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Attach local evidence files (PNG/JPG/GIF/MP4/PDF) to Slack threads so they
render INLINE — not as bare file paths or `MEDIA:/path` text tokens.
v1.3 cluster-anchor; v1.4 gist-raw-URL fallback; v1.5 Pre-Send Gate;
v1.6 OAuth preflight + f-string pitfall; v1.7 misrouted-fix; v1.8 gist
binary pitfall (clone-and-replace for PNG/MP4); v1.9 text-file gist delta
(`gh gist create` works directly for `.patch`/`.diff`/`.json`/`.md`/`.txt`);
v1.10 MP4 + curl-ARG_MAX quirks (subprocess.run arg-limit + clone-and-replace
content-type for MP4 = `application/octet-stream`);
v1.11 bot-side `files:write` scope closed (2026-07-19) — xoxp pending.
v1.12 added 2 new pitfall rows: (a) `chat.delete` destroys evidence
attachments if sorted by text-length heuristic (PR #8561 2026-07-24);
(b) `page.evaluate()` computed style ≠ visual proof — always
`vision_analyze` pixels before claiming visibility.
Anti-trigger: plain-text evidence → fenced code blocks.
when_to_use
When the user asks for "before/after", "screenshot", "GIF evidence", or
any message claims visual proof that lives as a local file. Also fires
when the user corrects you: "you forgot the screenshot" / "attach the
proof as media" / "use /harness and fix it".
**Auto-load contract (paired with SOUL.md `evidence-attach-presend-gate`):**
Load THIS skill before invoking `mcp__slack__conversations_add_message`
or `chat.postMessage` if your draft body contains any of:
- `MEDIA:/absolute/path` token referencing a binary file
- absolute path matching `/Users/[^ ]+\.(png|jpg|jpeg|gif|webp|mp4|pdf)`
- "BEFORE"/"AFTER"/"screenshot"/"see attached"/"here's what it looks like"
alongside any local path
triggers
["attach proof as media","you forgot the screenshot","post the screenshot to the thread","MEDIA: not rendering","evidence didn't show up in slack","use /harness and fix it","use /learn and fix it","show before after in thread","show screenshots here","show me screenshots","see screenshots","with screenshots","and screenshots","and show screenshots","All PRs need attached media in PR desc and slack","evidence should be in gist urls","put evidence in a gist","don't commit screenshots to the PR","I need the media evidence on this thread","where are the screenshots","post evidence to this thread","you keep skipping the upload","you keep skipping this","/learn and remember this","before/after","BEFORE/AFTER","PNG","MEDIA:/path with .png/.jpg/.gif in draft body","/Users/.../path/.png in draft body","draft body says BEFORE / AFTER / screenshot / here is what it looks like","you always forget","you always fail to attach","you always do Y","stop forgetting","stop doing X","why do you always","use /browser","use browser to fix your scopes","why are you struggling so much to show UI screenshots","fix your scopes"]
Attach local evidence files to Slack messages so the user can see them inline in the thread.
⚠️ Pre-Send Gate (NEW in v1.5.0 — paired with SOUL.md ## COMMIT: evidence-attach-presend-gate)
Before invoking mcp__slack__conversations_add_message or chat.postMessage, run this gate.
The gate regex (lives in SOUL.md, mirrored here for quick check):
import re
GATE_PATTERNS = {
"media_token": re.compile(
r"MEDIA:/[^\s]+\.(?:png|jpg|jpeg|gif|webp|mp4|pdf)\b", re.IGNORECASE),
"absolute_path": re.compile(
r"(?:^|\s)/Users/[^\s]+\.(?:png|jpg|jpeg|gif|webp|mp4|pdf)\b", re.IGNORECASE),
"phrase_with_path": re.compile(
r"(?i)\b(BEFORE|AFTER|screenshot|see attached|here'?s what it looks like)\b"),
}
defevidence_gate_fires(draft_text: str) -> bool:
returnany(p.search(draft_text) for p in GATE_PATTERNS.values())
If the gate fires, you MUST run the recipe below BEFORE calling the message tool. Skipping this gate is the exact failure mode that caused PRs #8139, #7953, #8337 to ship with text-only "evidence" that the user couldn't see. Verified by tests/test_evidence_attach_presend_contract.py (8 tests, all green).
The principle: when a recurring user correction has hit ≥3 times (as this one has), the fix must be a pre-send gate that intercepts the failure BEFORE the bad message is sent — not a post-hoc remediation rule that fires after the user complains. See references/recurring-failure-pattern-2026-07.md for the full 4-incident chain.
OAuth Scope Preflight (NEW in v1.6.0)
Before attempting Stage 1, run the scope preflight:
The script probes and against and returns one of three states:
HERMES_SLACK_BOT_TOKEN
SLACK_USER_TOKEN
auth.test
State
Token
Action
bot_has_scope
bot has files:write
Use canonical 3-stage flow with bot token
xoxp_has_scope
bot fails, xoxp has files:write:user
Use xoxp token for Stage 1-3
neither_has_scope
both fail
Skip straight to third-tier gist-raw-URL fallback
Why this matters (verified 2026-07-14 on PR #8139): I burned 4 curl calls discovering both tokens lacked files:write mid-flow. The preflight collapses that into 2 auth.test calls (one per token) and tells you the correct path upfront. Saves ~10s per evidence-attach cycle.
Re-verified 2026-07-15 (PR #7953 evidence run): OAuth scope state was unchanged — HERMES_SLACK_BOT_TOKEN (bot_id B0A3MS7G08P, user mcp_agent_mail) had chat:write, chat:write.public, channels:read/history/manage, groups:read/write, im:read/write, mpim:read/write, users:read — but no files:write. The bot-side gap was closed on 2026-07-19 via browser-driven OAuth reinstall — re-running check_token_scopes.py should now report bot_has_scope. The user-token xoxp side is unchanged (different personal app).
When to use
You MUST use this skill when all of these hold:
You have local binary evidence (PNG / JPG / WebP / GIF / MP4 / PDF) on disk — typically from a Playwright/Chromium screenshot, ffmpeg-converted GIF, browser recording, or audit-PDF generator.
The artifact is central to the claim in your Slack message — i.e. the message says "BEFORE/AFTER", "this is fixed", "here's what it looks like", etc., and the proof is the file itself, not the text around it.
The message will land in a Slack thread (most common case) or a Slack DM/channel where the user expects to see the evidence inline.
You MUST NOT use this skill when:
You have only textual evidence (logs, command output, JSON, code diffs). Paste fenced code blocks instead — Slack renders them as native monospace.
The file is on a remote URL. Markdown image syntax () works for public URLs.
The user already has the file (e.g. you uploaded it earlier and they're confirming receipt).
The recipe — Slack files.completeUploadExternal flow
The MEDIA:/path inline convention does NOT WORK through the
mcp__slack__conversations_add_message tool — it renders as plain text and
the Slack client never receives an attachment. The correct path is the
3-stage Slack files.upload API.
Stage 1 — Get upload URL
MUST use -F form fields, NOT -d JSON. Verified 2026-07-12 (PR #8337):
JSON-encoded body silently drops the form fields; the API returns invalid_arguments.
Or use scripts/upload_batch.py (canonical parallel uploader) which handles the form-encoding for you.
Stage 3½ — Post consolidated summary message (MANDATORY)
After uploading all the files, post a single text message in the thread that names each attachment file-by-file. Do NOT skip this step. The attachments themselves become separate bot messages in the thread, which — combined with the channel-bridge's tendency to echo internal narration into the same thread — can bury the screenshots under 4-8 bot messages. The summary message is the cluster anchor the user actually reads.
Stage 4 — Re-verify (Pattern A: verify BEFORE summary, NOT after)
Do NOT re-verify via mcp__slack__conversations_replies AFTER posting the summary — every call to that tool echoes a :gear: mcp__slack__conversations_replies: line into the thread as a bot message, which lands AFTER the summary and contradicts the "ignore the chatter above" anchor. Verified failure 2026-07-11 PR #8139.
Pattern A — verify before summary (preferred): Inside the same execute_code that ran Stage 1+2+3 for all N files, call conversations.replies ONE time and assert the file_ids landed, THEN post the summary. Pattern B (skip verification) is OK only if you can see uploads succeed in the same turn.
chat.getPermalink is the post-send verification that does NOT echo (verified 2026-07-15, PR #7953). After posting the summary, call chat.getPermalink?channel=X&message_ts=Y — it returns the public URL without spamming the thread with bot messages. Use this for "did my message land?" checks after the summary is posted.
conversations.repliesnot_in_channel is NOT a post failure (NEW v1.8.0). If the bot has chat:write.public but is not a member of the channel, chat.postMessage succeeds and chat.getPermalink returns the URL, but conversations.replies and conversations.history return not_in_channel. Don't treat that error as evidence the post failed — verify via permalink instead.
Token source
Use HERMES_SLACK_BOT_TOKEN from ~/.bashrc. Pull the token explicitly if subprocess.run(..., shell=False) doesn't auto-source bashrc.
For xoxp fallback: grep '^export SLACK_USER_TOKEN=' ~/.profile | sed 's/^export SLACK_USER_TOKEN=//;s/"//g' — it lives in ~/.profile, NOT ~/.bashrc (verified 2026-07-14, bashrc-profile-xapp-drift-blocks-launchd memory).
Scope state (updated 2026-07-19): The bot HERMES_SLACK_BOT_TOKEN now has files:write scope — closed via browser-driven OAuth reinstall (see ~/.hermes/skills/devops/slack-mcp-mail-bot-reinstall/references/files-write-scope-reinstall-2026-07-19.md). The xoxp SLACK_USER_TOKEN is from a separate personal app and still lacks files:write. For bot-token evidence (the common case), run the canonical 3-stage flow — check_token_scopes.py should now report bot_has_scope. For xoxp evidence, the third-tier gist-raw-URL fallback is still the path until the personal app gets files:write separately.
Historical gap (verified 2026-07-14, re-verified 2026-07-15, closed 2026-07-19): Both HERMES_SLACK_BOT_TOKEN and SLACK_USER_TOKEN lacked files:write scope. The bot token had files:read but not files:write; xoxp had chat:write but not files:write:user. The OAuth-scope gap was the root cause of the "3 tokens don't work → gist fallback" pattern in PR #7953, #8139, #8337, #8455. The bot-token half is now closed; the user-token half depends on a separate OAuth reinstall of the personal app.
Python 3.14 f-string pitfall (NEW in v1.6.0)
Symptom:SyntaxError: leading zeros in decimal integer literals are not permitted when an f-string contains {02,04,05} or similar.
# WRONG — Python 3.14 parses {02,04,05} as set literal with octal-int SyntaxError
text = f"Briefing dates: {02,04,05}.md"# RIGHT — three options:
text = f"Briefing dates: {'02,04,05'}.md"# Option A: quote the contents
text = f"Briefing dates: 02.md, 04.md, 05.md"# Option B: don't use set-literal-looking syntax
text = "Briefing dates: " + ",".join(["02", "04", "05"]) + ".md"# Option C: build outside f-string
Verified 2026-07-14 on PR #8139 evidence-post: this exact SyntaxError blocked my first chat.postMessage attempt. Quick fix, but the lesson generalizes — any time a date or ID list uses leading zeros, build the string outside the f-string or quote it.
Common failure modes
(Updated v1.6.0 — added rows for the 2 new failure modes observed today.)
Symptom
Cause
Fix
Post lands with [MEDIA:/path/to/file.png] as literal text and FileCount: 0
Used the wrong recipe — mcp__slack__conversations_add_message doesn't honor MEDIA: inline tokens
Run the Pre-Send Gate (top of this file) BEFORE invoking the message tool
User says "you always fail to attach" / "you always forget"
The pre-send gate was not enforced; agent defaulted to MEDIA:/path text tokens
The user's complaint IS the trigger — load this skill NOW and attach the missing media. Don't argue, don't ask, just upload.
Both bot AND xoxp tokens return missing_scope: files:write[:user] on Stage 1
OAuth scope gap (verified 2026-07-14, re-verified 2026-07-15). The user must reinstall files:write scope on the Slack app
Run scripts/check_token_scopes.py first to detect this WITHOUT burning 4 curl calls. If both fail, use third-tier gist-raw-URL fallback (Recipe below).
Stage 1 returns invalid_arguments with missing required field: length, filename
Used -d JSON instead of -F form fields. JSON silently drops form fields.
Use curl -F filename=... -F length=... (NOT -d '{"filename":...,"length":...}').
Stage 3 returns invalid_arguments with invalid additional property: initial_comment [json-pointer:/files/0]
initial_comment is a top-level Stage 3 parameter, NOT a per-file property
Move initial_comment from files[i] to the top-level payload.
SyntaxError: leading zeros in decimal integer literals when posting
Python 3.14 parses {02,04,05} inside f-string as set literal with octal-int
Quote the contents: f"{'02,04,05'}" or build the string outside the f-string.
conversations.replies shows my message has FileCount: 0 even though upload succeeded
Slack quirk — completeUploadExternal shares surface attachments via files array, not file_count integer
Verify via the files array, not file_count. Both are correct; file_count: 0 is just unreliable.
Attachments uploaded but user says "where's the screenshot?"
Internal narration (think-block, tool-call echoes) buried the attachments under 4-8 bot messages
Post the consolidated summary message AFTER the upload batch — it's the cluster anchor the user reads.
Agent chat.deletes an evidence attachment by mistake while trying to clean up thread noise (verified 2026-07-24 PR #8561)
conversations.replies returned short bot messages interleaved with file-bearing messages. Agent sorted by text length, picked the shortest message that "looked like terminal noise", and called chat.delete. That message's files array contained the captioned MP4 — Slack returned ok:true, the MP4 vanished from the thread, no recovery.
NEVER delete bot messages in a thread that contains evidence attachments. If the thread is cluttered, post ONE clean summary message that re-anchors on the existing attachments (by file name + gist URL) instead of deleting anything. If you absolutely must clean up, only delete messages whose files array is empty AND whose text does NOT contain BEFORE/AFTER/screenshot patterns. Verify intent with a precise ts filter — never pick by text-length heuristics.
User catches false visual evidence — page.evaluate() reported display:flex, opacity:1 but the element was visually hidden by a sibling (verified 2026-07-24 PR #8561, hit twice)
v2 evidence pitfall (added 2026-07-24).page.evaluate() returns computed style but does not account for stacking-context painting order, parent stacking contexts, or sibling z-index. An element can have display:flex, opacity:1, rect.y=X and still be visually hidden by another element on top. PR #8561 v1+v2 both shipped "chevron visible" claims where the chevron was hidden behind sticky .wizard-navigation (z-index:50). User: "lmao arrows / your screenshots are showing lmao arrows".
ALWAYS vision_analyze the actual screenshot pixel content before claiming visual proof. Two-step gate: (1) computed style confirms DOM presence, (2) vision_analyze confirms pixel-level visibility. If vision returns "I don't see X" or "the area is blank", the proof is invalid regardless of computed-style state. Companion gate belongs in ~/.hermes/skills/visual-evidence-with-playwright/ (proposed new skill). Also: Playwright is_mobile=False + viewport 390x844 matches matchMedia('(max-width: 768px)') but does NOT replicate mobile layout (sticky positioning, touch targets) — use is_mobile=True for mobile evidence.
Gist PNG URL serves content-type: text/plain; charset=utf-8 instead of image/png
NEW v1.8.0 — used api.github.com/gists POST with 'encoding': 'base64' directly, OR the gh gist create --public CLI. Both store bytes as utf-8 text.
Use the clone-and-replace recipe in scripts/upload_to_gist.sh: git clone https://${GH_TOKEN}@gist.github.com/<id>.git → git rm bad file → cp real binary bytes → add .gitattributes with *.png binary (and *.jpg binary, *.gif binary) → commit → git push origin HEAD. Verify new SHA via curl -I shows content-type: image/png.
conversations.replies returns not_in_channel error but my chat.postMessage succeeded
NEW v1.8.0 — Slack quirk: bots with chat:write.public can post to public channels they are NOT members of, but the read-side APIs (conversations.replies, conversations.history) require membership
Don't treat not_in_channel on conversations.replies as a post failure. Use chat.getPermalink?channel=X&message_ts=Y instead — it does NOT require channel membership and returns the public URL.
Gist raw URL renders as broken image in Slack thread
NEW v1.8.0 — Slack rendering is gated by the upstream Content-Type header. If it's text/plain (the broken-gist case above), Slack shows "broken image"
Fix the content-type per the row above. Slack only renders image/png, image/jpeg, image/gif, image/webp inline.
Verifying a text-file gist raw URL returns 404
NEW v1.9.0 — gh gist create <file> stores the file under its original name, but the public HTML URL guesses the wrong filename; the raw URL needs the actual commit-sha, not the filename-only path.
Run git clone https://gist.github.com/<id>.git /tmp/g, then git rev-parse HEAD inside the clone — the raw URL is https://gist.githubusercontent.com/<user>/<id>/raw/<sha>/<original-filename>. Verify with curl -fsI returns 200.
Gist raw URL serves a binary file as text/plain (the v1.4-v1.8 trap)
Posted a .png via api.github.com/gists POST with encoding: base64
Use the v1.8 clone-and-replace recipe — text files do NOT have this trap (v1.9.0), but PNGs/GIFs/MP4s/PDFs do
OSError: [Errno 7] Argument list too long on subprocess.run(["curl", "-d", payload])
NEW v1.10.0 — the JSON payload for api.github.com/gists was passed on argv; base64-encoded binaries >~3 MiB blow past macOS ARG_MAX (~256 KiB)
Use --data-binary @/tmp/payload.json so curl reads from disk. The argv path is fine for short JSON, but binary payloads MUST use the @file form.
curl -fsI <raw_url> shows content-type: application/octet-stream for an MP4
NEW v1.10.0 — expected behavior after the clone-and-replace recipe for MP4. Slack's URL unfurler still renders .mp4 inline (file extension is the signal, not the content-type).
Don't treat application/octet-stream as a failure for MP4. The killer content-type is text/plain (the v1.4/v1.8 trap).
Third-tier fallback: gist-raw-URL image embeds (when neither token has files:write)
When scripts/check_token_scopes.py reports neither_has_scope, do not stop and ask. Push PNGs to a public gist, then post a single chat.postMessage with markdown image embeds and unfurl_media: true. Slack renders image/png inline; the same gist URLs also serve the PR description.
Verified end-to-end 2026-07-14 on PR #8139: gist ID 7cfcc454079283f8973054271d19efe6, 4 files (3 PNGs + 1 GIF), all served content-type: image/png / image/gif via curl -fsI. Posted via chat.postMessage with xoxp user token, unfurl_media: true — all 4 rendered inline as attachments in the thread (attachments: 4 on the bot message ts 1784065062.654269).
Re-verified 2026-07-15 on PR #7953: gist 4ab1139eae87bde6102bc0961cb0168b, SHA 5547f92, 1 PNG (51KB) served image/png. Posted via chat.postMessage with bot token (the chat:write.public scope covers cross-channel posting). Both the gist and the Slack embed worked end-to-end.
Recipe — the ONLY path that produces content-type: image/png
Pitfall (verified 2026-07-15): the api.github.com/gists API endpoint POST with 'content': <base64 bytes>, 'encoding': 'base64' STORES the bytes as utf-8 text. The resulting raw URL serves content-type: text/plain; charset=utf-8, and Slack renders the markdown  as a broken-image icon. Same problem with the gh gist create CLI for binary files — it base64-encodes but the storage layer is still text.
The working path (clone + cp real bytes + gitattributes):
Or use scripts/upload_to_gist.sh — wraps the above with one call. Verify each raw URL after push:
for url in $(git ls-tree HEAD | awk '{print $4}' | xargs -I{} echo"https://gist.githubusercontent.com/jleechan2015/<id>/raw/$(git rev-parse HEAD)/{}"); do
curl -fsI "$url" | grep -i "^content-type:"done# Expected: content-type: image/png (or image/gif)
State the blocker explicitly in the summary so the user knows it's an OAuth-scope fallback, not the canonical path.
Update the PR description with the same gist raw URLs via gh pr edit <N> --body-file.
When to use third-tier vs the 3-stage flow
Situation
Path
Why
Bot token has files:write (default per preflight)
3-stage files.completeUploadExternal
Canonical — file appears as native Slack attachment
Bot fails but xoxp has files:write:user
SOUL.md xoxp fallback to SLACK_USER_TOKEN
Cross-workspace path; file attaches under user identity
Both tokens lack files:write/files:write:user
Third-tier gist-raw-URL embeds via chat.postMessage
Works without any OAuth scope; one evidence channel for PR desc + Slack thread
Why this skill exists (anti-pattern history, expanded v1.6.0)
The most common agent failure mode in this conversation log:
Agent captures a screenshot to disk, posts a Slack message that describes the screenshot ("here's the BEFORE/AFTER at /Users/.../evidence/foo.png"), but never attaches the file. User replies: "where's the screenshot?" or "stop forgetting use /harness and /learn and fix it."
Why this recurs despite the skill existing: The agent's default prompt encourages MEDIA:/path text tokens (lowest-friction path). The skill only loads if the agent remembers to skill_view it. The user's complaint is the trigger, not a pre-send check. The v1.5.0 Pre-Send Gate fixes this by intercepting the failure BEFORE the message tool is called.
The 4-incident chain (2026-07-02 → 2026-07-14):
Date
PR
What went wrong
Fix attempted
2026-07-02
#8139 (1st time)
4 PNGs + 1 GIF emitted as MEDIA:/path text tokens, all rendered as literal text
Skill v1.0 created
2026-07-10
#7953
4 PNGs uploaded successfully, but bridge chatter buried them under 6-8 bot messages
v1.1 added summary-as-cluster-anchor
2026-07-13
#8337
Stage 1 JSON body silently dropped form fields; Stage 3 strict about per-file extra properties
The lesson: when the same user correction hits ≥3 times across 12 days, the fix must move from "post-hoc documentation" to "pre-send invariant". This is encoded as a general principle in ~/.hermes/skills/harness-postmortem/SKILL.md Phase 1.5d (added 2026-07-14).
v1.8.0 addendum (PR #7953, 2026-07-15): The recurring evidence-attach failure recurred AGAIN today, but the actual cause was different from the 4-incident chain — the OAuth scope gap on HERMES_SLACK_BOT_TOKEN (files:write still missing) plus an additional gap in the gist recipe itself: api.github.com/gists POST with encoding: base64 does NOT preserve binary — the storage layer treats base64 content as utf-8 text, and the resulting raw URL serves content-type: text/plain; charset=utf-8. The fix is the clone-and-replace pattern documented above. The pattern holds: every PR cycle surfaces a new nuance in the same fall-back chain. v1.8.0 closes the gap that v1.4.0 left open (the "gist fallback works" claim was incomplete — it works only if you use clone-and-replace, NOT the direct API POST).
v1.10.0 addendum (Cloud Build E2E proof, 2026-07-16): Two new sub-pitfalls in the same fallback chain that v1.8 + v1.9 documented:
Pitfall A — subprocess.run argument-list limit on the curl+JSON base64 path. When uploading a binary via curl -X POST https://api.github.com/gists -d $(payload), the JSON payload is built in memory and passed on argv. For MP4s >~3 MiB the base64-encoded payload (≈1.33× size) approaches the macOS ARG_MAX (~256 KiB) and subprocess.run([...], ...) raises OSError: [Errno 7] Argument list too long. Symptom:
r = subprocess.run(["curl", "-d", open("/tmp/payload.json").read(), ...], ...)
# OSError: [Errno 7] Argument list too long: 'curl'
Fix: use --data-binary @file instead of -d <string>, so curl reads from disk and the payload never lands on argv:
Slack still renders MP4 inline when you paste https://gist.githubusercontent.com/.../raw/<sha>/file.mp4 directly in a message body (verified 2026-07-16, gist 77dd5406ec125ccb2a916c3a98787a4a, SHA c68173e, 3.6 MiB MP4 rendered inline in the user-facing Slack thread as an attachment). The application/octet-stream content-type is fine — what kills inline rendering is text/plain (the v1.4/v1.8 trap when you store the binary as utf-8 text via the API base64 path). application/octet-stream is treated as a generic binary stream and Slack's URL unfurler recognizes MP4 by file extension.
Why this still works for Slack thread delivery without the 3-stage API: once you have a public gist raw URL with the right SHA, you can either (a) embed it as a bare URL in a chat.postMessage body — Slack's unfurler detects .mp4 and renders inline, OR (b) try the files.completeUploadExternal 3-stage API first; if check_token_scopes.py reports neither_has_scope, fall straight to (a). For the Cloud Build proof run on 2026-07-16, both bot and xoxp tokens returned missing_scope: files:write[:user] on Stage 1 (verified in the chat log), so path (a) was the only working route.
v1.9.0 addendum (jleechanorg/disk_magician PR #17, 2026-07-15): The text-file gist delta. gh gist create <text-file> works directly for text evidence (.patch, .diff, .json, .md, .txt, .log, .yaml, .yml, .csv, .xml, .sh, .py). The clone-and-replace dance is unnecessary for text files — text is text, no base64-as-utf-8 corruption. The CLI base64-encodes on the way in but the storage is still text, and https://gist.githubusercontent.com/<user>/<id>/raw/<commit-sha>/<file> serves the text correctly with content-type: text/plain; charset=utf-8. The "raw URL" path requires git clone of the gist to discover the commit SHA (the public HTML URL guesses wrong — it's the SHA, not the filename), and the SHA-based URL with the exact filename works. Verified on PR #17 evidence run: 41,016-byte .patch file, diff between curl -fsS <raw-url> and local copy returned empty, byte count matched.
fenced code block (```lang) INLINE in the message body — Slack renders as native monospace
Same text types when too long for a single message (>~3500 chars)
gh gist create <file> --public, capture SHA via clone
raw URL inline in the message body — Slack makes it clickable
Any file the user explicitly wants as an attachment
all of the above
chat.getPermalink to verify, link to the raw URL in the message body
The principle: the only file types that need the third-tier gist-raw-URL trick for inline rendering are the binary types Slack won't render as native attachments (files:write is missing from both tokens). For text, Slack renders inline monospace from a fenced code block — there's no real problem to solve.
See references/recurring-failure-pattern-2026-07.md for the full transcripts and the trigger phrases the user actually typed.
Pairing with /harness and /learn
When the user says "use /harness and /learn", they are pointing at:
/harness → ~/.claude/plugins/marketplaces/claude-commands-marketplace/.claude/skills/harness-engineering.md — systematic infrastructure improvement vs one-off fix.
/learn → capture the lesson into a skill so the next agent reads it before posting evidence.
The right response when they say this about a missing-media failure is:
Now (immediate): attach the missing media to the thread using this skill's recipe. Don't argue, don't ask, just post.
Then (durable): the Pre-Send Gate (top of this file) prevents recurrence — if it's still being violated, the gate regex needs tuning, not the agent.
Then (audit): run scripts/check_token_scopes.py to verify the OAuth-scope gap hasn't been silently closed (it should report bot_has_scope once the user reinstalls files:write).
User suggestion "use /browser to fix your scopes" was PARTIALLY misrouted in 2026-07-15. The recurring evidence-attach failure's immediate cause WAS the OAuth-scope gap (correctly identified) but the suggested fix /browser was read as "go look at the Slack web UI manually" rather than "drive a browser-driven OAuth reinstall end-to-end." That left the gap unfixed for 4 days (PR #8139, PR #8337, PR #8455 all failed with missing_scope: files:write).
Correct interpretation (verified 2026-07-19):/browser IS the right vector for closing an OAuth-scope gap — the OAuth & Permissions page requires interactive JS-driven clicks (scope search → option click → Save → Reinstall → Allow in OAuth flow). The repair is via Playwright-driven cookie-injected Chromium session; full recipe in ~/.hermes/skills/devops/slack-mcp-mail-bot-reinstall/SKILL.md §11 + references/files-write-scope-reinstall-2026-07-19.md.
When the user suggests a different fix vector (different tool, different workflow), check whether the underlying intent is "make the gap stop recurring" — if so, drive it to completion even if the surface tool feels off-pattern for the symptom class. Don't dismiss it as misrouted; verify the actual end state.
Known Limitations
files.completeUploadExternal only handles binary files. Plain text (logs, JSON, code) should still be pasted as fenced code blocks in the message body.
1 GB hard limit per attachment (varies by workspace).
No inline preview on some Slack mobile clients. The attachment still uploads, but the user must tap to view.
Upload paths must be reachable from the gateway process. If you ran Playwright on your local machine but the gateway is on a different host, the path won't resolve.
Bot must be in the channel for Stage 3 file attachments. If completeUploadExternal returns channel_not_found for a private channel, invite the bot first. (Public-channel posts work without membership via chat:write.public, but conversations.replies returns not_in_channel — use chat.getPermalink for verification instead.)
Gist content-type is the gating factor for Slack inline rendering. If the raw URL serves text/plain, Slack will not render as an inline image — always verify with curl -fsI <raw_url> | grep -i content-type after pushing the gist.
Support files
scripts/upload_batch.py — parallel 3-stage upload loop for N PNGs in one execute_code call.
scripts/check_token_scopes.py — NEW v1.6.0 — OAuth scope preflight. Probes both tokens against auth.test and returns bot_has_scope / xoxp_has_scope / neither_has_scope. Run BEFORE Stage 1 to avoid wasted curl calls.
references/github-gist-binary-upload.md — GitHub gists API quirk catalog for binary uploads. UPDATED v1.8.0 — documents the base64-as-utf-8 storage pitfall and the clone-and-replace fix.
references/mp4-evidence-path.md — NEW v1.10.0 — MP4-specific path: gist clone-and-replace + content-type: application/octet-stream is fine + application/x-www-form-urlencoded workaround for --data-binary arg-list-limit. Verified 2026-07-16 on Cloud Build E2E proof (3.6 MiB MP4, byte-identical download).
references/recurring-failure-pattern-2026-07.md — NEW v1.6.0 — the 4-incident chain across 12 days with verbatim user trigger phrases. The data backing the pre-send gate.
references/false-evidence-incidents.md — NEW v1.12.0 — the v2 lesson: Pre-Send Gate catches missing-uploads but NOT false-claims. When agent asserts "X is visible" via page.evaluate(), vision_analyze the actual pixels before posting. Includes the two-step pre-claim gate recipe (PR #8561, 2026-07-24).
