diff --git a/agent/chat_completion_helpers.py b/agent/chat_completion_helpers.py
index 0fd556f8403d0..11a8c86535e98 100644
--- a/agent/chat_completion_helpers.py
+++ b/agent/chat_completion_helpers.py
@@ -39,6 +39,7 @@ from agent.message_sanitization import (
     _sanitize_surrogates,
     _repair_tool_call_arguments,
 )
+from agent.reasoning_summaries import separate_glued_reasoning_blocks
 from agent.stream_single_writer import claim_stream_writer, stream_writer_is_current
 from tools.terminal_tool import is_persistent_env
 from utils import base_url_host_matches, base_url_hostname, env_float, env_int
@@ -3256,6 +3257,14 @@ def interruptible_streaming_api_call(agent, api_kwargs: dict, *, on_first_delta=
             # Accumulate reasoning content
             reasoning_text = getattr(delta, "reasoning_content", None) or getattr(delta, "reasoning", None)
             if reasoning_text:
+                # Summary-part models (gpt-5.x and other Responses relays) send
+                # one complete markdown block per delta with no separator, so
+                # the parts glue into a single unreadable run. Only the tail of
+                # what's accumulated matters. See agent/reasoning_summaries.py.
+                reasoning_text = separate_glued_reasoning_blocks(
+                    reasoning_parts[-1] if reasoning_parts else "",
+                    reasoning_text,
+                )
                 reasoning_parts.append(reasoning_text)
                 _fire_first_delta()
                 agent._fire_reasoning_delta(reasoning_text)
diff --git a/agent/codex_runtime.py b/agent/codex_runtime.py
index c01084c4a4499..f36f1c78fedac 100644
--- a/agent/codex_runtime.py
+++ b/agent/codex_runtime.py
@@ -1027,6 +1027,10 @@ def _consume_codex_event_stream(
     first_delta_fired = False
     active_message_phase: str | None = None
     commentary_text_deltas: List[str] = []
+    # Last reasoning summary_index seen. The Responses stream delimits summary
+    # parts by this index and gives each part no separator of its own, so a
+    # change of index is where the blank line belongs.
+    active_summary_index: Any = None
     terminal_status: str = "completed"
     terminal_usage: Any = None
     terminal_response_id: str = None
@@ -1120,6 +1124,17 @@ def _consume_codex_event_stream(
         if "reasoning" in event_type and "delta" in event_type:
             reasoning_text = _event_field(event, "delta", "")
             if reasoning_text and on_reasoning_delta is not None:
+                # Summary parts stream one after another with no separator of
+                # their own; summary_index is the boundary the wire gives us.
+                summary_index = _event_field(event, "summary_index")
+                if (
+                    summary_index is not None
+                    and active_summary_index is not None
+                    and summary_index != active_summary_index
+                ):
+                    reasoning_text = f"\n\n{reasoning_text}"
+                if summary_index is not None:
+                    active_summary_index = summary_index
                 try:
                     on_reasoning_delta(reasoning_text)
                 except Exception:
diff --git a/agent/reasoning_summaries.py b/agent/reasoning_summaries.py
new file mode 100644
index 0000000000000..2dd1144fce992
--- /dev/null
+++ b/agent/reasoning_summaries.py
@@ -0,0 +1,67 @@
+"""Boundary repair for providers that stream reasoning as discrete summary parts.
+
+Reasoning-summary models (OpenAI's gpt-5.x family, and anything relaying the
+Responses API onto the OpenAI chat wire) do not stream a chain of thought token
+by token. They emit one ``reasoning_content`` delta per *completed* summary
+part, each opening with a bold markdown heading::
+
+    {"delta": {"reasoning_content": "**Investigating likely culprit PRs**"}}
+    {"delta": {"reasoning_content": "**Inspecting message schema**"}}
+
+On the Responses API those parts are delimited by ``summary_index``
+(``response.reasoning_summary_part.added`` / ``.done``). The OpenAI chat wire
+carries no such field — verified live against Nous Portal's
+``openai/gpt-5.6-sol``, whose reasoning chunks contain nothing but
+``delta.reasoning_content`` — so the boundary cannot be recovered from
+metadata, and consumers that concatenate deltas glue the parts together:
+
+    **Investigating likely culprit PRs****Inspecting message schema**
+
+That ``****`` run is neither a bold close nor a bold open to a markdown parser,
+so the whole trace renders as one unbroken, unspaced, half-bold paragraph.
+
+The AI SDK hit exactly this (vercel/ai#6742) and fixed it upstream by starting
+a new reasoning part per ``summary_index``. That route needs the index, which
+this wire does not give us, so we re-derive the boundary from the one signal it
+does carry: a delta opening a bold heading. Hermes' own Responses adapter
+already joins its summary parts with a blank line
+(``agent/codex_responses_adapter.py``), so this brings the chat-completions
+stream in line with the path that keeps the structure.
+"""
+
+from __future__ import annotations
+
+__all__ = ["separate_glued_reasoning_blocks"]
+
+
+def separate_glued_reasoning_blocks(previous: str, delta: str) -> str:
+    """Return *delta*, prefixed with a paragraph break when it glues onto *previous*.
+
+    *previous* is the reasoning text accumulated so far (only its tail matters).
+    A break is inserted when *delta* opens a bold heading and *previous* is
+    mid-line, which is the summary-part boundary the chat wire drops. Both
+    shapes the upstream issue reports are covered: a heading-only part butting
+    against the next heading (``**One****Two**``), and a part whose prose body
+    butts against the next heading (``...interaction!**Next**``).
+
+    Token-streamed reasoning is left alone: its deltas carry their own leading
+    whitespace, so *previous* ends mid-line only when the model really did run
+    two parts together.
+    """
+    if not previous or not delta:
+        return delta
+
+    if not delta.startswith("**"):
+        return delta
+
+    # Already separated — the provider (or an earlier part) ended the line.
+    if previous[-1].isspace():
+        return delta
+
+    # Require a *closed* heading. A token-streamed fragment that merely opens
+    # emphasis ("**" then "bold" then "**" across three deltas) is not a part
+    # boundary; a summary part always carries its whole heading in one delta.
+    if "**" not in delta[2:]:
+        return delta
+
+    return f"\n\n{delta}"
diff --git a/apps/desktop/src/components/assistant-ui/thread/message-parts.tsx b/apps/desktop/src/components/assistant-ui/thread/message-parts.tsx
index 8bcc946453871..df42bc175423c 100644
--- a/apps/desktop/src/components/assistant-ui/thread/message-parts.tsx
+++ b/apps/desktop/src/components/assistant-ui/thread/message-parts.tsx
@@ -16,6 +16,7 @@ import { GeneratedImage } from '@/components/chat/generated-image-result'
 import { SCAFFOLD_LABEL_CLASS, SCAFFOLD_META_CLASS, ScaffoldRow } from '@/components/chat/scaffold-row'
 import { useI18n } from '@/i18n'
 import { generatedImageFromResult } from '@/lib/generated-images'
+import { separateGluedReasoningBlocks } from '@/lib/reasoning-blocks'
 import { useEnterAnimation } from '@/lib/use-enter-animation'
 import { cn } from '@/lib/utils'
 
@@ -249,7 +250,7 @@ const ReasoningTextPart: ReasoningMessagePartComponent = () => {
       containerProps={{ 'data-slot': 'aui_reasoning-text' } as ComponentProps<'div'>}
       disableArtifacts
       isRunning={status.type === 'running' || messageRunning}
-      text={text.trimStart()}
+      text={separateGluedReasoningBlocks(text.trimStart())}
     />
   )
 }
diff --git a/apps/desktop/src/lib/reasoning-blocks.test.ts b/apps/desktop/src/lib/reasoning-blocks.test.ts
new file mode 100644
index 0000000000000..cab4ab90e25b9
--- /dev/null
+++ b/apps/desktop/src/lib/reasoning-blocks.test.ts
@@ -0,0 +1,48 @@
+import { describe, expect, it } from 'vitest'
+
+import { separateGluedReasoningBlocks } from '@/lib/reasoning-blocks'
+
+describe('separateGluedReasoningBlocks', () => {
+  it('splits heading-onto-heading parts (the `****` run)', () => {
+    const glued =
+      '**Investigating likely culprit PRs****Inspecting message schema****Analyzing interrupted tool call impact**'
+
+    expect(separateGluedReasoningBlocks(glued)).toBe(
+      [
+        '**Investigating likely culprit PRs**',
+        '',
+        '**Inspecting message schema**',
+        '',
+        '**Analyzing interrupted tool call impact**'
+      ].join('\n')
+    )
+  })
+
+  it('splits prose-onto-heading parts (vercel/ai#6742 repro)', () => {
+    const glued =
+      '**Simulating a greeting stream**\n\nIt feels like a streaming interaction!**Simulating a greeting stream**\n\nI want to meet the request.'
+
+    expect(separateGluedReasoningBlocks(glued)).toContain('interaction!\n\n**Simulating')
+    expect(separateGluedReasoningBlocks(glued)).not.toContain('interaction!**')
+  })
+
+  it('is idempotent on already-separated text', () => {
+    const separated = '**One**\n\n**Two**'
+
+    expect(separateGluedReasoningBlocks(separated)).toBe(separated)
+  })
+
+  it('leaves emphasis inside prose alone', () => {
+    const prose = 'Looking at the logs, the **signature** field is missing — so the replay 400s.'
+
+    expect(separateGluedReasoningBlocks(prose)).toBe(prose)
+  })
+
+  it('leaves an unclosed emphasis run alone', () => {
+    expect(separateGluedReasoningBlocks('weighing options **')).toBe('weighing options **')
+  })
+
+  it('does not split a heading that already opens the text', () => {
+    expect(separateGluedReasoningBlocks('**Only one part**')).toBe('**Only one part**')
+  })
+})
diff --git a/apps/desktop/src/lib/reasoning-blocks.ts b/apps/desktop/src/lib/reasoning-blocks.ts
new file mode 100644
index 0000000000000..a1122bd7478b2
--- /dev/null
+++ b/apps/desktop/src/lib/reasoning-blocks.ts
@@ -0,0 +1,31 @@
+/**
+ * Reasoning-summary models (OpenAI's gpt-5.x family, and anything relaying the
+ * Responses API onto the OpenAI chat wire) emit one delta per *completed*
+ * summary part, each opening with a bold markdown heading:
+ *
+ *   **Investigating likely culprit PRs**
+ *   **Inspecting message schema**
+ *
+ * The Responses API delimits those parts with `summary_index`; the chat wire
+ * carries no such field, so concatenated deltas glue into
+ * `...PRs****Inspecting...` — a `****` run markdown reads as neither a bold
+ * close nor a bold open, leaving one unbroken, unspaced, half-bold paragraph.
+ * The AI SDK hit the same bug (vercel/ai#6742).
+ *
+ * The backend now inserts the break as the deltas arrive. This repairs the text
+ * we display: reasoning persisted before that fix, and any provider still
+ * gluing its parts. Idempotent — a break already present is left alone.
+ */
+
+// A heading butting straight onto the previous part, in the two shapes the
+// wire produces:
+//   1. heading-onto-heading — `**One****Two**`, a bare `****` run.
+//   2. prose-onto-heading   — `interaction!**Two**`.
+// Emphasis that legitimately follows whitespace is left alone, and a heading
+// must close on its own line to count as a summary part.
+const GLUED_HEADING_RUN = /(?<!\*)\*{4}(?!\*)/g
+const GLUED_AFTER_PROSE = /(?<=[^\s*])(\*\*(?=[^\s*])[^\n]*?\*\*)/g
+
+export function separateGluedReasoningBlocks(text: string): string {
+  return text.replace(GLUED_HEADING_RUN, '**\n\n**').replace(GLUED_AFTER_PROSE, '\n\n$1')
+}
diff --git a/tests/agent/test_reasoning_summaries.py b/tests/agent/test_reasoning_summaries.py
new file mode 100644
index 0000000000000..6c93c534e3b75
--- /dev/null
+++ b/tests/agent/test_reasoning_summaries.py
@@ -0,0 +1,73 @@
+"""Reasoning summary-part boundary repair (agent/reasoning_summaries.py)."""
+
+from agent.reasoning_summaries import separate_glued_reasoning_blocks
+
+
+def _stream(deltas):
+    """Accumulate *deltas* the way the chat-completions stream loop does."""
+    parts: list[str] = []
+    for delta in deltas:
+        parts.append(
+            separate_glued_reasoning_blocks(parts[-1] if parts else "", delta)
+        )
+    return "".join(parts)
+
+
+def test_heading_only_parts_do_not_glue_into_one_run():
+    # The shape observed live on Nous Portal's openai/gpt-5.6-sol: each delta
+    # is a bare heading, so consecutive parts produce a `****` run.
+    text = _stream(
+        [
+            "**Investigating likely culprit PRs**",
+            "**Inspecting message schema and tool_calls content**",
+            "**Analyzing interrupted tool call impact**",
+        ]
+    )
+
+    assert "****" not in text
+    assert text.splitlines() == [
+        "**Investigating likely culprit PRs**",
+        "",
+        "**Inspecting message schema and tool_calls content**",
+        "",
+        "**Analyzing interrupted tool call impact**",
+    ]
+
+
+def test_prose_body_does_not_glue_onto_the_next_heading():
+    # vercel/ai#6742's repro: a part ends in prose and the next heading butts
+    # straight onto it, with no `****` run to key off.
+    text = _stream(
+        [
+            "**Simulating a greeting stream**\n\nIt feels like a streaming interaction!",
+            "**Simulating a greeting stream**\n\nI want to meet the request.",
+        ]
+    )
+
+    assert "interaction!**" not in text
+    assert "interaction!\n\n**Simulating" in text
+
+
+def test_token_streamed_reasoning_is_untouched():
+    deltas = ["Looking at", " the session", " logs, I see", " one bold word."]
+
+    assert _stream(deltas) == "".join(deltas)
+
+
+def test_bold_word_mid_sentence_is_not_a_boundary():
+    # Emphasis inside token-streamed prose arrives after a space.
+    assert separate_glued_reasoning_blocks("I see the ", "**signature**") == "**signature**"
+
+
+def test_unclosed_emphasis_fragment_is_not_a_boundary():
+    # A token stream splitting emphasis across deltas opens but never closes.
+    assert separate_glued_reasoning_blocks("weighing", "**") == "**"
+
+
+def test_boundary_needs_a_bold_opener():
+    assert separate_glued_reasoning_blocks("**Closing**", "plain head") == "plain head"
+
+
+def test_empty_operands_pass_through():
+    assert separate_glued_reasoning_blocks("", "**first**") == "**first**"
+    assert separate_glued_reasoning_blocks("**first**", "") == ""
diff --git a/tests/run_agent/test_run_agent_codex_responses.py b/tests/run_agent/test_run_agent_codex_responses.py
index 4b11b1fdb7db9..18dc13526b7dc 100644
--- a/tests/run_agent/test_run_agent_codex_responses.py
+++ b/tests/run_agent/test_run_agent_codex_responses.py
@@ -1985,11 +1985,59 @@ def test_duplicate_detection_uses_commentary_when_hidden_reasoning_changes(monke
 
 
 
+def test_consume_codex_stream_separates_reasoning_summary_parts():
+    """summary_index is the part boundary; the wire sends no separator itself."""
+    from agent.codex_runtime import _consume_codex_event_stream
+
+    reasoning_streamed = []
+
+    _consume_codex_event_stream(
+        _FakeCreateStream([
+            SimpleNamespace(type="response.created"),
+            SimpleNamespace(
+                type="response.reasoning_summary_text.delta",
+                summary_index=0,
+                delta="**Investigating culprit PRs**",
+            ),
+            SimpleNamespace(
+                type="response.reasoning_summary_text.delta",
+                summary_index=1,
+                delta="**Inspecting message schema**",
+            ),
+            SimpleNamespace(
+                type="response.reasoning_summary_text.delta",
+                summary_index=1,
+                delta=" and tool_calls content",
+            ),
+            SimpleNamespace(type="response.completed", response=SimpleNamespace(status="completed")),
+        ]),
+        model="gpt-5-codex",
+        on_reasoning_delta=reasoning_streamed.append,
+    )
+
+    joined = "".join(reasoning_streamed)
+    assert "****" not in joined
+    assert joined == (
+        "**Investigating culprit PRs**"
+        "\n\n**Inspecting message schema** and tool_calls content"
+    )
 
 
+def test_consume_codex_stream_leaves_unindexed_reasoning_untouched():
+    """Streams with no summary_index (plain reasoning_text) must not gain breaks."""
+    from agent.codex_runtime import _consume_codex_event_stream
 
+    reasoning_streamed = []
 
+    _consume_codex_event_stream(
+        _FakeCreateStream([
+            SimpleNamespace(type="response.created"),
+            SimpleNamespace(type="response.reasoning_text.delta", delta="Need to "),
+            SimpleNamespace(type="response.reasoning_text.delta", delta="inspect files."),
+            SimpleNamespace(type="response.completed", response=SimpleNamespace(status="completed")),
+        ]),
+        model="gpt-5-codex",
+        on_reasoning_delta=reasoning_streamed.append,
+    )
 
-
-
-
+    assert "".join(reasoning_streamed) == "Need to inspect files."