LangChain Content Blocks (Python)

Overview

On Claude, AIMessage.content is list[dict] even for pure text — so any code from an OpenAI-first tutorial that calls message.content.lower() or message.content.split() crashes with AttributeError: 'list' object has no attribute 'lower' on the first production Claude call (P02). Multi-modal code that works on GPT-4o breaks on Claude because pre-1.0 image-block shapes differed across providers (P64). Multi-turn Claude replay with extended thinking fails with anthropic.BadRequestError: missing signature when prior thinking blocks are stripped. Forced tool_choice prevents stop_reason="end_turn" and loops forever (P63).

This is the deep-dive companion to langchain-model-inference. That skill's references/content-blocks.md covers the str vs list[dict] divergence and a safe text extractor. This skill goes further:

• tool_use block iteration mechanics — IDs, args as dict vs JSON string, streaming deltas
• thinking blocks — signature, redaction, multi-turn replay semantics
• document blocks — Claude citations API, source types, citation extraction
• Multi-modal composition — universal 1.0 image shape, per-provider adapter behavior
• Per-provider size limits (Anthropic 5 MB/image up to 20 images, OpenAI 20 MB/image, Gemini 20 MB/request)

Pin: langchain-core 1.0.x, langchain-anthropic >= 1.0, langchain-openai >= 1.0, anthropic >= 0.40. Pain-catalog anchors: P02, P58, P63, P64.

Prerequisites

• Python 3.10+
• langchain-core >= 1.0, < 2.0
• At least one provider package: pip install langchain-anthropic langchain-openai
• For extended thinking: langchain-anthropic >= 1.0 and Claude Sonnet 4+ / Opus 4+
• For citations: anthropic >= 0.40 and Claude Sonnet 4+
• Familiarity with langchain-model-inference (reads references/content-blocks.md first)

Instructions

Step 1 — Learn the block-type taxonomy

LangChain 1.0 defines six typed content blocks on AIMessage.content (and on chunks during streaming):

| Block type | Produced by | Notes | |------------|-------------|-------| | text | All providers | On Claude, always wrapped as [{"type":"text","text":"..."}] | | tool_use | Claude, GPT-4o, Gemini | Always round-trip via msg.tool_calls, not hand-parsed | | tool_result | You (via ToolMessage) | One per tool_use; tool_call_id must match byte-for-byte | | image | Claude vision, GPT-4o, Gemini | Universal 1.0 shape; adapter handles wire format per provider | | thinking | Claude extended thinking only | Must preserve signature for replay | | document | Claude citations API (Sonnet 4+) | Input-side only; citations attach to output text blocks |

See Block-Type Matrix for the full table with streaming behavior and per-type gotchas.

Step 2 — Iterate mixed content safely

For most code, use the helpers:

python
text = msg.text()                    # concatenated text across all text blocks
tool_calls = msg.tool_calls          # normalized list[ToolCall]
usage = msg.usage_metadata           # input_tokens, output_tokens, cache_*

Hand-roll block iteration only when you need to (a) preserve order, (b) extract thinking blocks for replay, or (c) read citations metadata from text blocks. Order-preserving iteration:

python
from langchain_core.messages import AIMessage

def iter_blocks(msg: AIMessage):
    if isinstance(msg.content, str):
        yield "text", {"type": "text", "text": msg.content}
        return
    for block in msg.content:
        if isinstance(block, dict):
            yield block.get("type", "unknown"), block
        else:
            yield getattr(block, "type", "unknown"), block

Step 3 — Compose multi-modal messages with the universal image block

python
import base64
from pathlib import Path
from langchain_core.messages import HumanMessage

def image_block(path: str) -> dict:
    data = base64.standard_b64encode(Path(path).read_bytes()).decode("ascii")
    mime = {"png": "image/png", "jpg": "image/jpeg",
            "jpeg": "image/jpeg", "webp": "image/webp"}[
        Path(path).suffix.lstrip(".").lower()]
    return {
        "type": "image",
        "source_type": "base64",   # or "url"
        "data": data,
        "mime_type": mime,
    }

msg = HumanMessage(content=[
    image_block("screenshot.png"),                         # put image FIRST
    {"type": "text", "text": "What is broken here?"},      # instruction LAST
])
response = claude.invoke([msg])

Three invariants:

1. content must be list[dict] when including non-text blocks.
2. Put the image before the instruction — Claude attends most to trailing tokens.
3. Respect provider limits (Anthropic: 5 MB/image, up to 20 images; OpenAI: 20 MB/image; Gemini: 20 MB/request total).

LangChain's adapter translates the universal shape to each provider's wire format. See Multi-Modal Composition for the full adapter table, MIME-type compatibility, and the document/citations pattern.

Step 4 — Iterate tool_use correctly across stream deltas

Canonical non-streaming:

python
for tc in msg.tool_calls:
    output = tools[tc["name"]](**tc["args"])
    history.append(ToolMessage(content=str(output), tool_call_id=tc["id"]))

tc["args"] is already a parsed dict — do not json.loads it. tc["id"] is provider-shaped (toolu_* on Anthropic, call_* on OpenAI, 24+ chars) and must be copied verbatim to the ToolMessage.

Streaming is different. tool_use.input arrives as partial JSON fragments across on_chat_model_stream events. Buffer with tool_call_chunks, parse once at on_chat_model_end:

python
from collections import defaultdict
import json

partial = defaultdict(str)   # index -> accumulated JSON fragment
meta = {}                     # index -> {name, id}

async for event in model.astream_events({"messages": [...]}, version="v2"):
    if event["event"] != "on_chat_model_stream":
        continue
    for tc_chunk in getattr(event["data"]["chunk"], "tool_call_chunks", []) or []:
        idx = tc_chunk["index"]
        if tc_chunk.get("name"):
            meta[idx] = {"name": tc_chunk["name"], "id": tc_chunk["id"]}
        if tc_chunk.get("args"):
            partial[idx] += tc_chunk["args"]

completed = [{**meta[i], "args": json.loads(partial[i])} for i in meta]

See Tool-Use Iteration for multi-tool-per-turn handling, ToolMessage ordering, and the forced- tool_choice infinite-loop trap (P63).

Step 5 — Preserve Claude thinking blocks for replay

Claude extended thinking (Sonnet 4+, Opus 4+) returns thinking blocks carrying a cryptographic signature. The next turn must round-trip those blocks intact or Anthropic rejects the request:

anthropic.BadRequestError: messages.1.content.0: missing signature

The foot-gun: msg.text() strips thinking blocks. Never do:

python
# WRONG — thinking blocks lost, replay fails
history.append(AIMessage(content=ai_1.text()))

Correct — pass the AIMessage back verbatim:

python
history.append(ai_1)   # preserves full content list + signatures

For persistence across sessions, serialize with messages_to_dict(...) (not custom JSON), which preserves block structure:

python
import json
from langchain_core.messages import messages_to_dict, messages_from_dict

serialized = json.dumps(messages_to_dict([ai_1]))
restored = messages_from_dict(json.loads(serialized))

See Thinking Blocks for redaction handling, the budget-tokens rule, and the interaction with tool calls.

Step 6 — Provider-adapter checklist

Before sending any multi-modal or tool-using message:

1. Is content a list[dict] when it contains non-text blocks?
2. Are image blocks in the universal 1.0 shape (source_type, data, mime_type)?
3. Is each image under the target provider's limit? (5 MB / 20 MB / 20 MB total.)
4. If tool_use is involved, am I passing msg.tool_calls — not parsed content?
5. If extended thinking is on, am I returning the full AIMessage — not msg.text()?
6. System message at position 0 (P58) — not reordered by middleware?

Output

• Block-type matrix applied to a specific response (which types present, which helper used)
• Safe iteration that preserves order, citations, and thinking signatures
• Multi-modal HumanMessage in the universal 1.0 image shape, portable across Claude/GPT-4o/Gemini
• tool_use stream-delta accumulator that buffers partial input JSON and parses once at end
• Multi-turn Claude replay that keeps thinking blocks intact (no missing signature errors)
• document/citations extractor that reads citations metadata from text blocks

Error Handling

| Error | Cause | Fix | |-------|-------|-----| | AttributeError: 'list' object has no attribute 'lower' | Treating AIMessage.content as str on Claude (P02) | Use msg.text() or iterate blocks | | anthropic.BadRequestError: messages.N.content.M: missing signature | Stripped thinking block on replay | Pass AIMessage object back verbatim; never rebuild from text() | | anthropic.BadRequestError: tool_use_id not found in corresponding tool_result | Typo / case mismatch in ToolMessage.tool_call_id | Copy tc["id"] verbatim | | anthropic.BadRequestError: tool_use ids were found without tool_result blocks | Skipped a tool call | Emit one ToolMessage per tool_call (use status="error" on failure) | | anthropic.BadRequestError: image exceeds 5 MB limit | Un-resized screenshot | Pre-resize to < 5 MB (1024x1024 JPEG 85 is ~500 KB) | | openai.BadRequestError: Invalid image data | Hand-rolled image_url with wrong prefix | Use the universal block; adapter emits the data:image/...;base64, prefix | | Infinite agent loop | Forced tool_choice inside a loop (P63) | Use tool_choice="auto" for agents; forced-choice only for single-call extraction | | json.JSONDecodeError inside stream loop | Parsing partial tool_use.input fragment | Buffer in a defaultdict(str); parse once at on_chat_model_end | | Citations silently missing | Read via msg.text() which strips metadata | Iterate msg.content and read block["citations"] on text blocks |

Examples

Single-shot multi-modal on Claude + GPT-4o with one message object

python
msg = HumanMessage(content=[
    image_block("ui.png"),
    {"type": "text", "text": "Identify the broken UI element."},
])
# Same message works on both providers via adapter translation
claude_resp = claude.invoke([msg])
gpt4o_resp = gpt4o.invoke([msg])

Multi-turn Claude replay with extended thinking

python
claude = ChatAnthropic(
    model="claude-sonnet-4-6",
    max_tokens=8192,
    thinking={"type": "enabled", "budget_tokens": 4096},
)

ai_1 = claude.invoke([HumanMessage(content="What is the capital of France?")])
# ai_1.content == [{"type":"thinking",...,"signature":"..."}, {"type":"text",...}]

# Turn 2 — pass ai_1 VERBATIM
ai_2 = claude.invoke([
    HumanMessage(content="What is the capital of France?"),
    ai_1,                                                    # thinking preserved
    HumanMessage(content="And the population?"),
])

See Thinking Blocks for the full replay invariants and persistence pattern.

Extracting Claude citations from document input

python
doc_block = {
    "type": "document",
    "source": {"type": "base64", "media_type": "application/pdf", "data": pdf_b64},
    "title": "Q3 Earnings Report",
    "citations": {"enabled": True},
}
resp = claude.invoke([HumanMessage(content=[
    doc_block,
    {"type": "text", "text": "What drove revenue this quarter?"},
])])

for block in resp.content:
    if block.get("type") != "text":
        continue
    print(block["text"])
    for c in block.get("citations", []):
        print(f"  -> {c['document_title']}: {c['cited_text']!r}")

msg.text() flattens this — you lose citations. See Multi-Modal Composition for the full document block reference including supported source types.

Streaming tool_use with live argument rendering

See Tool-Use Iteration for the complete tool_call_chunks accumulator including multi-tool-per-turn handling and the ToolMessage ordering invariant.

Resources

• LangChain messages concept
• LangChain multimodality
• AIMessage API reference
• Anthropic content blocks / messages API
• Anthropic extended thinking
• Anthropic citations
• OpenAI vision
• Gemini multimodal
• Companion skill: langchain-model-inference (read its references/content-blocks.md for the str vs list[dict] fundamentals)
• Pack pain catalog: docs/pain-catalog.md (entries P02, P58, P63, P64)