diff --git a/src/plugin-handlers/agent-config-handler.ts b/src/plugin-handlers/agent-config-handler.ts
index 91cbf2a9..ca2e8d2c 100644
--- a/src/plugin-handlers/agent-config-handler.ts
+++ b/src/plugin-handlers/agent-config-handler.ts
@@ -23,9 +23,12 @@ type AgentConfigRecord = Record<string, Record<string, unknown> | undefined> & {
   plan?: Record<string, unknown>;
 };
 
-function hasConfiguredDefaultAgent(config: Record<string, unknown>): boolean {
+function getConfiguredDefaultAgent(config: Record<string, unknown>): string | undefined {
   const defaultAgent = config.default_agent;
-  return typeof defaultAgent === "string" && defaultAgent.trim().length > 0;
+  if (typeof defaultAgent !== "string") return undefined;
+
+  const trimmedDefaultAgent = defaultAgent.trim();
+  return trimmedDefaultAgent.length > 0 ? trimmedDefaultAgent : undefined;
 }
 
 export async function applyAgentConfig(params: {
@@ -107,11 +110,15 @@ export async function applyAgentConfig(params: {
   const plannerEnabled = params.pluginConfig.sisyphus_agent?.planner_enabled ?? true;
   const replacePlan = params.pluginConfig.sisyphus_agent?.replace_plan ?? true;
   const shouldDemotePlan = plannerEnabled && replacePlan;
+  const configuredDefaultAgent = getConfiguredDefaultAgent(params.config);
 
   const configAgent = params.config.agent as AgentConfigRecord | undefined;
 
   if (isSisyphusEnabled && builtinAgents.sisyphus) {
-    if (!hasConfiguredDefaultAgent(params.config)) {
+    if (configuredDefaultAgent) {
+      (params.config as { default_agent?: string }).default_agent =
+        getAgentDisplayName(configuredDefaultAgent);
+    } else {
       (params.config as { default_agent?: string }).default_agent =
         getAgentDisplayName("sisyphus");
     }
diff --git a/src/plugin-handlers/config-handler.test.ts b/src/plugin-handlers/config-handler.test.ts
index c8e25f58..ebba28ea 100644
--- a/src/plugin-handlers/config-handler.test.ts
+++ b/src/plugin-handlers/config-handler.test.ts
@@ -350,7 +350,55 @@ describe("Agent permission defaults", () => {
 })
 
 describe("default_agent behavior with Sisyphus orchestration", () => {
-  test("preserves existing default_agent when already set", async () => {
+  test("canonicalizes configured default_agent with surrounding whitespace", async () => {
+    // given
+    const pluginConfig: OhMyOpenCodeConfig = {}
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      default_agent: "  hephaestus  ",
+      agent: {},
+    }
+    const handler = createConfigHandler({
+      ctx: { directory: "/tmp" },
+      pluginConfig,
+      modelCacheState: {
+        anthropicContext1MEnabled: false,
+        modelContextLimitsCache: new Map(),
+      },
+    })
+
+    // when
+    await handler(config)
+
+    // then
+    expect(config.default_agent).toBe(getAgentDisplayName("hephaestus"))
+  })
+
+  test("canonicalizes configured default_agent when key uses mixed case", async () => {
+    // given
+    const pluginConfig: OhMyOpenCodeConfig = {}
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      default_agent: "HePhAeStUs",
+      agent: {},
+    }
+    const handler = createConfigHandler({
+      ctx: { directory: "/tmp" },
+      pluginConfig,
+      modelCacheState: {
+        anthropicContext1MEnabled: false,
+        modelContextLimitsCache: new Map(),
+      },
+    })
+
+    // when
+    await handler(config)
+
+    // then
+    expect(config.default_agent).toBe(getAgentDisplayName("hephaestus"))
+  })
+
+  test("canonicalizes configured default_agent key to display name", async () => {
     // #given
     const pluginConfig: OhMyOpenCodeConfig = {}
     const config: Record<string, unknown> = {
@@ -371,7 +419,32 @@ describe("default_agent behavior with Sisyphus orchestration", () => {
     await handler(config)
 
     // #then
-    expect(config.default_agent).toBe("hephaestus")
+    expect(config.default_agent).toBe(getAgentDisplayName("hephaestus"))
+  })
+
+  test("preserves existing display-name default_agent", async () => {
+    // #given
+    const pluginConfig: OhMyOpenCodeConfig = {}
+    const displayName = getAgentDisplayName("hephaestus")
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      default_agent: displayName,
+      agent: {},
+    }
+    const handler = createConfigHandler({
+      ctx: { directory: "/tmp" },
+      pluginConfig,
+      modelCacheState: {
+        anthropicContext1MEnabled: false,
+        modelContextLimitsCache: new Map(),
+      },
+    })
+
+    // #when
+    await handler(config)
+
+    // #then
+    expect(config.default_agent).toBe(displayName)
   })
 
   test("sets default_agent to sisyphus when missing", async () => {
@@ -396,6 +469,82 @@ describe("default_agent behavior with Sisyphus orchestration", () => {
     // #then
     expect(config.default_agent).toBe(getAgentDisplayName("sisyphus"))
   })
+
+  test("sets default_agent to sisyphus when configured default_agent is empty after trim", async () => {
+    // given
+    const pluginConfig: OhMyOpenCodeConfig = {}
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      default_agent: "    ",
+      agent: {},
+    }
+    const handler = createConfigHandler({
+      ctx: { directory: "/tmp" },
+      pluginConfig,
+      modelCacheState: {
+        anthropicContext1MEnabled: false,
+        modelContextLimitsCache: new Map(),
+      },
+    })
+
+    // when
+    await handler(config)
+
+    // then
+    expect(config.default_agent).toBe(getAgentDisplayName("sisyphus"))
+  })
+
+  test("preserves custom default_agent names while trimming whitespace", async () => {
+    // given
+    const pluginConfig: OhMyOpenCodeConfig = {}
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      default_agent: "  Custom Agent  ",
+      agent: {},
+    }
+    const handler = createConfigHandler({
+      ctx: { directory: "/tmp" },
+      pluginConfig,
+      modelCacheState: {
+        anthropicContext1MEnabled: false,
+        modelContextLimitsCache: new Map(),
+      },
+    })
+
+    // when
+    await handler(config)
+
+    // then
+    expect(config.default_agent).toBe("Custom Agent")
+  })
+
+  test("does not normalize configured default_agent when Sisyphus is disabled", async () => {
+    // given
+    const pluginConfig: OhMyOpenCodeConfig = {
+      sisyphus_agent: {
+        disabled: true,
+      },
+    }
+    const config: Record<string, unknown> = {
+      model: "anthropic/claude-opus-4-6",
+      default_agent: "  HePhAeStUs  ",
+      agent: {},
+    }
+    const handler = createConfigHandler({
+      ctx: { directory: "/tmp" },
+      pluginConfig,
+      modelCacheState: {
+        anthropicContext1MEnabled: false,
+        modelContextLimitsCache: new Map(),
+      },
+    })
+
+    // when
+    await handler(config)
+
+    // then
+    expect(config.default_agent).toBe("  HePhAeStUs  ")
+  })
 })
 
 describe("Prometheus category config resolution", () => {