Pi/gpt-6.1-sol

* 93ff29f  (2 minutes)  <Riza Fahmi>   fix: support IPv6 literal benchmark targets
* 30167d1  (8 minutes)  <Riza Fahmi>   feat: print partial summaries on terminal interruption
* 243ce83  (14 minutes)  <Riza Fahmi>   feat: report elapsed time and completed-response percentiles
* 107c650  (19 minutes)  <Riza Fahmi>   feat: report HTTP outcomes and transport failures
* 63b3248  (22 minutes)  <Riza Fahmi>   feat: pace bounded requests over a five-second window
* 1d97da5  (28 minutes)  <Riza Fahmi>   feat: send bounded streaming GET requests with verified TLS
* 6c3c203  (33 minutes)  <Riza Fahmi>   feat: validate benchmark arguments before networking
* ccb623b  (34 minutes)  <Riza Fahmi>   feat: launch CLI with terminal signal handling
* c90bc0c  (43 minutes)  <Riza Fahmi>   init

1. 1True Outside-In Red-Green TDD Across All 8 Commits (Complete Reversal from gpt-6-sol's Retroactive Commits)3 points▸

   • Strict Slice-by-Slice Workflow vs. gpt-6-sol Whereas gpt-6-sol (2026-10-24) spent 22.5 minutes writing the entire codebase with 0 commits and ran a Python script in the final 4 minutes to fabricate 8 retroactive commits, gpt-6.1-sol followed AGENTS.md and PLAN.md literally—committing organically at the end of every slice (18:28:05, 18:29:30, 18:33:57, 18:40:21, 18:43:17, 18:48:30, 18:54:19, and 19:00:09).
   • 12 of 26 Bash Errors Were Intentional Red-Phase TDD Runs Nearly half of all failed bash calls were deliberate failing-test executions before writing production code: S1 CLI help (:enoent at line 22), S2 arg validation (line 124), S3 HTTP/TLS server & cert checks (lines 148, 156, 172), S4 pure schedule & runner pacing (lines 198, 206), S5 status outcomes & CLI reporting (lines 244, 252), S6 percentile & body-receipt latency (lines 270, 278, 286), S7 foreground Ctrl-C partial summary (line 318), and the IPv6 literal regression test (line 344).
   • Bypassing Elixir 1.20's Two-Pass Formatter Cache Quirk (Lines 231–237 & 333–335) At line 231, mix format --check-formatted failed on test/http_test.exs even after running mix format at line 223, and running mix format again at line 233 was a no-op because Mix cached the file's mtime after pass 1 while Code.format_string!/2 needed a second pass on end, transport: :ssl, tls: tls). At line 237, the agent diagnosed that Mix's mtime cache was skipping the file and ran mix format test/http_test.exs explicitly—doing the same for test/support/cli.ex at line 335.
2. 2Decompiling OTP's erl_signal_handler.beam to Solve the SIGUSR1 Crash-Dump & PTY EOF Hang in S13 points▸

   • Discovering OTP's SIGINT Restriction & Building the POSIX SIGUSR1 Relay (Lines 25–52) The agent initially tried :os.set_signal(:sigint, :handle) in lib/capy_bench/signal.ex (line 25), which crashed at line 42 with ArgumentError: invalid signal name. After checking Erlang's os(3) and erl(1) man pages via ripgrep (line 45), it switched to a POSIX ./cb shell wrapper (emu_args: "-noinput +Bi") that traps INT, forwards SIGUSR1 to cb.escript, and loops on wait "$pid" until kill -0 "$pid" fails.
   • Decompiling erl_signal_handler.beam to Stop SIGUSR1 Crash Dumps (Lines 95–100) Even after calling :os.set_signal(:sigusr1, :handle) and adding CapyBench.Signal to :erl_signal_server, sending Ctrl-C caused the VM to print 'Received SIGUSR1 / Crash dump is being written to: erl_crash.dump...done' and exit 1 (line 96). At line 97, the agent decompiled OTP's erl_signal_handler.beam using beam_lib:chunks(..., [abstract_code]) and erl_prettypr:format/1, seeing that the default handler remained registered on :erl_signal_server and executed erlang:halt("Received SIGUSR1") on every sigusr1 event. At line 99, it added :gen_event.delete_handler(:erl_signal_server, :erl_signal_handler, []) before registering its own handler (while delegating non-SIGUSR1 events back to :erl_signal_handler).
   • Fixing the Python PTY Probe Deadlock & Cleaning Up Orphaned Artifacts (Lines 85–106, 355–359) Earlier PTY probes using subprocess.Popen(..., preexec_fn=foreground) hung because os.killpg failed with EPERM in the sandbox (line 72) and waitpid blocked while the PTY master stayed open. Switching to pty.fork() and draining the master fd via select.select until EOF before os.waitpid (line 103) yielded {"status": 130, "stdout": "", "stderr": "Interrupted\n"} at line 106. Before the final commit, the agent checked ps (line 355), found the orphaned background Python probe PID 63291 (started at line 61) and the 7.0 MB erl_crash.dump from S1, and cleaned both up at line 359.
3. 3Proving & Fixing Collector-Lag Latency Inflation in S6 Using :erlang.suspend_process/13 points▸

   • Subtle Timestamp Bug in Initial S3/S4 Client In the initial CapyBench.Client (line 157), the socket-owning worker sent {self(), {:ok, status}} upon receiving {:done, ref} from Mint, and the parent runner computed System.monotonic_time(:microsecond) - started inside timed/2 after receiving that message and waiting for {:DOWN, ...}.
   • Deterministic Process-Suspension Test (Lines 283–290) In S6, to verify the contract rule that 'latency ends at complete body receipt rather than result collection', the agent wrote a test in test/client_test.exs (line 283) that spawned Client.get/2 in a Task, waited for the server to receive the request, and called :erlang.suspend_process(task.pid) while the server finished sending the 100ms response body and slept another 500ms before resuming task.pid.
   • Red-to-Green Fix: The test failed on the first run (line 286) with left: 616558 >= 400000 because suspending the collector inflated the recorded latency by 500ms. At line 287, the agent moved the end timestamp into the worker's Mint {:done, ref} clause ({:ok, status, System.monotonic_time(:microsecond)}) and added a second :erlang.suspend_process test (line 303) proving that a response completing after the 2,000ms deadline while collection is suspended is still rejected as {:error, :timeout}.
4. 4Self-Initiated IPv6 Literal Bug Discovery & Fix During Release Verification (Commit 93ff29f)2 points▸

   • Auditing Mint's TCP Transport Source Code (Line 327) While reviewing edge cases at the end of S7, the agent ran ripgrep on deps/mint/lib/mint/core/transport/tcp.ex and noticed line 23: inet6? = Keyword.get(opts, :inet6, false). Even though CapyBench.Arguments.parse/1 accepted IPv6 URLs like http://[::1]:port/, Mint defaulted to IPv4-only :gen_tcp.connect unless transport_opts: [inet6: true] was passed.
   • Reproducing :nxdomain and Committing the 8th Fix (Lines 339–362) After committing S7 (30167d1 at line 337), the agent extended CapyBench.TestServer to bind to {0, 0, 0, 0, 0, 0, 0, 1} (::1) and wrote a CLI test against http://[::1]:<port>/ (line 341). Running mix test at line 343 failed with Transport failure: %Mint.TransportError{reason: :nxdomain}. At line 345, it updated CapyBench.Client.options/1 to pass transport_opts: [inet6: String.contains?(uri.host, ":")], turning the 21st test green and committing 93ff29f.
5. 595% Prefix Cache Discount ($0.10/1M vs. $0.20/1M on gpt-6-sol) Made a Longer Run 20% Cheaper ($1.37 vs. $1.72)3 points▸

   • Unit Pricing & $11.99 Cache Savings: Across 178 assistant turns, the session processed 6,454,732 prompt tokens (144,076 uncached at $2.00/1M = $0.2882; 6,310,656 cached at $0.10/1M = $0.6311; 97.77% cache hit rate) and 44,858 output tokens (including 17,026 reasoning tokens) at $10.00/1M = $0.4486. Because gpt-6.1-sol prices cacheRead at $0.10/1M (a 95% discount vs. gpt-6-sol's $0.20/1M 90% discount), this run cost $1.37 vs. $1.72 for gpt-6-sol despite using 14.5% more tokens (6.50M vs. 5.67M) and 18 more LLM calls. Without prefix caching, the prompt tokens alone would have cost $12.91 ($13.36 total, saving 89.8%).
   • Two Single-Turn Cache Evictions Cost $0.17 (Lines 65 & 361) Out of 177 post-initial turns, 175 hit the prefix cache cleanly, while 2 turns experienced a full cache drop (cacheRead = 0): Line 65 (14,186 uncached input tokens, costing $0.0284 instead of $0.0014) and Line 361 right before the final commit (74,636 uncached input tokens, costing $0.1493 instead of $0.0075). Those two turns alone accounted for 61.6% of all uncached input tokens (88,822 / 144,076) and $0.1688 of extra cost.
   • Wall-Clock Split & Selective High Reasoning Of the 40m 04s (2,403.7s) total runtime, 64.7% (25m 55s / 1,554.8s, avg 8.73s/turn) was spent in LLM inference and 35.3% (14m 09s / 848.8s, avg 4.80s/call) in tool execution (including five full-suite mix test runs taking 75–102s each). Despite thinkingLevel: "high", 104 of 178 turns (58.4%) used 0 reasoning tokens, reserving deep thinking bursts (up to 1,312 tokens / 73.3s at line 357 and 1,034 tokens / 51.2s at line 311) for architectural transitions and release verification.