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:

--- a/aider/args.py
+++ b/aider/args.py
@@
-    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

1. 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.
2. Capture wire.
3. Request body should contain cache_control: {"type": "ephemeral"} on the 4 breakpoint locations (system, repo-map, files, current).
4. 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.