sandbox.exec - streaming issues with pty

[paulgroves]

pty=True results in truncated stdout

Summary

When running commands with pty=True, stdout is truncated. The same command with pty=False returns complete output.

Reproduction

import modal                                                                                                                                                                    
                                                                                                                                                                                
SIZE = 102400  # 100KB                                                                                                                                                          
CMD = f"head -c {SIZE} /dev/zero | tr '\\0' 'X'"                                                                                                                                
                                                                                                                                                                                
app = modal.App.lookup("pty-bug-repro", create_if_missing=True)                                                                                                                 
                                                                                                                                                                                
print(f"Testing {SIZE} bytes output (3 runs)...\n")                                                                                                                             
                                                                                                                                                                                
for i in range(3):                                                                                                                                                              
    sb1 = modal.Sandbox.create("bash", "-c", CMD, app=app, timeout=60, pty=False)                                                                                               
    out1 = sb1.stdout.read()                                                                                                                                                    
    sb1.wait()                                                                                                                                                                  
                                                                                                                                                                                
    sb2 = modal.Sandbox.create("bash", "-c", CMD, app=app, timeout=60, pty=True)                                                                                                
    out2 = sb2.stdout.read()                                                                                                                                                    
    sb2.wait()                                                                                                                                                                  
    pty_bytes = out2.count("X")                                                                                                                                                 
                                                                                                                                                                                
    print(f"Run {i+1}: pty=False {len(out1)*100//SIZE}% | pty=True {pty_bytes*100//SIZE}%")                                                                                     
                                                                                                                                                                                
print("\nExpected: Both should be 100%")     

Output

Testing 102400 bytes output (3 runs)...

Run	pty false	pty true
1	100%	71%
2	100%	79%
3	100%	67%

Environment

• modal version: 1.3.1 (Python)
• Python: 3.14
• Also reproducible with Typescript and Go sdks

Expected behavior

Both pty=False and pty=True should return 100% of the command's output.

Observed behavior

With pty=True, only 67-79% of the output is received. The amount varies between runs.