diff --git a/packages/core/src/agent.ts b/packages/core/src/agent.ts
index 23a4e80..fe642c7 100644
--- a/packages/core/src/agent.ts
+++ b/packages/core/src/agent.ts
@@ -70,26 +70,6 @@ import type { ModelEntry } from "./state/index.js";
  */
 const MAX_SUBAGENT_DEPTH = 1;
 
-/** The five valid thinking level names (legacy session_meta additionally recorded the literal "default" for "no level"). */
-const THINKING_LEVEL_NAMES: readonly ThinkingLevelName[] = [
-  "none",
-  "low",
-  "medium",
-  "high",
-  "xhigh",
-];
-
-/**
- * Narrows a legacy session_meta `thinking_level` back to a ThinkingLevelName; the literal
- * "default" (no level recorded), a missing field (current Traces no longer record one), and
- * anything unknown are not levels and yield undefined.
- */
-function asThinkingLevelName(value: unknown): ThinkingLevelName | undefined {
-  return typeof value === "string" && (THINKING_LEVEL_NAMES as readonly string[]).includes(value)
-    ? (value as ThinkingLevelName)
-    : undefined;
-}
-
 export interface CreateAgentOptions {
   agentId?: string;
   projectId?: string;
@@ -394,15 +374,12 @@ export class Agent {
     // original history); the vault uses current values (it's injected into the
     // subprocess environment, not the history, so a resumed Session should get the
     // latest keys too).
-    // Back-compat: session_meta no longer records a thinking level (it became a per-turn
-    // run parameter), but OLD Traces still carry `thinking_level` in their meta JSON — when
-    // present, keep honoring it as this Session's default level (a resumed legacy subagent
-    // session keeps its inherited level instead of re-reading this Agent's config). The
-    // field is read loosely (it's gone from SessionMetaPayload); the legacy literal
-    // "default" — and any current Trace without the field — falls back to the Agent config.
-    const thinkingLevel =
-      asThinkingLevelName((meta as unknown as Record<string, unknown>).thinking_level) ??
-      this.state.systemConfig.model?.thinking_level;
+    // The default thinking level comes from this Agent's current config only: session_meta
+    // no longer records one (it became a per-turn run parameter), and a `thinking_level`
+    // still present in a legacy Trace's meta JSON is deliberately ignored — a resumed
+    // legacy subagent session falls back to this Agent's configured level instead of
+    // keeping the level it inherited at spawn time.
+    const thinkingLevel = this.state.systemConfig.model?.thinking_level;
 
     const rt = await this.buildRuntime({
       workspaceDir,
@@ -442,8 +419,7 @@ export class Agent {
     });
 
     return new Session({
-      // Invariants only (the legacy thinking_level, when honored above, feeds the LLM default
-      // but is not re-recorded — new meta writes never contain it).
+      // Invariants only — meta writes never contain a thinking level.
       meta: {
         session_id: sessionId,
         provider: modelEntry.provider,
diff --git a/packages/core/src/omnimessage/types.ts b/packages/core/src/omnimessage/types.ts
index 2093c53..3fe2cb0 100644
--- a/packages/core/src/omnimessage/types.ts
+++ b/packages/core/src/omnimessage/types.ts
@@ -83,8 +83,8 @@ export interface ToolDefinition {
 /**
  * Session metadata. Holds **per-session invariants only** — values fixed for the Session's
  * lifetime (model reference, assembled system prompt, tool schemas, paths, origin). Per-turn
- * parameters (e.g. the thinking level, passed with each run) never belong here; legacy Traces
- * may still carry a `thinking_level` field, which resume reads loosely for back-compat.
+ * parameters (e.g. the thinking level, passed with each run) never belong here; a
+ * `thinking_level` field still present in a legacy Trace's meta is ignored on resume.
  */
 export interface SessionMetaPayload {
   session_id: string;
diff --git a/packages/core/test/resume.test.ts b/packages/core/test/resume.test.ts
index 54ec2e9..b69abfe 100644
--- a/packages/core/test/resume.test.ts
+++ b/packages/core/test/resume.test.ts
@@ -175,11 +175,12 @@ describe("agent.resumeSession", () => {
     ).toEqual(["text", "text", "abort"]);
   });
 
-  it("honors a legacy trace's recorded thinking_level; new meta never re-records it", async () => {
-    // session_meta no longer carries a thinking level (it became a per-turn run parameter),
-    // but OLD traces still have it in their meta JSON: resume must keep honoring it as the
-    // session's default level (a legacy subagent session keeps its inherited level instead of
-    // re-reading this Agent's config). The seeded Agent config here pins "medium".
+  it("ignores a legacy trace's recorded thinking_level; the Agent config always wins", async () => {
+    // session_meta no longer carries a thinking level (it became a per-turn run parameter);
+    // a `thinking_level` still present in an OLD trace's meta JSON is deliberately ignored —
+    // resume always reads this Agent's current config, so a resumed legacy subagent session
+    // falls back to the config level instead of keeping the level it inherited at spawn
+    // time. The seeded Agent config here pins "medium".
     const agent = await createAgent({});
     expect(agent.state.systemConfig.model?.thinking_level).toBe("medium");
     const levelOf = (session: unknown): unknown =>
@@ -190,14 +191,14 @@ describe("agent.resumeSession", () => {
     // A legacy trace: inject the retired field loosely into the on-disk meta JSON.
     (recorded.payload as unknown as Record<string, unknown>).thinking_level = "xhigh";
     await writeTraceFile(tmpRoot, SID, [recorded, userText("hello")]);
-    const inherited = await agent.resumeSession({ sessionId: SID });
-    expect(levelOf(inherited)).toBe("xhigh");
-    // The rebuilt meta holds invariants only: the legacy field is honored but never re-recorded.
+    const ignored = await agent.resumeSession({ sessionId: SID });
+    expect(levelOf(ignored)).toBe("medium");
+    // The rebuilt meta holds invariants only: the legacy field is never re-recorded either.
     expect(
-      "thinking_level" in (inherited.metaMessage.payload as unknown as Record<string, unknown>),
+      "thinking_level" in (ignored.metaMessage.payload as unknown as Record<string, unknown>),
     ).toBe(false);
 
-    // A current trace (no field) — and the legacy literal "default" — fall back to the Agent config.
+    // A current trace (no field) — and the legacy literal "default" — read the Agent config too.
     const SID2 = "session-2026-07-06-11-00-00-abcdef02";
     await writeTraceFile(tmpRoot, SID2, [metaFor(SID2, workspace), userText("hi")]);
     const fallback = await agent.resumeSession({ sessionId: SID2 });
diff --git a/packages/docs/content/omni-message.en.md b/packages/docs/content/omni-message.en.md
index df6752d..4ded091 100644
--- a/packages/docs/content/omni-message.en.md
+++ b/packages/docs/content/omni-message.en.md
@@ -50,7 +50,7 @@ interface ToolDefinition {
 }
 ```
 
-session_meta holds **per-session invariants only** — the model, system prompt and Workspace are immutable for the Session's lifetime; on resume, the engine takes this Trace line as the runtime config. See [Sessions & Traces](/sessions-and-traces). The thinking level is a per-turn parameter (sent with each Task) and is not recorded here; legacy Traces may still carry a `thinking_level` field in their meta, which resume keeps honoring for back-compat.
+session_meta holds **per-session invariants only** — the model, system prompt and Workspace are immutable for the Session's lifetime; on resume, the engine takes this Trace line as the runtime config. See [Sessions & Traces](/sessions-and-traces). The thinking level is a per-turn parameter (sent with each Task) and is not recorded here; a `thinking_level` field still present in a legacy Trace's meta is ignored on resume — the resumed Session reads the Agent's current config instead.
 
 ## model_msg: complete payloads
 
diff --git a/packages/docs/content/omni-message.zh.md b/packages/docs/content/omni-message.zh.md
index d5609da..82c6c8a 100644
--- a/packages/docs/content/omni-message.zh.md
+++ b/packages/docs/content/omni-message.zh.md
@@ -50,7 +50,7 @@ interface ToolDefinition {
 }
 ```
 
-session_meta 只承载**会话级不变量**——模型、系统提示词、Workspace 在 Session 生命周期内不可变；恢复 Session 时引擎直接以 Trace 中的这条消息为运行时配置，见 [Session 与 Trace](/sessions-and-traces)。思考等级是逐轮参数（随每次 Task 下发），不记录在此；旧版 Trace 的 meta 里可能仍带 `thinking_level` 字段，恢复时按兼容逻辑继续生效。
+session_meta 只承载**会话级不变量**——模型、系统提示词、Workspace 在 Session 生命周期内不可变；恢复 Session 时引擎直接以 Trace 中的这条消息为运行时配置，见 [Session 与 Trace](/sessions-and-traces)。思考等级是逐轮参数（随每次 Task 下发），不记录在此；旧版 Trace 的 meta 里可能仍带 `thinking_level` 字段，恢复时会被忽略——恢复后的 Session 直接读取 Agent 当前配置。
 
 ## model_msg：完整消息