onlyterp/prompt-cache-skills/roo-bedrock-custom-arn

Roo Code: Bedrock custom ARN caching (#11983)

Target

src/api/providers/bedrock.ts → guessModelInfoFromId() in RooCodeInc/Roo-Code.

Tracked upstream as issue #11983; PR #11984 proposes a fix but status uncertain at audit time.

Symptom

When a user configures Bedrock with a custom ARN instead of one of Roo's hardcoded model IDs, guessModelInfoFromId() returns { supportsPromptCache: true } but omits cachableFields. Downstream the Bedrock provider checks for cachableFields and skips caching entirely when empty. Net effect: silent 0% cache hit on custom ARNs.

Users hit this when:

• Routing through a Bedrock-hosted Claude model with non-standard ARN
• Using cross-region inference profiles
• Provisioned throughput ARNs

Fix

--- a/src/api/providers/bedrock.ts
+++ b/src/api/providers/bedrock.ts
@@ guessModelInfoFromId(modelId: string)
   if (/* claude pattern match */) {
-    return { supportsPromptCache: true }
+    return {
+      supportsPromptCache: true,
+      cachableFields: ["system", "messages", "tools"],
+    }
   }

Apply this to every branch in guessModelInfoFromId() that returns supportsPromptCache: true without populating cachableFields.

If PR #11984 is already merged at apply time, verify the fix matches this shape and skip the diff.

Verify

1. Configure Roo with a custom Claude ARN (any ARN not in the

declared model list).

1. Enable prompt caching in Roo settings.
2. Capture wire via mitmproxy.
3. Inspect outbound Bedrock Converse API request body:

• Before fix: no cachePoint markers anywhere in the request
• After fix: cachePoint: { type: "default" } blocks on system

and last stable message

1. Inspect response usage:

• cacheWriteInputTokenCount > 0 on cold turn
• cacheReadInputTokenCount > 0 on warm turn

Background

Bedrock's Converse API uses a different field name (cachePoint) than direct Anthropic (cache_control). Roo handles the translation correctly for declared models but the custom-ARN detection path is incomplete.

See docs/concepts/bedrock.md. Full audit: audits/roo-code.md.