diff --git a/responder_test.mbt b/responder_test.mbt
new file mode 100644
index 0000000..3e97096
--- /dev/null
+++ b/responder_test.mbt
@@ -0,0 +1,122 @@
+///|
+async test "direct string responder sets text content type" {
+  let app = new()
+  app.get("/text", _ => "hello")
+  let response = dispatch_http(app, "GET", "/text", {}, b"")
+  inspect(response.status_code.to_int(), content="200")
+  @test.assert_eq(
+    response.headers.get("Content-Type"),
+    Some("text/plain; charset=utf-8"),
+  )
+  let body : String = response.read_body()
+  @test.assert_eq(body, "hello")
+}
+
+///|
+async test "direct json responder sets json content type" {
+  let app = new()
+  app.get("/json", _ => ({ "ok": true } : Json))
+  let response = dispatch_http(app, "GET", "/json", {}, b"")
+  inspect(response.status_code.to_int(), content="200")
+  @test.assert_eq(
+    response.headers.get("Content-Type"),
+    Some("application/json; charset=utf-8"),
+  )
+  let body : Json = response.read_body()
+  json_inspect(body, content={ "ok": true })
+}
+
+///|
+async test "direct html responder sets html content type" {
+  let app = new()
+  app.get("/html", _ => html("<h1>Hello</h1>"))
+  let response = dispatch_http(app, "GET", "/html", {}, b"")
+  inspect(response.status_code.to_int(), content="200")
+  @test.assert_eq(
+    response.headers.get("Content-Type"),
+    Some("text/html; charset=utf-8"),
+  )
+  let body : String = response.read_body()
+  @test.assert_eq(body, "<h1>Hello</h1>")
+}
+
+///|
+async test "custom status body infers text content type" {
+  let app = new()
+  app.get("/bad-request", _ => {
+    HttpResponse::new(BadRequest).body("Invalid JSON")
+  })
+  let response = dispatch_http(app, "GET", "/bad-request", {}, b"")
+  inspect(response.status_code.to_int(), content="400")
+  @test.assert_eq(
+    response.headers.get("Content-Type"),
+    Some("text/plain; charset=utf-8"),
+  )
+  let body : String = response.read_body()
+  @test.assert_eq(body, "Invalid JSON")
+}
+
+///|
+async test "custom status html body infers html content type" {
+  let app = new()
+  app.get("/not-found", _ => {
+    HttpResponse::new(NotFound).body(html("<h1>404</h1>"))
+  })
+  let response = dispatch_http(app, "GET", "/not-found", {}, b"")
+  inspect(response.status_code.to_int(), content="404")
+  @test.assert_eq(
+    response.headers.get("Content-Type"),
+    Some("text/html; charset=utf-8"),
+  )
+  let body : String = response.read_body()
+  @test.assert_eq(body, "<h1>404</h1>")
+}
+
+///|
+async test "json response infers json content type" {
+  let app = new()
+  app.get("/created", _ => {
+    HttpResponse::new(Created).json({ "ok": true })
+  })
+  let response = dispatch_http(app, "GET", "/created", {}, b"")
+  inspect(response.status_code.to_int(), content="201")
+  @test.assert_eq(
+    response.headers.get("Content-Type"),
+    Some("application/json; charset=utf-8"),
+  )
+  let body : Json = response.read_body()
+  json_inspect(body, content={ "ok": true })
+}
+
+///|
+async test "explicit content type wins over inferred body type" {
+  let app = new()
+  app.get("/custom-body", _ => {
+    HttpResponse::new(
+      OK,
+      headers={ "Content-Type": "application/vnd.custom" },
+    ).body("plain text")
+  })
+  app.get("/custom-json", _ => {
+    HttpResponse::new(
+      OK,
+      headers={ "Content-Type": "application/vnd.custom+json" },
+    ).json({ "ok": true })
+  })
+
+  let body_response = dispatch_http(app, "GET", "/custom-body", {}, b"")
+  @test.assert_eq(
+    body_response.headers.get("Content-Type"),
+    Some("application/vnd.custom"),
+  )
+  let body : String = body_response.read_body()
+  @test.assert_eq(body, "plain text")
+
+  let json_response = dispatch_http(app, "GET", "/custom-json", {}, b"")
+  @test.assert_eq(
+    json_response.headers.get("Content-Type"),
+    Some("application/vnd.custom+json"),
+  )
+  let json : Json = json_response.read_body()
+  json_inspect(json, content={ "ok": true })
+}
diff --git a/response.mbt b/response.mbt
index 9584449..42c0412 100644
--- a/response.mbt
+++ b/response.mbt
@@ -32,10 +32,25 @@ pub fn HttpResponse::new(
 }
 
 ///|
+/// Sets the response body from any `Responder`.
+///
+/// The body responder's default `Content-Type` is carried into this
+/// response only when no explicit `"Content-Type"` header was set on this
+/// response. Explicit response headers always take precedence.
+///
+/// Only the default `Content-Type` is inferred. The responder's status,
+/// other headers, cookies, and any other responder effects are not merged,
+/// so `self.status_code` is preserved.
 pub fn HttpResponse::body(
   self : HttpResponse,
   body : &Responder,
 ) -> HttpResponse {
+  let probe = HttpResponse::new(OK)
+  body.options(probe)
+  if probe.headers.get("Content-Type") is Some(content_type) &&
+    !self.headers.contains("Content-Type") {
+    self.headers["Content-Type"] = content_type
+  }
   let buf = Buffer()
   body.output(buf)
   self.raw_body = buf.to_bytes()
@@ -43,6 +58,12 @@ pub fn HttpResponse::body(
 }
 
 ///|
+/// Sets a JSON response body.
+///
+/// Infers `Content-Type: application/json; charset=utf-8` under the same
+/// rule as `HttpResponse::body`: an explicit `"Content-Type"` header already
+/// present on this response takes precedence, and `self.status_code` is
+/// preserved.
 pub fn HttpResponse::json(self : HttpResponse, obj : &ToJson) -> HttpResponse {
   self.body(obj.to_json())
 }