diff --git a/docs/HTTP_API.md b/docs/HTTP_API.md
index 6920db99e..343577d4e 100644
--- a/docs/HTTP_API.md
+++ b/docs/HTTP_API.md
@@ -211,6 +211,8 @@ Update task with partial payload.
 
 Configured TaskNotes user fields can be updated either by their frontmatter property key or via `customProperties`.
 
+Sending an empty array for `contexts` or `blockedBy` clears the corresponding frontmatter field.
+
 ```bash
 curl -X PUT "http://localhost:8080/api/tasks/TaskNotes%2FTasks%2FReview%20docs.md" \
   -H "Content-Type: application/json" \
diff --git a/docs/releases/unreleased.md b/docs/releases/unreleased.md
index 121110a03..cc1bd1185 100644
--- a/docs/releases/unreleased.md
+++ b/docs/releases/unreleased.md
@@ -31,3 +31,7 @@ When a change has user-facing documentation, include a canonical tasknotes.dev l
 ```
 
 -->
+
+## Fixed
+
+- Fixed `PUT /api/tasks/:id` ignoring empty arrays for `contexts` and `blockedBy`: sending `{"contexts": []}` or `{"blockedBy": []}` now clears the corresponding frontmatter field instead of silently leaving the previous value in place. The deletion pass previously fired only on a literal `undefined`, which JSON cannot express, so HTTP clients had no way to clear these fields. Thanks to @tgrosinger for the contribution.
diff --git a/src/services/task-service/taskUpdatePlanning.ts b/src/services/task-service/taskUpdatePlanning.ts
index 324608744..460569e00 100644
--- a/src/services/task-service/taskUpdatePlanning.ts
+++ b/src/services/task-service/taskUpdatePlanning.ts
@@ -307,7 +307,7 @@ function removeUnsetMappedFields(
 	}
 	if (
 		Object.prototype.hasOwnProperty.call(updates, "contexts") &&
-		updates.contexts === undefined
+		(!Array.isArray(updates.contexts) || updates.contexts.length === 0)
 	) {
 		delete frontmatter[fieldMapper.toUserField("contexts")];
 	}
@@ -356,7 +356,7 @@ function removeUnsetMappedFields(
 	}
 	if (
 		Object.prototype.hasOwnProperty.call(updates, "blockedBy") &&
-		updates.blockedBy === undefined
+		(!Array.isArray(updates.blockedBy) || updates.blockedBy.length === 0)
 	) {
 		delete frontmatter[fieldMapper.toUserField("blockedBy")];
 	}
diff --git a/tests/unit/services/taskUpdatePlanning.test.ts b/tests/unit/services/taskUpdatePlanning.test.ts
index 31215bfc9..5c83ca4fe 100644
--- a/tests/unit/services/taskUpdatePlanning.test.ts
+++ b/tests/unit/services/taskUpdatePlanning.test.ts
@@ -232,6 +232,85 @@ describe("taskUpdatePlanning", () => {
 		expect(result.finalTags).toEqual([]);
 	});
 
+	it("clears contexts and blockedBy when an update explicitly sets them to empty arrays", () => {
+		const frontmatter: Record<string, unknown> = {
+			title: "Old",
+			status: "open",
+			contexts: ["old"],
+			blockedBy: [{ uid: "[[Other]]", reltype: "FINISHTOSTART" }],
+			tags: ["task"],
+		};
+
+		applyTaskUpdateFrontmatterChange({
+			frontmatter,
+			originalTask: createTask(),
+			updates: { contexts: [], blockedBy: [] },
+			recurrenceUpdates: {},
+			dateModified: "2026-05-19T09:00:00.000Z",
+			fieldMapper: createFieldMapper(),
+			taskIdentification: {
+				method: "tag",
+				tag: "task",
+				propertyName: "",
+				propertyValue: "",
+			},
+			storeTitleInFilename: false,
+			updateCompletedDateInFrontmatter: jest.fn(),
+		});
+
+		expect(frontmatter).not.toHaveProperty("contexts");
+		expect(frontmatter).not.toHaveProperty("blockedBy");
+	});
+
+	it("leaves contexts and blockedBy alone when the update omits them or keeps values", () => {
+		const frontmatter: Record<string, unknown> = {
+			title: "Old",
+			status: "open",
+			contexts: ["old"],
+			blockedBy: [{ uid: "[[Other]]", reltype: "FINISHTOSTART" }],
+			tags: ["task"],
+		};
+
+		applyTaskUpdateFrontmatterChange({
+			frontmatter,
+			originalTask: createTask(),
+			updates: { title: "Renamed" },
+			recurrenceUpdates: {},
+			dateModified: "2026-05-19T09:00:00.000Z",
+			fieldMapper: createFieldMapper(),
+			taskIdentification: {
+				method: "tag",
+				tag: "task",
+				propertyName: "",
+				propertyValue: "",
+			},
+			storeTitleInFilename: false,
+			updateCompletedDateInFrontmatter: jest.fn(),
+		});
+
+		expect(frontmatter.contexts).toEqual(["old"]);
+		expect(frontmatter.blockedBy).toEqual([{ uid: "[[Other]]", reltype: "FINISHTOSTART" }]);
+
+		applyTaskUpdateFrontmatterChange({
+			frontmatter,
+			originalTask: createTask(),
+			updates: { contexts: ["new"] },
+			recurrenceUpdates: {},
+			dateModified: "2026-05-19T09:00:00.000Z",
+			fieldMapper: createFieldMapper(),
+			taskIdentification: {
+				method: "tag",
+				tag: "task",
+				propertyName: "",
+				propertyValue: "",
+			},
+			storeTitleInFilename: false,
+			updateCompletedDateInFrontmatter: jest.fn(),
+		});
+
+		expect(frontmatter.contexts).toEqual(["new"]);
+	});
+
 	it("builds the returned task state from the same planned mutation", () => {
 		const updated = buildUpdatedTaskFromPlan({
 			originalTask: createTask({ completedDate: undefined }),