diff --git a/.agents/retro.md b/.agents/retro.md
new file mode 100644
index 0000000..84311f4
--- /dev/null
+++ b/.agents/retro.md
@@ -0,0 +1,33 @@
+# Retrospectives
+
+Lessons from past tasks. Read before starting related work to avoid repeating mistakes.
+
+---
+
+## 2026-06-22: CUA send_message fix (v0.4.5 → v0.4.7)
+
+**Problem**: CUA smoke test `send_message` scenario failed — app sent `claude-sonnet-4-6` to Azure, but only `gpt-5.4` was deployed on the CI resource.
+
+**Mistake 1 — Wrong layer diagnosed first (PR #36)**: Initial fix assumed the app's model-selection *precedence* was wrong (preferring agent model over provider default). Reality: the provider registry default itself (`defaults["azure"] = "claude-sonnet-4-6"`) was the poison — it's a registry-wide default, NOT what's deployed on the user's resource. Fix was to stop auto-selecting entirely and let the server decide.
+
+**Mistake 2 — Trusted registry defaults as truth**: The `/provider` API returns 107 models for Azure including `claude-sonnet-4-6` (registry knows it exists), and `defaults` says it's the "default". But "exists in registry" ≠ "deployed on this resource". Never auto-select from registry defaults for actual inference calls.
+
+**Correct pattern**: When no user-explicit model choice exists, send `model: null` in the prompt request. The server's `opencode.json` `"model"` field is the only reliable source for what's actually deployed and reachable.
+
+**Time cost**: ~2h across two PRs (#36 then #37) because the first fix was plausible but wrong — it passed unit tests but failed the real E2E. Always validate against the actual server logs showing which `modelID` is used at inference time.
+
+---
+
+## Template for future entries
+
+```
+## YYYY-MM-DD: [task title] ([versions/PRs])
+
+**Problem**: [1 sentence]
+
+**Mistake(s)**: [what went wrong and why, 1-2 sentences each]
+
+**Correct pattern**: [what to do next time, 1-2 sentences]
+
+**Time cost**: [how much was wasted and what would have saved it]
+```
diff --git a/AGENTS.md b/AGENTS.md
index eb8a777..c721d17 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -1,5 +1,9 @@
 # Mobile App - AGENTS.md
 
+## Retrospectives (read before related work)
+
+See [`.agents/retro.md`](.agents/retro.md) for lessons from past tasks. Read entries related to your current work before starting — they document mistakes that cost hours and how to avoid them.
+
 ## Agent Operating Rules (read first)
 
 **Failure taxonomy — classify every failure before acting. Escalate cheapest-first; never skip a rung or repeat one:**