onlyterp/prompt-cache-skills/cline-pin-timestamp

Cline: pin the system-prompt timestamp

Target

src/core/prompts/system.ts in cline/cline. PR #1168 added a current-time field to the system prompt.

Symptom

The system prompt includes the current time. If the time is computed per-request (e.g. new Date().toISOString() evaluated at every call site), then the system prompt bytes differ on every request, which invalidates the system-prompt cache breakpoint. The cache write premium gets paid every turn for zero reads, on what's usually the largest single block in the prefix.

PR #1168 was merged despite a reviewer flagging exactly this:

"Just be aware that if the time is being calculated every time the

user sends a request, it invalidates Anthropic's cache."

Verify on the wire: if the system text changes between consecutive calls in the same task, this bug is present.

Fix

Compute the timestamp ONCE at task start, store it on the task, pass through to every system prompt build:

--- a/src/core/prompts/system.ts
+++ b/src/core/prompts/system.ts
@@
-export const SYSTEM_PROMPT = (...args) => `
-  ...
-  Current time: ${new Date().toISOString()}
-  ...
-`
+export const SYSTEM_PROMPT = (args: SystemPromptArgs) => `
+  ...
+  Current time: ${args.taskStartedAt}
+  ...
+`

Then in the task initialization code:

+  this.taskStartedAt = new Date().toISOString()

And pass this.taskStartedAt everywhere SYSTEM_PROMPT is built.

If date awareness is critical for the agent and you want it to drift during a long session, round to the nearest day or hour rather than the second — that bounds the cache invalidation rate.

Verify

1. Start mitmproxy.
2. Run a Cline task. Wait 10 seconds. Send a second prompt in the

same task.

1. Diff the system prompt bytes between the two requests.

• Before fix: bytes differ (timestamp incremented)
• After fix: bytes identical

1. Confirm response usage.cache_read_input_tokens covers the system

prompt on turn 2.

Background

Anthropic caches the entire prefix UP TO AND INCLUDING the breakpoint. A single byte change before or at the breakpoint invalidates everything. Timestamps are the most common cache killer because they look innocent and few people think to check.

OpenAI's automatic prefix caching has the same vulnerability — same fix applies if Cline targets OpenAI as well.

See docs/gotchas.md #5 (byte-identity of tool definitions, same principle for system prompts) and #7 (OpenAI byte-identical prefix requirement).

Full audit: audits/cline.md.