claude-code-session-jsonl-whitespace-text-block

Claude Code session JSONL — whitespace-only text block repair

Problem

A Claude Code session becomes permanently stuck: every API request — typing a new prompt, /compact, or claude --resume <id> — returns:

API Error: 400 messages: text content blocks must contain non-whitespace text

The Anthropic API validates every {"type":"text","text":...} block in incoming messages and rejects any whose text is empty or whitespace-only. The broken block lives in the on-disk transcript, so it gets replayed every time — the session is hostage until the file is edited.

Context / Trigger Conditions

Any one of these:

1. Active session refuses new input with 400 ... text content blocks must contain non-whitespace text.
2. /compact fails with the same 400. Important: /compact does NOT bypass the validator — it sends the full history to the model first, so the same broken block kills it before any compaction happens.
3. claude --resume <session-id> exits with the same 400 on first turn.
4. The session has used extended thinking at least once (this is the dominant trigger — see Root Cause).

Root Cause

When an assistant turn starts with an extended-thinking block, Claude Code's serializer emits a synthetic leading text block whose content is a single space " " before the thinking block, presumably to satisfy some internal content-ordering constraint. Each content block is stored as its own JSONL line, so you end up with assistant lines like:

{"message":{"role":"assistant","content":[{"type":"text","text":" "}]}, ...}

The Anthropic API rejects these on replay. A long extended-thinking session can silently accumulate dozens of these — one user's session had 88 in a single file, another had 20 — and they only surface when something replays the full history (new prompt, /compact, resume).

Solution

Step 1 — locate candidate JSONLs

Session JSONLs live at:

~/.claude/projects/<encoded-cwd>/<session-uuid>.jsonl

The encoded-cwd directory replaces / with - in the working directory path. The most recently modified .jsonl in the relevant project dir is usually the broken one.

Step 2 — confirm the diagnosis (walk JSON, do NOT just grep)

The empty block is typically deeply nested inside message.content[0].text. Plain grep '"text":""' works for exactly empty strings but misses " "/"\n"/etc. Use a recursive JSON walk:

python3 - <<'PY'
import json, glob, pathlib
home = pathlib.Path.home()
def w(o):
    if isinstance(o, dict):
        if o.get("type") == "text":
            t = o.get("text")
            if isinstance(t, str) and t.strip() == "":
                yield 1
        for v in o.values(): yield from w(v)
    elif isinstance(o, list):
        for v in o: yield from w(v)
hits = []
for fp in sorted(glob.glob(str(home / ".claude/projects/*/*.jsonl"))):
    n = 0
    with open(fp, errors='ignore') as f:
        for line in f:
            line = line.strip()
            if not line: continue
            try:
                n += sum(w(json.loads(line)))
            except: pass
    if n: hits.append((n, fp))
for n, fp in hits: print(f"{n:5d}  {fp}")
PY

Step 3 — distinguish API-bound vs local-only empty blocks

Not every whitespace text block causes the 400. Two locations matter:

Path in JSONL	Sent to API?	Action
.message.content[*].text	Yes	Must fix
.toolUseResult (and nested)	No — local	Leave alone

A fixer that targets only the API-bound location avoids unnecessary mutation.

Step 4 — run the fixer

Idempotent script in scripts/fix-empty-text-blocks.py (also installable to ~/.claude/bin/). It:

• Walks each line's message.content array
• Replaces any whitespace-only text block's text with "." (the smallest non-whitespace placeholder — preserves message-shape alternation; deleting the block could orphan adjacent tool_use_ids)
• Writes a backup at <path>.bak.<epoch> before mutating
• Skips files with no fixes (no backup created)

~/.claude/bin/fix-empty-text-blocks.py PATH/TO/session.jsonl
~/.claude/bin/fix-empty-text-blocks.py --all   # scan every session file

Step 5 — re-launch and verify

The on-disk file is now valid. Quit the broken Claude Code session (Ctrl+D / close terminal) and re-launch:

claude --resume <session-id>

The first turn will succeed. /compact will now work.

Verification

After fixing, re-run the Step-2 detector. Files with remaining empty text blocks in message.content should be 0. (Stragglers in toolUseResult are benign — they don't cross the API boundary.)

Example

$ /Users/x/.claude/bin/fix-empty-text-blocks.py --all
/Users/x/.claude/projects/.../<uuid-A>.jsonl
  fixed 20 block(s); backup at .../<uuid-A>.jsonl.bak.1779986308
/Users/x/.claude/projects/.../<uuid-B>.jsonl
  fixed 88 block(s); backup at .../<uuid-B>.jsonl.bak.1779986328

Total fixes across 1326 file(s): 108

The two affected sessions resumed cleanly; /compact ran end-to-end on both.

Notes

• Why replace with "." instead of deleting the block? Deleting an entire content block can violate alternation invariants if the block is the only one in a content array (it was, in observed cases). Replacement keeps the message structurally identical and the placeholder character is inconsequential to model behaviour on replay.
• Why does this differ from claude-code-session-jsonl-orphan-advisor-tool-result? Same family of failure (broken JSONL → all API calls fail → /compact blocked) but the offending block type and root cause are different. That skill repairs orphan server_tool_use / *_tool_result pairs caused by mid-stream interruption. This skill repairs whitespace-only text blocks caused by extended-thinking serialization. The diagnosis path branches on the exact 400 message:
  • text content blocks must contain non-whitespace text → this skill
  • unexpected tool_use_id found in advisor_tool_result blocks → that skill
• Optional auto-scrub via Stop hook: writing a global Stop hook in ~/.claude/settings.json that calls the fixer with the active session's JSONL path on every session end will prevent recurrence cost-free. Not enabled by default because it mutates session state on every stop; recommend only after the user understands the trade-off.
• The 0 MB free red herring: an unrelated symptom that can appear on the same broken session is the temp filesystem at /private/tmp/claude-<uid>/.../tasks is full (0MB free). This is Claude Code's per-session tasks-dir quota, not actual disk pressure (df -h will show plenty of room). Clearing /private/tmp/claude-<uid>/* or starting a new session resets the quota. Do not assume disk is full when you see this — verify with df -h first.

See also

• claude-code-session-jsonl-orphan-advisor-tool-result — sibling skill for the orphan tool_use_id variant of the broken-JSONL failure family.
• claude-code-session-jsonl family more broadly — the JSONL on disk is the single source of truth for replay; any structural defect in it is hostage until repaired in place.

References

• Anthropic Messages API content-block validation: empty / whitespace-only text blocks are rejected with HTTP 400 at validation time, before the request reaches the model.
• Observed on Claude Code (macOS) — Opus 4.7, May 2026. Reproduced across two user-active sessions, 108 total whitespace blocks repaired without loss of conversation state.