agents: reduce prompt token bloat from exec and context (#16539)

Merged via /review-pr -> /prepare-pr -> /merge-pr.

Prepared head SHA: 8e1635fa3f
Co-authored-by: CharlieGreenman <8540141+CharlieGreenman@users.noreply.github.com>
Co-authored-by: gumadeiras <5599352+gumadeiras@users.noreply.github.com>
Reviewed-by: @gumadeiras

src/agents/bash-tools.e2e.test.ts

@@ -221,6 +221,28 @@ describe("exec tool backgrounding", () => {
     expect(status).toBe("completed");
   });
 
+  it("defaults process log to a bounded tail when no window is provided", async () => {
+    const lines = Array.from({ length: 260 }, (_value, index) => `line-${index + 1}`);
+    const result = await execTool.execute("call1", {
+      command: echoLines(lines),
+      background: true,
+    });
+    const sessionId = (result.details as { sessionId: string }).sessionId;
+    await waitForCompletion(sessionId);
+
+    const log = await processTool.execute("call2", {
+      action: "log",
+      sessionId,
+    });
+    const textBlock = log.content.find((c) => c.type === "text")?.text ?? "";
+    const firstLine = textBlock.split("\n")[0]?.trim();
+    expect(textBlock).toContain("showing last 200 of 260 lines");
+    expect(firstLine).toBe("line-61");
+    expect(textBlock).toContain("line-61");
+    expect(textBlock).toContain("line-260");
+    expect((log.details as { totalLines?: number }).totalLines).toBe(260);
+  });
+
   it("supports line offsets for log slices", async () => {
     const result = await execTool.execute("call1", {
       command: echoLines(["alpha", "beta", "gamma"]),
@@ -239,6 +261,29 @@ describe("exec tool backgrounding", () => {
     expect(normalizeText(textBlock?.text)).toBe("beta");
   });
 
+  it("keeps offset-only log requests unbounded by default tail mode", async () => {
+    const lines = Array.from({ length: 260 }, (_value, index) => `line-${index + 1}`);
+    const result = await execTool.execute("call1", {
+      command: echoLines(lines),
+      background: true,
+    });
+    const sessionId = (result.details as { sessionId: string }).sessionId;
+    await waitForCompletion(sessionId);
+
+    const log = await processTool.execute("call2", {
+      action: "log",
+      sessionId,
+      offset: 30,
+    });
+
+    const textBlock = log.content.find((c) => c.type === "text")?.text ?? "";
+    const renderedLines = textBlock.split("\n");
+    expect(renderedLines[0]?.trim()).toBe("line-31");
+    expect(renderedLines[renderedLines.length - 1]?.trim()).toBe("line-260");
+    expect(textBlock).not.toContain("showing last 200");
+    expect((log.details as { totalLines?: number }).totalLines).toBe(260);
+  });
+
   it("scopes process sessions by scopeKey", async () => {
     const bashA = createExecTool({ backgroundMs: 10, scopeKey: "agent:alpha" });
     const processA = createProcessTool({ scopeKey: "agent:alpha" });
@@ -300,6 +345,49 @@ describe("exec notifyOnExit", () => {
     expect(finished).toBeTruthy();
     expect(hasEvent).toBe(true);
   });
+
+  it("skips no-op completion events when command succeeds without output", async () => {
+    const tool = createExecTool({
+      allowBackground: true,
+      backgroundMs: 0,
+      notifyOnExit: true,
+      sessionKey: "agent:main:main",
+    });
+
+    const result = await tool.execute("call2", {
+      command: shortDelayCmd,
+      background: true,
+    });
+
+    expect(result.details.status).toBe("running");
+    const sessionId = (result.details as { sessionId: string }).sessionId;
+    const status = await waitForCompletion(sessionId);
+    expect(status).toBe("completed");
+    expect(peekSystemEvents("agent:main:main")).toEqual([]);
+  });
+
+  it("can re-enable no-op completion events via notifyOnExitEmptySuccess", async () => {
+    const tool = createExecTool({
+      allowBackground: true,
+      backgroundMs: 0,
+      notifyOnExit: true,
+      notifyOnExitEmptySuccess: true,
+      sessionKey: "agent:main:main",
+    });
+
+    const result = await tool.execute("call3", {
+      command: shortDelayCmd,
+      background: true,
+    });
+
+    expect(result.details.status).toBe("running");
+    const sessionId = (result.details as { sessionId: string }).sessionId;
+    const status = await waitForCompletion(sessionId);
+    expect(status).toBe("completed");
+    const events = peekSystemEvents("agent:main:main");
+    expect(events.length).toBeGreaterThan(0);
+    expect(events.some((event) => event.includes("Exec completed"))).toBe(true);
+  });
 });
 
 describe("exec PATH handling", () => {