diff --git a/src/plugin/tool-execute-before.test.ts b/src/plugin/tool-execute-before.test.ts
new file mode 100644
index 000000000..1b5b80788
--- /dev/null
+++ b/src/plugin/tool-execute-before.test.ts
@@ -0,0 +1,138 @@
+import { describe, expect, test } from "bun:test"
+import { createToolExecuteBeforeHandler } from "./tool-execute-before"
+import type { CreatedHooks } from "../create-hooks"
+
+describe("createToolExecuteBeforeHandler", () => {
+  describe("task tool subagent_type normalization", () => {
+    const emptyHooks = {} as CreatedHooks
+
+    function createCtxWithSessionMessages(messages: Array<{ info?: { agent?: string; role?: string } }> = []) {
+      return {
+        client: {
+          session: {
+            messages: async () => ({ data: messages }),
+          },
+        },
+      } as unknown as Parameters<typeof createToolExecuteBeforeHandler>[0]["ctx"]
+    }
+
+    test("sets subagent_type to sisyphus-junior when category is provided without subagent_type", async () => {
+      //#given
+      const ctx = createCtxWithSessionMessages()
+      const handler = createToolExecuteBeforeHandler({ ctx, hooks: emptyHooks })
+      const input = { tool: "task", sessionID: "ses_123", callID: "call_1" }
+      const output = { args: { category: "quick", description: "Test" } as Record<string, unknown> }
+
+      //#when
+      await handler(input, output)
+
+      //#then
+      expect(output.args.subagent_type).toBe("sisyphus-junior")
+    })
+
+    test("preserves existing subagent_type when explicitly provided", async () => {
+      //#given
+      const ctx = createCtxWithSessionMessages()
+      const handler = createToolExecuteBeforeHandler({ ctx, hooks: emptyHooks })
+      const input = { tool: "task", sessionID: "ses_123", callID: "call_1" }
+      const output = { args: { subagent_type: "plan", description: "Plan test" } as Record<string, unknown> }
+
+      //#when
+      await handler(input, output)
+
+      //#then
+      expect(output.args.subagent_type).toBe("plan")
+    })
+
+    test("sets subagent_type to sisyphus-junior when category provided with different subagent_type", async () => {
+      //#given
+      const ctx = createCtxWithSessionMessages()
+      const handler = createToolExecuteBeforeHandler({ ctx, hooks: emptyHooks })
+      const input = { tool: "task", sessionID: "ses_123", callID: "call_1" }
+      const output = { args: { category: "quick", subagent_type: "oracle", description: "Test" } as Record<string, unknown> }
+
+      //#when
+      await handler(input, output)
+
+      //#then
+      expect(output.args.subagent_type).toBe("sisyphus-junior")
+    })
+
+    test("resolves subagent_type from session first message when session_id provided without subagent_type", async () => {
+      //#given
+      const ctx = createCtxWithSessionMessages([
+        { info: { role: "user" } },
+        { info: { role: "assistant", agent: "explore" } },
+        { info: { role: "assistant", agent: "oracle" } },
+      ])
+      const handler = createToolExecuteBeforeHandler({ ctx, hooks: emptyHooks })
+      const input = { tool: "task", sessionID: "ses_123", callID: "call_1" }
+      const output = { args: { session_id: "ses_abc123", description: "Continue task", prompt: "fix it" } as Record<string, unknown> }
+
+      //#when
+      await handler(input, output)
+
+      //#then
+      expect(output.args.subagent_type).toBe("explore")
+    })
+
+    test("falls back to 'continue' when session has no agent info", async () => {
+      //#given
+      const ctx = createCtxWithSessionMessages([
+        { info: { role: "user" } },
+        { info: { role: "assistant" } },
+      ])
+      const handler = createToolExecuteBeforeHandler({ ctx, hooks: emptyHooks })
+      const input = { tool: "task", sessionID: "ses_123", callID: "call_1" }
+      const output = { args: { session_id: "ses_abc123", description: "Continue task", prompt: "fix it" } as Record<string, unknown> }
+
+      //#when
+      await handler(input, output)
+
+      //#then
+      expect(output.args.subagent_type).toBe("continue")
+    })
+
+    test("preserves subagent_type when session_id is provided with explicit subagent_type", async () => {
+      //#given
+      const ctx = createCtxWithSessionMessages()
+      const handler = createToolExecuteBeforeHandler({ ctx, hooks: emptyHooks })
+      const input = { tool: "task", sessionID: "ses_123", callID: "call_1" }
+      const output = { args: { session_id: "ses_abc123", subagent_type: "explore", description: "Continue explore" } as Record<string, unknown> }
+
+      //#when
+      await handler(input, output)
+
+      //#then
+      expect(output.args.subagent_type).toBe("explore")
+    })
+
+    test("does not modify args for non-task tools", async () => {
+      //#given
+      const ctx = createCtxWithSessionMessages()
+      const handler = createToolExecuteBeforeHandler({ ctx, hooks: emptyHooks })
+      const input = { tool: "bash", sessionID: "ses_123", callID: "call_1" }
+      const output = { args: { command: "ls" } as Record<string, unknown> }
+
+      //#when
+      await handler(input, output)
+
+      //#then
+      expect(output.args.subagent_type).toBeUndefined()
+    })
+
+    test("does not set subagent_type when neither category nor session_id is provided and subagent_type is present", async () => {
+      //#given
+      const ctx = createCtxWithSessionMessages()
+      const handler = createToolExecuteBeforeHandler({ ctx, hooks: emptyHooks })
+      const input = { tool: "task", sessionID: "ses_123", callID: "call_1" }
+      const output = { args: { subagent_type: "oracle", description: "Oracle task" } as Record<string, unknown> }
+
+      //#when
+      await handler(input, output)
+
+      //#then
+      expect(output.args.subagent_type).toBe("oracle")
+    })
+  })
+})
diff --git a/src/plugin/tool-execute-before.ts b/src/plugin/tool-execute-before.ts
index c7fefb0a6..9d07e5efa 100644
--- a/src/plugin/tool-execute-before.ts
+++ b/src/plugin/tool-execute-before.ts
@@ -3,6 +3,7 @@ import type { PluginContext } from "./types"
 import { getMainSessionID } from "../features/claude-code-session-state"
 import { clearBoulderState } from "../features/boulder-state"
 import { log } from "../shared"
+import { resolveSessionAgent } from "./session-agent-resolver"
 
 import type { CreatedHooks } from "../create-hooks"
 
@@ -34,8 +35,13 @@ export function createToolExecuteBeforeHandler(args: {
       const argsObject = output.args
       const category = typeof argsObject.category === "string" ? argsObject.category : undefined
       const subagentType = typeof argsObject.subagent_type === "string" ? argsObject.subagent_type : undefined
-      if (category && !subagentType) {
+      const sessionId = typeof argsObject.session_id === "string" ? argsObject.session_id : undefined
+
+      if (category) {
         argsObject.subagent_type = "sisyphus-junior"
+      } else if (!subagentType && sessionId) {
+        const resolvedAgent = await resolveSessionAgent(ctx.client, sessionId)
+        argsObject.subagent_type = resolvedAgent ?? "continue"
       }
     }