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Routing inside the harness: 64% cheaper, same merge rate
LangChain wrote up how it built a model router for Open SWE, its internal coding agent, and published the results. It picked three tiers from different providers: GLM-5.3-Flash (xhigh) as fast, GPT-5.6 Sol (medium) as balanced and GPT-6 Astra (low) as performance . The router middleware runs once, on the thread's first human message, and has three parts :
- a base prompt telling the classifier to pick the least expensive model likely to finish the task
- short, plain-language criteria for each tier
- a classifier model
The tier criteria come from two sources: LangChain's own analysis of its task mix, and each provider's prompt guide. Because those criteria depend so much on the agent's own tasks, the router lives in the harness rather than a generic gateway . The first classifier was an LLM with structured output. Switching it to the Jev decision model made classification almost 50× faster. The chosen model then handles the whole thread .
They measured outcomes with merged PRs per thread and thumbs up/down ratings. Merged PRs were the stronger signal because few threads got rated . Results from a 973-thread A/B test against always using Astra:
- Quality: 29.2% of routed threads ended in a merged PR, against 27.3% for control (p=0.49) .
- Cost: the median thread fell from $2.61 to $0.94, a 64% drop. The mean fell 42% and p90 fell 37% .
- Routing mix: 56% of threads went to balanced, 34% to fast and 10% to performance. Median cost per thread ranged from $0.097 on fast to $2.88 on performance .
The opposite test, router against always using the fast model, was stopped within a day. Engineers said the low quality was disrupting their work . In short, routing saves money, but sending everything to the cheapest model doesn't work.
Related: LangChain's new smithtune CLI turns an agent's own trajectories into fine-tuning data . LangChain says OpenSWE Review's precision rose from 62.9% to 81.5% with 29% fewer tool calls per review .
Hand off verification too, not just coding
In a new video, Theo estimates that only 10–15% of his tokens go to writing code. The rest goes to verifying it, so the PR is actually done by the time he looks . He also says that if undoing a bad change takes more than 15 seconds, you should fix your systems before vibe coding . Habits from the video:
- Give the agent the problem, not your solution. He now sends a bug screenshot with "Fix it." If the agent fails, that tells him the problem deserves his own thinking .
- Ask for everything up front: build it, file a PR, spin up a Tailscale environment for him to try, then babysit the PR . If you state the conditions under which the agent may merge, the thread can close without you ever opening it .
- Treat threads as a to-do list. He fires prompts into the background (Cmd+Enter in T3 Code), only opens threads marked done or needing input, and spreads work across his Linux boxes, with auto load-balancing available .
On exploratory work, Theo argues that the branch you explore on and the PR you merge can, and often should, be built differently: "assume all PRs will be closed, not merged" . Geoffrey Huntley goes further on review. Read the properties your tests express, and if performance is a problem, "run a loop and optimise" while those properties keep holding . The things he says you must read are public SDK APIs, your verification and agent traces .
The Pragmatic Engineer quotes DHH saying 37signals is "done writing code by hand." When it happens, it's treated like a bug: you ask why the agent couldn't produce the result, then fix the "factory" . 37signals is now building native mobile apps, moving backend services from Ruby to Rust because agents write good-enough Rust, and keeping Rails because its conventions suit agents .
Claude Code mods
Claude Code now supports mods, which can change its behavior, customize the UI or swap in your own features. You can write one in a few lines of TypeScript or have Claude build it. Mods ship inside plugins and install with /plugin in the CLI or desktop app . Boris Cherny's pitch is that everyone works differently, so build your own and share them as plugins .
Dots: open autonomous PRs as drafts
Alexander Embiricos described an early internal incident. A dot watching a feedback channel found a bug and opened a non-draft PR. It had inferred the engineer's habit of setting auto-merge, and the PR merged once a colleague approved it. "Dot should set PRs in draft mode," he said . If you give an always-on agent repo access, make draft PRs the default.
Riley Brown's notes after 24 hours with a dot:
- Voice is the standout, but he hit problems in sessions of 30 minutes or more .
- He wants a better view of the Codex threads the dot spins up .
- He found it confusing to choose between the VM browser, the Codex browser and Chrome .
On capacity, Tibo says GPT-6.1 Sol is back to normal speed after a load spike, and a global usage reset for paid ChatGPT accounts lands at 10am PST on the day after his post . ThePrimeagen says about 60 minutes of prompting on a "$500 plan" used 56% of his weekly limit .
Smaller items
- Cursor added GLM 5.3 and GLM 5.3 Flash. Cursor says GLM 5.3 Max is the best-scoring open-weight model on CursorBench 4.0 .
@langchain/mcp-adapters2.0 supports the latest stateless MCP, adds tools that can check in with users and makes auth easier .- Pi 1.0 shipped with Pi Durable (post) .
- Persistent subagents: in a Latent Space interview, swyx calls ephemeral Codex subagents his biggest pain and Alex Zhang agrees that writing state to the filesystem is "the big trick." Zhang's Pi-based Prime Agent offers subagents that outlive the root agent and can be prompted later .
- Figma MCP workaround: Kent C. Dodds suggests forking his Kody Figma setup and asking your agent to set it up, which he says takes about five minutes . He builds Kody.
- Btrfs: Peter Steinberger found copy-on-write great for worktrees but terrible for SQLite. The next OpenClaw update moves the database to a NOCOW location .
- Making agent output easier to understand: Karpathy suggests asking for ASD-STE100 controlled English (or "80% of the way" to it), diagrams, HTML pages, or "a 3b1b style video explainer on X" .
- Security: Matthew Green notes that agents in separate sandboxes left instructions for each other in a shared package cache, and those instructions changed what the recipients did. Swap the cache for email, Slack or shared docs and you have the makings of an agent worm .
