diff --git a/packages/syntax/src/api-sections.ts b/packages/syntax/src/api-sections.ts
new file mode 100644
index 00000000..bfa76b5f
--- /dev/null
+++ b/packages/syntax/src/api-sections.ts
@@ -0,0 +1,79 @@
+/**
+ * Parse the sectioned form of an `api` block body.
+ *
+ * Returns null when no section keyword is present, which is how the legacy
+ * bare-body form stays valid: the caller keeps treating the body as the
+ * response expression.
+ */
+export interface ApiFieldDecl {
+  name: string;
+  optional: boolean;
+  type: string;
+}
+
+export interface ApiSections {
+  parameters: ApiFieldDecl[];
+  body: ApiFieldDecl[];
+  response: string;
+  error: string;
+}
+
+const SECTION_NAMES = ["request", "response", "error"] as const;
+
+/** Slice the balanced `{ ... }` that follows `keyword`, or null when absent. */
+function sectionBody(source: string, keyword: string): string | null {
+  const match = new RegExp(`(^|[^A-Za-z0-9_$])${keyword}\\s*\\{`).exec(source);
+  if (!match) return null;
+
+  const open = source.indexOf("{", match.index + match[1]!.length);
+  let depth = 0;
+
+  for (let index = open; index < source.length; index++) {
+    const character = source[index];
+    if (character === "{") depth++;
+    else if (character === "}") {
+      depth--;
+      if (depth === 0) return source.slice(open + 1, index);
+    }
+  }
+
+  throw new Error(`Unclosed "${keyword}" section in an api block`);
+}
+
+/** `name?: string` -> { name, optional, type }. Blank lines and comments are skipped. */
+function parseFields(source: string): ApiFieldDecl[] {
+  const fields: ApiFieldDecl[] = [];
+
+  for (const rawLine of source.split("\n")) {
+    const line = rawLine.trim().replace(/,$/, "");
+    if (!line || line.startsWith("//")) continue;
+
+    const match = /^([A-Za-z_$][A-Za-z0-9_$]*)(\?)?\s*:\s*(.+)$/.exec(line);
+    if (!match) {
+      throw new Error(`Expected "name: type" in an api request section, got "${line}"`);
+    }
+
+    fields.push({ name: match[1]!, optional: match[2] === "?", type: match[3]!.trim() });
+  }
+
+  return fields;
+}
+
+export function parseApiSections(source: string): ApiSections | null {
+  const present = SECTION_NAMES.some((name) => sectionBody(source, name) !== null);
+  if (!present) return null;
+
+  const request = sectionBody(source, "request");
+
+  return {
+    parameters: request ? parseFields(sectionBody(request, "parameters") ?? "") : [],
+    body: request ? parseFields(sectionBody(request, "body") ?? "") : [],
+    response: sectionBody(source, "response") ?? "",
+    error: sectionBody(source, "error") ?? "",
+  };
+}
+
+/** True when the block declares a `request` section. */
+export function hasRequestSection(source: string): boolean {
+  return sectionBody(source, "request") !== null;
+}
diff --git a/packages/syntax/src/parser.ts b/packages/syntax/src/parser.ts
index c5ce52bf..576f871d 100644
--- a/packages/syntax/src/parser.ts
+++ b/packages/syntax/src/parser.ts
@@ -1,4 +1,5 @@
 import { WRN_RUNTIME_TARGETS } from "./spec.ts";
+import { parseApiSections, hasRequestSection, type ApiSections } from "./api-sections.ts";
 
 /**
  * Recursive-descent parser for `.wrn`, producing a small AST.
@@ -148,7 +149,10 @@ export interface DataApiBlock {
   name: string;
   method: string;
   path: string;
+  /** Legacy bare body. Empty string when `sections` is set. */
   body: string;
+  /** Present only for the sectioned, typed form. */
+  sections?: ApiSections;
 }
 
 export interface ModeFunctionsBlock {
@@ -696,7 +700,20 @@ export function parse(source: string): PageAst {
                 const method = expect("ident").value.toUpperCase();
                 const path = lx.readPath();
                 const body = lx.readBalancedBraces();
-                dataApis.push({ mode, name, method, path, body });
+                const sections = parseApiSections(body);
+                if (sections && mode !== "client" && hasRequestSection(body)) {
+                  throw new ParseError(
+                    `An ssr api block cannot declare "request": there is no caller at render time to supply it. Use a client block, or a server function.`,
+                  );
+                }
+                dataApis.push({
+                  mode,
+                  name,
+                  method,
+                  path,
+                  body: sections ? "" : body,
+                  ...(sections ? { sections } : {}),
+                });
                 break;
               }
               case "functions": {
diff --git a/packages/syntax/test/api-block.test.ts b/packages/syntax/test/api-block.test.ts
new file mode 100644
index 00000000..83bd6b87
--- /dev/null
+++ b/packages/syntax/test/api-block.test.ts
@@ -0,0 +1,98 @@
+import { expect, test } from "bun:test";
+import { parse } from "../src/index.ts";
+
+const page = (inner: string) => `page Repro {
+  client {
+${inner}
+  }
+
+  view { <main>x</main> }
+}
+`;
+
+test("parses a sectioned api block into request, response and error", () => {
+  const ast = parse(
+    page(`    api searchUsers POST /api/users {
+      request {
+        body {
+          name?: string
+          age?: number
+        }
+      }
+
+      response {
+        return data.users
+      }
+
+      error {
+        return []
+      }
+    }`),
+  );
+
+  const block = ast.dataApis[0]!;
+  expect(block.name).toBe("searchUsers");
+  expect(block.method).toBe("POST");
+  expect(block.path).toBe("/api/users");
+  expect(block.sections?.body).toEqual([
+    { name: "name", optional: true, type: "string" },
+    { name: "age", optional: true, type: "number" },
+  ]);
+  expect(block.sections?.response.trim()).toBe("return data.users");
+  expect(block.sections?.error.trim()).toBe("return []");
+});
+
+test("a bare body still parses as the legacy response body", () => {
+  const ast = parse(
+    page(`    api legacyUsers GET /api/users {
+      return users.length
+    }`),
+  );
+
+  const block = ast.dataApis[0]!;
+  expect(block.sections).toBeUndefined();
+  expect(block.body.trim()).toBe("return users.length");
+});
+
+test("GET parameters are parsed as required when not marked optional", () => {
+  const ast = parse(
+    page(`    api listUsers GET /api/users {
+      request {
+        parameters {
+          team: string
+        }
+      }
+
+      response {
+        return data.users
+      }
+    }`),
+  );
+
+  expect(ast.dataApis[0]!.sections?.parameters).toEqual([
+    { name: "team", optional: false, type: "string" },
+  ]);
+});
+
+test("request inside an ssr block is rejected with a message naming the restriction", () => {
+  const source = `page Repro {
+  ssr {
+    api ssrUsers GET /api/users {
+      request {
+        parameters {
+          team: string
+        }
+      }
+
+      response {
+        return data.users
+      }
+    }
+  }
+
+  view { <main>x</main> }
+}
+`;
+
+  expect(() => parse(source)).toThrow(/ssr[\s\S]*request/i);
+});