| name | aider-cache-default-on |
| description | Aider's --cache-prompts is off by default. Flip it on for supported models. |
| target_harness | Aider |
| target_repo | Aider-AI/aider |
| target_files | ["aider/args.py"] |
| target_commit | 3ec8ec5a |
| estimated_savings | 90% input discount for every user who didn't know to set the flag |
Aider: default --cache-prompts to on for supported models
Target
aider/args.py in Aider-AI/aider. The --cache-prompts flag is
defined here with default=False.
Symptom
Aider implements the canonical 4-breakpoint Anthropic caching pattern
correctly, but --cache-prompts is off by default. Users have to
know to set it (in CLI args, .aider.conf.yml, or env var) to get
any cache benefit. The result is that a well-implemented caching path
goes unused by most users.
Fix
Flip the default to True, with a per-model gate so it only activates
for models that support caching:
@@
- group.add_argument(
- "--cache-prompts",
- action=argparse.BooleanOptionalAction,
- default=False,
- help="Enable caching of prompts (default: False)",
- )
+ group.add_argument(
+ "--cache-prompts",
+ action=argparse.BooleanOptionalAction,
+ default=True,
+ help="Enable caching of prompts on models that support it. "
+ "90%% input-token discount on cache hits. Pays for itself "
+ "after ~3 reads. Disable with --no-cache-prompts.",
+ )
The existing per-model cache_control: true|false gate in
aider/models.py already prevents this from being sent to unsupporting
models, so flipping the default is safe — it's just opt-out instead of
opt-in.
Add a CHANGELOG note.
Verify
- Run
aider --model claude-3-7-sonnet-20250219 with NO mention of
--cache-prompts on the command line and no entry in
.aider.conf.yml.
- Capture wire.
- Request body should contain
cache_control: {"type": "ephemeral"}
on the 4 breakpoint locations (system, repo-map, files, current).
- Confirm 2nd-turn
usage.cache_read_input_tokens > 0.
Background
Defaults matter. The 1.25x cache-write premium is real but small, and
it pays back after the second cache hit — which any multi-turn agent
session clears trivially. A caching system that requires opt-in is a
caching system most users won't use.
Cline / Roo / OpenCode all default-on caching. Aider being default-off
is an anomaly more than a deliberate design choice.
Full audit: audits/aider.md.
