Python: fix(core): treat a missing text delta as empty when adding text content

[Anish Mehta (anishmehta24)]

Motivation and Context

text is optional on a text content — Content("text") and Content.from_dict({"type": "text"}) both produce one with text=None, and test_chat_response_value_handles_text_content_without_text already covers that shape existing. But _add_text_content concatenates the attributes directly:

text=self.text + other.text,  # type: ignore[attr-defined, operator]

so adding one raises TypeError: can only concatenate str (not "NoneType") to str.

That's reachable from the public API rather than just from +. _coalesce_text_content merges consecutive text contents with + during _finalize_response, so a single text part carrying no delta (annotations or metadata only) takes down the whole response:

ChatResponse.from_updates([
    ChatResponseUpdate(role="assistant", contents=[Content("text", text="Hello ")]),
    ChatResponseUpdate(role="assistant", contents=[Content("text")]),
    ChatResponseUpdate(role="assistant", contents=[Content("text", text="world")]),
])
# TypeError: can only concatenate str (not "NoneType") to str

The same stream built out of text_reasoning parts returns "Hello world" and always has, because _add_text_reasoning_content handles this case deliberately — there's a comment there about Anthropic emitting a thinking block with thinking="". The two content types just disagreed about what a missing value means.

Description

_add_text_content now follows the same rule as its reasoning counterpart: a missing delta counts as empty, and None survives only when neither side has text. That keeps None (no text) distinguishable from "" (an empty one), which matters for the same replay reason as in the reasoning path.

One line of behaviour change, no signature change. The # type: ignore[operator] comment is gone too, since the operands are strings now.

Contribution Checklist

• The code builds clean without any errors or warnings
• I didn't break anyone 😄

Tests in packages/core/tests/core/test_types.py:

• test_text_content_add_handles_missing_text — adding in either direction
• test_text_content_add_preserves_none_and_empty_string — None + None stays None, "" + None stays ""
• test_chat_response_from_updates_coalesces_text_update_without_text — the reachable path, asserting text and text_reasoning now behave the same

All three fail on main. tests/core/test_types.py passes in full, and the whole tests/core directory is green. ruff check and ruff format clean.

🤖 Generated with Claude Code

https://claude.ai/code/session_01WyCUJcDPnCnQMmuDwsnDmD

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[Eduard van Valkenburg (eavanvalkenburg)]

Anish Mehta (@anishmehta24) Reviewed the current head and found no blocking issues.