diff --git a/packages/coding-agent/src/core/agent-session.ts b/packages/coding-agent/src/core/agent-session.ts
index b75febf0..fe450e5e 100644
--- a/packages/coding-agent/src/core/agent-session.ts
+++ b/packages/coding-agent/src/core/agent-session.ts
@@ -40,6 +40,7 @@ import {
 	prepareCompaction,
 	shouldCompact,
 } from "./compaction/index.js";
+import { DEFAULT_THINKING_LEVEL } from "./defaults.js";
 import { exportSessionToHtml, type ToolHtmlRenderer } from "./export-html/index.js";
 import { createToolHtmlRenderer } from "./export-html/tool-renderer.js";
 import {
@@ -1094,6 +1095,8 @@ export class AgentSession {
 		this._followUpMessages = [];
 		this._pendingNextTurnMessages = [];
 
+		this.sessionManager.appendThinkingLevelChange(this.thinkingLevel);
+
 		// Run setup callback if provided (e.g., to append initial messages)
 		if (options?.setup) {
 			await options.setup(this.sessionManager);
@@ -2303,9 +2306,19 @@ export class AgentSession {
 			}
 		}
 
-		// Restore thinking level if saved (setThinkingLevel clamps to model capabilities)
-		if (sessionContext.thinkingLevel) {
+		const hasThinkingEntry = this.sessionManager.getBranch().some((entry) => entry.type === "thinking_level_change");
+		const defaultThinkingLevel = this.settingsManager.getDefaultThinkingLevel() ?? DEFAULT_THINKING_LEVEL;
+
+		if (hasThinkingEntry) {
+			// Restore thinking level if saved (setThinkingLevel clamps to model capabilities)
 			this.setThinkingLevel(sessionContext.thinkingLevel as ThinkingLevel);
+		} else {
+			const availableLevels = this.getAvailableThinkingLevels();
+			const effectiveLevel = availableLevels.includes(defaultThinkingLevel)
+				? defaultThinkingLevel
+				: this._clampThinkingLevel(defaultThinkingLevel, availableLevels);
+			this.agent.setThinkingLevel(effectiveLevel);
+			this.sessionManager.appendThinkingLevelChange(effectiveLevel);
 		}
 
 		this._reconnectToAgent();
diff --git a/packages/coding-agent/src/core/sdk.ts b/packages/coding-agent/src/core/sdk.ts
index beca4417..6b191d2c 100644
--- a/packages/coding-agent/src/core/sdk.ts
+++ b/packages/coding-agent/src/core/sdk.ts
@@ -185,6 +185,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 	// Check if session has existing data to restore
 	const existingSession = sessionManager.buildSessionContext();
 	const hasExistingSession = existingSession.messages.length > 0;
+	const hasThinkingEntry = sessionManager.getBranch().some((entry) => entry.type === "thinking_level_change");
 
 	let model = options.model;
 	let modelFallbackMessage: string | undefined;
@@ -222,7 +223,9 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 
 	// If session has data, restore thinking level from it
 	if (thinkingLevel === undefined && hasExistingSession) {
-		thinkingLevel = existingSession.thinkingLevel as ThinkingLevel;
+		thinkingLevel = hasThinkingEntry
+			? (existingSession.thinkingLevel as ThinkingLevel)
+			: (settingsManager.getDefaultThinkingLevel() ?? DEFAULT_THINKING_LEVEL);
 	}
 
 	// Fall back to settings default
@@ -329,6 +332,9 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 	// Restore messages if session has existing data
 	if (hasExistingSession) {
 		agent.replaceMessages(existingSession.messages);
+		if (!hasThinkingEntry) {
+			sessionManager.appendThinkingLevelChange(thinkingLevel);
+		}
 	} else {
 		// Save initial model and thinking level for new sessions so they can be restored on resume
 		if (model) {