diff --git a/builtin-modules/doc-core.json b/builtin-modules/doc-core.json
index 1e78973..630c0c6 100644
--- a/builtin-modules/doc-core.json
+++ b/builtin-modules/doc-core.json
@@ -3,8 +3,8 @@
   "description": "Format-agnostic document infrastructure — themes, colour validation, contrast utilities, input guards. Used by ha:pptx, ha:pdf, and other format modules.",
   "author": "system",
   "mutable": false,
-  "sourceHash": "sha256:b9119a600839812d",
-  "dtsHash": "sha256:1f311b99f56fdcbb",
+  "sourceHash": "sha256:dffdb06f7812466e",
+  "dtsHash": "sha256:b39c60e35b4359bd",
   "importStyle": "named",
   "hints": {
     "overview": "Shared utilities for all document formats. Provides themes, colour validation (WCAG AA contrast), and input guards. You rarely import this directly — ha:pptx and ha:pdf re-use it internally.",
diff --git a/builtin-modules/src/doc-core.ts b/builtin-modules/src/doc-core.ts
index 0998f5e..c791371 100644
--- a/builtin-modules/src/doc-core.ts
+++ b/builtin-modules/src/doc-core.ts
@@ -12,16 +12,35 @@
 
 // ── Colour Utilities ─────────────────────────────────────────────────
 
+/** Regex for a valid 6-character hex colour (with optional #). */
+const HEX_RE = /^#?[0-9A-Fa-f]{6}$/;
+
 /**
- * Convert a hex colour string to normalised format (strip leading #, uppercase).
- * This is the **lenient** version — it does NOT throw on bad input.
- * Prefer `requireHex()` at public API boundaries; this is kept for
- * internal paths where the value has already been validated.
+ * Normalise and validate a hex colour string.
+ *
+ * Throws on invalid input (non-hex strings, XML fragments, named
+ * colours, rgb() notation, etc.) to prevent malformed OOXML output.
+ * Prefer `requireHex()` at public API boundaries for more descriptive
+ * error messages with parameter names.
  *
  * @param hex - Colour like "#2196F3" or "2196F3"
  * @returns Normalised colour like "2196F3"
+ * @throws {Error} If hex is not a valid 6-character hex colour
  */
 export function hexColor(hex: string): string {
+  if (typeof hex !== "string" || !HEX_RE.test(hex)) {
+    // Safely render non-string values without risking Symbol/object toString
+    const display =
+      typeof hex === "string"
+        ? hex.length > 30
+          ? hex.slice(0, 30) + "..."
+          : hex
+        : `[${typeof hex}]`;
+    throw new Error(
+      `Invalid hex colour: "${display}". ` +
+        `Expected a 6-character hex string like "2196F3" or "#FF9800".`,
+    );
+  }
   return hex.replace(/^#/, "").toUpperCase();
 }
 
@@ -314,8 +333,7 @@ export function isDark(hex: string): boolean {
 // three layers deep. Every error message is LLM-actionable: it tells
 // the caller WHAT is wrong, WHY, and HOW to fix it.
 
-/** Regex for a valid 6-character hex colour (with optional #). */
-const HEX_RE = /^#?[0-9A-Fa-f]{6}$/;
+// HEX_RE is defined at the top of the file alongside hexColor().
 
 /**
  * Validate and normalise a hex colour string.
diff --git a/builtin-modules/src/types/ha-modules.d.ts b/builtin-modules/src/types/ha-modules.d.ts
index fe9538e..0d88510 100644
--- a/builtin-modules/src/types/ha-modules.d.ts
+++ b/builtin-modules/src/types/ha-modules.d.ts
@@ -43,13 +43,16 @@ declare module "ha:crc32" {
 
 declare module "ha:doc-core" {
   /**
-   * Convert a hex colour string to normalised format (strip leading #, uppercase).
-   * This is the **lenient** version — it does NOT throw on bad input.
-   * Prefer `requireHex()` at public API boundaries; this is kept for
-   * internal paths where the value has already been validated.
+   * Normalise and validate a hex colour string.
+   *
+   * Throws on invalid input (non-hex strings, XML fragments, named
+   * colours, rgb() notation, etc.) to prevent malformed OOXML output.
+   * Prefer `requireHex()` at public API boundaries for more descriptive
+   * error messages with parameter names.
    *
    * @param hex - Colour like "#2196F3" or "2196F3"
    * @returns Normalised colour like "2196F3"
+   * @throws {Error} If hex is not a valid 6-character hex colour
    */
   export declare function hexColor(hex: string): string;
   /**
diff --git a/tests/docgen-modules.test.ts b/tests/docgen-modules.test.ts
index 41617f8..6585030 100644
--- a/tests/docgen-modules.test.ts
+++ b/tests/docgen-modules.test.ts
@@ -51,6 +51,50 @@ describe("ooxml-core", () => {
     it("should handle already-clean input", () => {
       expect(core.hexColor("ABCDEF")).toBe("ABCDEF");
     });
+
+    it("should throw on XML fragment input", () => {
+      expect(() =>
+        core.hexColor(
+          '<p:bg><p:bgPr><a:solidFill><a:srgbClr val="FF0000"/></a:solidFill></p:bgPr></p:bg>',
+        ),
+      ).toThrow("Invalid hex colour");
+    });
+
+    it("should throw on named colour", () => {
+      expect(() => core.hexColor("red")).toThrow("Invalid hex colour");
+    });
+
+    it("should throw on 3-char shorthand", () => {
+      expect(() => core.hexColor("FFF")).toThrow("Invalid hex colour");
+    });
+
+    it("should throw on rgb() notation", () => {
+      expect(() => core.hexColor("rgb(255,0,0)")).toThrow("Invalid hex colour");
+    });
+
+    it("should throw on empty string", () => {
+      expect(() => core.hexColor("")).toThrow("Invalid hex colour");
+    });
+
+    it("should truncate long strings in error message", () => {
+      const longXml = "<a:gradFill>" + "x".repeat(100);
+      try {
+        core.hexColor(longXml);
+      } catch (e: unknown) {
+        const msg = (e as Error).message;
+        expect(msg).toContain("...");
+        expect(msg.length).toBeLessThan(200);
+      }
+    });
+
+    it("should safely handle non-string input", () => {
+      expect(() => core.hexColor(42 as unknown as string)).toThrow(
+        "Invalid hex colour",
+      );
+      expect(() => core.hexColor(null as unknown as string)).toThrow(
+        "Invalid hex colour",
+      );
+    });
   });
 
   describe("themes", () => {