## Summary
Large pasted text that the UI collapses into an attachment chip was
completely invisible to the LLM. Providers only accept specific MIME
types (images, PDFs) in file content blocks — a `text/plain` `FilePart`
is silently dropped, so the model received nothing for pasted content.
## Fix
Detect paste-originated text files by their
`pasted-text-{timestamp}.txt` filename pattern and convert them to
`fantasy.TextPart` with a bounded 128 KiB inline body and truncation
notice. Binary uploads and real uploaded text files keep their existing
`FilePart` semantics.
The detection uses the existing frontend naming convention
(`pasted-text-YYYY-MM-DD-HH-MM-SS.txt`) combined with a text-like MIME
check for defense-in-depth. A TODO marks this for migration to explicit
origin metadata.
<details>
<summary>Review notes: intentionally skipped findings</summary>
A 10-reviewer deep review was run on this change. The following findings
were raised and intentionally dropped after cross-check. Documenting
them here so future reviewers do not re-flag the same concerns:
**"Unresolved file IDs cause silent data loss" (Edge Case Analyst P1)**
— When a file ID is not in the resolver map, `name` stays empty and
paste detection fails. This is pre-existing behavior for ALL file types
(not introduced by this change). The resolver calls `GetChatFilesByIDs`
which returns whatever rows exist; missing IDs simply fall through to an
empty `FilePart`. The Contract Auditor independently traced this path
and confirmed the fallback is safe. If the file was deleted between
message construction and conversion, the model already saw nothing
before this patch — this change does not make it worse.
**"String builder pre-allocation overhead" (Performance Analyst P1)** —
Misidentified scope. `formatSyntheticPasteText` is only called when
`isSyntheticPaste` returns true (actual synthetic pastes), not for every
file part. The `Grow()` call is correct and efficient.
**"Constant naming violates Uber style" (Style Reviewer P1)** —
Over-severity. `syntheticPasteInlineBudget` is standard Go camelCase for
unexported constants, consistent with the Uber guide and surrounding
code.
**"`IsSyntheticPasteForTest` naming is misleading" (Style Reviewer P2)**
— This is the standard Go `export_test.go` pattern. The `ForTest` suffix
is conventional.
</details>
Michael Suchacz
4f571f8fff