diff --git a/packages/coding-agent/CHANGELOG.md b/packages/coding-agent/CHANGELOG.md
index eaf139ae..a77e38da 100644
--- a/packages/coding-agent/CHANGELOG.md
+++ b/packages/coding-agent/CHANGELOG.md
@@ -13,7 +13,8 @@
   - `saveCompaction(entry)` replaced with `appendCompaction(summary, firstKeptEntryId, tokensBefore)`
   - `getEntries()` now excludes the session header (use `getHeader()` separately)
   - New methods: `getTree()`, `getPath()`, `getLeafUuid()`, `getLeafEntry()`, `getEntry()`, `branchWithSummary()`
-  - New `appendCustomEntry(customType, data)` for hooks to store custom data
+  - New `appendCustomEntry(customType, data)` for hooks to store custom data (not in LLM context)
+  - New `appendCustomMessageEntry(customType, content, display, details?)` for hooks to inject messages into LLM context
 - **Compaction API**:
   - `CompactionEntry<T>` and `CompactionResult<T>` are now generic with optional `details?: T` for hook-specific data
   - `compact()` now returns `CompactionResult` (`{ summary, firstKeptEntryId, tokensBefore, details? }`) instead of `CompactionEntry`
diff --git a/packages/coding-agent/docs/session-tree-plan.md b/packages/coding-agent/docs/session-tree-plan.md
index 8dfa9bbb..b6ac5f45 100644
--- a/packages/coding-agent/docs/session-tree-plan.md
+++ b/packages/coding-agent/docs/session-tree-plan.md
@@ -118,14 +118,14 @@ export interface CustomMessageEntry<T = unknown> extends SessionEntryBase {
 ```
 
 Behavior:
-- [ ] Participates in context and compaction as user messages (after messageTransformer)
-- [ ] Not displayed as user messages in TUI
-- [ ] Display options:
+- [x] Type definition matching plan
+- [x] `appendCustomMessageEntry(customType, content, display, details?)` in SessionManager
+- [x] `buildSessionContext()` includes custom_message entries as user messages
+- [x] Exported from main index
+- [ ] TUI rendering:
   - `display: false` - hidden entirely
   - `display: true` - baseline renderer (content with different bg/fg color)
-  - Custom renderer defined by the hook that contributes it
-- [ ] Define injection mechanism for hooks to add CustomMessageEntry
-- [ ] Hook registration for custom renderers
+  - Custom renderer defined by the hook that contributes it (future)
 
 See also: `CustomEntry<T>` for storing hook state that does NOT participate in context.
 
diff --git a/packages/coding-agent/src/core/session-manager.ts b/packages/coding-agent/src/core/session-manager.ts
index 79d610c7..02e43194 100644
--- a/packages/coding-agent/src/core/session-manager.ts
+++ b/packages/coding-agent/src/core/session-manager.ts
@@ -1,4 +1,4 @@
-import type { AppMessage } from "@mariozechner/pi-agent-core";
+import type { AppMessage, Attachment } from "@mariozechner/pi-agent-core";
 import { randomUUID } from "crypto";
 import {
 	appendFileSync,
@@ -86,6 +86,26 @@ export interface LabelEntry extends SessionEntryBase {
 	label: string | undefined;
 }
 
+/**
+ * Custom message entry for hooks to inject messages into LLM context.
+ * Use customType to identify your hook's entries.
+ *
+ * Unlike CustomEntry, this DOES participate in LLM context.
+ * The content is converted to a user message in buildSessionContext().
+ * Use details for hook-specific metadata (not sent to LLM).
+ *
+ * display controls TUI rendering:
+ * - false: hidden entirely
+ * - true: rendered with distinct styling (different from user messages)
+ */
+export interface CustomMessageEntry<T = unknown> extends SessionEntryBase {
+	type: "custom_message";
+	customType: string;
+	content: (string | Attachment)[];
+	details?: T;
+	display: boolean;
+}
+
 /** Session entry - has id/parentId for tree structure (returned by "read" methods in SessionManager) */
 export type SessionEntry =
 	| SessionMessageEntry
@@ -94,6 +114,7 @@ export type SessionEntry =
 	| CompactionEntry
 	| BranchSummaryEntry
 	| CustomEntry
+	| CustomMessageEntry
 	| LabelEntry;
 
 /** Raw file entry (includes header) */
@@ -140,6 +161,35 @@ export function createSummaryMessage(summary: string, timestamp: string): AppMes
 	};
 }
 
+/** Convert CustomMessageEntry content to AppMessage format */
+function createCustomMessage(entry: CustomMessageEntry): AppMessage {
+	// Convert content array to AppMessage content format
+	const content = entry.content.map((item) => {
+		if (typeof item === "string") {
+			return { type: "text" as const, text: item };
+		}
+		// Attachment - convert to appropriate content type
+		if (item.type === "image") {
+			return {
+				type: "image" as const,
+				data: item.content,
+				mimeType: item.mimeType,
+			};
+		}
+		// Document attachment - use extracted text or indicate document
+		return {
+			type: "text" as const,
+			text: item.extractedText ?? `[Document: ${item.fileName}]`,
+		};
+	});
+
+	return {
+		role: "user",
+		content,
+		timestamp: new Date(entry.timestamp).getTime(),
+	};
+}
+
 /** Generate a unique short ID (8 hex chars, collision-checked) */
 function generateId(byId: { has(id: string): boolean }): string {
 	for (let i = 0; i < 100; i++) {
@@ -308,8 +358,12 @@ export function buildSessionContext(
 			if (entry.id === compaction.firstKeptEntryId) {
 				foundFirstKept = true;
 			}
-			if (foundFirstKept && entry.type === "message") {
-				messages.push(entry.message);
+			if (foundFirstKept) {
+				if (entry.type === "message") {
+					messages.push(entry.message);
+				} else if (entry.type === "custom_message") {
+					messages.push(createCustomMessage(entry));
+				}
 			}
 		}
 
@@ -318,15 +372,19 @@ export function buildSessionContext(
 			const entry = path[i];
 			if (entry.type === "message") {
 				messages.push(entry.message);
+			} else if (entry.type === "custom_message") {
+				messages.push(createCustomMessage(entry));
 			} else if (entry.type === "branch_summary") {
 				messages.push(createSummaryMessage(entry.summary, entry.timestamp));
 			}
 		}
 	} else {
-		// No compaction - emit all messages, handle branch summaries
+		// No compaction - emit all messages, handle branch summaries and custom messages
 		for (const entry of path) {
 			if (entry.type === "message") {
 				messages.push(entry.message);
+			} else if (entry.type === "custom_message") {
+				messages.push(createCustomMessage(entry));
 			} else if (entry.type === "branch_summary") {
 				messages.push(createSummaryMessage(entry.summary, entry.timestamp));
 			}
@@ -623,6 +681,34 @@ export class SessionManager {
 		return entry.id;
 	}
 
+	/**
+	 * Append a custom message entry (for hooks) that participates in LLM context.
+	 * @param customType Hook identifier for filtering on reload
+	 * @param content Message content (strings and attachments)
+	 * @param display Whether to show in TUI (true = styled display, false = hidden)
+	 * @param details Optional hook-specific metadata (not sent to LLM)
+	 * @returns Entry id
+	 */
+	appendCustomMessageEntry<T = unknown>(
+		customType: string,
+		content: (string | Attachment)[],
+		display: boolean,
+		details?: T,
+	): string {
+		const entry: CustomMessageEntry<T> = {
+			type: "custom_message",
+			customType,
+			content,
+			display,
+			details,
+			id: generateId(this.byId),
+			parentId: this.leafId || null,
+			timestamp: new Date().toISOString(),
+		};
+		this._appendEntry(entry);
+		return entry.id;
+	}
+
 	// =========================================================================
 	// Tree Traversal
 	// =========================================================================
diff --git a/packages/coding-agent/src/index.ts b/packages/coding-agent/src/index.ts
index acfc766a..55909056 100644
--- a/packages/coding-agent/src/index.ts
+++ b/packages/coding-agent/src/index.ts
@@ -111,6 +111,8 @@ export {
 	buildSessionContext,
 	type CompactionEntry,
 	CURRENT_SESSION_VERSION,
+	type CustomEntry,
+	type CustomMessageEntry,
 	createSummaryMessage,
 	type FileEntry,
 	getLatestCompactionEntry,