diff --git a/.changeset/late-kids-visit.md b/.changeset/late-kids-visit.md
new file mode 100644
index 000000000..c199fe651
--- /dev/null
+++ b/.changeset/late-kids-visit.md
@@ -0,0 +1,5 @@
+---
+"@emdash-cms/admin": patch
+---
+
+Fixes plugin block defaults so initial values are seeded without overriding later edits.
diff --git a/packages/admin/src/components/PortableTextEditor.tsx b/packages/admin/src/components/PortableTextEditor.tsx
index 7826fda0b..ac243fbf9 100644
--- a/packages/admin/src/components/PortableTextEditor.tsx
+++ b/packages/admin/src/components/PortableTextEditor.tsx
@@ -970,6 +970,33 @@ function SlashCommandMenu({
 	);
 }
 
+function getPluginBlockDefaultValues(fields?: Element[]): Record<string, unknown> {
+	const defaults: Record<string, unknown> = {};
+
+	for (const field of fields ?? []) {
+		const initialValue = "initial_value" in field ? field.initial_value : undefined;
+		if (initialValue !== undefined) {
+			defaults[field.action_id] = initialValue;
+		}
+	}
+
+	return defaults;
+}
+
+function buildPluginBlockFormValues(
+	block: PluginBlockDef | null,
+	initialValues?: Record<string, unknown>,
+): Record<string, unknown> {
+	const defaults = getPluginBlockDefaultValues(block?.fields);
+	return initialValues ? { ...defaults, ...initialValues } : defaults;
+}
+
+function hasPluginBlockFormData(values: Record<string, unknown>): boolean {
+	return Object.values(values).some(
+		(value) => value !== undefined && value !== null && value !== "",
+	);
+}
+
 /**
  * Plugin block insertion/editing modal.
  * When the block has `fields`, renders Block Kit elements.
@@ -992,11 +1019,7 @@ function PluginBlockModal({
 
 	React.useEffect(() => {
 		if (block) {
-			if (initialValues) {
-				setFormValues({ ...initialValues });
-			} else {
-				setFormValues({});
-			}
+			setFormValues(buildPluginBlockFormValues(block, initialValues));
 			if (!block.fields || block.fields.length === 0) {
 				setTimeout(() => inputRef.current?.focus(), 0);
 			}
@@ -1025,7 +1048,7 @@ function PluginBlockModal({
 	// For simple URL mode, check if the URL is non-empty
 	// For Block Kit fields, require at least one field to have a value
 	const canSubmit = hasFields
-		? Object.values(formValues).some((v) => v !== undefined && v !== null && v !== "")
+		? hasPluginBlockFormData(formValues)
 		: typeof formValues.id === "string" && formValues.id.trim().length > 0;
 
 	return (
@@ -1255,6 +1278,10 @@ export type { PluginBlockDef } from "./editor/PluginBlockNode";
 // Exported for unit testing (pure functions, no React dependencies)
 export { prosemirrorToPortableText as _prosemirrorToPortableText };
 export { portableTextToProsemirror as _portableTextToProsemirror };
+export {
+	buildPluginBlockFormValues as _buildPluginBlockFormValues,
+	hasPluginBlockFormData as _hasPluginBlockFormData,
+};
 
 // =============================================================================
 // Editor Footer with Writing Metrics
diff --git a/packages/admin/tests/editor/PortableTextEditor.test.tsx b/packages/admin/tests/editor/PortableTextEditor.test.tsx
index 7a2f54d7a..31d5d0517 100644
--- a/packages/admin/tests/editor/PortableTextEditor.test.tsx
+++ b/packages/admin/tests/editor/PortableTextEditor.test.tsx
@@ -11,7 +11,11 @@ import * as React from "react";
 import { describe, it, expect, vi } from "vitest";
 
 import type { PluginBlockDef } from "../../src/components/PortableTextEditor";
-import { PortableTextEditor } from "../../src/components/PortableTextEditor";
+import {
+	_buildPluginBlockFormValues,
+	_hasPluginBlockFormData,
+	PortableTextEditor,
+} from "../../src/components/PortableTextEditor";
 import { render } from "../utils/render";
 
 // ---------------------------------------------------------------------------
@@ -173,7 +177,123 @@ function textBlock(
 }
 
 // =============================================================================
-// 1. Portable Text ↔ ProseMirror Conversion (via component)
+// 1. Plugin block helpers
+// =============================================================================
+
+describe("plugin block helpers", () => {
+	it("builds form state from field initial_value defaults", () => {
+		const block: PluginBlockDef = {
+			type: "readingTime",
+			pluginId: "reading-time",
+			label: "Reading Time",
+			fields: [
+				{
+					type: "select",
+					action_id: "variant",
+					label: "Style",
+					options: [
+						{ label: "Inline", value: "inline" },
+						{ label: "Compact", value: "compact" },
+					],
+					initial_value: "inline",
+				},
+				{
+					type: "toggle",
+					action_id: "includeHeadings",
+					label: "Include headings",
+					initial_value: true,
+				},
+			],
+		};
+
+		expect(_buildPluginBlockFormValues(block)).toEqual({
+			variant: "inline",
+			includeHeadings: true,
+		});
+		expect(_hasPluginBlockFormData(_buildPluginBlockFormValues(block))).toBe(true);
+	});
+
+	it("merges existing block data over defaults when editing", () => {
+		const block: PluginBlockDef = {
+			type: "readingTime",
+			pluginId: "reading-time",
+			label: "Reading Time",
+			fields: [
+				{
+					type: "select",
+					action_id: "variant",
+					label: "Style",
+					options: [
+						{ label: "Inline", value: "inline" },
+						{ label: "Compact", value: "compact" },
+					],
+					initial_value: "inline",
+				},
+				{
+					type: "toggle",
+					action_id: "includeHeadings",
+					label: "Include headings",
+					initial_value: true,
+				},
+			],
+		};
+
+		expect(
+			_buildPluginBlockFormValues(block, {
+				variant: "compact",
+				customLabel: "Custom label",
+				includeHeadings: false,
+			}),
+		).toEqual({
+			variant: "compact",
+			customLabel: "Custom label",
+			includeHeadings: false,
+		});
+	});
+
+	it("keeps explicit existing values over defaults", () => {
+		const block: PluginBlockDef = {
+			type: "readingTime",
+			pluginId: "reading-time",
+			label: "Reading Time",
+			fields: [
+				{
+					type: "number_input",
+					action_id: "minutes",
+					label: "Minutes",
+					initial_value: 5,
+				},
+				{
+					type: "text_input",
+					action_id: "label",
+					label: "Label",
+					initial_value: "Default label",
+				},
+				{
+					type: "toggle",
+					action_id: "includeHeadings",
+					label: "Include headings",
+					initial_value: true,
+				},
+			],
+		};
+
+		expect(
+			_buildPluginBlockFormValues(block, {
+				minutes: 0,
+				label: "",
+				includeHeadings: false,
+			}),
+		).toEqual({
+			minutes: 0,
+			label: "",
+			includeHeadings: false,
+		});
+	});
+});
+
+// =============================================================================
+// 2. Portable Text ↔ ProseMirror Conversion (via component)
 // =============================================================================
 
 describe("Portable Text ↔ ProseMirror conversion", () => {