diff --git a/src/features/builtin-commands/commands.ts b/src/features/builtin-commands/commands.ts
index 053c3eed..d4e10903 100644
--- a/src/features/builtin-commands/commands.ts
+++ b/src/features/builtin-commands/commands.ts
@@ -55,7 +55,7 @@ ${REFACTOR_TEMPLATE}
   },
   "start-work": {
     description: "(builtin) Start Sisyphus work session from Prometheus plan",
-    agent: "Atlas",
+    agent: "atlas",
     template: `<command-instruction>
 ${START_WORK_TEMPLATE}
 </command-instruction>
diff --git a/src/hooks/atlas/index.test.ts b/src/hooks/atlas/index.test.ts
index 4a90f9ae..6683f7c7 100644
--- a/src/hooks/atlas/index.test.ts
+++ b/src/hooks/atlas/index.test.ts
@@ -123,7 +123,7 @@ describe("atlas hook", () => {
      test("should append standalone verification when no boulder state but caller is Atlas", async () => {
        // #given - no boulder state, but caller is Atlas
        const sessionID = "session-no-boulder-test"
-       setupMessageStorage(sessionID, "Atlas")
+       setupMessageStorage(sessionID, "atlas")
       
       const hook = createAtlasHook(createMockPluginInput())
       const output = {
@@ -149,7 +149,7 @@ describe("atlas hook", () => {
      test("should transform output when caller is Atlas with boulder state", async () => {
        // #given - Atlas caller with boulder state
        const sessionID = "session-transform-test"
-       setupMessageStorage(sessionID, "Atlas")
+       setupMessageStorage(sessionID, "atlas")
       
       const planPath = join(TEST_DIR, "test-plan.md")
       writeFileSync(planPath, "# Plan\n- [ ] Task 1\n- [x] Task 2")
@@ -188,7 +188,7 @@ describe("atlas hook", () => {
      test("should still transform when plan is complete (shows progress)", async () => {
        // #given - boulder state with complete plan, Atlas caller
        const sessionID = "session-complete-plan-test"
-       setupMessageStorage(sessionID, "Atlas")
+       setupMessageStorage(sessionID, "atlas")
       
       const planPath = join(TEST_DIR, "complete-plan.md")
       writeFileSync(planPath, "# Plan\n- [x] Task 1\n- [x] Task 2")
@@ -225,7 +225,7 @@ describe("atlas hook", () => {
      test("should append session ID to boulder state if not present", async () => {
        // #given - boulder state without session-append-test, Atlas caller
        const sessionID = "session-append-test"
-       setupMessageStorage(sessionID, "Atlas")
+       setupMessageStorage(sessionID, "atlas")
       
       const planPath = join(TEST_DIR, "test-plan.md")
       writeFileSync(planPath, "# Plan\n- [ ] Task 1")
@@ -261,7 +261,7 @@ describe("atlas hook", () => {
      test("should not duplicate existing session ID", async () => {
        // #given - boulder state already has session-dup-test, Atlas caller
        const sessionID = "session-dup-test"
-       setupMessageStorage(sessionID, "Atlas")
+       setupMessageStorage(sessionID, "atlas")
       
       const planPath = join(TEST_DIR, "test-plan.md")
       writeFileSync(planPath, "# Plan\n- [ ] Task 1")
@@ -298,7 +298,7 @@ describe("atlas hook", () => {
      test("should include boulder.json path and notepad path in transformed output", async () => {
        // #given - boulder state, Atlas caller
        const sessionID = "session-path-test"
-       setupMessageStorage(sessionID, "Atlas")
+       setupMessageStorage(sessionID, "atlas")
       
       const planPath = join(TEST_DIR, "my-feature.md")
       writeFileSync(planPath, "# Plan\n- [ ] Task 1\n- [ ] Task 2\n- [x] Task 3")
@@ -335,7 +335,7 @@ describe("atlas hook", () => {
      test("should include resume and checkbox instructions in reminder", async () => {
        // #given - boulder state, Atlas caller
        const sessionID = "session-resume-test"
-       setupMessageStorage(sessionID, "Atlas")
+       setupMessageStorage(sessionID, "atlas")
       
       const planPath = join(TEST_DIR, "test-plan.md")
       writeFileSync(planPath, "# Plan\n- [ ] Task 1")
diff --git a/src/hooks/atlas/index.ts b/src/hooks/atlas/index.ts
index 9852ed8b..b8bcdf5f 100644
--- a/src/hooks/atlas/index.ts
+++ b/src/hooks/atlas/index.ts
@@ -498,7 +498,7 @@ export function createAtlasHook(
        await ctx.client.session.prompt({
          path: { id: sessionID },
          body: {
-            agent: "Atlas",
+            agent: "atlas",
            ...(model !== undefined ? { model } : {}),
            parts: [{ type: "text", text: prompt }],
          },
diff --git a/src/hooks/start-work/index.test.ts b/src/hooks/start-work/index.test.ts
index df4a2063..32067f29 100644
--- a/src/hooks/start-work/index.test.ts
+++ b/src/hooks/start-work/index.test.ts
@@ -395,7 +395,7 @@ describe("start-work hook", () => {
       )
 
       // #then
-      expect(updateSpy).toHaveBeenCalledWith("ses-prometheus-to-sisyphus", "Atlas")
+      expect(updateSpy).toHaveBeenCalledWith("ses-prometheus-to-sisyphus", "atlas")
       updateSpy.mockRestore()
     })
   })
diff --git a/src/hooks/start-work/index.ts b/src/hooks/start-work/index.ts
index dc03d3c7..4f3d528b 100644
--- a/src/hooks/start-work/index.ts
+++ b/src/hooks/start-work/index.ts
@@ -71,7 +71,7 @@ export function createStartWorkHook(ctx: PluginInput) {
         sessionID: input.sessionID,
       })
 
-      updateSessionAgent(input.sessionID, "Atlas")
+      updateSessionAgent(input.sessionID, "atlas")
 
       const existingState = readBoulderState(ctx.directory)
       const sessionId = input.sessionID