diff --git a/packages/studio/src/utils/sourcePatcher.test.ts b/packages/studio/src/utils/sourcePatcher.test.ts
index 7697d24e6..67c01b3a7 100644
--- a/packages/studio/src/utils/sourcePatcher.test.ts
+++ b/packages/studio/src/utils/sourcePatcher.test.ts
@@ -517,17 +517,72 @@ describe("motion attribute round-trip via sourcePatcher", () => {
   });
 });
 
-// T3 — id-based targeting (spec for R1).
-// R1 adds `hfId?: string` to PatchTarget and a `[data-hf-id="…"]` lookup branch
-// in findTagByTarget. Convert from it.todo to real assertions in the R1 PR.
+// T3 — id-based targeting (R1).
 describe("T3 — hfId targeting (spec for R1)", () => {
-  it.todo("updates inline style by data-hf-id");
+  it("updates inline style by data-hf-id", () => {
+    const html = `<h1 data-hf-id="hf-x7k2" style="color: red">Hello</h1>`;
+    const result = applyPatchByTarget(
+      html,
+      { hfId: "hf-x7k2" },
+      {
+        type: "inline-style",
+        property: "color",
+        value: "blue",
+      },
+    );
+    expect(result).toContain("color: blue");
+    expect(result).toContain('data-hf-id="hf-x7k2"');
+  });
 
-  it.todo("updates text content by data-hf-id");
+  it("updates text content by data-hf-id", () => {
+    const html = `<p data-hf-id="hf-a1b2">Old text</p>`;
+    const result = applyPatchByTarget(
+      html,
+      { hfId: "hf-a1b2" },
+      {
+        type: "text-content",
+        property: "",
+        value: "New text",
+      },
+    );
+    expect(result).toContain(">New text<");
+  });
 
-  it.todo("updates attribute by data-hf-id");
+  it("updates attribute by data-hf-id", () => {
+    const html = `<div data-hf-id="hf-c3d4" data-start="0"></div>`;
+    const result = applyPatchByTarget(
+      html,
+      { hfId: "hf-c3d4" },
+      {
+        type: "attribute",
+        property: "start",
+        value: "2.5",
+      },
+    );
+    expect(result).toContain('data-start="2.5"');
+  });
 
-  it.todo("data-hf-id attribute is preserved after a style patch");
+  it("data-hf-id attribute is preserved after a style patch", () => {
+    const html = `<h1 data-hf-id="hf-x7k2" style="color: red">Hello</h1>`;
+    const patched = applyPatchByTarget(
+      html,
+      { hfId: "hf-x7k2" },
+      {
+        type: "inline-style",
+        property: "color",
+        value: "blue",
+      },
+    );
+    expect(readAttributeByTarget(patched, { hfId: "hf-x7k2" }, "data-hf-id")).toBe("hf-x7k2");
+  });
 
-  it.todo("hfId lookup falls through to selector when hfId not found");
+  it("hfId lookup falls through to selector when hfId not found", () => {
+    const html = `<h1 class="headline" style="color: red">Hello</h1>`;
+    const result = applyPatchByTarget(
+      html,
+      { hfId: "hf-missing", selector: ".headline" },
+      { type: "inline-style", property: "color", value: "blue" },
+    );
+    expect(result).toContain("color: blue");
+  });
 });
diff --git a/packages/studio/src/utils/sourcePatcher.ts b/packages/studio/src/utils/sourcePatcher.ts
index 2d89eea2e..9d19114d4 100644
--- a/packages/studio/src/utils/sourcePatcher.ts
+++ b/packages/studio/src/utils/sourcePatcher.ts
@@ -94,6 +94,7 @@ export interface PatchOperation {
 
 export interface PatchTarget {
   id?: string | null;
+  hfId?: string;
   selector?: string;
   selectorIndex?: number;
 }
@@ -232,61 +233,67 @@ function replaceTagAtMatch(html: string, match: TagMatch, newTag: string): strin
   return `${html.slice(0, match.start)}${newTag}${html.slice(match.end)}`;
 }
 
-export function findTagByTarget(html: string, target: PatchTarget): TagMatch | null {
-  if (target.id) {
-    const idPattern = new RegExp(`(<[^>]*\\bid=(["'])${escapeRegex(target.id)}\\2[^>]*)>`, "i");
-    const match = idPattern.exec(html);
-    if (match?.index != null) {
+function execDataAttrPattern(html: string, attr: string, value: string): TagMatch | null {
+  const pattern = new RegExp(`(<[^>]*\\b${attr}=(["'])${escapeRegex(value)}\\2[^>]*)>`, "i");
+  const match = pattern.exec(html);
+  if (match?.index == null) return null;
+  // Defensive: a second exact match means a duplicate id/attr in the source
+  // (id drift). Don't silently patch the first while leaving the other stale —
+  // surface it. By the mint contract this should never fire.
+  const all = html.match(new RegExp(`<[^>]*\\b${attr}=(["'])${escapeRegex(value)}\\1[^>]*>`, "gi"));
+  if (all && all.length > 1) {
+    // eslint-disable-next-line no-console
+    console.warn(
+      `sourcePatcher: ${attr}="${value}" matched ${all.length} elements; patching the first. ids/attrs must be unique per document.`,
+    );
+  }
+  return { tag: match[1], start: match.index, end: match.index + match[1].length };
+}
+
+function findTagByClass(html: string, target: PatchTarget): TagMatch | null {
+  const classMatch = target.selector?.match(/^\.([a-zA-Z0-9_-]+)$/);
+  if (!classMatch) return null;
+  const cls = classMatch[1];
+  const pattern = new RegExp(
+    `(<[^>]*\\bclass=(["'])[^"']*\\b${escapeRegex(cls)}\\b[^"']*\\2[^>]*)>`,
+    "gi",
+  );
+  const selectorIndex = target.selectorIndex ?? 0;
+  let match: RegExpExecArray | null;
+  let currentIndex = 0;
+  while ((match = pattern.exec(html)) !== null) {
+    if (currentIndex === selectorIndex && match.index != null) {
       return {
         tag: match[1],
         start: match.index,
         end: match.index + match[1].length,
       };
     }
+    currentIndex += 1;
+  }
+  return null;
+}
+
+export function findTagByTarget(html: string, target: PatchTarget): TagMatch | null {
+  if (target.hfId) {
+    const result = execDataAttrPattern(html, "data-hf-id", target.hfId);
+    if (result) return result;
+  }
+
+  if (target.id) {
+    const result = execDataAttrPattern(html, "id", target.id);
+    if (result) return result;
   }
 
   if (!target.selector) return null;
 
   const compositionIdMatch = target.selector.match(/^\[data-composition-id="([^"]+)"\]$/);
   if (compositionIdMatch) {
-    const compId = compositionIdMatch[1];
-    const pattern = new RegExp(
-      `(<[^>]*\\bdata-composition-id=(["'])${escapeRegex(compId)}\\2[^>]*)>`,
-      "i",
-    );
-    const match = pattern.exec(html);
-    if (match?.index != null) {
-      return {
-        tag: match[1],
-        start: match.index,
-        end: match.index + match[1].length,
-      };
-    }
+    const result = execDataAttrPattern(html, "data-composition-id", compositionIdMatch[1]);
+    if (result) return result;
   }
 
-  const classMatch = target.selector.match(/^\.([a-zA-Z0-9_-]+)$/);
-  if (classMatch) {
-    const cls = classMatch[1];
-    const pattern = new RegExp(
-      `(<[^>]*\\bclass=(["'])[^"']*\\b${escapeRegex(cls)}\\b[^"']*\\2[^>]*)>`,
-      "gi",
-    );
-    const selectorIndex = target.selectorIndex ?? 0;
-    let match: RegExpExecArray | null;
-    let currentIndex = 0;
-    while ((match = pattern.exec(html)) !== null) {
-      if (currentIndex === selectorIndex && match.index != null) {
-        return {
-          tag: match[1],
-          start: match.index,
-          end: match.index + match[1].length,
-        };
-      }
-      currentIndex += 1;
-    }
-  }
-
-  return null;
+  return findTagByClass(html, target);
 }
 
 export function readAttributeByTarget(