Long content fails with "Prompt is too long"; manually saving it to a local file and asking the agent to read it fails identically

[BlessedChild]

When pasting a long block of text (e.g. a long document, log dump, or chat transcript) into the chat input and sending, the message fails with the error "Prompt is too long".

I tried a workaround: manually save the same content into a local file in the workspace and ask the agent to read it. This triggered the same "Prompt is too long" error — because the file content ultimately still gets injected into the prompt, so it is no different from pasting directly.

So this is really a context-window-overflow issue, not a paste / input-box issue. Currently Finch gives no actionable hint and no automatic fallback path.

Steps to reproduce

1. Copy a long block of content (e.g. several thousand characters of prose or logs).
2. Paste it into the Finch chat input and press send → fails with "Prompt is too long".
3. Manually save that same content as a local file in the workspace (e.g. long-content.md).
4. Ask the agent in Finch to read that file → same error: "Prompt is too long".

Expected behavior

When content (either directly pasted or pulled in via file read) would push the prompt past the model's context limit, Finch should at minimum:

1. Show a clearer error: explicitly tell the user this is a "context window exceeded" condition, not a generic "send failed";
2. Provide a fallback path, for example:
   • Chunked/streaming processing for long files, only injecting relevant chunks into the prompt;
   • Automatic summarization before injection;
   • Prompt the user to split the content / start a new conversation;
3. Do not imply that "save it to a file" is a workaround — because reading the file hits the same ceiling.

Actual behavior

• Sending long content directly → "Prompt is too long" error; the conversation does not progress.
• Saving the content to a local file + asking the agent to read it → same "Prompt is too long" error.
• No in-app fallback. The user has to manually split the content and send it across multiple turns.

Suggested fix

• In the send / file-read pipeline, detect when the prompt is about to exceed the limit and surface a dedicated "context window exceeded" error with a message that names the actual cause;
• Provide at least one automatic fallback: chunking, summarization, or an "I'll split this into N messages — proceed?" confirmation dialog.

Screenshots

• 03-long-paste-fail-1.png — "Prompt is too long" error when sending long content

Environment

• App: Finch 1.3.0 (build 262) — Desktop
• OS: macOS 26.4.1 (25E253)