returns,
+ * because the whole point is what the real router actually hands a module handler.
+ */
+describe("module route parameters are decoded, untrusted input", () => {
+  function paramsApp() {
+    const seen: Array<Readonly<Record<string, string>>> = [];
+    const handler = vi.fn<ModuleRouteHandler>(async (_request, context) => {
+      seen.push(context.params);
+      return Response.json({ id: context.params.id });
+    });
+    const { app } = buildModuleApp({
+      registry: permissionRegistry([{ id: readPermission, baseline: "public" }]),
+      routes: [
+        moduleRoute({
+          handler,
+          method: "GET",
+          moduleId: "calendar",
+          path: "/items/:id",
+          permission: readPermission,
+        }),
+      ],
+    });
+    return { app, handler, seen };
+  }
+
+  it.each([
+    // A single encoded segment can carry characters the declaration grammar forbids outright.
+    ["an encoded path separator and dot segments", "%2F..%2Fadmin", "/../admin"],
+    ["an encoded backslash", "a%5Cb", "a\\b"],
+    ["an embedded NUL", "a%00b", "a\u0000b"],
+    ["encoded CR/LF", "a%0D%0Ab", "a\r\nb"],
+    ["a percent sign", "100%25", "100%"],
+    ["non-ASCII text", "%C3%A9v%C3%A9nement", "événement"],
+    ["a statement separator", "1%3B%20drop", "1; drop"],
+  ])("delivers %s decoded, exactly as the router produced it", async (_label, encoded, decoded) => {
+    const { app, seen } = paramsApp();
+
+    const response = await app.request(`/api/v1/modules/calendar/items/${encoded}`);
+
+    expect(response.status).toBe(200);
+    expect(await response.json()).toEqual({ id: decoded });
+    // The declared path grammar (`/items/:id`) permits none of these characters — validating the
+    // *declaration* says nothing about what a request may send, which is exactly why a handler
+    // must validate the decoded value against its own domain grammar before using it.
+    expect(seen[0]?.id).toBe(decoded);
+    expect(Object.isFrozen(seen[0])).toBe(true);
+  });
+
+  it("keeps parameter delivery behind authorization", async () => {
+    const handler = vi.fn<ModuleRouteHandler>(async () => new Response("never"));
+    const { app, evaluator } = buildModuleApp({
+      evaluator: { authorize: vi.fn().mockResolvedValue({ allowed: false }) },
+      registry: permissionRegistry([{ id: readPermission, baseline: "restricted" }]),
+      routes: [
+        moduleRoute({
+          handler,
+          method: "GET",
+          moduleId: "calendar",
+          path: "/items/:id",
+          permission: readPermission,
+        }),
+      ],
+    });
+
+    const response = await app.request("/api/v1/modules/calendar/items/%2F..%2Fadmin");
+
+    expect(response.status).toBe(403);
+    expect(evaluator.authorize).toHaveBeenCalled();
+    // An unauthorized caller can never observe how the handler would treat a parameter, because
+    // the handler never runs.
+    expect(handler).not.toHaveBeenCalled();
+  });
+
+  it("lets a handler reject a decoded value against its own domain grammar", async () => {
+    // The platform supplies no generic parameter validator; this is the module's own check, which
+    // is exactly the documented contract.
+    const canonicalId = /^[0-9a-f]{8}$/u;
+    const handler = vi.fn<ModuleRouteHandler>(async (_request, context) => {
+      const id = context.params.id;
+      if (id === undefined || !canonicalId.test(id)) {
+        return Response.json({ code: "INVALID_REQUEST" }, { status: 400 });
+      }
+      return Response.json({ id });
+    });
+    const { app } = buildModuleApp({
+      registry: permissionRegistry([{ id: readPermission, baseline: "public" }]),
+      routes: [
+        moduleRoute({
+          handler,
+          method: "GET",
+          moduleId: "calendar",
+          path: "/items/:id",
+          permission: readPermission,
+        }),
+      ],
+    });
+
+    expect((await app.request("/api/v1/modules/calendar/items/deadbeef")).status).toBe(200);
+    const rejected = await app.request("/api/v1/modules/calendar/items/%2F..%2Fadmin");
+    expect(rejected.status).toBe(400);
+    expect(await rejected.json()).toEqual({ code: "INVALID_REQUEST" });
+  });
+});
+
+describe("module route overlap can never reach mounting", () => {
+  it("would otherwise let registration order choose the governing permission", async () => {
+    // Both orders are mounted directly here — bypassing the runtime rejection — to show precisely
+    // what `MODULE_ROUTE_OVERLAP` prevents: the same request reaching a different handler *and* a
+    // different permission depending only on the order the routes were registered in.
+    const createPermission = parsePermissionId("calendar.events.create");
+    const byId = vi.fn<ModuleRouteHandler>(async () => Response.json({ handler: "byId" }));
+    const create = vi.fn<ModuleRouteHandler>(async () => Response.json({ handler: "new" }));
+    const registry = permissionRegistry([
+      { id: readPermission, baseline: "public" },
+      { id: createPermission, baseline: "public" },
+    ]);
+    const paramRoute = moduleRoute({
+      handler: byId,
+      method: "GET",
+      moduleId: "calendar",
+      path: "/events/:id",
+      permission: readPermission,
+    });
+    const staticRoute = moduleRoute({
+      handler: create,
+      method: "GET",
+      moduleId: "calendar",
+      path: "/events/new",
+      permission: createPermission,
+    });
+
+    const parameterFirst = buildModuleApp({ registry, routes: [paramRoute, staticRoute] });
+    const staticFirst = buildModuleApp({ registry, routes: [staticRoute, paramRoute] });
+
+    expect(
+      await (await parameterFirst.app.request("/api/v1/modules/calendar/events/new")).json(),
+    ).toEqual({ handler: "byId" });
+    expect(
+      await (await staticFirst.app.request("/api/v1/modules/calendar/events/new")).json(),
+    ).toEqual({ handler: "new" });
+
+    // Core rejects this pair during staged registration, so no such snapshot can ever be published
+    // to `mountModuleHttpRoutes` — see `core/tests/module-runtime.test.ts`.
+    expect(moduleRoutesOverlap(paramRoute, staticRoute)).toBe(true);
+  });
+});
diff --git a/core/src/modules/runtime.ts b/core/src/modules/runtime.ts
index f6ecd94..9395b65 100644
--- a/core/src/modules/runtime.ts
+++ b/core/src/modules/runtime.ts
@@ -12,6 +12,7 @@ import {
   type ModuleRuntimeDefinition,
   type ModuleRuntimeServices,
   moduleRouteShapeKey,
+  moduleRoutesOverlap,
   parseModuleRuntimeDefinition,
   type RegisteredCapability,
   type RegisteredModuleHttpRoute,
@@ -37,6 +38,7 @@ export type ModuleRuntimeIssueCode =
   | "MODULE_ROUTE_DUPLICATE"
   | "MODULE_ROUTE_MAX_BODY_BYTES_INVALID"
   | "MODULE_ROUTE_METHOD_INVALID"
+  | "MODULE_ROUTE_OVERLAP"
   | "MODULE_ROUTE_PATH_INVALID"
   | "MODULE_ROUTE_PERMISSION_INVALID"
   | "MODULE_ROUTE_PERMISSION_UNDECLARED"
@@ -331,6 +333,22 @@ class StagedModuleHttpRoutes {
             ),
           );
         }
+        // Distinct from a duplicate: these are *different* routes that can both match at least one
+        // request path. With an order-sensitive matcher, whichever registered first would decide
+        // which permission governs that path — a permission-policy decision no routing order may
+        // make. Core rejects the ambiguity here instead of ranking by specificity, so the module
+        // publishes none of its staged routes and no request can reach an ambiguous match.
+        const overlapping = local.find((existing) => moduleRoutesOverlap(existing, parsed.data));
+        if (overlapping) {
+          throw new ModuleRuntimeSignal(
+            runtimeIssue(
+              "MODULE_ROUTE_OVERLAP",
+              "register",
+              module.manifest.id,
+              `Module ${module.manifest.id} registered HTTP routes that can both match the same request path.`,
+            ),
+          );
+        }
         shapes.add(shapeKey);
         local.push(
           Object.freeze({
diff --git a/core/tests/module-runtime.test.ts b/core/tests/module-runtime.test.ts
index 239d22c..048e837 100644
--- a/core/tests/module-runtime.test.ts
+++ b/core/tests/module-runtime.test.ts
@@ -562,6 +562,141 @@ describe("module HTTP routes", () => {
     expect(runtimeIssueCodes(error)).toEqual(["MODULE_ROUTE_DUPLICATE"]);
   });
 
+  it.each([
+    ["parameter registered first", "/events/:id", "/events/new"],
+    ["static registered first", "/events/new", "/events/:id"],
+    ["multi-position, parameter first", "/events/:id/edit", "/events/new/:action"],
+    ["multi-position, static first", "/events/new/:action", "/events/:id/edit"],
+    ["parameters on both sides", "/a/:left/c", "/a/b/:right"],
+  ])("rejects routes that can both match one request path (%s) regardless of registration order", async (_label, firstPath, secondPath) => {
+    const modulesDirectory = await createModulesDirectory();
+    // Deliberately *different* permissions: the defect this rejects is that whichever route
+    // registered first would decide which permission governs the ambiguous path.
+    await createRuntimeModule(modulesDirectory, "overlap", {
+      manifest: manifestFor("overlap", {
+        permissions: [
+          permissionDeclaration("overlap.events.read"),
+          permissionDeclaration("overlap.events.create"),
+        ],
+      }),
+      serverSource: `export default {
+        id: "overlap", version: "0.1.0",
+        register({ http }) {
+          http.route({ method: "GET", path: "${firstPath}", permission: "overlap.events.read", maxBodyBytes: 1024, handler: ${noopHandlerSource()} });
+          http.route({ method: "GET", path: "${secondPath}", permission: "overlap.events.create", maxBodyBytes: 1024, handler: ${noopHandlerSource()} });
+        }
+      };`,
+    });
+
+    const error = await captureRuntimeError(modulesDirectory);
+    expect(runtimeIssueCodes(error)).toEqual(["MODULE_ROUTE_OVERLAP"]);
+  });
+
+  it.each([
+    ["two different static literals", "/events/new", "/events/archive"],
+    ["different segment counts", "/events/:id", "/events/:id/edit"],
+    ["a different static prefix", "/events/:id", "/people/:id"],
+    ["module root against a one-segment path", "/", "/events"],
+  ])("still allows disjoint routes (%s) and preserves their registration order", async (_label, firstPath, secondPath) => {
+    const modulesDirectory = await createModulesDirectory();
+    await createRuntimeModule(modulesDirectory, "disjoint", {
+      manifest: manifestFor("disjoint", {
+        permissions: [permissionDeclaration("disjoint.events.read")],
+      }),
+      serverSource: `export default {
+        id: "disjoint", version: "0.1.0",
+        register({ http }) {
+          http.route({ method: "GET", path: "${firstPath}", permission: "disjoint.events.read", maxBodyBytes: 1024, handler: ${noopHandlerSource()} });
+          http.route({ method: "GET", path: "${secondPath}", permission: "disjoint.events.read", maxBodyBytes: 1024, handler: ${noopHandlerSource()} });
+        }
+      };`,
+    });
+
+    const runtime = await startModuleRuntime({
+      moduleSdkVersion: "0.1.0",
+      modulesDirectory,
+      platformVersion: "0.1.0",
+      services: fakeModuleRuntimeServices(),
+    });
+    expect(runtime.routes.map((route) => route.path)).toEqual([firstPath, secondPath]);
+    await runtime.stop();
+  });
+
+  it("allows the same overlapping local path under different methods", async () => {
+    const modulesDirectory = await createModulesDirectory();
+    await createRuntimeModule(modulesDirectory, "methods", {
+      manifest: manifestFor("methods", {
+        permissions: [permissionDeclaration("methods.events.read")],
+      }),
+      serverSource: `export default {
+        id: "methods", version: "0.1.0",
+        register({ http }) {
+          http.route({ method: "GET", path: "/events/:id", permission: "methods.events.read", maxBodyBytes: 1024, handler: ${noopHandlerSource()} });
+          http.route({ method: "POST", path: "/events/new", permission: "methods.events.read", maxBodyBytes: 1024, handler: ${noopHandlerSource()} });
+        }
+      };`,
+    });
+
+    const runtime = await startModuleRuntime({
+      moduleSdkVersion: "0.1.0",
+      modulesDirectory,
+      platformVersion: "0.1.0",
+      services: fakeModuleRuntimeServices(),
+    });
+    expect(runtime.routes).toHaveLength(2);
+    await runtime.stop();
+  });
+
+  it("allows two different modules to declare the same overlapping local paths", async () => {
+    const modulesDirectory = await createModulesDirectory();
+    for (const moduleId of ["alpha", "beta"]) {
+      await createRuntimeModule(modulesDirectory, moduleId, {
+        manifest: manifestFor(moduleId, {
+          permissions: [permissionDeclaration(`${moduleId}.events.read`)],
+        }),
+        serverSource: `export default {
+        id: "${moduleId}", version: "0.1.0",
+        register({ http }) {
+          http.route({ method: "GET", path: "/events/:id", permission: "${moduleId}.events.read", maxBodyBytes: 1024, handler: ${noopHandlerSource()} });
+        }
+      };`,
+      });
+    }
+
+    // Each module has its own platform-owned mount prefix, so these can never collide.
+    const runtime = await startModuleRuntime({
+      moduleSdkVersion: "0.1.0",
+      modulesDirectory,
+      platformVersion: "0.1.0",
+      services: fakeModuleRuntimeServices(),
+    });
+    expect(runtime.routes.map((route) => route.moduleId).sort()).toEqual(["alpha", "beta"]);
+    await runtime.stop();
+  });
+
+  it("publishes no route from a module whose registration was rejected for an overlap", async () => {
+    const modulesDirectory = await createModulesDirectory();
+    await createRuntimeModule(modulesDirectory, "partial", {
+      manifest: manifestFor("partial", {
+        permissions: [permissionDeclaration("partial.events.read")],
+      }),
+      serverSource: `export default {
+        id: "partial", version: "0.1.0",
+        register({ http }) {
+          http.route({ method: "GET", path: "/healthy", permission: "partial.events.read", maxBodyBytes: 1024, handler: ${noopHandlerSource()} });
+          http.route({ method: "GET", path: "/events/:id", permission: "partial.events.read", maxBodyBytes: 1024, handler: ${noopHandlerSource()} });
+          http.route({ method: "GET", path: "/events/new", permission: "partial.events.read", maxBodyBytes: 1024, handler: ${noopHandlerSource()} });
+        },
+        start() { throw new Error("start must never run for a rejected registration"); }
+      };`,
+    });
+
+    const error = await captureRuntimeError(modulesDirectory);
+    expect(runtimeIssueCodes(error)).toEqual(["MODULE_ROUTE_OVERLAP"]);
+    // The earlier, perfectly valid `/healthy` route is not published either: registration is
+    // all-or-nothing, and the start hook above would have thrown had it ever run.
+  });
+
   it("fails a malformed route with its specific structured issue code", async () => {
     const badMethodDirectory = await createModulesDirectory();
     await createRuntimeModule(badMethodDirectory, "bad-method", {
diff --git a/docs/module-development/README.md b/docs/module-development/README.md
index dd9f51d..4ff5f8b 100644
--- a/docs/module-development/README.md
+++ b/docs/module-development/README.md
@@ -219,6 +219,65 @@ registration outright rather than being ignored. `method` is one of `GET`, `POST
 `DELETE`, always uppercase; there is no `OPTIONS` (the platform owns CORS preflight) and no
 wildcard/all-method registration.
 
+### Two routes may not both match one request path
+
+Within one module and one HTTP method, two route patterns must never be able to match the same
+request path. `/events/:id` and `/events/new` can — `/events/new` matches both — and that pair is
+rejected during registration with `MODULE_ROUTE_OVERLAP`. So are `/events/:id/edit` against
+`/events/new/:action`, and `/a/:left/c` against `/a/b/:right`. Two identical shapes, including paths
+differing only in parameter name (`/events/:id` and `/events/:eventId`), keep the existing
+`MODULE_ROUTE_DUPLICATE` result.
+
+This is rejected rather than resolved by a "static beats parameter" rule because the routes may
+carry *different permissions*, and letting declaration order decide which one guards `/events/new`
+would make a permission policy depend on the order lines appear in your `register` hook. A rejected
+registration publishes **none** of that module's routes or capabilities and runs no start hook, so
+the failure is loud at startup rather than subtle at request time.
+
+Still perfectly legal: two different static literals (`/events/new` and `/events/archive`),
+different segment counts (`/events/:id` and `/events/:id/edit`), the same path under different
+methods, and the same local path in two different modules — each module has its own
+platform-owned mount prefix. Declaration order among disjoint routes is preserved exactly.
+
+### Route parameters are decoded, untrusted input — validate them yourself
+
+`context.params` holds the router's **decoded** path values. Decoding is not validation, and the
+fact that your *declared* path passed the platform's path grammar says nothing about what a caller
+may *send*. A single encoded segment routinely carries characters no declared path could:
+
+| Request segment | `context.params` value |
+| --- | --- |
+| `%2F..%2Fadmin` | `/../admin` |
+| `a%5Cb` | `a\b` |
+| `a%00b` | `a`, NUL, `b` |
+| `a%0D%0Ab` | `a`, CR, LF, `b` |
+| `1%3B%20drop` | `1; drop` |
+| `%C3%A9v%C3%A9nement` | `événement` |
+
+**Validate every parameter you use against your own exact domain grammar** — a UUID, slug, date,
+numeric identifier, or whatever your module owns — before any database, filesystem, command,
+identifier, or other side-effecting use, and reject a missing, repeated, malformed, or semantically
+invalid value with your own sanitized response:
+
+```ts
+const eventId = /^[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}$/u;
+
+async handler(request, context) {
+  const id = context.params.eventId;
+  if (id === undefined || !eventId.test(id)) {
+    return Response.json({ code: "INVALID_REQUEST" }, { status: 400 });
+  }
+  // ...only now is `id` safe to use as a domain identifier.
+}
+```
+
+Re-encoding the value, or a decode/re-encode round trip, is **not** a substitute for that check. The
+platform deliberately supplies no generic parameter validator: your parameters are your domain
+types, and one platform-wide pattern would either reject legitimate values or approve meaningless
+ones. The platform continues to own authentication, authorization, `Origin`, CORS, body limits,
+cache behavior, and uncaught-handler sanitization, and permission enforcement always completes
+before your handler runs — an unauthorized caller can never reach your validation at all.
+
 `maxBodyBytes` is a conscious, required request-body contract, not an optional or defaulted field:
 `0` means the route accepts no body at all, and a non-empty declared (`Content-Length`) or streamed
 body is rejected before the handler ever runs; any other value is the exact maximum number of bytes
diff --git a/packages/contracts/src/module-http.ts b/packages/contracts/src/module-http.ts
index 9fccc6f..8208314 100644
--- a/packages/contracts/src/module-http.ts
+++ b/packages/contracts/src/module-http.ts
@@ -141,6 +141,49 @@ export function moduleRouteShapeKey(method: ModuleHttpMethod, path: ModuleRouteP
   return `${method} /${shapeSegments.join("/")}`;
 }
 
+/** `"/"` is the module root and has no segments; every other canonical path is its `/`-separated
+ * segments. Takes an already-validated `ModuleRoutePath`. */
+function moduleRoutePathSegments(path: ModuleRoutePath): readonly string[] {
+  return path === "/" ? [] : path.slice(1).split("/");
+}
+
+/**
+ * Whether two routes of the same module can both match at least one request path.
+ *
+ * Two routes overlap when they share an HTTP method and a segment count, and at every position
+ * either both segments are the same static literal or at least one is a parameter. Under this
+ * route grammar — static kebab-case literals or `:name` parameters, with no wildcard, regex,
+ * optional, or catch-all segment — that condition is exactly "some concrete path matches both".
+ *
+ * This exists because an overlap is a *permission* ambiguity, not a routing preference:
+ * `GET /events/:id` and `GET /events/new` can carry different permissions, and with an
+ * order-sensitive matcher whichever registered first decides which permission governs
+ * `/events/new`. Core rejects the overlap at registration instead of picking a winner, so no
+ * router's matching order can ever select the governing permission. Identical shapes (including
+ * paths differing only in parameter name) remain the pre-existing duplicate result; callers check
+ * that first.
+ *
+ * Deliberately framework-neutral: path semantics already live in this package, and nothing about
+ * this rule may depend on Hono, which stays confined to `apps/api`.
+ */
+export function moduleRoutesOverlap(
+  first: { readonly method: ModuleHttpMethod; readonly path: ModuleRoutePath },
+  second: { readonly method: ModuleHttpMethod; readonly path: ModuleRoutePath },
+): boolean {
+  if (first.method !== second.method) {
+    return false;
+  }
+  const firstSegments = moduleRoutePathSegments(first.path);
+  const secondSegments = moduleRoutePathSegments(second.path);
+  if (firstSegments.length !== secondSegments.length) {
+    return false;
+  }
+  return firstSegments.every((segment, index) => {
+    const other = secondSegments[index] as string;
+    return segment.startsWith(":") || other.startsWith(":") || segment === other;
+  });
+}
+
 /**
  * What a module's route handler receives about the caller, the match, and the platform services
  * available to it — never an authorization-store, grant-management, storage, jobs, configuration,
@@ -149,6 +192,28 @@ export function moduleRouteShapeKey(method: ModuleHttpMethod, path: ModuleRouteP
  * before ever invoking its handler; this contracts-layer module never invokes a handler itself.
  * `services` is the same frozen `ModuleRuntimeServices` object the owning module's `start`/`stop`
  * hooks receive for this API runtime — never a fresh one built per request.
+ *
+ * ## `params` values are untrusted input
+ *
+ * `params` holds the HTTP router's **decoded** path values, frozen alongside this context. Decoding
+ * is not validation, and the declaration grammar above constrains only what a module may *declare*
+ * — it says nothing about what a request may *send*. A caller can percent-encode anything into a
+ * single segment, so a decoded value routinely contains characters no declared path could:
+ * verified against the pinned router, `%2F..%2Fadmin` arrives as `/../admin`, `a%5Cb` as `a\b`,
+ * `a%00b` with an embedded NUL, `a%0D%0Ab` with CR/LF, and `%C3%A9v%C3%A9nement` as `événement`.
+ *
+ * Every handler must therefore validate each parameter it uses against its own exact domain
+ * grammar — a UUID, slug, date, numeric id, or whatever that module owns — **before** any
+ * database, filesystem, command, identifier, or other side-effecting use, and reject a missing,
+ * repeated, malformed, or semantically invalid value with its own sanitized response. Re-encoding,
+ * decode/re-encode round-tripping, or inspecting the raw URL is not a substitute for that check.
+ * The platform deliberately supplies no generic parameter grammar: module parameters are
+ * module-owned domain types, and one platform-wide pattern would either reject legitimate values
+ * or approve meaningless ones.
+ *
+ * The platform continues to own authentication, authorization, `Origin`, CORS, body limits, cache
+ * behavior, and uncaught-handler sanitization; permission enforcement always completes before a
+ * handler — and therefore any parameter validation it performs — can run.
  */
 export interface ModuleRouteHandlerContext {
   readonly moduleId: ModuleId;
diff --git a/packages/contracts/tests/module-http.test.ts b/packages/contracts/tests/module-http.test.ts
index 90e1e45..4f2d3c2 100644
--- a/packages/contracts/tests/module-http.test.ts
+++ b/packages/contracts/tests/module-http.test.ts
@@ -6,6 +6,7 @@ import {
   type ModuleRouteDefinitionInput,
   moduleRoutePathSchema,
   moduleRouteShapeKey,
+  moduleRoutesOverlap,
   parseModuleRouteDefinition,
   parseModuleRoutePath,
   safeParseModuleRouteDefinition,
@@ -294,3 +295,42 @@ describe("parseModuleRouteDefinition", () => {
     }
   });
 });
+
+describe("moduleRoutesOverlap", () => {
+  const route = (method: "GET" | "POST", path: string) => ({
+    method,
+    path: parseModuleRoutePath(path),
+  });
+
+  it.each([
+    ["a parameter against a static literal", "/events/:id", "/events/new"],
+    ["a later position", "/events/:id/edit", "/events/new/:action"],
+    ["parameters on both sides at different positions", "/a/:left/c", "/a/b/:right"],
+    ["identical parameter shapes with different names", "/events/:id", "/events/:eventId"],
+    ["identical static paths", "/events/new", "/events/new"],
+    ["root against root", "/", "/"],
+  ])("reports an overlap for %s", (_label, first, second) => {
+    expect(moduleRoutesOverlap(route("GET", first), route("GET", second))).toBe(true);
+    // The relation is symmetric, so registration order cannot change the verdict.
+    expect(moduleRoutesOverlap(route("GET", second), route("GET", first))).toBe(true);
+  });
+
+  it.each([
+    ["two different static literals", "/events/new", "/events/archive"],
+    ["different segment counts", "/events/:id", "/events/:id/edit"],
+    ["a static prefix that differs earlier", "/events/:id", "/people/:id"],
+    ["root against a one-segment path", "/", "/events"],
+  ])("reports no overlap for %s", (_label, first, second) => {
+    expect(moduleRoutesOverlap(route("GET", first), route("GET", second))).toBe(false);
+    expect(moduleRoutesOverlap(route("GET", second), route("GET", first))).toBe(false);
+  });
+
+  it("never reports an overlap across different methods", () => {
+    expect(moduleRoutesOverlap(route("GET", "/events/:id"), route("POST", "/events/new"))).toBe(
+      false,
+    );
+    expect(moduleRoutesOverlap(route("GET", "/events/new"), route("POST", "/events/new"))).toBe(
+      false,
+    );
+  });
+});